Roles of AOT molecules on the adhesive force between surfaces in cyclohexane with water contamination

The roles of water and sodium bis (2-ethylhexyl) sulphosuccinate (AOT) molecules on the adhesive force between a mica plate and a SiO2-surface in cyclohexane are investigated at the molecular level using an atomic force microscope (AFM). A small amount of water which is able to be dissolved into cyclohexane generates a strong adhesion force between hydrophilic surfaces, which is considered to be caused by the water bridging between surfaces. When the AOT is added to cyclohexane contaminated by water, the adhesive force decreases with the concentration of AOT, because of the removal of water available to form the water bridging. However, when a large amount of water is dissolved into cyclohexane with AOT, excessive water accumulates to form thin layers on the surfaces on which AOT adsorb standing to form firm layers. As long as the AOT layers are not broken, the adhesive force is negligible, but once they are broken by a force greater than a critical pushing force, the adhesive force appears. A possible mechanism for the adhesive force is illustrated schematically.