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Demulsification is a fundamental process in many industries, including food, petroleum production and refining, with ongoing research into cheaper and more efficient processes. Microwaves have been considered as a low-cost and efficient method to speed demulsification: they can pass through the non-polar oil phase to be absorbed in the polar aqueous phase, effectively targeting the liquid-liquid interface. This study uses in-situ measurement of interfacial tension to investigate the effect of repeated short duration microwave irradiation on an oil-water interface in the presence of the non-ionic surfactant, Triton X-100. It was previously shown that a high energy concentration at the interface – achieved by microwave irradiation – resulted in rapid increases in interfacial tension. In this paper, the mechanism behind the change in interfacial tension induced by microwave irradiation is discussed: we propose that the rapid switching of the induced electromagnetic field causes disorder and desorption of surfactant adsorbed at the oil-water interface, leading to the short-term increase in interfacial tension. Management of the overall energy concentration is achieved by patterns of short-term pulsed irradiation to avoid boiling. The results indicate that disruption of the bonding network in the interfacial layer, and hence effective modification of interfacial tension, can be achieved by repeated irradiation of high power microwaves without leading to boiling. A previously discussed dimensionless number was used to characterize the extent of energy concentration at the interface in each experiment.

The behaviour of a long-tailed surfactant (Triton X-405) at the water-octane interface was monitored in-situ during microwave heating in the presence of an electrolyte (NaCl). The interfacial tension and bulk temperature were analysed during and after pulsed microwave irradiation. The energy concentration at the interface, which we propose affects the molecular behaviour at the interface, was characterised by a dimensionless number recently proposed by our group. Although the non-ionic surfactant layer was stabilised by the electrolyte solutions, rapid changes to the interfacial conditions, particularly the movement and desorption of surfactants at the interfacial layer, were triggered by pulsed (short, intermittent) irradiation of high microwave power. The dimensionless number, which incorporates both interfacial and bulk properties, can be used as an effective indicator for determining the heating behaviour by accounting for the ability for energy to dissipate through the bulk solutions. The results can be useful to determine an optimal condition for a rapid disruption of the surfactant layer without causing boiling at the interface, making it possible to target conditions desirable for the de-emulsification process.

In our recent study, a new dimensionless number for microwave local heating at an interface was proposed for explanation of surfactant desorption behaviour in water and oil system. In this study, interfacial behaviour for both boiling and surfactant desorption at water/decane interface was investigated. In particular, a system of water phase with a non-ionic surfactant (Triton X-100) and NaCl was used. Several pulsed irradiation patterns were employed to vary the microwave energy density, and interfacial tension was measured at each condition. It was found that Triton X-100 layer in NaCl aqueous solution is more resistant to interfacial boiling than the Triton X-405 layer in a previous study. The dimensionless number can be used as an indicator to distinguish stable heating and boiling regions. For industrial practices, preventing boiling is desirable to get surfactant desorption efficiently and stably. The results present a practical method to maximum microwave effect on oil/water interface.

In previous study, the interfacial tension during microwave irradiation was measured in the presence of the non-ionic surfactant, Triton X-100. If the surfactant is short, quick increase during the pulse irradiation was observed due to desorption caused by the quick thermal response of microwave. Although operation of microwave irradiation would be useful for demulsification processes, combinations of two liquids and surfactant are various and need further studies. Accordingly, optimization of microwave local heating is required for industrial application of this technique. In this study, desorption of longer surfactant, Triton X-405 as a result of microwave irradiation was investigated at different power, and dimensionless number for the local heating around the interface was proposed to estimate surfactant behaviour for the desorption and disorder quantitatively.