Langmuir 2004, 20, 4977-4988
4977
Interactions between Nonpolar Surfaces Coated with the
Nonionic Surfactant Hexaoxyethylene Dodecyl Ether C12E6
and the Origin of Surface Charges at the Air/Water
Interface
Cosima Stubenrauch,*,† Orlando J. Rojas,‡ Judith Schlarmann,† and
Per M. Claesson§,|
Institut für Physikalische Chemie, Universität zu Köln, Luxemburger Str. 116,
D-50939 Köln, Germany, Department of Wood and Paper Science, North Carolina State
University, Box 8005, Raleigh 27695, North Carolina, Royal Institute of Technology,
Department of Chemistry, Surface Chemistry, Drottning Kristinas väg 51,
Stockhom, SE-100 44, Sweden, and Institute for Surface Chemistry,
Box 5607, Stockholm, SE-114 86, Sweden
Received November 6, 2003. In Final Form: March 10, 2004
The interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl
ether C12E6 were investigated using two techniques and three different types of surfaces. As nonpolar
surfaces, the air/water interface, silanated negatively charged glass, and thiolated uncharged gold surfaces
were chosen. The interactions between the air/water interfaces were measured with a thin film pressure
balance in terms of disjoining pressure as a function of film thickness. The interactions between the
solid/liquid interfaces were determined using a bimorph surface force apparatus. The influence of the
nature of the surface on the interaction forces was investigated at surfactant concentrations below and
above the cmc. The adsorption of the nonionic surfactant on the uncharged thiolated surface does not, as
expected, lead to any buildup of a surface charge. On the other hand, adsorption of C12E6 on the charged
silanated glass and the charged air/water interface results in a lowering of the surface charge density. The
reduction of the surface charge density on the silanated glass surfaces is rationalized by changes in the
dielectric permittivity around the charged silanol groups. The reason for the surface charge observed at
the air/water interface as well as its decrease with increasing surfactant concentration is discussed and
a new mechanism for generation of OH- ions at this particular interface is proposed.
1. Introduction
To understand interaction forces generated between
surfactant coated surfaces, several topics have to be
addressed. First, the adsorption of the surfactant at the
surface is of outstanding importance as each adsorbed
molecule alters the properties of the surface and thus the
respective interaction forces. Second, the nature of the
surface has to be considered because it influences the
interaction forces. This is due to both forces generated
between the surfaces themselves (e.g., double-layer forces
generated by charged surface groups) and the influence
of the surface on the structure of the adsorption layers.
For example, the adsorption of surfactants at hydrophilic
and hydrophobic surfaces, respectively, results in adsorption layers of completely different structure, which, in
turn, generates different interaction forces. Additionally,
the nature of the surfactant is crucial for the nature and
the magnitude of the interaction forces. These forces are
mainly due to repulsive short-range confinement, repulsive long-range electrostatic, and attractive short-range
van der Waals interactions.1,2 In addition, the presence of
an attractive long-range force, often referred to as the
“hydrophobic interaction”, is observed between nonpolar
* Corresponding author.
† Universität zu Köln.
‡ North Carolina State University.
§ Royal Institute of Technology.
| Institute for Surface Chemistry.
(1) Israelachvili, J. In Intermolecular and Surface Forces, 2nd ed.;
Academic Press: San Diego, CA, 1991.
(2) Israelachvili, J. N.; Wennerström, H. J. Phys. Chem. 1992, 96,
520.
surfaces in aqueous solutions.3,4 For the investigation of
these forces, different surface force techniques can be
employed. The choice of the appropriate technique depends
on the type of surface dealt with. For solid/liquid interfaces,
the atomic force microscope (AFM), the surface force
apparatus (SFA), and the MASIF-technique5-7 and for
both air/water and oil/water interfaces the thin film
pressure balance (TFPB) (refs 8 and 9 and references
therein) have proved to be suitable techniques. In this
study, we used the MASIF and TFPB techniques to probe
the interactions between surfactant coated surfaces across
the corresponding aqueous surfactant solution.
The present study is concerned with the interactions
between nonpolar surfaces coated with the nonionic
surfactant hexaoxyethylene dodecyl ether, C12E6. One aim
is to clarify the influence of the nature of surface on the
interaction forces. Our approach is to directly measure
the interaction forces between three different kinds of
nonpolar surfaces, namely, air/water, silanated glass, and
(3) Israelachvili, J. N.; Pashley, R. M. Nature 1982, 300, 341.
(4) Christenson, H. K.; Claesson, P. M. Adv. Colloid Interface Sci.
2001, 91, 391.
(5) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans.
1 1978, 74, 975.
(6) Parker, J. L. Prog. Surf. Sci. 1994, 47, 205.
(7) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv.
Colloid Interface Sci. 1996, 67, 119.
(8) (a) Mysels, K. J.; Jones, M. N. Discuss. Faraday Soc. 1966, 42,
42. (b) Exerowa, D.; Scheludko, A. C. R. Acad. Bulg. Sci. 1971, 24, 47.
(c) Exerowa, D.; Kruglyakov, P. M. In Foam and Foam Films - Theory,
Experiment, Application; Möbius, D., Miller, R., Eds.; Elsevier: Amsterdam, 1998.
(9) Stubenrauch, C.; Klitzing, R. J. Phys. Condens. Matter 2003, 15,
R1197.
10.1021/la0304060 CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/04/2004
4978
Langmuir, Vol. 20, No. 12, 2004
Figure 1. Surface tension σ as a function of the C12E6
concentration c at T ) 22 ( 1 °C. The error is about the size
of the symbols. The line represents the best LangmuirSzyskowski fit (eq 3), leading to a cmc of 7.3 10-5 M.
thiolated gold surfaces. Investigations with solid surfaces
are of particular interest since we are able to modify the
surface. For example, the silanization of glass surfaces
leads to a nonpolar surface that is slightly charged,
whereas the thiolated gold surfaces are completely
uncharged. We took advantage of the possibility to adjust
the surface charge of the (nonpolar) solid surfaces to
compare the interaction forces with those existing between
air/water interfaces. For that purpose, the interaction
forces across aqueous solutions of C12E6 were investigated
at three different surfactant concentrations (two below
and one above the cmc). These measurements shed further
light on the origin of the surface charges at the air/water
surface which is one of the central topics in this study.
First proposed by Exerowa in 1969,10 it is widely
accepted, but not proved, that the surface charge at the
air/water interface is produced by excess OH- ions (the
latest results and related discussions are reviewed in ref
9). These charges are responsible for the long-range
electrostatic forces observed in thin liquid foam films
stabilized by nonionic surfactants. It has also been noted
that the adsorption of nonionic surfactant leads to a
decrease of the surface charge.11-15 However, one important question remains to be answered: What is the
mechanism which generates an excess of negative charges
at the air/water interface? We will address this question
and propose a new mechanism for the charging of the
air/water interface.
2. Experimental Section
2.1 Solution Preparation. The nonionic surfactant hexaoxyethylene dodecyl ether (C12E6) was purchased from Fluka
(Germany) and used as received. The purity was checked by
measuring the surface tensions as a function of the concentration
c at 22 °C by the DuNoüy ring method using a Krüss K10ST
tensiometer (see Figure 1). Sodium chloride was obtained from
Merck (Germany) and roasted at 500 °C before use to remove
organic impurities. Water used for the preparation of all solutions
was purified with a Millipore Milli-Q Plus 185 water purification
(10) Exerowa, D. Kolloid-Z. 1969, 232, 703.
(11) Exerowa, D.; Zacharieva, M.; Cohen, R.; Platikanov, D. Colloid
Polym. Sci. 1979, 257, 1089.
(12) Manev, E. D.; Pugh, R. J. Langmuir 1991, 7, 2253.
(13) (a) Waltermo, A° .; Manev, E.; Pugh, R.; Claesson, P. J. Dispersion
Sci. Technol. 1994, 15, 273. (b) Bergeron, V.; Waltermo, A.; Claesson,
P. M. Langmuir 1996, 12, 1336.
(14) Karraker, K. A.; Radke, C. J. Adv. Colloid Interface Sci. 2002,
96, 231.
(15) Stubenrauch, C.; Schlarmann, J.; Strey, R. Phys. Chem. Chem.
Phys. 2002, 4, 4504; 2003, 5, 2736.
