Direct Measurement of Solvophobic Force between Particles in Liquids and Its Origin

Abstract

The interaction forces between hydrophobized silica surfaces with no nanobubbles were measured using atomic force microscopy (AFM). To obtain hydrophobic surfaces without nanobubbles, an aqueous solution was introduced between the surfaces following an exchange process involving several solvents. In the obtained approaching force curves, an attractive force was observed from a distance of 10–20 nm, which is an additional attractive force stronger than typical van der Waals attractions. We also conducted the force measurements between the fluorinated silica surfaces in formamide and found an attractive force that is stronger than the van der Waals attraction. This result confirms that the ‘solvophobic attraction’, which is very similar to the hydrophobic attraction, exists in some organic solvents. The interaction forces were also measured between a hydrophobized substrate and an AFM tip, which has the small radius of curvature. The range of the attraction was found to become smaller abruptly when the radius of curvature of the tip is around 100–200 nm, implying that the origin of the force is different depending on the size of the surfaces.