Langmuir 2005, 21, 11213-11219
11213
Microscopic Structure of Crystalline Langmuir Monolayers
of Hydroxystearic Acids by X-ray Reflectivity and GID: OH
Group Position and Dimensionality Effect
Luigi Cristofolini,*,†,‡ Marco P. Fontana,†,‡ Carla Boga,§ and Oleg Konovalov|
INFM and Physics Department, University of Parma, Parco Area delle Scienze 7/A, I-43100
Parma, Italy, Centro Ricerca e Sviluppo SOFT, Physics Department, University of Rome I,
Department of Organic Chemistry “A. Mangini”, University of Bologna, Viale Risorgimento 4,
40136 Bologna, Italy, and European Synchrotron Radiation Facility, 6 rue Jules Horowitz,
F-38043 Grenoble, France
Received May 31, 2005. In Final Form: September 5, 2005
Hydroxystearic acid (HSA) molecules at the air-water interface present an interesting bicompetitive
adsorption between primary and secondary hydrophilic groups on either end of an alkyl chain, which,
depending on the position of the second hydrophilic group, may lead to a sharp transition from an expanded
phase to a crystalline condensed morphology as surface pressure is increased. Here we report a set of
measurements on a series of hydroxystearic acids in which the position of the secondary competing hydrophilic
group position is varied along the whole extent of the alkyl chain from position 2 (i.e., close to the primary
hydrophilic group) to positions 7, 9 and 12, the latter being the compounds mostly studied in the literature.
We show here direct microscopic evidence, obtained by synchrotron radiation reflectometry and grazing
incidence diffraction, that the position of the secondary hydrophilic group not only strongly influences the
phase diagram as determined by compression isotherms and ellipsometry but also induces different
crystallization patterns in the 2D system of the Langmuir monolayer. In particular, we report for the first
time the existence of a turning point in the effects of the hydroxyl position on the monolayers structure
at 7-HSA.
Introduction
Insoluble monolayers of fatty acids containing two
hydrophilic moieties have been studied by several groups
in recent years.1-4 Such molecules present interesting
complex behavior at the air-water interface because of
the competition between the adsorption of the primary
and secondary hydrophilic groups;2 furthermore, the
presence of an additional hydroxyl group might lead to
hydrogen bonding between adjacent molecules, thus
producing different packing arrangements and interactions at the air-water interface.
In the studies present in the literature,1,2,4 the isotherms
of the hydroxystearic acids (HSA) show a plateau region
that is commonly understood to be due to the coexistence
of two distinct phases: at large areas per molecule, with
the surface pressure below the plateau value, the molecules lie approximately flat with both of the hydrophilic
groups on the water surface, whereas at a smaller area
per molecule, with the surface pressure higher than the
plateau, the molecules assume a more or less vertical
conformation,2 although precise structural determination
is available only for hydroxystearic acids with the second
hydrophilic group well separated from the polar head.4
Indirect structural determination by in-situ polarized
FT-IR external-reflection spectra of 12-hydroxystearic acid
* Corresponding author. E-mail: [email protected].
† University of Parma.
‡ University of Rome I.
§ University of Bologna.
| European Synchrotron Radiation Facility.
(1) Sakai, H.; Umemura, J. Langmuir 1998, 14, 6249-6255.
(2) Yim, K. S.; Rahaii, B.; Fuller, G. G. Langmuir 2002, 18, 65976601.
(3) Meurk, A. Tribol. Lett. 2000, 8, 161-169.
(4) Vollhardt, D.; Siegel, S.; Cadenhead, D. A. Langmuir 2004, 20,
7670-7677.
Figure 1. Langmuir isotherms for the different hydroxystearic
acids (indicated in the legend) used in the present work.
Conditions: 30 µL of a 0.33 mg/mL chloroform solution, T )
21°C. (Top inset) Time evolution of film thickness, probed by
ellipsometry during the compression along the Π-A plateau,
for 9-HSA. (Bottom inset) Structure of 2-HSA. Positions of the
second hydroxyl group in 7-HSA, 9-HSA, and 12-HSA are
indicated by the arrows..
(12-HSA) suggests that the molecular orientation angles
vary at different points on the isotherm: at the final stage
of the plateau region of the compression isotherm (Figure
1), the orientation angle of the hydrocarbon chain was
found to lie about 55° from the surface normal; however,
upon monolayer compression, the angle decreased to about
28° in the solid-phase region.1 This finding suggests that
some phase transition might take place in the solid phase.
Additional interest is due to the fact that hydroxystearic
acids are widespread in nature and in industrial products.
