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Studies in amphipathic adsorption. I. The adsorption of polyvinyl alcohol on silver bromide
STUDIES IN AMPHIPATHIC ADSORPTION. I. THE ADSORPTION OF POLYVINYL ALCOHOL ON SILVER BROMIDE * S. E. Sheppard~ A. S. O'Brien and G. L. Beyer Communication No. 1067 from the Kodak Research Laborotories, Rochester 4, N. Y. Received February/~, 19/~6 INTRODUCTION
The term "amphipathic" was originally employed by Hartley (1) to refer to the dual character of particles, micelles and (macro) molecules which have both hydrophile and hydrophobe groups operative in the same structure or system. We have used the term in reference to adsorption for cases where the adsorption of an amphipathic molecule or micelle to a solid or liquid involves a primary step of one-sided irreversible adsorption (sometimes termed pseudo-adsorption), which we will distinguish as ¢~-adsorption, followed by n secondary step of reversible adsorption, which we will distinguish as 0-adsorption, effeeted by the antithetic side or aspect of the molecule or micelle in question. In most cases so far studied the primary ¢~-adsorption has occurred by attachment of polar hydrophile groups to a polar surface, leaving a nonpolar hydrophobe grouping exposed as the new surface to which similar groups of free amphipathic molecules could attach themselves, in general reversibly, but with development of a new polar hydrophile surface. The primary stage generally gives rise to coagulation and precipitation of the coated particles (to which is attributed the phenomenon of the "sensitization" of hydrophobic inorganic colloids by hydrophile organic colloids (2)), while the secondary 0-adsorption leads to a peptization or deflocculation of the precipitated material. The first example of such behavior examined in these laboratories was the adsorption of basic cyanine dyes to silver broraJde (3), which gave a method for determining the specific surface of the dispersed phase of the silver bromide sol and, furthermore, estimates of the orientation of the adsorbed molecules to the surface. This procedure, in which the adsorbate consists, in selected cases, of molecularly dispersed particles of relatively low molecular weight (ca. M.W. 400), and of rather accurately * Beside that of gelatin (Sheppard, S. E., Lambert, R. H., and Swinehart, D., J. Chem. Physics 13, 372 (1945)), a number of orientating investigations have beer~ made by these Laboratories and will be reported on later. 213
2]4
S.E.
SHEPPARD, A. S. O'BRIEN AND G. L. BEYER
known composition and constitution, is of great assistance in studying the C-adsorption of high molecular substances. EXPERIMENTAL
The silver bromide sol was prepared by slight modification of a procedure already described (4). It was 0.020 M to AgBr and contained 10-a M bromide ion. Another sol containing 10-3 M silver ion was prepared by a similar method. Since PVA * ~s not a material of unique and identically reproducible composition and constitution,~ some particulars are in order as to the specimens used. Material A was prepared by the Organic Research Department of these Laboratories by alkaline deacetylation of a polyvinyl acetate of relatively low viscosity correspolading to a M.W. of about 20,000-25,000. It still contained 1.5% of undecomposed polyvinyl acetate and was unfractionated. Material B was a fractionated portion (by ace-
2°F
Z
2
--'---
~o 4~
'
0
20
o
40
60
60
~
I00
120
140 . 160
180
200
RESIDUAL CONCENTRATION P.V.A.,mgm. PER LITER Fro. 1
Adsorption of Polyvinyl Alcohol on the Dispersed Phase of the Silver Bromide Sol [] 10-3 M to Bromide Ion; (9 10 -3 M to Silver Ion
tone-water) of M.W. 47,000 prepared by the same laboratory by acid hydrolysis (deacetylation) of a polyvinyl acetate of correspondingly high M.W. It likewise contained a residual 1.5% of acetate and also about 3% of polyvinyl acetal. Material A was used for the analytical adsorption e x p e r i m e n t s - Table I and Fig. 1, and Table II while the coagulation and peptization cycles were carried out with both A and B. No significant difference of behavior was observed between them. It may be noted that these ad* PVA is the abbreviation for polyvinyl alcohol. t We t h a n k Dr. C. J. Staud for calling our a t t e n t i o n to this question and Dr. G. Waugh for supplying the data on composition.
