Interaction of monodisperse poly(N-isopropylacrylamide) microgel particles with sodium dodecyl sulfate in aqueous solution

A thermodynamic analysis of ionic surfactant solutions in equilibrium with polymer solutions, polymer microgels, and macrogels is given. It is concluded that the surfactant binding isotherm cannot be exactly calculated only from the total and equilibrium surfactant concentrations. An additional trace probe electrolyte method is suggested for the determination of the binding isotherm. The interaction of sodium dodecyl sulfate with poly(N-isopropylacrylamide) microgel particles was investigated by dynamic light scattering, electrophoretic mobility, and surfactant binding isotherm measurements. The interaction of the surfactant with the microgel particles takes place in a two-step process. The surfactant binds above a critical surfactant concentration; then, after a plateau is reached a new binding step evolves in the isotherm. The hydrodynamic diameter of the particles also increases in a stepwise manner in correlation with the binding isotherm. The microgel-surfactant interaction is interpreted by means of polymer-surfactant interaction theories. The surfactant binds in the gel as small aggregates with different aggregation numbers in the two binding steps, which is a consequence of the inhomogeneous inner structure of the particles. The binding starts in an outer shell of the particles followed by surfactant binding in the particle core.