THE INTERFACE SYMPOSIUM - IS
Structure of Macromolecules at Liquid-Solid Interfaces
F. ROWLAND R. BULAB E. ROTHSTEIN F. R. EIRICH
a
he hypothcsia underlying much of the work reported T below is that the adsorption of polymm is the primary step of contact-making in the formation of all polymeric interfacial bonds, such as occur in organic adhesion, in coatings, in dispersion-stabhtion (decting viscosity), and in biological membranes. It is further assumed that there is good reason for the observation that all adhcaivea are macromolecular in charactemacromolecules can adhere to the interfaces by multiple pint8 of physical adsorption as well as by extending into the adjacent phase. They thereby form a bridge of multiple van der Waah strength, a if they had diffused into the adjacent phases (which may or may not actually be true), and thus can accomplish a stitching action. The function of primers or the adherence of various paint8 or coatings can be undmtood on the same basis, a can the action of Wen. Stabilization of a dispersion is assumed to function via solvated protective jackets formed by adlorbed macromolecules. Finally, many biological structures arc of composite nature and contain folded or helical macromolecules in contact with solid materials--e.g., inorganic constituents of bone and teeth. Thus, the study of adsorption of polymers from solution is of both academic and practical value. In such a study, the experimental techniques should measure the amount of solute removed by the solid, and should recognize whether the adsorbate is present directly in the interface. Apart from the problem of how much is adsorbed, there is also the problem of finding out how the material is adsorbed. On an idealized plane surface with idealized spherical hard molecules a adsorbate,one can aeaume at first a monolayer, as described by Langmuir’s theory, which can be a densely or loosely packed condensed solid or liquid layer, or a gaseous one, with changca between the types either continuous or marked by sharp phase transitions. If more material is to be adsorbed it has to be accommodated in multilayers. When the molecules have a complicated shape, are triaxial, for instance, they can be packed on the surfaces with respect to any of their three axes and thus exhibit a 46
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tures of the mcdayen.
Inasmuch ill in cdwrptilm from solution the solvent also playa an important part, the d v e n t will also influence the structure ofthe adwDbed layer. solvent har to be desorbed from the rolid wfacc before any solute adsorption can take place and keeps on interacting with the adsorbate. For a full understanding of the total system, therefore, it would be nccarrary to know either the complete thermodynamic functions of all components More and after adsorption, as well as the relations of these functions to the molecular stata encountered; or it would be nexamy, in addition to a measure of the polymer runoved from the supernatant, to know also the fraction lying directly in the interface and, if the fraction is much less than 1, either the density or the thickness of the interfacial boundary zone. This situation will be greatly accentuated in the case of linear maeromolacules which m a y be ammed to pack poorly under many adsorptive wnditiom and to lie rather ineomplctely on an interface (Figure 1). We have wnducted studies to develop information of this kind. Our main concern w ~ dto deternine the thidmarces of adsorbed layers, in addition to measuring amounts of polymer removed from the supernatant liquid. Because we were intematcd in the c o ~ u c n c e s of adsorption for transport p~eesses,for the packing of partida, and in interactiom between adsorption-coated solid surfaces it will natural to lwk for methods which would measure adsorbed thicknesses by mccbanical ~CBIIII. In particular, we were applying mahods which measure the so-called hydrodynamically effective thickncd(lc(I or dimensions of dispersed particles or of cap& lariesaa such and in solid 6lterbcds (Figure 2). Our d t s leave, in our opinion, no doubt that under many conditimpolymers adsorb from dilute solution on r~lid-liquidinin the form of monolayers ofmolesular coils whose dimdm are proportional to thwc of the free coils in that partidar solution. Thus, the thickness of the adsorbed layer, its density, and the nature ofthemacromolccularwnformationscanbederivedfrom intrinsic VisCasity mulsunments. The adsorbed molesular coils seem to retain also the tendency of the free coils to be mutually repelling within the monolayer, as well as between monolayers on different partides. This repulsion lead& among other things, to an i n c