Binding of Sodium Dodecyl Sulfate to a Cationic Polymer of High Charge Density

An investigation was made into the binding of sodium dodecyl sulfate (SDS) to a cationic polymer of high charge density, the homopolymer of diallyldimethylammonium chloride (PDADMAC), in 1 mM (1 M = 1 mol dm^-3) NaCl solution using a potentiometric titration method utilizing a surfactant-selective electrode at 15, 20, 25, 30, 35, and 45 °C. The concentrations of PDADMAC examined were 100, 200, and 400 ppm. SDS ions bound very cooperatively to the PDADMAC polymer. Variation of the polymer concentration had almost no effect on the binding parameters at the same temperature, while increasing the temperature had a large effect on the binding system at the same polymer concentration. The intrinsic binding constant (K) and the critical aggregation concentration (cac) decreased and then increased with increasing temperature, having minimum values of 17.9 × 10^-2 mM^-1 and 0.19 × 10^-2 mM at 25 °C, respectively. However, the cooperativity parameter (u) and the binding affinity (uK) had a maximum value of 20.0 × 10^2 and 357 at the same temperature, respectively. The polymer-concentration-independence of the binding parameters for the anionic surfactant (SDS) to the cationic polymer (PDADMAC) corresponds to the binding of a cationic surfactant to an anionic polymer, and the maximum in the binding affinity at 25 °C is quite similar to the binding of a cationic surfactant to an anionic polymer, and also to the micellization of ionic surfactants having a minimum cmc value at 25 °C.