Thermal response of a non-ionic surfactant layer at the water/oil interface during microwave heating

Abstract

This study measured in-situ the interfacial tension of the decane/water interface during microwave radiation, in the presence of the non-ionic surfactants Triton X-100 and X-405. It has been found that the two surfactants, with different hydrophilicity, displayed contrasting responses to the microwave heating. For X-100, the interfacial tension increased with the solution temperature. However, for X-105, the tension decreased as the solution temperature rose. The tension-temperature trends were consistent for a range of different microwave pulsing patterns. The results can be explained by considering the molecular origins of the interfacial forces. In particular, a semi-quantitative analysis verified that the hydrophobic interaction between water in the bulk and water associated with the surfactant tails increases with rising temperature. This thermal response is opposite to the relationship between temperature and hydrophilic interaction. The results demonstrated that rigorous thermodynamic analysis must be applied in combination with the conventional HLB in designing targeted surfactant systems.