J . Phys. Chem. 1992, 96, 6698-6707
6698
Phase Behavior and Structures of Mlxtures of Anionic and Catlonlc Surfactants Eric W. Kaler,* Kathleen L. Herrington, A. Kamakkara Murthy, Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
and Joseph A. N. Zasadzinski Department of Chemical and Nuclear Engineering and Materials Engineering, University of California, Santa Barbara, California 93106 (Received: August 30, 1991)
Spontaneous, singlewalled,equilibrium vesicles of controlled size and surfacecharge can be prepared from aqueous mixtures of simple, commercially available, single-tailed cationic and anionic surfactants. We present detailed phase behavior and structural studies of one such mixture, sodium dodccylbenzenesulfonate (SDBS) and cetyl trimethylammonium tosylate (CTAT) as well as results of less complete surveys of other mixtures. The SDBS/CTAT mixture has many features that in HzO, appear to be common to aqueous mixtures of asymmetric cationic and anionic surfactants. Vesicle formation apparently results from the production of an anion-cation surfactant pair which then acts as a double-tailed zwitterionic surfactant. Although unilamellar vesicles have been created by numerous physical and chemical techniques from multilamellar dispersions, all such vesicle systems revert to the equilibrium, multilamellar phase over time. These catanionic vesicles are stable for periods as long as several years and appear to be the equilibrium form of aggregation.
Introduction More than a quarter of a century ago, Bangham et al.' showed that phospholipids dispersed in water formed closed, multibilayer aggregates capable of separating an internal compartment from the bulk solution. As bilayers are relatively impermeable to many ions and nonelectrolytes, it became simple to create small domains of different composition within such a bilayer aggregate or lip o m e and mimic many of the properties that nature has designed into cells and organelles?" As a result, there has been great effort devoted to using liposomes as model membranes2 and, more recently, as drug delivery systems3and microreactors for specialized chemistry." Vesicles,6 which are single-bilayer closed shells that encapsulate an aqueous interior, have become the preferred structure for use in most application^.^*^-' While vesicles often form spontaneously in vivo, they have only rarely been observed to form in vitro without the input of considerable mechanical energy or elaborate chemical treatment^.^ Hence, a variety of methods have been developed to create unilamellar vesicles, Le. single-bilayer compartments, with sizes ranging from about 20 nm to more than 20 pm.697 The most common method of forming unilamellar vesicles is by mechanical disruption of a dispersion of phospholipid liposomes in water, usually by sonication. Depending on the length and intensity of sonication, an optically clear suspension of small, 10-20 nm. Experimentally, it is observed that the minimum radius of a sonicated vesicle is about 10-1 5 nm, and this is also of the order of the minimum radius found for these spontaneous vesicles (data not shown). Vesicles formed by this mechanism should be extremely polydisperse as the free energy of curvature, ~ U K is , independent of the vesicle radius above R,. However, small amounts of "edge-active" materials, that is surfactant molecules that prefer highly curved interfaces, could act to lower the edge energy substantially and result in the stabilization of the flat bilayer fragments. One such example was shown by Fromherz and Rappel for the case of egg lecithin and taurochenodeoxycholate, a micelle-forming bile salt detergent.57 The highly convoluted surfaces we observe in the multilamellar aggregates (Figure 6) suggest that the excess singletailed surfactant in these catanionic vesicles might be acting as an edge-actant to reduce the energy of the bilayer edges, and enhance the exchange of surfactant between multilamellar aggregates and vesicles. Also, the corrugated surfaces might be the result of vesicles budding off the multilamellar surfaces or fusing with the multilamellar aggregate, although it is difficult to make kinetic arguments based on a static image. If this were true, it suggests that the exchange rate of vesicles with the multilamellar aggregates is a more dynamic process in the catanionic system in comparison to the phospholipid dispersions. This enhanced exchange rate is likely to be important to the stability of the vesicle phase. A recent model by Safran and c o - ~ o r k e r sfocuses ~ ~ on the spontaneous curvature of the two monolayers making up the bilayer for the case of surfactant mixtures. This model is directly relevant to the mixed vesicles explored in this work. They rewrite eq 1 above to reflect the possibility of different spontaneous curvatures in either monolayer in a spherical vesicle of curvature c (c = R', where R is the radius of the vesicle):
