STUDIES OF POLYELECTROLYTE SOLUTIONS .3. CORRELATIONS BETWEEN THE 2ND AND THE 3RD VIRIAL-COEFFICIENT

Abstract

Combined Donnan equilibria and static light scattering experiments have been performed on aqueous solutions of poly(styrene sulfonate), PSS, in the presence of low molar mass electrolytes. Two systems were investigated, namely PSS/Na and PSS/Pb/H. The coion was nitrate. Additionally, the molar mass, M(w), of PSS was varied in the range of 3.20 x 10(5) to 1.22 x 10(6) Da. Donnan equilibria experiments yield the degrees of binding, THETA1 and THETA2, for the uni- and the divalent counterions, THETA1 and THETA2 increase with increasing ionic strength, but they are independent on M(w) within the experimental error. Static light scattering yields the molar mass, M(w) the radius of gyration, [S2]z1/2, and the second and third virial coefficients, A2 and A3. Data analysis is made in two ways: (1) by using the brute-, and (2) by using the net polyion concentrations. Here, the term `'brute'' stands for the naked polyion skeleton, while the net polyion concentration is the concentration which includes the counterions bound to the polyelectrolytes. C(brute) is known from the sample preparation. C(net) can be calculated from THETA1 and THETA2. It is found: (1) M(w,net) is significantly larger than M(w,brute); (2) [S2]z,net1/2 and [S2]z,brute1/2 are identical within the experimental error, and (3) A2,net and A3,net are significantly smaller than A2,brute and A3,brute. The molar mass dependencies of [S2]z,net1/2, A2,net, and A3,net can be well described by power laws [S2]z,net1/2 = K[S]z . M(w,net)v[S]z, A2,net = K(A2) . M(w,net)v(A2) and A3,net = K(A3) . M(w,net)v(A3). The exponents, v[S]z and v(A3), decrease with increasing ionic strength while v(A2) increases. Interestingly, v(A3) is significantly larger than 2v(A2) 1. This suggests that A3 is not proportional to A2(2) . M(w) as predicted by some theories. The nature of the mentioned effects could be established, however, only in part. An interpretation by the conventionally excluded volume theory was not successful. Indeed, a universal functon, PSI, incorporating effects such as molar mass, degree of dissociation, and solvent quality, is found; but none of the present theories is suitable to describe the experimental data satisfactory. In conclusion, a completely new theoretical concept will be necessary for the future.