Surface pressure and surface potential measurements of polydimethylsiloxane substituted undercanoic acids on aqueous surfaces

Surface Pressure and Surface Potential Measurements of Polydimethylsiloxane Substituted Undecanoic Acids on Aqueous Surfaces R. F. W I L L I S I

Surface Physics, Cavendish Laboratory, University of Cambridge, Cambridge, England l%eeeived December 8, 1969; accepted July 20, 1970 Surface pressure and surface potential measurements of m.onomolecular fihns of polydimethylsiloxane substituted ll-undecanoic acids, (CII~)3SiO((CI-I3)2SiO)~-

(CH~)2Si (CH~)IoCOOtt, in which n = 0 and n = 6, on aqueous surfaces are described, and their behavior is compared with that of stearie acid and linear polydimethylsiloxane dodecamer films under similar conditions. At low surface pressures, these siloxane substituted fatty acids orient horizontally on the aqueous surface. Under compression, the molecules come together and the siloxane-alkane chains fold up out of the aqueous phase so that, at low areas per molecule, the terminal siloxane units are as closely packed as possible and the molecules oriented almost vertically to the surface. The limiting cross-sectional area of the close-packed siloxane units was found to be 28 ~2. The larger value of 40/~, deduced from viscosity studies of the bulk polydimethylsiloxane fluids by previous workers, includes the area swept out by the thermally agitated siloxane units. The films collapsed by a rather complicated mechanism involving both time-dependent molecular rearrangement and intermoleeular slippage of molecules within the monolayer. The condensing effect of multivalent cations in solution was studied and the results for stearie acid appear to corroborate the polynuelear surface complex theory of Abramson and Ottewill for hydrated thorium cations in solution. The results of this investigation are discussed in terms of the configuration and packing of the adsorbed molecules and illustrate the low intermolecular cohesive forces associated with the polysiloxanes. INTRODUCTION

limits their use as boundary lubricants since the adsorbed interfaeial films are easily disrupted and rendered ineffective. I n view of their technological importance as hydraulic fluids and hydrodynamic lubricants, various attempts have been made to form effective boundary lubricating silicone layers. For example, monolayers formed on metal surfaces by the hydrolysis in situ of halosilanes containing long-chain hydrocarbon radicals (15) and by the deposition of methyl alkyl silicone films by the Langmuir-Blodgett technique (14) were found to be similar in behavior to the longchain hydrocarbon surfactants such as the f a t t y acids, esters, and alcohols. However, such films cannot be formed by adsorption from solution; this limits their use as practical boundary lubricants and additives.

The polydimethylsiloxanes or "silicones", ((CH3)2SiO)~, find wide use as surface-active agents (1) and, in consequence, the behavior of polyorganosiloxane molecules at interfaces has been the subject of much interest. Monomoleeular films have been studied on aqueous (2-10), organic liquid (11-13), and solid surfaces (14-16). It appears that the free rotation of the substituent organic radicals relative to the siloxane bonds and the high flexibility of the polysiloxane chains together with their extremely low intermolecular cohesive forces make the formation of compact oriented films at the interphase boundaries difficult. This severely 1 Present address: Space Sciences Dept., Surface Physics Division, European Space Research Organisation, Noordwijk, Holland. Copyright @ 1971 b y Academic Press, Inc.

Journal of Colloid and Interface Science, VoL 35, No. 1, J a n u a r y 1971

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WILLIS

Newing (3) attempted to overcome this difficulty by introducing polar-CH2OH and -CHIC1 groups at the end of the polydimethylsiloxane molecule. By analogy with the hydrocarbon surfactants, it was envisaged that these molecules would be adsorbed on metal surfaces and orient to form a close-packed boundary lubricating interracial layer. However, although these fluids showed a marked improvement in lubricating properties over the unsubstituted polymers, the friction and wear of heavily loaded steel surfaces was still considerably higher than that observed with conventional hydrocarbon additives. The structure and configuration of the polysiloxane molecules were such that no close-packed, protective monomolecular films were formed on the interacting metal surfaces. The orientation, configuration, and molecular packing of the molecules is of paramount importance if the interracial film is to function as a protective boundary lubricating layer. In view of the known correlation between molecular orientation and configuration observed on aqueous and on solid surfaces (15, 16), recent work in our laboratory has been directed towards a detailed understanding of the behavior of similar "tailored" silicone molecules at the gas/ water interface in a n attempt to understand their subsequent boundary lubricating properties on metal surfaces. This paper describes an investigation of the surface pressure-area and surface potential-area characteristics of monomolecular films of polydimethylsiloxane substituted ll-undecanoic acids, (CH3)sSiO. ((CH3)=SiO)~(CH~)2Si(CH2) ~oCOOH, in which n -- 0 and n = 6, spread on aqueous surfaces of varying pH and ionic content. Their behavior is compared with that of stearic acid and with that of a low molecular weight polydimethylsiloxane dodecamer fluid under similar conditions. The results are discussed in terms of the orientation, configuration, and packing of the adsorbed molecules, and in terms of the steric hindrance and low intermolecular cohesive forces associated with the siloxane units.

EXPERIMENTAL

Materials Two siloxane substituted undecanoie acids, (CH3)3SiOSi(CH3)~(CH~)loCOOH (11-pentamethyldisiloxy-undecanoic acid) and (CH3) ~SiO((CH3)2SiO)~(CH~)~Si. (CH2)IoCOOH,n = 6 (ll-heptadecamethyloetasiloxyundecanoie acid) were kindly prepared for us by Drs. R. W. J. Williams and F. C. Saunders, Midland Silicones Ltd., Wales. They were prepared by adding together equimolar quantities of as pure as possible samples of the reactants, (CH3) ~SiO((CH3)2SiO)~Si(CH~)2H and CH2 = CH(CH2)sCOOH, the reaction being catalyzed by chloroplatinic acid, which is known to give 100% addition in such a reaction with no siloxane bond cleavage. The small amount of unreacted matter and impurities were carefully removed by distillation in the presence of a cold finger under vacuum. The resulting low viscosity fluids were better than 99.5 % pure by glc analysis. The disiloxy derivative was a single tool. wt. compound. Owing to the difficulty in preparing and isolating single mol. wt. components of the polysiloxane reactant, it is probable that the octasiloxy fraction contained small quantities of derivatives corresponding to n = 4-8. Nevertheless, the surface pressure-area and potential-area characteristics of this fluid were very similar to those which were expected for the pure n = 6 single mol. wt. "oetasiloxy" acid. For convenience, these two polydimethylsiloxane substituted ll-undecanoie acids will be referred to by the abbreviated names "disiloxy" and "oetasiloxy" acids or collectively, "siloxy acids". The stearie acid, CH~(CH~)16COOH, was of the highest purity available from Fluka A. G., Switzerland, and had a m p of 69.6°C. The polydimethylsiloxane dodecamer was a highly pure single tool. wt. derivative, which was kindly supplied by Dr. R. Baney of the Dow Corning Company. The volatile solvent used to spread these compounds on the aqueous subphase was twice-distilled Analar quality petroleum ether (boiling range 60°-80°C). Concentrations ranged from 1 X 10-4 to 5 X 10-4 gm/