Stubenrauch et al.
system. For the MASIF measurements, the water was deaerated
using a water jet pump for 2 h immediately before use which is
essential to minimize the formation of air bubbles in the vicinity
of highly hydrophobic solid surfaces. All glassware (except the
film holders of the TFPB) was cleaned with deconex from Borer
Chemie (as a replacement for chromic sulfuric acid) and rinsed
thoroughly with Milli-Q water before use. The film holders used
in the TFPB (see below) were boiled two times in acetone, six
times in water, and at least 0.5 L of hot water was sucked through
each disk. Three different surfactant solutions at concentrations
of 10-5 M, 5 10-5 M, and 10-4 M, respectively, were prepared in
10-4 M NaCl background electrolyte concentration. It was made
sure that the pH of the investigated solutions was equal to the
usual pH of 5.5 ( 0.3, which is due to the dissolution of CO2
present in air.
2.2 Thin Film Pressure Balance. The disjoining pressure
Π was measured as a function of the film thickness h with the
TFPB technique. Experimental details have been published only
recently.15 In brief, with a TFPB free-standing horizontal liquid
films are investigated. These films are formed in a film holder
that consists of a glass tube, which is connected to a porous glass
frit so that the solution is free to move. A hole of 1-2 mm diameter,
in which the film is formed, is drilled in the frit. The film holder
is sealed in a cell in which the gas pressure can be adjusted in
a controlled manner. Π(h)-curves are generated by interferometrically measuring the equivalent film thickness heq after
applying a fixed pressure in the cell. The true film thickness h
is obtained according to the three-layer model16 where the film
is considered as a solution layer of refractive index ns plus two
adsorbed surfactant layers of different refractive index, that is,
(
h ) heq - 2htail
)
ntail2 - ns2
2
ns - 1
(
- 2hhead
nhead2 - ns2
ns2 - 1
)
(1)
where htail and hhead are the thicknesses of the hydrocarbon tail
and the headgroup, respectively, ntail and nhead are the corresponding refractive indices, htail ) 1.3 nm and hhead ) 1.2 nm
were derived from neutron reflectivity measurements,17,18 and
ntail was approximated to that of ndodecane (1.4216) and nhead to
that corresponding to hexaoxyethylene nE6 ) 1.4650. The error
margins in the reported Π(h)-curves are (30 Pa which is a
result of uncertainty in the measurement of hydrostatic pressure
in the glass tube of the film holder and (5% for the film thickness.
The latter results from the uncertainty in the determination of
the equilibrium intensity from which the film thickness is
calculated.15
2.3 Surfaces Used for MASIF Measurements. Supporting
Glass Surfaces. The hydrophobic surfaces on which the adsorption
of C12E6 was studied were obtained by silanization and thiolization, respectively, of hydrophilic glass surfaces. The supporting
glass surfaces were prepared from rods of borosilicate glass
(Pyrex) with 2-mm diameter that were cut into pieces of 25-30
mm in length, rinsed with ethanol, and melted at one end with
a butane-oxygen burner. The treatment was conducted until a
surface of spherical shape with a radius of approximately 2 mm
was obtained. It has been shown that flame-polished glass
surfaces are smooth enough to enable accurate measurements
of surface forces down to molecular separations.19
Silanated Glass. Flame-polished glass surfaces were hydrophobized through a vapor-phase silanization with (3,3-dimethylbutyl) dimethylchlorosilane (Petrarch). For the silanization procedure, the flame-polished glass surfaces were used
immediately after they had been prepared and then secured on
a holder. This holder was placed in a silanization glass chamber
containing a few milliliters of silane solution. The chamber was
blown with nitrogen and tightly closed for at least 18 h to allow
a complete (vapor) deposition. After silanization, the surfaces
were thoroughly rinsed with ethanol and then blown dry with
(16) Duyvis, E. M. Ph.D. Thesis, Universiteit Utrecht, 1962.
(17) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.;
Penfold, J. J. Phys. Chem. 1993, 97, 8012.
(18) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Binks, B. P.; Crichton, D.;
Fletcher, P. D. I.; McNab, J. R.; Penfold, J. J. Phys. Chem. B 1998, 102,
5785.
(19) Ederth, T.; Claesson, P.; Liedberg, B. Langmuir 1998, 14, 4782.
Interactions between Nonpolar Surfaces
a gentle stream of nitrogen. This vapor-phase silanization
procedure was chosen because it was shown to give good quality
surfaces with satisfactory reproducibility.20-22
Thiolated Glass. In this study, thin gold films were prepared
on the above-mentioned glass surfaces to provide substrates
suitable for self-assembly of alkanethiolate monolayers. The gold
substrates onto which the thiols are adsorbed are polycrystalline,
with typical peak-to-valley roughness of about 1.5 nm and rms
roughness in the 0.15-0.20 nm range (as measured over 1 × 1
µm2).19 The roughness of the gold layer is low enough to make
them suitable for direct measurements of surface forces. Since
the wetting of silica by gold is poor (which might lead to patchy
layers of this metal), we promoted the adhesion between gold
and the glass substrate during evaporation by adding an
“adhesion” layer of titanium. This was accomplished by vapor
deposition of Ti in the form of a thin (1 nm) layer, preceding
vapor deposition of a thicker layer of gold (10 nm).
The metal evaporation was conducted in an electron-beam
ultrahigh vacuum evaporation system (Auto 306 Turbo, BOC
Edwards, U.K.) at evaporation rates of 0.1 and 0.5 nm s-1 for
titanium and gold, respectively, and the deposition was carried
out until the required film thickness was obtained as measured
with a quartz film thickness monitor with 0.1 nm resolution.
The base pressure in the evaporation unit was typically below
7 10-7 torr before the evaporation started, and the pressure during
the gold evaporation step rarely exceeded 7 10-6 torr.
Since the gold surface is highly surface active and attracts
contaminants, we minimized the exposure to air by quickly
immersing the surface in the alkanethiol solution and leaving
the system for adsorption for at least 18 h to ensure the buildup
of a well-ordered alkanethiol monolayer on gold. The alkanethiol
solution was a 10-3 M 1-hexadecanethiol (Fluka, >95%) solution
in 99.5% ethanol (Kemetyl, Stockholm). The thiolated surfaces
are less prone to contamination then the gold surfaces.
Adsorption as well as transportation and storage of the
substrates were carried out in polypropylene containers fitted
with poly(tetrafluoroethylene) holders. Before use, the surfaces
were removed from the thiol solution, ultrasonicated in ethanol
for 10 min to remove excess thiols physisorbed on the surface,
and finally dried in a gentle flow of dry nitrogen.
2.4 MASIF Surface Force Measurements. The device used
for the surface force measurements is based on a single cantilever
bimorph deflection sensor.6,7,24 The apparatus that is known as
MASIF (Measurement and Analysis of Surface Interaction
Forces) can be used to measure forces between any two bodies,
irrespective of composition and shape, provided they are smooth
enough to make data interpretation possible in the separation
range of interest. As explained before, we used two spherical
surfaces with radii (R1 and R2) of approximately 2 mm. One
surface is mounted onto a bimorph deflection sensor, and the
other is attached to a piezoelectric displacement transducer. The
two surfaces are driven to a separation of some hundred
nanometers by using a motor. After this distance is reached, or,
to be more precise, when the onset of interaction forces is about
to take place, the upper surface is moved toward the lower one
in a continuous manner, using the piezoelectric tube. When the
surfaces make “hard wall” contact, the upper surface is withdrawn
again. During the process, both the bimorph output signal, which
is proportional to the deflection of the lower surface, and the
displacement of the upper surface, which is measured by a linear
variable displacement transducer (LVDT), are monitored.
Since the displacement of the upper surface is known, the
bimorph output signal can be calibrated (on the assumption that
the surfaces are incompressible). Hence, the deflection of the
lower surface is known at each instant, and the force can be
calculated by multiplying the deflection with the spring constant
of the bimorph. The spring constant was determined by adding
weights (0.1-0.5 g) to the spring and measuring the deflection
with a microscope. Typical spring constants are in the range of
(20) Fadeev, A. Y.; McCarthy T. J. Langmuir 1999, 15, 3759.
(21) Persson, C. M.; Claesson, P. M.; Lunkenheimer, K. J. Colloid
Interface Sci. 2002, 251, 182.
(22) Kjellin, U. R. M.; Claesson, P. M. Langmuir 2002, 18, 6745.
(23) Kjellin, U. R. M.; Claesson, P. M. Langmuir 2002, 18, 6754.
(24) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci.
Instrum. 1989, 60, 3135.
Langmuir, Vol. 20, No. 12, 2004 4979
80-100 N m-1. The distance resolution of the instrument is ca.
