Evaluation of interaction forces between hydrophobic surfaces in surfactant solutions by atomic force microscopy

The interaction forces between silica surfaces with various degrees of hydrophobicity were measured using colloidal probe atomic force microscopy to clarify the relationship between the hydrophobicity of the surfaces and hydrophobic attraction in detail. The interaction forces between silica surfaces pre-hydrophobised with octadecyltrichlorosilane were measured in aqueous solutions with three different (cationic, anionic, and nonionic) surfactants. Hydrophobic attraction was observed between surfaces from 10 to 15 nm without surfactant, and the range of this attraction decreased when the surface hydrophobicity decreased with increasing surfactant concentration. The hydrophobic attraction disappeared when the surface contact angle was below 80°–90° in the presence of all surfactants used in this study. The range of the hydrophobic attraction was found to depend strongly on the contact angle and not on the surfactant type and concentration. The adhesion force was also highly dependent on the contact angle and comparable to the theoretical capillary force. The possible origin of this force is discussed with reference to the obtained results. Graphical Abstract