Probing effects of molecular-level heterogeneity of surface hydrophobicity on hydrophobic interactions in air/water/solid systems

Abstract

HypothesisHydrophobic interaction is crucial in various colloidal phenomena and engineering processes involving air/water/solid systems where polar and nonpolar substances usually mingle with each other. The molecular-level heterogeneity of surface hydrophobicity could potentially disorder the interfacial water molecules and thus influence the hydrophobic interaction strength, but the underlying mechanism is not completely clear. ExperimentsThe hydrophobic interactions between air bubbles and self-assembled monolayer surfaces bearing methyl and different polar moieties were precisely quantified via bubble probe atomic force microscope coupled with theoretical modeling analysis. FindingsIncreasing coverage of surface polar moieties on binary-component surfaces can apparently lower surface hydrophobicity θc, but hardly affect hydrophobic interaction’s decay length D0. Changing pH has no detectable effect on hydrophobic interactions involving CH3 and CH3/OH-ended surfaces. In contrast, θc and D0 decrease monotonically with pH rising for CH3/COOH-ended surface, because high pH tends to dissociate –COOH, enhancing the surface hydrophilicity and weakening the hydrophobic interaction. For CH3/NH2-ended surface, θc and D0 decrease sequentially with pH declining, because protonation of –NH2 can lower the surface hydrophobicity and weaken the hydrophobic interaction. This work improves the fundamental understanding of hydrophobic interactions in air/water/solid systems and provides useful insights into the interfacial assembly processes in relevant engineering applications.