Journal of Colloid and Interface Science, VoL 35, No. 1, J a n u a r y 1971

POLYDI1VIETHYLSILOXANE SUBSTITUTED FATTY ACID MONOLAYERS ml solvent. Each solution was delivered dropwise to a clean water surface from a specially constructed glass capillary weighing pipet which minimized evaporation effects during spreading (17). The aqueous substrates were prepared from ion-exchange water which was twice distilled from dilute potassium permanganate solution (to remove the organic impurities from the ion-exchange resin), and then twice distilled in a two-stage all-quartz distillation apparatus prior to use. The water produced had a specific conductivity of about 0.5 X 10.6 ohm-1 and a pH of 6.8. Variations in pH were made by additions of Analar quality HC1 and KOI-I. High-purity calcium chloride and thorium nitrate salts were employed as sources of divalent and tetravalent cations at concentrations of 10-2 M and 5 X 10-~ M, respectively (18).

3

surface and positioned 10 em from the mica boom. The circuit was completed via the electrometer with a saturated KC1 saltbridge electrode immersed in the aqueous subphase solution containing 0.01 M KC1. The whole apparatus was mounted on a vibration-free table, completely enclosed and electrically isolated, all measurements being made externally to limit contamination. A stream of moist nitrogen was continuously circulated through the enclosure in order to stabilize the environment of the reference air electrode (19). With these precautions, potentials were stable and reproducible to within 4-3 my over periods of several hours. All experiments were conducted at 18°-22°C, and, over this range, temperature had little effect on the films studied.

Procedure

Apparatus The surface pressure (~r) of the spread monolayers was measured with a Cenco Hydrophil Balance2 of the conventional Langmuir-Adam type. The apparatus consisted of a Teflon-coated aluminum trough (65 X 14 X 15 cm) and a Cenco du Notiy torsion head sensitive to surface pressure changes of 0.05 dyne/cm. The pressuresensing barrier was a lightly waxed mica boom, which was suspended from a steel torsion wire (torsion constant 0.431 dyne/ em/degree) and attached to the sides of the trough by thin platinum foil end-loops. The area occupied by the film was varied by a Teflon-coated glass compression barrier (30 X 2.5 X 0.5 cm) which was attached to a variable-speed motor drive. Surface potential changes (AV) due to the adsorption of the monolayer at the air/water interface were recorded by a high-impedance electrometer (Keithley Model 610 B) by means of the air-ionizing electrode method (19). The complex cell consisted of a polonium a-source electrode (10 millieurie Po 21° a-source, type PDC-10 obtainable from the Radiochemical Centre, Amersham, England), placed vertically at a working distance of approximately 2 mm above the water 2Cenco Instruments Corporation, Chicago, Illinois.

The usual precautions were taken to prevent contamination interfering with the behavior of the monolayers. Before a series of experiments was begun, all parts of the film balance were thoroughly cleaned and reassembled. The trough was filled with purified water and the surface swept many times with small barriers before the spreading solution was introduced to the surface. In view of the recent controversy (20) concerning the effect of spreading solvent, particularly benzene, on the subsequent behavior of monolayers on aqueous surfaces, the above compounds were spread from n-hexane, petroleum ether, and benzene and their ~-A isotherms compared. The drops were introduced at regularly spaced intervals on the surface and 15 rain. elapsed before the onset of compression. In the presence of Teflon-coated compression barriers and providing the spreading solution was introduced as far away as possible from the lightly waxed float system, all three solutions gave identical and reproducible results within the limits of experimental error. If a heavily waxed compression barrier was employed, however, the benzene solutions gave rise to a significant expansion in the ~r-A isotherms, particularly at low film pressures (16). Clearly, this suggests that benzene is able to spread and dissolve wax

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from the compression barrier (and possibly also from the float system in the present case)--a conclusion reached by Walker and Ries (21). Consequently, in view of this and in view of the fact that the polysiloxanes have a low solubility in n-hexane, petroleum ether (boiling range 60°-80°C) was employed exclusively as the spreading solvent. The stability of the monolayers at high film pressures was determined by observing changes in film pressure with time at constant area. Film instability was detected by the movement of a beam of light directed at a galvanometer mirror attached to the pressure-sensing torsion arm. Irreversible changes in film structure during compression were determined in some cases by recording the ~r-A expansion curve as well as the compression curve and noting any hysteresis. The criterion for film collapse was taken as that point at which the pressure fell significantly, and continued to fall, when the film was held at constant surface area; or the pressure remained constant with decreasing o area down to "zero" A2/molecule. In some cases, the pressure fell for a short time after compression had ceased but then became stable suggesting some time-dependent molecular rearrangement in the films. The rate of film compression was normally 0.1 to 0.4 cm/min, but, in order to study such kinetic effects in more detail, the adopted procedure was to compress the films very slowly by reducing the area in small decrements (approximately 1.5 to 3.0 cm2) and to observe the changes in surface pressure with time at constant area. In the absence of these kinetic effects, monolayers of the siloxane substituted acids could be left on the surface for several hours, very little change in their behavior being observed after this time. Hence, it was considered that the surface pressure-area isotherms obtained were stable and free of solution or evaporation effects during the time taken for an experiment. The air-ionizing electrode was attached to a concentric arm mounted directly through the roof of the enclosure such that it could be swept over the film-covered surface to check film homogeneity. In addition, this arm was hinged in such a manner that the electrode could be swung across the eom-

pression barrier in order to obtain a direct differential reading of the surface potential difference, AV millivolts, between the filmcovered and the film-free aqueous surfaces at any time in the course of an experiment. The technique of dusting the film-covered surface with lyeopodium powder was used to provide some indication of film rheology (19). After each experiment, the films were removed with a fine glass capillary suction device. With care, the apparatus could be used for as many as six experiments before dismantling and cleaning again became necessary. All experiments were triplicated and the results superimposed. Estimated errors were 4-0.1 dyne/em in the surface pressure ~r, 4-0.2 A/molecule in the area A, and 4-3 my in the surface potential changes AV. RESULTS I. SURFACE P R E S S U R E VS. A R E A ISOTHERMS FOR MONOLAYERS ON ])ISTILLED W A T E R

