Download Packing and Molecular Orientation of Alkanethiol Monolayers on

1147
Langmuir 1989,5, 1147-1152
Packing and Molecular Orientation of Alkanethiol
Monolayers on Gold Surfaces
Abraham Ulman,* James E. Eilers,* and Nolan Tillman
Corporate Research Laboratories and the Information and Computer Technology Division,
Eastman Kodak Co., Rochester, New York 14650
Received February 15, 1989
Preliminary calculations based on a simple model give a good description of the molecular orientation
and packing of akanethiol monolayers on gold surfaces. These calculations suggest that alkanethiol molecules
on gold have a total alkyl chain axis tilt of approximately38' in a plane that bisects the methylene H-C-H
angles, followed by a rotation about the alkyl chain axis of -46'. The alkyl chain tilt is a function of the
sulfur-sulfur spacing in a hexagonal crystal lattice layer and maximizes the attractive interactions between
neighboring molecules. These results are in agreement with the molecular orientations obtained from grazing
angle FTIR experiments on dodecanethiol monolayers on a (111)gold surface.
Introduction
The understanding of the interrelationships between the
molecular structure of a surfactant and its organization
on different surfaces is a fundamental problem in today's
surface science. The packing and orientation of such
molecules affect the surface chemistry of the monolayer
and are responsible for boundary lubrication, corrosion
inhibition, adhesion, and catal~sis.l-~Furthermore, such
understanding is essential for the future development of
multilayer systems with useful properties. For example,
an organic film for nonlinear optics applications should
have a noncentrosymmetric arrangement of the molecular
dipoles, preferably perpendicular to the monolayer surface.
Thus, a better understanding of the monolayer bulk
structure relationships should allow for molecular design
to specifically engineer the packing, orientation, and stability of such monolayer films.
The packing and orientation of individual molecules in
a monolayer of long-chain alkyl compounds have been
suggested to be a function of the spacing between the
molecular head groups'5 and the van der Waals and dipole
interactions between these mole~uls.~~'In monolayer
assemblies where the head-head spacing is greater than
the touching van der Waals distance of the alkyl chains,
these tails tilt in such a way as to maximize attractive van
der Waals interactions between molecules and thus minimize the free energy of the
Alkanethiols form well-ordered monolayers when adsorbed from solution onto metal s u r f a ~ e s . ~ JIn
~ ' 1987,
~
(1)Zisman, W. A. In Friction and Wear; Davis, R., Ed.; Elsevier: New
York, 1959;p 118.
(2)Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New
York, 1976 and references cited therein.
(3)Somorjai, G. A. Chemistry of Two Dimensions: Surfaces; Cornell
University Press: Ithaca, NY, 1981 and references cited therein.
(4)Safran, S. A.; Robbins, M. 0.;
Garoff, S. Phys. Reu. A 1986,33,
2188.
(5)Porter, M.D.; Bright, T. B.; Allara, D. L.; Chidsey, C. F. D. J. Am.
Chem. SOC.1987,109,3559.
(6)Kitaigorodskii, A. 1. Organic Chemical Crystallography; Consultanta Bureau: New York, 1959;pp 177-217.
(7)Garoff, S.h o c . Natl. Acad. Sci. U.S.A. 1987,84,4729and references cited therein.
(8) Langmuir, 1. J. Chem. Phys. 1933,1 , 756.
(9)Epstein, H.T.J. Colloid Chem. 1950,54,1053.
(IO) Stewart, K. R.; Whitesides, G. M.; Godfried, H. P., Silvera, I. F.
Reo. Sci. Znstrom. 1986,57,1381.
(11)Finklea, H.0.;
Avery, S.; Lynch, M.; Furtach, T. Langmuir 1987,
3, 409.
(12)Strong, L.; Whitesides, G. M. Langmuir 1988,4,546.
1987,
(13)Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J . Am. Chem. SOC.
109, 2358.
