Oct., 3962
ADSORPTION OF
A
COPOLYMEIZ POLYELECTROLYTE
of freedom of a molecule (including those due to internal rotation) into the original partition function of the polymer molecule? If one does that in the manner of Prigogine ("Molecular Theory of Solution") I suspect one will find that the major chauge will be the form of z, namely that x = E/flcT, where f = number of external degrees of freedom per chain link ( = 3c/r in Prigogine's nomenclature).
A. SILBERBERG.--The seeming irreversibility of polymer adsorption can, I believe, be explained in this may. Your bash suggestion of a method for introducing flexibility has in fact been followed in the present treatment, although in a manner rigidly tied to the lattice coordination arrangement assumed (parameters 2 and 8). One can vary these parameters or introduce additional ones, such as g (Part 11), but my purpose so far has been to keep the number of adjustable parameters down. I hope to discuss these and other structural effects in a paper which I have in preparation.
M. J. VOLD(University of Southern California).--The findings that pendant loops of an adsorbed polymer extend only a very few segment lengths into the solution and that the thickness of adsorbed film is independent of molecular weight has a very devastating effect upon at least two heretofore relatively well accepted explanations of experimental effects. One is the flocculation of suspensions by bridging as discussed by La Mer and the other is the stabiliRation of traditional colloids against coagulation by electrolytes (for example, the gold sols of Heller and Pugh). Both these effects are sensitive t o the molecular weight of the polymer and seem to require long pendant loops. Can you providt! us with a way out of thii! seeming dilemma? A. SrLmtsnRc,.--Large pendant loops whose size depends on molerular weight can be expected during the period which
1007
follows upon the successful attachment of a macromolecule to the surface, Only very gradually will the equilibrium configuration predicted in this treatment be ap roached. I n fact, the transition period may be of considerahy larger duration than the relaxation times of the other phenomena studied. Long loops in equilibrium are, moreover, not excluded by the treatment. Under certain conditions, some covered here (see Part I), others not (polyelectrolyte adsorption, for example), the loops are redicted t o be long. The observations quotecfby Dr. Vold thus do not necessarily stand in contradiction to the picture here proposed. It should be noted, moreover, that a molecular weight. d e pendence of the amount adsorbed is one of the conclusions of the present treatment. M. L. HUQGINS(Stanford Research Institute).--I am not clear a8 to how your theory can relate the observable properties to two inolecular parameters which I have shown to be important in three-dimensional polymer solutions (and which also enter into my treatment of polymeric monolayers, presented a t Wiesbaden in 1959). One of these parameters is the difference between the molecular interaction energy per unit molecular surface area between unlike molecules and the average of that between like molecules. The other is the change with concentration of the average flexibility per joint in the polymer chain. A. SILBERBERO.--Of the two parameters you mention the first is, I believe, considered by the treatment and is most closely related to the parameters zo and z0p introduced in Part 11. Restrictions on the flexibility of links are changed by changing the lattice coordination numbers 2 and S, by varying the parameter g (Part 11), and by introducing some of the special effects considered in Part I. A dependence on concentration has, however, not yet been envisaged.
ADSORPTION OF A COPOLYMER POLYELECTROLYTE BY W. SCHMIDT AND F. R. EIRICH Institute of Polymer Research, Polytechnic Institute of Brooklyn, Brooklyn, N . Y. R G C E ~March U E ~ f 8, f 989
A series of vinyl acetate-crotonic acid copolymers was synthesiaed and characterized by light scattering and viscosity measurements. The adsorption behavior of these copolymers on anatase was investigated as a function of polymer charge and charge density a t several pH values. The results demonstrate a large increase in adsorption capacity with decreasing crotonic acid content and decreasing pH. Formation of a common function for all copolymers connecting adsorption capacity with their states in solution as a function of ApH (pH units above the point of preci itation) was observed. The effect of the function of ApH was interpreted as producing equal ratios of adsorbing (VA) anznon-adsorbing, or even repelling, (COO-) groups.