215
A M P H I P A T H I C ADSORPTION
s o r p t i o n e x p e r i m e n t s a r e c a r r i e d o u t on m a t e r i a l in d i l u t e a q u e o u s s o l u tion. I t is p r o b a b l e t h a t t h e s i g n i f i c a n t differences o b s e r v e d w i t h d i f f e r e n t b a t c h e s of P V A , w h i c h ~re c o n n e c t e d w i t h differences in t h e m o d e s of p r e p a r a t i o n a n d , t o s o m e e x t e n t , w i t h s m a l l differences in c o m p o s i t i o n , a p p e a r w i t h h y d r o g e l s a n d w i t h sols a n d d o p e s of h i g h c o n c e n t r a t i o n . TABLE I Adsorption of P VA to the Dispersed Phase of the Silver Bromide Sol Mg. PVA Added per 25 ml. Sol 0.50 0.80 1.10 1.60 3.00 3.40 3.70 4.00 5.00 6.00 7.00 8.00 0.80 1.10 1.60
2.00 3.00 4.00 5.00
Mg. PVA Adsorbed Residual Conc. per 25 ml. Sol PVA mg./liter In 10-~ M Bromide Ion 0.50 0.0 0.80 0.0 1.05 1.2 1.24 8.9 1.1..9 45.3 1.24 54.0 1.58 53.0 1.24 69.0 1.29 92.7 1.28 118 1.30 142 1.22 169 In 10-~ M Silver Ion 0.57 5.7 0.86 6.0 1.20 9.7 ! .30 17.4 1.32 42.0 1.48 62.5 1.50 87.5
x/m mg./g. 5.3 8.5 l 1.2 13.2 12.7 13.2 16.8 13.2 13.7 13.6 13.8 13.0 6.1 9.2 12.9 13.8 14.0 15.7 15.9
TABLE II Displaccment of Adsorbed P V A by Dye I Vb big. l)yc Added 0.48 0.80 1.12 1.60 2.08 2.56 3.20
PVA Desorbcd ~Ig./G. 2.4 6.6 9.0 -9.4 -9.9
Per C~entPVA Desorbed 18 5l 69 -72 -76
T h e a d s o r p t i o n of P V A t o t h e sol was t e s t e d as f o l l o w s : T o 50-ml. g l a s s - s t o p p e r e d c e n t r i f u g e t u b e s w e r e a d d e d k n o w n a m o u n t s of P V A s o l u t i o n , a c o n s t a n t a m o u n t of buffer a t p H 6 a n d w a t e r t o m a k e a t o t a l v o l u m e of 15 ml. T h e n 25 ml. of silver b r o m i d e sol ( c o n t a i n i n g 0.094 g. A g B r p e r 25 ml.) were a d d e d a n d t h e t u b e s were t u m b l e d a t r o o m t e m p e r a -
216
s.E.
SHEPPARD,
A.
S.
O'BRIEN
AND
G.
L.