Porter et al.5 reported on a detailed investigation of
monolayers of alkanethiols adsorbed on evaporated gold
films. They used ellipsometry and grazing-angle FTIR
spectroscopy to characterize the monolayers and establish
molecular orientation. Both in their report and in a previous report by Nuzzo et aLl3 on the orientation of alkyl
chains in monolayers of dialkyl disulfides, the authors
discuss their spectroscopic results in terms of molecular
coordinates, with no consideration of the molecular packing
in the two-dimensional assemblies.
In this report, we present our efforts to predict the expected chain tilt for an assembly of alkanethiols with a very
simple model and compare our results with tilt angles that
have been estimated by IR spectroscopy. We attempt to
account for the observed chain tilting in terms of the likely
crystallographic packing of the monolayer film and the
spacing of the sulfur atom head group, and we suggest
some general conclusions.
The Model
Electron diffraction studies of monolayers of docosane
thiol on gold single-crystal foils with an exposed (111)
surface have shown that the alkanethiols have hexagonal
packing.12 The S-S spacing in these samples was found
to be 4.97 A, a distance that would leave substantial free
volume between molecules if they "stood up" normal to
the plane of the lattice.
One might investigate this packing problem in many
ways and at several levels of complexity and sophistication.
For example, one could consider a small patch (e.g., 10 X
10) of alkanethiols, with imposed periodic boundary conditions in two dimensions to eliminate edge effects, undergoing molecular dynamics until they fall into an ordered
pattern and stay that way.14J6 However, while very sophisticated, complex, and expensive computations may
eventually be required for a thorough understanding of the
process of monolayer formation, we think that the question
"what is the most favorable packed arrangement for these
molecules?" should be amenable to relatively simple
analysis.
FTIR spectroscopy and electron diffraction experiments
suggest that alkanethiol monolayers on gold surfaces have
crystal-like periodicity in two dimensions (for alkyl chain
(14)van der Ploeg, P.; Berendsen, H. J. C. J. Chem. Phys. 1982,76,
3271.
(15)Cardini, G.; Bareman, J. P.; Klein, M. L. Chem. Phys. Lett. 1988,
145,493.
0743-7463/89/2405-ll47$01.50/0 0 1989 American Chemical Society
1148 Langmuir, Vol. 5, No. 5, 1989
cs-
c
dlman et al.
-
-50
-25.20-15.iO
-
0
-5
5
1 0 15 20 25
a
Figure 4. Sum of interaction energies (01, a + 6
0
'
.
a
+ 120').
*Y
X
Figure 1. Hexagonal arrangement of alkanethiol molecules on
a (111)gold surface (DDT/Au).
z:i
CBO
0 0 ~ ~ 0 0
o ~ @@m-"
a o o o
m
i:"-.?
Figure 5. 1 X 3 assembly at 4.24-A spacing (left)and at 4.97-A
spacing (right).
a000
X
E p r a 2. Hexagonal doaspackedarrangementof mncylindrical
objecta. Note the equivalence of the lattice directions and our
choice of cwrdinate system.
+ Epmr
Y
0
4.8
4
v
4.8
-15
-
4 s
E
-12
z
-16
Y
~
4.4
4.2
K
-20
5
0
10
20
15
25
30
3 5
e
Figure 6. Interaction energy in a single row as a function of the
in-pane tilt (0) for d = 4.97 A.
-m
4.0
3.8
45.30-15
I
o
1 5 30 4 5 6 0 1 5
eo
-24
10~120135
a
Figure 3. Optimal interacton energy and the corresponding
spacing as a function of the twist angle (a)for a single row.
length 2 C,&5*12 Thus,it should be reasonable, to a first
approximation, to treat all the molecules in the system
as having identical conformations and identical orientations relative to a hexagonal close packed grid of S atoms.
Figure 1presents our representation of an alkanethiol on
a (111)surface. Note that the S atoms form a hexagonal
lattice in the XY plane and that we have chosen to orient
the Y-axis along one of the lattice axes of the hexagonal
array of S atoms. This scheme is chosen to facilitate the
investigation and discussion of the packing of rigid rods
whose 'roots" are locked in a repeating planar lattice and
where every molecule will have an identical orientation
relative to that lattice. This rigid rod model would probably be too much of an idealization if we were modeling
the dynamic proeess of forming the monolayer, but a rigid
rod should he adequate for merely identifying favorable
packing arrangements. The molecular conformation in the
42-1
4.0
.