1. Introduction The adr;,orptionof high polymers on liquid-solid interfaces lately has become the subject of detailed experimental and theoretical studies. Among the pertinent investigations, Claesson,' Jenckel and Rumbach,2 Hobden and jell in el^,^ Lifland,4 Kraus and Dugonel6Koral, Ullman, and Eirich,6aLopatin and EiricIh,6b Ullman, et U Z . , ~ ~ and others have ascertained that polymers are readily adsorbed in quantities which would amount to multilayers if complet,ely flat accommodation at the surface (1) I. Claesion and S. Claesson, Arkiv. Ksm., Mineral.. Ueol., 19A, No. 5 (1945). (2) E. Jenclrel and B. Rumbaoh, Z. ElekLrochem., 66, 612 (1951). (3) J. F. Hobden and H. H. G. Jellinek, J . Polymer. Sci.. 11, 365 (1953). (4) ,J. Liflanid, Master's Thesis, Polyteohnia Institute of Brooklyn, 1954. (5) G. Kraus and J. Dugone, Ind. Eng. Chem., 47, 1809 (1955). (6) (a) J. Koral, R. Ullman, and F. R. Eirieh, J . Phy8. Chsm., 62, 541 (1958); (b) G.Lopatin and F. R. Eirich, P T ~ C 3rd Intern. Congr. Surface Activity, 2, 97 (1960); (c) R. Perkd and R. Ullman, J . Polymer Sci. 64. 127 (1901).
were assumed, but would signify incomplete adsorption if the polymer chains were to pack standing end on. The amounts adsorbed as a function of concentration conformed, surprisingly, to Langmuir type isotherms, and the dependence on molecular weight, temperature, type of adsorbent, and solvent was found to be very complex. On porous surfaces fractionation and time effects were found whenever the molecules were as large or larger than a characteristic diameter of the pores. On nonporous adsorbents, the amounts adsorbed rose slowly with molecular weight, and poorer solvents magnified this trend. This tendency, together with the general observation that poorer solvents lead to increased adsorption, have been explainedaa in terms of the effect of the solvent on the configuration of the polymer in solution. The effect of temperature on adsorption is in most cases small and depends on the nature of the system. Besides the more usual decreases of adsorption with temperature, increases often are ob-
1908
W. SCI~MIDT a m B. R. RIRICH
served which have been related to entropy effects connected with solvent desorption.6a Jenrkel and Rumbachz postulated that the polymer chain be anchored onto the solid surface at several sites along the chain with the remainder of the coil extending into the solution in the form of loops. This cancept led to a detailed statistical adsorption theory for a random coil polymer put forth by Frisch, Simha, and Eirich' and further developed by Friach and S i m h 8 This theory considers the changes in free energy of all the participants (surface, solute, solvent) including the mixing of the loops of the enforced chain configurations with solvent during the adsorption process, The isotherm predicted by this theory is of the Fowler type but under certain conditions can approach the Langmuir type. Until recently, very little experimental work had been carried out on the adsorption of polyelectsolytes. Michaels and Morelos,R investigating the adsorption of sodium polyacrylate and a partially hydrolyzed polyacrylamide on kaolinite iii the pH range 5 to 8, found adsorption to decrease with increasing pH. Lopatinlo and Lauria," investigating the adsorption of PMAAlia and PAA on anatase, also found the isotherm? to be alronglg pH dependent, with both affinity and rapacity passing through maxima, but at different pH values. Both investigatorslO l1 have reported a rather small molecular weight dependence of the order of M oO5 to z, The adsorption of PMAA 011 lon7er dielectric materida, PS and PMMA beads, also demonstrated capacity dependence on pH, but the capacities on these adsorbents were appreciably higher than for TiO,.ll A new theoretical derivation by Frisch and StillingerlZa which relates the rising slope of the adsorption isotherms, i e . , the "affinity" with the state of charge of polymer and surface, dielectric constant, and ionic strength, hax been employed to explain some of the results of Lauria, and an unpublished extension of this theory may be applicable to the adsorption "capacities" of this work.i2b The purpose of the preaent investigation was to continue the study of the adsorption behavior of polyelectrolytes on a nonporous surface (anatase) as B function of polymer charge and charge distribution. The variation of polymer charge at constant pH was to be accomplished by the expedient of copolymerization using a hydrophobic comonomer from which a series of increasingly water soluble polyelectrolytes could be derived. The copolymer chosen 'was crotonic acid-vinyl acetate and the compositions to be studied were (7) H. L. Frisoh, R. Simha, and F. R Eirich, J . Phys. Chern., 67, 684
(1953).
