Interaction between Polyelectrolyte and Oppositely Charged Surfactant:  Effect of Charge Density

Abstract

The binding of dodecyltrimethylammonium bromide (DoTab) to poly(acrylic acid) (PAA) at different degrees of neutralization (α) was investigated using isothermal titration calorimetry (ITC) and laser light scattering (LLS) techniques. The surfactant binds to the polymer at all α; however, the mechanism varies. When α is lower than a critical value (αC), the hydrocarbon chains of DoTab cooperatively bind to the apolar segments of PAA driven by hydrophobic interaction at very low DoTab concentration (CDoTab ≤ 0.2 mM). In this binding region, the ITC profile exhibits a significant exothermic peak and the mixture precipitates, which is attributed to the interchain complexation via hydrogen bonding induced by the binding. The precipitation is soon resolubilized with further addition of surfactant as more DoTab micelles are bound to the polymer backbones with their ionic headgroups extending outward. When α > αC, the hydrophobic binding ceases as the polymer is progressively ionized and DoTab binds to the charged polymer chains driven by electrostatic attraction. The condensed counterions on the charged polymer chains are released via the ion-exchange process, resulting in an endothermic maximum on the ITC profile. The value of αC determined from ITC is approximately 0.3, which is reasonably close to the theoretical value derived from the Manning's counterion condensation theory (approximately 0.35).