The siloxane substituted undecanoic acids ("siloxy acids") formed stable insoluble monolayers on distilled water surfaces, their surface pressure-area (Tr-A) isotherms being compared with that for stearic acid on twice-distilled water, pH -- 6.8, at 20°C in Fig. 1. Molecular models of stearic (I), disiloxy (II), and oetasiloxy (III) acid molecules in the linear chain-extended configuration are shown in the inset figure. Stearie acid (I) formed a typically rigid "solid-condensed" film in which the molecules were close-packed and oriented vertical to the surface. The shape of the ~r-A curve for the siloxy acid monolayer (II) was typical of that of a "liquid condensed" film (19). Increasing the overall length of the molecule did not give lise to enhanced intermolecular cohesion and a more stable film; the behavior of the octasiloxy acid film (III) was essentially the same as that of a shortchain linear polydimethylsiloxane (2-10), particularly at large surface areas. At low surface areas per molecule, however, the octasiloxy acid monolayer was slightly more stable owing to the presence of the polar carboxylic acid unit, (CH2)10COOH, which increased the adhesion between the film and the aqueous substrate.

Journal of Colloid and Interface Science, Vol. 35, No. 1, January 1971

POLYDIMETtIYLSILOXANE SUBSTITUTED FATTY ACID MONOLAYERS

5

E SO

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FIG, 1. Surface pressure-area (z-A) isotherms of monolayers of stearic acid (I), disiloxy sul~stituted ll-undeeanoie acid (II), and oetasiloxy substituted ll-undecanoic acid (III) on twice-distilled water, pH 6.8, at 20°C. Inset: Molecular models of stearic acid (I), disiloxy substituted ll-undecanoie acid (II), and octasiloxy substituted ll-undeeanoic acid (III) molecules in the linear chain-extended configuration.

Film Stability and Reversibility

50

The stearic acid monolayer exhibited a sudden and rapid fall in surface pressure when the film was compressed beyond 42 dynes/cm and 19.3 M/molecule. The collapse point at which the siloxy acid monolayers become unstable, however, was less well defined. For example, the disiloxy acid ~r-A isotherm was reversible and the film remained stable at surface areas greater t h a n 28 ~2/ molecule. At smaller areas per molecule, the film became unstable, irreversible collapse occurring above approximately 32 dynes/cm pressure. T h a t is, the ~r-A curve "foldsover," the surface pressure remaining constant at approximately 36 dynes/cm with further compression down to zero area per molecule. Crisp (22) observed similar behavior with polyacrylate monolayers and attributed this kind of collapse to the molecules in the monolayer being squeezed up out of the surface to form art "overfilm" of molecules on top of the underlying highly compressed interracial film. The molecules remaining in the surface are as close-packed as possible and the film is in a state of maximum density. Upon releasing the pressure and increasing the surface available to the film, the ~r-A curve exhibited hystere-

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Fro. 2. ~r-A isotherm hysteresis behavior of disiloxy acid film on twice-distilled w~ter, pH 6.8. ABCD, compression curve; DE, decompression curve. sis of the type shown in Fig. 2, A B C D E. This type of irreversible behavior is thought to be due to the time required for the molecules in the "overfilm" to relax and respread on the aqueous substrate. Irt some cases, it was observed that if sufficient time was allowed to elapse (1-3 hours) after decompression to zero pressure (E) it was possible to repeat the original cycle, A B C D E. This behavior was not at all reproducible,

Journal of Colloid and Interface Science, Vol. 35, No. 1, J a n u a r y 1971

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WILLIS

so however, the second cycle often being shifted to lower areas per molecule, possibly owing CO.eRESS~O. to solution effects of the type described by "~hol . . . . . . . . . Fox et al. (4) for polydimethylsiloxane monolayers aged for long periods on aqueous surfaces. The octasiloxy acid monolayer exhibited irreversible behavior at pressures below that required for collapse (Fig. 3). Upon taking the film through a cycle of compression followed by decompression, hysteresis 10 50 70 90 110 130 150 170 7~0 210 AREA(~2/ Molecule) was observed particularly at large areas per molecule indicating some irreversible change FIG. 3. Irreversible r-A behavior of octasiloxy in the configuration of the molecules in the acid film on twice-distilled water, pH 6.8. monolayer during compression. In this this study being conducted in this pH respect, the behavior of the octasiloxy acid range. film was again very similar to that which The disiloxy acid formed stable insoluble has been observed with the unsubstituted monolayers at all three pH values, the ~-A linear polydimethylsiloxanes on aqueous curves superimposing within the limits of surfaces (7-9). The octasiloxy acid mono- experimental error. No significant changes layer showed signs of instability below 28 were observed at high or low film pressures A2/molecule, overfilm formation occurring at over a period of several hours. approximately 25 dynes/cm pressure. The octasiloxy acid monolayers were also That the collapse of these monolayers did stable and gave reproducible results over not, in fact, involve any dissolution effects periods of 1 to 2 hr. On acidic substrates, was checked by carrying out experiments in pI-I 2.2, however, the ~r-A isotherm was the presence of a piece of carefully cleaned displaced to slightly higher pressures (Fig. 4) platinum foil (10 × 10 cm square) situated but still exhibited the same compressionapproximately 1 mm below the water sur- decompression behavior as that described face. Even after as many as a dozen or more previously (Fig. 3). Noll et al. (8, 9) reported experiments, this foil remained completely similar behavior for the polydimethylhydrophilic indicating that the films did not siloxanes at this pit level and proposed an dissolve in the aqueous substrate and adsorb explanation involving hydrogen bonding on the foil to any significant degree during with the aqueous subphase which is enthe course of the above experiments. hanced by the presence of tt + ions in solution. That is, at low surface pressures, the Effect of Substrate pH polydimethylsiloxane molecules lie flat on Fox, Solomon, and Zisman (4) observed the aqueous surface with all the Si and O that the r-A curves of the polydimethyl- atoms in the water and the methyl groups siloxanes on aqueous surfaces changed close-packed in a plane above the surface. slowly with time and that the behavior was The siloxane units are heavily hydrated dependent on substrate pH. Strongly acidic owing to the strongly polar nature of the and strongly alkaline solutions (i.e., below SP+--O L bonds. Increasing the H + conpH 2.0 or above pH 12.0) gave rise to ir- centration of the subphase strengthens reversible behavior due to hydrolytic cleav- hydrogen bonding between the water moleage of the siloxane bonds followed by dis- cules and siloxane units so that a greater solution and evaporation of the resultant force is required to squeeze the polysiloxane siloxanol fragments. In order to establish chain up out of the water surface and the the stability of the siloxy acid monolayers on ~r-A curve is displaced to higher surface aqueous surfaces, therefore, the properties pressures. of these films on substrates of pH 2.2, 6.8, The 7r-A curves shown in Fig. 4 were and 9.2 were investigated, all experiments in obtained for compression times of the order v

