Contact angle of surfactant solutions on precipitated surfactant surfaces. IV. calcium dodecyl sulfate system

Contact angles of calcium dodecyl sulfate (CDS) precipitate in equilibrium with its saturation solution and subsatured surfactantssolutions were studied. The second surfactants were anionic sodium decyl sulfate (SDeS), anionic sodium noctanoate (NaC8), and nonionic nonylphenol polyethoxylate (NPE, EO9). The results show the wettability of the second surfactants as NaC8>NPE>SDeS corresponding to contact angles at CMC of 41, 52, respectively. Surface tension at the CMC increased in order NaC8 < NPE <SDeS corresponding to 28 mN/m,39 mN/m respectively and subsaturated surfactant adsorption increased in the order of NPE << SDeS, whereas the adsorption of the NaC8 was immeasurable. Interfactial tension rduction at the solid/liquid interface due to subsaturated surfactant adsorption contributed to contact angle reduction in addition to surface tension reduction at the air/water interface. Greasiness, or slipperiness, or scummy feel of precipitated CDS does not significantly imply ahydrophobic surface. It was found out that C10H21O3SNa has low carbon chain length of an alkyl group, which considerably dissolved the CDS solid with remarkable reduction of surface tension at very low concetration then suddenly increased until it reaches plateau at 50 mM subsaturated SDeS concentration.