Structure Features and Phase Behavior of 1-(12-Hydroxy)stearoyl-rac- glycerol Monolayers

Article abstract of DOI:10.1021/jp0361677

The effect of the alkyl-chain substitution on ordering and phase behavior of monoglycerol esters is studied by coupling the results of surface pressure-area (??-A) isotherm measurements, Brewster angle microscopy, and grazing incidence X-ray diffraction. In the present work, structure features and phase behavior of 1-(12-hydroxy)stearoyl-rac-glycerol monolayers are investigated and compared with those of the nonsubstituted 1-stearoyl-rac- glycerol and 12-hydroxystearic acid monolayers. The dominating effect of the alkyl-chain substitution by a hydroxyl group in the 12 position on the monolayer properties is demonstrated. Phase behavior, domain morphology, and two-dimensional lattice structure of the two 12OH-substituted amphiphiles are similar to each other but different from those of the usual nonsubstituted amphiphiles. Main characteristics of the 1-(12-hydroxy)stearoyl-rac-glycerol monolayers are an extended flat phase-transition (plateau) region of the ??-A isotherms related with a small temperature dependence of the main phase-transition points, domains with several arms which reflect homogeneously and grow rather irregularly with different tendencies to form curvatures and oblique lattice structures. The bipolar character of amphiphiles OH substituted in the midposition of the alkyl chain is obviously responsible for the special monolayer characteristics deviating from those of typical amphiphilic monolayers.

Full text of DOI:10.1021/jp0361677

J. Phys. Chem. B 2004, 108, 3781-3788
3781
Structure Features and Phase Behavior of 1-(12-Hydroxy)stearoyl-rac-glycerol Monolayers
D. Vollhardt,*,† G.Weidemann,‡ and S. Lang§
Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany, Federal Institute for
Materials Research and Testing, Unter den Eichen 87, D-12200 Berlin, Germany, and Institute for
Biochemistry and Biotechnology, Technical UniVersity Braunschweig, D-38106 Braunschweig, Germany
ReceiVed: July 24, 2003; In Final Form: January 5, 2004
The effect of the alkyl-chain substitution on ordering and phase behavior of monoglycerol esters is studied
by coupling the results of surface pressure-area (π-A) isotherm measurements, Brewster angle microscopy,
and grazing incidence X-ray diffraction. In the present work, structure features and phase behavior of 1-(12-
hydroxy)stearoyl-rac-glycerol monolayers are investigated and compared with those of the nonsubstituted
1-stearoyl-rac-glycerol and 12-hydroxystearic acid monolayers. The dominating effect of the alkyl-chain
substitution by a hydroxyl group in the 12 position on the monolayer properties is demonstrated. Phase behavior,
domain morphology, and two-dimensional lattice structure of the two 12OH-substituted amphiphiles are similar
to each other but different from those of the usual nonsubstituted amphiphiles. Main characteristics of the
1-(12-hydroxy)stearoyl-rac-glycerol monolayers are an extended flat phase-transition (plateau) region of the
π-A isotherms related with a small temperature dependence of the main phase-transition points, domains
with several arms which reflect homogeneously and grow rather irregularly with different tendencies to form
curvatures and oblique lattice structures. The bipolar character of amphiphiles OH substituted in the midposition
of the alkyl chain is obviously responsible for the special monolayer characteristics deviating from those of
typical amphiphilic monolayers.
Introduction
acid) has shown that the absence of long-range tilt orientational
order observed in these systems is correlated to a disordered
packing of alkyl chains.8
In the recent decade, it has been established that the
unsuspected morphological variety of condensed phase domains
depends sensitively on the chemical structure of the amphiphiles,
particularly on the chemical structure of the polar headgroups.