Journal of Colloid and Interface Science, Vol. 35, No. I, J a n u a r y 1971

I

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FIe. 4. 7t-_A_isotherms of octasiloxy acid films on aqueous substrates of varying pH. Acidic solutions, pH 2.2, displace the curves to slightly

higher pressures believed to be due to increased H-bonding between the monolayer and the subphase. of ~ - 1 hr. No significant aging effects were detected over these periods. However, irreversible changes of the type described by earlier workers (4, 8, 9) were'observed after 4 or 5 hr if the films were aged at large surface areas per molecule. Slightly accelerated changes were observed on acidic substrates. At high film pressures no irreversible changes due to hydrolysis were detected at all during this period indicating complete removal of the siloxane units from the aqueous surface during compression. The behavior of the siloxy acids, therefore, was not affected significantly during the time usually taken for an experiment at the pH levels employed in these experiments. II.

S U R F A C E P R E S S U R E VS. A R E A ISOTHERMS

FOR MONOLAYERS ON AQUEOUS SOLUTIONS OF MULTIVALENT CATIONS

In view of the condensing effect of multivalent cations in solution on adsorbed monolayers of the higher fatty acids (23, 24), the siloxane substituted undecanoic acids were spread on aqueous solutions containing small quantities of multivalent cations with the object of stabilizing and possibly condensing them at high surface pressures.

Dilute Calcium Chloride Solution The siloxy acids were spread on aqueous solutions containing 10-: M CaC12 at pH 2.2, 6.8, and 9.2, and their 7r-A isotherms were

7

compared with those of stearic acid. On alkaline substrates, pH 9.2, the carboxylic acid group ionizes and reacts with the aqueous Ca 2+ ions in solution to form the corresponding soap (25). Stearic acid formed a close-packed monolayer even at low film pressures under these conditions and was stable up to pressures as high as 50 dynes/ era. The siloxy acids, however, were little affected by the presence of divalent Ca 2+ ions in the subphase, even at high pH. The collapse pressures increased slightly with increasing pI-I but the compressibilities of the films remained the same. The disiloxy acid monolayer remained stable up to increased surface pressures of the order 40 dynes/era on dilute calcium chloride substrates at pH 9.2 compared with 32-36 dynes/em on pure distilled water, pH 6.8. The oetasiloxy acid film collapsed at 30 dynes/em compared with 25 dynes/era on the pure substrate. In both eases, there was no evidence of any condensing effect, the films remaining liquid and highly compressible compared with the fatty acids. The presence of the divalent Ca 2+ ions in solution was not sufficient to overcome the effect of the bulky terminal siloxane units.

Dilute Thorium Nitrate Solution The tetravalent aqueous Th 4+ cation is an extremely powerful condensing agent for monomoleeular films on aqueous surfaces. For example, Zisman (24) discovered that dilute solutions containing a small quantity of a tetravalent thorium salt were able to solidify monolayers of the straight-chain fatty acids at the oil/water interface and to condense branched-chain aliphatic acids containing ten or more carbon atoms per molecule. More recently, it has been shown that as little as 5 X 10-4 M thorium nitrate in solution was sufficient to completely condense monolayers of myristic acid (26) on acidic solutions and to overcome the electrostatic repulsive forces between the terminal halogen atoms in 16-halogen hexadecanoic acid molecules at the air/water interface (18). With this in mind, the siloxy acids were spread on aqueous substrates containing 5 X 10.4 M Th(NO~)4 at an equilibrium

Journal of Colloid and Interface Science, Vol. 35, No. 1, January 1971

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WILLIS

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F~o. 5. ~--A, AV-A, and t~-A curves of ste~ric acid monolayers on aqueous substrates containing 0.01 M KC], pH 6.8, and 0.01 M KC1 4- 5 X 10-a M Th(NO~)~, pH 3.8. p H of 3.8 and their behavior at 20°C was compared with that of stearic acid films. The presence of aqueous T h 4+ cations in solution produced a marked effect on both the stability and the compressibilities of the monolayers. As reported by other workers (18, 26), the stearic acid monolayers were expanded and were somewhat less stable on dilute thorium nitrate substrates, Fig. 5. The siloxy acid films were con siderably more condensed, however, and collapsed at much higher surface pressures when 5 X 10-4 M T h ( N Q ) was introduced to the subphase. The monolayers could be compressed up to pressures as high as 52 dynes/cm before collapsing to form an overfilm, hysteresis again occurring on decompression as described earlier, Figs. 6 and 7. The siloxy acid films behaved in a rather complex manner during compression at elevated film pressures below 28 A2/molecule surface area on dilute thorium nitrate solution. The 7r-A curves shown in Figs. 6 and 7 were obtained at compression rates of the order of 3 A2/molecule/minute. When compression was stopped at any point below 28 A2/molecule area, however, the film

20

3'0 AREA ( ~21Mo[ecu{e)

FIG. 6. 7r-A, AV-A, and #-A curves for disiloxyacid mono]syers on aqueous substrates contain-. ing 0.01 M KC], p H 6.8, and 0.01 M KC1 4- 5 X 10-~ Th(NO3)4, p H 3.8.

