Phase separation phenomena and thermodynamics of octylphenol polyoxyethylene ether in micellar solutions: Effect of high concentration of inorganic salts and linear correlation

[Display omitted] •Phase separation of octylphenol polyoxyethylene ether in micellar solution.•Effect of high concentration of inorganic salt and linear correlation.•Depression or increase of cloud point induced by salt and mechanism.•Thermodynamics and related theoretical description. The phase separation phenomena or cloud points (CP) behavior of nonionic octylphenol polyoxyethylene ether with oxyethylene glycol ether unit of 15 (OP-15) were investigated in aqueous micellar solutions containing high concentrations of inorganic salts including NaCl, KCl, MgCl2, CaCl2, BaCl2, AlCl3, FeCl3, Na2SO4 and Na2SiO3. The change in CP induced by salts and its mechanism are discussed theoretically. In aqueous solutions containing high concentration of salt, for the first time the linear relationship between CP and salt concentration is well established to characterize the depression of CP. Based on pseudo phase separation model, thermodynamic parameters are estimated and adopted to further describe the phase separation process and the interactions between OP-15 molecules or the OP-15/water interactions. Thermodynamic parameters including standard Gibbs energy change (ΔGCP0), enthalpy change (ΔHCP0) and entropy change (ΔSCP0) indicate that the additions of different salts result in the change in entropy-driven nonspontaneous process of phase separation. These data, which are from the compensation temperatures and the change of molar heat capacity, also prove that different ions induce the change in the interactions between surfactant molecules or the surfactant–water interaction and thus affect the depression of CP. These findings help with understanding the effect of inorganic salts, especially the high concentration of salt, on CP of nonionic surfactant and then broadening the application of nonionic surfactant based on CP such as the extraction of biomaterials in biological samples derived from animals or plants, etc.