11836
Langmuir 2005, 21, 11836-11843
Interactions between Nonpolar Surfaces Coated with the
Nonionic Surfactant n-Dodecyl-β-D-maltoside
Orlando J. Rojas,*,† Cosima Stubenrauch,‡ Judith Schulze-Schlarmann,§ and
Per M. Claesson|,⊥
Forest Biomaterials Laboratory, College of Natural Resources, North Carolina State
University, Box 8005, Raleigh, North Carolina 27695, Department of Chemical and
Biochemical Engineering, University College Dublin, Belfield, Dublin 4, Ireland, Engelhard
Process Chemicals, Freundallee 23, 30173 Hannover, Germany, Department of Chemistry,
Surface Chemistry, Royal Institute of Technology, Drottning Kristinas väg 51,
Stockholm, SE-100 44, Sweden, and Institute for Surface Chemistry, Box 5607,
Stockholm, SE-114 86, Sweden
Received July 18, 2005. In Final Form: September 9, 2005
The forces acting between nonpolar surfaces coated with the nonionic surfactant n-dodecyl-β-D-maltoside
(β-C12G2) were investigated at concentrations below and above the critical micelle concentration. The
long-range and adhesive forces were measured with a bimorph surface force apparatus (MASIF). It was
found that the effect of hydrodynamic interactions had to be taken into account for an accurate determination
of the short-range static interactions. The results were compared with disjoining pressure versus thickness
curves that were obtained earlier with a thin film pressure balance (TFPB). This comparison led to the
conclusion that the charges observed at the air-water interface are not due to charged species present
in the surfactant sample. In addition, it was observed that the stability of thin liquid films crucially
depends on the surfactant’s bulk concentration (c) and thus on the packing density in the adsorbed layer.
The force barrier preventing removal of the surfactant layer from between two solid-liquid interfaces
increases with increasing c, while for foam films it is the stability of the Newton black film that increases
with c. Finally, the results obtained for β-C12G2 were compared with those obtained for the homologue
n-decyl-β-D-maltoside (β-C10G2) as well as with those obtained for nonionic surfactants with polyoxyethylene
moieties as polar groups.
1. Introduction
Nonionic surfactants of the alkyl polyglycoside type,
also called “sugar” surfactants, are becoming an attractive
alternative to other nonionic surfactants as demonstrated
by their increasing use. Besides their distinctive properties, one of the reasons that makes this type of surfactant
attractive is that they can be produced on a large scale
from renewable raw materials. Alkyl-β-D-glucosides, alkylR-D-glucosides, alkyl-β-D-maltosides, and other derivatives
have been incorporated in a broad variety of commercial
formulations.1 Wide-ranging research work has been
carried out on the bulk and interfacial properties of sugar
surfactants.1-6 A characteristic feature of sugar surfactants is their insensitivity to changes in temperature,
* Corresponding author. Telephone: (919) 513-7494. Fax: (919)
515-6302. E-mail: [email protected].
† North Carolina State University.
‡ University College Dublin.
§ Engelhard Technologies.
| Department of Chemistry, Surface Chemistry, Royal Institute
of Technology.
⊥ Institute for Surface Chemistry.
(1) Nickel, D.; Forster, T.; von Rybinski, W. In Alkyl Polyglycoside,
Technology, Properties and Applications; Hill, K., von Rybinski, W.,
Stoll, G., Eds.; VCH: Weinheim, Germany, 1996; Chapter 4.
(2) von Rybinski, W. Curr. Opin. Colloid Interface Sci. 2001, 6, 146.
(3) Aveyard, R.; Binks, B. P.; Chen, J.; Esquena, J.; Fletcher, P. D.
I.; Buscall, R.; Davies, S. Langmuir 1998, 14, 4699.
(4) Stubenrauch, C. Curr. Opin. Colloid Interface Sci. 2001, 6, 160.
(5) Claesson, P. M. Interactions between Surfaces Coated with
Carbohydrates, Glycolipids and Glycoproteins. In Interfacial Behaviour
of Biomolecules and Biopolymers; Malmsten, M., Ed.; Marcel Dekker:
New York, 1998; Vol. 75; p 281.
(6) Claesson, P. M.; Kjellin, U. R. M. Sugar Surfactants. In Encyclopedia of Surface and Colloid Science; Hubbard, H., Ed.; Marcel
Dekker: New York, 2002; p 4909.
which distinguishes them from the most common nonionic
surfactants having polyoxyethylene headgroups. The
reason for the temperature insensitivity of the properties
of sugar surfactants is suggested to be due to the strength
of the hydrogen bonds between the hydroxyl groups of the
polar sugar headgroups and surrounding water, which
significantly reduces their dehydration with increasing
temperature.4 In fact, sorption calorimetry studies have
shown that the hydration of the glucose unit in n-octylβ-D-glucoside is entropy driven and results in a repulsive
pressure in the lamellar phase that increases with
increasing temperature.7 Further, sum-frequency generation spectroscopy has identified water molecules forming
exceptionally strong hydrogen bonds next to the sugar
moiety of several glucoside and maltoside surfactants
adsorbed at the air-water interface.8
Many applications, including particle dispersion and
stabilization,9-12 utilize the adsorption of (sugar) surfactants on solid surfaces to induce the desired changes in
long-range and adhesion forces. Not surprisingly, significant differences have been observed between the
adsorption behavior of nonionic surfactants at hydrophilic
and hydrophobic solid surfaces, respectively. The adsorp(7) Kocherbitov, V.; Söderman, O.; Wadsö, L. J. Phys. Chem. B 2002,
106, 2910.