0.2 nm, the minimum detectable force is of the order of 2 10-8
N, and the corresponding normalized force resolution is about
10 µN m-1. All forces were normalized with the harmonic radius
R of the spheres (R ) 2R1R2/(R1 + R2)). The radii of the spherical
surfaces were measured using a micrometer screw after each
experiment. The resulting normalized force F/R is related to the
interaction free energy per unit area G between parallel plates
of unit area through the Derjaguin approximation25 F/R ) πG.
F/R versus distance D curves are presented without any
averaging, if not otherwise stated. The point of zero separation
corresponds to the location of the “hard wall”, as selected in the
analysis. In some cases, hydrodynamic effects are considered as
explained below.
All the procedures for assembling the measuring chamber and
preparing the solutions were carried out inside a laminar flow
cabinet. At the beginning of each set of experiments, the
interaction profiles were determined in air as well as in Milli-Q
water to ensure that the system (i.e., the surfaces and the
measuring chamber) was free of contamination. Concentrated
stock surfactant solution was then introduced into the waterfilled measuring chamber until the desired concentration was
attained. The surfactant concentration in the chamber was varied
consecutively by replacing a fraction of the total volume in the
chamber with a surfactant solution of appropriate composition.
Thus, in these measurements, each new solution comes in contact
with a surface preequilibrated with the previous solution.
2.5 DLVO Calculations. We compared the measured ∏(h)
and F/R(D) curves with interaction curves calculated within the
framework of the classical DLVO theory,26,27 taking into account
repulsive double-layer forces and attractive van der Waals forces.
To obtain the electrostatic component, the nonlinear PoissonBoltzmann equation was solved. The calculations were done with
the algorithm of Chan et al.,28 using either constant charge or
constant potential boundary conditions and the theoretical Debye
length κ-1. The van der Waals component of the interaction should
ideally be calculated using (at least) a five-layer model (e.g., airsurfactant-water-surfactant-air or solid-surfactant-water-surfactant-solid). However, for our purpose it is sufficient to use a
simple nonretarded Hamaker constant (A) which for the foam
films was set to A ) 3.7 10-20 J1, and for the silanated glass
surfaces to A ) 1 10-20 J. The Hamaker constant for the thiolated
gold surfaces was estimated from the measurements leading to
a value of A ) 2.5 10-20 J (see discussion in connection with
Figure 10).
The parameters extracted from these calculations are the
apparent surface potential ψo from which the corresponding
surface charge density q0 can be calculated using the Grahame
equation.1 The parameters extracted from the DLVO calculations
are summarized in Table 1.
Hydrodynamic Effects. For the MASIF measurements, we
considered the effect of hydrodynamic forces in the interaction
profile since this can become an important issue, especially in
cases where the electrostatic repulsion is minimal. The hydrodynamic interactions result in an extra repulsion when the
surfaces approach each other and in an extra attraction when
they are separated. To eliminate this effect, we used low enough
driving rates, both on approach and on separation. In some cases
(which will be indicated), the hydrodynamic effects were considered by subtracting the calculated hydrodynamic force FH
from the measured total interaction force F. The hydrodynamic
forces between two approaching surfaces are given by29,30
FH 3πηR d(D - 2Ds)
)
R
2D
dt
(2)
where η is the viscosity of the solution (assumed to be identical
(25) Derjaguin, B. Kolloid-Z. 1934, 69, 155.
(26) Derjaguin, B. V.; Landau, L. D. Acta Physicochim. USSR 1941,
14, 633.
(27) Verwey, E. J. W.; Overbeek, J. T. G. In Theory of the Stability
of Lyophobic Colloids; Elsevier: Amsterdam, 1948.
(28) Chan, D. Y.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci.
1980, 77, 283.
(29) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311.
(30) Vinogradova, O. I. Langmuir 1996, 12, 5963.
4980
Langmuir, Vol. 20, No. 12, 2004
Stubenrauch et al.
Table 1. Surfactant Concentration c and Corresponding Surface Concentration Γ at the Air/Water Interfacea
c(C12E6) M
10-5
5 10-5
10-4
Γ 10-6 mol m-2
ψ0,1 mV
q0,1 mC m-2
area/charge1 nm2
ψ0,2 mV
q0,2 mC m-2
area/charge2 nm2
3.0
3.2
3.2
60
24
b
1.70
0.57
b
95
280
b
100
30
b
4.03
0.72
b
40
220
b
a Γ was obtained from fitting the surface tension isotherm according to eq 3. Surface potentials ψ , surface charges q , and area/charge
0
0
are from DLVO calculations for the foam films (index 1) and the silanated surfaces (index 2) as described in section 2.5. The calculated
Debye length is κ-1 ) 30 nm at the given electrolyte concentration of 10-4 M NaCl. b No measurable electrostatic repulsion under the given
experimental conditions.
Figure 2. Surface excess Γ as a function of the bulk
concentration c for aqueous solutions of the surfactant C12E6
calculated according to the Gibbs’ adsorption equation (eq 4).
Inset: Calculated surface pressures π as a function of the area
per molecule A derived from the fitted surface tension isotherm.
to that of water) and D is the surface separation at time t. Ds is
a distance that is fitted to account for the position of the plane
of no shear. Thus, Ds is a measure of the thickness of an “immobile”
region of liquid adjacent to each solid surface (the stick boundary
conditions apply at a distance Ds out from each surface). The
local rate of approach of the surfaces (d(D-2Ds)/dt) changes as
the surfaces start to interact and it is calculated directly from
the experimental data.
3. Results
3.1 Surface Tension Measurements. In Figure 1,
the surface tensions σ of aqueous C12E6 solutions are
presented as a function of the surfactant concentration c
at 22 °C. The surface tension decreases with increasing
surfactant concentration down to the cmc, which is
indicated by a sharp bend at 7.3 10-5 M.
At concentrations below the cmc, the data is fitted to
the Langmuir-Szyszkowski equation
(
σ ) σ0 - RT Γ∞ln 1 +
c
a
)
(3)
where σ0 ) 72.2 mN m-1 is the surface tension of pure
water, Γ∞ is the maximum surface concentration of
surfactant, and a represents the concentration at which
50% of Γ∞ has been reached. The best fitting parameters
are Γ∞ ) 3.18 10-6 mol m-2 and a ) 4.67 10-7 M. With this
set of data, the surface excess Γ of C12E6 was calculated
using the Gibbs equation (eq 4):
Γ)-
c ∂σ
RT ∂c
( )
(4)
Knowing Γ, one can calculate the area per molecule at the
interface according to A ) (NAΓ)-1. In Figure 2, the surface
excess Γ is plotted as a function of the surfactant
concentration c. In addition, the dependence of the surface
pressure π ) σ0 - σ on the area per molecule A is shown
in the inset.
As expected, the surface excess increases with increasing
bulk concentration and comes close to the saturation value
at concentrations around 1/2 cmc (Figure 2). The area per
molecule at the surface extracted from this plateau equals
0.52 nm2, which is in perfect agreement with results
derived from neutron reflectivity measurements.17 A
consequence of the increasing surface coverage is an
increasing surface pressure π, which can be considered as
a measure for the interactions in the surface layer. The
higher the surface pressure the stronger the repulsive
forces between surfactant molecules, which results in a
strong resistance against further adsorption of surfactant
molecules at the surface. At concentrations around the
cmc, a sharp increase in the surface pressure is observed
(see Figure 2, inset), demonstrating strong repulsive shortrange interactions in densely packed monolayers.31
3.2 Interactions between Air/Water Interfaces. To
investigate the interactions between the surfactant-loaden
air/water interfaces, the disjoining pressure Π has been
measured as a function of the film thickness h. More
precisely, the Π(h)-curves have been examined for the
effect of the C12E6 concentration at a fixed electrolyte
concentration of 10-4 M NaCl. Film thicknesses range
from more than 80 nm to less than 5 nm, depending on
the composition of the sample and the applied pressure,
which ranges from 200 to 9000 Pa. The model DLVO
calculations using constant charge boundary conditions
and the theoretical Debye length of κ-1 ) 30 nm1 provide
strong support for the conclusion that the long-range
repulsive force is a double-layer force. The parameters
extracted from the DLVO calculations are summarized in
Table 1.
In Figure 3, the Π(h)-curves for three different C12E6
concentrations are shown, two of which were below and
one above the cmc (cmc ) 7.3 10-5 M).
