Hydrophobic repulsion and its origin

TL;DR

This study uses advanced atomic force microscopy to reveal that hydrophobic interactions are attractive at longer distances but become strongly repulsive at very short ranges, challenging previous assumptions.

Contribution

The paper provides direct experimental evidence of short-range hydrophobic repulsion and links it to air accumulation near hydrophobic surfaces, offering new insights into hydrophobic forces.

Findings

Hydrophobic interactions switch from attraction to repulsion at sub-3 nanometers.

Short-range repulsion is linked to air accumulation near surfaces.

Mid-range attraction is also explained by air presence.

Abstract

The fundamental role of hydrophobic interactions in nature and technology has motivated decades long research aimed at measuring the distance-dependent hydrophobic force and identifying its origin. This quest has nevertheless proved more elusive than anticipated and the nature of the interaction at distances shorter than 2-3 nanometers, or even its sign, have never been conclusively determined. Employing an ultra-high resolution frequency-modulation atomic force microscope (FM-AFM) we succeeded in measuring the interaction at all distances and discovered that the commonly observed attraction at 3-10 nanometer distances turns into pronounced repulsion below 0.3-3 nanometers, depending on the hydrophobicity of the AFM tip and the surface. This generic short-range repulsion disproves the prevailing dogma that hydrophobic interactions are attractive at all distances, hence bearing on practically all biological and technological hydrophobic phenomena. The short-range repulsion, as well as the mid-range attraction, are traced by experiment and theory to the prevalent air accumulation near hydrophobic surfaces.