Abstract
The contact angles of saturated calcium dodecanoate (CaC12) solution containing a second subsaturated surfactant on its precipitated surfaces were measured by using the drop shape analysis technique. The second subsaturated surfactants used were sodium dodecylsulfate (NaDS), sodium octanoate (NaC8), and nonylphenol ethoxylate nonionic surfactant (NPE). The results show that the second surfactants can act as a wetting agent. NaC8 is the most effective wetting agent, while NPE is the least effective one. The graphical plot between contact angles and surfactant concentrations can be used in determination of the CMC value. Addition of NaCl can reduce the CMC of the anionic surfactant mixture of subsaturated NaDS and saturated CaC12 solution by reducing electronic repulsion between surfactant molecules. The subsaturated NaDS system provides the highest effectiveness in spreading pressure and its value decreases with increase in NaCl concentration, while the subsaturated NPE system gives the lowest value. Different values of the critical surface tension obtained from the Zisman method of three subsaturated surfactants of NaDS, NPE, and NaC8 were due to molecular structures and specific adsorption effects.