(8) H. L. Frisch and R. Simha, ( b i d , 68, 607 (19541, J . Chem P h y 6 , 24, 651 (1955); 27, 701 (1957). (9) A S. hfichaels and 0. Morolos, Ind E n g Chem., 47, 1801 (1955). (10) G. Lopatin, Dissertation, Polytechnio Institute of Brooklyn, 1961. (11) R. Lauria, Dissertation, Polytechnic Institute of Brooklyn, 1962. ( l l a ) Poly(methacry1io acid), F R I A 4 , poiy(aory1ic acid), PA*; polystyrene, PS, poly(methy1 methacrylate) Ph? 31.1. (12) (a) 13. L. Frisch and F. H Stillinger, 4merican Chemical Society, 140th National Meeting, Chicago, 1961; (b) private oommunication,
Vol. 66
similar to those discussed by Katchalsky and Giliis.13
11. Experimental Procedures A. Materials. Solvent.-The water used in O ~ Vst8udies was obtained by distillation of deionized water. Solutions were adjusted and maintained at the desired pH by addition of 0.05 A' sodium acetate buffer. Polymers.-The vinyl acetate-crotonic acid copolymers were prcpared by bulk polymeriention M ith henzoj 1 prroxide as the initiator. The crotonic acid was used as received. The vinyl acctate was distilled prior to use. Tho monomer mixtures were reacted in sealed Pyrex tubes a t a temperature of 'io", using an initiator concentration of 0.035 mole % with respect t o monomq. The monomer concentrations used, polymerization times, and % conversion are shown in Table I. The copolymers formed were isolated by precipitation in 500 ml. of heptane. After decanting the heptane, the copolyrncrs were dissolved in methyl ethyl ketone (10 parts MEK to 1 part polymer), and precipitated by dropwise addition t P hot (60") heptane, with vigorous stirring. This procedure was repeated sevesal tmefl, Copolymer8 A and R were further treated by dissolving io benzene and freeze-drying. The copolymers then were dried in a vacuum oven at 68'. Copolymer composition was determined by titration with 0.1 N sodium hydroxide in Qj%.ethanol to a phenolphthalein end-point. The results are given in Table I, The molecular weights of the copolymers as determined by light scattering (in XEK) are given in Table 11. The refractive index increments, dnldc, as measured by a Ra leigh interferometer, were found t o be 0.110, 0.116, anJ0.123 for copolymers B, C, and D, respectively. The intrinsic viscosities in MEK and 0.05 N sodium acetate solutions also are shown in Table II. The poly( 4-vinyl N,n-butyl pyridinium bromide) used for the copolymer titration was prepared from poly(nvinyl pyridine) following the method of Fuoss and Strauss l 4 Adsorbent Surface -A sample of anatase especially prepared by the Titaniu.m Pigment Gorp. was used as thP adaorbent, The Rpecific surface area as determined by the BET method (nitrogen) on n sample outgassed a t 400' was 10 9 m.P/g. The anatase powder was further treated in our Laboratory before adsorption measurements were carried ouB, by a slightly modified version of the procedure outlined by Lopatin.lO Adsorption-desorption cleaning then was carried out by adsorbing co ol mer C at pH 0.0, withdrawing the solution from the a&or{ent, and adding to the adsorbent NaOH solution t o p H 12. Desorption of the copolymer was found t o be 96% complete. The adsorbent then wag washed with distilled water until neutral, washed with acid, dkali, and water as before, and freeze-dried. Adsorption of the copolymers was determined on this surface. B. Adsorption Procedure.-A weighed quantity of anatase was shaken with 10 ml. of polymer solution of known concentration for a minimum of 24 hr., centrifuged, and the supernatant analyzed. The difference in concentration Rnd the amount of adsorbent were used to calculate the adsorption gram ( z l m )at the equilibrium concentration. Glass tu es stoppered with polyethylene caps were used. They were weighed, .put on a thermostated rotator (30') for 24 hr., and rewelghed to check for leakage. The pH of the supernatant solutions was determined on a Reckman pH meter after each adsorption run. Adsorption runs were set u a t pH 7.6, 6.5, 6.0, and 5.5 in the presence of 0.05 so$um acetate-acetic acid. C. Analytical Method.-lVith only sIight modification, the method outlined by Lopatinlo was used to analyze for copolymer concentration before and after each adsorption run. Solutions of known copolymer concentration ( 2 to 100 p.p.m.) were titrated with poly(4-vinyl-X,n-butyl pyridinium bromide) solutions (3.4 to 6.8 m ./IO0 nil.). The copolymer solutions were buffered to p€f 10 with NaOB. The titrant 'c1'as added in 0.02-ml. increments and the solution allowed t o mix for 15-20 sec. (by evolution of X2) (13) A. Katchalsky and J. Gillis, Rec. Trav. Chim., 68, 879 (1949). (14) R. F U o S S and R. StrQusS, J. PoZy~&er Sei., 8 , 246 (1948).