10.1021/la0514213 CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/13/2005
11214
Langmuir, Vol. 21, No. 24, 2005
It is apparent that in most cases hydroxystearic acids act
on the rheological properties and the structure of the
systems with which they interact and that a determining
role is played by the OH group on the alkyl tail and the
ensuing degrees of motional and structural freedom of
the molecules. It is therefore of some interest to study, in
a simple system, the effect of OH position along the alkyl
tail on the structure and morphology of HSA assemblies,
such as monolayers at the air-water interface.
In addition, nanotribology measurements by atomic
force microscopy on glass surfaces covered by 2-HSA and
12-HSA pointed out the importance of the position of the
OH group in determining the rheological properties of
hydroxystearic acid confined to the surfaces. Two different
mechanical regimes are found, depending on the position
of the OH group in the alkyl chain: whereas 2-HSA
exhibits a static-dynamic transition (steady sliding
replaced by a stick-slip regime as the velocity is decreased), 12-HSA has no such transition. This is attributed
to the different configurations adopted by the molecules
due to the OH bridging groups.3
In this work, we wish to address the specific point of the
relative roles of hydrophobic and hydrophilic equilibriums
on the molecular scale in determining the aggregation
and morphology of molecules at the air-water interface
in an effort to understand the generalized mechanical
effects that seem to be associated with the presence of
n-HSA. The functionalized n-HSA molecules studied here
seem particularly interesting from this point of view
because the molecules can be engineered so as to position
the hydroxyl group any distance from the interface, thus
varying practically continuously the relative hydrophilicity
of the normally hydrophobic alkyl chain.
In this article, we shall concentrate on the roomtemperature behavior, addressing the issue of whether
the position of the OH group along the chain, leading to
the different isotherms observed, also influences the
crystal morphology and the crystallographic structure of
the monolayer. We do this by X-ray reflectometry and
grazing incidence diffraction (GID), using synchrotron
radiation, and by ellipsometric imaging. From these
measurements, we obtain information on the formation,
structure, and molecular orientation of the microscopic
2D crystallites that form in the monolayer at sufficiently
high pressures and for specific positions of the OH group
along the alkyl chain. This information is then related to
data from the compression isotherms and from nullellipsometric measurements. An important result we have
obtained is the identification of a critical role of 7-HSA
that seems to separate two different regimes on the basis
of the effects of the OH group on the structure and
morphology of the monolayer. The overall information we
obtain allows us to propose a comprehensive interpretation
of the role of competing hydrophilic interactions on the
structure and morphology of the Langmuir monolayer of
this family of fatty acids at the air-water interface.
Experimental Section
2-Hydroxystearic acid (2-HSA) and 12-hydroxystearic acid (12HSA) were purchased from Sigma-Aldrich. 7-Hydroxystearic acid
(7-HSA) and 9-hydroxystearic acid (9-HSA) were prepared
according to the literature,5 with modifications. The corresponding structures are sketched in the bottom inset of Figure 1.
For the ellipsometric measurements, we used a small trough
made by us (84 × 305 mm2 surface), which was placed directly
in the laser light path and was positioned through micrometric
controls. The ellipsometer was aligned to operate in the vertical
(5) Bergstrom, S.; Aulin-Erdtman, G.; Rolander, B.; Stenhagen, E.;
Osting, S. Acta Chem. Scand. 1952, 6, 1157.
Cristofolini et al.
plane, at an incidence angle of Φ ) 48° from the normal to the
water surface, on an antivibration table with a He-Ne laser
source (λ ) 632.8 nm). The same computer was used to drive the
electronics controlling the Langmuir trough and to perform the
ellipsometric measurement. Null-ellipsometric imaging measurements have been performed using the same setup while the
light was collected with an M-PLAN APO 20× (Mitutoyo) objective
and a CCD camera detector; images were captured with a frame
grabber.
In all of the experiments, the water surface was routinely
characterized prior to film dispersion to ensure purity, by a
complete compression cycle while measuring the surface pressure,
to detect the presence of any contaminant. For the ellipsometric
measurement, angles ∆ and Ψ were measured on the pure water
surface (i.e., before film dispersion) to provide the final alignment
and quantification of capillary waves, thus providing an accurate
reference point. A standard inversion scheme, based on Fresnel
formulas, was applied to convert ellipsometrically measured
angles ∆ and Ψ to film thickness. For this, the refractive index
is needed and was directly measured for all of the hydroxystearic
acids from ellipsometric measurements on thick multilayers,
obtaining the value n ) 1.51(1). We note that this value is sensibly
larger than that tabulated6 for stearic acid (n ) 1.4299) probably
because of the different, more dense packing of the HSA molecules
with respect to the normal stearic acid, which is possibly related
to the existence of extra hydrogen bonding between the additional
OH groups.