r e a s e d stability of dispersed particles' thus coated; the more wmpletc the monolayer is, the better the stability. Given a certain degm of coating, the dispvsive paver then depends further on the degree of wetting of the uncoated particles by the solvent; on the state of solution of the macromolecules; and on the molecular weight, which must not be too low. If the polymer coils arc well extended in a good solvent, the adsorbed layer is of low density and yet protects well @ n s t inelastic collisions. Conversely, dense adsorbed layas in nearly precipitating solvents are poorly protective. H o w m , a polymer may &end far into the solvent because it has a low &ity for (few segments in) the surface. In that case, because it is more readily 48
demrbed, it wilI not stabilize &penions well. corub qucntly, an adsorption isotherm on the partida to be diapaaed and in the dmt to be d, which exhibits a high affinity but ody d a t e capacity, combined with an intrinaic vireosity of the polymer in the same solvent about twice that in a theta solvent, lhould +e the best conditions for a atablc nonionic dispersion. Exxpwimontal M.lhods
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. I -
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Adsorption from solution of low and high molecular weight materials alike is, as a rule, measured by ditference. The adsorbent is finely divided and shaken with a solution of the adhorbate. The system is centrifuged and it is m m e d that the state of adsorption is not altered thereby. The amount taken down with the adsorbent is calculated from the change of concentration in the
-- -
Fi&n 2. Changm in bydraaynamik dintnrriarr ab to aa!m@tion. FwcapiUq&u, PohiII#slnruappIM.pplirsi.r.,q sp(r ArW8qi. For tfucwtalporliclas,Btutm?; law h o l H . $ . , q H ( I 4- 2.5# +1W+ ...) ~ a d - - / Y X ~ / 3 ( d / 2 + ~ ' .Q'-poW rate; p = flcsswe; q = fl vimsiiy; = disprmd dum;I number of pntidcrw wnl dunu of dirpdsim; d PrmiJSdihnuta
-
-
supernatant liquid, and the further assumption is made that the adsorbent is not bridged by the adsorbate. The amount of solute runond, when calculated on the basic ofthe two above assumptions, is not tantamount to adsorption, except in the specific caw in which all removed matdal lies in the interface. However, to elucidate the state of dispersion created by a protective :olloid, this methcd does show how much of the latter move8 with the disprmed particles and how much is independently wntaincd in the intermicellar serum. Data arc usually presented in the form of adsorption isotherms (Figure 3) and may have to be qualified with respect to the quantity removed from the supernatant but held by bridging or entanglement.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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I n principle, one could think of methods which recognize whether the adsorbate is present directly in the interface, provided the latter can be molecularly defined, as is usually true for the interfaces between liquids, when surface tension and optical reflection methods have been applied. If the adsorbent is particulate, the latter methods cannot be used. I n the case of smooth metallic plates, ellipsometry has given good results (25, 26), as has infrared spectroscopy (9,27); the latter also can be applied to dispersions. However, as can be readily understood, these methods count the material within 3 to 30 A. from the surface, in addition to that within the van der Waals bonding distances. CONCENTRATION, c, MOLES/LITER
Figure 3. Various kinds of adsorption isotherms. Monolayers exhibit an initial straight line teith slope equal to the a&ity constant, k , and a saturation; capacity (complete monolayer) of am. Multilayers are indicated by a subscript; concentration, c, represents equilibrium concentration of supernatant I-Langmuir type, where a = (amkc)/(7 f kc) ?I-Intralayer r e p h i o n III-Intralayer condensation Poiymers follow a Tybe 11 isotherm
TABLE I. METHODS Particle Mean Diameters -
OF AVERAGING Znidi
di
Arithmetic mean
d,
Surface mean
d3
Volume-surface mean
1
~
Znidi3 ZnidS2 or 6
Pore Size Averaees dv
1
Volume mean
V,,,,
MOLECULAR WEIGHT,
M, x
= pore volume; 19 = VDore/Vtotal; C =
specific surface area.
10-5
Figure 4. Relatiue adsorbed film thickness us. molecular weight. Curve A , PS in cyclohexane; curve B, PS in benzene; curue C, V A in benzene; curve D, M M A in benzene
1
2
1
2
__ S = pore surface; Znz = K ; R % ; R = disk radius; A,, =
1
2
INTRINSIC VISCOSITY, ['7]
Figure 5. Change in adsorbed jilm thickness with intrinsic uiscosity. A , benzene at 50' C.; B, benzene at 30' C.; C, methylethyl ketone at 30' C.; D, cyclohexane at 34.2' C.