E/A =
f/ZK[(C
+ C O ) ~+ (C - s)']
(3)
q and c, are the spontaneous curvatures of the inner and outer monolayers, respectively. For the case of symmetric, singlecomponent monolayers, ci = c,, and the minimum curvature energy occurs for c = 0, or flat bilayers. However, for surfactant mixtures, nonideal mixing of surfactant molecules in the vesicle could allow the inner and outer monolayers to have equal and opposite spontaneous curvatures;that is q = -co = c. For this to happen, the effective head group size of the mixed surfactants on the inside monolayer must be different than on the outside monolayer. This can be achieved either with a mixture of amphiphiles with widely different areas per head group or in a mixture in which surfactant complexes form such that the complex has a small area per head group. By placing more of the smaller head group component (or complex) on the inside monolayer (and more of the larger head
group or uncomplexed surfactants on the outer monolayer) of the vesicle, the spontaneous curvatures could be adjusted to suit a particular curvature of the vesicle. Vesicles with such a curvature would then be stable with respect to flat bilayers in the limit that the bending modulus of the bilayer is large compared to ksT. In combination with the elimination of residual edge energy as discussed above, the net result is a compositiondependent spontaneouscurvature for the bilayer which determines the size and size distribution of the vesicles. However, the theory does not yet deal with bilayer interactions, which are undoubtedly important at higher surfactant concentrations. Extensions of this model can account for several features of the phase although not for micelle-vesicle equilibria. Finally, note that the higher translational entropy of many small vesicles as opposed to a single large bilayer also helps to stabilize the vesicle phase!** It is these entropic contributions that alone may stabilize vesicles in equilibrium with lamellar sheets in aqueous dispersions of GM3.30 Bilayer Interactions. The stability of vesicles as opposed to multilamellar liposomes depends primarily on the detailed nature of bilayer-bilayer interactions, especially as the surfactant concentration is increased. In mixtures of CTAT and SDBS, vesicles appear to be stable up to a concentration at which steric interactions between vesicles become important, and volume constraints require that the surfactants form a multilamellar phase. This suggests that the interaction between these bilayers is repulsive and that they are clearly different from typical zwitterionic phospholipid bilayers that usually have a net A number of forces, some of which have not been generally recognized, play a role in bilayer-bilayer interactions. In addition to the well-known attractive van der Waals and repulsive electrostatic double-layer and "hydration" the importance of repulsive thermal "undulation" forces5'*65~66 and the recently proposed attractive "hydrophobic" are only now being appreciated. The influence of hydration, hydrophobic, and undulation forces on bilayer adhesion and fusion is not yet completely understood, especially in free vesicles.61v62 The phase diagram for the SDBS/CTAT system suggests that the interaction between the vesicles is repulsive, in that the vesicles are stable almost to the point at which the vesicles touch in solution. Hence, it seems necessary to look for an additional repulsive interaction that is important in catanionic vesicles that is absent in conventional double-tailed surfactant vesicles. In the mixed anionic/cationic vesicles, there is always an excess of one type of surfactant; consequently, the bilayers should always have a net charge. Nonetheless, experiments show that CTAT/SDBS vesicles are stable up to a NaCl concentration of about 1 M (Figure lo), which corresponds to a Debye length of less than 0.3 nm." Even though the vesicles are charged, electrostatic interactions do not appear to be responsible for their surprising stability against aggregation. In addition, equilibrium vesicles are also found in uncharged surfactant mixtures, and stable multilayered liposomes are found in highly charged systems. Thus it seems that simple electrostatic interactions are probably not responsible for the stability of equilibrium vesicles. Another possible explanation is the hydration repulsion. Israelachvili and Wenner~trbm~~ have recently proposed a different origin for the short-ranged, but highly repulsive, hydration forces based on the thermal "protrusion" of surfactant molecules from the bilayer into the aqueous space between layers, in analogy to Helfrich's model of bilayer thermal undulations. Experimentally, it is known that the hydration pressure is the dominant repulsion at small separations:6345