Two different kinds of films were observed: thick
common black films (CBF), stabilized by electrostatic
repulsion, and thin Newton black films (NBF), stabilized
by short-range repulsion. The thicknesses of the CBFs
decrease monotonically as the disjoining pressure increases. While the slope d logΠ/d h is independent of the
surfactant concentration, a significant shift of the curves
toward lower disjoining pressures is observed by increasing the surfactant concentration from 10-5 M to 5 10-5 M.
Apart from this shift, a CBF-NBF transition takes place
for the film stabilized by the 5 10-5 M solution, an observation which was not made for the lower concentration.
The increasing tendency to form an NBF with increasing
surfactant concentration is demonstrated by the fact that
for the highest surfactant concentration investigated (10-4
M), the transition pressure is so low that under the
experimental conditions only an NBF was seen.
Common Black Film (CBF). The present study provides additional supporting evidence for the general
(31) Persson, M.; Kumpulainen, A. J.; Eriksson, J. C. Langmuir 2003,
19, 6110.
Interactions between Nonpolar Surfaces
Figure 3. Disjoining pressure Π as a function of the film
thickness h for three concentrations of C12E6 in 10-4 M NaCl
solution, namely, 1 10-5 M (0), 5 10-5 M (O), and 1 10-4 M (9).
The two lower concentrations are below, the highest is above
the cmc ()7.3 10-5 M). The solid lines are calculated according
to the DLVO theory assuming interactions at constant charge.
The upper one corresponds to a surface potential of ψ0 ) 60 mV
and the lower one to ψ0 ) 24 mV.
phenomenon reported previously, namely, that surfaces
of foam films stabilized by nonionic surfactants are
charged.8c,13-15,21,32,33 Analyzed within the framework
of the DLVO theory assuming interactions at constant
charge (solid lines in Figure 3), the surface potentials are
ψ0 ) 60 and 24 mV for c ) 10-5 and 5 10-5 M, respectively, whereas for 10-4 M no electrostatic repulsion was
measurable under the given experimental conditions.
Thus, the surface potential drops from 60 mV to very low
values34 when the cmc is exceeded (see Table 1). This
finding is in perfect agreement with the observation of a
nearly constant surface charge density at low surfactant
concentrations, which decreases only significantly above
a certain concentration.12-15 An attempt to specify this
concentration is made in ref 13a. We will come back to
this point in section 4.4. The decrease in q0 leads to a
destabilization of the CBF until it is completely unstable
for c > cmc under the chosen experimental conditions.
The destabilization of the CBF is accompanied by the
stabilization of a NBF because of increased short-range
repulsion and higher monolayer cohesion. This change of
the stabilizing mechanism is clearly seen in Figure 3 for
the 5 10-5 M solution. Whereas the CBF is unstable already
at 1000 Pa, the NBF remains stable up to 5000 Pa. We
will come back to this point below.
Although the experimental data are adequately described by the DLVO theory down to thicknesses of around
15 nm for c ) 10-5 M, a discrepancy between measured
and calculated Π(h)-curves is observed at smaller thicknesses. Although the reason for this discrepancy is not
known, the experimental observations are in accordance
with previous results obtained for n-dodecyl-β-D-maltoside
(β-C12G2)15 and the shorter homologue tetraoxyethylene
decyl ether (C10E4).33 Several suggestions to explain these
(32) Persson, C. M.; Claesson, P. M.; Johansson, I. Langmuir 2000,
16, 10227.
(33) Schlarmann, J.; Stubenrauch, C.; Strey, R. Phys. Chem. Chem.
Phys. 2003, 5, 184.
(34) In a Scheludko-Exerowa cell (Scheludko, A.; Exerowa, D. Comm.
Department Chem. Bulg. Acad. Sci. 1959, 7, 123), where the pressure
does not exceed values around 40 Pa, CBFs of the nonionic surfactant
n-dodecyl-β-D-maltoside (β-C12G2) are stable at surfactant concentrations far above the cmc (Müller, H.-J., personal communication). Thus,
it is only for the pressures applied in the present work (which are g200
Pa) that the surface charge density is too low to stabilize a CBF.
Langmuir, Vol. 20, No. 12, 2004 4981
findings have been reported in the literature, for example,
changes in the adsorption density with film thickness35
or effects due to an inhomogeneous charge distribution.36
To this we would like to add the suggestion that fluctuations within the film may result in an extra repulsion
when the adsorption density is high (in contrast to film
rupture that occurs because of these fluctuations at low
surfactant adsorption densities). Further work to clarify
this is under way. The fit between calculated and measured
forces across the 5 10-5 M C12E6 solution is less convincing.
However, noting the difficulty of measuring weak forces
accurately, we propose that indeed also in this case the
force observed at long distances is a double-layer force.
Newton Black Films (NBF). As has already been
mentioned, the tendency of the NBF to form increases
with increasing surfactant concentration, that is, the
magnitude of the applied pressure needed to induce the
CBF-NBF transition decreases. In fact, for the highest
surfactant concentration (10-4 M), an equilibrium CBF
was not formed and only an NBF was seen under the
given experimental conditions. The NBFs are very thin
with an aqueous core of 1-2 nm assuming a length of 2.5
nm17 for the surfactant. Hence, these films consist of two
surfactant monolayers with only small amounts of water
separating the headgroups (mainly hydration water). The
thickness of the NBF decreases from around 7 to 5 nm
with increasing pressure, which indicates large flexible
headgroups that can change their conformation or packing
in the NBF. This is consistent with previous surface force
data using C12E5 adsorbed to nonpolar solid surfaces.37
It is important to realize that for a stable NBF to form
not only short-range repulsive interactions normal to the
surfaces are required but also a densely packed adsorption
layer (see chapter 3.4.3 and 3.4.4 in ref 8c). The packing
density increases with increasing alkyl chain length.38 In
the alkyl polyglycolethers CiEj, for example, at least 10
carbon atoms in the nonpolar tail are needed to stabilize
an NBF.39,40 Thus, stable C12E6-NBFs are not surprising.
However, looking at Figure 3, a CBF-NBF transition can
clearly be seen for c ) 5 10-5 M at pressures around 1000
Pa, whereas for the lowest concentration only a CBF was
observed. To check if this is because the surface coverage
at c ) 10-5 M is too low to stabilize an NBF, we performed
a series of measurements at an electrolyte concentration
of 10-1 M to screen electrostatic forces (the results are not
shown). In the absence of electrostatic forces, only NBFs
were, as expected, formed at all three concentrations (10-5,
5 10-5, and 10-4 M). However, the NBF formed at the
lowest concentration ruptured immediately after its
formation, verifying the assumption that the surface
coverage is too low to stabilize an NBF (see ref 41 for
further details with regard to the rupture of NBFs). Thus,
at concentrations far below the cmc only CBF films
stabilized by repulsive electrostatic forces are formed.
However, at concentrations close to the cmc the surface
coverage is high enough to stabilize NBFs, whereas the
lower surface charge reduces the stability of the CBF.
Above the cmc, the charge at the surface is too small to
support a CBF under the given experimental conditions
and only the NBF is sufficiently stable to be seen.34
(35) Müller, H.-J. University of California, Berkeley, personal
communication.
(36) Foret, L.; Kühn, R.; Würger, A. Phys. Rev. Lett. 2002, 89, 156102.
(37) Claesson, P. M.; Kjellander, R.; Stenius, P.; Christenson, H. K.
J. Chem. Soc., Faraday Trans. 1 1986, 82, 2735.
(38) Bergeron, V. Langmuir 1997, 13, 3474.
(39) Karraker, K. A. Ph.D. Thesis, Berkeley, 1999.
(40) Schlarmann, J.; Stubenrauch, C. Tenside Surf. Det. 2003, 40,
190.
(41) Kashchiev, D.; Exerowa, D. J. Colloid Interface Sci. 1980, 77,
501.
4982
Langmuir, Vol. 20, No. 12, 2004
Figure 4. Force F normalized by the harmonic radius R as a
function of surface separation D. The forces were measured
between silanated glass surfaces across aqueous solutions
containing 10-4 M NaCl in the absence (9) and in the presence
of 1 10-5 M C12E6 (0). The lines are calculated according to the
DLVO theory for a surface potential of ψ0 ) 100 mV assuming
interactions at constant charge (full line) and at constant
potential (dashed line), respectively.
Another interesting result of the present study is the
fact that the NBF is formed via a stepwise CBF-NBF
transition. In an extensive disjoining pressure study,39 it
has been shown that the CBF-NBF transition is discrete
for C10E4 and continuous for C10E8 because in the latter
case the film is stabilized by short-range repulsive forces
before attractive van der Waals forces dominate the
interactions.39 In other words, it is between E4 and E8
that a crossover from a discrete to a continuous NBF
formation is expected. With the results obtained in the
present work, we can define the range for this crossover
more precisely, namely, between E6 and E8.
