Abstract
The short-range attractive forces between hydrophobic surfaces are key factors in a wide range of areas such as protein folding, lipid self-assembly, and particle-bubble interaction such as in industrial flotation. Little is certain about the effect of dissolved (well-controlled) gases on the interaction forces, in particular in those systems where the formation of surface nanobubble bridges is suppressed. Here, we probe the short-range attractive force between hydrophobized silica surfaces in aqueous solutions with varying but well-controlled isotherms of gas solubility. The first contact approach force measurement method using AFM shows that decreasing gas solubility results in a decrease of the force magnitude as well as shortening of its range. The behavior was found to be consistent across all four aqueous systems and gas solubilities tested. Using numerical computations, we corroborate that attractive force can be adequately explained by a multilayer dispersion force model, which accounts for an interfacial gas enrichment (IGE), that results in the formation of a dense gas layer (DGL) adjacent to the hydrophobic surface. We found that the DGL on the hydrophobic surface is affected only by the concentration of dissolved gases and is independent of the salt type, used to control the gas solubility, which excludes the effect of electrical double-layer interactions on the hydrophobic force.
Highlights
Hydrophobic interactions are known to be critical in a wide range of areas, including protein folding and aggregation, a lipid assembly, particle-bubble interactions in both biological systems, and industrial processes such as froth flotation separation of hydrophobic particles1
We note that the attractive force in different salts with the same concentration is different, which excludes the possibility that the observed changes are dominated by non-linear polarizability effects observed in the concentrated 1:1 salt solutions
We have observed that the range and magnitude of the short-range force between the hydrophobic surfaces decreased with decreasing concentration of dissolved gases in the aqueous solutions
Summary
Hydrophobic interactions are known to be critical in a wide range of areas, including protein folding and aggregation, a lipid assembly, particle-bubble interactions in both biological systems, and industrial processes such as froth flotation separation of hydrophobic particles. In most systems under normal atmospheric conditions, the nucleation of surface nanobubbles can hardly be avoided, resulting in a large range attraction force with a span of up to several hundred nanometres as measured by the conventional AFM (Atomic Force Microscopy) technique using the AFM colloid probes In this conventional technique, the solid particle on the probe was first brought into contact with the solid substrate to establish the “zero” inter-surface separation distance and cyclically separated from the contact and brought to the contact for a few times to allow an averaging of the actual data for the force curve which is expected to be statistically representative. Some of the most important existing theories include; the entropic origin originating from the configurational rearrangement of the vicinal water molecules between the hydrophobic surfaces, cavitation or separation induced phase transition, fluctuation correlation mechanisms, whereby confinement induces large density fluctuations that may result in anomalous hydrodynamic pressure, aqueous charge, or dipole correlations, Anomalous polarization of vicinal water molecules, and bridging of submicron bubbles referred to the nanobubble bridging capillary force
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