Foam films from thermosensitive PNIPAM and SDS solutions

Thin liquid foam films stabilized with the addition of a temperature-sensitive neutral polymer, poly(N-isopropylacrylamide) (PNIPAM), in the presence of varying amounts of an anionic surfactant, SDS, were investigated using the thin-film balance. Data were analyzed in light of new and previously reported neutron reflectivity data describing the adsorption behavior of PNIPAM/SDS mixtures at the single air-water interface. In the case of thin films prepared from pure polymer solutions, unexpectedly thick and stable films were observed, and increases in solution temperature or polymer molecular weight induce corresponding increases in the thin-film thickness. This behavior is attributed to a stabilization of the films by repulsive entropic forces between long dangling polymer tails adsorbed at the interfaces. The addition of SDS causes a striking decrease in the thin-film thickness that is very well correlated with the desorption of polymer chains from the interface resulting from polymer-surfactant interactions in the bulk. Upon SDS introduction, repulsive forces stabilizing the films vary from purely entropic forces for small amounts of surfactant to supramolecular oscillatory forces at high SDS concentrations, whereas a combination of electrostatic and entropic forces is expected in intermediate concentration regions. Properties of thin films made from PNIPAM and SDS are thus highly sensitive to parameters such as the polymer-surfactant ratio, temperature, and molecular weight, offering precise control of the thin-film thickness that could be used to tailor foam characteristics.