Binding of Dodecyltrimethylammonium Bromide to pH-Responsive Nanocolloids Containing Cross-Linked Methacrylic Acid−Ethyl Acrylate Copolymers

Abstract

The binding of dodecyltrimethylammonium bromide (DoTab) to cross-linked methacrylic acid-ethyl acrylate (MAA-EA) copolymers with various MAA/EA molar ratios at different degrees of neutralization (alpha) was quantitatively studied using isothermal titration calorimetry, dynamic light scattering, surfactant selective electrode, and electrophoresis techniques. The surfactant binds to the polymers at all degrees of neutralization, but via different mechanisms. When alpha is sufficiently high, the binding is primarily electrostatic interaction between the surfactant and ionized polymer chains, which is reinforced by the micellization of electrostatically bound surfactant molecules. The saturation takes place at charge ratio ([DoTa(+)]/[ approximately COO(-)]) close to 1, indicating that the binding is a one-to-one charge neutralization between the cationic surfactant headgroups and anionic carboxylate sites of the polymers. When alpha is low, the binding of DoTab to the unneutralized polymers is driven by the hydrophobic interaction. The onset of hydrophobic binding takes place at DoTab concentration as low as 0.01 mM in 0.05 wt % polymer solution, where the saturation occurs at C(DoTab) approximately 0.19 mM and the amount of bound surfactant is approximately 0.09 mmol of DoTab/(g of polymer) at saturation concentration. The binding results in the formation of the polymer-surfactant complex. For the polymer with low MAA/EA molar ratio, the complex coagulates at a higher DoTab concentration that leads to phase separation; however, for polymers with high MAA/EA molar ratio, the complex remains dispersed and the mixture is stable even at high DoTab concentration.