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Langmuir 1995,11,1078-1080
Spontaneous Self-Organizationvia Cation Exchange in Fatty Acid Films Immersed in Aqueous Media P. Ganguly," S. Pal, Murali Sastry, and M. N. Shashikala Physical Chemistry Division, National Chemical Laboratory, Pashan Road, Pune 411008,India Received October 11, 1994. I n Final Form: January 17,1995@ The spontaneous self-organization of vacuum-deposited films of fatty acids immersed in electrolyte solutions yield c-axis-orientedmultilayer films of their metal salts. X-ray diffraction, electron takeoff angle dependent photoelectron spectroscopic studies, infrared spectroscopy, as well as ellipsometric studies have been used to demonstrate the self-organization via cation exchange. The solvation of the exchanged cation causes a swelling which in turn brings about the reorganization. c-axis oriented films of metal salts of lauric acid which cannot be deposited by the Langmuir-Blodgett technique may now be deposited. The deposition of N bilayer of Y-type c-axis-oriented films of metal salts of fatty acids,l deposited by the Langmuir-Blodgett (LB) technique, involves -N/2 dipping steps which is laborioudtime-consuming when N is large. We demonstrate for the first time that there is an alternative route requiring a spontaneous self-organization of vacuum-deposited films of fatty acids in one single step via cation e ~ c h a n g ewithout ~ - ~ the organizing agency of a solid-condensed phase of the Langmuir monolayer used in the LB technique. These results, besides raising important questions related to hydrophobic5or solvation6 pressure effects that may be important in the organization/ deposition of LB films, also expand considerably the scope of using Y-type c-axis-oriented multilayer films of metal salts of fatty acids. Large or selected areas may now be deposited by such films including those (such as lauric acid) that are conventionally inaccessible by the LB technique. When a vacuum-deposited film of arachidic acid (trace A of Figure l a , details are given in legend for all figures) is dipped in aqueous PbCl2 solution, we obtain intense X-ray diffraction (XRD) lines (trace B ofFigure la),typical of c-axis oriented Y-type Langmuir-Blodgett (LB) films of lead arachidate. The observed XRD pattern of a thick vacuum-deposited film of lauric acid (C12)dipped in PbCl2 solution compares well with that calculated7for a c-axis oriented Y-type film (Figure lb). X-ray photoelectron spectroscopy (XPS)studies show a lead 1aurate:lauric acid Abstract published in Advance A C S Abstracts, March 1,1995. (1)Blodgett, K. B. J . Am. Chem. SOC.1936,57,1007.Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. Langmuir Blodgett Films; Roberts, G. G., Ed.; Plenum Press: New York, 1990. (2)Langmuir, I. J . Chem. Phys. 1938,6,873.Delville, A.Langmuir 1991,7,547. Ellipsometric evidence for the swelling of LangmuirBlodgett films of amphiphilic molecules such as phosphatidylserine has been reported by Cuypers, P. A.; Corsel, J. W.; Janssen, M. P.; Kop, J. M. M.; Hermens, W. Th.; Hemker, H. C. J . B i d . Chem. 1983,258, 2426. (3)Ganguly, P.; Paranjape, D. V.; Pal, S.; Sastry, Murali Langmuir 1994,10, 1670. (4)This method of self-organization is quite distinct from the selforganization of lipid amphiphiles in monolayers (Netzer, L.; Sagiv, J. J . Am. Chem. SOC. 1983,105,674.)or during smearing of surfaces by fatty acids (Menter, J. W.; Tabor, D. J. Proc. R. SOC.London, A 1960, 204,514). (5) Clunie, J. S.; Goodman, J. M.; Symons, P. C. Nature 1967,216, 1203. Leneveu, D. M.; Rand, R. P.; Parsegian, V. A.Nature 1976,259, 601. Israelachvili, J. N.;Wennerstrom, H. Langmuir 1990,6, 873. Helfrich, W.2.Naturforsch. 1978,33a, 305. (6)Israelachvili, J. N.; Pashley, R. Nature 1982,300,29. Helm, C. A.;Israelachvili, J. N.; McGuiggan, P. M. Science 1989,246,919.Bailey, S.M.; Chiruvolu, S.;Israelachvili,J. N.; Zasadzinskii, J. A.N. Langmuir 1990,6 , 1326.