= Ho exp(-d/Ah) (4) Here Ah is a decay length which ranges from about 0.1-1 nm, and Ho is a prefactor of order 108-109 Pa. Israelachvili and Wennerstrbm predict that the decay length Ah is roughly proportional to the surfactant solubility: Phyd
Ah
= k e T / a , a = uay
(5)
in which uay is the energy associated with a molecule protruding
6706 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 from the bilayer into the aqueous phase. For relatively soluble surfactants, such as the single-tailed surfactants present in equilibrium vesicles, the hydration force may well be of much longer range than in typical vesicles of double-tailed surfactants, which are almost insoluble in water. This additional repulsion could stabilize these vesicles against adhesion. Marsha has shown that the hydration interactions between more soluble, singletailed lysolecithins are substantially longer ranged than the less soluble, double-tailed lecithins with similar head groups. These forces might be measured using the osmotic pressure-X-ray technique pioneered by Parsegian and co-workers in the stable multilamellar phases found at lower water contents in the CTAT/SDBS diagram.69
Conclusions Equilibrium vesicles form in a variety of mixtures of anionic and cationic surfactants. The regions of vesicle stability in the phase diagram are bounded by lamellar and micellar phases. An enhanced repulsive interaction between bilayers in these unique surfactant mixtures stabilizes these vesicles in comparison to typical double-tailed phospholipids. Electrostatic interactions alone are clearly not responsible for the enhanced stability-both charged and uncharged mixtures form equilibrium vesicle phases, and the charged vesicles are stable against flocculation with added electrolyte. The other possible explanations are an enhanced undulation or hydration repulsion. What is necessary is to experimentally determine important parameters, especially the bending modulus of the bilayers, K. The second possible explanation, that enhanced steric hydration forces are responsible for the surprising stability of these vesicles, should also be tested. Acknowledgment. We acknowledge many useful discussions with Jacob Israelachvili, Fyl Pincus, Sam Safran, and Fred MacKintosh on the theoretical aspects of spontaneous vesicles. Bety Rodriguez prepared the samples for the TEM analysis and offered many useful experimental suggestions. J.A.N.Z. acknowledges financial support from the National Science Foundation (Grant CBT86-57444) and the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. E.W.K.acknowledges support from the University of Delaware Research Foundation, The Clorox Co., and the National Science Foundation (Grant PYIA 8351 179). Registry NO. SDBS,25155-30-0; CTAT, 138-32-9.
References and Notes (1) Bangham, A. D.; Standish, M. M.; Watkins, J. C. J. Mol. Biol. 1965, 13, 238. (2) Fendler, J. Membrane Mimetic Chemistry; Wiley: New York, 1983. (3) Ostro, M. J., Ed. Liposomes: From Biophysics to Therapeutics; Dekker: New York, 1987. (4) Bhandarkar, S.;Bose, A. J . Colloid Interface Sci. 1990, 135, 531. (5) Zasadzinski, J. A. N.; Scriven, L. E.; Davis, H. T. Philos. Mag. A . 1985, 51, 287. (6) Both uni- and multilamellar lipid and surfactant aggregates are called
liposomes. Within this classification are three acronyms-MLV for multilamellar vesicle, SUV for small unilamellar vesicle (C100-nm diameter), and LUV for large unilamellar vesicle (>IO0 nm). Sce: Papahadjopolous,D. Ann. N.Y. Acad. Sci. 1978,308, 367. (7) Szoka, F.; Papahadjopoulos, D. Annu. Rev. Biophys. Bioeng. 1980, 9, 461. (8) Papahadjopoulos, D.; Miller, N. Biochim. Biophys. Acta 1967, 135, 624. (9) Huang, C. H. Biochemistry 1969, 8, 344. (10) Nozaki, Y.; Lasic, D. D.; Tanford, C.; Reynolds, J. A. Science 1982, 217, 366. (1 1) Mayer, L. D.; Hope, M. J.; Cullis, P. R. Biochim. Biophys. Acta 1986, 858, 161. (12) Zasadzinski, J. A. N. Biophys. J . 1986.49, 1 119. (13) Kagawa, Y.; Racker, E. J . Biol. Chem. 1971, 246, 5477. (14) Hauser, H.; Mantsch, H. H.; Casal, H. L. Biochemistry 1990, 29, 2321. (15) Virdon, J. Private communication. (16) Rydhag, L.; Stenius, P.; &berg, L. J . Colloid Interface Sci. 1982, 86, 275. Rydhag, L.; Gabrln, T. Chem. Phys. Lipids 1982, 30, 309. (17) Hauser. H.; Gains, N.; Eibl, H. J.; Miiller, M.; Wehrli, E. Biochemistry 1986, 25, 2126. (18) Gros, L.; Ringsdorf, H.; Schupp, H. Angew. Chem., Inr. Ed. Engl. 1981, 20, 305. (19) Zasadzinski, J. A. N.; Vosejpka, P.; Miller, W. G. J . Colloid Interface Sei. 1986, 110, 347.
