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 5–6nm. 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 6nm. 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 0.3–1.0nm. In particular, attractive forces have been measured on small radius surfaces (8–50nm) consistent with a hydrophobic force with a decay length of 0.5–0.6nm, and a variety of net repulsive measurements are consistent with an attractive hydrophobic force that has a decay length of 0.3–1.0nm. We also discuss some new measurements, which are consistent with cavitation, and not a surface force that acts at a distance.