pressure was observed to fall slowly by 2 or 3 dynes/cm before stabilizing. This apparent "relaxation" behavior points to some kind of time-dependent molecular rearrangement or intermolecular slippage occurring within the highly compressed monolayers in t h e presence of aqueous T h 4+ ions in the subphase. Subsequent surface potential measurements to be described appear to support this view. Thus, even in the presence of aqueous T h 4+ cations and at surface pressures ir~ excess of 50 dynes/era, the siloxane substituted undecanoic acid monolayers remained fluid and viscous with compressibilities of the order of those of "liquid-condensed" fatty acids. Clearly, the bulky terminal siloxane units prevent complete ordering of the molecules and the formation of a compact crystalline monolayer. III. ELECTRICAL PROPErTiES In order to investigate molecular configuration and orientation in more detail, the difference in electrostatic potential ("surface potential"), AV, between the

Journal of Colloid and Interface Science, Vo]. 35, N o , 1, J a n u a r y 1971

P O L Y D I M E T H Y L S I L O X A N E S U B S T I T U T E D F A T T Y A C I D MONOLNYEI~S

9

The surface potential difference AV of adsorbed acidic or basic compounds de~.~a ~ o.o~°~ o "600"700~Q' pends not only on the molecular structure -500 I ...... ~....... ""CJJ vS A '400~ but also on the pH and salt concentration ~01 / : Co'~o .... pH= 3.8 .30C-of the aqueous substrate (19, 25). The -200 I ,~/~ _,_,_,.0 01M KC[+5.10-~ M Th(NO3)4 -too results for stearic acid, Fig. 5, are in agree" I -0 50I pH = 6.8 -400 merit with those reported in the literature "% I -o-o-o- 0.01MKCI (25, 28) for the variation of the surface -350 potential of fatty acid monolayers with pH. ~o- 1 ' \ -~oo *,l "% In the presence of aqueous Th 4+ cations ~i "., -250 ~'1 "L. AVvsA in solution at an equilibrium pH 3.8, the uJ ,, "-e-,~,~, // stearic acid film is expanded and AV inu~ 1 ~°'c-~-o--°--o-- o-o "~ '~. o.. , ~m o \ • 150 ~creases substantially becoming more posio_ ~L 28~2 ".v . \ r ~+ tive relative to the clean water surface 2 °'x° \ 100 oz ~ o ~ 50 (18, 26). Abramson and Ottewill (26) suggested that the expansion of myristic acid monolayers at low surface pressures on dilute thorium nitrate substrates occurs because o Io ~bo ~o go of the interaction of the acid molecules with AREA (A2I Motecule) hydrated thorium cations in solution. The :FIG. 7. n-A, AV-A, and t~-A curves for oetaresults of their work suggest that at low siloxy acid monolayers on aqueous substrates pH the thorium ions form chains of linear containing 0.01 M KCI, p H 6.8, and 0.01 M polynuclear complexes of the form KC1 + 5 X 10-4 M Th(N03)4, pH 3.8. (Th(OH)2)~ + at the film-water interface, the distance between the thorium ions o clean and the monolayer-covered aqueous being of the order of 6 A. This being the surfaces was monitored as a function of area. case, at pH 3.8 expansion of the fatty acid The surface dipole moments of the molecules monolayer occurs since the spacing of the at the aqueous interface, t~ milliDebyes (1 acid molecules in the surface is determined mD = 10-2T cgs units), were computed by the position of the thorium cations in this from the equation (19): polynuclear complex structure. That is, the = AAV/12% [1] adsorbed molecules would tend to occupy sites in the surface of area approximately o where A is the molecular area, A2/molecule, 29 A2/molecule, corresponding to an interand AV is the change in surface potential in molecular spacing of 6 A. millivolts. The resulting A V vs. A and The surface potential readings for the vs. A relationships for stearic acid, disiloxy stearic acid monolayer on dilute thorium acid, and octasiloxy acid monolayers on nitrate solution were observed to fluctuate aqueous substrates containing 0.01 M KC1, erratically with timeo at areas greater than pH 6.8, and 0.01 M KCI + 5 X 10 -4 M approximately 35 A2/molecule indicating Th(N03)4, pH 3.8, are shown superimposed inhomogeneous surface coverage. Upon comon their r-A isotherms in Figs. 5-7. pression, AV stabilized and exhibited a The 0.01 M KC1 did not appear to affect sudden increase at approximately 29 ~2/ the behavior of the films appreciably, molecule indicative of some kind of surface 7r-A curves identical to those obtained ordering as proposed above. If this "crystalwithout pbtassium chloride in solution lographic" model of aqueous surfaces conbeing obtained in all cases. The small taining Th 4+ cations ois true, however, comquantity of 0.01 M KC1 in the substrate pression below 29 A2/molecule might be solution merely served to stabilize the expected to produce some distortion of the trough KC1 sMt-bridge electrode against (Th(OH2)2fi+ complex which would in turn any fluctuations in electrode voltage which presumably have a disruptive effect on the might produce erroneous results. fatty acid film. The behavior of the stearic r

t

"~'-

.lOOO -900

Journal of Colloid and Interface Science, V o l . 35, N o . I , J a n u a r y

1971

10

WILLIS

acid monolayer, Fig. 5, appeared to corroborate this view. The film exhibited a marked degree of° instability when compressed below 29 A2/molecule, the surface pressure and surface potential readings varying with time when the film was held at constant area, i.e., the actual slope of the v-A curve was dependent on the compression rate. The 7r-A and AV-A curves shown were obtained at faMy fast compression rates of the order of 2 A2/molecule/minute and were irreversible on thorium nitrate solution. The surface potential data for the disiloxysubstituted - undecanoic - acid monolayer (Fig. 6) emphasized the condensing effect of the presence of aqueous Th 4+ ions in solution. In the absence of these tetravalent cations at pH 6.8, the disiloxy acid films were gaseous and homogeneous, the surface potential readings remaining stable and invariant at large surface areas per molecule. A sudden transition in ~V at approxio mately 45 AS/molecule indicated some kind of molecular reorientation effect. The introduction of 5 × 10-4 M Th(N03)4 to the subphase solution produced an immediate condensation of the film, hV fluctuato ing at areas greater than 45 A2/molecule in a manner indicative of inhomogeneous or patchy surface coverage. Upon compression, AV stabilized and increased in the manner shown in Fig 6. At point C, the surface potential readings became very unstable as collapse or overfilm formation occurred. On the other hand, the surface pressure of the monolayer showed signs of monolayer instability or relaxation during compression over the region ABC, o i.e., at areas below 28 A~/moleeule (point A). The behavior of the film was indicative of some sort of molecular rearrangement occuring just prior to collapse. The oetasiloxy substituted undecanoic acid films (Fig. 7) exhibited similar instability or "molecular relaxation" effects below 28 A2/moleeule on dilute thorium nitrate substrates. At larger surface areas, their behavior resembled that of the unsubstituted linear polydimethylsiloxanes. The behavior of the octasiloxy acid monolayer is compared with that of a linear polydimethylsiloxane dodecamer on a 0.01 M KC1 substrate in Fig. 8. The results for