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Figure 1. (a) X-ray diffractograms from vacuum-deposited film (-650 A thick) of arachidic acid on 111 oriented polished
Si wafer (deposition rate 100 kmin) in the following sequence: M trace A, as deposited; trace B, after a dip for 20 min in PbClz solution (pH = 5.0);trace C, after subsequent annealing in air at 70 "C for 1 h; trace D, after further dipping in water (pH = 6.5)for 20min. (b)Observed (dashedline)and calculated (full line) in the experimentally significant region of the X-ray diffraction pattern of a 2500 k 20 A thick vacuum deposited M PbClt solution (pH lauric acid film after immersion in = 4) for 20 min. ratio close to 1:2. The method is quite general and all the films of metal salts of fatty acids deposited by the usual LB technique may be formed by this technique without affecting significantly the meta1:carboxylate ratio. The
0743-7463/95/2411-1078$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 4, 1995 1079
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Figure3. (a)Transmission unpolarized infrared spectra from -650 A thick vacuum-deposited film of arachidic acid as M PbClz solution for 60 deposited and after dipping in min. (b) Infrared spectra in the -CH2 scissoring band region for the same film (top) as in (a) and a 2500 A thick vacuumdeposited film of lauric acid (bottom)before and after dipping (20 min) in M PbClz solution. Full lines correspond to before dipping and broken lines to that after dipping.
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20(O) Figure 2. X-ray diffractograms from vacuum-deposited film (thickness 700A) of equimolar amounts of behenic and stearic acid: trace A, as deposited; trace B, after dipping 20 min in M PbClz solution(pH = 5);trace C, a h r subsequent dipping in distilled water (pH= 6.5)for 3 h; trace D, Langmuir-Blodgett film using a solid condensed monolayer of a 50:50 mixture of behenic and stearic acid trace E, LB film deposited by dipping alternately in troughs covered by Langmuir monolayers of behenic and stearic acid.
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versatility for the deposition of various metal ions increases with decreasing chain length. Several metal ions including mercury and potassium have been incorporated by this technique. On heating the film corresponding to trace B of Figure l a a t 70 "C for 1h, the room-temperature XRD pattern shows a broadening (trace C, Figure la). When subsequently dipped in pure water, the XRD lines become more intense and narrowel.8 (trace D, Figure la). This highlights the influence of the aqueous medium alone in bringing about spontaneous organization. A vacuumdeposited film of equimolar mixture of behenic and stearic acids (Figure 2) immersed in PbC12 solution shows weak XRD lines (trace B, Figure 2). The intensification of the XRD lines when the film is reimmersed in pure water for 3 h (trace C, Figure 2) shows that cation exchange had taken place in the earlier step and that it is the aqueous medium that brings about the subsequent reorganization of the hydrocarbon chains. The XRD pattern compares well with thatg from an LB film with mixed bilayers deposited using a Langmuir monolayer of equimolar (7) Ganguly, P.; Paranjape, D. V.; Chaudhari, S. K.; Patil, K. R. Langmuir 1992, 8, 2365. The relative X-ray intensities of the 001 reflections match well with that calculated from the model given in this paper. (8)The intermediate step of heating is not required to bring about the improved organization when dipped in pure water. (9) Ganguly, P.; Paranjape, D. V.; Chaudhari, S. K. J . Phys. Chem. 1999,97, 11965.