Kaler et al. (20) Talmon, Y.; Evans, D. F.; Ninham, B. W. Science 1983,221, 1047. Ninham, B. W.; Evans, D. F.; Wei, G. J. J . Phys. Chem. 1983, 87, 5020. (21) Brady, J. E.; Evans, D. F.; Kachar, R.; Ninham, B. W . J . Am. Chem. Soc. 1984,106, 4279. Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 226. (22) Brady, J. E.; Evans, D. F.; Warr, G. G.; Grieaer, F.;Ninham, B. W. J . Phys. Chem. 1986, 90. 1853. Miller, D. D.; Bellare, J. R.; Kaneko, T.; Evans, D. F. Lungmuir 1988, 4, 1363. (23) Miller, D. D.; Magid, L. J.; Evans, D. F. J . Phys. Chem. 1990, 94, 5921. (24) Murthy, A. K.; Kaler, E. W.; Zasadzinski, J. A. N. J . Colloid Interface Sci. 1991, 145, 598. (25) Gebicki, J. M.; Hicks, M. Narure 1973, 243, 232. Gebicki, J. M.; Hicks, M. Chem. Phys. Lipids 1976, 16, 142. (26) Cistola, D. P.; Atkinson, D.; Hamilton, J. A.; Small, D. M. Biochemistry 1986, 25, 2804. (27) Hargreaves, W. R.; Deamer, D. W. Biochemistry 1978, 17, 3759. (28) Jain, M. K.; van Echteld, C. J. A.; Ramirez, F.; de Gier, J.; de Haas, G. H.; van Deenen, L. L. M. Nature 1980, 284,486. Jain, M. K.; DeHaas, G. H. Biochim. Biophys. Acta 1981, 642, 203. (29) Hauser, H. Chem. Phys. Lipids 1987, 43, 283. (30) Cantu, L.; Corti, M.; Musolino, M.; Salina, P. Europhys. Lett. 1990, 13, 561. (31) Fukuda, H.; Kawata, K.; Okuda, H.; Regen, S.L. J . Am. Chem. Soc. 1990,112, 1635. (32) Kaler, E. W.; Murthy, A. K.; Rodriguez, B.; Zasadzinski, J. A. N. Science 1989, 245, 137 1. (33) Chang, N. J. Thesis, University of Washington, Seattle, 1986. (34) Zasadzinski, J. A. N.; Bailey, S . J . Electron. Microsc. Tech. 1989, 13,309. Robards, A. W.; Sleytr, V. B. Low temperature methods in biological
electron microsoopy. In Practical Methods in Electron Microscopy; Glauert, A. M., Ed.; Elsevier: Amsterdam, 1985; Vol. 10, pp 5-133. (35) van VenetiB, R.; Leunissen-Bijvelt, J.; Verkleij, A. J.; Ververgaert, P. H. J. Th.; J . Microsc. (Oxford) 1980, 118, 401. (36) MacKintosh, F. C., Safran, S.A. To be published. (37) Hemngton, K. L.; Kaler, E. W.; Miller, D.D.; Zasadzinski, J. A. N. Manuscript in preparation. (38) (a) Rehage, H.; Hoffmann, H. J . Phys. Chem. 1988,92,4712. (b) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987,3, 1081. (c) Gamboa, C.; Sepulveda, L. J. Colloid Interface Sci. 1986, 113, 566. (d) Candau, S. J.; Hirsch, E.; Zana,R. J. Colloid Interface Sci. 1985,105, 521. (e) Candau, S. J.; Hirsch, E.; a n a , R.; Adam, M. J. Colloid InrerfaceSci. 1988,122,430. (39) Ween, J.; Kaler, E. W.; Herrington, K. L. Manuscript in preparation. (40)Rosevear, F. B. J. Am. Oil Chem. SOC.1954, 31, 628. (41) Chang, N.J.; Kaler, E. W . J. Phys. Chem. 1985,89, 2996. (42) Tanford, C. The Hydrophobic Effect; Wiley: New York, 1980. (43) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem. SOC., Faraday Trans. 2 1976, 72, 1525. (44) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1985. (45) Helfrich, W. Z. Narurforsch. 