I
4.2
.
I
4.4
.
I
4.6
.
I
4.8
,
I
5.0
,
I
5.2
SPkm (AI
Figure 7. Begt interaction energy (for optimal in-plane tilt) 88
a function of the spacing (d).
formed monolayer would not be expected to differ significantly from the favored conformation for an isolated
molecule, since the molecular shapes are simple and van
der Waals interactions provide most of the packing forces.
Such a model haa only 3 degrees of freedom, and we fmd
it convenient to define these as follows. We begin by
placing a molecule of dodecanethiol (CI2HZ5SH)
perpendicular to the plane of the lattice, with its S at the origin
Packing and Orientation of Alkanethiol Monolayers
Langmuir, Vol. 5, No. 5, 1989 1149
Figure 8. View of a 3 X 3 assembly of in-plane-tilteddodecanethid moleeulas down the Y-axis(top). View ofthe same assembly
down the axes of the molecules (sidetop view), showing the
remaining free volume after the in-plane tilt (bottom).
and oriented so that the YZ plane bisects the methylene
H-C-H bond angles (Figure 1). Exact duplicates are
placed on all other lattics points under consideration. The
first degree of freedom is a twist (a)around the long axis
of the molecule 0.
The second we take to be an in-plane
tilt of the molecule in the YZ plane or, equialently, a r e
tation of every molecule around a local X-axisby 8. The
last is taken as an out-of-plane tilt or, equivalently, a rotation of each molecule around a local Y-axis by 8’.
Computation
One then needs a method for evaluating the quality of
the packing afforded by various combinations of these
three males. To do this. we need to consider the energy
of interaction of one molecule, somewhere in the infinite
sheet,with all of ita neighbors. Sincs all the molecules are
identical in every way, minimizing this energy will also
minimize the energy of the sheet. For situations dominated by electrostatics or situations with strong dipoles,
Figure 9. View of the fmal assembly from the X-direction
showing the in-plane tilt (top) and from the Ydirection showing
the o u t - o f - p ~ tilt
e (mid&). me88meview looking down n e
dthe
(sidetop ~ e w showing
)
the dose paekmg
and h k of free volume is shown in the bottom part ofthe fm.
one would have to include the interactions with distant
molecules. However, for these simple alkyl chains,
shorbrange van der Waals forces will dominate, and consideration of only close neighbors should be adequate. In
Ulman et al.
1150 Langmuir, Vol. 5, No. 5, 1989
practice, we find that specific inclusion of either the interactions with the six nearest neighbors or, alternatively,
the interactions with the eight neighbors of a 3 X 3 patch
gave the same qualitative result.
Standard molecular mechanics methods were used to
evaluate the quality of the packing afforded by various
combinations of these angles. First, the geometry of the
isolated molecule was optimized by using the MM2 force
field16 in the MACROMODEL program.17 Small hexagonal
assemblies were then constructed of rigid molecules (each
in this optimized geometry), and different combinations
of the orientation angles were examined. The energy of
interaction of a molecule with its neighbors was examined
as a function of the angles. The energetics considered only
the two-body terms (van der Waals and electrostatics) and
only intermolecular terms.18 This simple model does not
permit the molecules to adjust their geometries to accommodate the presence of their near neighbors. However,
molecular dynamics calculations of the Langmuir-Blodgett
monolayer suggest that at 306 K only 1% of the torsional
angles is gauche.15 Thus, our suggested model should
afford considerable insight as to the packing and orientation of alkyl chains in self-assembling monolayers.