(8) Tyrode, E.; Johnsson, M.; Kumpulainen, A.; Rutland, M. W.;
Claesson, P. M. Submitted for publication.
(9) Osipow, L.; Snell, F. D.; Marra, D.; York, W. C. Ind. Eng. Chem.
1956, 48, 1462.
(10) Weerawardena, A.; Boyd, B. J.; Drummond, C. J.; Furlong, D.
N. Colloids Surf., A: Physicochem. Eng. Aspects 2000, 169, 317.
(11) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Weerawardena,
A.; Furlong, D. N.; Grieser, F. Langmuir 2001, 17, 6100.
(12) Stradner, A.; Mayer, B.; Sottmann, T.; Hermetter, A.; Glatter,
O. J. Phys. Chem. B 1999, 103, 6680.
10.1021/la051938e CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/21/2005
Interactions of Nonpolar β-C12G2-Coated Surfaces
tion of nonionic surfactants with different headgroups on
hydrophilic surfaces (e.g., silica) is very sensitive to the
nature of both the surface and the surfactant headgroup.13,14 This indicates that the adsorption is strongly
influenced by specific short-range interactions such as
hydrogen bonding and Lewis acid-base interactions. On
the other hand, adsorption of different nonionic surfactants
on hydrophobic surfaces is expected to be less dependent
on the nature of the headgroup since it is the nonpolar
part of the molecule that interacts with the hydrophobic
surface. However, interactions between the headgroups
within the layer will influence the adsorption and the
interactions between nonpolar surfaces coated with nonionic surfactants. The effect of the headgroup is difficult
to predict, and accurate measurements are needed in order
to better understand the influence of different polar and
nonpolar groups on the interfacial properties of a given
system.
Recently, the adsorption and wetting behavior as well
as the surface forces acting between adsorbed layers of
n-decyl-β-D-maltoside (β-C10G2) have been investigated.14-21
Since notable differences in interfacial properties are
expected even with small changes in the surfactant
structure, this study addresses the surface forces generated by another important representative of the alkyl
maltoside series, namely n-dodecyl-β-D-maltoside (βC12G2). There are some studies dealing with the properties
of adsorbed β-C12G2 focusing on either the single air/water
interface22 or on foam films.23-26 To our knowledge there
are only two studies14,27 in which the adsorption of β-C12G2
on hydrophilic solid surfaces was investigated, and
likewise only a few reports address the adsorption of
β-C12G2 on hydrophobic surfaces.27,28 Moreover, no studies
of the interaction forces between solids coated with β-C12G2
have been reported. This lack of information is rather
surprising since sugar surfactants are used in several
applications involving solid surfaces.
The present study deals with the adsorption of β-C12G2
on hydrophobic solid surfaces and the resulting long-range
and adhesive forces between the respective surfactantcoated surfaces. Thiolated gold was used as hydrophobic
solid surface. The interaction forces were measured with
a bimorph surface force apparatus (the measurement and
analysis of surface interaction forces (MASIF) tech(13) Kiraly, Z.; Findenegg, G. H. Langmuir 2000, 16, 8842.
(14) Matsson, M. K.; Kronberg, B.; Claesson, P. M. Langmuir 2004,
20, 4051.
(15) Persson, C. M.; Claesson, P. M.; Lunkenheimer, K. J. Colloid
Interface Sci. 2002, 251, 182.
(16) Persson, C. M.; Kjellin, U. R. M.; Eriksson, J. C. Langmuir 2003,
19, 8152.
(17) Persson, C. M.; Kumpulainen, A. J.; Eriksson, J. C. Langmuir
2003, 19, 6110.
(18) Persson, C. M.; Kumpulainen, A. J. Colloids Surf., A 2004, 233,
43.
(19) Kumpulainen, A. J.; Persson, C. M.; Eriksson, J. C. Langmuir
2004, 20, 10935.
(20) Kumpulainen, A. J.; Persson, C. M.; Eriksson, J. C. Langmuir
2004, 20, 10534.
(21) Kumpulainen, A. J.; Persson, C. M.; Eriksson, J. C.; Tyrode, E.
C.; Johnson, C. M. Langmuir 2005, 21, 305.
(22) Liljekvist, P.; Kjellin, M.; Eriksson, J. C. Adv. Colloid Interface
Sci. 2001, 293, 89.
(23) Stubenrauch, C.; Schlarmann, J.; Strey, R. Phys. Chem. Chem.
Phys. 2002, 4, 4504; Phys. Chem. Chem. Phys. 2003, 5, 2736 (erratum).
(24) Schlarmann, J.; Stubenrauch, C. Tenside Surfactants Deterg.
2003, 40, 190.
(25) Muruganathan, R. M.; Krustev, R.; Ikeda, N.; Müller, H.-J.
Langmuir 2003, 19, 3062.
(26) Muruganathan, R. M.; Krustev, R.; Müller, H. J.; Möhwald, H.;
Kolaric, B.; von Klitzing, R. Langmuir 2004, 20, 6352.
(27) Zhang, L.; Somasundaran, P.; Maltesh, C. Colloid Interface Sci.
1997, 191, 202.
(28) Dedinaite, A.; Bastardo, L. Langmuir 2002, 18, 9393.