T o provide the more specific information needed in this study, the experimental methods consisted of two kinds of measurements. I n the first series, the flow rate of polymer solutions and of the corresponding pure solvents through capillaries (77) or sintered glass disks (21) was measured. In the second series, Bulas and Rothstein measured viscosities of coated and uncoated dispersions (3, 20). The average pore size of the Corning disks was obtained by a gas flow technique (22), and the pore size distribution by the mercury intrusion method (28). The amounts adsorbed per unit area were determined by independent adsorption studies on the same glass powder that was sintered into the disks. The isotherms were determined by the methods of Koral et al. (7, 74, 16) and of Ellerstein and Ullman (8, 18)and were in good agreement with those reported. The isotherms of polystyrene showed a markedly lower affinity and rapid partial reversibility. The amounts adsorbed at the surface at saturation were equivalent to approxiVOL. 5 7
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600
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VOLUME CONCENTKATION COCO,,
G
p
20 PER 100 ML
Figure 6. variation in spec@ viscosity with concentration of calcium carbonate dispersions in tetrachloroethylene. E,, Einstein curve f o r hard spheres; E,, (‘Einstein‘’ curve f o r fully dispersed CaCO,, (iriegular purticles plus surfactant); 0 , curve f o r unstabilized CuCO3 dispersion; I, II, III, IV, curves of CaCO, disperrion with O2y0 M M A of increasing molecular rveipht added
mately 2 to 8 dense segmental (monomeric) monolayers for all polymers. Great care was taken to ensure reproducibility and internal consistency of the porous disk method. A fourth power averaging calculation (Table I) was worked out (21), blanks and standards were run concurrently, the applicability of Poiseuille’s law was verified, and additional adsorption isotherms were run with stearic acid to determine the effective surface area for nonpolymeric solutes. As a final check, the adsorption of stearic acid in the filter disk showed, in fact, an average narrowing of the capillary radius by approximately 30 A . For an independent method, study of changes of hydrodynamic particle radii in dispersion, based on Einstein’s equation, was designed along the lines described by Amborski and Goldfinger ( Z ) , in collaboration with Bulas ( 3 ) . Calcium carbonate was suspended in organic solvents, and the average particle size and surface area Lvere determined by adsorption of stearic acid tagged with carbon-14, b>- sedimentation analysis, and by microscopic particle counting. An average cube root diameter (Table I) was determined and the hydrodynamically effective volume measured from a viscosity-concentration scries of suspensions which Miere made to be fully dispersed (no particle aggregation) by surface active agents. These agents could be replaced by polymers, in this case by fractions of polymethylmethacrylate, of which the lowest fraction (mol. wt. E 75,000) produced a dispersion of a viscosity almost identical with that obtained using the surface active agents; the higher fractions exhibited increasingly larger suspensoid viscosities. From these data the increase of the dispersed volume due to adsorption could be calculated according to Einstein’s equation and, by dividing it through the surface area, an effective thickness of the adsorbed layer could be calculated (Figure 2). Whereas the efficiency of the adsorbed MMA layer at full coverage as a dispersant for calcium carbonate in organic solvents under conditions of equilibrium adsorp50
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0
1
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3
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INTRINSIC VISCOSITY, [’i]yy4
Figure 7. Increase in particle radius by adsorption of M M A of increasing intrinsic aisco.rit~, I , calrulufpd US &/’A; IZ: calculated as A d J 2