BEYER
ture for 16 hours, then centrifuged at 2200 r.p.m, for 1 hour. Portions of the supernatant liquid were analyzed for their PVA content by a method of permanganate oxidation to be described elsewhere (5). Data for two series of experiments at pH 6, one at 10-~ M silver ion and one at 10-3 M bromide ion, are given in Table I, and graphically presented in Fig. 1. For comparison, Fig. 2 represents the data obtained in the adsorption of Dye IVb to silver bromide sol which is 10-3 M to bromide ion. 12
m
~,~o
~
(o
.... G
6
4
E"
2
0
0
t I I I I I ! 2 3 4 S 6 RESIDUAL CONCENTRATION D.YF "1~ b, morn. PER LITER
FIG. 2 Adsorption of Dye IVb to AgBr Sol 10-3 M to Bromide Ion
EXPERIMENTS ON REVERSIBILITY Attempts were made to reverse the adsorption by shaking in water at 25°C. After 45 hours, not more than 0.1-0.2 mg. of polyvinyl alcohol was removed per g., which is c a . 1-1.5% of the total adsorbed. Boiling the same quantities of polymer-coated silver bromide for 3 hours removed only 1.8 mg./g. AgBr, or c a . 13% of the 13-14 mg. originally adsorbed. BEHAVIOR OF BASIC CYANINE DYE
Experiments in a medium 10-3 M to bromide ion and at pH 6 showed that, when the silver bromide surface had Dye IVb (3,3'-diethyl-9-methyl thiacarbocyanine bromi.de) adsorbed just sufficient to saturate the surface, PVA, in any concentration, was quite unable to displace any dye. Varying amounts of PVA were added to the dyed silver bromide in centrifuge tubes which were tumbled for 16 hours. Not only was no dye displaced from the silver bromide but no polymer was adsorbed from the
217
AMPHIPATHIC ADSORPTION
solution on the dye coating at any concentration of PVA. Thus, the dye completely "protected" the silver bromide against the PVA. Similar experiments indicate also that the dye very largely (up to 75% at least) displaces PVA on the silver bromide surface, when just sufficient PVA had been added to saturate the surface. AMPHIPATHIC ADSORPTION
In several respects the behavior of PVA on silver bromide is similar to that of gelatin (4). Some experiments, therefore, were made on the coagulation and peptization of the silver bromide sol by dilute PVA solutions by the procedure described in the reference above. The results are presented graphically in Fig. 3, and it will be seen that the curve is
1"2 1.0 f
o
~
e
0.8 J ~ 0.6 o ~ 0.4 ~0.2
0
0
2
III
4
t l l l l l l I I
6 S I0 12 14 16 mgm. PVA ADDED PER grn. AgBr
IS
20
22
Fro. 3 Flocculation-Peptizationof AgBr Sol by PolyvinylAlcohol;pH = 4.9 generally similar to those obtained with gelatin but without pH effect. The peptization point is at 6.5-7.0 mg./g. AgBr for both Materials A and B, a value which must be taken as provisional until more data are obtained on different specimens of PVA. EFFECT OF ADSORPTION TO ~ILVER ]:~ROMIDE ON 1)OLYVINYL ALCOHOL
When t,he eoagul'~ted (precipitated) PVA-eoated silver bromide was (~entrifuged prior to peptization in excess solution of PVA, it was observed lhat this material dissolved much more slowly in a saturated solution of potassium bromide than the original silver bromide particles uncoated with PVA, several hours in the cold plus about one hour at 80 ° to 90°C. being necessary. When the silver bromi(le w ~ finally dissolved, the
218
s.E.
StIEPPARD, A. S. O'BRIEN A.ND G. L. BEYER
polymer coating retained the form of the precipitated and centrifuged mass and floated in the solution of potassium bromide as a semitransparent disk. This was insoluble in cold water but dissolved readily when the temperature was raised to 60°C. The observations suggest that, not only is PVA made relatively insoluble as and when adsorbed to a silver bromide surface, but it may have been made permanently less soluble after removal (or solution) of the silver bromide. The solution of potassium bromide, from the position of its ions in the lyotrope series, might be expected to have a peptizing rather than an insolubilizing effect on the PVA; the experiments will be repeated and diversified in an effort to specify and clarify more completely the meaning of "solubility," "melting point" and "minimum solution temperature" when applied to such substances as gelatin and polyvinyl alcohol. DISCUSSION Examination of the data given in Table I and Fig. 1 shows that the adsorption of PVA to silver bromide resembles that of several eyanine dyes (3) in proceeding to an irreversible saturated layer. The saturation value found at 25°C. for the standard AgBr sol was 13-14 mg. PVA/g. AgBr, which is near that found, I0 mg./g., for the basic eyanine dye, IVb. The dye has a M.W. of c a . 400, and the PVA one of c a . 