Langmuir, Vol. 21, No. 25, 2005 11837
Figure 1. Molecular structures of n-dodecyl-β-D-maltoside (βC12G2) and hexaoxyethylene dodecyl ether (C12E6).
nique).29,30 The results are discussed in the light of previous
studies in which the interactions between the respective
water/air surfaces, i.e., the disjoining pressure of foam
films stabilized with β-C12G2, were investigated.23,24 To
elucidate the influence that the molecular structure of
the surfactant has on the interaction forces, we compared
our results with those obtained for hexaoxyethylene
dodecyl ether (C12E6) (see Figure 1).31 Key features of sugar
surfactants (CnGm) and polyoxyethylene alkyl ethers (CiEj)
will be juxtaposed and discussed.
2. Experimental Section
2.1. Solution Preparation. The nonionic surfactant n-dodecyl-β-D-maltoside (β-C12G2) was used as received from Sigma
(>98% GC). The purity was checked by measuring the surface
tension as a function of the concentration at 22 °C by the Du
Nouy ring method using a Krüss K10ST tensiometer.23 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 system. For the surface force
experiments 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 was cleaned with Deconex from
Borer Chemie and rinsed thoroughly with Milli-Q water before
use. Three different surfactant solutions at concentrations of
0.035, 0.14, and 0.175 mM were prepared in 0.1 mM NaCl
background electrolyte. The first two concentrations are below
and the last one is above the critical micelle concentration (cmc)
of this surfactant (0.15 mM). In Table 1 some characteristic
parameters of β-C12G2 as well as those of some other selected
nonionic surfactants are listed.
2.2. MASIF Technique. Surface Preparation. The hydrophobic surfaces used for force measurements were obtained by
thiolation of gold-coated glass. The glass surfaces were prepared
from rods of borosilicate (Pyrex) with 2 mm diameter that were
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. Details about the subsequent thiolation are provided in previous reports.31,35,36
Surface Forces. The forces between surfactant-coated solid
surfaces were measured with the MASIF technique,29,30,37 which
basically employs an LVDT and a bimorph sensor to measure
the relative distance between the surfaces (D) and the forces of
interaction (F). The experimental data are presented as (F/R)-D
(29) Parker, J. L. Prog. Surf. Sci. 1994, 47, 205.
(30) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv.
Colloid Interface Sci. 1996, 67, 119.
(31) Stubenrauch, C.; Rojas, O. J.; Schlarmann, J.; Claesson, P. M.
Langmuir 2004, 20, 4977.
(32) Shinoda, K.; Yamaguchi, T.; Hori, R. Bull. Chem. Soc. Jpn. 1961,
34, 237.
(33) Drummond, C. J.; Warr, G. G.; Grieser, F.; Ninham, B. W.; Evans,
D. F. J. Phys. Chem. 1985, 89, 2103.
(34) Kjellin, U. R. M.; Claesson, P. M.; Linse, P. Langmuir 2002, 18,
6745.
(35) Stubenrauch, C.; Schlarmann, J.; Rojas, O. J.; Claesson, P. M.
Tenside Surfactants Deterg. 2004, 41, 174.
(36) Ederth, T.; Claesson, P.; Liedberg, B. Langmuir 1998, 14, 4782.
(37) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci.
Instrum. 1989, 60, 3135.
11838
Langmuir, Vol. 21, No. 25, 2005
Rojas et al.
Table 1. Critical Micelle Concentrations (cmc), Area per Molecule (Acmc), and Surface Tensions (γcmc) of Different
Nonionic Surfactants in Aqueous Solution at 298 Ka
cmc/mM
Acmc/Å2
γcmc/mN m-1
2
0.19
2.1
0.15, 0.2 (295-297 K)22
0.86
0.73
0.05
0.064
0.073,35 0.064 mM (295-297 K)22
41
36
56
50,33 48.5,27 38 (295-297 K),22 55 (on graphite)27
50
47
42
47
52,35 50 (295-297 K)22
29.9
40.9
37.0
37.633
29.4
29.9
27.6
29.7
34.035
surfactant
n-decyl-β-glucoside3
β-C10G1,
β-C12G1, n-dodecyl-β-glucoside32
β-C10G2, n-decyl-β-maltoside3
β-C12G2, n-dodecyl-β-maltoside
C10E424
C10E534
C12E434
C12E534
C12E6
a
Acmc and γcmc refer to the values at the cmc.
curves, where R is the harmonic radius R of the two surfaces (R
) 2R1R2/(R1 + R2)). In the MASIF experiments the distance D
measured between the (bare or surfactant-coated) surfaces is
not absolute but relative to the zero compliance or “hard wall”
contact. Therefore, if two surfactant-coated surfaces are in
contact, D ) 0 means that the two surfactant layers (one on each
surface) are in direct contact. This has to be taken into account
when the MASIF results are evaluated and compared with results
from other techniques, e.g., the interferometric surface force
apparatus (SFA). A detailed description of the experimental
procedure and the data evaluation is provided in ref 31.
Hydrodynamic Effects. In the MASIF the interactions are
measured under dynamic conditions; i.e., the data are automatically acquired while the surfaces are moving with respect to
each other (approaching or separating). Therefore, hydrodynamic
effects arising from the movement of the liquid medium during
the approach or the separation of the surfaces need to be taken
into account, especially when high rates of displacement are
used.38,39 The hydrodynamic interactions result in an extra
repulsion when the surfaces approach each other and in an extra
attraction when they are separated. To minimize this effect, low
driving rates (during surface approach and separation) are
employed. The hydrodynamic force FH between two approaching
surfaces is given by38,39
FH 3πηR d(D - 2Ds)
)
R
2D
dt
where η is the viscosity of the solution (assumed to be identical
to that of water) and D is the surface separation at time t. Ds is
the position of the plane of no shear and thus 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 location of the slipping plane is not known
independently, and Ds is treated as a fitting parameter. 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. The hydrodynamic force FH is subtracted
from the measured “dynamic” interaction force F, leading to the
“static” interaction force.