Correlations have been substantiated between morphological
texture, lattice structure, phase behavior of amphiphilic mono-
layers, and the headgroup structure of the monolayer material.1,2
On the other hand, such systematic information does not exist
on the effect of alkyl-chain substitution by a second polar group
although previous studies particularly Cadenhead et al. draw
attention on selected hydroxyfatty acids and esters.3-5 They
found on the basis of surface pressure-area isotherms a
remarkable dependence of the two-dimensional phase properties
on the position of the OH group in the alkyl chain. Obviously
there are large differences in the surface pressure-area isotherms
between fatty acids OH-substituted in the 2 position and those
where the OH substitution is far from the COOH group.
Fluorescence microscopy and Brewster angle microscopy
(BAM) studies of selected hydroxyfatty acid monolayers
provided first information on the morphology of their condensed
phases.6,7 The combination of BAM and GIXD (grazing
incidence X-ray diffraction) allowed the conclusion that OH
substitution in the 2 position gives rise to a loss of ordering
which has been attributed to a misfit of the alkyl chain and the
headgroup enlarged by the neighboring OH group in the 2
position. Another monolayer study of amphiphiles OH substi-
tuted in the 2 position (1,2-hexadecandiol, 2-hydroxypalmitic
Completely different behavior is indicated when the OH group
is positioned in the alkyl chain farther from the polar headgroup.
Then, bipolar behavior of the molecules should be expected in
the expanded state and the monolayer should undergo a
conformational change at compression. Accordingly, well-
shaped condensed phase domains and a defined lattice structure
were found after the phase transition in 9-hydroxypalmitic acid
monolayers.7
Langmuir monolayers of monoglycerolesters are not only
good candidates for systematic structure and texture studies but
they have also an interesting application potential.9-13 Therefore
extensive studies on the structure and morphology of these
amphiphiles with nonsubstituted alkyl chains have been per-
formed. In 1-monopalmitoyl-rac-glycerol (or 1-stearoyl-rac-
glycerol) monolayers, the BAM studies revealed circular or
cardioid-shaped domain textures subdivided into defined seg-
ments of different uniform brightness. The molecules are tilted
along the bisector of each segment, and alkyl chains are tilted
toward the nearest neighbors (NN) at room temperature.
The objective of this work is to obtain some knowledge how
the monolayer features of nonsubstituted monoglycerol esters
are affected by the substitution of 1 OH group in the 12 position
of the alkyl chain. Such functionalized monoglycerides and their
biotechnological synthesis have been of interest in real applica-
tion systems.14 The effect of chain substitution will be demon-
strated by comparison of the surface pressure-area per molecule
(π-A) isotherms, domain morphology, and lattice structure of
1-monostearoyl-glycerol and 1-(12-hydroxy-stearoyl)glycerol.
To corroborate the conclusion on the effect of OH substitution
* Author to whom correspondence may be addressed. E-mail: vollh@
mpikg-golm.mpg.de.
† Max Planck Institute of Colloids and Interfaces.
‡ Federal Institute for Materials Research and Testing.
§ Technical University Braunschweig.
10.1021/jp0361677 CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/27/2004

3782 J. Phys. Chem. B, Vol. 108, No. 12, 2004
Vollhardt et al.
The experimental setup for the surface pressure-area (π-
A) isotherm and the BAM studies consisted of a self-made
computer-interfaced film balance coupled with a Brewster angle
microscope (BAM1+, NFT, Go¨ttingen). The surface pressure
measured with the Wilhelmy method and using a roughened
glass plate was reproducible to (0.1 mNm-1. The monolayer
morphology was studied by BAM. An image-processing
software was used to correct the distortion of the digitized
images as the BAM images are distorted due to the observation
at the Brewster angle. The lateral resolution of the BAM1+ is
approximately 4 µm. More detailed information on the experi-
mental setup and BAM method is given elsewhere; see, e.g.,
refs 1 and 15.
