Studies on Surfactant-Ionic Liquid Interaction on Clouding Behaviour and Evaluation of Thermodynamic Parameters

The present study investigates the effect of tetraethyl ammonium tetrafluoroborate [TEA(BF4)] ionic liquid (IL) on the cloud point (CP) of the following nonionic surfactants in aqueous solution: ter-octylphenol ethoxylates with 9.5 and 4.5 ethylene oxide groups (abbreviated TOPEO9.5 and TOPEO4.5, respectively), cetyl alcohol ethoxylate with 10 ethylene oxide groups (C16EO10), and sorbitan monolaurate and monooleate both with 20 ethylene oxide groups (SMLEO20 and SMOEO20, respectively) in aqueous solutions. The thermodynamic parameters of these mixtures were calculated at different IL concentrations. The CP of most of the tested nonionic surfactants increased with the increment of IL concentrations with the exception of C16EO10 for which it decreased. The solubility of a nonionic surfactant containing polyoxyethylene (POE) hydrophilic chain was considered as maximum at the CP, hence the thermodynamic parameters were calculated at the same temperature. The results showed that the standard Gibbs free energy (Delta G(CP)(0)), the enthalpy (Delta H-CP(0)) and the entropy (Delta S-CP(0)) of the clouding phenomenon were found to be positive for ethoxylated octylphenol and sorbitan esters, whereas Delta H-CP(0) and Delta S-CP(0) were found to be negative for C16EO10. It was found that the overall clouding process is endothermic for ethoxylated octylphenol and sorbitan esters and exothermic for C16EO10. For all the studied systems, Delta H-CP(0) > T Delta S-CP(0) indicated that the process of clouding is guided by both enthalpy and entropy. The positive value of standard Gibbs free energy (Delta G(CP)(0)) for the all mixed systems indicated that the process proceeds non-spontaneously. The Delta G(CP)(0) decreased with increasing IL concentration for all the nonionic surfactants; however, it decreased with increasing surfactant concentration for TOPEO9.5, C16EO10, and SMOEO20, and increased with increasing surfactant concentration for TOPEO9.5 and SMLEO20.