Monolayer films of poly(dimethylsiloxane) on aqueous surfactant solutions

Spreading at the air-solution interface, of poly(dimethylsiloxane) (PDMS), a widely used ingredient in commercial antifoaming agents, depends strongly on the type of surfactant dissolved in solution. To isolate which polymer-surfactant interactions control the process, we measure surface pressure isotherms for PDMS on a wide variety of surfactant solutions. Moreover, we directly probe the influence of electrostatic forces by addition of a 1:1 electrolyte to the solutions. Although surfactant type, anionic, cationic, or nonionic, and level of charge play only a minor role, the length of the surfactant tail can control the PDMS spreading behavior. For the homologous series of alkyltrimethylammonium bromides, a critical carbon chain length of 14 is found, above which PDMS displays ''complete'' wetting behavior. However, shorter chain lengths can induce a metastable condition of ''pseudopartial'' wetting. We utilize both two dimensional polymer scaling concepts and generalized spreading coefficients to rationalize our results.