Langmuir 1994,10, 3390-3394
3390
Art ides Phase Behavior of Polymers with Concentrated Dispersions of Surfactants Stuart M. Clegg,? Peter A. Williams,+ Patrick Warren,* and Ian D. Robb*pt Water Soluble Polymer Group, North East Wales Institute, Deeside, Clwyd CH5 4BR, U.K., and Unilever Research, Port Sunlight, Merseyside L63 3JW, U.K. Received May 20, 1993. In Final Form: January 18, 1994@ Mixtures of polymers and surfactants have been prepared at relatively high concentration and the regions of phase separationmeasured. The polymers were both charged and uncharged,while the surfactants were nonionic. The separated phases were analyzed to obtain the tie lines for the two-phase region. The separated phases were either surfactant rich or polymer rich. Higher molecular mass polymers induced separation at lower concentration and the phase boundary did not depend strongly on the persistence length of the polymer. This indicates that the main origin of the separation was depletion flocculation. A theory describing such phase separation has been developed and shows qualitative agreement with the experimental results.
Introduction The phase separation of dispersions of colloidal particles in the presence of nonadsorbing polymers has been the subject of many studies since the early work of Asakura and 0osawa.l Flocculation of the colloidal particles takes place in the presence of these polymers, largely as a result of attraction arising from the overlap of the depletion layers around the approaching particles.2 This attractive potential, in addition to the two DLVO potentials, is usually sufficient to predict the onset of phase separation, especially in dilute colloidal system^.^^^ The phase separation can be considered analogous to that of hard spheres giving gas-solid or gas to liquid transitionsindeed the experimental data of S p e d was largely in agreement with this theoretical approach. Scheutjens, Fleer, and Vincent6 (SFV)used a lattice theory for noninteracting polymer near a hard surface and found that A, the depletion layer thickness, remains constant until the bulk concentration of polymer, e,, reaches the semidilute region, i.e. e*,, the overlap concentration. At polymer concentrations higher than this, A decreases until it is zero at 8, = 1. V i n ~ e n tde , ~ Gennes,8 and the SFV6 theory estimate how A decreases with 8,. Ausserre et aL9 have measured the effect of epon A and found that it was approximately constant until the critical overlap concentration and then decreased with Ap-0.79, close to that predicted.8 * To whom correspondence should be addressed. Water-Soluble Polymer Group. 0 Unilever Research. * Abstract published in Advance A C S Abstracts, September 15, 1994. (l)Asakura, S.; Oosawa, F. J . Polym. Sci. 1958,33, 183. (2) de Hek, H.; Vrij, A. J . Colloid Interface Sci. 1981,84, 409. (3) Sperry, P. R. J . Colloid Interface Sci. 1982,87, 375. (4) Gast, A. P.; Russel, W. B.; Hall, C. K. J . Colloid Interface Sci. 1986,161,109. ( 5 ) Sperry, P. R. J. Colloid Interface Sci. 1984,99,97. (6)Scheutjens, J. M. H. M.; Fleer, G. J.;Vincent, B. A.C.S. Symp. Ser. 1984,No. 240, 245. ( 7 )Vincent, B. Colloid Surf. 1990,50, 241. (8)Joanny, J. F.; Leibler, L.; de Gennes, P. G. J . Polym. Sci. Polym. Phys. 1979,17, 1073. (9)Ausserre, D.; Hervet, H.; Rondelez, F. Macromolecules l986,19, 85.