The GIXD experiments were performed using the liquid-
surface diffractometer on the undulator beamline BW1 at
HASYLAB, DESY, Hamburg, Germany. To reduce the back-
ground in the X-ray scattering experiments, the film trough was
located in a sealed He-flushed container. A monochromatic
synchrotron X-ray beam was adjusted to strike the helium/water
interface at a grazing incidence angle Ri ) 0.85Rc, where Rc is
the critical angle for total reflection. The diffracted intensity is
detected by a linear position-sensitive detector (PSD) (OED-
100-M, Braun, Garching, Germany) as a function of the vertical
scattering angle Rf. A Soller collimator located in front of the
PSD provides the resolution for the horizontal scattering angle
2θxy. The scattering vector Q ) kf - ki has an in-plane
component Qxy ≈ (4π/λ)sin θxy and an out-of-plane component
Qz ≈ (2π/λ)sin Rf, where λ is the X-ray wavelength.16,17 The
diffracted intensities are corrected for polarization, effective area,
and Lorentz factor. The lattice parameters are obtained from
Figure 1. Surface pressure-area isotherms of 1-(12-hydroxy)-
monostearoyl-rac-glycerol monolayers at different temperatures.
in the 12 position, a comparison is performed with some results
of 12-hydroxystearic acid monolayers.
Experimental Section
1-(12-Hydroxy)monostearoyl-rac-glycerol was synthesized
according to a procedure which provides exclusively monoglyc-
erides using glycerol as polyhydroxy substrate and fatty acids
as acyl donors during lipase catalysis.14 The direct enzymatic
monoacylation of glycerol with 12-hydroxystearic acid was
performed in n-hexane in the presence of phenylboronic acid.
A purity of g99% was obtained by crystallization from
methanol, liquid chromatography, and thin-layer chromatogra-
phy.
1-Monostearoyl-rac-glycerol (purity < 99%) and 12-hydroxy-
stearic acid (purity > 99%) were purchased from Sigma and
used without further purification. Deionized water used for the
experiments was made ultrapure by a Millipore desktop system
(Millipore, Eschborn, Germany). In the case of 12-hydroxy-
stearic acid monolayers, pH 3 subphase water adjusted by the
addition of 0.1 M HCl was used. The monolayer-forming
substances were dissolved in a 9:1 (v:v) mixture of heptane (for
spectroscopy, Merck) and ethanol (p.a. Merck) to a 1 mM
spreading solution.
the peak positions. The lattice spacing is given by d(hk) ) 2π/
hk
xy
Q , where (h,k) denotes the order of the reflection. The polar
tilt angle t of the long molecule axis and the tilt azimuth ψxy
are calculated from the positions of the Qxy and Qz maxima,18
according to Qhzk ) Qhk cos ψhk tan t. The lattice parameters a,
xy
b, and γ were calculated from the lattice spacing dhk, and from
these the unit cell area Axy ) ab sin γ. The cross section per
alkyl chain A0 ) Axy cos t is related to the unit cell area Axy
(area per molecule parallel to the interface) and the tilt angle t.
Figure 2. Surface pressure-area isotherms of 1-monostearoyl-rac-glycerol monolayers at different temperatures.

1-(12-Hydroxy)stearoyl-rac-glycerol Monolayers
J. Phys. Chem. B, Vol. 108, No. 12, 2004 3783
consisting of a nonsubstituted alkyl chain and a headgroup with
a somewhat larger cross section area. In particular, this concerns
the temperature dependence of the “plateau” (two-phase coexist-
ence) region. The coexistence region of the nonsubstituted
1-monostearoyl-rac-glycerol monolayers is essentially smaller
than that of 1-(12-hydroxy)monostearoyl-rac-glycerol and shows
an increasing inclination with increasing temperature. The
nonhorizontal phase transition of the π-A isotherms and its
dependence on the temperature has been theoretically described
by consideration of the formation of two-dimensional aggregates
on the basis of new equations of state.19,20 Also the phase-
transition pressures are in a completely different temperature
region. It is seen in Figure 2 how the phase-transition point
changes with temperature; at 30 °C Ac ) 0.64 nm2/molecule
and πc ) 1.4 mN/m, at 35 °C Ac ) 0.55 nm2/molecule and πc
) 4 mN/m, at 40 °C Ac ) 0.48 nm2/molecule and πc ) 7.7
mN/m, and at 45 °C Ac ) 0.44 nm2/molecule and πc ) 11.9
mN/m.
