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Langmuir 1992,8, 630-636
Divalent Cation-Stearic Acid Monolayer Interactions at the Air/Water Interface Mehran Yazdanian,+ Hyuk Yu,*and George Zografi*t+ School of Pharmacy and Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
Mahn Won Kim Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received July 25, 1991. I n Final Form: November 15,1991 The structure and dynamics of stearic acid monolayers at the airlwater interface have been examined by means of in situ ellipsometry, surface potential, and surface light scattering. The results of the ellipsometry and surface potential experiments are completely in accord with our model proposed earlier that Pb2+and Cd2+,unlike those of alkaline-earth ions, are specific in modifying the aqueous head group structure of fatty acid monolayers without affecting the thickness of the hydrocarbon tail layer. Pursuing the head group structure modifications by these ions, we determined the viscoelastic parameters of the monolayer with various cations, Na+,Mg2+,Ca2+,Ba2+,Co2+,Cd2+,and Pb2+,with the technique of surface light scattering. We conclude that Pb2+and Cd2+are anomalous compared to all others, and both ions give rise to larger dilational elastic and loss moduli. In addition, compared to all other ions, lead ion has a greater loss tangent, the ratio of loss modulus to elastic modulus, while cadmium ion has a smaller loss tangent. Hence, we attribute the unique property of the cadmium salt of stearic acid in forming LangmuirBlodgett films to the least loss tangent in conjunction with an enhanced elastic modulus.
Introduction The presence of divalent cations and the pH of the subphase are important factors in determining the properties and stability of Langmuir-Blodgett (L-B) films formed from spread monolayers of saturated fatty acids.l-" Different divalent ions have been shown to have very different effects on the properties of such monolayers and on the L-B films subsequently formed, under otherwise identical condition^.^^-'^ Recently, Yazdanian et al.I6 studied the effects of various divalent cations on the surface potential of some fatty acid monolayers spread at the air/ water interface as a function of cation concentration, pH, and counterion of the cation. Although all divalent cations studied caused the fatty acid monolayers to be more condensed relative to that found in the presence of only Na+,significantly greater negative surface potentials were found, relative to that seen with only Na+, in the order Pb2+> Cd2+> Co2+,while correspondingly less negative School of Pharmacy. Department of Chemistry. (1) Blodgett, K. B. J . Am. Chem. SOC.1935,57, 1007. (2) Blodgett, K. B. Langmuir, I. Phys. Rev. 1937,51, 964. ( 3 ) Ellis,J. W.; Pauley, J. L. J. Colloid Sci. 1964, 19, 755. (4) Vogel, C.; Corset, J.; Dupeyrat, M. J. Chim. Phys. 1979, 76, 909. (5) Vogel, C.; Corset, J.; Billoudet, F.; Vincent, M.; Dupeyrat, M. J. Chim. Phys. 1980, 77,947. (6) Miyano, J. D.; Abraham, B. M.; Xu, S. Q.;Ketterson, J. B. J. Chem. Phys. 1982, 77, 2190. (7) Laxhuber, L.; MBhwald, H. Colloids Surf. 1984, 10, 225. (8) Petrov, J. G.; Kuleff, I.; Platikanov, D. J. Colloid Interface Sci. 1982,88,29. (9) Buhaenko, M. R.; Goodwin, J. W.; Richardson, R. M. Thin Solid Films i1988,159,171. s m , 159,171. (10)Grundy, M. J.; Richardson, R. M.; Roser, S. J.; Penfold, J.; Ward, R. C. Thin Solid F i l m 1988, 159,43. (11) Kobavashi, K.; Takaoka, K.; Ochiai, S. Thin Solid Films 1988, 159, 267. (12) Vogel, C.; Corset, J.; Dupeyrat, M. J. Chim. Phys. 1979, 76,903. (13) Hasmonay, H.; Vincent, M.;Dupeyrat, M. Thin Solid Films 1980, 68,21. (14) Rabe. J. P.:Swalen, J. D.: Outka, D. A.: Stahr, J. ThinSolidFilm 1988,159, 275. (15) Outka, D. A,; Stahr, J.; &be, J. P.; Swalen, J. D.; Rotermund, H. H. Phys. Reu. Lett. 1987, 59, 1321. (16) Yazdanian, M.; Yu, H.; Zografi, G. Langmuir 1990,6, 1093. t
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(more positive) surface potentials were found for the alkaline-earth ions in the order Ba2+> Ca2+> Mg2+.With the former series, particularly with Pb2+and Cd2+,there appeared to be dependency of surface potential on the type of counterion used. Also, with Pb2+such effects could be noted much below pH 5.0, whereas the same effects with Ba2+, Ca2+, Mgz+, Co2+, and Cd2+ only became measurable above pH 5.0, near the intrinsic pKa of the spread fatty acids. The results of this study appeared to support a model proposed by Vogel et al.4J2in which more specific types of binding of metal ions to fatty acids occur with ions like Cd2+and Pb2+, but not with the others. These differences appear to arise because of the ability of Cd2+and Pb2+,particularly Pb2+,to interact with the carboxylate group through specific covalent binding, while the other ions interact primarily through nonspecific electrostatic attraction. Preliminary surface ellipsometric measurements of fatty acid monolayers in the presence of Cd2+ and Pb2+ also produced results that suggested specific interactions between these cations and the carboxylate group, with Pb2+ again producing much greater effects than Cd2+.17 The ellipsometric phase angle, 6A, at constant fatty acid chain length, for example, was shown to increase in the presence of Cd2+and Pb2+,relative to a subphase of only 0.01 M HC1, and to be dependent on cation concentration. It was concludedthat these effects most likely arise because of a change in the refractive index of the surface due to a specific carboxylate-metal ion interaction. This appeared to strengthen some of the conclusions drawn from the earlier surface potential study.'6 In the work presented below these surface ellipsometric studies have been extended to include all of the systems and conditions used in the surface potential study. It was also of interest to more directly examine the tendency of various divalent cations to form specific complexes with fatty acids. This was accomplished by carrying out po~
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(17) Kim, M. W.; Sauer, B. B.; Yu, H.; Yazdanian, M.; Zografi, G . Langmuir 1990,6, 236.
