Origin of the apparent long-range attractive force between surfaces in cyclohexane

The origin for the long-range attractive force between hydrophilic surfaces, which appears frequently in non-polar and hydrophobic media, is investigated using an atomic force microscope. It is found that (i) the frequency distribution of the long-range attractive force decreases monotonically with increasing range of force, (ii) the interaction cannot be explained by the water-bridging between hydrophilic surfaces and (iii) the decay of the interaction force follows the prediction given by the spontaneous discharging process of surfaces. In conclusion, the origin of the long-range interaction between hydrophilic surfaces in a hydrophobic medium is the initial charge of the surfaces.