ADSORPTION OF SMALL SILICONE POLYETHER SURFACTANTS AT THE AIR/WATER INTERFACE

Silicone polyether copolymers are used in a diverse number of applications, for instance, as foam stabilizers, wetting agents, and emulsifiers. We have explored the effect of surfactant structure, specifically the ethylene oxide (EO) chain length of a series of(Me(3)SiO)(2)Si(Me)-(CH2)(3)(EO)(x)OH (or Bis-(EO)(x)OH) molecules, on (1) the surfactant adsorption at the air/water surface, (2) the minimum surface tension of the surfactant solution, and (3) the critical aggregation concentration(CAC). The surface adsorption of these species does not vary with EO lengths x = 4-16, but decreases when x > 16. The results for x = 4-16 are consistent with a tightly packed surface monolayer whose area/molecule is determined by the dimensions and conformation of the Bis group, and the close packing of the Bis methyl groups is responsible for the low surface tension. For x > 16, the EO chain penetrates the surface and disrupts the close packing. Other findings include that the minimum surface tension increases as x increases. The CAC also increases as x increases, reflecting the increase in hydrophilicity of the surfactant as the EO content of the copolymer increases.