Langmuir 1989,5, 1147-1152 1147

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 approximately 38' 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 sta- bility 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 min- imize the free energy of the

Alkanethiols form well-ordered monolayers when ad- sorbed from solution onto metal s u r f a ~ e s . ~ J ~ ' ~ In 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; Consult- anta Bureau: New York, 1959; pp 177-217.

(7) Garoff, S. h o c . Natl. Acad. Sci. U.S.A. 1987,84,4729 and refer- ences 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.

(11) Finklea, H. 0.; Avery, S.; Lynch, M.; Furtach, T. Langmuir 1987,

(12) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (13) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J . Am. Chem. SOC. 1987,

Reo. Sci. Znstrom. 1986,57, 1381.

3, 409.

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 pre- vious 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 ex- pected 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 con- ditions in two dimensions to eliminate edge effects, un- dergoing molecular dynamics until they fall into an ordered pattern and stay that way.14J6 However, while very so- phisticated, 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,

(15) Cardini, G.; Bareman, J. P.; Klein, M. L. Chem. Phys. Lett. 1988, 3271.

145, 493.

0743-7463/89/2405-ll47$01.50/0 0 1989 American Chemical Society

1148 Langmuir, Vol. 5, No. 5, 1989 dlman et al.

c s - c - *Y

X

Figure 1. Hexagonal arrangement of alkanethiol molecules on a (111) gold surface (DDT/Au).

CBO 0 0 ~ ~ 0 0 z:i o ~ a o o o @@m-"

m a000 X

E p r a 2. Hexagonal doaspacked arrangement of mncylindrical objecta. Note the equivalence of the lattice directions and our choice of cwrdinate system.

0

4.8 4

4.8 - 4 s

E

-12

~

- z K

4.4 v

-16 Y 4.2

4.0 -m

3.8 -24 4 5 . 3 0 - 1 5 o 1 5 30 4 5 6 0 1 5 e o 1 0 ~ 1 2 0 1 3 5

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 orienta- tions relative to a hexagonal close packed grid of S atoms. Figure 1 presents 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 prob- ably 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

-50 - - 2 5 . 2 0 - 1 5 . i O - 5 0 5 1 0 1 5 2 0 2 5

a Figure 4. Sum of interaction energies (01, a + 60'. a + 120').

Figure 5. 1 X 3 assembly at 4.24-A spacing (left) and at 4.97-A spacing (right).

I + Epmr i:"-.? Y -15

-20 0 5 1 0 1 5 2 0 2 5 3 0 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.

42-1 . I . I . I . I , I , I

SPkm (AI 4.0 4 . 2 4 .4 4 . 6 4 . 8 5 .0 5 . 2

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 sig- nificantly 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) perpen- dicular to the plane of the lattice, with its S at the origin

Packing and Orientation of Alkanethiol Monolayers

Figure 8. View of a 3 X 3 assembly of in-plane-tilted dodecan- ethid 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-axis by 8. The last is taken as an out-of-plane tilt or, equivalently, a ro- tation of each molecule around a local Y-axis by 8’.

Langmuir, Vol. 5, No. 5, 1989 1149

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

Figure 9. View of the fmal assembly from the X-direction showing the in-plane tilt (top) and from the Ydirection showing the out -of -p~e tilt (mid&). me 88me view 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.

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 domi- nated by electrostatics or situations with strong dipoles,

one would have to include the interactions with distant molecules. However, for these simple alkyl chains, shorbrange van der Waals forces wil l dominate, and con- sideration of only close neighbors should be adequate. In

1150 Langmuir, Vol. 5, No. 5, 1989

practice, we find that specific inclusion of either the in- teractions 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 accom- modate 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 orien- tation of alkyl chains in self-assembling monolayers.

Ulman et al.

Results and Discussion

I t was found that a twist of a = lo, 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 in- teraction 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 nega- tive). The most energetically favored twist for one row o f alkane thiols is a = 0'. There are also minima at a = 70' and cy = 110'. These minima are crucial to under- standing 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 a t (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 di- rections for which optimal van der Waals interactions are

(16) Allinger, N. L. J. Am. Chem. SOC. 1977,99, 8127; J. Org. Chem.

(17) Still, C. Department of Chemistry, Columbia University, New 1987,52, 5162.

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.

-38.6 -t- Eprox

- 2 - 1 0 1 2 a

-36 Y, x -37 e a w -38

-39

-40 b

2 8 2 9 3 0 3 1 3 2 3 3

e -36

C -40 , I . 1 . , . , .

2 0 2 1 2 2 2 3 2 4 2 5 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 4 A and with the molecules normal to the surface.