40,000, so that it is probable that the p.olymer axis or backbone would be lying parallel to the AgBr surface but the rest of the molecule more or less edgewise thereto. It is noteworthy that there seemed no tendency to formation of further layers over the primary layer at saturation. The fact that the weight/g, of this coating appears to be lust twice the value for the "peptization point" (6-7 mg./g. AgBr) may be fortuitous, otherwise there are two interpretations: (I) The peptization point represents a completed monolayer with a hydrophobe exterior, in which case the saturation level indicates a completed duplex layer, but both compound monolayers are irreversibly adsorbed; or (2) The saturation level represents a monolayer, but one such that hydrophile properties appear at 50v/v coverage. The structure suggested from Xoray diffraction data (6) for orientated PVA is not inconsistent with a duplex layer (7) which might have an alternate folding, whereby - - O H polar groups are effectively presented to both the silver bromide surface and to water.* Some quasi-electronic * A further, perhaps equivalent, kind of hydrogen bonding might be occurring somewhat in the fashion suggested by L. Michaelis (8) for the polymerization of the free
AMPHIPATHIC ADSORPTION
219
continuity of the - - O H groups of PVA is suggested by recent observations that the intensity of the electrical anisotropy of stretched PVA films is considerably increased by previous impregnation of the PVA sheet with precipitated silver bromide.* The behavior of PVA on silver bromide shows that the phenomenon of agglutination --~ peptization consequent on addition of an amphipathic molecule or macromolecule to a hydrophobe sol may involve variants in the mechanism, even though this seems to be fundamentally a matter of alternation of polar and nonpolar interactions. So far, the rate of adsorption in the primary phase (¢~-adsorption) has appeared, in absence of other interfering bodies, to be very rapid, but it is likely that the later phases of the adsorption act involve relatively slow processes of diffusion and molecular accommodation (orientation) on the surface of the solid. It is hoped to gain some information about these by time-studies of the interracial tension and contact angle of water and, e.g., cyclohexane, at the evolving surface. The pseudo-adsorption or irreversible adsorption dealt with in this and related studies need not be considered as contradicting the broad principle embodied in the Gibbs Theorem, but as standing in much the same relation thereto as crystallization or precipitation to the behavior of a solution, i.e., essentially as a discontinuity in any functional representation. The adsorbed dye or colloid, which becomes insoluble in the liquid from which it is deposited, is bonded to the adsorbent by chemical forces of high magnitude, greater than any existing between water an(l the hydrophobe organic part of the adsorbed molecule. SUMMARY
The adsorption of polyvinyl alcohol to silver bromide has been measured and found to be irreversible (C-adsorption). No secondary reversible ~adicals of W(irster dyes. I n the case of PVA, it would have to be assumed t h a t
/
H
\ H
groups attached to --OH groups, which themselves,when in orientated chains (ill the polymer), form resonating systems, take oll properties similar to the alternate of / ~N +
\
H
I[ an d
\ /
N---
H H Michaelis's configuration. Naturally, this remains speculation unfit electron diffraction studies can be made of the adsorbed PVA. Something similar m a y be present in the polymeric structures of boron hydride and of aluminum trimethyl (9). * Unpublished experiments of S. E. Sheppard and P. T. Newsome.
220
s. E, SHEPPARD~ A. S. O~BRIEN AND G. L. BEYER
adsorption appeared to occur but there was precipitation of silver bromide sol followed by peptization with excess polyvinyI alcohol. The peptization point was found to occur at an adsorption value 6.5 to 7.0 mg./g., just half that at which saturation occurred. Adsorbed basic (cyanine) dye is not displaced from silver bromide by PVA but the latter is very largely displaced by basic dye. I~EFERENCES 1. HARTLEY, G. S., Trans. Faraday Soc. 37, 130 (1941).
2. 3. 4. 5. 6. 7. 8. 9.
SHm,PAPm, S. E , Nature 155, 266 (1945). SHEPPARD,S. E., LAMBERT,R. H., ANDWALKER,R. D., J. Chem. Phys. 7, 265 (1939). SHEI~PARD,S. E., L£~IBERT,R. H., AND.SwINEHAR%D., J. Chem. Phys. 13, 372 (1945). Paper in preparation, by A. S. O'Brien. MOONEY,R. C. L., J. Am. Chem. Soc. 63, 2828 (1941). SHEPPARD,S. E., AND Nv.WSOME,P. T., J. Chem. Phys. 12, 513 (1944). MICHAESm,L., AND GRANICK,S., J. Am. Chem. Soe. 65, 1747 (1943). BURAWOY,A., Nature 155, 328 (1945).