Figure 3. Surface pressure-area isotherms of 12-hydroxystearic acid
monolayers at different temperatures.
The remarkable effect of the alkyl-chain substitution on the
thermodynamic behavior of amphiphilic monolayers can be
demonstrated by the π-A isotherms of 12-hydroxystearic acid
monolayers (Figure 3). Despite the large differences in the
monolayer properties of 1-monostearoyl-rac-glycerol and stearic
acid, the π-A isotherms of both 12OH-substituted amphiphiles
show striking similarities. The π-A isotherms of 12-hydroxy-
stearic acid have a similar flat “plateau” over an even larger
area region between about 110 and 30 Å2/molecule at 5 °C and
between about 95 and 30 Å2/molecule at 30 °C as those of 1-(12-
hydroxy)monostearoyl-rac-glycerol. At all temperatures, the
isotherms show also a small overshoot of the surface pressure
around the break point of the transition to the condensed phase,
indicating a small supersaturation with fluid phase.
The dominating effect of the OH substitution in the 12
position of the alkyl chain on the thermodynamic behavior of
the amphiphilic monolayers is corroborated by the direct
comparison of the π-A isotherms of the three amphiphiles at
the same temperature (e.g., 30 °C in Figure 4) and by the
dependence of their break point for the main phase transition
on temperature (Figure 5). Figure 4 shows that the shape of the
π-A isotherms of the two 12OH-substituted amphiphiles is very
similar; only the phase-transition pressure of the 12-hydroxy-
Results and Discussion
The large effect of the OH substitution in the 12 position of
the alkyl chain on the thermodynamic monolayer properties can
be demonstrated by a comparison of the π-A isotherms of
1-(12-hydroxy)monostearoyl-rac-glycerol with nonsubstituted
1-monostearoyl-rac-glycerol and 12-hydroxystearic acid, which
has the same alkyl-chain substitution but another headgroup (a
carboxyl group instead of glycerol). Figure 1 shows the π-A
isotherms of 1-(12-hydroxy)monostearoyl-rac-glycerol mono-
layers at 5, 10, 15, 20, 25, and 30 °C. The characteristic shape
of these isotherms is noteworthy. For all temperatures, a flat
“plateau” over a large area region between about 90 and 30
Å2/molecule at 5 °C and about 80 and 25 Å2/molecule at 30 °C
indicates a broad two-phase coexistence region between the
fluidlike and the condensed phase. The phase-transition pressure
increases with the temperature, as expected. It is interesting to
note that a small overshoot of the surface pressure around the
break point of the transition to the condensed phase indicates a
small supersaturation with fluid phase.
The π-A isotherms of the nonsubstituted 1-monostearoyl-
rac-glycerol monolayers are completely different (Figure 2) and
show the characteristic monolayer features of usual amphiphiles
Figure 4. Surface pressure-area isotherms of the monolayers of 1-(12-hydroxy)monostearoyl-rac-glycerol, 1-monostearoyl-rac-glycerol, and 12-
hydroxystearic acid at 30 °C.

3784 J. Phys. Chem. B, Vol. 108, No. 12, 2004
Vollhardt et al.