3. Results
3.1. Interactions between Thiolated Surfaces.
Nonpolar thiolated gold surfaces were prepared, and the
interactions across aqueous solutions in the absence and
presence of β-C12G2 were determined. The (dynamic and
static) forces acting between the pure thiolated gold
surfaces immersed in 0.1 mM aqueous NaCl solution are
illustrated in Figure 2. This figure also includes the static
force profile that is calculated by subtracting the hydrodynamic interactions from the measured (dynamic) forces.
No long-range electrostatic double-layer force is present,
demonstrating that the thiolated surfaces are uncharged.
A long-range attractive force becomes detectable at a
surface separation of about 15 nm, and the gradient of the
force exceeds the spring constant when the separation
has decreased to 10 nm. From this position the surfaces
(38) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311.
(39) Vinogradova, O. I. Langmuir 1996, 12, 5963.
Figure 2. Force F normalized by the harmonic radius R as a
function of surface separation D. The forces were measured on
approach (at a rate of 17 nm/s) between thiolated gold surfaces
across aqueous solutions containing 0.1 mM NaCl in the absence
of surfactant (circles). The solid line corresponds to the
experimental static surface forces (calculated after subtracting
hydrodynamic forces from the measured surface forces). The
dashed line is the van der Waals force assuming a 2-nm-thick
thiol layer on gold calculated with a distance-dependent
Hamaker constant (according to Ederth40). The arrow represents
the jump into contact when the force gradient d(F/R)/dD exceeds
the spring constant, k/R.
jump into contact, and the magnitude of the attraction in
this distance regime cannot be quantified. The calculation
of the van der Waals force acting between thiolated gold
surfaces is rather complex since the magnitude of the
effective Hamaker constant varies with the surface
separation. However, a situation identical to that in the
MASIF experiment has been considered in detail by
Ederth,40 and we utilized his results for calculating the
van der Waals forces. The results, which are displayed in
Figure 2 as a dashed line, show that the measured
attraction is slightly larger than the calculated van der
Waals force. Note that Ederth obtained significantly larger
attractive forces for similar surfaces36 which were identified as being due to the formation of vapor cavities, a
process that did not occur in the present study. In other
words, the origin of the attractive force observed for the
present system is not quite clear. However, the presence
of attractive forces is consistent with previous reports
describing the forces acting between nonpolar solid
surfaces in water. For an introduction to the topic of the
long-range attraction between such surfaces, the reader
is referred to the review by Christenson and Claesson.41
Once in contact, the surfaces adhere strongly to each other.
The adhesion force is in fact too strong to be measured
with the bimorph spring used in these measurements,
which means that it exceeds 80-100 mN m-1.
(40) Ederth, T. Langmuir 2001, 17, 3329.
(41) Christenson, H. K.; Claesson, P. M. Adv. Colloid Interface Sci.
2001, 91, 391.
Interactions of Nonpolar β-C12G2-Coated Surfaces
Langmuir, Vol. 21, No. 25, 2005 11839
Figure 4. Force F normalized by the harmonic radius R as a
function of surface separation D measured 8 h after the β-C12G2
surfactant had been injected. The forces were measured on
approach (filled circles) and on separation (open circles) between
thiolated gold surfaces across aqueous solutions containing 0.1
mM NaCl in the presence of 0.14 mM β-C12G2. The driving
velocity was 4.3 nm/s. The black line that follows the trace of
the force profile on separation corresponds to the static forces
(after subtracting hydrodynamic effects).
Figure 3. (a) Force F normalized by the harmonic radius R
as a function of surface separation D between thiolated gold
surfaces across aqueous solutions containing 0.1 mM NaCl in
the presence of 0.035 mM β-C12G2. The surface forces represented by the open circles were measured on approach (25 nm/
s; see inset) 2.5 h after injection of β-C12G2. An inward step at
around 4 mN m-1 was observed in this case. The respective
pulloff or adhesion forces measured on separation were too
strong to be depicted in the force diagram. The solid line
represents the static force computed after subtracting hydrodynamic effects. A slip plane located at 3.2 nm was assumed
in the calculation to ensure a perfect match between the static
forces and the experimental data in the zero-compliance zone.
The dashed line represents the van der Waals forces for a threelayer model (gold/thiol layer/surfactant) calculated with a
distance-dependent Hamaker constant according to ref 40. The
surface force profile obtained after 19 h equilibration (for the
same system) is represented by the filled circles (offset by 3.2
nm from the origin). In this case no step-in was observed. (b)
Schematic diagrams of the situations depicted in (a) for the
force curves between thiolated gold surfaces across 0.1 mM
NaCl in the presence of 0.035 mM β-C12G2 after 2.5 and 19 h
equilibration time. After 2.5 h the adsorbed surfactant molecules
are driven out from between the interfaces at a compressive
load of ca. 4 mN m-1. A strong adhesion due to thiol-thiol
contact is observed. After 19 h the adsorbed surfactant molecules
are not squeezed out (under the conditions of the experimental
compressive loads applied); i.e., the surfactant polar groups on
each surface are in contact.
The force profiles obtained after addition of β-C12G2 to
a low concentration (0.035 mM) are reported in Figure 3.