LMoo + ~ "*~b~*-u-*-q

,-'" ...~b/./1..C.N.~('-"~w"~'m"dr; jJ v s ~

1000 900 80O70o ~ .400'~.i 500600 .300

E ~o-

o@..(:.~--

.zoo .I00

* Po[ydimethytsiloxane Oodecamer -o-o-o- Octasiloxyundecanoic

Acid

.2OO

Ld 30~ tn

°'°'°~Q~°-o. o

-200

~o-

.O..o

. ....

a:

.,~o

5o y: lo-

0

~+

s'o

~o

1;o

2'oo

2'~o

30o

AREA (2k~l Motecule )

FzG. 8. Comparing the behavior of an octasiloxy acid monolayer with that of a single in. wt. linear polydimethylsiloxane dodecamer, (CH3)~Si[OSi(CH3)~h0OSi(CH~)3, on an aqueous substrate containing 0.01 M KC1, pH 6.8.

the p.d.m.s, dodecamer are in agreement with those of other workers (2, 7, 8, 9, 12) at pH 6.8, i.e., at large areas per molecule the ?r-A curve extrapolates to a limiting area of 246 A2/molecule or 20.5 ,&2/monomer unit and ~ = 1060 roD/molecule or 89 roD/monomer unit. The octasiloxy acid film follows a similar pattern of behavior at large surface areas. With decreasing area, however, there is an increase in both the surface pressure and the surface potential owing to the presence of the substituent (CH2)10COOH unit. Below 28 A2/mo]ecule, the slope of the ~-A curve of the octasiloxy acid film changes slightly, extrapolating the zero moment with compression to zero area per omolecule. This "break" in the curve at 28 A2/molecule was even more pronounced for films on dilute thorium nitrate solution (Fig. 7). Similar results have been reported by Newing (3) for linear p.d.m.s, hexamer monolayers in which the molecules contained terminal CH:OH substituent units. DISCUSSION As Gaines (19) has recently pointed, out, there has been considerable controversy over the actual molecular arrangement in an expanded or liquid monolayer. In condensed films, the molecules have a close-

Journal of Colloid and Interface Science, Vol. 35, No. 1, J a n u a r y 1971

POLYDIMETHYLSILOXANE SUBSTITUTED FATTY ACID ]VIONOLAYERS packed, well-aligned configuration, whereas in gaseous films at liquid/gas interfaces, they probably lie flat and are widely separated. In expanded films, the configuration is in some way intermediate between these extremes. Laagmuir (29) introduced the coneept that an expanded film could be thought of as a very thin liquid phase in which the hydrophobic portions of the molecule are in a random, rather than regular, orientation, the polar functional groups being constrained to be in contact with the subphase. The results of the above experiments lend support to this view. At large surface areas per molecule, the siloxane substituted undeeanoic acid molecules orient horizontally with both their hydrophilic siloxane and earboxylic acid units in contact with the aqueous subphase. Under compression, the molecules come together and energy is expended partly in overcoming the repulsive forces between the molecules and partly in forcing the molecular chains up out of the water surface. The alkane chain (CH2)10 buckles up out of the surface at very low pressures (0.2 dyne/em or less) since it is weakly adsorbed. The siloxane units are next displaced in preference to the earboxylic acid group since the attractive forces betweeu them and the aqueous surface are much weaker, as evidenced by the low collapse pressures of the linear polydimethylsiloxanes (12), Fig. 8. At low surface areas the molecules are oriented, on average, with their siloxane-alkaue chains above the surface and their polar carboxylie acid groups in the substrate phase. With increasing compression, the molecules are forced closer together such that the terminal siloxane units are as closely packed as possible and the molecular chains are aligned almost vertical to the interface. The limiting area per molecule is dependent on the cross-sectional area of the bulkier terminal siloxane units. The compressibility and viscosity of these monolayers are what might be expected for such structures. There appears to be some confusion in the literature as to the cross-sectional area of an extended siloxane chain taken at right angles to its long axis. From his studies of

11

the vapor pressure-viscosity relationships of the polydimethylsiloxane fluids, Wilcock ~0) deduced a value of approximately 40 A2/moleeule as the cross-sectional area of the siloxane chain flow unit. Other workers (2) have employed this value in the interpretation of the behavior of the linear polydimethylsiloxane monolayers at the air water interface. Newing (3), however, calculated the somewhat smaller value of 28 ~2 from molecular models of the extended siloxane chain. In Figs. 6 aud 7 it can be seen that both the disiloxy and the octasiloxy undecanoie acids can be compressed to a limiting area of 28 A2/molecule before the films exhibit instability. These results suggest that the cross-sectional area of the close-packed siloxane unit, ((CH3)~SiOSi(CH3)2), is 28 As and that, in the case of the octasiloxy derivative, the siloxane chain takes up a linear-extended configuration at high surface pressures. Molecular models of stearic (I), disiloxy (II), and oetasiloxy (III) acid molecules in the vertical, chain-extended configuration are compared in Fig. 1. The larger cross-sectional area deduced from viscosity studies of the bulk polydimethylsiloxane fluids (30) includes the area swept out by the thermally agitated siloxane units, which will be particularly pronounced in the liquid state. The siloxy acid monolayers were condensed to some degree by the presence of aqueous Th 4+ cations in solution since the limiting area of the siloxy acid molecules was of the order of magnitude of that of the hydrated thorium ions in the polynuclear surface complex, (Th(OH2)2+)~. That is, the spacing between the thorium ions (6 A) compares closely with the mean intermolecular spacing of the siloxy acid moleculeso (5.96 A) at their limiting areas of 28 A2/moleeule. Compression below 28 o AS/molecule causes the film pressure to rise sharply and the siloxy acid molecules slip past each other to form an "overfilm." The ion-dipole and dipole-dipole interactions between the COOH groups and the hydrated thorium ions in solution at pit 3.8 (26) are not sufficient to completely overcome the steric effects of the terminal