Figure 5. Dependence of the main phase-transition point of 1-(12-hydroxy)monostearoyl-rac-glycerol, 1-monostearoyl-rac-glycerol, and
12-hydroxystearic acid monolayers on temperature.
stearic acid monolayer is somewhat lower than that of the 1-(12-
hydroxy)monostearoyl-rac-glycerol monolayer. On the other
hand, it is clearly seen that there exist considerable differences
between the 12OH-substituted and the nonsubstituted mono-
stearoyl-rac-glycerol. In the same way, the presentation of
temperature dependence of the break point for the main phase
transition demonstrates the similarity of both the 12OH-
substituted amphiphiles, despite the essential difference of the
headgroups, and the complete difference between the 12OH-
substituted and the nonsubstituted monostearoyl-rac-glycerol
(Figure 5). In all cases, it exists a linear increase of the phase-
transition pressure with temperature. For the two 12OH-
substituted amphiphiles, the slope is notably small and nearly
the same in the same temperature range. The break point of the
main phase transition is obviously shifted to lower temperatures
by 12OH substitution of the alkyl chain. In the case of the
nonsubstituted 1-monostearoyl- rac-glycerol monolayers, the
much stronger increase of the break point for the main phase
Figure 6. BAM image of a characteristic 1-monostearoyl-rac-glycerol
domain. T ) 30 °C.
transition is convincingly demonstrated by the stronger slope
of the adequate straight line.
The BAM studies provide information on the morphological
features of the condensed phase domains formed in the two-
phase coexistence region. So, it is interesting to show in which
way the 12OH substitution of the alkyl chain affects the
morphology of the condensed phase domains of 1-monostearoyl-
rac-glycerol. For a comparison, a single domain of 1-mono-
stearoyl-rac-glycerol formed in the two-phase coexistence region
at 30 °C is shown in Figure 6. It is seen that the domain is
subdivided into seven segments of different uniform brightness
that meet approximately in the center. The domain morphology
is generally the same as that of the homologous 1-monopalmi-
toyl-rac-glycerol.21,22 On the basis of the geometric analysis of
the domain texture performed for 1-monopalmitoyl-rac-glycerol,
it can be concluded that, in each segment, the alkyl chains have
the same azimuthal tilt along the bisector jumping with a defined
angle (∼51° for same segment size) at the segment boundaries.
The domain morphology of the 1-(12-hydroxy)monostearoyl-
rac-glycerol monolayers is completely different. At all temper-
atures, the domain shapes consist of several arms more or less
curved, sometimes forming closed rings. This is demonstrated
in Figure 7, which shows selected domain shapes for different
temperatures between 6 and 25 °C. The tendency to form
curvatures and to develop main arms from a center decreases
somewhat with increasing temperature. Despite the curvature
of the arms, the domain morphology looks more crystalline than
that of the nonsubstituted monogycerol. However, the rather
irregular domain shapes combined with differences in the growth
direction indicate rather low line tension. All domains reflect
homogeneously, which means the molecules within the domains
have the same azimuthal orientational order. According to the
racemic character of 1-(12-hydroxy) monostearoyl-rac-glycerol,
domains with clockwise and anticlockwise curvatures of the
domain arms have been found; see Figure 8. It is interesting to
note that all arms within a domain have the same direction of
curvature.
The 12OH substitution affects various monolayer properties,
as demonstrated by comparison with the domain morphology
of 12-hydroxystearic acid. Figure 9 shows typical domain
textures of 12-hydroxystearic acid monolayers measured at 30,
20, and 10 °C. In agreement with the similarities of the π-A
isotherms, the morphological features of both 12OH-substituted
amphiphiles resemble each other considerably, despite the
different headgroups. Also, the domains of 12-hydroxystearic

1-(12-Hydroxy)stearoyl-rac-glycerol Monolayers
J. Phys. Chem. B, Vol. 108, No. 12, 2004 3785
Figure 7. BAM images of typical domain shapes of 1-(12-hydroxy)monostearoyl-rac-glycerol monolayers at different temperatures.
acid are homogeneously reflecting, develop several arms,
especially in the medium temperature region, and grow rather
irregularly with differences in the growth direction. On the other
hand, the tendency to form curvatures is not so strongly
pronounced as in the case of 1-(12-hydroxy)monostearoyl-rac-
glycerol.