The surfactant molecules adsorb with the nonpolar group
directed toward the surface and the polar group toward
the aqueous medium. After addition of the surfactant the
surfaces thus become hydrophilic and the long-range
attraction present prior to addition of β-C12G2 vanishes,
as expected. It takes less than 2.5 h to develop a surfactant
monolayer that produces a steep repulsive force barrier
at separations below 5 nm. To overcome this force barrier,
a compressive force of ca. 4 mN m-1 is needed. At short
separations the surfactant monolayers (one on each
surface) exert a steric repulsion as the two surfaces are
brought together, producing the force barrier illustrated
in Figure 3. If a sufficiently large compressive force is
applied, the surfactant is squeezed out from the contact
zone, leading to a contact of the two bare thiolated surfaces.
Thus, the steep steric repulsion at D ∼ 3-4 nm is
equivalent to twice the length of the adsorbed surfactant,
and the thickness of a single sugar surfactant monolayer
can be calculated to be 1.7-2 nm. The influence of
hydrodynamic effects is clearly seen in Figure 3 (static
force profile), where an adhesion force can be detected
after subtracting hydrodynamic forces. In this case the
position of the slipping plane was chosen from the
separation distance at which the measured approaching
velocity suddenly drops (see inset in Figure 3). This
distance is estimated to be 3.2 nm, which coincides with
the separation distance where a steric barrier comes into
play (see step in the force profile). The observed attraction
is attributed to van der Waals forces, which were calculated
by assuming a distance-dependent Hamaker constant (as
a multiple-layer system) according to ref 40. Indeed, the
static force profile nearly matches the calculated van der
Waals interactions up to the onset of the steric repulsion.
Another observation worth mentioning is the fact that
the magnitude of the force barrier increases with time
and that a “hard wall” contact is observed after the
adsorption equilibrium is reached (see force curve recorded
after 19 h from injection of the surfactant solution). This
steep force is due to steric repulsion forces between the
maltoside headgroups of the surfactant molecules adsorbed on each surface.
At a β-C12G2 concentration of 0.14 mM the cohesion of
the monolayer is sufficiently strong to prevent the
displacement of the surfactant from between the interfaces
using forces up to F/R ) 10 mN m-1 (see Figure 4). Thus,
for the 0.14 mM surfactant concentration we observe a
hard wall (or MASIF constant compliance zone) due to
the repulsion between the surfactant headgroups up to
the maximum measurable force, which indicates that the
surfactant layer is densely packed.
Note that the maximum forces applied were significantly
larger than the ones that can be measured accurately
(limited by the saturation of the bimorph force sensor).
11840
Langmuir, Vol. 21, No. 25, 2005
However, the applied loads can be estimated from the
known spring constant and piezo displacement. The
outward force curve displayed in Figure 4 was obtained
after applying a high load (F/R ≈ 50 mN m-1) and shows
an interesting feature: when the compressive force was
decreased to F/R ) 2.2 mN m-1, a sudden increase in
thickness by 2-3 nm was observed.
The sudden increase in the surface separation distance
observed in the outward force profile has also been reported
by Kjellin et al.42 and is explained by the diffusion of
surfactant molecules (that were driven out from between
the surfaces upon approach at a high, but not measurable,
force) into the contact region due to the intralayer surface
pressure at the solid-liquid interface outside the contact
position. A rapid readsorption of the surfactant occurs
once the repulsive force has been reduced sufficiently.
Since the adsorbed surfactants were removed at high
compression, the approach (inward) force curve in Figure
4 was shifted outward by 3 nm (as was the case in Figure
3). The adhesion observed after separating the surfaces
(ca. 0.5 mN m-1 in the outward run shown in Figure 4)
is suggested to be due to the van der Waals forces between
the surfactant layers (see Figure 3).
By further increasing the surfactant concentration to
0.175 mM, one observes interaction curves similar to the
one obtained for 0.14 mM. An example is given in Figure
5a. Thus, for all three concentrations (below and above
the cmc) the adsorbed surfactant layers are sufficiently
dense to provide a stabilizing force barrier at a separation
of ca. 3 nm. The magnitude of this force barrier increases
with increasing surfactant concentration as the adsorption
density, and thus the monolayer cohesion, is increased.
An estimate of the adsorption density at the three
surfactant concentrations can be obtained using the data
reported by Dedinaite for β-C12G2 on silanated silica.28
The value at 0.035 mM can be estimated to be 1.4 mg/m2
(2.7 µmol/m2), whereas a value of 1.7 mg/m2 (3.3 µmol/m2)
was obtained at concentrations close to the cmc.
An important issue to be mentioned at this point is that
Figures 3, 4a, and 5a show “featureless” inward curves up
to the development of the steep steric barrier due to the
contact of the adsorption layers. However, after subtracting hydrodynamic forces, it became clear that for thiolated
glass surfaces the van der Waals attraction dominates at
surface separations below ca. 7 nm up to the onset of the
steep steric barrier (at 3-4 nm separation). In other words,
the hydrodynamic correction led to a slight but important
change in the force profiles revealing an additional
attractive force, i.e., the van der Waals force, close to the
steric repulsion. There is thus no need to include a
contribution from hydrogen bonding to explain the experimental data. We note here that the adhesion between
n-decyl-β-D-maltoside surfactant (β-C10G2) layers has been
reported to be 0.9 mN m-1, which can also be explained
as a van der Waals attraction.15
3.2. Interactions between Air/Water Surfaces. In
a previous study, the disjoining pressure Π (i.e., the
interaction between air/water surfaces across aqueous
solutions) of foam films stabilized by β-C12G2 was measured
as a function of the film thickness h, using the thin film
pressure balance (TFPB) technique.23,43 Surfactant concentrations similar to those employed in the present study
were used, and the background electrolyte concentration
was also kept constant at 0.1 mM NaCl. Details with
regard to the technique, the experimental procedure, and
(42) Kjellin, U. R. M.; Claesson, P. M. Langmuir 2002, 18, 6754.