This optimal packing will only be achieved if the at- tachment to the underlying substrate is compatible with such a lattice. If the attachment to the underlying sub- strate 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 mole- cule 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 inter- locking contacts for most of the chain length (Figure 5). They have simply slipped one notch and interlocked 11 of the 12 CH2 units. Remarkably, a t a tilt of 32' the neighbor-neighbor interaction energy is 92% (or 11/ 12!) of that found for the tightly packed vertical row of dode- canethiols at 4 . 2 4 4 spacing. Figure 6 shows the behavior of this interaction energy as a function of tilt. The re- pulsive 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 inter- action 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 si- multaneous 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 neighbor- neighbor interaction energies versus a, 6, and 8' near the minimum for the 3 X 3 cluster a t 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 three- dimensional array for the minimum energy is a t 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 ~ 4 6 ' .

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 Lang- muir-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 sug- gest 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

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 neighbor- neighbor interactions energy in the alkyl chain hexagonal assembly. Since the total energy of the system is com- prised of both the sulfur-gold interaction and the inter- actions 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 = nR/D (where 6 is the tilt angle from the normal, R = 2.52 A is the distance between second nearest-neighbor carbon at- oms 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 hydro- carbon 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 ab- sorptin 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 observa- tion 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 av- erage 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

(21) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. Reu. Let t .

(22) Dote, J. L.; Mowery, R. L. J. Phys. Chem. 1988, 92, 1571.

1988, 159, 275.

1987,59, 1597. (19) Allara, D. L., private communication.

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 con- ventional wetting studies or simple spectroscopic tech- niques. 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 mon- olayers, when the I#J = 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 surface23 and for the LB monolayer of tricosanoic acid on aluminum oxide.24

In this model, we have assumed a perfectly flat and uniform surface and perfect crystalline packing. In actu- ality, deviations from this perfect order may be expected due to defects in the metallic surface which are compen- sated 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 calcu- lational 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 op- timized 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. In Final Form: April 6, 1989

A preliminary study of the 2H and 13C NMR 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 13C NMR 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 Unsaturated fatty acidlsoap combinations form liquid

crystals with water at room temperature, whereas com- binations of saturated fatty acids tend to form crystals because of the Kraft point of the soap being above the room temperature-l Small-angle X-ray diffraction and differential scanning calorimetry (DSC) have shown2 that partially neutralized fatty acidfsoap forms a lamellar structure with water. Such a structure has been ~ s e d ~ - ~ as a model of the layered structure containing the lipids of the stratum corneum.

Surprisingly, saturated and unsaturated fatty acid/soap combinations have not been investigated for structure and

(1) Ekwall, P. In Advances in Liquid Crystals; Brown, G. H., Ed.;

(2) Cistala, D. P.; Atkinson, D.; Hamilton, J. A.; Small, D. M. Bio-

(3) Elias, P. M.; Brown, B. E.; Fritsch, P.; Goerke, J.; Gray, M.; White,

(4) Swanbeck, G. Acta Dermata-Veneralogica 1959, 39, 3. (5) Friberg, S. E.; Osborne, D. W. J. Dispersion Sci. Technol. 1985,

Academic Press: New York, 1975; Vol. 1, p 1.

chemistry 1986,25, 2804.

R. J. J. Inuest. Dermatol. 1979, 73, 339.

6, 485.

mobility in spite of their obvious role in biological tissue, except for some preliminary determinationsY6 which in- dicated that the fatty acids penetrate the layered structure. The present paper describes a study of the interaction of saturated fatty acids with a liquid crystalline lamellar structure, using a combination of 2H and 13C NMR spec- troscopy.

Dynamic order profiles for lamellar liquid crystals have earlier been obtained7-13 for a wide variety of surfactant and lipid lyotropic liquid crystal systems by using the

(6) Klasson, T.; Henriksson, U. In Solution Behauiour Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 1.

(7) Ward, A. J. I.; Phillippi, M. A.; Marie, C. Mol. Cryst. Liq. Cryst.

(8) Friberg, S. E.; Ward, A. J. I.; Larson, D. W. Langmuir 1987,3,735. (9) Seelig, J. Prog. Colloid Polym. Sci. 1978, 65, 172. (10) Smith, I. C. P.; Stockton, W.; Tulloch, A. P.; Polnaszek, C. F.;

(11) Tiddy, G. J. T. Phys. Rep. 1980, 57, 1. (12) Seelig, J . Q. Reu. Biophys. 1977, 10, 353. (13) Mely, B.; Charvolin, J.; Keller, P. Chem. Phys. Lipids 1975, 15,

1988,616, 323.

Johnson, K. G. J . Colloid Interface Sci. 1977, 93, 560.

161.

0743-7463/89/2405-1152$01.50/0 0 1989 American Chemical Society