Journal of Colloid and Interface Science, Vol. 35, N o . 1, J a n u a r y 197

WILLIS

12

TABLE I SURFACE DIPOLE MOMENTS PER SIO(CH~)2 UNIT FOR THE P.D.M.S. SUBSTITUTED ll-UNDECANOIC ACIDS ON DISTILLEDWATER, PI-I = 6.8 Compound

o Area (A2/molecule)

Pressure 7r (dyncs/cm)

AV (my)

45 185 246

1.0 1.5 1.0

+74 +150 + 160

Disiloxy acid Octasiloxy acid Polydimethylsiloxane dodecamcr TABLE II

SURFACE D I P O L E MOMENTS OF THE P.D.M.S. SUBSTITUTED ll-UNDEC~kNOIC ACIDS AT T H E I R .~kREAS OF CLOSEST APPROACH

Compound

Limiting area

(~/

molecule)

Stearic acid Disiloxy acid Octasiloxy acid

19.3 28.0 28.0

,imitin *ressur, (dynes cm)

tt (mD) XV (mE (10~I c gs units)

42.0

+280 + 142 + 175 +131 +205 +1~.5

32.0

20.5

siloxane units and their associated thermal agitation. The surface potential data corroborate these views and provide specific information regarding molecular orientation and configuration during compression. In the absence of multivalent ions in solution and at large areas per molecule, the siloxy acid molecules orient horizontally. Comparison with molecular models suggests that the disiloxy ando the oetasiloxy acids would occupy 95 A2/molecule and 235 ~2/lnoleeule, respectively, in the chainextended configuration. A significant surface pressure was first detected at surface areas of this order of magnitude. In the ease of the disiloxy acid film on distilled water p H = 6.8, Fig. 6, the surface dipol%moment was zero at areas greater than 70 A2/mole cule suggesting a random orientation of the molecular dipoles in the surface. Further compression produces some molecular orientation, u increasing to 30 mD at 47 A~/ molecule and then increasing sharply to 88 mD at 45 A2/molecule. An examination of a molecular model suggests that this area can be obtained by the molecule's adopting a configuration in which the polar siloxane and earboxylic acid groups remain con-

g(mD) (10-~1 cgs units)

+88 +720 + 1065

g(mD) per SiO(CtI3)2 unit

+88 +90 +89

strained in the surface with the alkane chains in random coils above. The octasiloxy acid, Fig. 7, takes up a similar configuration at 185 A2/moleeule. The molecules adopt a close-packed configuration with the siloxane chain and the COOIt units in the surface and the alkane chain folded above. The random coiling of the alkane chain out of the surface at surface pressures of the order 0.2 d y n e / c m or less produces a considerable amount of strain in the siloxy acid molecules. The configuration of the molecule is due to the combined effect of two opposing forces: a) The attraction of the (CH2)10C00H unit for the aqueous phase and its tendency to orient perpendicular to the surface. b) The attraction of the siloxane units, S i - - 0 , for the water also, tending to orient with all the Si and 0 atoms in the surface and the methyl groups in a plane immediately above. The flexible siloxane bonds, Si0Si, are more able to accommodate the stress imposed b y the buckling up of the alkane chain and remain oriented in the surface. However, the carboxylic acid units are forced to take up random configurations oriented almost horizontally in the surface and, as such, make no contribution to the net vertical surface dipole moment. This view is endorsed b y the results shown in Table I, in which t~ per (CH3)2SiO unit was calculated assuming the carboxylic acid unit made no contribution to the vertical component of the surface dipole moment of the siloxy acid molecules in the above configuration. It can be seen from these results that at a surface pressure of approximately 1.0 dyne/era, the molecules are closely packed such that the values of u per siloxane unit

Journal of Colloid and Interface Science, Vol. 35, No. 1, January 1971

POLYDIIVIETIIYLSILOOANE SUBSTITUTED FATTY ACID MONOLAYERS are very close to that, observed for the unsubstituted polydimethylsiloxanes, i.e., 89 roD/monomer. When we take t h e limiting area per siloxane unit to be 20 A s in the aqueous surface (7, 12), the Coarboxylic acid unit occupies an area of 25 A 2 in this configuration; this compares favorably with the value estimated from the molecular models. Under further compression, the siloxane units are forced up out of the surface and this allows the (CH2)10COOH unit to orient vertically. In the case of the disiloxy acid film, Fig. 6, the decrease in u due to the disorientation of the terminal siloxane unit is compensated by an increase in u due to the vertical orientation of the carboxylic acid unit. Under pressure, the molecular chains are forced closer together and the surface dipole moment approaches that for the carboxylic acids since the moment of the siloxane unit is horizontal in this configuration. With the octasiloxy acid derivative, Fig. 7, t~ is much higher initially owing to the additive effect of the siloxane units in the surface. As the siloxane chain folds up out of the surface, u decreases and approaches a value at 28 ~_~/molecule corresponding to a close-packed, vertical configuration. In Table II, n g and tL for the stearic, disiloxy, and octasiloxy acid films are compared at their limiting areas per molecule on aqueous substrates, pI-I 6.8. Both siloxy acids have surface dipole moments which approximate that of the f a t t y acid, i.e., approximately 142 inD. On dilute thorium nitrate solution, the disiloxy acid film (Fig. 6) can beoeompressed down to approximately 25 AS/molecule before the film collapses completely to form an overfilm. The surface dipole curve indicates rather eoomplieated molecular behavior below 28 A2/molecule involving timedependent molecular rearrangements within the monolayer such that the (CH2)10COOH units become more aligned and closer packed. It is possible that over the region 28.0 to 26.0 A s this is achieved b y some sort of intermeshing of the siloxane units of the type described recently by Noll et al. (7-9). l~/Ioleeular models (Fig. 9) indicate such a configuration which would allow the (CH2)10COOH units to become more

13

FIG. 9. Molecular model illustrating possible interIneshing oi the terminal substituent siloxane units in the disiloxy undecanoic acid monolayer over the region 26.0 to 28.0 _A2/molccnte on dilute thorium nitrate solution. Such a configuration allows the (CH2)10COOtt units to become more aligned and vertically oriented giving rise to an increase in the surface dipole moment over this region 0rior to film collapse. aligned and t~ to increase over this region (region AB, Fig. 6). The strong ion-dipole and dipole-dipole interactions at the thorium nitrate solution interface stabilize the films at the high film pressures required. Below 26 A2/molecule, however, the applied surface pressure is sufficient to overcome these forces and the molecules slip past each other up out of the film with increasing pressure (region BC, Fig. 6). The octasiloxy acid monolayer exhibited

