Calorimetric determination of surfactant/polyelectrolyte binding isotherms

Mixing of oppositely charged surfactants and polyelectrolytes in aqueous solutions leads to surfactant adsorption onto the polyelectrolyte chain. Experimental determination of the binding isotherm is normally achieved by ion-selective electrode methods, which require time consuming experiments with sensitive, custom-built equipment. Here we present a simple method for approximating binding isotherms from isothermal titration calorimetry (ITC) data. The Satake-Yang model is used to fit the binding constant (Ku), cooperativity parameter (u), and specific heats of cooperative and non-cooperative surfactant adsorption to the total measured heat of adsorption. The fitted binding parameters, Ku and u, are then used to estimate the model polyelectrolyte coverage (β) as a function of free surfactant concentration. This approach is applied to two surfactant/polyelectrolyte mixtures: dodecyltrimethylammonium bromide (DTAB) and poly(styrene sulfonate) (PSS), and sodium perfluorooctanoate (FC₇) and N,N,N- trimethylammonium derivatized hydroxyethyl cellulose (JR-400™). The results suggest that while both systems exhibit cooperative surfactant binding, the interactions in DTAB/PSS mixtures favor the formation of intrapolymer surfactant aggregates, while the FC₇/JR-400 mixtures form interpolymer surfactant/polyelectrolyte networks.