Interaction of monodisperse poly(N-isopropylacrylamide) microgel particleswith 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 conclud ed that the surfactant binding isotherm cannot be exactly calculated only f rom the total and equilibrium surfactant concentrations. An additional trac e probe electrolyte method is suggested for the determination of the bindin g isotherm. The interaction of sodium dodecyl sulfate with poly(N-isopropyl acrylamide) microgel particles was investigated by dynamic light scattering , electrophoretic mobility, and surfactant binding isotherm measurements. T he interaction of the surfactant with the microgel particles takes place in a two-step process. The surfactant binds above a critical surfactant conce ntration; then, after a plateau is reached a new binding step evolves in th e isotherm. The hydrodynamic diameter of the particles also increases in a stepwise manner in correlation with the binding isotherm. The microgel-surf actant interaction is interpreted by means of polymer-surfactant interactio n theories. The surfactant binds in the gel as small aggregates with differ ent 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 parti cle core.