Langmuir 1994,10, 830-836
830
Packing Characteristics of Crystalline Monolayers of Fatty Acid Salts, at the Air-Solution Interface, Studied by Grazing Incidence X-ray Diffraction C. Bbhm,? F. Leveiller,t D. Jacquemain,$ H. Mbhwald,? K. Kjaer,s J. Als-Nielsen,s I. Weissbuch,! and L. Leiserowitz*J Department of Materials & Interfaces, The Weizmann Institute of Science, 76100 Rehovot, Israel, Institute fiir Physikalische Chemie, Johannes Gutenberg Universitat, Jacob Welder Weg l l , D - 6 5 0 0 Mainz, Germany, and Physics Department, Rise National Laboratory, DK4000 Roskilde, Denmark Received August 2,1993. In Final Form: November 22, 199P Grazing incidence X-ray diffraction (GID) studies were performed on self-assembled monolayers of carboxylic acids C,&2,+1C02H (n = 13,19,29)spread over CdC12 or CaCl2 solutions at pH values 8.85 and 10, respectively, adjusted with ammonia, and subphase temperatures in the range 5 to 9 "C. The GID crystalline aggregation of the molecules resulta show the tendency for spontaneous two-dimensional (2-D) which pack in a rectangular cell with the hydrocarbon chains tending to be vertically aligned with respect to the solution surface. The molecular chains form a herring bone arrangement with the two molecules in the unit cell related essentially by glide symmetry. Detailed Bragg rod fitting analysis shows however, that the two molecules are crystallographically independent so that the plane group symmetry is chiral p l . For n = 29, it was possible to determine the extent of molecular thermal motion parallel to the water surface and which proved to be in the same range as in layered 3-Dcrystal structures containing aliphatic chains. The GID data did not provide definite evidence for ordered ion binding although the extent of crystallinity is enhanced by the presence of the bound counterion. For example, myristic acid ( n = 13) yielded the GID pattern only when spread over CdClz but not on pure water. A simple packing model is provided for ordered binding of counterions with high degree of coordination to the oxygen atoms of the carboxylate head groups. The proposed model is also in keeping with the observed lower area per amphiphilic molecule than over pure water. Introduction
Results
The interaction of a charged surfactant with counterions is important for many processes in biophysics and interfacial science. In the preceding paper1 we determined the ordered arrangement of cadmium ions bound to an arachidic acid monolayer although the relative positions of the CdOH+ ions and the carboxylate COZ-head gorups of the amphiphile were not established with certainty. The question now arises whether such an ordered bilayer arrangement is unique and, if not, whether we may still obtain information of a general nature as to ordering properties within the counterionic layer. In order to address this problem we varied the chain length of the fatty acid and the chemical nature of the counterion. We thus embarked on a grazing incidence X-ray diffraction (GID) study of the two-dimensional (2-D) crystalline selfaggregating properties of carboxylic acid monolayers CnHzn+1C02Has a function of chain length ( n = 13, 19, 29), when spread over CdClz or CaC12 solutions at high pH values of 8.85 and 10, respectively, adjustedwith ammonia. All the GID data were recorded for monolayers at a surface ressure lI = 0 mN/m, with surface coverage of 67 5% (28 J t 2 per molecule), and a subphase temperature in the range of 5 to 9 "C. The experimental setup for the GID measurements of the monolayers is described in the previous paper.' The measurements were performed on the bending magnet beamline D4, but for the GID data on the films of calcium triacontanoate, which were measured on the wiggler beamline BW1 at Hasylab DESY.
1. Monolayer of Cadmium Triacontanoate. The GID Data. The GID powder pattern of the monolayer of the cadmium salt of triacontanoic acid CzsH&O2H over CdClz solution at high pH measured in the range 0.62 Iqry I1.96A-l, where qryis the horizontal X-ray scattering vector (47r sin WA), yielded two low-order reflections (Figure 1) a t qry = 1.53 and 1.69 A-l (with d-spacings of 4.12 and 3.71 A, respectively). Two higher-order peaks were recorded at qrr = 2.54 and 2.84 A-l, with d-spacings of 2.48 and 2.21 A, respectively (Figure 1). All the observed reflections arose primarily from contribution of the anionic Cz9H&Oz- acid chains as indicated by the shape of the intensity profiles of their Bragg rods (Figure 2). In other words, no reflections were observed whose Bragg rod profiles were flat and extended, indicative of contribution only from the counterionic layer.' The integrated intensity of the peak at q x y = 1.53 A-l is about 3.3 times larger than that of the peak at q r y = 1.69 A-l. The first peak is attributed to the degenerate { l , l ]and {l,Qreflections; the second peak to the {0,2]reflection. This assignment ields a rectangular unit cell with dimensions a = 4.95 b = 7.42 A, and y = 90°, corresponding to a molecular area (ab/2)of 18.4 A2. Naturally, the higher-order reflections appearing at qxy = 2.54 A-l and 2.84 A-l were indexed as {2,0] and {1,3] {l,$],respectively. No diffraction signal from the ions present in the subphase was detected in this system. Structure Refinement o f t he C&&OZMolecules. The Bragg rod intensity profiles peak at qz values close to 0 A-1 for all the observed reflections (Figure 21, indicating that the molecular chains are aligned (almost) vertically on the water surface.2 This result is compatible with the unit cell area per molecule of 18.4 A2, a value comparable to the cross-sectionalarea per chain observed in the densest
t Johannes Gutenberg Universitiit. 8 Weizmann Institute of Science.
Ria0 National Laboratory. a Abstract publishedin Advance ACSAbstracts, January 15,1994.
(1) Leveiller, F.; B o b , C.; Jacquemain,D.;MBhwald, H.; Leieerowitz, L.; Kjaer, K.; Ala-Nielaen,J. Previous paper in this issue.
0743-746319412410-0830$04.50/0
1,
+
0 1994 American Chemical Society
Packing of Monolayers of Fatty Acid Salts
Langmuir, Vol. 10, No. 3, 1994 831
performed in which the glide symmetry of the system was slightly relaxedlo in a manner already described for the structure refinement of fatty acid2 and alcoholmonolayers' on water. The best fitfor the structure of CmH&Oz- was obtained with pseudo pl lg glide symmetry,which yielded a molecular packing arrangement, shown in Figure 3, in which all molecular chains are tilted by an angle t = 1.5' from the vertical in the ac plane and then made to lean 1000 in the direction of the b axis by an angle 6 = 1.7' which 1.40 L48 1.56 1.64 1.72 1.00 is the dihedral angle between the planes of the pure and the pseudoglide.1° The plane group of this structure is actually pl, so the packing arrangement is, in principle, chiral. This "pseudoglide" model yielded a reliability v) index" lower by 4.8% than the pure glide arrangement c and calculated Bragg rod profiles which compare well with 3 2 = 1200 0 1300/J
