Fig. 5.

Possible mechanisms for long-range attraction between hydrophobic surfaces. (a) Although a depletion layer exists next to a hydrophobic surface, the range of thickness of this layer is typically only one to two water molecules, suggesting that only a short-range force should be operating. (b) The presence of a hydrophobic solute (or ion) also affects the local orientation of the surrounding water molecules, an effect that can propagate many molecular layers into the bulk. (c and d) Local charge fluctuations at one surface can influence the charge density of the opposing surface, causing a long-range attractive electrostatic interaction, such as that seen with patchy bilayers. (e) When present on hydrophobic surfaces, nanobubbles can coalescence, leading to an attractive Laplace pressure at large range.