Journal of Colloid and Interface Science, ¥oi. 35, No. i, January 1971

WILLIS

14 o

no increase in ~ below 28 A2/molecule (Fig. 7). The thermal agitation associated with the polysiloxane chain is such as to prevent any completely ordered cooperativ~ intermeshing of the molecules. Below 28 A~/mole cule, the surface pressure increases sharply owing to the strong stabilizing effect of the aqueous T h 4+ ions in solution, but the molecules in the film become progressively o disoriented. Below 28 A2/molecule on thorium nitrate substrates, the siloxy acid films appear to be in what might be termed a highly compressed metastable state. CONCLUSIONS

ACKNOWLEDGMENTS The author wishes to thank Drs. D. Tabor and H. E. Ries, Jr., for their encouragement and helpful discussion, Drs. R. W. J. Williams and F. C. Saunders, Midland Silicones Ltd., Wales, U. K., and Dr. I~. Baney, DoT Corning Corp., Michigan, U. S. A., for kindly supplying materials; and the Turner & Newall postdoctoral fellowship fund for financial support. REFERENCES 1. SCHWABZ,E. G., AND REID, W. G., Ind. Eng. Chem. 56, 26 (1964). 2. Fox, H. W., TAYLOR, P. W., AND ZISMAN, W. A., Ind. Eng. Chem. 39, 1401 (1947). 3. NEWING, M. J., Trans. Faraday Soc. 46, 755 (1950).

The van der Waals forces of attraction 4. FOX, H. W., SOLOMON, E. M., ~ND ZISMAN, associated with the polydimethylsiloxanes W. A., J. Phys. Chem. 54,723 (1950). are known to be very low as~evidenced by 5. JARVlS, N. L., J. Phys. Chem. 70, 3027 (1966). their low boiling points, low surface ten6. KAKIItARA, Y., HIMMELRLAU, D. M., AND sions, low surface viscosities, and low activaSCHECHTER, I~. S., J . Colloid Interface Sei., tion energy for viscous flow (31). The short30, No. 2, 200 (1969). range intermolecular forces are prevented 7. NOLL, W., STEINBACI-I,H., AND SUCKER, CHR., from being strongly operative by the free Ber. Bunsenges. Phys. Chem. 67, 407 (1963). 8. NOLL, W., STEINDACH,H., AND SUCKER, CHIt., rotations of the substituent methyl radicals Kolloid-Z. 204, 94 (1965). and the high degree of thermal motion of the 9. NOLL, W., Kolloid-Z. 211, 98 (1966). flexible siloxane bonds. The behavior of the above dimethylsiloxane substituted un- 10. TRAPEZNIKO¥, A. A., ZATSEPINA, T. I., GRACHEVA, W. A., StICHERRA~OVA, 1:~. N.~ decanoic acids on aqueous surfaces supports ANn OGAREV,V . A., Proe. Aead. Sci. U S S R these views. The weak cohesive forces Phys. Chem. Sect. (English Transl.) 160, between the terminal siloxane units pre174 (1965). vent the formation of a clos~::p~ked, stable 11. ELLISON, A. H., AND ZISMAN,W . A., J . Phys. monolayer even in the presence of strongly Chem. 60,416 (1956). condensing tetravalent T h 4+ cations in solu- 12. JARVlS, N. L., J. Colloid Interface Sei. 29, No. 4, 647 (1969). tion. At high surface pressures, the molecules 13. BANKS, W. H., Nature 174,365 (1954). oriented vertically to the surface. The cross- 14. HUNTER, M. J., GORDON, M. S., BARItY, A. J., HYDE, J. F., AND HEIDENREICH, R. D., sectional area of the close-piacked siloxane Ind. Eng. Chem. 39, 1389 (1947). units was found to be 28 A ~ rather than 15. GREGORY,J. N., ANDNEWING,M. J., Aust. ,f. 40 A 2. The films collapsed to form an overSei. Res. A1, 85 (1948). film by a mechanism involving both molecu- 16. WILLIS,R. F., Ph.D. Dissertation, Cambridge lar rearrangement and slippage of the University, Cambridge, England, 1967. molecules past each other within the mono- 17. RIES, H. E., JR., AND COOK, I-I. D., J. Phys. layer. Chem. 60, 1533 (1956). The results of these experiments show 18. BERNETT, M. K., JARVIS, N. L., AND ZISMAN, W. A., J. Phys. Chem. 68, 3520 (1964). clearly that the nature of the polysiloxanes is such that it is unlikely that they will 19. GAINES,G. L., JR., "Insoluble Monolayers at Liquid-Gas Interfaces," Wiley, New York, form compact oriented films at interphase 1966. boundaries even if they contain strongly 20. BARNES, G. T., ELLIOT, A. J., AND GRIGG, adsorbing chemical substituent groups. This E. C. M., J. Colloid. Interface Sci. 26, 230 imposes a severe limitation on their use as (1968) for a review. protective monomolecular films between 21. WALKER,D. C., AND RIES, I~I.E., JR., Nature interacting solid surfaces. 203,292 (1964). Journal of Colloid and Interface Science, ¥ol. 35, No. 1, January 1971

POLYDIMETHYLSILOXANE SUBSTITUTED FATTY ACID MONOLAYERS 22. CRISP, D. J., J . Colloid Sci. i, 49 (1946). 23. LANGMUI:R,I., AND SCIIAEFFER,V. J., J . Amer. Chem. Soc. 59, 2400 (1937). 24. ZIS~AN, W . A., J . Chem. Phys. 9,534 (1941). 25. SPINE:, J. A., AND SANDERS, J. V., Trans. Faraday Soe. 51, 1154 (1955). 26. A~RAMSON,M. B., AND OTTEWIL~, 1%. H., J. Colloid Sci. 17,883 (1962). 27. I-IARKINS,W. D., "The Physical Chemistry of

28. 29. 30. 31.

15

Surface Films," p 135. Reinhold, New York, 1952. GODDARD, E. D., AND ACKILLI, J. A., J. Colloid Sei. 18,585 (1963). LANGMUIR,I., J . Chem. Phys. 1,756 (1933). WILCOCK, D. F., J . Amer. Chem. Soc. 68, 691 (1946). EABORN, C., "Organosilicon Compounds," Chapters 8, 15. Butterworths, London, 1960.

Journal of Colloidand Inter/aceScience, Vol.35, No. I, January 1971