X-ray reflectivity and GID were measured at the Troika II
(ID10B) beamline of ESRF operating at wavelength λ ) 1.542
Å. A Langmuir trough made by the ESRF workshops, with a
single moving barrier (maximum surface 418 × 170 mm2) was
mounted on an active antivibration support and was surrounded
by a helium atmosphere to reduce diffuse scattering from air.
Before all of the measurements on Langmuir films, the X-ray
reflectivity from the bare water surface was measured to check
the alignment and to provide a reference.
The reflectivity data were subsequently analyzed both with
our own software and with the program PARRATT,7 which
calculates theoretical reflectivity curves according to the so-called
Parratt recursive approach.8,9 In this method, one calculates the
transmission and reflection Fresnel coefficients at each interface,
starting from the approximation of independent smooth layers.
To reproduce the reflectivity curve of a real multilayer, a
structured model of independent layers, each with uniform
electron density, has to be provided. The roughness of real layers
is accounted for using the Névot-Croce approximation,10 which
implies a scaling of the reflectivity at the interface between layers
a and b by the factor exp(-2kakbσ2), with σ being the roughness
rms value and ka kb representing the z component of the wave
vector for layers a and b, respectively.
The incident beam had vertical size of 0.1 mm, and for the
GID measurements, we selected an angle of incidence Ri ) 0.12°
(i.e., 75% of the critical angle for the air-water interface at this
wavelength), resulting in a 5 cm beam footprint. The scattered
beam was spatially filtered by means of Soller slits, which give
an angular resolution of 0.08° in the horizontal plane yielding
the intrinsic width in the momentum space of fwhm ) 0.0056
Å-1, almost constant in our measurement range. The intensity
of the scattered X-ray beam was subsequently recorded by means
of a linear position-sensitive detector. The incident beam was
attenuated to keep the total count below 5000 counts per second,
thus avoiding pileup and detector saturation. Because we are
not interested in absolute intensities but only in position and
shapes, data have not been corrected for polarization effects.
In the analysis of GID data, the isotropic 2D powder average
can be safely assumed because the typical crystallite sizes, as
observed also by ellipsometric imaging, are much smaller than
the impinging beam footprint, that is, 5 cm long.
(6) CRC Handbook of Chemistry and Physics, 79th ed.; Lide, D. R.,
Ed.; CRC Press: New York, 1998.
(7) Braun, C. Parrat32 Software for Reflectivity; HMI: Berlin, 1999.
(8) Parratt, L. G. Phys. Rev. 1954, 95, 359-369.
(9) Daillant, J.; Gibaud, A. X-ray and Neutron Reflectivity: Principles
and Applications; Springer: Berlin, 1999.
(10) Nevot, L.; Croce, P. Rev. Phys. Appl. 1980, 15, 761-779.
OH Group Position and Dimensionality Effect
Figure 2. Ellipsometric imaging pictures of the different
crystalline morphologies, all recorded at the end of the plateau
in the isotherm, for the different HSAs. The length scale (same
for all of the pictures) is indicated in the bottom right panel.
Results and Discussion
a. Langmuir Isotherms, Ellipsometry, and Imaging. In Figure 1, we report the isotherms for the different
HSAs used in the present work, all recorded at T ) 21 °C
with the same aliquot of 0.33 mg/mL chloroform solution
spread on the surface of the Langmuir trough. We clearly
note that different positions of the OH group influence
the extent and the pressure of the plateau in the isotherm.
The isotherms of 9- and 12-HSA are consistent with those
present in the literature, and 7-HSA also shows similar
behavior. On the contrary, 2-HSA shows very different
behavior, characterizing a remarkably robust film, with
a steep onset of surface pressure, no sign of collapse up
to 66 mN/m, and a final collapse pressure very close to
(but below) the bare water surface tension. We double
checked this result both by varying the dispersed aliquot
and by checking the Wilhelmy balance calibration.
Null-ellipsometry measurements performed on all of
the Langmuir monolayers during compression invariably
Langmuir, Vol. 21, No. 24, 2005 11215
show an increase in film thickness as pressure is increased,
as indicated by the continuous increase of the angle ∆,
which is directly related to film thickness via the refractive
index. The values for the final thickness of the compressed
monolayers agree with those more precisely determined
by X-ray reflectivity (vide infra). However, as shown in
the inset of Figure 1, the time evolution of film thickness
along the plateau shows discrete jumps between “high”
and “low” film thickness. This indicates that the film is
laterally inhomogeneous (i.e., it consists of dense patches
separated by areas of lower density). This is also confirmed
by the ellipsometric imaging data of Figure 2.
