Thermodynamic parameters of clouding phenomenon in nonionic surfactants: The effect of the electrolytes

In this study, the thermodynamics of the cloud point (CP) of the two non-ionic surfactants, polyoxyethylene (40) iso octylphenyl ether, (TX-405) and polyoxyethylene (23) lauryl ether, Brij-35 were investigated in different electrolyte concentrations. In the measurements of cloud point temperatures, UV-visible spectrophotometer was used instead of visual observation. Li2SO4, Na2SO4, KNO3, K2SO4, K3PO4 and (NH4)2SO4 were used as electrolytes. Thermodynamic parameters were calculated from cloud point values of nonionic surfactant-electrolyte mixtures. The CP values in the presence of KNO3 were obtained by an extrapolation of the straight line. The results of the energetics of clouding showed that the standard Gibbs free energy (ΔGCPo) was positive but the enthalpy (ΔHCPo) and the entropy (ΔSCPo) were negative in all cases. Furthermore, the enthalpy-entropy compensation relationship was estimated and the compensation temperature values (Tc) values were found as 389.70 K and 391.19 K for TX-405 and Brij-35, respectively.