The GIXD data confirm the conclusions on the dominant
effect of the 12OH substitution on the structure of the condensed
monolayer phase. In earlier work, evidence has been provided
that the microscopic textural features of the condensed mono-
layer phases are related to the two-dimensional lattice struc-
ture.23-25 The contour plots of the GIXD measurements reveal

3786 J. Phys. Chem. B, Vol. 108, No. 12, 2004
Vollhardt et al.
Figure 8. 1-(12-hydroxy)monostearoyl-rac-glycerol domains with counterclockwise (left) and clockwise (right) curvatures at 6 °C.
Figure 9. BAM images of typical domain shapes of 12-hydroxystearic acid monolayers at different temperatures.
considerable differences between the nonsubstituted and the
12OH-substituted monostearoyl-rac-glycerol (Figure 10). Ac-
cording to the two reflexes of 1-monostearoyl-rac-glycerol (Qz
) 0 and Qz > 0), the alkyl chain pack in a centered rectangular
lattice tilted toward the NN direction. On the other hand, the
three reflexes (Qz > 0) of 1-(2-hydroxy)monostearoyl-rac-
glycerol indicate that, in this case, the alkyl chains form an
oblique lattice. In agreement with the π-A isotherms and the
BAM results discussed above with respect to the dominant effect
of the 12OH substitution of the alkyl chain, the contour plots
of the 12-hydroxystearic acid monolayers show also three
reflexes (Qz > 0) characteristic of the oblique lattice structure.
The structure data calculated for different surface pressures of
the three amphiphiles are listed in Table 1 wherein a, b, and γ
are the unit cell parameters, Axy is the in-plane molecule area,
ψa is the angle between azimuthal tilt direction and a axis, t is

1-(12-Hydroxy)stearoyl-rac-glycerol Monolayers
J. Phys. Chem. B, Vol. 108, No. 12, 2004 3787
Figure 11. Reflex profiles of 1-monostearoyl-rac-glycerol at 31 °C,
10 mN/m (top image) and 1-(12-hydroxystearoyl)-glycerol at 16 °C,
12 mN/m (bottom image).
mental differences in the lattice structure between the nonsub-
stituted and the 12OH-substituted monostearoyl-rac-glycerol,
whereas most of the characteristic two-dimensional structure
data of the two 12OH-substituted amphiphiles are very similar.
In the latter case, solely some differences exist in the change
of the polar tilt angle with the surface pressure in agreement
with differences of the adequate nonsubstituted amphiphiles.
Whereas the alkyl chains of stearic acid can be erected to the
normal at high surface pressures, those of the 1-monostearoyl-
rac-glycerol remain tilted even at highest pressures.23 Steric
effects may be the main reason for this difference because of
the smaller cross-section area of the -COOH headgroup
compared with that of monoglycerol.
The changes of the lattice data of the substituted and
nonsubstituted monoglycerols in dependence on the surface
pressure are also different. This concerns particularly the polar
tilt, t, the change of which is much smaller in the case of the
12OH-substituted monoglycerol. The hydroxyl group reduces,
obviously, the erection of the alkyl chains. Accordingly,
hydrogen bonds between the hydroxyl groups of adjacent alkyl
chains may be formed. Such a possibility is indicated by the
reflex profile of the GIXD measurements. Figure 11 shows the
reflex profiles of 12OH-substituted and the nonsubstituted
1-monostearoyl-rac-glycerol monolayers. It is seen that one of
the three reflexes of 1-(12-hydroxy)stearoyl-rac-glycerol has a
lower width than the other one. That would be expected for the
lattice direction along which the hydrogen bonds run. In
agreement with the BAM studies, a more crystalline packing
of the 1-(12-hydroxy)stearoyl-rac-glycerol monolayers can be
Figure 10. Contour plots of 1-monostearoyl-rac-glycerol at 30 °C (top)
and 1-(12-hydroxystearoyl)-glycerol at 16 °C (middle) and 12-hydrox-
ystearic acid at 10 °C (bottom) monolayers.
