Thermodynamic Effects of Alcohol Additives on the Cooperative Binding of Sodium Dodecyl Sulfate to a Cationic Polymer

The binding isotherms for the interaction of sodium dodecyl sulfate (SDS) with poly(diallyldimethylammonium chloride) in aqueous solutions of 1,8-octanediol, 1,2-octanediol, or 1-butanol (BuOH) were determined by potentiometry, using a functional membrane responsive to anionic surfactants. Each alcohol caused the binding isotherm to shift to a lower SDS concentration. The magnitude of the effect was 1,2-octanediol > 1,8-octanediol ∼ BuOH. The binding isotherms were analyzed by a linear lattice model for cooperative binding. The effect of the alcohols was analyzed by thermodynamic treatment of mixtures based on the regular mixing model, and the interchange energies between alcohol and water (4.06 kT for 1,8-octanediol, 4.82 kT for 1,2-octanediol, and 3.63 kT for BuOH) and between alcohol and SDS (−1.21 kT for 1,8-octanediol, −3.52 kT for 1,2-octanediol, and −1.65 kT for BuOH) were estimated. The pronounced effect of 1,2-octanediol on the binding isotherms was ascribed to its instability in the water phase and its strong hydrophobic interaction with the SDS anion in the polyion domain. The effect of 1,8-octanediol was comparable to that of BuOH, but 1,8-octanediol was more unstable in both the water phase and the polyion domain by about kT ln 2 compared to BuOH. This phenomenon can be attributed to the fact that binding of one 1,8-octanediol molecule structurally approximates the binding of two BuOH molecules.