Hydrophobic interactions in salt solutions

Abstract

The fundamental understanding of hydrophobic interactions and forces between hydrophobic (water-repellent) surfaces in water is central to many areas of daily activities in many industries. Examples are broad and range from dissolving oily drugs into aqueous solutions for delivery to the body to the coalescence or aggregation of air bubbles, droplets and solid particles dispersed in water. Recent studies show controversial effects of dissolved (hydrophobic) gases in the water on hydrophobic forces. This PhD project aims to further investigate the role of dissolved gases in hydrophobic interactions. The concentration of dissolved gases in water can be controlled using inorganic salt solutions, which possess different levels of gas solubility at different salt concentrations. The use of salt solutions can reduce the effect of the electrical repulsion between charged surfaces in water, allowing a better quantification of hydrophobic forces. Atomic Force Microscopy (AFM), with surface force measurement and surface imaging abilities, was used to study the interaction between hydrophobic surfaces. Force measurements were carried out in salt solutions and were quantified by comparing experimental data with the classical theory of colloid stability. The electrical double-layer force was quantified using numerical solutions of the Poisson-Boltzmann equation. Van der Waals forces were calculated by applying the Lifshitz-Hamaker theory, which is based on quantum mechanics. It was hypothesised that dissolved gases would accumulate at the interface between water and hydrophobic surface in the form of dense gas layer changing the structure of interfacial water molecules and their hydrogen bond network, and cause the hydrophobic attraction. The research also examined the effect of dissolved gases on the force between hydrophobic surfaces in various salt types including NaCl, LiCl, KCl, and CsCl. The results showed that increasing salt concentration, or equivalently decreasing gas solubility, decreased the range and magnitude of the force between hydrophobic surfaces. The behaviour was found to be consistent across different salts tested. There is limited evidence of the presence of an interactive force between hydrophobic solid surfaces in non-aqueous solutions. The interaction between hydrophobic surfaces was also studied in non-aqueous solutions including formamide and dimethyl sulfoxide (DMSO). The AFM force measurement results showed that there is a short-range attractive force between the hydrophobic solid surfaces in formamide, while no attraction was found in the DMSO solution. However, an attractive force was observed between the hydrophobic solid surfaces in DMSO solutions mixed with water at 50-50% and 25-75% DMSO-water mixtures. The results suggested that the solution bonding structure and the presence of dissolved gas molecules affect and control the interaction between the hydrophobic solid surfaces. This research provides further evidence that the short-range attraction between hydrophobic surfaces is affected by the presence of dissolved gases. The recommendations for future research include: studying the hydrophobic interactions in non-aqueous solutions, using ellipsometry to study the possibility of interfacial gas enrichment, using capillary tube and high speed camera to study hydrophobic interactions and effect of dissolved gases, and using in-situ surface enhance Raman spectroscopy to probe the possibility of presence of gas layer in the vicinity of hydrophobic surface.