Direct Force Measurement of Silicone- and Hydrocarbon-Based ABA Triblock Surfactants in Alcoholic Media by Atomic Force Microscopy

Abstract

The surface force profiles between an atomic force microscopy (AFM) tip and a self-assembled monolayer of n-octadecyltrichlorosilane on oxidized silicon were measured in an ABA triblock series of silicone and Pluronic surfactant solutions. Changes in steric force barrier thickness and height were examined as a function of hydrophobic and hydrophilic block size and ethanol to water ratio in mixed solvents. In water, all the surface force profiles exhibited repulsive forces except for the Pluronic surfactants with short polyoxyethylene blocks. The steric barrier thickness increased with increasing hydrophilic block size, with an estimated force decay length proportional to (number of oxyethylene units) 0.62. However, the same data yielded no clear correlation between the hydrophobic chain block size and the steric barrier. The silicone surfactants provided steric repulsion up to 80% ethanol in solution, while the Pluronic surfactants lost their surface activity at approximately 40% ethanol in solution. The difference between the Pluronic surfactants and the silicone surfactants is attributed to the polysiloxane block being hydrophobic as well as oleophobic. In addition, the solubility and surface activity of silicone surfactants in alcoholic media are tunable by the ratio of oxyethylene to oxypropylene units.