The term "amphipathic" was originally employed by Hartley (1) to refer to the dual character of particles, micelles and (macro) molecules which have both hydrophile and hydrophobe groups operative in the same structure or system. We have used the term in reference to adsorption for cases where the adsorption of an amphipathic molecule or micelle to a solid or liquid involves a primary step of one-sided irreversible adsorption (sometimes termed pseudo-adsorption), which we will distinguish as ¢~-adsorption, followed by n secondary step of reversible adsorption, which we will distinguish as 0-adsorption, effeeted by the antithetic side or aspect of the molecule or micelle in question. In most cases so far studied the primary ¢~-adsorption has occurred by attachment of polar hydrophile groups to a polar surface, leaving a nonpolar hydrophobe grouping exposed as the new surface to which similar groups of free amphipathic molecules could attach themselves, in general reversibly, but with development of a new polar hydrophile surface. The primary stage generally gives rise to coagulation and precipitation of the coated particles (to which is attributed the phenomenon of the "sensitization" of hydrophobic inorganic colloids by hydrophile organic colloids (2)), while the secondary 0-adsorption leads to a peptization or deflocculation of the precipitated material. The first example of such behavior examined in these laboratories was the adsorption of basic cyanine dyes to silver broraJde (3), which gave a method for determining the specific surface of the dispersed phase of the silver bromide sol and, furthermore, estimates of the orientation of the adsorbed molecules to the surface. This procedure, in which the adsorbate consists, in selected cases, of molecularly dispersed particles of relatively low molecular weight (ca. M.W. 400), and of rather accurately * Beside that of gelatin (Sheppard, S. E., Lambert, R. H., and Swinehart, D., J. Chem. Physics 13, 372 (1945)), a number of orientating investigations have beer~ made by these Laboratories and will be reported on later. 213
2]4
S.E.
SHEPPARD, A. S. O'BRIEN AND G. L. BEYER
known composition and constitution, is of great assistance in studying the C-adsorption of high molecular substances. EXPERIMENTAL
The silver bromide sol was prepared by slight modification of a procedure already described (4). It was 0.020 M to AgBr and contained 10-a M bromide ion. Another sol containing 10-3 M silver ion was prepared by a similar method. Since PVA * ~s not a material of unique and identically reproducible composition and constitution,~ some particulars are in order as to the specimens used. Material A was prepared by the Organic Research Department of these Laboratories by alkaline deacetylation of a polyvinyl acetate of relatively low viscosity correspolading to a M.W. of about 20,000-25,000. It still contained 1.5% of undecomposed polyvinyl acetate and was unfractionated. Material B was a fractionated portion (by ace-
2°F
Z
2
--'---
~o 4~
'
0
20
o
40
60
60
~
I00
120
140 . 160
180
200
RESIDUAL CONCENTRATION P.V.A.,mgm. PER LITER Fro. 1
Adsorption of Polyvinyl Alcohol on the Dispersed Phase of the Silver Bromide Sol [] 10-3 M to Bromide Ion; (9 10 -3 M to Silver Ion
tone-water) of M.W. 47,000 prepared by the same laboratory by acid hydrolysis (deacetylation) of a polyvinyl acetate of correspondingly high M.W. It likewise contained a residual 1.5% of acetate and also about 3% of polyvinyl acetal. Material A was used for the analytical adsorption e x p e r i m e n t s - Table I and Fig. 1, and Table II while the coagulation and peptization cycles were carried out with both A and B. No significant difference of behavior was observed between them. It may be noted that these ad* PVA is the abbreviation for polyvinyl alcohol. t We t h a n k Dr. C. J. Staud for calling our a t t e n t i o n to this question and Dr. G. Waugh for supplying the data on composition.
