Energy versus Entropy in Cooperative Electrostatic Interactions: Comparative Study of Binding of Sodium Poly(Styrenesulfonate), Dodecylbenzenesulfonate, and Methylbenzenesulfonate to Polycations

Abstract

Electrostatic coupling of sodium 4-dodecylbenzenesulfonate (DDBS), sodium 4-methylbenzene-sulfonate (MBS), and sodium poly(styrenesulfonate) (PSS) with the homopolymers of N-diallyldimethylammonium chloride (DADMAC) and chloride of 2-N-trimethylammonioethylmethacrylate (TMAEMA) or their copolymers with neutral hydrophilic groups were studied using 1H and 35Cl (single- and double-quantum) NMR, relaxations, and pulsed field-gradient (PFG) diffusivity measurements. Both DDBS and PSS were found to react cooperatively with the polycations, in contrast to MBS. However, PSS binds more strongly to the polycations and is thus able to substitute DDBS in its complex with an appropriate polycation. The binding isotherms of DDBS and MBS with DADMAC polymer agree well with the theoretical model suggested by Kuhn, Levin, and Barbosa, providing that the hydrophobic energy parameter χ has the value −3.5kBT for DDBS and 0.0 for MBS. Energy stabilization by hydrophobic interactions is thus suggested to be the determining factor in the cooperative binding of DDBS. This conclusion is supported by clusters of bound DDBS with a mean length of about 30 molecules in its complex with DADMAC polymer, which were indirectly found using 35Cl T23 relaxation. In conclusion, the interaction of DDBS with polycations is suggested to be an example of cooperative electrostatic binding with unfavorable entropy but strong energy stabilization by, e.g., hydrophobic interactions. Entropy change due to liberation of bound counterions into a disordered state, found by us earlier1-3 to be operative in the couplings of complementary polyions, is thus not the only possible driving force of cooperative electrostatic interactions.