mixtures of behenic and stearic acid (trace D of Figure 2). "he XRD pattern of the film obtained by dipping alternately in two different troughs with behenic and stearic acid monolayers is very different (trace E, Figure 2). Ellipsometric studies show the films to be uniformly thick (within f 2 0 A) both before and after dipping. A permanent increase in the thickness of the dry film by -10% due to the reorganization process is always seen after the first dip in the aqueous solution. Subsequent in situ ellipsometric studieslO on the cation-exchanged films show reversible swelling on immersion and contraction on withdrawal. The as-deposited films themselves show very little swelling in pure water. The Pb2+-ion-exchanged films of C, fatty acids (650 f 20 A thick) when immersed in M PbCl2 solution swell by nearly 15, 11, and 7 A ( f lA)per estimated monolayer thickness for n = 18,20, and 22, respectively. LB films with isotropic refractive index close to the ideal value of 1.51 f 0.01 show similar swelling. The observed dependence on chain length suggests the increasing influence of a cohesive "hydrophobic pressuren5countering the expanding influence of a "hydration pressuren6with increasing chain length. The rate of swelling for both the LB films and the cationexchanged films as well as the extent of cation exchangell (determined by XPS) decreases with increasing n . For thin films or longer dipping times, however, the exchange can be complete as shown by infrared (IR)spectroscopic studies (Figure 3a). The refractive indices, n, and nxy,perpendicular and parallel, respectively, to the substrate for the C, acid films have been measured ellipsometrically.12 The anisotropy, M (=n, - nxy),of the as-deposited film always increased aRer cation exchange and self-organization due to dipping. The change, M , in the anisotropy (=M,, - Mini,where ME, (10)The methodology of Tiberg, F.; Landgren, M. Langnuir 1999, 9, 927, was adopted. (11) Sastry, Murali; Ganguly, P.; Badrinarayanan, S.; Mandale, A. B.; Sainkar, S.R.; Paranjape, D. V.; Patil, K. R.; Chaudhary, S. K. J . Chem. Phys. 1991, 95, 8631. Sastry, Murali; Badrinarayanan, S.; Ganguly,P. Phys. Rev. B 1992,45,9320. The normalized XPS intensity ratiosatETOA=52"ofPb4EC lsforn = 12,18,and22(initialthickness . . = 650 & 25 A; dipped for 20 min in M PbClz solution) are, respectively, 1:22, 1:51, and 1:140 compared to the calculated ratio of 1:24, 1:43, and 150.
y:LbF'E/ 1080 Langmuir, Vol. 11, No. 4, 1995 I
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Figure 4. X-ray photoemission spectra of the 2500 A thick lauric acid film dipped in Lac13 solution for 20 min (La:lauric acid ratio 1:8)at two different ETOAs of 55 and 35". The low ETOA spectra have been multiplied by an additional factor of 1.9 t o match the intensities of the higher ETOA spectra. (a) C 1scorelevel with the resolved carboxylate carbon peak shown in a dashed line and indicated by an arrow. (b) 0 1s core level spectrum and (c) La 3d5/2core level. In all the spectra, the low ETOA spectra (dashed lines) have been multiplied by an additional factor of 1.9.
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and Mini are the anisotropies after and before dipping, respectively) for n = 20, 18, and 12, (thicknesses of 250, 275, and 1450 8,respectively) are 0.031,0.045 and 0.15,
Letters
respectively, with M,, being 0.050, 0.051, and 0.18, respectively. IR studies on the vacuum-deposited films show an increased splitting of the -CH2 scissoringbands13around 1470 cm-l after dipping in metal salt solution, the extent of splitting being maximum for lauric acid films (Figure 3b). The splitting in cation exchanged films is close to 12 cm-I which is character is ti^'^ of the organized orthrohombic cell of n-paraffins. The XPS spectra of ETOAll of 35" (dashed line) and 55" (full line) from a partially ionized lauric acid film of lanthanum after immersion in LaC13 aqueous solution is shown in Figure 4. The C 1s and 0 1s intensities have been normalized with respect to the La 3d512intensities. The superimposable0 1s and carboxylate C 1s spectra a t the two tilt angles as well as the -1.9 times increase of the hydrocarbon C 1s spectra at the lower tilt angles (comparedto the calculated'l value of -1.85) clearly show a Y-type c-axis oriented film involvingall the hydrocarbon chains.
Acknowledgment. We thank the Indo-French collaborative project (CEFIPRA) for financial support. LA940794T
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(12) (a) The methodology of den Engelsen, D. J.Opt. SOC.Am. 1971, 61, 1460, has been followed. (b) den Englesen, D. Surf. Sci. 1976,56, 272. The value of M for barium arachidate as reported by this author is 0.055.
(13) Snyder,R. G.J.MoZ.Spectros.1961,7,116. Rabolt, J.F.;Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78,946. Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986,2, 96.