215
A M P H I P A T H I C ADSORPTION
s o r p t i o n e x p e r i m e n t s a r e c a r r i e d o u t on m a t e r i a l in d i l u t e a q u e o u s s o l u tion. I t is p r o b a b l e t h a t t h e s i g n i f i c a n t differences o b s e r v e d w i t h d i f f e r e n t b a t c h e s of P V A , w h i c h ~re c o n n e c t e d w i t h differences in t h e m o d e s of p r e p a r a t i o n a n d , t o s o m e e x t e n t , w i t h s m a l l differences in c o m p o s i t i o n , a p p e a r w i t h h y d r o g e l s a n d w i t h sols a n d d o p e s of h i g h c o n c e n t r a t i o n . TABLE I Adsorption of P VA to the Dispersed Phase of the Silver Bromide Sol Mg. PVA Added per 25 ml. Sol 0.50 0.80 1.10 1.60 3.00 3.40 3.70 4.00 5.00 6.00 7.00 8.00 0.80 1.10 1.60
2.00 3.00 4.00 5.00
Mg. PVA Adsorbed Residual Conc. per 25 ml. Sol PVA mg./liter In 10-~ M Bromide Ion 0.50 0.0 0.80 0.0 1.05 1.2 1.24 8.9 1.1..9 45.3 1.24 54.0 1.58 53.0 1.24 69.0 1.29 92.7 1.28 118 1.30 142 1.22 169 In 10-~ M Silver Ion 0.57 5.7 0.86 6.0 1.20 9.7 ! .30 17.4 1.32 42.0 1.48 62.5 1.50 87.5
x/m mg./g. 5.3 8.5 l 1.2 13.2 12.7 13.2 16.8 13.2 13.7 13.6 13.8 13.0 6.1 9.2 12.9 13.8 14.0 15.7 15.9
TABLE II Displaccment of Adsorbed P V A by Dye I Vb big. l)yc Added 0.48 0.80 1.12 1.60 2.08 2.56 3.20
PVA Desorbcd ~Ig./G. 2.4 6.6 9.0 -9.4 -9.9
Per C~entPVA Desorbed 18 5l 69 -72 -76
T h e a d s o r p t i o n of P V A t o t h e sol was t e s t e d as f o l l o w s : T o 50-ml. g l a s s - s t o p p e r e d c e n t r i f u g e t u b e s w e r e a d d e d k n o w n a m o u n t s of P V A s o l u t i o n , a c o n s t a n t a m o u n t of buffer a t p H 6 a n d w a t e r t o m a k e a t o t a l v o l u m e of 15 ml. T h e n 25 ml. of silver b r o m i d e sol ( c o n t a i n i n g 0.094 g. A g B r p e r 25 ml.) were a d d e d a n d t h e t u b e s were t u m b l e d a t r o o m t e m p e r a -
216
s.E.
SHEPPARD,
A.
S.
O'BRIEN
AND
G.
L.
BEYER
ture for 16 hours, then centrifuged at 2200 r.p.m, for 1 hour. Portions of the supernatant liquid were analyzed for their PVA content by a method of permanganate oxidation to be described elsewhere (5). Data for two series of experiments at pH 6, one at 10-~ M silver ion and one at 10-3 M bromide ion, are given in Table I, and graphically presented in Fig. 1. For comparison, Fig. 2 represents the data obtained in the adsorption of Dye IVb to silver bromide sol which is 10-3 M to bromide ion. 12
m
~,~o
~
(o
.... G
6
4
E"
2
0
0
t I I I I I ! 2 3 4 S 6 RESIDUAL CONCENTRATION D.YF "1~ b, morn. PER LITER
FIG. 2 Adsorption of Dye IVb to AgBr Sol 10-3 M to Bromide Ion
EXPERIMENTS ON REVERSIBILITY Attempts were made to reverse the adsorption by shaking in water at 25°C. After 45 hours, not more than 0.1-0.2 mg. of polyvinyl alcohol was removed per g., which is c a . 1-1.5% of the total adsorbed. Boiling the same quantities of polymer-coated silver bromide for 3 hours removed only 1.8 mg./g. AgBr, or c a . 13% of the 13-14 mg. originally adsorbed. BEHAVIOR OF BASIC CYANINE DYE
Experiments in a medium 10-3 M to bromide ion and at pH 6 showed that, when the silver bromide surface had Dye IVb (3,3'-diethyl-9-methyl thiacarbocyanine bromi.de) adsorbed just sufficient to saturate the surface, PVA, in any concentration, was quite unable to displace any dye. Varying amounts of PVA were added to the dyed silver bromide in centrifuge tubes which were tumbled for 16 hours. Not only was no dye displaced from the silver bromide but no polymer was adsorbed from the
217
AMPHIPATHIC ADSORPTION
solution on the dye coating at any concentration of PVA. Thus, the dye completely "protected" the silver bromide against the PVA. Similar experiments indicate also that the dye very largely (up to 75% at least) displaces PVA on the silver bromide surface, when just sufficient PVA had been added to saturate the surface. AMPHIPATHIC ADSORPTION