(43) Stubenrauch, C.; von Klitzing, R. J. Phys.: Condens. Matter
2003, 15, R1197.
Rojas et al.
Figure 5. (F/R)-D curves between thiolated gold surfaces (a)
and corresponding Π-h curves (b) for three different concentrations of β-C12G2 in 0.1 mM NaCl solution. For the cmc a
value of 0.15 mM was determined. All (F/R)-D curves were
measured on approach and after the equilibrium adsorption
had been established. The different surfactant concentrations
correspond to 0.035 (diamonds), 0.14 (open circles), and 0.17
mM (filled circles). In (a) a representative static force profile
(after subtraction of hydrodynamic effects) is shown for
completeness (solid line). The solid lines in (b) are calculated
according to the DLVO theory assuming interactions at constant
charge. Data for the Π-h curves are taken from ref 23.
the interpretation of the results are given in refs 23 and
43. General conclusions can be drawn with regard to the
influence of the surfactant concentration on the force
profiles in the case of thiolated (see Figure 5a) surfaces
as well as on the corresponding Π-h curves (Figure 5b).
Since the situation displayed in Figure 5a was already
discussed, we now turn our attention to the Π-h curves.
As is seen in Figure 5b, film thicknesses were found to
range from more than 80 nm to less than 5 nm, depending
on the surfactant concentration and the applied pressure,
which ranges from 200 to 9000 Pa. 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 Π
increases. While the slope d(log Π)/dh is independent of
the surfactant concentration, a significant shift of the
curves toward lower disjoining pressures is observed by
increasing the surfactant concentration from 0.034 to 0.137
mM. Moreover, at the highest concentration no CBF is
formed at all, but the foam film drains directly down to
the NBF.
It is established that-but not yet completely understood
whysthe air/water surface is negatively charged. This
charge is responsible for the long-range electrostatic
repulsive forces observed in foam films stabilized by
Interactions of Nonpolar β-C12G2-Coated Surfaces
Table 2. Surface Excess of β-C12G2 at the Air-Water and
Silanated Silica Interfaces
surface
concn (mM)
surf. excess (µmol/m2)
air-water
silanated silica
air-water
silanated silica
air-water
silanated silica
0.034
0.035
0.137
0.14
0.17
0.17
4.0b
2.7a
4.5b
3.3a
4.7b
3.3a
a
From ref 28. b From ref 23.
nonionic surfactants. An increase in nonionic surfactant
concentration leads to a decrease of the surface charge
density as more uncharged molecules (i.e., nonionic
β-C12G2 surfactant) adsorb at an originally charged
surface. A detailed discussion about the origin of the
surface charge at the bare air/water surface can be found
in refs 31 and 43-48. The electrostatic forces acting in
foam films stabilized by β-C12G2 were quantified by means
of the DLVO theory using constant charge boundary
conditions and the theoretical Debye length of κ-1 ) 30
nm.49 These calculations led to surface charge densities
of q0 ) 1.55 mC m-2 for the 0.035 mM solution and q0 )
0.95 mC m-2 for the 0.137 mM solution, respectively. The
decrease in surface charge density destabilizes the CBF
until no CBF is observed for c > cmc under the given
experimental conditions. At these concentrations it is the
immediate formation of an NBF that is observed. The
NBFs are very thin (ca. 5 nm) with an aqueous core of 1-2
nm, assuming a length of ∼2 nm for the surfactant. In
other words, these films consist of two surfactant monolayers with only small amounts of water separating the
headgroups (mainly hydration water). As was the case for
the force barrier between nonpolar solid surfaces, densely
packed monolayers are required to stabilize an NBF and
thus to prevent contact between the two bare surfaces
(i.e., film rupture). Hence the presence of an NBF signifies
the existence of a force barrier between two air/water
surfaces in analogy to the force barrier between two solid
surfaces (see Figure 5a).
4. Discussion
4.1. Influence of the Surface on Interfacial Forces.
The driving force for adsorption of nonionic surfactants
to nonpolar surfaces is the reduction in contact area
between the surface and water, as well as between the
nonpolar surfactant tails 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 adsorption at different
nonpolar surfaces, however, varies depending on the
nature of the surface. For instance, Kjellin et al. showed
that the surface excesses of a range of nonionic ethoxylate
surfactants were slightly larger at the air-water interface
than at silanated silica.34 The value reported for the surface
excess of β-C12G2 at the air-water interface23 is also
slightly larger than that on silanated silica28 as shown in
Table 2. Unfortunately, the surface excess of this surfactant on thiolated gold has not been reported, but as a
first approximation one can assume that it is similar to
that on silanated silica.
(44) Karraker, K. A.; Radke, C. J. Adv. Colloid Interface Sci. 2002,
96, 231.
(45) Exerowa, D.; Kruglyakov, P. M. Foam and Foams FilmssTheory,
Experiment, Application; Elsevier: Amsterdam, 1998.
(46) Carruthers, J. C. Trans. Faraday Soc. 1938, 34, 300.
