Effect of SHN on the Phase Behaviour of Some Cationic Surfactants

The physical properties of ionic surfactants are very sensitive to the nature of the counterions. In particular, counterions that have a tendency to adsorb on the surfactant micelle are known to dramatically modify the viscoelastic properties of these solutions through the formation of long wormlike micelles [1, 2, 3, 4]. One extreme limit of an adsorbed counterion is an oppositely charged surfactant, and aqueous solutions of such surfactant mixtures are known to show interesting properties, such as low critical micellar concentration (CMC), high surface activity, formation of liquid crystalline phases at higher water content and spontaneous formation of vesicles [5, 6, 7, 8, 9]. This chapter deals with the effect of the strongly bound hydroxy naphthoate (HN−) counterion on the phase behaviour of concentrated aqueous solutions of three cationic surfactants, namely, cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC) and dodecyltrimethylammonium bromide (DTAB). This counterion is introduced by adding 3-sodium2-hydroxy naphthoate (SHN) to the surfactant solution. Earlier studies on similar systems are described in section 2.2. In section 2.3, a brief description of the experimental techniques used in these studies has been given. Our results are described in section 2.4. At low amounts of SHN, the phase behavior of CTAB-SHN-water system is similar to that of the CTAB-water binary system, which exhibits a hexagonal (HI) phase over a wide range of water content. With increasing SHN concentration a lamellar phase with curvature defects is found instead of the HI phase. At