Phase diagrams'O for these systems of polymers with colloidal particles have been calculated, assuming the polymer and solvent are one "pseudophase" and simply considering the interaction between the particles. This gives the two phases in equilibrium having a high and a low particle concentration but having equal polymer concentration. More recently Lekkekerker et al." have allowed both the particle and polymer to distribute freely between the two phases, and this approach has been used here to calculate the composition of phases at equilibrium. The approach is at present the simplest model, but sufficiently close to the experimental system to make initial comparisons useful. Improvements to the theory are in progress. The experimental system is of polyelectrolytes or uncharged polymers mixed with nonionic surfactants. These surfactants form roughly spherical micelles until about 30%(v/v),when they change from an isotropic solution to a hexagonal or lamellar mesophase. As nonionic surfactants, these micelles will have little or no surface charge and a steep repulsion on contact with polymer chains. They are quite small, so that van der Waals attraction will also be small. Thus although not exactly hard spheres, their interaction potential is reasonably approximated by a hard sphere model. The polymers used were either polysaccharides, having similar chemical composition but different molecular flexibility, or polyelectrolytes to elucidate any effects of polymer type on the phase behavior.
Experimental Section The polymers used were as follows: (a)a copolymer of acrylic acid and maleic anhydride (PAMA)with a molecular mass of 70 000 compared to poly(styrenesu1fonate) as measured by aqueousGPC. The molar ratio of acrylic acid to maleic anhydride was 3:l. (b)Poly(acry1icacid) (PAA)was obtained from BDH as a 25% solution and had a quoted molecular mass of 230 000. (c) Hydroxyl ethyl cellulose (HEC)was obtained from Hoechst UK, Ltd.,coded Tylose H20P. (d) Dextrans T500 and T70 were obtained from Pharmacia-the numbers refer to their normal ~
~
~~
~~
~~
(10)Gast, A. P.;Hall, C. K.; Russel, W. B. J . Colloid Interface Sci. 1983,96,251. (11)Lekkerkerker,H. N.W.; Poon, W. C.-K.; Pusey, P. N.; Stroobants, S.;Warren, P.W. Europhys. Lett. 1992,20, 559.
0743-7463/94/2410-3390$04.50/00 1994 American Chemical Society
Polymer /Surfactant Phases
Langmuir, Vol. 10,No. 10, 1994 3391
Table 1 ~
polymer PAMA PAA DextranT70 Dextran T500 HEC
solvent 0.125 mol dm-3 NaCl 0.125 mol dm4 NaCl H2O HzO HzO
Rdnm 11.5 17.0 7.5 19.5 19.0
~~
Rdnm 12.7 33.1
molecular mass in kilodaltons. The average hydrodynamic radii (Rh) of the various polymers were determined using the Oros Model 801photon correlation spectrometer. The values are given in Table 1, all solutions being adjusted to pH 9. Values of R, have been calculated using the relation R, = 1.7Rh, based on recent experimental12 and theoretical13 work of polymers in solution. C* for the dextrans can be obtained14J5fromtherelation C*= 4 / ~giving , C*values for T70 of -10% and for T500 of 4.3%. The ethoxylated nonionic surfactants were obtained from Vista UK and had compositions ClOE6 and C13E7, where the C, represents the average number ofcarbon atoms n the alkyl chain and E, represents the average number of ethylene oxide groups. For the CloE6 the ethylene oxide length had a distribution ranging from 2 to 12 units with a maximum at 7. The phase behavior of the surfactants in the presence and absence of polymer was studied using an Olympus microscope with polarized light. The temperature dependence of the phase diagram was measured by heating and cooling on a host stage of the microscope. The diameter of the ClOE6 micelle was found to be 6.8 nm, using the Oros Model 801 photon correlation spectrometer. Phase diagrams were constructed in the following way. Polymer and surfactant solutions, both at pH 9, were mixed by weight and 1 mL of the mixture drawn into a syringe and left to come to equilibrium at room temperature. The volumes of any separate phases were measured in the syringe. The compositions of the separate phases were determined by (a)using GPC peak heights to measure polymer concentrations in an eluent of 0.25 m NaCl at pH 9 or (b) using lH NMR to measure the concentration of surfactant (compared against a standard of trioxan). In addition, the volumes of the separate phase, and the compositions of the original and separated phases were checked by the ratio method to help locate the phase boundary.
