ИСТИНА

The influence of a low molecular weight electrolyte, viz., NaCl, on behavior of water-soluble interpolyelectrolyte complexes (IPECs) formed by star-shaped poly(acrylic acid) ((PAA)N, N is the number of arms, N = 5, 8, 21, DPn(arm) = 100) interacting with exhaustively quaternized poly(4-vinylpyridine) (P4VPQ, DPn = 290, quaternized with ethyl bromide) or exhaustively quaternized poly(2-vinylpyridine) (P2VPQ, DPn = 270, quaternized with demethyl sulfate) was examined by means of turbidimetry, light scattering, and analytical cetrifugation. Aqueous solutions of such macromolecular co-assemblies with desired base-molar stoichiometry Z (Z = [N+]/[COO- + COOH] < 1) in which (PAA)N plays a role of lyophilizing polymeric component being present in an excess were prepared by direct mixing of the necessary amounts of salt-free aqueous solutions of (PAA)N and P4(2)VPQ followed by fast neutralization of (PAA)N with the equivalent amount of an aqueous solution of NaOH and subsequent dilution with an aqueous solution of TRIS*HCl (pH 7). On addition of NaCl into aqueous solutions of IPECs prepared according to the above procedure, they typically become turbid at a certain threshold salt concentration, [NaCl]1, and can even undergo macroscopic phase separation, the values of [NaCl]1 increasing with the rising N while decreasing with the rising Z. It is remarkable that aqueous solutions of IPECs based on (PAA)N are distinctly more stable with respect to their salt-induced phase separation compared to those of IPECs based on linear PAA of DPn = 75 and even DPn = 500, thereby manifesting a significant role of non-linear topology of the lyophilizing polymeric component. A further increase of the concentration of NaCl results in a gradual decrease of turbidity which disappears at a certain threshold salt concentration, [NaCl]2, the values of [NaCl]2 being nearly independent of N and Z. Analysis of homogeneous solutions at [NaCl] > [NaCl]2 obtained through addition of NaCl into the aqueous solutions of IPECs by means of analytical ultracentrifugation and light scattering indicates that (PAA)N and P4(2)VPQ at sufficiently high salt concentrations do not interact each other because of the screening effect of small ions suppressing electrostatic interaction between the oppositely charged polymeric components.