Contact angle of surfactant solutions on precipitated surfactant surfaces

In this study, the contact angle of a saturated aqueous surfactant solution onto the surface of a precipitate of that surfactant is investigated. Thos e precipitates include fatty acids (C-10, C-12, C-14, C-16, and C-18), sodi um salts of fatty acids (C-14, C-16, and C-18), calcium salts of fatty acid s (C-8, C-10, C-12, C-14, C-16, and C-18), 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 (theta (A)) between 77 and 92 degrees, with little dependence on alkyl chain length fo r C-12 and higher alkyl chains. The sodium salt of a fatty acid has a lower theta (A) than the free fatty acid or the calcium salt of the soap. The ca lcium salt of dodecyl sulfate has a lower theta (A) than the calcium salt o f dodecanoic acid (theta (A) = 46 vs. 82 degrees), 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 su rface.