-38.6
-t- Eprox
-2
0
-1
1
2
a
-36
Y,
x
e
-37
wa -38
-39
b
-40
28
29
30
31
32
33
e
-36
Results and Discussion
I t was found that a twist of a = l o ,an in-plane tilt of
6 = 32O, and an out-of-plane tilt of 6' = 22' give the best
(Le., most negative) interaction energy for an S-S spacing
of 4.97 A. This result may be easier to visualize, and some
insight may be gained as to the physical reasons for these
optimal angles, if we start by closely examining just one
row of the assembly, such as the B's in Figure 2. We begin
by considering three molecules located in the YZ plane,
with each molecule normal to the underlying hexagonal
lattice. The energy of interaction between the central
molecule and its two neighbors will depend upon the twist
angle, a,and the lattice spacing, d. For each a in the set
(5, 10, 15, 170, 1751, we varied d until the optimal interaction energy was found for that a. Figure 3 shows both
the optimal interaction energy and the corresponding
spacing as a function of a. Perhaps the most striking
observation is the parallel between compactness of
structure (small d ) and favorable energetics (more negative). T h e most energetically favored twist for one row
of alkane thiols is a = 0'. There are also minima at a =
70' and cy = 110'. These minima are crucial to understanding hexagonal close packing, since if the molecules
have a twist angle of a relative to one lattice axis (i.e., the
B's in Figure 21, they must have twist angles of a + 60'
and a + 120' relative to the other row directions (C's and
D's of Figure 2). Thus, one is tempted to examine the s u m
of the interaction energies at (a,a + 60°,and a + 120°),
as in Figure 4. This would imply that a = 0' is favored
with a broad minimum. Actually, the preference for a =
0" is much more pronounced, since in hexagonal closest
packing one cannot have different spacing along these
three directions. As can be seen in Figure 3, only for a z
0' do we find a similar optimal spacing for all three row
directions. For any other twist angle, we would either have
to pack in a less symmetric space group or have row directions for which optimal van der Waals interactions are
(16)Allinger, N. L. J.Am. Chem. SOC.
1977,99, 8127; J. Org. Chem.
1987,52, 5162.
(17) Still, C. Department of Chemistry, Columbia University, New
York. NY.
(18)All energies in the figures refer to the energy of interaction of the
central molecule in the cluster with ita near neighbors. If one wants to
consider the stabilization energy of an infinite sheet, one should divide
these energies by 2 in order to avoid double counting.
C
-40
,
20
I
21
.
1
22
.
,
23
.
,
24
.
25
8'
Figure 10. Interaction energy for a 3 X 3 assembly near the global
minimum: (a) as a function of CY for B = 32' and W' = 22O, (b)
as a function of B for CY = lo and 8' = 22O, and (c) as a function
of B' for CY = lo and 0 = 32O.
not achieved for molecules normal to the lattice. Thus,
it would seem that the best way to arrange a monolayer
of alkanethiols is close-packed hexagonal with spacing of
~ 4 . 2 4A and with the molecules normal to the surface.
This optimal packing will only be achieved if the attachment to the underlying substrate is compatible with
such a lattice. If the attachment to the underlying substrate were to enforce a lattice spacing significantly larger
than 4.24 A, as appears to be the case on gold, there would
be no close intermolecular contacts at all for molecules
oriented normal to the surface, and we would expect the
molecules to "tip" away from the normal in such a way as
to again establish optimum van der Waals contacts. As
noted earlier, the best van der Waals contacts for a row
of tightly packed alkyl chains normal to some substrate
lattice occur for a twist angle of a z 0'. This orientation
results in a near perfect interlock of bumps on one molecule into depressions on its neighbors (Figure 5). This
"front-to-back" arrangement provides for both a vertical
and horizontal interlocking of van der Waals surfaces. At
a lattice spacing of 4.97 A, it is possible to tilt the molecule
along the row and establish these same kinds of interlocking contacts for most of the chain length (Figure 5).
They have simply slipped one notch and interlocked 11
of the 12 CH2 units. Remarkably, at a tilt of 32' the
neighbor-neighbor interaction energy is 92% (or 11/ 12!)
of that found for the tightly packed vertical row of dodecanethiols at 4 . 2 4 4 spacing. Figure 6 shows the behavior
of this interaction energy as a function of tilt. The repulsive wall for tilts > 35' is quite steep, and there are no
barriers to this in-plane tilting.