Moreover, it is interesting that when the plateau
coexistence range is well defined its temperature dependence in the isotherms follows the 3D behavior of the van
der Waals fluid very closely (data not shown). This
particular aspect deserves further investigation.
The first two pictures of Figure 2 confirm the morphology
previously observed,4 with the characteristic acicular
morphology, whereas the other pictures reveal a new,
unexpected morphology featuring round islands for 7-HSA
and large uniform patches for 2-HSA.
b. X-ray Reflectivity. As expected, very little structure
is seen in the low-pressure reflectivity curves of the HSA
(data not shown). On the contrary, at high pressure (Π )
25 mN/m) 2-HSA shows a well-defined pattern that could
be fitted (quality of the fit χ2 ) 1.7) by the model depicted
in Figure 3, in which, coming from the water side (electron
density ) 0.33 e- Å-3) we have a somewhat extended polar
head region of thickness 9.36 Å (electron density ) 0.42
e- Å-3), followed by a shorter alkyl chain region that is
only 15.6 Å in thickness (electron density ) 0.31 e- Å-3).
We note that it is not possible to distinguish, in the electron
density profile, a separate region with the side OH group.
The overall film thickness (25.0(1) Å) is in good agreement
with the results reported in the literature11-13 for Langmuir films of Cd or Ba-stearate by X-ray reflectivity (25.15(5) Å and 25.3(4) Å, respectively). We note, however, that
in 2-HSA the polar head region is more extended;
conversely, the alkyl chain region is shorter, most probably
because of the presence of the additional OH group in the
2 position.
Figure 3. X-ray reflectivity pattern for 2-HSA at Π ) 25 mN/m. The error bars are the experimental data, and the continuous
line between the points is the reflectivity calculated from the model illustrated in the inset.
11216
Langmuir, Vol. 21, No. 24, 2005
Figure 4. X-ray reflectivity pattern for 12-HSA acid at Π )
15 mN/m. The error bars are the experimental data, and the
continuous line is the reflectivity calculated from the threeslab model illustrated in the inset.
If we refer to the commonly accepted rule14,15 predicting
that in the crystal structures of most saturated hydrocarbons the CH2-CH2 spacing projected onto the chain
axis is d ) 1.265 (0.010)Å and that the final CH3 group
counts for 9/8 of such value, we are led to the conclusion
that in Langmuir monolayers of 2-HSA 6 carbon atoms
are submerged and only 12 carbon atoms are left out of
the water surface, which is reasonable given the very
strong hydrophilic nature of the polar head with 2 OH
groups.
On the contrary, the reflectivity curve from 12-HSA at
Π ) 15 mN/m shows a less-structured pattern, which in
principle could be fitted with a simple two-slab model of
overall thickness of 25.16 Å. However, the quality of this
fit is not satisfying, particularly in the higher exchanged
momentum region. This prompted the formulation of a
more complex model containing three layers. The best fit
of the experimental data was thus obtained (χ2 ) 1.29)
with the model depicted in Figure 4, which implies a total
thickness of 23.35 Å. This value, compared with that of
2-HSA previously discussed (25.0 Å), implies a tilting angle
from the water surface normal of θ ) 20.9°, which is in
excellent agreement with the GID determination as
discussed in the following section.
Looking at our model in more detail, we find first a
region of polar heads extending for 3.88 Å. This value is
in good agreement with what is commonly found for other
fatty acid polar headgroups (e.g., in arachidic acid15 at Π
) 15.9 mN/m, t ) 3.88 Å). Next, we find a slab of alkyl
chains extending for 8.85 Å and characterized by an
electron density comparable to that of pure water and
finally a region of anomalously high electron density
(thickness 10.62 Å) that could be related to the presence
of the OH group in the 12 position. Again, if we take the
commonly accepted rule14,15 for the extension of the alkyl
(11) Langmuir Blodgett Films; Roberts, G., Ed.; Plenum Press: New
York, 1990; p 139.
(12) Srivastava, V. K.; Ram Verma, A. Solid State Comm. 1966, 4,
367-371.
(13) Matsuda, A.; Sugi, M.; Fukuj, T.; Izima, S.; Miyahara, M.; Otsubo,
Y. J. Appl. Phys. 1977, 48, 771-774.
(14) Tanford, C. The Hydrophobic Effect: Formation of Micelles and
Biological Membranes; Wiley: New York, 1973.
(15) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippman-Krayer, P.;
Mohwald, H. J. Phys. Chem. 1989, 93, 3200-3206.