1973, 28c, 693. (46)Helfrich, W. Z. Naturforsch 1978, 33a, 305. Helfrich, W.; Servuss, R. M. Nuouo Cimento 1984, 30, 137. (47) Safran, S. A.; Pincus, P.; Andelman, D. Science 1990, 248, 354. Safran, S.A.; Pincus, P.; Andelman, D.; MacKintosh, F. C. Phys. Rev. A 1991, 43, 1071. (48) Safinya, C. R.; Roux, D.; Smith, G. S.;Sinha, S. K.; Dimon, P.; Clark, N. A,; Bellccq, A. M. Phys. Reu. Lett. 1986, 57, 2718. (49) Semss, R. M.; Harbich, W.; Helfrich, W. Biochim. Biophys. Acta 1976,436, 900. (SO) Schneider, M. B.; Jenkins, J. T.; Webb, W. W. J . Phys. (Paris) 1984, 45, 1457; Biophys. J . 1984, 45, 891. (51) Helfrich, W. J. Phys. (Paris) 1985, 46, 1263. Peliti, L.; Leibler, S. Phys. Rev. Lett. 1985, 54, 1690. (52) Safran. S. A.; Roux, D.; Cates, M. E.; Andelman, D. Phys. Rev. Lett. 1986,57,49 1. (53) Evans, E. A. Biophys. J. 1974,14,923. Evans, E. A.; Skalak, R. CRC Crir. Rev. Bioeng. 1979, 3, 181. (54) Szleifer, I.; Kramer, D.; Ben-Shaul, A.; Roux, D.; Gelbart, W. M. Phys. Rev. Lett. 1988, 60, 1966. Szleifer, I.; Kramer, D.; Ben-Shaul, A,; Gelbart, W. M.; Safran, S. A. J . Chrm. Phys. 1990, 92, 6800. (55) Safinya, C.; Sirota, E. B.; Roux, D.; Smith, G. S . Phys. Reu. Lett. 1989, 62, 1134. (56) Leermakers, F. A. M.; Scheutjens, J. M. H. M. J . Phys. Chem. 1989, 93, 7417. (57) Fromhen, P. Chem. Phys. Lerr. 1983,94,259. Fromherz, P.; Rappel, D. FEBS Lett. 1985, 179, 155. (58) Zasadzinski, J. A. N. Biophys. J . 1986. 49, 1119. (59) Harbich, W.; Servuss, R.M.; Helfrich, W. Z . Narurforsch. 1979, 33a, 1013. Harbich, W.; Helfrich, W. 2.Naturforsch. 1979, 34a, 1063. (60) Semss, R.M.; Helfrich, W. In Physics of Complex and Supramolecular Fluids; Safran, S.A., Clark, N. A,, Eds.; Wiley: New York, 1987, pp 85-100. Servuss, R. M.; Helfrich, W. J. Phys. (Paris) 1990, 50, 809. (61) Bailey, S.M.; Chiruvolu, S.;Israelachvili, J. N.; Zasadzinski, J. A. N. Lungmuir 1990,6, 1326. (62) Helm, C. A.; Israelachvili, J. N.; McGuiggan, P. M. Science 1989, 246, 9 19. (63) Rand, R. P.; Parsegian, V. A. Annu. Rev. Physiol. 1986, 48, 201. LeNeveu, D. M.; Rand, R. P.; Panegian, V. A.; Gingell, D. Nature 1976,259, 601. Rand, R. P.; Parsegian, V. A. Blochim. Biophys. Acta 1989,988, 351.
6707
J. Phys. Chem. 1992,96,6707-6712 (64) (65)
McIntosh, T. J.; Simon, S . A. Biochemistry 1986, 25, 4058. Israelachvili, J. N.; Pashley, R. M. Nature 1982,300,341. Pashley,
R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985,229, 1088. Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sei. 1986, 114, 234. (66) Marra, J.; Israelachvili, J. N. Biochemistry 1985, 24,4608. Marra,
J. J. Colloid Interface Sei. 1985, 107,446. Marra, J. Biophys. J. 1986, 50,
815. (67) Israelachvili, J. N.; Wennerstrbm, H. Longmuir 1990, 6, 873. (68) Marsh, D. Biophys. J. 1989, 55, 1093. (69) LeNeveu, D. M.; Rand, R. P.; Parsegian, V. A.; Gingell, D. Biophys. J . 1977, 18, 209.