3.3 Interactions between Solid/Liquid Interfaces.
The interactions between nonpolar surfaces across aqueous solutions in the presence of C12E6 were determined
with the MASIF surface force apparatus, using two
different substrate surfaces. The first substrate chosen
was silanated glass that, despite being hydrophobic,
carries a significant net negative charge. This charge
comes from the dissociation of unreacted silanol groups.42
The other substrate chosen was a gold surface modified
by hexadecanethiol. This surface is uncharged in aqueous
solutions.
Silanated Surfaces. The forces acting between a pair of
silanated glass surfaces immersed in a 10-4 M aqueous
NaCl solution are shown in Figure 4 (filled squares).
The long-range forces are dominated by an electrostatic
double-layer force having the expected decay length (30
nm) for the given ionic strength. The apparent surface
potential according to DLVO calculations is 100 mV,
corresponding to an area per charge of 40 nm2. The force
measured at separations below ∼20 nm is less repulsive
than predicted by theory assuming either interactions at
constant charge (full line) or at constant potential (dashed
line). In fact, this is not surprising but in line with a
multitude of measurements demonstrating the presence
of a long-range attractive force, significantly larger than
the van der Waals force, between nonpolar surfaces in
aqueous solutions. For robust hydrophobic surfaces, that
is, surfaces that are intrinsically nonpolar or where a
strongly anchored film generates the surface hydropho(42) Iler, R. K. In The Chemistry of Silica; John Wiley: New York,
1979.
Stubenrauch et al.
bicity, there is growing evidence that this force is due to
capillary evaporation and bridging because of microbubbles adsorbed to the surfaces.19,43,44 Both silanated glass
and thiolated gold surfaces fall within this category. For
such robust hydrophobic surfaces, it has been clearly
demonstrated that a hydrophobic force due to capillary
evaporation and bridging bubbles is present only when
the (advancing) contact angle is above 90°. This has been
proved by modifying the wetting properties both by tuning
the chemical composition of the surface layer and by
varying the solvent composition.44 Thus, it is natural that
a reduction in contact angle because of surfactant adsorption also reduces the long-range attraction. Our observation is consistent with other studies that have demonstrated the reduction and subsequent removal of the
“hydrophobic interaction” because of surfactant adsorption.21,45 This report will not discuss the molecular origin
of this attractive force, often referred to as the “hydrophobic
interaction”, any further, but we note that the molecular
mechanism is less clear when less robust nonpolar surface
coatings are used, for example, those prepared by Langmuir-Blodgett deposition or adsorption of cationic surfactants on negatively charged surfaces. The interested
reader is referred to a rather recent review4 and a paper
by Ederth.46 Once in contact, the silanated glass surfaces
adhere strongly to each other. The adhesion force is in
fact too strong to be measured with the spring used for
these measurements, which means that it is above 80100 mN m-1.
Addition of C12E6 to a concentration of 10-5 M has a
rather limited effect on the long-range interactions (see
Figure 4, unfilled squares). The double-layer force is
virtually unchanged. However, the strong non-DLVO
attraction is significantly reduced, which is mirrored in
a clear inward shift of the force maximum. However, the
experimental maximum is still located at a larger separation than predicted by DLVO theory. The decrease in the
attractive force component indicates that the hydrophobicity of the surface has decreased as a result of the
adsorption of some surfactant. However, the adhesion force
remains too strong to be measurable.
The forces between the silanated surfaces change
significantly when the C12E6 concentration is increased
to 5 10-5 M. In Figure 5a, the F/R curves measured 30 min
and 18 h after increasing the surfactant concentration
are shown. For comparison, the equilibrium F/R curve for
the 1 10-5 M solution is shown once again. In addition, the
time development of the interaction forces acting across
the 5 10-5 M solution is shown in Figure 5b on a linear
scale.
It is seen in Figure 5a that the forces measured after
30 min are well described by the DLVO theory down to
a separation of 3 nm.The double-layer force falls between
that predicted for constant surface charge and constant
surface potential, respectively, as is expected for chargeregulating surfaces.47 The magnitude of the double-layer
force is the same after 30 and 50 min (see Figure 5b),
respectively, and it is similar to the double-layer force
measured at 10-5 M (see Figure 5a). (We attribute the
difference in force observed between the two surfactant
concentrations at distances above 40 nm to experimental
uncertainties, and this difference does not persist when
(43) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. B 1994,
98, 8468.
(44) Ederth, T.; Liedberg, B. Langmuir 2000, 16, 2177.
(45) Herder, P. C. J. Colloid Interface Sci. 1990, 134, 336.
(46) Ederth, T. J. Phys. Chem. B 2000, 104, 9704.
(47) Chan, D. Y. C.; Healy, T. W.; White, L. R. J. Chem. Soc., Faraday
Trans. 1 1976, 72, 2844.
Interactions between Nonpolar Surfaces
Langmuir, Vol. 20, No. 12, 2004 4983
Figure 6. Force F normalized by the harmonic radius R as a
function of surface separation D. The forces were measured
between silanated glass surfaces across aqueous solutions
containing 10-4 M NaCl and 5 10-5 M C12E6. The upper curve
(b) was measured on approach and the lower one (O) on
separation.
Figure 5. Force F normalized by the harmonic radius R as a
function of surface separation D. The forces were measured
between silanated glass surfaces across aqueous solutions
containing 10-4 M NaCl and 5 10-5 M C12E6. (a) Measurements
30 min (9) and 18 h (O) after increasing the surfactant
concentration. In addition, the equilibrium curve for the 1 10-5
M C12E6 solution is shown (0). The two upper lines are calculated
according to the DLVO theory for a surface potential of ψ0 )
100 mV assuming interactions at constant charge (full line)
and at constant potential (dashed line), respectively. The lower
line is calculated for a surface potential of ψ0 ) 30 mV assuming
interactions at constant charge. (b) Measurements 30 min (9),
50 min (4), and 18 h (O) after increasing the surfactant
concentration. For the sake of clarity the F/R data are presented
on a linear scale.
inspecting several force curves measured under these two
conditions.) Thus, the charge does not decrease during
the initial state of surfactant adsorption. However, the
forces observed after an 18-h exposure to the 5 10-5 M
surfactant solution, that is, after the equilibrium adsorption state was obtained, are significantly different. The
decay length of the double-layer force remains the same,
but the magnitude of the apparent double-layer potential
has decreased in magnitude to 30 mV, corresponding to
an area per charge of 220 nm2. Clearly, adsorption of the
surfactant is now sufficiently large to affect the charging
behavior of the silanated glass surfaces.
It is not only interesting to follow the change of the
double-layer force with time but also how the increased
surfactant adsorption affects the height of the force barrier
associated with the removal of surfactants from the contact
zone. (The solution was not stirred since stirring affects
the force measurements. Thus, the adsorption occurs
significantly slower than, e.g., in an ellipsometric measurement.) It is seen in Figure 5b that at 30 min a small
force barrier is present at small separations which is
overcome at a compressive force (F/R) of 1.8 mN m-1. The
extra repulsion is due to the work needed to remove the
adsorbed surfactants from between the approaching
surfaces. This force barrier increases from 1.8 mN m-1
after 30 min to 2.5 mN m-1 after 50 min. In the equilibrium
state, that is, after 18 h, the surfactant layer can no longer
be displaced even under a high compressive force (F/R ≈
30-40 mN m-1). In other words, it is no longer possible
to reach contact between the hydrophobic surfaces. The
adhesion is the third parameter that is changed as a
function of time. With increasing time the adhesion is
reduced, for example, from 17 mN m-1 after 30 min to 6
mN m-1 after 50 min (data are not shown). After
equilibrium adsorption had been established, only a weak
adhesion of 0.7 mN m-1 was observed on retraction (see
Figure 6).
However, unlike the situation at lower surfactant
concentrations and prior to equilibrium, the attractive
minimum is located outside the position where the
surfactant layers come into direct contact. This reflects
the short-range interaction between the surfactant layers.
We note a weak trend of a slightly larger adhesion force
between the surfactant layers with increasing compressive
load prior to separation.
As can be seen in Figure 7, a further increase of the
C12E6 concentration to 10-4 M has only a small effect on
the interactions. For the sake of clarity, the same F/R
data are shown on a logarithmic (Figure 7a) as well as on
a linear scale (Figure 7b).