TABLE 1: Lattice Structure Data of
1-(12-Hydroxy)monostearoyl-rac-glycerol,
1-Monostearoyl-rac-glycerol, and 12-Hydroxystearic Acid
Monolayersa
a
(Å)
b
(Å)
γ
Axy
Ψa
t
Ao
(deg) (Å2) (deg) (deg) (Å2)
1-(12-hydroxy)stearoyl-rac-glycerol, 16 °C
12 mN/m 4.817 4.980 112.3 22.2
21
21
21
21
27
25
24
24
19.8
19.9
19.8
19.8
18 mN/m 4.752 5.010 113.3 21.9
25 mN/m 4.731 5.005 113.5 21.7
35 mN/m 4.721 5.007 113.7 21.6
1-monostearoyl-rac-glycerol, 31 °C
10 mN/m 5.095 5.623
20 mN/m 5.039 5.525
30 mN/m 4.991 5.227
40 mN/m 4.928 5.099
23.9
22.9
22.2
21.5
NN
NN
NN
NN
33
29
26
22
20.0
20.1
19.9
20.0
12-hydroxyoctadecanoic acid, 10 °C
6 mN/m
4.61
5.00
5.00
5.01
112.3 21.3
112.9 20.9
113.9 20.3
25
26
20.0
15.8
5.8
20.0
20.0
20.2
10 mN/m 4.53
20 mN/m 4.43
a a, b, γ are lattice constants, t is the polar tilt angle, Axy is molecular
area, A0 is cross-section area of alkyl chain, and ψa is the angle between
the azimuthal tilt direction and the a axis.
the polar tilt angle, and A0 is the cross-section area of alkyl
chain. The characteristic lattice data show clearly the funda-

3788 J. Phys. Chem. B, Vol. 108, No. 12, 2004
Vollhardt et al.
concluded from a comparison of the different reflex profiles of
the two monoglycerol monolayers because of the fact that the
intensity distribution of 1-monostearoyl-rac-glycerol is close to
a Lorentzian, whereas the intensity distribution of 1-(12-
hydroxy)stearoyl-rac-glycerol is close to a Gaussian. Finally,
it is interesting to note that, despite the 12OH substitution, the
cross-section area of the alkyl chain is not increased compared
to that of the nonsubstituted alkyl chain.
and grow rather irregularly with different tendency to form
curvatures. They have oblique lattices with very similar
characteristics. The analysis of the reflex profiles suggests
hydrogen bonds between the hydroxyl groups of adjacent alkyl
chains.
Acknowledgment. The authors thank Dr. S. Siegel for
valuable discussions.
References and Notes
Conclusions
(1) Vollhardt, D. AdV. Colloid Interface Sci. 1996, 64, 143.
(2) Vollhardt, D. In Encyclopedia of Surface and Colloid Science;
Hubbard, A., Ed.; Marcel Dekker: New York, 2001; p 3585.
(3) Kellner, B. M.; Cadenhead, D. A. J. Colloid Interface Sci. 1978,
63, 452.
(4) Kellner, B. M.; Cadenhead D. A. Chem. Phys. Lipids 1979, 23,
41.
(5) Matuo, H.; Rice, D. K.; Balthasar, D. M.; Cadenhead, D. A. Chem.
Phys. Lipids 1982, 30, 367.
(6) Asgharian, B.; Cadenhead, D. A. Langmuir 2000, 16, 677.
(7) Siegel, S.; Vollhardt, D.; Cadenhead, D. A. Prog. Colloid Polym.
Sci. 2002, 121, 67.
Substitution of the alkyl chain in the 12 position by an OH
group has a dominating effect on the main characteristics of
long-chain 1-monoalkanoyl-rac-glycerol monolayers. This is
demonstrated by a comparison of the monolayer features of
1-(12-hydroxy)monostearoyl-rac-glycerol with nonsubstituted
1-monostearoyl-rac-glycerol. The conclusions can be cor-
roborated by the results of an amphiphile of another homologous
series such as fatty acids but with the same alkyl-chain length
and the same OH-substitution in the 12 position (12-hydroxy-
stearic acid in the present work).
