Direct force measurement of comb silicone surfactants in alcoholic media by atomic force microscopy

The interaction forces provided by silicone polyether (SPE) surfactants, ad sorbed at the interface of octadecyltrichlorosilan monolayer and aqueous su rfactant solution, were measured by atomic force microscopy. Changes in the forces in the presence of ethanol were examined for a series of comb-type SPE surfactants. It was found that the steric repulsion provided by the SPE molecules persisted up to an 80% or higher ethanol level. Conversely, conv entional hydrocarbon surfactants such as dodecyl polyoxyethylene ether lose their interfacial activity at much lower ethanol levels. The force measure ment results were compared favorably with the findings from emulsion-stabil ity and interfacial-tension measurements. The difference between the conven tional hydrocarbon surfactants and the SPE surfactants is attributed to the polysiloxane moiety being hydrophobic but not oleophilic. The polyoxypropy lene units in the polyether chain were found to add unique characteristics to the surface behavior of the SPE surfactants by modifying the hydrophile' s solubility in the alcoholic media. The surface activity and colloidal sta bility can be fine-tuned by the ratio of polyoxypropylene to polyoxyethylen e. In general, emulsion and colloidal products and processes that contain a lcohol or other nonaqueous media will benefit from the unique properties of this special class of SPE surfactants. (C) 2001 Academic Press.