tion was thus demonstrated as an incidental but important result, it became also of interest to find at which density of surface coverage--i.e., at which fraction of the surface coated, or at which coating thickness-full particle dispersion could be achieved. This study was undertaken by Rothstein (20). With the investigation slanted toward the problem of pigment dispersion in alkyd paints, a silica, an anatase, and a phthalocyanine powder were chosen as adsorbents; three molecular weights of three fatty acid-extended glycerophthalic polyesters were chosen as polymers; and xylol, n-butyl acetate, and methylethylketone xvere chosen as solvents. All adsorption isotherms were determined and the viscosities of a given concentration of the dispersions measured as a function of relative surface coverage as derived from the adsorption isotherm in a given system of pigment, polymer, and solvent. The details of the results have been discussed and published (79). Some facts particularly relevant to our present discussion shall be discussed later in conjunction with the other results. The materials used, apart from the already mentioned Corning glass, were six fractions of bulk polymerized polyvinyl acetate, M , 75,000 to 1,200,000; six fractions of a commercial polystyrene, M , 60,003 to 1,400,000; and a Sartomer polymethylmethacrylate freed from gel and fractionated into five fractions from M r v 75,000 to 1,400,000. The latter material and some
F . R. Eirich is Professor o f Chemistry at Polytechnic Institute of Brooklyn. n f a t e r i a l i n this article i s taken f r o m dissertations submitted by the other authors in partial f u l j l l ment of requirementsfor the degrees of M S c . and Ph.D. at that college. R. B u l a s is now Senior Chemist, International Gorp., CliJton, N . J.; E . Rothstein is Research Engineer, K e u f e l and Esser Co., Hoboken, iV. J.; and F . Rowland i s Research Chemist, Carothers Research Laboratory, E. I. Du Pont de Nemours and Co. F . Rowland wishes to acknowledge Jnancial subjort f r o m the AFOSR. AUTHORS
slightly different fractions of it were also used by Bulas, who employed a suspension in benzene or tetrachloroethane of a calcium carbonate No. 2924 supplied by Whittaker, Clark, & Daniels of a volume average particle diameter d, = 4.4 x cm. T h e alkyds and pigments used by Rothstein are described in detail (79, 20). The Mw varied from 4 to 12,000, the oil length from 30 to 60%. Well defined fractions were used. T h e silica was Berkshire ground quartz, the anatase was Titanox ANO, and the phthalocyanine was GAF heliogen. All polymers were characterized by their intrinsic viscosity and all pigments by particle size. Results and Discussion
T h e adsorption of polymers from solution on liquidsolid interfaces exhibits isotherms which closely resemble Langmuir’s. T h e explanation for this curious finding is probably that if adsorbed particles have a tendency to repel one another within the surface layer, this produces an isotherm similar to Langmuir’s but with lower capacity. Adamson (7) and De Boer (5)have shown that this can be quantitatively explained by a repulsion term in the equation of state for the surface layer. This is briefly explained by Figure 3 , Generally, any adsorption process which yields a monolayer will have an initial linear slope proportional to affinity times concentration and show saturation or a plateau, so that a t least the low and high concentration sectors of any isotherm can be characterized by two constants as if it were a Langmuir isotherm. I n addition, one is interested in the fraction of the adsorbing surface which is covered in a particular case, and in the thickness and structure of the adsorbed layer. Previous work elsewhere (4, 77-73) and at the Polytechnic Institute of Brooklyn (7, 14, 76) has established a number of important basic facts. First, there is the preponderance of isotherms whose shapes indicate the presence of monolayers of some kind. Second, the amounts held at the interface at or near equilibrium saturation are, in most cases, appreciably larger than can be accounted for by a monolayer of monomer units, with the exception of the adsorption of polyelectrolytes on surfaces of opposite charge when capacities of mono-
mer monolayers are often approached. Third, the saturation values depend weakly on molecular weight. Fourth, there is a weak dependence on temperature which can be positive or negative. Compensations of the accompanying enthalpy and entropy changes probably account for this small temperature dependence. Finally, there is a strong dependence on the solvent. A number of theories have been developed to explain these observations-notably those by Frisch, Simha, and Eirich (70,24), by Silberberg (23),and, more recently, by DiMarzio and coworkers (6). These theories differ primarily in the way the polymer is pictured at the interface. T h e first theory considers that the polymer will, in most cases, be only partially adsorbed, with short sequences of segments held on the surface and random loops extending into the solvent. Should the forces of surface-polymer attraction become strong, or the solvent be poor, then the polymer will lie more and more in the