In several respects the behavior of PVA on silver bromide is similar to that of gelatin (4). Some experiments, therefore, were made on the coagulation and peptization of the silver bromide sol by dilute PVA solutions by the procedure described in the reference above. The results are presented graphically in Fig. 3, and it will be seen that the curve is
1"2 1.0 f
o
~
e
0.8 J ~ 0.6 o ~ 0.4 ~0.2
0
0
2
III
4
t l l l l l l I I
6 S I0 12 14 16 mgm. PVA ADDED PER grn. AgBr
IS
20
22
Fro. 3 Flocculation-Peptizationof AgBr Sol by PolyvinylAlcohol;pH = 4.9 generally similar to those obtained with gelatin but without pH effect. The peptization point is at 6.5-7.0 mg./g. AgBr for both Materials A and B, a value which must be taken as provisional until more data are obtained on different specimens of PVA. EFFECT OF ADSORPTION TO ~ILVER ]:~ROMIDE ON 1)OLYVINYL ALCOHOL
When t,he eoagul'~ted (precipitated) PVA-eoated silver bromide was (~entrifuged prior to peptization in excess solution of PVA, it was observed lhat this material dissolved much more slowly in a saturated solution of potassium bromide than the original silver bromide particles uncoated with PVA, several hours in the cold plus about one hour at 80 ° to 90°C. being necessary. When the silver bromi(le w ~ finally dissolved, the
218
s.E.
StIEPPARD, A. S. O'BRIEN A.ND G. L. BEYER
polymer coating retained the form of the precipitated and centrifuged mass and floated in the solution of potassium bromide as a semitransparent disk. This was insoluble in cold water but dissolved readily when the temperature was raised to 60°C. The observations suggest that, not only is PVA made relatively insoluble as and when adsorbed to a silver bromide surface, but it may have been made permanently less soluble after removal (or solution) of the silver bromide. The solution of potassium bromide, from the position of its ions in the lyotrope series, might be expected to have a peptizing rather than an insolubilizing effect on the PVA; the experiments will be repeated and diversified in an effort to specify and clarify more completely the meaning of "solubility," "melting point" and "minimum solution temperature" when applied to such substances as gelatin and polyvinyl alcohol. DISCUSSION Examination of the data given in Table I and Fig. 1 shows that the adsorption of PVA to silver bromide resembles that of several eyanine dyes (3) in proceeding to an irreversible saturated layer. The saturation value found at 25°C. for the standard AgBr sol was 13-14 mg. PVA/g. AgBr, which is near that found, I0 mg./g., for the basic eyanine dye, IVb. The dye has a M.W. of c a . 400, and the PVA one of c a . 40,000, so that it is probable that the p.olymer axis or backbone would be lying parallel to the AgBr surface but the rest of the molecule more or less edgewise thereto. It is noteworthy that there seemed no tendency to formation of further layers over the primary layer at saturation. The fact that the weight/g, of this coating appears to be lust twice the value for the "peptization point" (6-7 mg./g. AgBr) may be fortuitous, otherwise there are two interpretations: (I) The peptization point represents a completed monolayer with a hydrophobe exterior, in which case the saturation level indicates a completed duplex layer, but both compound monolayers are irreversibly adsorbed; or (2) The saturation level represents a monolayer, but one such that hydrophile properties appear at 50v/v coverage. The structure suggested from Xoray diffraction data (6) for orientated PVA is not inconsistent with a duplex layer (7) which might have an alternate folding, whereby - - O H polar groups are effectively presented to both the silver bromide surface and to water.* Some quasi-electronic * A further, perhaps equivalent, kind of hydrogen bonding might be occurring somewhat in the fashion suggested by L. Michaelis (8) for the polymerization of the free
