Effects of sodium dodecyl sulfate on poly(N-isopropylacrylamide) adsorption at the air-water interface above the lower critical solubility temperature

The behavior of poly (N-isopropylacrylamide) (PNIPAM) at the air-water interface above the lower critical solubility temperature and the effects of sodium dodecyl sulfate (SDS) have been studied by neutron reflectivity. In the absence of SDS, PNIPAM adsorbs at the air-water interface and forms a thick and dense multilayer which can be characterized by two regions: a first region made up of pure polymer, and a second containing the same volume fraction of polymer as in the bulk precipitate. In the presence of SDS, polymer adsorption is decreased, and the structure of the adsorbed polymer can be described as a layer of colloidal particles of collapsed PNIPAM stabilized by SDS molecules. At higher SDS concentrations where the PNIPAM is completely resolubilized into charged chains, most, but not all, of the polymer is displaced from the surface, and the remaining adsorbed polymer forms a thin and dilute layer.