Packing and Orientation of Alkanethiol Monolayers
At lattice spacings less than about 4.8 A, however, it is
not possible for the "racheting effect" to establish this
special interlocking. Figure 7 shows how the best interaction energy (at the best tilt for this spacing) varies as
a function of the lattice spacing. The minima near 4.24
A requires a nearly vertical orientation, and that near 4.95
A requires tilts near 30'. Thus it seems that spacings other
than 4.1-4.3 and 4.8-5.05 A and tilt angles other than
approximately 0' or approximately 30' are unlikely for
rows of alkyl chains.
The twist angle being near 0' and the in-plane tilt being
about 30' are apparently the two most important factors
in establishing good van der Waals contacts. However, a
monolayer with a =,'O 6 = 30°, and 6' = 0' would have
close packing within parallel planes perpendicular to the
lattice but still have appreciable free volume between the
adjacent planes of packed molecules (Figure 8). A simultaneous tilt (in the X-direction) of these tilted rows
closes the gap between these planes and provides the final
structure, which is tightly packed in all direction (Figure
9). The out-of-plane tilt (8') provides approximatley 25%
of the total stabilization energy. Plots of the neighborneighbor interaction energies versus a, 6,and 8' near the
minimum for the 3 X 3 cluster at 4.97 A are shown in
Figure 10. Figure 10a presents the energy as a function
of a for 6 = 32' and 6' = 22'. We see that for the threedimensional array for the minimum energy is at a = lo,
which is a rather small change from the a = 0' found for
one row of molecules. A plot of the neighbor-neighbor
interaction energy for the 3 X 3 assembly as a function of
6 for a = 1' and 6' = 22' (Figure lob) reveals that the
minimum energy is at 8 = 32'. Finally, Figure 1Oc presents
a plot of the neighbor-neighbor interacton energy as a
function of 6' for the assembly, with a = 1' and 6 = 32'.
Here we find a minimum between 6' = 22 and 24'.
The result of the 32' in-plane and 22' out-of-plane tilts
in the present case is equivalent to a total chain axis tilt
of 38' from the normal in a plane containing the Z-axis
and rotated 59' from the XZ plane followed by a rotation
about the alkyl chain axis so that the angle between this
plane and the plane bisecting the methylene units becomes
~46'.
It is interesting to note that the same general picture
of molecules tilted more in-plane and less out-of-plane was
found in molecular dynamics calculations of the Langmuir-Blodgett mon01ayer.l~In their model, the interchain
separation was 4.9 A (very similar to the 4.97 A found for
the S 4 separation on (111) gold surface12)and the mean
chain tilt angle was 4 = 40'.
Furthermore, the calculated chain axis tilts compare very
favorably with chain axis tilts measured by FTIR. Using
grazing-angle FTIR data with a comprehensive approach
which involves comparison of observed band intensities
with calculated, expected band intensities, Allara and
Nuzzo have measured chain tilts and rotation angles for
a variety of alkanethiol mon01ayers.l~ Their results suggest an average tilt angle of 34' and rotation angle of 55'.
There were only small changes in tilt angle or rotation
depending upon the particular surface functionality (e.g.,
OH, COOH, CONH2). These results strongly suggest that
the packing and orientation of alkanethiol monolayers are
dominated by the hydrocarbon chain and not by a surface
functional group.
Conclusions
In conclusion, the results of this study show that the tilt
of alkyl chains observed in oriented thiol monolayers may
(19) Allara, D. L., private communication.
Langmuir, Vol. 5, No. 5, 1989 1151
be accounted for by a calculation that considers only
chain-chain interaction and the spacing and symmetry of
the lattice. Furthermore, we suggest that the S-43 spacing
of 4.97 A found in monolayers of docosyl mercaptan
(CzzH&3H) on (111) gold12 may be a result of both the
sulfur-gold interfacial interactions and the neighborneighbor interactions energy in the alkyl chain hexagonal
assembly. Since the total energy of the system is comprised of both the sulfur-gold interaction and the interactions of the alkyl chains with each other, one could
speculate that if the sulfur-gold interactions should favor
a S.S spacing of, for example, 4.7 A, this would be a highly
inefficient way to pack the alkyl chains.