(47) Dickinson, W. Trans. Faraday Soc. 1941, 37, 140.
(48) Taylor, A. J.; Wood, F. W. Trans. Faraday Soc. 1957, 53, 523.
(49) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.;
Academic Press: San Diego, CA, 1991.
Langmuir, Vol. 21, No. 25, 2005 11841
One important difference between the two hydrophobic
surfaces used in this study, the air-water interface and
thiolated gold, is that the bare air/water surface is charged
while the thiolated surface is uncharged (see Figure 2).
The charge at the air/water surface leads to the presence
of a long-range electrostatic repulsion well described by
the nonlinear Poisson-Boltzmann model. Thus, the (F/
R)-D and the Π-h curves have nothing in common with
regard to long-range interactions: no long-range electrostatic interactions are observed for the thiolated surfaces,
while it is exactly these interactions that dominate the
properties of the foam films. On the other hand, the shortrange repulsive interactions between air/water and thiolated surfaces, respectively, are similar. In both cases a
direct contact between the surfaces is not observed (in the
latter case a direct contact is equivalent to a film rupture).
The force barrier that is built up between the thiolated
surfaces is comparable to the formation of an NBF, as
was discussed in detail in our previous study.31 Note that,
in contrast to the bilayer formation between the thiolated
surfaces, NBF formation was not observed at the lowest
surfactant concentration under the given experimental
conditions. However, it is not unlikely that an NBF is
formed at pressures higher than 10 000 Pa. In both cases,
the short-range repulsion is due to a combination of a
hydration repulsion that originates from the removal of
water molecules tightly bound to the polar sugar group,50
a protrusion repulsion due to the restriction in movement
of surfactants in and out from the surface,51 and a
conformational repulsion originating from the restricted
conformation of the polar group.52
4.2. Influence of the Surfactant Polar Group on
Interfacial Forces. The interaction forces and the
disjoining pressures between surfaces coated with either
β-C12G2 or C12E6 are compared in Figure 6. Two different
surfaces are considered: thiolated gold (Figure 6a) and
the air/water surface (Figure 6b). Since one can extract
more information from the force profiles obtained at
concentrations below the cmc, i.e., for nonsaturated
adsorption layers, the respective data of the current study
and of the previous C12E6 study31 are juxtaposed. Note
that in both cases we are not far from the maximum air/
water surface concentration, which allows ussat least
qualitativelysto compare the results. [Actual surface
coverage of β-C12G2 at 0.034 mM is 4.0 × 10-6 mol m-2
compared to the maximum surface coverage of 4.7 mol
m-2. Actual surface coverage of C12E6 at 0.01 mM is 3.0
× 10-6 mol m-2 compared to the maximum surface
coverage of 3.2 mol m-2.31]
A major difference is worth mentioning with regard to
the adsorption of β-C12G2 and C12E6 at the solid surfaces.
Under equilibrium conditions very high forces are needed
to expel β-C12G2 from between thiolated gold surfaces,
while for C12E6 compressive loads below 1 mN m-1 are
sufficient (see Figure 6a). The most reasonable explanation
for this observation is a higher adsorption density of
β-C12G2 which results in a significant force barrier already
at low concentrations. Similar observations were made in
the case of silanated surfaces, where a steric force between
β-C12G2-coated surfaces was evident even at low concentrations (unpublished data), whereas in the case of C12E6coated surfaces no steric force was observed at similar
concentrations.31 This observation is in line with the
adsorption behavior at the air/water surface: at the chosen
(50) Parsegian, V. A.; Fuller, N.; Rand, R. P. Proc. Natl. Acad. Sci.
U.S.A. 1979, 76, 2750.
(51) Israelachvili, J. N.; Wennerstrom, H. J. Phys. Chem. 1992, 96,
520.
(52) de Gennes, P. G. Adv. Colloid Interface Sci. 1987, 27, 189.
11842
Langmuir, Vol. 21, No. 25, 2005
Figure 6. (F/R)-D curves between thiolated gold surfaces (a)
and corresponding Π-h curves (b) for two different nonionic
surfactants (C12E6 and β-C12G2) in 0.1 mM NaCl solution. The
cmc values are given in Table 1. All (F/R)-D curves were
measured on approach and after the equilibrium adsorption
had been established. The solid lines in (b) are calculated
according to the DLVO theory assuming interactions at constant
charge. Data for C12E6 are taken from 31. No correction for
hydrodynamics is included in the force profiles shown in (a).
concentrations the surface concentration of C12E6 is around
25% less than that of β-C12G2. This difference is suggested
to be due to the lower flexibility of the maltoside group
compared to an oligooxyethylene group.22 This leads to a
larger loss of conformational entropy for C12E6 layers than
for β-C12G2 layers due to the confinement between two
surfaces, which, in turn, provides a larger driving force
for the desorption of C12E6 molecules. At higher concentrations, however, both β-C12G2 and C12E6 produce a
monolayer that is very difficult to remove from the gap
between the surfaces upon compression, indicating formation of densely packed surfactant layers.
The layer thickness can be determined with the MASIF
technique only when the surfactant is removed by
compression. On the basis of these results we estimate
that the β-C12G2 barrier is located at a distance of 3-4
nm, whereas a value of 4-5 nm is observed for C12E6 (see
Figure 6). It is therefore concluded, as expected from the
molecular structure, that under similar experimental
conditions the effective length of the β-C12G2 molecule
confined at the interface is shorter than that of C12E6.