AJXNRPTIOX
oct,., 196!2
OF A
COPOLYNER POLYELECTROLYTE
1909
before recording turbidity on a Brice-Phoenix scatteriDg instrument. The peak of turbidity served as a characteristic end-point. I n the low croficerrthtion %glm (0.6 t o 2 p.p.m.) results were obtsined by a direct comparison with a known copolymer solution ( I p.p.m.) rather than from the calibration curve.
Results and Discussion The monomer concentrationa used in t8he copolymerizations, as previously men$oned, were calculatedl to provide copolymers similar to those studied by Katchalsky and Gillis.I3 Our analysis (Table 1) shows that, in fact, quite similar compositions were obtpined. Katclhalsky and GiIlis were concerned with the variation of pK of polymeric acidls with concentration, degree of ionization, and distance between carboxyl groups. The use of their data for the calculation of the number of charged groups on our copolymers for various adsorption runs will be discussed more fully below.
7-
6 -
9-
L
I
I
I
20
30
40
I
% Crotonic acid.
Fig,
TABLE I
I
10
l.-Solubililies
of vinyl aretnte-rrotonic copolymers.
acid
COPOLYMERIZATION DATA 70 Monomer oonon. CDVinyl Crotonic polymer acetate, g. acid, g.
A 33
c
D
48.2 46.4 42.5 28.0
1.8 3.6 7.5 22.0
Rearticjn % ’ time, Converhr. sion
14 22 26
14 13 8
26
K
Crotonic acid in etrpolyrncw
5.5
12.9 26.1 42.4
Ih the present syntheses, attempts were made to prepare compounds of approximately the same molecular weight by using coiiistant catalyst concentration and tempera%tlre. While the molecular weights as determined by lighd scattering differ by a factor of 3, the intrinsic viscosities in water vary much less and, e.g., those of copolymers B and C a t pH 7‘8 and 6 5 are almost the same (see Table 11). This must be interpreted as showing that the two copolymers encompass comparable volumes which ought to be due to the greater relative number of carboxyl groupls in copolj7mcr C providing a more expanded configuration.
TABLE I1 INTRINSIC vIK!OfilW COPOLYMERS B, C, AND D
! v ~ O I , ~ C U L A BWEIGHT AND
DATA FOR Nol. It.
x
Copolymer
B(l2.9% CA) C(26,1% CA) D(42.4ojO CJ4)
[?I
I
5
10-8)
[ q ] in Ha0 pH7d ~ H 6 5
in MEK
(Le&ifi
0.563
0.635
0.82
,577
.550
.40
.365
.20 .lo
61 31 18
MEX)
The decision of which pH range would be most profitable for our studies was made with reference to the solubility pattern of the copolymers. It was noted that, the latter precipitated from solution as the p H was lowered, independently of which acids were used. The pH values for the precipitation varied from 4.8 to 7.0, depending on the copolymer composition, giving ? smooth function as shown in Pig. 1. It was decided that adsorption runs would be attempted a t several pH’s and a t appropriate pIf intervals in such fashion that approximately “corresponding states” of solution would be bbtained (see also Fig, 11, ke., a t definite
Concentration, g./100 @e. X 102.
