Surface Forces in Thin Liquid Films of H-Bonding Liquids Confined between Hydrophobic Surfaces
Hydrophobic interaction plays an important role in biology, daily lives, and a variety of industrial processes such as flotation. While the mechanisms of hydrophobic interactions at molecular scale, as in self-assembly and micellization, is relatively well understood, the mechanisms of macroscopic hydrophobic interactions have been controversial. It is, therefore, the objective of the present work to study the mechanisms of interactions between macroscopic hydrophobic surfaces in H-bonding liquids, including water, ethanol, and water-ethanol mixtures.
The first part of the present study involves the measurement of the hydrophobic forces in the thin liquid films (TLFs) confined between two identical hydrophobic surfaces of contact angle 95.3o using an atomic force microscope (AFM). The measurements are conducted in pure water, pure ethanol, and ethanol-water mixtures of varying mole fractions. The results show that strong attractive forces, not considered in the classical DLVO theory, are present in the colloid films formed with all of the H-bonding liquids tested. When an H-bonding liquid is confined between two hydrophobic surfaces, the vicinal liquid molecules form clusters in the TLFs and give rise to an attractive force. The cluster formation is a way to minimize free energy for the molecules denied of H-bonding with the substrates. Thus, solvophobic forces are the result of the antipathy between the CH2- and CH3-coated surface and H-bonding liquid confined in the film. A thermodynamic analysis of the solvophobic forces measured at different temperatures support this mechanism, in which solvophobic interactions entail decreases in the excess film enthalpy and entropy. The former represents the energy gained by building clusters, while the latter represents loss of entropy due to structure building.
Thus, hydrophobic interaction may be a subset of solvophobic interaction. The solvophobic forces are strongest in pure water and pure ethanol, and decrease when one is added to the other. Adding a very small amount of ethanol to water sharply reduced the solvophobic force due to the adsorption of the former with an inverse orientation. An exposure of the OH-group toward the aqueous phase decreases the antipathy between the surface and H-bonding liquid and hence causes the hydrophobic (or solvophobic) forces to decrease.
The second part of the study involves the measurement of the hydrophobic forces in the wetting films of water using the force apparatus for deformable surfaces (FADS). This new instrument recently developed at Virginia Tech is designed to monitor the deformation of bubbles to determine the surface forces in wetting films. In effect, an air bubble is used a force sensor. The measurements have been conducted with gold, chalcopyrite, and galena as substrates. The results obtained with all three minerals show that hydrophobic force increases with increasing water contact angle, suggesting that hydrophobic forces are inherent properties of hydrophobic surfaces rather than created from artifacts such as preexisting nanobubbles and/or cavitation. A utility of the intrinsic relationship between hydrophobic force and contact angle is to predict flotation kinetics from the hydrophobicity of the minerals of interest.