O743-7463/92/2408-0630$03.00/0 0 1992 American Chemical Society
Langmuir, Vol. 8, No. 2, 1992 631
Divalent Cation-Stearic Acid Monolayer Interactions tentiometric titrations of acetic acid as a model carboxylic acid in the absence and presence of the various divalent cations, under conditions related to the monolayer studies. In the final part of this study we have conducted a series of experiments to determine the effects of cations on the surface viscoelasticity of stearic acid monolayers, under the conditions of the earlier studies. Surface rheological properties of a monolayer should play an important role in the formation of L-B films, particularly during transfer from the aidwater interface to the solid. To our knowledge, however, only one series of studies, attempting to relate surface rheology t o formation of L-B films, has been r e p ~ r t e d . ~ItJ was ~ shown, for example, with calcium docosanoate monolayers that when surface shear viscosity became too high relative to surface elasticity, the monolayer did not deposit effectively as an L-B film. On the other hand, when the elasticity became too high relative to the surface viscosity, deposition was also quite poor, presumably because the monolayer transferred was too rigid and brittle. Hence, we direct our focus on the relationships between surface viscosity and surface elasticity for the various metal ion-fatty acid systems that have been previously studied by the surface potential and surface ellipsometry techniques. In this study we have chosen to use a relatively noninvasive technique, electrocapillary wave diffraction (ECWD), implemented in this laboratory,lg to determine the dynamic surface storage and loss moduli of stearic acid monolayers in the presence of various divalent cations, over a frequency range of 0.25 kHz. A particular emphasis has been placed on differences in relative contributions of storage and loss moduli between Pb2+,Cd2+,and Co2+versus Ba2+,Ca2+, and Mg2+. Since in our previous study16 we observed no effects of ionic strength above 0.01, unless otherwise specified, all studies were carried out at 0.01 ionic strength. Since above pH 6 monolayer stability was significantly reduced, some of the divalent cations form hydroxy complexes and even precipitate, and buffers are required to maintain pH, all studies were carried out in the range of pH 2-6 where these problems can be avoided. We chose to use pH 6 only, for some comparisons, since the most significant changes in cation effects previously were shown to occur between pH 5 and pH 6.
Experimental Section Materials. The water used as the subphase liquid in all experiments was the house-distilled water purified with a Millipore Milli-& filtering system with two carbon to two ionexchange stages. Stearic acid ((218) (Applied Sciences Laboratories, State College, PA) and arachidic acid ((220) (Fluka, Renekonkoma, NY) had a stated purity of >99%. They were recrystallized three times from hexane for further purification. Docosanoicacid (C22) (Aldrich) had a stated purity of 99% and was used as received. Hexane (99.9%spectrophotometricgrade, Aldrich) was used to spread C18 and C20, and chloroform (99.9+ % , ACS spectrophotometric grade) for C22. The concentrationof spreading solutions ranged from 1-2 mg/mL. The compounds BaC12 (dihydrate, 99+ % ), CaCl2 (hexahydrate, 99.99%), CdCL (99.999%), CoCl2 (Anhydrous, 97%), MgClz (hexahydrate, 99.999%),NaCl(99.99%), and PbClz (99.999%) were obtained from Aldrich and used as received. The ionic strength of all solutions was brought to 0.01 with NaCl except when stated otherwise. The pH values of the subphasesolutions from 2.0 to 6.0 were adjusted with HCl and NaOH. The pH of solutionschanged by less than 0.1 pH unit during the time course of any experiment. (18) Buhaenko, M. R.; Goodwin, J. W.; Richardson, R. M.; Daniel, M. F. Thin Solid F i l m 1985, 134, 217. (19) Ito,K.; Sauer, B. B.; Skarlupka, R. J.; Sano, M.; Yu, H. Langmuir 1990,6, 1379.
Methods. Surface Ellipsometry. The instrument design was based on modulation ellipsometryprinciples and was reported previously.17p20s21 It was further improved by addition of a beam splitter before the reflection of the beam from the surface in order to account for the fluctuations in the beam properties and to improve the calibrations of phase angle changes. The experimental quantity is the change in the ellipsometric phase angle, 6A, defined as SA=A'-A
(1)
where A and A' represent the phase differences of the clean and monolayer-covered surfaces. The change in the amplitude attenuation$ for nonadsorbing substrateshas been shown to be approximately zero and was not measured here.= As previously reported, we determined only the difference,"-21ps i.e., A' - A, instead of the absolute values of each. The relationship between 6A and the thickness dl is given by the Drude equation: 6A = ad,
(2)
where a is a proportionality constant that dependson the incident angleand the birefringentrefractiveindicesand may be calculated from eq 1. For all the systems studied the 6A values were found to be constant within the experimentalerror between the surface pressures of 1-30 dyn/cm (or
