Polydimethylsiloxane monolayers at an air-water interface

Abstract

A part of our studies relating to the dynamics of interfaces has been an investigation of diffusional processes along gasliquid interfaces. These studies are conducted under conditions of uniform surface pressure as contrasted to spreading experiments which are associated with gradients of surface pressure and as a consequence, hydrodynamic flows. The diffusional processes arise from random thermal motions and should partly characterize the interaction of the substrate with the surface molecules. Also, it should be noted that surface diffusion coefficients appear in many formulations of macroscopic transport problems (1) although such coefficients have never been measured. The measurement of surface diffusion coefficients may, in fact, never be achieved owing to the structure exhibited by monolayers. This note describes the strange and largely unexplained observations made during an attempt to study diffusive mixing of polydimethylsiloxane (PMS) monolayers on water. Such monolayers seem to have a structure which is of macroscopic dimensions, and, in a surface diffusion experiment, these structures apparently migrate as a unit. If all monolayers of importance are similarly structured, then the formulations of surface transport based on the concept of a surface diffusion “driven” by variations in surface concentration are likely to be of only academic importance. A secondary result has been the measurement of the bulk diffusivity of a 40 centistoke radioactive PMS in water. This quantity should be of interest in considering the applications of PMS as an antifoamant or as a water repellent (2).