and sodium and calcium salts of alkyl sulfates (C 12 , C 14 , and C 18 ). On virgin surfaces, free fatty acids and calcium salts of fatty acids have advancing contact angles (θ A ) between 77 and 92°, with little dependence on alkyl chain length for C 12 and higher alkyl chains. The sodium salt of a fatty acid has a lower θ A than the free fatty acid or the calcium salt of the soap. The calcium salt of dodecyl sulfate has a lower θ A than the calcium salt of dodecanoic acid (θ A = 46 vs. 82°), but the calcium salt of the 18-carbon hydrophobes showed nearly the same contact angle for the soap and the alkyl sulfate. Greasiness, or slipperyness, or a scummy feel of a precipitated surfactant does not necessarily correspond to a hydrophobic surface.

The contact angle of a saturated aqueous surfactant solution on the precipitate of that surfactant was measured by using the sessile drop method. The sodium and calcium salts of alkyl sulfates (C 12 , C 14 , and C 18 ) had advancing contact angles higher than those of alkyl trimethylammonium bromides (C 14 , C 16 , and C 18 ). The measured advancing contact angles for several surfactant solutions did not substantially change with varying surfactant/counterion ratios; therefore, the precipitating counterion concentration (e.g., water hardness) had little effect on the wettability. The contact angles of fatty acid (C 12 and C 16 ) solutions did not show any dependence on pH between a pH of 4 and 10. The contact angles of saturated calcium dodecanoate (CaC 12 ) solutions containing a second subsaturated surfactant (sodium dodecyl sulfate: NaDS) decreased with increasing NaDS concentrations until reaching the critical micelle concentration of the surfactant mixture. These results show that the second suractant can act as a wetting agent in this saturated surfactant system. Application of Young's equation to contact angles showed that the solid/liquid surface tension can change substantially with surfactant concentration and be important in addition to the liquid/vapor surface tension in reducing contact angles. Application of the Zisman equation results in a "critical" surface tension for the CaC 12 or soap scum of 25.5 mN/m, which is comparable to difluoroethene.