Controlling the thermodynamic stability of intermediate phases in a cationic-amphiphile-water system with strongly binding counterions.

We have studied the influence of two structurally isomeric organic salts, namely, 2-sodium-3-hydroxy naphthoate (SHN) and 1-sodium-2-hydroxy naphthoate (SHN1), on the phase behavior of concentrated aqueous solutions of the cationic surfactant cetylpyridinium chloride (CPC). Partial phase diagrams of the two systems have been constructed using polarizing optical microscopy and x-ray diffraction techniques. A variety of intermediate phases is seen in both systems for a range of salt concentrations. The CPC-SHN-water system exhibits the rhombohedral and tetragonal mesh phases in addition to the random mesh phase, whereas the CPC-SHN1-water system shows only the tetragonal and random mesh phases. The CPC-SHN-water system also exhibits two nematic phases consisting of cylindrical and disk-like micelles at relatively low and high salt concentrations, respectively. These results show that the concentration of the strongly bound counterion provided by the organic salt can be used as a control parameter to tune the stability of different intermediate phases in amphiphile-water systems.