Cristofolini et al.
Figure 5. (Main panel) Grazing incidence diffraction intensity
map (counts are represented as gray intensity) from a Langmuir
layer of 12-HSA at Π ) 7 mN/m as measured on the ID10B
beamline. The letters A, B, and C mark the three diffraction
peaks discussed in the text. The graph in the top panel is the
GID curve (i.e., the projection of the 2D map on the Qxy axis).
A small artifact can be seen at Qxy ) 1.466Å-1, which appears
as extra intensity at a single particular value of Qxy. The graph
on the right panel is the Qz dependence (rod) of the three peaks.
chain as a function of the number of CH2 units, we are led
to the conclusion that only seven CH2 units are left
unaffected by the OH group in the 12 position. We also
tried to improve the model by the addition of a fourth
layer, which could in principle account for the remaining
part of the alkyl chain above the second OH group.
However, the quality of the fit did not improve significantly
upon this addition, which is related to the fact that there
is little structure present in our reflectivity data, so we
discarded this model. Comparing these data with those
obtained for 2-HSA, we note that the submerged part
contains only the polar head: thus for 12-HSA essentially
all of the carbons are above the water surface. Furthermore, the electron density profile is qualitatively different
in 12-HSA and in 2-HSA; namely, in 12-HSA the maximum
electron density is found in the topmost layer. There is a
minimum in the electron density apparently at the position
of the second OH group and an anomalous increase in
electron density for the remaining CH2 units. Comparing
the profiles for 2-HSA and 12-HSA, respectively, we note
that the average electron density is higher for 12-HSA
(∼0.4 vs ∼0.35 e- Å-3). Considering that the first seven
carbons in 12-HSA have essentially the same electron
density as water, we can infer from this result that there
is a kink in the alkyl chain at the 12 position, which is
consistent with the average tilt angle of θ ) 20.9° that we
have measured and which would yield a more compact
alkyl chain structure for n > 12, and hence a higher
electron density.
c. Grazing Incidence Diffraction. 12-Hydroxystearic
Acid. In Figure 5, we show a typical 2D GID pattern
obtained for 12-HSA at Π ) 7 mN/m. We note that for all
of the HSAs measured diffraction peaks from 2D ordered
structures were found only when the surface pressure
was higher than the plateau pressure. In particular, the
minimum surface pressure required to observe diffraction
peaks was 16 mN/m for 7-HSA, 12 mN/m for 9-HSA, and
7 mN/m for 12-HSA.
This is in contrast to the situation encountered in normal
fatty acids, where GID peaks are measured virtually at
any pressure (e.g., in arachidic acid albeit with reduced
intensity when the area per molecule is very large15). As
a matter of fact, fatty acids show molecular self-aggrega-
OH Group Position and Dimensionality Effect
Langmuir, Vol. 21, No. 24, 2005 11217
Table 1. In-Plane and Out-of-Plane Exchanged Momenta
and Corresponding Widths of the Three GID Peaks for
12-HSA at Increasing Surface Pressure, Π ) 7, 10, and 15
mN/m, as Indicated in the First Column
Qxy
(Å-1)
fwhm
Qxy (Å-1)
Qz
(Å-1)
7 mN/m A
7 mN/m B
7 mN/m C
1.363(1)
1.476(2)
1.574(1)
0.006(1)
0.014(4)
0.007(2)
0.20(4)
0.52(6)
0.31(6)
10 mN/m A
10 mN/m B
10 mN/m C
1.363(1)
1.481(2)
1.574 (2)
0.006(1)
0.024(4)
0.011(1)
0.22(3)
0.48(4)
0.29(4)
15 mN/m A
15 mN/m B
15mN/m C
1.371(1)
1.485(1)
1.573(1)
0.010(1)
0.020(1)
0.019(2)
0.15(3)
0.45(5)
0.32(16)
tion, and this gives GID Bragg peaks even at zero pressure.
On the contrary, phospholipids do not show self-aggregation, and Bragg peaks are present only at pressures above
the plateau. Clearly in the case of HSA, the presence of
side groups prevents them from aggregating. This indirectly confirms our model for the molecular conformation
on the water surface, which assumes that at low pressure
the second OH group resides on the water surface.
The GID curve of 12-HSA acid (top panel of Figure 5)
consists of three peaks that could be fitted by a Lorentzian
shape (peaks A-C). The peak positions and widths are
reported in Table 1 also as a function of the applied
pressure. The widths of peaks A and C appear to be
resolution-limited, implying crystalline coherence extending at large distances, at least over 1000 Å, whereas peak
B appears to be broader because of a geometrical distortion
of the experimental geometry.