The differences in the short-range interactions across
the 10-4 M and the 5 10-5 M surfactant solution,
respectively, are within the scattering of the experimental
data (see Figure 7b). However, the magnitude of the
double-layer force is further reduced at the higher
surfactant concentration (see Figure 7a). This result
indicates a small increase in the adsorption density when
the C12E6 concentration is increased from 5 10-5 M to 10-4
M. Looking at Table 1, one sees that the situation at the
air/water interface is similarsthe surface concentration
Γ increases slightly from 5 10-5 M to 10-4 M. Moreover,
as was discussed in connection with Figure 3, it is in this
concentration range where NBFs are formed, whereas
the stability of the CBF decreases significantly. We will
come back to this comparison in the discussion.
Thiolated Surfaces. There are some differences between
the thiolated gold surfaces and the silanated glass surfaces
4984
Langmuir, Vol. 20, No. 12, 2004
Stubenrauch et al.
Figure 8. Force F normalized by the harmonic radius R as a
function of surface separation D. The forces were measured
between thiolated gold surfaces across aqueous solutions
containing 10-4 M NaCl in the absence (b) and in the presence
of 1 10-5 M C12E6 (squares). Measurements in the presence of
surfactant were performed 1 h (0) and 24 h (9) after increasing
the surfactant concentration.
Figure 7. Force F normalized by the harmonic radius R as a
function of surface separation D. The forces were measured
between silanated glass surfaces across aqueous solutions
containing 10-4 M NaCl and different surfactant concentrations,
namely, 1 10-5 M (0), 5 10-5 M (O), and 1 10-4 M (9). All curves
were measured on approach and after the equilibrium adsorption had been established. For the sake of clarity, the same F/R
data are shown on a logarithmic (Figure 7a) as well as on a
linear scale (Figure 7b). The solid lines in Figure 7a are
calculated according to the DLVO theory assuming interactions
at constant charge. The upper one corresponds to a surface
potential of ψ0 ) 100 mV and the lower one to ψ0 ) 30 mV.
that should be kept in mind. First, the hexadecanethiol
layer is in a crystalline state whereas the silane layer is
disordered. Second, the hydrophobicity of the thiolated
layer is larger than that of the silanated glass surfaces.
The advancing and receding contact angles of the thiolated
gold surface are 110° and 104°, respectively, whereas the
corresponding values for the silanated surfaces are 100°
and 70°. Third, the silanated surfaces are, as shown in
Figure 4, strongly charged whereas the thiolated gold
surfaces are uncharged, which will be demonstrated in
the following.
It can be seen in Figure 8 that no long-range repulsive
forces act between the hexadecanethiol coated gold
surfaces but that the forces are attractive below separations of about 30 nm.
This attraction has a similar origin as the one seen for
the silanated glass surfaces and thus is due to capillary
evaporation and bridging bubbles.19,44 Like the silanated
surfaces, the adhesion force between the thiolated surfaces
is too high to be measurable (F/R > 80 mN m-1). Addition
of C12E6 to a concentration of 10-5 M removes the longrange attraction completely. Instead, a weak repulsive
force dominates at large separations. This force is of
Figure 9. Force F normalized by the harmonic radius R as a
function of surface separation D. The forces were measured
between thiolated gold surfaces across aqueous solutions
containing 10-4 M NaCl and 10-5 M C12E6. Measurements were
performed at different driving speeds, namely, at 15 nm s-1
(O), 36 nm s-1 (0), and 55 nm s-1 (9).
hydrodynamic origin (see Figure 10 below). After a short
equilibration time of 1 h, a weak repulsive barrier can
just be distinguished at separations below 5 nm. Similar
to the results presented in Figure 5, the magnitude of this
barrier increases with time and reaches about 0.8 mN
m-1 after 24 h. The force barrier can clearly be attributed
to the removal of surfactants from between the two
surfaces approaching each other. The adhesion force
decreases from about 16 mN m-1 after 1 h to 2-3 mN m-1
after 24 h.
It is demonstrated in Figure 9 that the magnitude of
the force barrier present at 4-5 nm separation shows
some variability, typically between 0.7 and 1.2 mN m-1.
We see no clear effect of the driving rate of approach,
but the magnitude of the barrier is on average somewhat
larger when the surfaces are left undisturbed for longer
times (more than 2 min). However, this correlation is also
weak. The most plausible explanation for the variation in
the magnitude of the force barrier is that the adsorbed
layers are not homogeneous. Inspecting the structure of
the thin evaporated gold surface with AFM one sees that
it is polycrystalline, despite being very smooth (see
Interactions between Nonpolar Surfaces
Figure 10. Force F normalized by the harmonic radius R as
a function of surface separation D. The forces were measured
between thiolated gold surfaces across aqueous solutions
containing 10-4 M NaCl and 10-5 M C12E6. Measurements were
performed at different driving speeds, namely, at 9 nm s-1
(squares), 36 nm s-1 (circles), and 55 nm s-1 (triangles). The
filled symbols represent the total force and the unfilled symbols
the force obtained after subtracting the hydrodynamic contribution. The bump in the corrected force curve obtained at 36
nm s-1 is an artifact (see text).
Experimental Section). The presence of grain boundaries
is likely to affect the homogeneity of the adsorbed
surfactant layer away from the adsorption plateau.
That the weak long-range repulsion is due to hydrodynamic interactions is demonstrated in Figure 10. In
this figure, the measured forces at different driving speeds
are reported together with the surface forces calculated
by subtracting the hydrodynamic force from the measured
force (see Section 2.5).
The resulting surface force is attractive from a separation of about 20 nm and unaffected by the driving speed.
The attractive interaction is well described by a van der
Waals force with an effective Hamaker constant of A )
2.5 10-20 J. The high value of A (compared to A ) 0.5 10-20
J for two spheres of hydrocarbon interacting in water1) is
due to the presence of the gold layer.48 In fact, the deduced
effective Hamaker constant is lower than that calculated
by Ederth for SAM-coated gold surfaces48 but identical to
that found by Persson and Kumpulainen for thiolated gold
surfaces coated with n-decyl-β-D-glucoside,49 which can
be rationalized by the presence of the adsorbed surfactant
layer. The value of the Hamaker constant for a multilayer
system depends on the separation. However, this refinement of the calculation was not considered in the present
case. Also, the subtraction of the hydrodynamic force from
the measured force is a subtle exercise since the magnitude
of the hydrodynamic force is relatively large compared to
the other force contributions. This means that measured
force curves prior to calculations of the local speed and
the hydrodynamic force must be smoothened (by a running
average) to remove noise. Nevertheless, artifacts may
occur. One such artifact is illustrated in Figure 10 by the
local maximum observed in the static forces for one of the
curves at a separation of 7-8 nm. It is due to the small
noise seen in the corresponding measured curve of the
total force.
The influence of the surfactant concentration on the
interaction forces between the surfactant coated thiolated
surfaces is shown in Figure 11.
(48) Ederth, T. Langmuir 2001, 17, 3329.
(49) Persson, C. M.; Kumpulainen, A. J. Colloids Surf., A 2004, 233,
43.
Langmuir, Vol. 20, No. 12, 2004 4985
Figure 11. Force F normalized by the harmonic radius R as
a function of surface separation D. The forces were measured
between thiolated gold surfaces across aqueous solutions
containing 10-4 M NaCl and different surfactant concentrations,
namely, 1 10-5 M (0), 5 10-5 M (O), and 1 10-4 M (9). All curves
were measured on approach and after the equilibrium adsorption had been established.
An increase of the C12E6 concentration to 5 10-5 M and
10-4 M, respectively, has no significant effect on the
interaction forces. The most striking change which is
observed with increasing surfactant concentration is the
fact that it is no longer possible to remove the adsorbed
layer from between the surfaces even under high forces
(F/R ≈ 30 mN m-1). In other words, the barrier for
squeezing out the adsorbed layers increases enormously.
It is important to realize that the results obtained for the
two higher concentrations are very similar to those
measured between silanated glass surfaces at the same
surfactant concentrations (see Figure 7). However, there
is one noticeable difference. The force minimum measured
on separation for the silanated surfaces was around 0.50.7 mN m-1, whereas a significantly smaller value, namely,
0.2-0.1 mN m-1, was observed when the thiolated gold
surfaces were separated.