(8) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; DeWolf, C.;
Mo¨hwald, H. Langmuir 1999, 15, 2901.
In case of OH substitution in the 12 position of the
nonsubstituted amphiphile 1-monostearoyl-rac-glycerol, a com-
plete change of phase behavior and ordering of the monolayer
has been demonstrated. On the other hand, the two 12OH-
substituted amphiphiles have a similar phase behavior different
to that of the usual nonsubstituted amphiphiles. The bipolar
character of the OH-substituted amphiphile in midposition of
the alkyl is obviously responsible for the special monolayer
characteristics deviating from those of typical amphiphilic
monolayers. Striking characteristics of the monolayers of 12OH-
substituted amphiphiles are an extended flat phase-transition
(plateau) region of the π-A isotherms related with a small
temperature dependence of the main phase-transition points,
whereas the nonsubstituted 1-monostearoyl-rac-glycerol behaves
as a typical amphiphile with stronger temperature dependence
of the phase-transition point as well as the phase coexistence
region.
Analogous conclusions concerning the effect of 12OH
substitution of the alkyl chain can be drawn from the domain
morphology and the two-dimensional lattice structure. 1-Mono-
stearoyl-rac-glycerol monolayers form well-shaped circular
domains subdivided by sharp boundaries into seven segments
for different reflecting. They have a centered rectangular lattice
tilted toward NN. The domains of both 12OH-substituted
amphiphiles develop several arms which reflect homogeneously
(9) Vollhardt, D.; Gehlert, U.; Siegel, S. Colloids Surf., A 1993, 76,
187.
(10) Weidemann, G.; Gehlert U.; Vollhardt, D. Langmuir 1995, 11, 864.
(11) Gehlert, U.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. Langmuir
1996, 12, 4892.
(12) Brezesinski, G.; Scalas, E.; Struth, B.; Mo¨hwald, H.; Bringezu, F.;
Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Phys. Chem. 1995, 99, 8758.
(13) Krasteva, N.; Vollhardt, D.; Brezesinski, G. J. Phys. Chem. B 2000,
104, 8704.
(14) Multzsch, R.; Lokotsch, W.; Steffen, B.; Lang, S.; Metzger, J. O.;
Scha¨fer, H. J.; Warwell, S.; Wagner, F. J. Am. Oil Chem. Soc. 1994, 71,
721.
(15) Meunier, J. Colloids Surf., A 2000, 171, 33.
(16) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav,
M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251.
(17) Kjaer, K. Physica B 1994, 198, 100.
(18) Als-Nielsen, J.; Kjaer, K. In Phase Transitions in Soft Condensed
Matter; Riste, T., Sherrington, D., Eds.; NATO ASI Series B; Plenum
Press: New York, 1989; Vol. 211, p 113.
(19) Fainerman, V. B.; Vollhardt, D.; Melzer, V. J. Phys. Chem. 1996,
100, 15478.
(20) Fainerman, V. B.; Vollhardt, D. J. Phys. Chem. 1999, 103, 145.
(21) Weidemann, G.; Gehlert, U.; Vollhardt, D. Langmuir 1995, 11,
864.
(22) Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Colloid Interface Sci.
1995, 174, 392.
(23) Brezesinski, G.; Scalas, E.; Struth, B.; Mo¨hwald, H.; Bringezu, F.;
Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Phys. Chem. 1995, 99, 8758.
(24) Melzer, V.; Vollhardt, D.; Weidemann, G.; Brezesinski, G.; Wagner,
R.; Mo¨hwald, H. Phys. ReV. E 1998, 57, 901.
(25) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; DeWolf, C.;
Mo¨hwald, H. Langmuir 1999, 15, 2901.