Forces between extended hydrophobic solids: Is there a long-range hydrophobic force?

Thirty years ago there was considerable excitement over the first report of a long-ranged "hydrophobic force" between solids that were not wet by water (Israelachvili and Pashley, Nature 1982, 300, 341-342). Many of the subsequent measurements have been reexamined and found not to support the existence of a long-range hydrophobic force. The principal difficulty was that hydrophobic solids frequently experience other forces, which obscured or were mistaken for a hydrophobic force. In this paper, we review the surviving evidence for a long-range hydrophobic force and find that there is only supporting evidence in a total of two papers, one old and one new, where net attractive forces were measured at separations greater than about 5-6 nm. Thus the evidence is scarce. In contrast there are new experiments showing no such force, thereby arguing against the universality of a measureable hydrophobic force beyond about 6 nm. Since solvent water is common to the experiments, such evidence makes it difficult to describe a universal mechanism for a long-ranged hydrophobic force based on water structure. There are also new measurements that are consistent with a hydrophobic force with a decay length in the range 0.3-1.0 nm. In particular, attractive forces have been measured on small radius surfaces (8-50 nm) consistent with a hydrophobic force with a decay length of 0.5-0.6 nm, and a variety of net repulsive measurements are consistent with an attractive hydrophobic force that has a decay length of 03-1.0 nm. We also discuss some new measurements, which are consistent with cavitation, and not a surface force that acts at a distance. (C) 2016 Elsevier Ltd. All rights reserved.