We have repeated the measurements after increasing
the surface pressure (Π ) 7, 10, and 15 mN/m), and the
results are summarized in Table 1. No major differences
are found between 7 and 10 mN/m. On the contrary, at
15 mN/m we observe the onset of collapse and 3D
crystallization, as indicated by the periodic oscillations
present in the rods of peaks B and C at this pressure. This
is accompanied by a measurable reduction of the out-ofplane components of peaks A and B, indicative of a
reduction of the tilt angle under increased lateral pressure.
For the collapsed phase, from the period of the oscillation
of the rods (dQ ) 0.143 (4) Å-1) we extract a value for the
d spacing of d ) 44(1)Å that is slightly smaller than the
spacing expected for a bilayer from the reflectivity data
(46.7 Å), perhaps indicating some form of interdigitation
of the alkyl chains in the 3D collapsed phase.
The lattice parameters and molecular tilting angles
obtained from the GID data are shown in Table 2, where
we report the parameters of both the primitive, oblique
cell and of an equivalent body-centered, pseudo-rectangular cell, which is useful for the following comparison
with the crystal structures of the other compounds. The
angle γPRIM refers to the angle between a and b in the
primitive cell, and γRECT is the distortion of the pseudorectangular cell (which would be truly rectangular if γRECT
) 90°). Tilt angle θ refers to the molecular inclination
Figure 6. (Main panel) Grazing incidence diffraction intensity
map (counts are represented as gray intensity) from a Langmuir
layer of 9-HSA at Π ) 12 mN/m as measured on the ID10B
beamline. The graph in the top panel is the GID curve (i.e., the
integrated projection of the 2D map on the Qxy). The right panel
shows the Bragg rods (i.e., Qz dependence of the GID intensity
integrated in the Qxy range around the peak).
with respect to the vertical, whereas the azimuth ψ is the
angle between aPRIM and the projection of the tilted
molecule, measured clockwise.
Our results for the values of lattice parameters are
consistent with those of ref 4 even if the data are not
taken at the same temperature and pressures; note that
in ref 4 lattice parameters a and b are exchanged.
Furthermore, we observe a small increase in γPRIM and a
decrease in tilt angle θ as pressure is increased, in
agreement with the cited reference.4
It is also interesting that tilt angle θ directly measured
by GID, in agreement with that from X-ray reflectivity,
is smaller (θ ) 20°) than that deduced by in-situ polarized
FT-IR external-reflection spectra (θ ) 28°) in the same
region of the isotherm1. Because FT-IR analysis is based
on the CH2 modes, it seems reasonable to conclude that
the molecules are not straight rods and different parts
experience different orientations.
9-Hydroxystearic Acid. In Figure 6, we report the GID
pattern of 9-HSA at Π ) 12 mN/m (i.e., at a pressure just
above its plateau). The pattern consists of two peaks only,
as indicated in Figure 6, indicating rectangular symmetry.
In this case, we have also repeated the scan three times,
with a 54-min interval between each scan, to detect any
time evolution in the GID pattern. We find that whereas
the position of peak A, at Qxy ) 1.465(1) Å-1, is timeindependent within the experimental accuracy, peak B
shows a temporal evolution, especially in the out-of-plane
component, as shown in Table 3. Note in particular the
narrowing of the half width of both peak positions and a
correlated increase in the out-of-plane position. This
implies that in time the system evolves toward a better-
Table 2. Lattice Parameters and Tilting Angles for 12-HSA as a Function of Applied Pressurea
Π ) 7 mN/m
Π ) 10 mN/m
Π ) 15 mN/m
aPRIM
(Å)
bPRIM
(Å)
γPRIM
(deg)
area
(Å2)
aRECT
(Å)
bRECT
(Å)
γRECT
(deg)
tilt
θ (deg)
azimuth
ψ (deg)
5.00(1)
5.00(1)
5.00(1)
4.62(1)
4.61(1)
4.61(1)
112.8
112.9
113.3
21.3(1)
21.3(1)
21.7(1)
5.33(1)
5.32(1)
5.29(1)
8.01(1)
8.01(1)
8.03(1)
85.0
84.8
84.9
19.8
18.0
17.2
43.1
37.4
46.0
a Results for the primitive (oblique) cell are shown, together with the equivalent body-centered, pseudo-rectangular cell. Tilt angle θ
refers to the molecular inclination with respect to the vertical, whereas azimuth ψ is the angle between aPRIM and the projection of the
tilted molecule, measured clockwise.
11218
Langmuir, Vol. 21, No. 24, 2005
Cristofolini et al.
