Interactions between methacrylic acid/ethyl acrylate copolymers and dodecyltrimethylammonium bromide

The interactions between dodecyltrimethylammonium bromide (DoTab) and methacrylic acid/ethyl acrylate copolymers were studied using isothermal titration calorimetry (ITC) and laser light scattering (LLS) techniques. Individual cationic surfactant molecules first bind to negatively charged carboxylate groups on the polymer chains due to electrostatic attraction. In this regime, a pronounced endothermic peak is detected in the enthalpy curve and a slight reduction in the particle size is observed. When C' (indicated by another endothermic peak) is reached, the micellization of electrostatically bound surfactant commences and the particle size increases by several orders of magnitude. It then precipitates, producing a large hydrophobic polymer/surfactant complex. With further addition of surfactant, free micelles begin to form, and the precipitates resolublize (for the polymers with lower charge density) or gelatinize (for the polymers with higher charge density). The thermodynamic parameters derived from ITC measurements suggest that the electrostatic binding is an endothermic process driven by entropy. The positive entropy is attributed to the recovery of translational entropy of released counterions from bound surfactant molecules. Addition of salt screens the electrostatic repulsion between surfactant headgroups and attraction between oppositely charged polymer chains and surfactant molecules, which weakens the binding of surfactant onto the polymers favoring the formation of free micelles.