Influence of hofmeister anions on the krafft temperature, surface and bulk behaviour of ionic surfactants in aqueous solution

Abstract

In the present work, the influence of Hofmeister anions on the Krafft Temperature (TK) of cetylpyridinium chloride (CPC) and cetylpyridinium bromide (CPB) has been investigated by means of conductometric, surface tensiometric methods, and visual inspection. The surface adsorption and bulk micellization as well as the solubilization of water insoluble 1–(2–pyridylazo)–2–naphthal (PAN) and curcumin in aqueous solution of surfactants CPB and CPC have been investigated. It was found that the TK of these surfactants depends on the nature and ionic strength of the counter-ion present in the solution. The results show that the TK of these surfactants decreases in the presence of stronger kosmotropes as well as moderate chaotropes but increases in the presence of stronger chaotropes and the common ion compared to that of CPB and CPC in pure water.Added electrolytes also affect the surface adsorption and bulk micelization of both these surfactants. It was found that in the presence of all the anions,the CMC of both these surfactants decrease more than the CMC of these surfactants in pure water due to the neutralization of the micelle surface charge by the associated counter-ions. The CMC of these surfactants in pure water and in the presence of potassium salts was found to increase gradually with increasing temperature. The surface excess concentration (max) of both these surfactants was found to decrease gradually with increasing temperature and the values are much lower in the presence of electrolyte than the corresponding values of these surfactants in pure water. The thermodynamic parameters such as free energy change(∆G_(mic.)^°,∆G_(ad.)^°), enthalpy change (∆H_(mic.)^°,∆H_(ad.)^°), and entropy change (∆S_(mic.)^°,∆S_(ad.)^°) were calculated by measuring their CMC over a wide range of temperature. The values of 〖∆G〗_(mic.)^°,〖∆G〗_(ad.)^° are found to be negative which indicates that both the processes are spontaneous. The enthalpy and entropy terms for both adsorption and micellization are found to compensate each other. In addition, the solubilization behavior of water-insoluble PAN and curcumin in micellar system of CPB and CPC respectively was examined at 30 °C as well as complexation of Cu(II), Ni(II), Zn(II) with the PAN and Cu(II) with the curcumin in the micellar system were also studied by the UV-visible spectrophotometric technique. The location of solubilized ligands and complexes has also been measured by 1HNMR spectra of CPC and CPB in aqueous solution. It has also been found that in the presence of potassium salt, the solubilization capacity is higher than that of the surfactantin pure water.