We furthermore suggest that in the tilted alkanethiol
monolayers there is no free rotation around the molecular
axis ("freely rotating methylene units").12 When the chains
are close packed (the chainwhain distance is 4.24 A) they
encounter a substantial energy barrier for rotation.
The fact that tilt angles may have distinct values has
been suggested before.20 In their investigation of the
orientation of arachidate chains in Langmuir-Blodgett
monolayers on Si (1111, Rabe et al. have found that the
alkyl chains in a monolayer of cadmium arachidate are
practically perpendicular to the surface, while in calcium
arachidate the chains are tilted by 33' f 5' (i.e., as in
DDT/Au). They suggested the formula tan 4 = n R / D
(where 6 is the tilt angle from the normal, R = 2.52 A is
the distance between second nearest-neighbor carbon atoms in the alkyl chain, D is the minimum van der Waals
separation between the chains, which we found to be 4.24
A, and n = 0, 1, 2) to describe these distinct tilt angles.
Accoridng to this formula, the tilt angles should be only
,'O 30.7", 49.9', etc. Of course, since the tilt can be coupled
with a rotation around the molecular axis (a),the tilt angle
values can somewhat fluctuate around the exact expected
value. Evidence for a stable assembly of perpendicular
alkyl chains can be found in the work of Guyot-Sionnest
et
who examined the LB film of pentadecanoic acid
on the water surface. On the basis of their sum-frequency
vibrational spectroscopy,they calculated a tilt angle of 35'
from the normal for the methyl group, which suggests that
the alkyl chains are nearly perpendicular to the water
surface.
It is also interesting to note that molecular dynamics
simulation suggests tilt angles of ca. 30' for the hydrocarbon chains of molecules in bilayer membranes.15 This
similarity in the alkyl chain tilt angle of the liquidlike
bilayers and the crystallinelike alkanethiol monolayers
further supports the suggestion that a collective tilt of the
alkyl chains in a monolayer assembly can have only specific
values.
Recently, Dote and Mowery reported a tilt angle of 4
= 12' in freshly prepared Langmuir-Blodgett stearic acid
monolayers on aluminum, measured by reflectance absorptin spectra.22 They have also looked at the spectra
after 168 h and found a significant diminution in the
methylene absorption (v,CH2). This interesting observation is consistent with the idea that the tilt of alkyl chains
can have only specific values, determined by the spacing
and orientation of their head groups. Two mechanisms
can be proposed in this case: (a) The monolayer is close
packed and crystalline and the 12' tilt is a weighted average of two kinds of domains, i.e., one having an average
tilt angle of 4 i= 0' and other 4 = 32'. When these do(20) Rabe, J. P.; Swalen, J. D.; Outka, D. A.; Stahr, J. Thin Solid Films
1988, 159, 275.
(21)Guyot-Sionnest, P.;Hunt, J. H.; Shen, Y. R. Phys. Reu. Lett.
1987,59, 1597.
(22)Dote, J. L.;Mowery, R. L. J.Phys. Chem. 1988, 92, 1571.
1152
Langmuir 1989,5, 1152-1154
mains have dimensions smaller than 100 X 100 A (= 20
X 20 molecules), it is not possible to detect them by conventional wetting studies or simple spectroscopic techniques. If this is the case, then there may be a dynamic
process in which one domain grows at the expense of the
other. It is apparent that, if these are close-packed monolayers, when the = Oo domain grows at the expense of
the 4 = 32O domain, there should be a shrinkage of the
monolayer away from the edges. (b) The monolayer is
amorphous, there is more free rotation of the alkyl chains,
and the 1 2 O tilt is a weighted average of different molecular
tilts. If this is the case, then the authors report on a slow
crystallization process in the monolayer, where domains
with 6 = 0" are being formed.