Monomolecular Layers and Thin Films of Silane Coupling Agents by Vapor-Phase Adsorption on Oxidized Aluminum Dirk G.Kurth and Thomas Bein* Department of Chemistry, Purdue University, West hfayette. Indiana 47907 (Received: November 25, 1991; In Final Form: April 13, 1992)
Thin films of tetraethoxysilane [TEOS], (3-bromopropyl)trimethoxysilane [BPS], trimethoxyvinylsilane [VS], and 3-(trimethoxysily1)propyl methacrylate [TPM] on oxidized aluminum surfaces have been investigated by reflection-absorption FTIR spectroscopy, ellipsometry, contact angle, and quartz crystal microbalance (QCM) measurements. Gravimetric measurements with the QCM can reveal quantitative aspects of adsorption and film formation, even for films as thin as monolayers. Adsorption of these silane coupling agents from solution typically produces multilayer films. Vapor-phase adsorption of TEOS and TPM at room temperature results in monomolecular layers. The coupling agents VS and BPS require additional heating after the vapor-phase adsorption to initiate the hydrolysis and condensation reactions necessary for the surface attachment, which produces one to three layers. For vapor adsorbed films a packing density of 4-7 molecules/nm2 was found. The data strongly suggest that the organic moieties in several of these films have a preferential orientation on the surface; they can be viewed as two-dimensional, oligomeric siloxane networks with oriented organic chains. Subsequent heating of TPM films results in structural rearrangements; heating of TEOS results in complete condensation to Si02films.
Introduction The molecular modification of surfaces is a field of current interest.' Applicationsare found in areas such as thin film optics,2 sensor^,^ chemically modified electrodes? and protective layems Methods of surface modification include the Langmuir-Blodgett technique, self-assembly of various precursor molecules, and adsorption or grafting of polymers. The Langmuir-Blodgett technique permits reproducible formation of well-ordered mono- and multilayers.6 Self-assembly of organic monolayers includes the adsorption of alkanethiols,' dialkyl disulfides,8 and dialkyl sulfides9 on gold; fatty acids on alumina;I0 and alcohols and amines on platinum." Organosilicon compounds (silane coupling agents, SCA) on appropriate substrates provide another route to surface derivatization.I2 The structure of the resulting siloxane film is complex and depends on the functionalityof the silane and the experimental ~onditi0ns.I~ The covalent bond between the native oxide surface layer of the underlying substrate and the organosilicon compound makes these films particularly robust. SCAs are generally applied from solutions. Many functionalized organosilicon compounds are readily a~ai1able.l~ Our research on these systems has two objectives. First, we explore ways to generate single layers of immobilized functional groups on surfaces. Second, we investigate the experimental parameters that affect the structure of SCA thin films. Because short-chain molecules, such as the ones used here, do not selfassemble into crystalline films,the functional groups are expected to be more exposed at the solid/ambient interface, which makes these systems attractive for surface-chemistry related studies. To gain more insight into quantitative aspects of adsorption and film formation, we have introduced the quartz crystal micr~balance'~ (QCM) in our studies. The relation between the frequency change, Aft and the mass adsorbed on one face, Am, has been derived by Sauerbrey:I6 Af = -2f02Am/A& Am = -cfAf (1) where A = piezoelectricactive area, fo = fundamental frequency, *Author for correspondence.
TABLE I: Perk Positions and Band Assignmentsa of TEOS Mowlaver Adsorbed on an Aluminum Surface ~~
freq 2977 2886 1394
assignments -CH3, asym str rPa -CH3, sym str rt C H 3 , sym str 6(r) - S i c - asym str (doublet), -SiUSi- str
1091
OLitcrature sources are found in refs 21 and
24.
= shear modulus, p = density of quartz, and cf = sensitivity factor. For a 9-MHz crystal operated at its fundamental frequency the sensitivity factor cf is 5.4 ng Hz-'cm-2. The QCM provides enough sensitivity even for measurements on monomolecular films." A molecular layer is very thin compared to the wavelength of the shear wave in the resonator, such that the adsorbed layer should entirely exist at the antinode of the shear wave. Therefore, the adsorbed layer should experience no significant shear deformation, and its viscoelastic properties should have no influence on the measurement. If the change in frequency caused by the mass loading is less than 2% of the fundamental frequency and the mass is equally distributed over the electrode surface, eq 1 can be used, as has been demonstrated by Wardla and Jones and Mieure.I9 p
Results and Discussion Adsorption of Tetraetboxysilane (TEOS) on Oxidized MumiTEOS films are interesting for two reasons. Fmt, remaining ethoxy groups on the surface can react with other reagents via condensation. Second, fully condensed films of TEOS form SiOl layers, which can serve as insulating or protecting layers. The vapor-phase adsorption of TEOS provides a convenient route for the formation of extremely thin SiOz films with nanometer thickness. This approach complements the well-developed sol-gel techniques that typically result in much thicker films.20 Figure 1 shows the R4IR spectrum of a TEOS film,generated by vapor-phase adsorption on a water vapor-exposed substrate.
".I
0022-3654/92/2096-6707%03.00/0 0 1992 American Chemical Society