Table 3. Position and Width of the GID Peaks for 9-HSA
at Constant Surface Pressure as a Function of Time
first scan B
second scan B
third scan B
Qxy
(Å-1)
fwhm
Qxy (Å-1)
Qz
(Å-1)
fwhm Qz
(Å-1)
1.487(3)
1.488(1)
1.489(1)
0.017(6)
0.008(2)
0.010(2)
0.32(6)
0.38(4)
0.38(4)
0.36(16)
0.27(10)
0.27(9)
defined crystalline structure, in agreement with the
images shown in Figure 2.
The twofold splitting, with both of the peak maxima at
Qz > 0, implies rectangular symmetry with molecular
tilting toward the next nearest neighbors (NNN), in
agreement with the data in the literature4 in which the
rectangular symmetry deduced from the GID pattern
seems to be distorted if one takes the lattice parameters
reported in Table 1 of the paper.
Given the 2:1 rule for the ratio of the Qz component of
the nondegenerate to degenerate peak, we identify peak
A with (1, 1) and (1, -1) and peak B with (0, 2). We therefore
extract the lattice parameters as summarized in Table 4.
We note that the area per molecule is stable with time
and is comparable to the value from the isotherm at this
pressure and slightly smaller than for the 12-HSA. On
the contrary, from the reported increase of the position of
the out-of-plane component (Table 3), we calculate an
increase in the tilt angle upon annealing of the film,
reaching the equilibrium value of θ ) 14.3°. If we further
increase the surface pressure, 3D crystallization occurs,
with Debye rings appearing as a signature (data not
shown).
7-Hydroxystearic and 2-Hydroxystearic Acids. The GID
intensity from 7-HSA at Π ) 16 mN/m is shown in Figure
7. It consists of a single peak at position Qxy ) 1.4578(4)Å-1
(quality factor for the fit χ2 ) 1.1), implying the packing
of molecules in hexagonal symmetry. To compare with
the other HSA crystalline structures, we chose to represent
the cell with an equivalent centered rectangular lattice
with a side-length ratio of b/a ) x3. The lattice parameters
are then a ) 4.977Å and b ) 8.620 Å. The peak width
fwhm ) 0.007(1) Å-1 is almost resolution-limited, indicating large crystalline domains. Hexagonal symmetry
correlates nicely with the round shape of the crystallites
observed in the ellipsometric imaging of 7-HSA, in contrast
to the acicular shape observed for 9-HSA and 12-HSA
(Figure 2).
It is remarkable, however, that this single peak in the
GID pattern is characterized by a measurably nonzero
out-of-plane component, as shown in the inset of Figure
7, where the dots represent the measured integrated
intensity and the continuous line is a fit of the peak with
a Lorentzian shape whose maximum is located at Qz )
0.25(7) Å-1. Assuming a rigid rodlike molecule, this value
should correspond to a tilt angle of θ ) 9.7°, which,
however, cannot be ascribed to the undistorted hexagonal
Figure 7. (Main frame) Grazing incidence diffraction intensity
map (counts are represented as gray intensity) from a Langmuir
layer of 7-HSA at Π ) 16 mN/m as measured on the ID10B
beamline. (Top graph) GID curve (i.e., projection of the 2D map
on the Qxy axis). (Right panel) Bragg rod (i.e., Qz dependence
of the GID intensity integrated in the Qxy range around the
peak, between 1.42 and 1.47 Å-1). The continuous line is the
fit of the peak. (See the text for details.)
phase or to the so-called rotator phase,16 which should
have maximum intensity at Qz ) 0.
Such an apparently self-contradictory result could be
explained by assuming that the 7-HSA molecule is not a
rigid rod but presents a kink in the correspondence of the
second OH group. This would allow truly hexagonal
packing of the centers of the molecules, which would
interact via the submerged polar heads, together with the
presence of a tilt in the alkyl chains pointing out of the
water surface. To test this hypothesis, additional measurements are needed (e.g., by neutron reflectivity with
partially deuterated HSA).
Finally, the GID pattern from 2-HSA, measured at Π
) 30 mN/m, as shown in Figure 8, is similar to that of
7-HSA, consisting of a single peak implying hexagonal
symmetry, but with a much broader width (position Qxy
) 1.480(3)Å-1, width fwhm ) 0.05(1) Å-1), the hexagonal
cell is equivalent to a centered rectangular lattice with
lattice parameters of a ) 4.90(1)Å and b ) 8.49(1) Å. From
the large value of the width of the peak, we can estimate
the coherence length to be only about 30 lattice parameters
in each direction, in contrast to the much more ordered
7-HSA lattice.