4. Discussion
4.1 Attractive Forces between Adsorbed Nonionic
Surfactant Layers. In most cases, a weak attractive force
with a typical magnitude of 0.1-1 mN m-1 is observed
between nonpolar surfaces coated with a monolayer of
nonionic surfactant. The magnitude of this force depends
on the nature of the nonpolar part.50,51 It is determined
by a subtle interplay of van der Waals forces, hydration
forces, and confinement forces (e.g., protrusion forces2).
The attraction is larger when the adsorbed chains are in
a frozen state as compared to in a fluid state,52 which is
due to a reduction in the repulsive confinement forces.
For surfactants in the fluid state, the attractive force is
generally larger when the polar group consists of sugar
units as compared to ethylene oxide units.23 One effect
contributing to this is the larger refractive index of sugar
compared to ethylene oxide, which increases the van der
Waals force. In the particular case of ethylene oxide-based
surfactants, it has been noted in one study37 that an
increase in temperature results in a larger attraction, an
(50) Claesson, P. M.; Eriksson, J. C.; Herder, C.; Bergenstahl, B. A.
Faraday Discuss. Chem. Soc. 1990, 90, 129.
(51) Claesson, P. M.; Kjellin, U. R. M. In Modern Characterization
Methods of Surfactant System; Binks, B. P., Ed.; Marcel Dekker: New
York, 1999; p 255.
(52) Pezron, I.; Pezron, E.; Claesson, P. M.; Bergenstahl, B. A. J.
Colloid Interface Sci. 1991, 144, 449.
4986
Langmuir, Vol. 20, No. 12, 2004
effect related to the temperature dependence of the
hydration interaction. Similar temperature-dependent
hydration forces have also been observed for amine oxide
and phosphine oxide surfactants.53,54 Work with respect
to a more detailed analysis of the attractive forces between
layers of sugar-based and ethylene oxide-based surfactants
is under way.
In the study at hand, we have demonstrated that the
adhesion force between nonpolar surfaces coated with the
same surfactant, C12E6, is significantly different at the
two substrates used, silanated glass and thiolated gold.55
The adhesion is significantly larger when silanated
surfaces are used. This cannot be due to the difference in
van der Waals attraction since this force contribution is
larger for the thiolated gold substrate (the Hamaker
constants for thiolated and silanated surfaces are about
2.5 10-20 J and 1.0 10-20 J, respectively). Furthermore, it
cannot be due to any difference in hydration since the
same temperature was used in both experiments. Instead,
it appears that differences in surface roughness and the
presence of less hydrophobic patches on the silanated
surface is the cause of the observed substrate effect.
4.2 Comparison of Adsorption of Nonionic Surfactants to Different Nonpolar Surfaces. The driving
force for adsorption of nonionic surfactants to nonpolar
surfaces is the removal of contacts between the surface
and water as well as between the nonpolar surfactant tail
and water. This leads to an adsorption with the surfactant
tail directed toward the hydrophobic surface and the polar
group directed toward the aqueous solution. The extent
of the adsorption at different nonpolar surfaces will,
however, vary depending on the exact nature of the surface.
In general, one would expect that the adsorption would
be larger on a more nonpolar surface. Hence, in the present
case one would expect that the adsorption is largest on
the air/water interface, followed by that at the thiolated
gold, and lowest adsorption is expected on the silanated
surface. In fact, Kjellin et al. have demonstrated that the
adsorbed amount of C12E5 is larger at the air/water
interface than at silanated glass surfaces.22 Moreover, we
found a repulsive force barrier (i.e., the work needed to
remove surfactants from between the surfaces) at a C12E6
concentration of 10-5 M when thiolated gold was used as
substrate (see Figure 8), whereas no such barrier was
observed for silanated glass (see Figure 4). Indeed, this
observation indicates that the adsorption is, as was
assumed above, larger at the thiolated surfaces, at least
at low surfactant concentrations. For the two higher
surfactant concentrations investigated, no such conclusion
can be drawn. It was impossible to squeeze out the
adsorbed surfactant layer for both the silanated (see Figure
7) and the thiolated (see Figure 11) surfaces indicating
densely packed surfactant layers on both surfaces.
4.3 The Effect of Surfactant Adsorption on the
Surface Charge. As can be seen in Figure 8, the thiolated
gold surface is uncharged in 10-4 M NaCl. This conclusion
is based on the absence of any repulsive double-layer force.
(We thus reject the possibility of the presence of a doublelayer force exactly balanced by the presence of an attractive
force with equal magnitude but opposite sign at all large
(53) Herder, C. E. J. Colloid Interface Sci. 1991, 143, 1.
(54) Mol, L.; Bergenstahl, B. A.; Claesson, P. M. Langmuir 1993, 9,
2926.
(55) In both cases, silanated and thiolated surfaces, there is some
scatter in the measured adhesion data. In the thiolated surfaces, the
addition of the titanium and gold layers increases slightly the surface
rougness of the glass substrate (the total thickness of both metal layers
is approximately 11 nm). Generally, this roughness causes some spread
in the pull off forces because the effective surface area upon contact
varies over the surfaces.
Stubenrauch et al.
surface separations.) The double-layer force is absent also
when surfactant is added to concentrations of 10-5 M, 5
10-5 M, and 10-4 M (see Figure 11). Thus, the adsorption
of C12E6 does not result in any charging of the surface.
Hence, we conclude that, in keeping with what is expected,
the surfactant is nonionic and no charged and surfaceactive impurities are adsorbing to the surface. We can
also draw the conclusion that neither sodium nor chloride
ions associate with the surfactant layer (even though in
principle we could have equal amounts of sodium and
chloride associated with the surfactant, which would lead
to a net zero charge). The situation for the silanated surface
is different since it is inherently charged because of
dissociation of silanol groups. In this case, the initial
adsorption does not affect the charging of the surface.
However, once a significant surfactant adsorption is
reached, the surface charge is decreased to a very low
value. We attribute this to the decrease in the dielectric
constant of the environment close to the silanol groups,
which reduces the dissociation degree and thus the charge
density. Let us now turn to the situation at the air/water
interface, where we have, similar to the silanated surface,
a net charge. Like the silanated glass surfaces, the charge
of the air/water interface is reduced when the surfactant
concentration is increased. However, as the origin of the
surface charge has not been established yet, the interpretation of the experimental observation is not selfevident. What is clear, however, is the fact that the
decrease cannot be put down to the shielding of fixed
charges anchored at the surface. We will discuss this point
in the following section.
4.4 The Origin of the Charge at the Air/Water
Interface. From the results presented for the solid/liquid
interfaces coated with C12E6, we draw the conclusion that
the charge at the air/water interface does not arise as a
result of the presence of the surfactant or any charged
surface-active impurities in the sample. This last point is
difficult to prove, but it is hard to envisage an impurity
with a very high affinity for the air/water interface but
no affinity for the nonpolar solid/liquid interface. A
possibility is, of course, a charged and surface-active
impurity that is present in the frit material used in TFPB
measurements. Such an impurity could give rise to the
charges observed. It could then be solubilized into micelles
once the cmc is reached, which would explain the decrease
in charge with surfactant concentration. However, this
explanation seems to be far-fetched considering the large
number of results showing the same trend as reported
here. This includes other TFPB measurements as well as
measurements of the electrophoretic mobility of gas
bubbles in aqueous solutions (references and a detailed
discussion are given in ref 9).
At present, the widely accepted explanation for the
origin of the charges in thin liquid films stabilized by
nonionic surfactants is the presence of excess OH- ions
at the air/water interface.8c,9,10,14,56 However, lately it has
been shown by simulations that halide ions accumulate
at the air/water interface,57 the amount of which increases
with increasing size and polarizability of the anion. Thus,
fluoride is depleted at the interface, whereas bromide and
iodide concentrations are enhanced.57 In light of this, it
is obvious that OH- ions are not the only anions the
concentration of which is enhanced at the air/water
interface. Experimentally, it has been observed repeatedly
that the surface charge is constant at low surfactant
(56) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.;
Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045.
(57) Garrett, B. C. Science 2004, 303, 1146.
Interactions between Nonpolar Surfaces
Langmuir, Vol. 20, No. 12, 2004 4987
concentrations and decreases significantly above a certain
concentration, which is proposed to be connected with the
total number of ethylene oxide units.13a The results
obtained so far are reviewed in ref 9. On the basis of these
observations, Exerowa10 postulated a specific adsorption
of OH-, which was further developed by Marinova et al.56
and Karraker and Radke14 only recently. They argue that
the decrease of the surface charge contrasts sharply with
the adsorption of the surfactant, which changes significantly at low concentrations and stays close to constant
already far below the cmc. Thus, they exclude a typical
competitive adsorption between surfactant and hydroxide
and propose a two-site mechanism for the adsorption.14
If such sites do exist, they should be considered as highly
dynamic and being related to the specific water structure
as discussed above. The magnitude of the observed surface
charge density, even though it is small, is too large to be
explained by negative adsorption of cations only.