The results presented in this report sugest that in a
monolayer of dodecanethiol on gold (at 0 K) there is a
barrier to rotation of the alkyl chains about their molecular
axes, and thus it is more appropriately described as
pseudohexagonal (face-centered orthorhombic). This is
not surprising since similar packing arrangements were
reported for the LB monolayer of 1-heneicosanol on the
water surface23and for the LB monolayer of tricosanoic
acid on aluminum oxide.24
#IJ
In this model, we have assumed a perfectly flat and
uniform surface and perfect crystalline packing. In actuality, deviations from this perfect order may be expected
due to defects in the metallic surface which are compensated for by elastic distortions of the hydrocarbon tails as
the monolayers bridge over and cover such defect^.^
This work represents an initial effort to develop calculational procedures that can predict the orientation of
molecular features in monolayer assemblies. Continued
development of such methods should allow for the design
of organic monomolecular films with predicted and optimized properties.
Acknowledgment. We acknowledge the contribution
of Jon Littman of the Corporate Research Laboratories,
Eastman Kodak Co., for preparing the gold substrates.
Registry No. Au, 7440-57-5.
(23)Barton, S. W.; Thomas, B. N.;Flom, E.B.; Rice, S. A.; Lin, B.;
Peng, J. B.; Ketterson, J. B.; Dutta, P. J. Phys. Chem. 1988,89,2257.
(24)Heard, D.; Roberts, G. G.; Holcroft, B.; Goringe, M. J. Thin Solid
Films 1988,160,491.
Carboxylic Acid Solutions in Soap Bilayers
Anthony J. I. Ward, Ibrahim H. Kayali, and Stig E. Friberg*
Department of Chemistry, Clarkson University, Potsdam, New York 13676
Received November 21, 1988. I n Final Form: April 6, 1989
A preliminary study of the 2H and 13CNMR spectra of n-hexadecanoic acid solubilized in a lamellar
liquid crystal of sodium oleatefoleic acidfwater was made as a function of temperature and solubilizate
content. At low contents a spectrum consistent with an isotropic fraction of the acid was found, while
increasing the hexadecanoic acidoleic acid ratio lead to the appearance of a complex spectrum comprising
16 overlapping powder spectra in addition to the isotropic component. The order profile of the palmitate
chains showed no enhanced order for the 2,2'-position, and order parameter values were less than those
found in comparable bilayers made from fully saturated soaps. The picture of a thin layer of acid solubilized
at the center of the bilayer is supported by the 13CNMR spectrum of the two acids in the system. Differing
environments for the carboxylic acid groups were observed which may be assigned to those in solution
in the biiayer center and those which are interdigitated between the surfactant molecules forming the bilayer
structure.
Introduction
mobility in spite of their obvious role in biological tissue,
except for some preliminary determinationsY6which inUnsaturated fatty acidlsoap combinations form liquid
dicated that the fatty acids penetrate the layered structure.
crystals with water at room temperature, whereas comThe present paper describes a study of the interaction of
binations of saturated fatty acids tend to form crystals
saturated fatty acids with a liquid crystalline lamellar
because of the Kraft point of the soap being above the
structure, using a combination of 2Hand 13C NMR specroom temperature-l Small-angle X-ray diffraction and
troscopy.
differential scanning calorimetry (DSC) have shown2that
Dynamic order profiles for lamellar liquid crystals have
partially neutralized fatty acidfsoap forms a lamellar
earlier been obtained7-13for a wide variety of surfactant
structure with water. Such a structure has been ~ s e d ~ - ~
and lipid lyotropic liquid crystal systems by using the
as a model of the layered structure containing the lipids
of the stratum corneum.
Surprisingly, saturated and unsaturated fatty acid/soap
(6) Klasson, T.; Henriksson, U. In Solution Behauiour Surfactants;
combinations have not been investigated for structure and
Mittal, K. L., Fendler, E.J., Eds.; Plenum Press: New York, 1982;Vol.
1.
(1) Ekwall, P. In Advances in Liquid Crystals; Brown, G. H., Ed.;
Academic Press: New York, 1975;Vol. 1, p 1.
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