interface. Silberberg’s theory assumes, on the contrary, a preponderance of positions in or near the surface with looping into the solvent only in the case of weak interactions. There are other physical and mathematical differences which, however, need not be considered here. More recent theories concern themselves largely with the choice of the right statistics of conformation counting. All findings to date say little with respect to the structure of the polymeric layer. Certain of the possible structures can be excluded-for instance, a dense packing of the polymer molecules, standing on end like soap molecules in a condensed water-air interface, would lead to a capacity dependence on molecular weight much higher than is observed. O n the other hand, the low molecular weight dependence of the capacity found is ambiguous because the flat model as well as the random coil or looped model give a dependence of M 0 . T h e thickness of adsorbed polymer layers would be independent of molecular weight in the case of Silberberg’s model; nearly proportional to it for stiff, end-on packed rods; and to M112for random coils. T h e models are therefore better distinguished by thickness than by capacity or affinity measurements (Figure 1). Our results show now that the thicknesses in a wide variety of physical adsorptions are generally of the order
0 RELATIVE ADSORPTION, A,e,
Figure 8. Specific viscosity of silica-alkyd suspensions as a function of solvent and surface coverage. A , from xylol; B , from n-but$ acetate; C, from methylethyl ketone
0.2
04
06
08
10
RELATIVE ADSORPTION, A,e,
Figure 9. Specific viscosity of alkyd-anatase suspensions as a function of solvent and surface coverage. A , from xylol; B, from n-but31 acetate; C, from methylethyl ketone VOL. 5 7
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of the diameter of the free coils in solution-that is, approximately proportional to LW’’~. A convincing way to express this is to plot a relative thickness, such as the ratio R” of the thickness adsorbed, Ar, to the mean square diameter of the free molecule in solution 23-i.e., R* = Ar/2p. If this is done, an interesting difference can be observed between polystyrene (PS) on the one hand, and methylmethacrylate (MMA) and vinyl acetate (VA) on the other. AS shown in Figure 4, the ratio falls off with molecular weight for PS, but rises for MMA and VA. Another interesting fact is that the values of Ar always rise faster than the adsorption isotherms, so that the final thicknesses reach their limiting value prior to the saturation of the surface. When the solvent power changes, the thicknesses reflect this in a sensitive manner-that is, the coils reach further into the solvent the better it is, and contract toward the surface with a poorer solvent. Interestingly enough, in theta solvents the relative thickness R* no longer varies with molecular weight. Changes of the temperature change R* relatively little. Altogether, the changes for Ar and R* closely follow those which the free molecules would undergo in solution (as measured by intrinsic viscosity)SO much so, in fact, that a plot of Ar us. intrinsic viscosity, whether affected by solvent, temperature, or molecular weight, is linear over a wide range for the ester polymers and for polystyrene in the theta solvent (Figure 5). When adsorbed thicknesses at constant molecular weight are plotted against amounts adsorbed, an almost constant value of Ar was found even though the amounts adsorbed almost triple from the best to the poorest solvent. This means that adsorbed layers become much more compact as the solvent power decreases. Because this is about the same factor of densification which the free polymer coil experiences when the solvent changes in the above manner, the increase of compactness found on the surface does not necessarily indicate coil interpenetration. On the other hand, depending on the validity of the surface area determinations, the coils are about two to six times denser at the surface than in free solution, a state which is compatible with the view of coils adsorbed on multiple sites, but still reaching extensively into the solution. If thicknesses and amounts adsorbed are compared in the same solvent at rising molecular weight, fairly straight lines are obtained which do not point to the origin of the diagram. Because adsorption measurements at lo\+molecular weight are difficult, an extrapolation may be permissible, the interpretation of which indicates that low molecular weights of all polymers form relatively short loops and lie more directly on the surface, but that longer l o o p occur above a critical molecular weight. I n the case of PS on a polar surface, Ar rises rapidly at first to almost the free coil value at a molecular weight of 50,000-100,000 and then it declines; in the case of the ester pol>-mers, Ar rises eventually to approximately one half of the free coil diameter. All of these data are compatible with the view that these polymers are adsorbed in the form of monolayers of coils which are somewhat, but not much, denser than in solution and which 52
