Physicochemistry of the binary interacting mixtures of cetylpyridinium chloride (CPC) and sodium dodecylsulfate (SDS) with special reference to the catanionic ion-pair (coacervate) behavior

The interaction between the cationic surfactant cetylpyridinuim chloride (CPC) and the anionic surfactant sodium dodecylsulfate (SDS) has been studied both at the air/water interface and in the bulk along with the formation of ‘catanionic coacervate’ or ion-pair amphiphile (IPA) by the coulombic interaction between the binary oppositely charged surfactants. The ternary phase behavior of the CPC/SDS/H2O system has been investigated. Results have been analyzed in the light of Gibbs adsorption isotherm and the regular solution theory of Rubingh at five different CPC:SDS mole ratios. Of the studied combinations, the equimolar composition has evidenced maximum interaction with a large synergistic effect. The isolated coacervate has been characterized by FTIR, NMR, XRD, and DSC measurements. The Langmuir monolayer formation and its π–A isotherm has been studied for the coacervate film under compression–expansion conditions. The interfacially disposed film has formed morphology differences under compressions. Polarizing microscopic measurements of the transferred monolayer on glass surface have evidenced diffusion limited fractal structures with concentric circular growths whereas the epifluorescence images showed poly and monodisperse domains at the air/water interface.