The attractive forces acting between the surfactant
layers when they are close to each other is mainly due to
van der Waals interactions. However, the depth of the
attractive minimum depends on the nature of the polar
part of the surfactant53-55 and is the result of an interplay
(53) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994,
98, 8468.
(54) Ederth, T.; Liedberg, B. Langmuir 2000, 16, 2177.
Rojas et al.
of van der Waals, hydration, and confinement forces.51
The attractive minimum is marginally deeper for the
surfactant with the maltoside headgroup (ca. 0.5-1 mN
m-1 attractive minimum for β-C12G2 adsorbed on thiolated
glass) compared to C12E6 with the same hydrophobic chain
(attractive minimum of less than 0.5 mN m-1).31 One effect
contributing to this is the larger refractive index of a
glucoside unit compared to that of ethylene oxide, which
increases the van der Waals force. Moreover, the shortrange repulsive force contributions are expected to be
different for the two surfactants, which will also influence
the depth of the attractive minimum. For the sake of
completeness we note that in the particular case of
ethylene oxide based surfactants an increase in temperature results in a larger attraction, an effect related to
the temperature dependence of the hydration interaction.56
While the adsorption behaviors of β-C12G2 and C12E6 at
solid surfaces are obviously different, astonishing similarities with regard to the Π-h curves are found. As is
seen in Figure 6b, the curves nearly lie on top of each
other, which means that the surface charge densities q0
are nearly equal. Indeed, surface charge densities of q0 )
1.70 mC m-2 for C12E6 and q0 ) 1.55 mC m-2 for β-C12G2
were calculated from the experimental data.23,31 As the
surface charge density is a property of the bare air/water
surface, these values mean that a surface concentration
of 3.0 × 10-6 mol m-2 C12E6 reduces the surface charge
density to the same amount as a surface concentration of
4.0 × 10-6 mol m-2 β-C12G2. Moreover, the surface
concentration is in both cases enough to stabilize a foam
film up to 10 000 Pa. We suggest that it is the effective
surface cross-sectional area covered per molecule, which
is different from the average area per molecule at the
interface that determines the reduction in surface charge
density. Since the headgroup of C12E6 is larger than that
of β-C12G2, a smaller number of the former surfactant is
needed to achieve a given reduction in the repulsive doublelayer force. Similar arguments have been presented
before.57,58 We conclude that for each surfactant the surface
charge density is reduced with increasing adsorption.
However, when comparing different surfactants, a given
adsorption reduces the surface charge density to a different
degree.
Finally, a comparison of various oligomers of polyglycoside-type surfactants is not attempted here, but it is
expected that the glucoside (β-C12G1) surfactant will form
(on nonpolar surfaces) an adsorbed monolayer of higher
cohesive energy and therefore will offer greater resistance
against compression compared to the maltoside counterpart (β-C12G2). This is based simply on the fact that β-C12G1
is expected to adsorb more extensively due to a smaller
repulsion between the headgroups. These assumptions
are validated by results reported for a similar system (βC10G2 and β-C10G1 surfactants).18
5. Conclusions
In conclusion, 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. The fact that no
(55) Claesson, P. M.; Kjellin, U. R. M. Modern Characterization
methods of surfactant systems. In Surfactant Science Series; Binks, B.
P., Ed.; Marcel Dekker: New York, 1999; Vol 83, p 255.
(56) Claesson, P. M.; Kjellander, R.; Stenius, P.; Christenson, H. K.
J. Chem. Soc., Faraday Trans. 1 1986, 82, 2735.
(57) Manev, E. D.; Pugh, R. J. Langmuir 1991, 7, 2253.
(58) Waltermo, A.; Manev, E.; Pugh, R.; Claesson, P. J. Dispersion
Sci. Technol. 1994, 15, 273.
Interactions of Nonpolar β-C12G2-Coated Surfaces
double-layer force was observed between the β-C12G2coated thiolated surfaces proves that the surfactant did
not contain any charged surface-active impurities, and
thus that the repulsive double-layer force observed
between air-water interfaces is due to charges present
at the bare air-water interface.
The measurements performed in this study contribute
to elucidating the influence of the molecular structure,
i.e., the polar headgroup and the hydrophobic chain, on
interaction forces. Comparing data obtained for β-C12G2
and C12E6, for example, demonstrates that for similar bulk
concentrations the sugar surfactant forms a denser and
more robust monolayer with higher cohesiveness. On the
other hand, compared with the shorter tail homologue
β-C10G2, β-C12G2 is anchored more strongly to thiolated
gold. This is explained by a stronger interaction between
the nonpolar tails within the monolayer.
Our measurements indicate a thickness of the β-C12G2
monolayer under compressive loads marginally lower than
Langmuir, Vol. 21, No. 25, 2005 11843
2 nm. This value is in agreement with the surfactant
molecular dimensions. It is smaller than the (compressed)
monolayer thickness measured for C12E6. For both surfactants, the most distinctive feature in the force curve
between surfactant-coated solid surfaces is the buildup of
a steep and short-range repulsion as the surfactant
adsorption approaches saturation.
Acknowledgment. O.J.R. and P.M.C. are grateful to
the SSF program “Colloid and Interface Technology” and
the European Commission (Marie Curie RTN SOCON)
for their support. C.S. is indebted to the DFG, the Fond
der Chemischen Industrie, and the European Commission
(Marie Curie RTN SOCON) for financial support. J.S.-S.
acknowledges financial support from the DFG and the
DAAD. O.J.R. acknowledges support from the National
Textile Center grant C05-NS09.
LA051938E