Attractive Forces between Hydrophobic Solid Surfaces Measured by AFM on the First Approach in Salt Solutions and in the Presence of Dissolved Gases

Abstract

Interfacial gas enrichment of dissolved gases (IGE) has been shown to cover hydrophobic solid surfaces in water. The atomic force microscopy (AFM) data has recently been supported by molecular dynamics simulation. It was demonstrated that IGE is responsible for the unexpected stability and large contact angle of gaseous nanobubbles at the hydrophobic solid-water interface. Here we provide further evidence of the significant effect of IGE on an attractive force between hydrophobic solid surfaces in water. The force in the presence of dissolved gas, i.e., in aerated and nonaerated NaCl solutions (up to 4 M), was measured by the AFM colloidal probe technique. The effect of nanobubble bridging on the attractive force was minimized or eliminated by measuring forces on the first approach of the AFM probe toward the flat hydrophobic surface and by using high salt concentrations to reduce gas solubility. Our results confirm the presence of three types of forces, two of which are long-range attractive forces of capillary bridging origin as caused by either surface nanobubbles or gap-induced cavitation. The third type is a short-range attractive force observed in the absence of interfacial nanobubbles that is attributed to the IGE in the form of a dense gas layer (DGL) at hydrophobic surfaces. Such a force was found to increase with increasing gas saturation and to decrease with decreasing gas solubility.

Mechanisms Governing the Transfer of Pure and Plant Matrix Carotenoids Toward Emulsified Triglycerides

“…To explain this observation, we hypothesize that the transfer efficiency depends both on the collisions between the agitated TG droplets and the film of carotenoids stuck to the glass tubes, and on the hydrophobic attraction between carotenoids and TG molecules when TG droplets enter in contact with the solid pure carotenoid film. Indeed, the transfer of nonpolar molecules from a polar environment to a nonpolar environment is driven both by the free energy gain and the hydrophobic force . The involvement of the hydrophobic force is further supported by previous finding and by the fact that the maximal transfer efficiency of LUT was lower than that of βC and LYC, i.e., 59% versus 77% and 72% respectively, which can be related to their respective hydrophobicity schematically reflected by their Log P , i.e., 11.78 versus 15.51 and 15.19, respectively (http://www.chemspider.com).…”

Influence of Interfacial Gas Enrichment on Controlled Coalescence of Oil Droplets in Water in Microfluidics

“…Except for the extensively documented effect of bridging nanobubbles and cavitation on the long-range attractive forces, a recent study found the interfacial gas enrichment (IGE) of dissolved gases significantly contributes to the short-range attractive forces between solid hydrophobic surfaces of an atomic force microscopy (AFM) probe and a hydrophobic solid substrate. 21 The existence of IGE is also examined by molecular dynamics simulation 22 and experimentally confirmed by the reduced water density at hydrophobic surfaces in the presence of dissolved gases observed via direct non-invasive neutron reflectivity measurements. 23 In addition, the intrinsic structure of water next to the oil phase has been proved to be similar to the bare water-vapor interface using molecular dynamics simulations of realistic models of alkanes and water.…”

Solute particle near a nanopore: influence of size and surface properties on the solvent-mediated forces

“…Depletion forces are observed in numerous fundamental processes such as colloidal self-assembly, [27][28][29][30] protein stabilization in helix 31,32 and the effective attraction between hydrophobic surfaces. 33,34 Microscopically, depletion forces result from two effects: (a) at very short separations, solvent molecules are excluded from the gap between the macroscopic objects ( particles, molecular chains and flat walls) thus generating a force; and (b) the formation of low-density phases in the gap between the objects due to capillary effects. The first contribution to the depletion force only depends on the geometry and the surface properties of the macroscopic objects whereas the second depends as well on the interaction between the surfaces and the solvent.…”