Effects of Anionic Surfactant on the Coil-to-Globule Transition of Interfacial Poly(N-isopropylacrylamide)

Abstract

The temperature dependence of the dimensions of poly(N-isopropylacrylamide) (PNIPAM) chains attached to electrosterically stabilized polystyrene latex particles in the presence of sodium dodecyl sulfate (SDS) was studied by dynamic light scattering. The range of SDS concentrations studied increased from zero to the critical micellar concentration. Three types of transition curves were observed as the SDS concentration was increased. At lower SDS concentrations (<∼150 mg/L), most of the interfacial PNIPAM chains collapsed to their globular state in much the same fashion as in the absence of surfactant, with the collapse commencing under better than θ-conditions and being completed under worse than θ-conditions. The magnitude of the chain collapse under better than θ-solvency conditions, however, was reduced continuously and finally eliminated as the SDS concentration was further increased (>∼150 mg/L). The collapse transition accordingly became sharper and approached a cooperative first-order-like transition at the higher SDS concentrations studied. An unexpected finding was that at higher SDS concentrations (>∼800 mg/L), the collapse of the interfacial chains proceeded through a relatively weak transition before the onset of a much sharper collapse at the corresponding transition temperature. The temperature at which this weak transition commenced was about 32 °C, close to the θ-temperature and independent of the SDS concentration. The weak transition was interpreted as involving the formation of ion pairs between the charges on the PNIPAM−SDS complexes and their counterions, as well as perhaps the multiplet structure of ion pair aggregates.