Fig. Z.-Adsorption
of copolymers on anatase.
pH increments between a given solution and the limiting pH a t which precipitation occurred. Insofar as the solubility range permitted, the adsorption behavior of the copolymer was further compared at constant pH. The adsorption curves obtained in this way are given in Fig. 2. The results demonstrate a large increase in capacity with decreasing crotonic acid content. The capacity increases markedly in all rases with decreasing pH as found by earlier workers. The adsorption capacities as a function of pH have been summarized in Fig. 3. Comparing the adsorption of copolymers B, C, and D in the “corresponding states” of solution mentioned, and taking the pH a t which the copolymers precipitated as o, reference point, the capacities were plotted next us. ApH above the point of precipitation. The results, shown in Fig. 4, show formation of a common function for all copolymers, This relationship is indicated similarly
W. SCHMIDT AND F. R. EIRICH
3 910
Vol. 66
TABLE I11 VARIATION OF COPOLYMER DEGREE OF IONIZATION ( 0 1 ) WITH pH Copolymer B Copolymer C Copolymer D (pK = 6.8,'n = 1.4a) (pK 6.5,b n = 1.3") ( p K xi 7.0,' ??r = 1.5") coo Coocoo coo __-coo-
-
PH VA Molecule OL VA Molecule 7.6 0.88 0.13 80 0.79 0.28 74 6.5 .50 ,074 40 .38 .13 36 6.3 .39 .057 35 .* .. 6.0 .29 .043 (ppt.) 26 .21 .074 20 5.5 ,. .. .lo5 .037 (ppt.) 10 5.0 *. .* .. a Taken from Katchalslcy and Gillis.'B Estimated from 0 t h and Doty.16 (x
I .
..
..
.*
..
I
coo -
a
VA
Molecule
0.72 .32
0.53 .23
..
..
64 28 ..
.18 .09
*
13 .067 .033 (ppt.)
16 8 4
,045
I
I
I
1
2
3
("Solubility") A ~ H = pH units above limiting pH 1 I
I
6
6
Fig. 3.-Adsorption
I
I
7
8
1
I
1
7
1 7
1
9
of copolymers on anatase solution.
1 __
us. pH of
by Fig. 3, where a constant shift in pH units for the individual copolymer curves will lead to the formation of a single curve like that shown in Fig. 4. This is all the more striking as the copolymers possess rather different molecular weights. Earlier work, however, has shown that the influence of molecular weight on capacities is small, and the mere existence of such a smooth dependence as seen in Fig. 4 points toward the absence of substantial molecular weight effects. In a first attempt to interpret Fig. 4, one would assume a variation in adsorption capacity due to changes in charge density on the polymer. The degree of ionization (CY)may be calculated by means of the equation16 1--a pH = p K - %logCY
By assigning pK and 12 values to copolymers B, C , and D as obtained by extrapolation from the data of Katchalsky and Gillis,l3 the values for -a were obtained and used to prepare a plot of the (15) A. Katchalsky and
I
I
P.Spitnik, J . Polymer Sci., 2, 432 (1947).
d. P H
Fig. 4.-Adsorption
of copolymers on anatase vs. A ~ H .
adsorption capacity as a function of charge d!nsity. However, the coincidence of the curves jn thls plot is poor. The discrepancies are greatly reduced if one assumes higher pK values for the copolymers than those of ref. 13. Based on the work of 0 t h and Doty'% regarding variation in pK with concentration, pK values approximately 1.0 unit higher than those given by Katchalsky and Gillis would be quite in order. This increase in pK was estimated by an extrapolation (of our pK according to ref. 13) toward lower concentrations, parallel to the 0 t h and Dot,y curve for their change in pK with concentration. Using these new pK values for calculation of a! (see Table 111)) t.he adsorption capacities were plotted again as functions of charge density (expressed in #COO-/lo0 monomer units) and now approached very closely a unified relationship as is shown in Fig. 4. A slightly modified version was obtained by plotting adsorption capacity as a (16) A. 0 t h and
P. Doty. J . Phys.
Ckem., 66, 43 (1962).
Oct., 1962
MONOLAYER PERMEABILITY AND PROPERTIEB OF NATURAL MEMBRANES
1911
function of the ratio of number of charges per vinyl a replot of our data according to x/m = K acetate group, which might characterize the “solu- VA/ACOOb reproduces the straight line in bility state” of the copolymers somewhat better Fig. 4, lending a certain ?mount of interpretation than charge density alone. The result is such a to the primary dependence of our adsorption close reproduction of Fig. 4 that it supports the capacities on l/ApI