Theory Recently Lekkekerker et aZ.ll have developed an alternative approach to depletion flocculation which takes account of polymer partitioning between the two phases. Their approach is based on the free energy of the whole system. In their model, they treat the colloidal particles as uncharged hard spheres of radius a and the polymer as freely interpenetrable coils. The interaction between the colloid and polymer in this initial theory is purely an excluded volume effect; the center of mass of the polymer cannot approach closer than a distance r, to the surface of the colloid. While this initial approach is the simplest possible, it will be a reasonable approximation to the system of polymer plus nonionic micelles, the latter having no charge and a steep steric repulsion potential, approximating to a simple hard sphere. In this study rda =- 1,so that overlap between three or more depletion layers is important. This is taken into account in the theory, where a mean field van der Waals approximation16 is used in place of pairwise overlap of excluded volume shells. The mean field van der Waals approximation uses the free volume fraction a,i.e. the proportion of the total volume not occupied by the colloid particles and their excluded volume shells. An expression (12)Devanand, K.; Selser, J. C. Macromolecules 1991,24, 5943. (13)Burchard, W.;Schmidt, M.; Stockmayer,W. H. Macromolecules 1980,13, 1265.
(14)Morris, E . R.; Cutler, A. N.; Ross-Murphy, S.; Rees, D. A. Carbohydr. Polym. 1981, 1 , 5. (15) Furukawa, R.; Arauz-Lara,J. L.; Ware, B. R. Macromolecules 1991,24, 599.
(16)Widom, B. J . Chem. Phys. 1986,39, 2808.
for a may be derived from the Percus-Yevick approximation i.e.
where y = y,/(l - y,)
+
+ c3,
3c2 B = 9c212,C = 3f3, f' = rda, and rp is the volume fraction of colloid particles.
A = 3f
Lekkererker et al. calculated the chemical potentials of colloid and polymer as a function of the volume fraction of each, together with osmotic pressure of the system, or solvent chemical potential.
(up - pUpo)/kT = log I4
a
4naW3kT = qZ
(3)
+ -31 n a3n a
where pc and pp are the chemical potentials of the colloid and polymer, pCoand ,upoare the reference potentials, Z is the hard sphere compressibility, rp = volume fraction of colloid particles, n = number of polymer coilslunit volume, and ll is the osmotic pressure of the polymer in solution. The calculation of the phase behavior is made by eliminating p, from eqs 2-4. This gives, at high rp, two values of n and rp which represent the compositions of phases at either end of a tie line. Numerical routines, which solve these equations, thus generate the phase diagram. For this system, we initially took r, to be the same as R,, though as Vincent7has shown, this will only be so up to rp*, after which r, will decrease with increasing polymer concentration. We have calculated the phase diagrams using the experimentally-measured polymer and micelle sizes and then compared the result with experiment. To estimate differences between theory and experiment, we have altered the value of rpto obtain agreement between them. Given the polydispersity of the polymers and the simple assumptions in this initial theory, we expect (and find) only qualitative agreement between theory and experiment. Results
The phase behaviors for the two surfactants are given in Figures 1 and 2 as a function of composition and temperature. They show that at 25 "C, and at about 30% surfactant, hexagonal phase is formed. In the case of the C13E7, a lamellar phase is formed at about 50%and persists to higher concentrations (about so%), after which an isotropic solution with reversed micelles is formed. This
Clegg et al.
3392 Langmuir, Vol. 10, No. 10, 1994
t w P
r:w
c
I
50 40
30
-
1 I
'
20 20
30
40
50
60
70
80
YO
I00
Figure 1. Phase behavior O f CioE6 surfactant as a function of composition and temperature. H1 denotes hexagonal phase, L1 denotes isotropic phase.
Figure 3. Ternary phase diagram for PAMA and CIOE~.L1 and HI refer to the surfactant structures in the surfactant-rich phase. I Y I I . ~n i l it (PAMA)
r
211' 20
W A 1 I I
