Study of the long-range hydrophobic attraction in concentrated salt solutions and its implications for electrostatic models

Direct force measurements of the interaction between silica surfaces in low concentrations of the soluble cationic surfactant cetyltrimethylammonium bromide reveal a long-range strongly attractive hydrophobic force extending out to 100 nm. Similar forces observed for cetylpyridinium chloride in 0.1 M electrolyte demonstrate that the hydrophobic attraction is undiminished in the presence of concentrated electrolyte. These results are discussed in relation to other investigations of the long-range hydrophobic attraction and lead to the conclusion that electrostatic double layer forces are unable to account for these attractive forces.