AMPHIPATHIC ADSORPTION
219
continuity of the - - O H groups of PVA is suggested by recent observations that the intensity of the electrical anisotropy of stretched PVA films is considerably increased by previous impregnation of the PVA sheet with precipitated silver bromide.* The behavior of PVA on silver bromide shows that the phenomenon of agglutination --~ peptization consequent on addition of an amphipathic molecule or macromolecule to a hydrophobe sol may involve variants in the mechanism, even though this seems to be fundamentally a matter of alternation of polar and nonpolar interactions. So far, the rate of adsorption in the primary phase (¢~-adsorption) has appeared, in absence of other interfering bodies, to be very rapid, but it is likely that the later phases of the adsorption act involve relatively slow processes of diffusion and molecular accommodation (orientation) on the surface of the solid. It is hoped to gain some information about these by time-studies of the interracial tension and contact angle of water and, e.g., cyclohexane, at the evolving surface. The pseudo-adsorption or irreversible adsorption dealt with in this and related studies need not be considered as contradicting the broad principle embodied in the Gibbs Theorem, but as standing in much the same relation thereto as crystallization or precipitation to the behavior of a solution, i.e., essentially as a discontinuity in any functional representation. The adsorbed dye or colloid, which becomes insoluble in the liquid from which it is deposited, is bonded to the adsorbent by chemical forces of high magnitude, greater than any existing between water an(l the hydrophobe organic part of the adsorbed molecule. SUMMARY
The adsorption of polyvinyl alcohol to silver bromide has been measured and found to be irreversible (C-adsorption). No secondary reversible ~adicals of W(irster dyes. I n the case of PVA, it would have to be assumed t h a t
/
H
\ H
groups attached to --OH groups, which themselves,when in orientated chains (ill the polymer), form resonating systems, take oll properties similar to the alternate of / ~N +
\
H
I[ an d
\ /
N---
H H Michaelis's configuration. Naturally, this remains speculation unfit electron diffraction studies can be made of the adsorbed PVA. Something similar m a y be present in the polymeric structures of boron hydride and of aluminum trimethyl (9). * Unpublished experiments of S. E. Sheppard and P. T. Newsome.
220
s. E, SHEPPARD~ A. S. O~BRIEN AND G. L. BEYER
adsorption appeared to occur but there was precipitation of silver bromide sol followed by peptization with excess polyvinyI alcohol. The peptization point was found to occur at an adsorption value 6.5 to 7.0 mg./g., just half that at which saturation occurred. Adsorbed basic (cyanine) dye is not displaced from silver bromide by PVA but the latter is very largely displaced by basic dye. I~EFERENCES 1. HARTLEY, G. S., Trans. Faraday Soc. 37, 130 (1941).
2. 3. 4. 5. 6. 7. 8. 9.
SHm,PAPm, S. E , Nature 155, 266 (1945). SHEPPARD,S. E., LAMBERT,R. H., ANDWALKER,R. D., J. Chem. Phys. 7, 265 (1939). SHEI~PARD,S. E., L£~IBERT,R. H., AND.SwINEHAR%D., J. Chem. Phys. 13, 372 (1945). Paper in preparation, by A. S. O'Brien. MOONEY,R. C. L., J. Am. Chem. Soc. 63, 2828 (1941). SHEPPARD,S. E., AND Nv.WSOME,P. T., J. Chem. Phys. 12, 513 (1944). MICHAESm,L., AND GRANICK,S., J. Am. Chem. Soe. 65, 1747 (1943). BURAWOY,A., Nature 155, 328 (1945).