As it was for 7-HSA, it is remarkable that this single
peak is characterized by a measurably nonzero out-ofplane component, as shown in the inset of Figure 8. The
peak maximum is located at Qz ) 0.29(1)Å-1. One could
assume a rigid rodlike molecule with a tilt angle of 11°;
Table 4. Structure Parameters Extracted from GID Data for 9-HSA at Constant Surface Pressure
a ) 4π/ (4Qxy(11))2 - (Qxy(02))2 (Å)
b ) 4π/Qxy(02) (Å)
area per
molecule (Å2)
tilt angle
θ ) a tan(Q02z/Q02xy)
4.98(1)
8.44(1)
21.0(1)
14.3°
x
Table 5. Summary of the Results for the Different 2D Crystalline Structures Found for the Different HSAs Studied
molecule
plateau
Π (mN/m)
symmetry
aRECT
(Å)
bRECT
(Å)
area
(Å2)
cell angle
γ (deg)
molecular
tilt θ (deg)
2-HSA
7-HSA
9-HSA
12-HSA
?
14
11
6.5
HEX
HEX
RECT
OBL
4.90(1)
4.98(1)
4.98(1)
5.33(1)
8.49(1)
8.62(1)
8.44(1)
8.01(1)
20.8(1)
21.4(1)
21.0(1)
21.3(1)
90
90
90
85.0
11
9.7
14.3
19.8
OH Group Position and Dimensionality Effect
Figure 8. (Main frame) Grazing incidence diffraction intensity
map (counts are represented as gray intensity) from a Langmuir
layer of 2-HSA at T ) 6.5 °C and Π ) 30 mN/m as measured
on the ID10B beamline. (Top graph) GID curve (i.e., projection
of the 2D map on the Qxy axis). (Right panel) Bragg rod (i.e.,
Qz dependence of the GID intensity integrated in the Qxy range
around the peak, between 1.42 and 1.52 Å-1).
however, the possibility of a kink near the water surface
should also be considered.
Conclusions
We have measured the crystalline structure of different
HSA monolayers at the air-water interface, as summarized in Table 5, and have demonstrated a specific
correlation between the position of the secondary OH group
and the crystalline symmetry of the monolayer, which
decreases with decreasing separation between the two
competing hydrophilic groups, reaching hexagonal symmetry for 7-HSA.
These data, together with some discrepancies in the
data present in the literature, indicate that (i) the
secondary OH groups might form a hydrogen bond network
(16) Kenn, R. M.; Biihm, C.; Bib, A. M.; Peterson, I. R.; Mohwald, H.;
Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092-2097.
Langmuir, Vol. 21, No. 24, 2005 11219
even when the molecules are confined as Langmuir
monolayers and (ii) the molecules are not rigid rods but
rather might present a kink in correspondence of the
second OH group. Both conclusions are clearly confirmed
by the observed behavior of the electron density profiles
obtained by reflectometry. Furthermore, the change in
symmetry at 7-HSA, with 2-HSA and 7-HSA both showing
hexagonal packing, indicates very clearly the importance
of the competing interactions: 2-HSA and 7-HSA show a
highly symmetric hexagonal phase; thereafter, the symmetry changes, accompanied by a change in the molecular
tilt, leading to similar behavior for the 9-HSA and 12HSA members of this family. Thus, it is around n ) 7
(further data are necessary to pinpoint the position more
precisely) that the competition between hydrophobic and
hydrophilic interactions equilibrates: For small values
of n, hydrophilic interaction will dominate, and the
behavior will tend to that of pure HSA. For higher values
of n, the OH will be too far from the water interface;
therefore, hydrogen bonding or hydrophobic interactions
will become more important, and 2D crystallization will
take place. The symmetry lowering as the secondary OH
group is placed further away from the interface can be
interpreted as being partially due to entropic effects. In
fact, if the alkyl chain is tilted at the position of the
secondary OH group, then the remaining alkyl chain will
have from 11 (n ) 7) to 6 carbons. It is reasonable to
assume that the longer residual chains will be merely
flexible and hence sample more orientational conformations, leading to the higher hexagonal symmetry of 7-HSA
monolayers; conversely, the more rigid short residual
chains of 12-HSA lead to tilted, more compact molecular
arrangements of lower symmetry.
We feel that our data and the analysis presented here
put on a more quantitative, microscopic footing ideas about
the correlation between the position of the secondary OH
group and the general phenomenology of these interesting
systems, with particular reference to the role of morphology on several space scales in the mechanical and
viscoelastic behavior. Clearly our data should be supplemented by further studies, particularly of the lower n
members of this interesting family of fatty acids.
LA0514213