Nowadays, vibrational sum frequency generation spectroscopy allows us to obtain a fairly good idea of the
structure of the air/water interface.58-60 Generally speaking, the water spectrum of the surface includes features
from water molecules in three different environments.
There is a sharp peak due to the presence of free OHgroups pointing out toward the air side (non-hydrogen
bonding). It is the OH-groups from water rather than OHions that are seen. The other features are two broad peaks
assigned to weakly hydrogen bonding water (“liquidlike”)
and strongly hydrogen bonding water (“icelike”).58-60 When
considering the charging behavior of the air/water interface, one must be aware that we are considering a highly
dynamic interface. In fact, it has been estimated that even
close to 0 °C, about 3 million monolayers of water per
second are exchanged between the liquid and the vapor
phase!59 It is conceivable that it is in this dynamic exchange
that some OH- ions are created. For instance, consider
just two of the possible reactions that can occur at the
surface:
H2Obi + H2Ovi S OHbi- + H3Ovi+
H2Obi + H2Ovi S H3Obi+ + OHvi-
(5)
where subscript “bi” stands for molecules at the interface
on the bulk water side and subscript “vi” for molecules at
the interface on the bulk vapor side. Provided that the
equilibrium constants for these reactions (K1 for the first
and K2 for the second reaction) were slightly different, a
surface having a net charge would be created.
This is one possible mechanism that we suggest should
be considered and further elaborated on. First, it avoids
the difficulty of ion adsorption at specific sites. Second, it
explains how surfactant adsorption can reduce the charge
at the surface, namely, by switching off the reactions
postulated in eq 5 when no free air/water interface is left.
(However, just as with the ion adsorption model this
suggestion retains the difficulty of explaining why the
charge density of the air/water interface is not proportional
to the area of the free air/water interface.) The pHdependence would in this scheme be related to changes
in the dynamic water structure at the interface that would
influence the equilibrium constants for the exchange
reactions. The model also has obvious disadvantages. It
cannot explain the charging of an oil/water interface.
(58) Gragson, D. E.; Richmond, G. L. J. Phys. Chem. B 1998, 102,
3847.
(59) Shultz, M. J.; Baldelli, S.; Schnitzer, C.; Simonelli, D. J. Phys.
Chem. B 2002, 106, 5313.
(60) Richmond, G. L. Chem. Rev. 2002, 102, 2693.
Further, just as with the specific ion adsorption mechanism, it does not couple the OH- concentration at the
surface to the bulk water ion concentration through a
simple Boltzmann distribution in the mean electrostatic
potential on the waterside. It seems that further understanding can only be obtained by increasing the knowledge
of the dynamic water structure at the air/water interface
and how this influences ion adsorption and surface
reactions.
5. Conclusions
In this study, we considered the interactions between
nonpolar surfaces coated with the nonionic surfactant
hexaoxyethylene dodecyl ether, C12E6. As nonpolar surfaces, the air/water interface, silanated glass, and thiolated
gold surfaces were chosen. To investigate the influence of
the surface on the interaction forces, three different
surfactant concentrations were investigated, namely, 10-5
M, 5 10-5 M, and 10-4 M (cmc ) 7.3 10-5 M). The most
important results with respect to the comparison of
different surfaces are presented in Figure 3 (air/water
interface), Figure 7 (silanated glass surface), and Figure
11 (thiolated gold surface) and can be summarized as
follows.
(a) Air/Water Interface. Although the mechanism for
generating the surface charge is not fully clarified, the
available data strongly points to that the pure air/water
interface is negatively charged because of the presence of
anions, particularly OH- ions. Ion adsorption driven by
the specific dynamic water structure at the interface or
surface reactions are two possible mechanisms for the
origin of the surface charge. The adsorption of the nonionic
surfactant C12E6 leads to a decrease of the surface charge.
This decrease eventually results in a transition from an
electrostatically stabilized common black film (CBF) to a
Newton black film (NBF) that is stabilized by short-range
repulsive forces (see Figure 3). In the NBF, a surfactant
bilayer consisting of two densely packed monolayers
creates a “force barrier”ssimilar to the force barriers
observed in MASIF measurementssthat prevents the film
from rupturing. Sufficient monolayer cohesion is also a
necessary criterion for the formation of a long-lived NBF.
(b) Silanated Glass Surface. The silanated surface
is negatively charged because of the dissociation of
unreacted silanol groups. Although the origin of the charge
is different, the interaction forces between two silanated
surfaces and two air/water interfaces, respectively, are
very similar (see Figure 3 and Figure 7). At the lowest
surfactant concentration investigated (10-5 M), the interaction forces are dominated by long-range electrostatic
double-layer forces. In analogy to the air/water interface,
further adsorption of C12E6 reduces the net charge until
long-range forces are negligible. Comparing Figure 3 and
Figure 7, one sees that the NBF formation (Figure 3)
corresponds to the appearance of a force barrier between
the silanated surfaces (Figure 7). This barrier is a measure
for the force that is needed to remove surfactant from
between the two surfaces. As this barrier is located at a
distance corresponding to the thickness of a surfactant
bilayer, the analogy to the NBF formation is obvious. Thus,
in both cases, the adsorption of C12E6 leads to a decrease
of the surface charge resulting in the formation of a densely
packed bilayer which generates a force barrier. A “removal”
of this bilayer results in film rupturing in foam films,
whereas in solid/liquid interfaces it leads to a direct contact
of the solid surfaces.
(c) Thiolated Gold Surface. The situation is different
for the interaction forces between thiolated gold surfaces
4988
Langmuir, Vol. 20, No. 12, 2004
coated with C12E6 (see Figure 11). The substrate, that is,
the thiolated surface, is uncharged and double-layer forces
do not play a role either in the absence or in the presence
of C12E6. However, the adsorption of C12E6 significantly
influences the short-range interaction forces. In contrast
to the silanated surfaces (see Figure 7), it is already at the
lowest surfactant concentration of 10-5 M that a force
barrier is observed. An increase in the surfactant concentration leads to an increase of this barrier. Comparing
Figure 7 and Figure 11 one clearly observes the similarities
between the C12E6 coated silanated and thiolated surfaces
at surfactant concentrations of 5 10-5 M and 10-4 M,
respectively. Electrostatic forces are absent (or at least
negligible) and the force barrier is so high that the
surfactant cannot be removed from between the surfaces
under the given experimental conditions.
To conclude, one can say that the nature of the surface
at which the surfactant adsorption takes place mainly
influences the interaction forces at low surface coverages.
Once a densely packed surfactant layer is formed, it is the
surfactant itself that determines the interaction forces. It
would be of interest to quantify the surface concentration
Γ, the area per molecule A, and the conformation of C12E6
at the different surfaces. Detailed studies for the air/water
interface have already been made. Lu et al. used neutron
reflection to study the surface excess as well as the
monolayer thickness and its roughness.17 Moreover, from
sum-frequency spectroscopy and ellipsometry, the con-
Stubenrauch et al.
formation of the C12E6 molecules at the air/water interface
is known.61 What is still missing, however, are the
corresponding data for the adsorption of C12E6 at silanated
glass and thiolated gold surfaces, respectively. The fact
that we did not observe any double-layer forces between
the C12E6 coated thiolated surfaces proves that the
surfactant did not contain any charged surface-active
impurities. This is an important result with respect to the
discussion of the charge origin at the air/water interface.
Work in progress deals with interaction forces between
nonpolar surfaces coated with the nonionic sugar surfactant n-dodecyl-β-D-maltoside (β-C12G2). These measurements are aimed to clarify the influence of the
headgroup on the interaction forces.
Acknowledgment. Pleasant and fruitful cooperation
with Prof. R. Strey is gratefully acknowledged. C. Stubenrauch is indebted to the FCI, the Ministerium für
Wissenschaft und Forschung des Landes NRW, and the
Deutsche Forschungsgemeinschaft (DFG) for financial
support. J. Schlarmann thanks the DAAD for finacial
support. P. Claesson’s contribution to this work was carried
out within the competence center Surfactants Based on
Natural Products.
LA0304060
(61) Goates, S. R.; Schofield, D. A.; Bain, C. D. Langmuir 1999, 15,
1400.