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
have dimensions perpendicular to the surface which are substantial fractions of the free coil diameters. I n view of the many assumptions on which these conclusions had to be based, the results obtained by Bulas ( 3 ) may be considered as independent confirmation. He found that the increase in radius of dispersed particles caused by adsorption, calculated according to Einstein’s equation, agreed extremely well with the A7 values established by Rowland, and, moreover, the same h e a r dependence was found between adsorbed thicknesses and intrinsic viscosities (Figures 6 and 7). Rothstein (79, 20) slanted his investigation toward the problem of pigment dispersion in alkyd paints and compared states of dispersion, as disclosed by reduced viscosities, with degree of surface coverage by adsorbed alkyd molecules, as established by adsorption isotherms. The adsorbed thicknesses were again a linear function of intrinsic viscosity, even though in this case the molecular weights were low. The adsorption could be shown to be a sensitive function of details of polymer structure and of solvent-again, the amounts adsorbed decreased with solubility. The thicknesses increased with the fatty acid content (oil length) of the esters, which may be explained by decreasing affinity toward polar surfaces of increasingly hydrocarbon-rich polyesters. There is, again, a uniform and rapid increase in the density of the adsorbed layer as the film thickness drops with decreasing solvent power. Most interestingly, there is a striking decrease in viscosity-that is, increase in dispersing power-with relative surface coverage. The critical percentage of coverage which is required to produce complete dispersion (minimum viscosity of the dispersed system) varies greatly, as shown in Figures 8 and 9. When thc wetting power of the solvent for the pigment is smaller, or when the alkyd solubility in a given solvent is poorer, the surface coverage must be more complete if good dispersion is to be achieved. Similar findings have been discussed recently by LaMer and Healy (75). REFERENCES (1 ) Adamson, A,, “Physical Clicinistiy of Surfaces,” Inrerscicnce, S e w Y o l k . 1960. (2) Amtorski, L. E., Goldfinger, P ~ cInt. . Coil. dMacromo!ecules: Xmstcrdnm, 1949. (3) Bulas, R., thesis, Polytechnic Institute, Brooklyn, X. Y . , 1963. (4) Claesson, I., Claesson, S., Phys. Review 7 3 , 1221 (1948). (5) D e Boer H., “ D y n a m i c C h a r a c t e r of Adsorption,” Ciarendon Press, O X ~ O I ’ ~ , 1953. ( 6 ) DiMarzio, E. A,: Peyser, P., Hoeve, C . A . J., J. Cirem. Phq5. 42; 2558 (1965,. (7) Eirich, F. R., Consiglio Naz. Ricerce, R o m a (1963). (8) Ellerstein, S., Ullman, R., J.Polymrr Sci. 5 5 , 123 (19611. ( 9 ) Fontana, B. J., Thomas, J. R., J.P h p Chrm. 6 5 , 480 (1961). (10) Frisch, H., Simha, R., J. Chem. Phy5. 27, 702 (1957). (11) Gottlieb, M, J.Phjs. Chem. 64, 427 (1960). (12: Hobden, J., Jellinek, H., J.Polymer Sci. 11, 365 (1953). (13) Jenkel, E., R u m b a c h . B.. Z. Elekirochemie 5 5 , 612 (1951). Koral, J,, Ullman, R., Eirich, F. R . . J. Pizj~s.Ciiem. 62, 541 (1958). LaMer, V. K., Healy, T. I\‘,, Rea. Putt. A@!. Chem. (Australia) 13, 112 (1963). (16) Lauria, R., thesis. Polytechnic Institute, Brooklyn, S . Y . ,1962. (17) O h m , O., AI& Kemi 12, 397 (1958). (18; Perkel, R., Ullman, R., J.PolytnerSci. 54, 127 (1961 j. (19) Rorhstein. E., Of, Dipsst Paint M a n u f . 3 6 , 479 (1964). ( 2 0 ) Rothstein, E.: thesis, Polytechnic Institute, Brooklyn, N. Y . , 1964. (21 j Rowland, F., Ibid., 1963. (22) Schwartz, F. A., J. A p p l . Pitjs. 20, 1070 (1949). (23) Silberberg, A,, J.Phys. Chem. 6 6 , 1872, 1884 (1962). (24) Simha, R., Frisch, H., Eirich, F. R., Zbid.,57, 584 (1953,. (25) Stromberg, R . R . , Passaglia, E., J.Res. .Vostl. Bur. Stand. (1964). (26) Stromberg, R . R., Passaglia, E.. Tutea. D . J., Ibid.,67A, 431 (1963). (27) Thies, C., Peyser. P., Ullman. R., Proceedings, 4th International C>onSr.e\\on Surface Activity, 1964. ( 2 8 ) TVinsiow: N., Shapiro, J.. A S T M Bu!!. 236, Philxdelphia. Pa., 1959.
e.,
