MONOLAYER FILMS OF POLY(DIMETHYLSILOXANE) ON AQUEOUS SURFACTANT SOLUTIONS

Spreading at the air-solution interface, of poly(dimethylsiloxane) (PD MS), a widely used ingredient in commercial antifoaming agents, depend s 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 s olutions. Moreover, we directly probe the influence of electrostatic f orces by addition of a 1:1 electrolyte to the solutions. Although surf actant type, anionic, cationic, or nonionic, and level of charge play only a minor role, the length of the surfactant tail can control the P DMS spreading behavior. For the homologous series of alkyltrimethylamm onium bromides, a critical carbon chain length of 14 is found, above w hich PDMS displays ''complete'' wetting behavior. However, shorter cha in lengths can induce a metastable condition of ''pseudopartial'' wett ing. We utilize both two dimensional polymer scaling concepts and gene ralized spreading coefficients to rationalize our results.