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Microwaves, long used as a convenient household appliance, have been increasingly used in industrial processes such as organic synthesis and oil processing. It has been proposed that microwaves can enhance these chemical processes via a non-thermal effect. Here we report the instantaneous effect of microwaves on the permittivity and phase velocity of light in water through the in-situ measurement of changes in refractive index. Microwave irradiation was found to reduce the water refractive index (RI) sharply. The reduction increased as a function of microwave power to a far greater extent than expected from the change in temperature. The phase velocity of light in water increases up to ~ 5% (RI of 1.27) during microwave irradiation. Upon stopping irradiation, the return to the equilibrium RI was delayed by up to 30 min. Our measurement shows that microwaves have a profound non-thermal and long-lasting effect on the properties of water. Further investigation is planned to verify if the observed RI reduction is restricted to the region near the surface or deep inside water bulk. The observation suggests a relationship between microwave-induced and the enhanced aqueous reactions., (© 2022. The Author(s).)

Microwave-assisted de-emulsification is attractive in the processes of petroleum production and refining. The main advantage of microwaves is their direct influence on the surfactant layer at the oil/water interface. Previously, an effective interfacial modification was demonstrated by pulsed microwave irradiation. However, the effect of the modification diminished during the off interval of the pulse irradiation. In this study, two-stage microwave irradiation with different powers and durations was applied as a method to maintain an interfacial effect. The power of the second stage was changed to optimise the modification. Quick modification was obtained by high-power irradiation followed by low-power irradiation. It was confirmed a sustained modification was maintained by a moderate power of the second irradiation. This observation indicates a re-adsorption or re-structure process after the first irradiation is suppressed by the second irradiation. The results open new opportunities to optimise microwave operation in oil/water systems. [ABSTRACT FROM AUTHOR]

Microwave (MW)‐assisted de‐emulsification has been attractive in processes of petroleum production and refining. However, the mechanism and optimal operation remain poorly understood. In this study, the behaviour of decane‐in‐water emulsion under MW irradiation was monitored in‐situ through a size measurement system equipped with the reactor and surface tension profiles. The results indicated that the bubble was formed around the oil droplet during MW irradiation. The MW‐induced bubbles can enhance de‐emulsification in a similar mechanism as the flotation column. The efficiency of MW de‐emulsification depends on emulsion content and MW power. Although a higher MW power is more effective, boiling caused by excessive energy must be considered due to the higher local heating. As a result, moderate power is more desirable. For the decane‐in‐water emulsions in this study, the optimal condition was determined to be around 500 W. [ABSTRACT FROM AUTHOR]

Aggregate shape was predicted through aggregate size and number of particles in aggregate at different concentrations of electrolyte solution. • Bubble size in suspension of aggregates was measured during microwave irradiation. • Shape of aggregate was predicted through bubble size during microwave. • Growth of aggregate was influenced by initial number density. To efficiently operate microwave assisted nanoparticle synthesis, thermal behavior around aggregate during irradiation must be well-understood to ensure stable operation. The key information in this regard are aggregate size and shape. In this study, aggregate size before the irradiation and maximum bubble size during the irradiation were measured at the different induction time and varying number density of primary particle. It was found that maximum bubble size is more closely related to the number density of aggregate than the number density of primary particles. Further analysis on the results indicates that aggregate size and shape can be estimated using known data of bubble size. These findings provide useful information on how microwave assisted nanoparticle formation can be better operated. [ABSTRACT FROM AUTHOR]

Recently, it has been found that surface tension of a colloid solution maintained lower value by microwave irradiation even after the microwave was turned off. The results verified bubbles around particles due to microwave. In this study, the surface tension and particle-bubble were monitored during and after the irradiation for suspension from an aqueous solution of ferric chloride. The surface tension decreased with temperature during the irradiation. Furthermore, the tension did not recover to the original value. Simultaneously, it was confirmed that nano-bubble was formed around the particle when nanoparticles are formed during microwave heating. The effect of an anti-solvent, ethylene glycol, on the process was also investigated. It was found that ethylene glycol affects the bubble growth significantly during microwave irradiation. In contrast, the particle size was slightly affected by ethylene glycol. The results can be explained in terms of the H-bond networks of molecules around the liquid–air interface. [ABSTRACT FROM AUTHOR]

Recently, special behavior of interfacial tension during and after microwave irradiation was found. To understand the behavior in more details, microwave irradiation pattern was changed in this study. Higher power and longer irradiation time, which do not cause the boiling at the interface, are better for the reduction of interfacial tension and irradiation retention. Accordingly, the quick thermal response of microwave is essential for modification of interface compared to the conventional heating. In this way, interfacial tension does not return quickly to the original value, and effect of the reduction slowly disappeared for a few weeks. [ABSTRACT FROM AUTHOR]

Graphical abstract 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. [ABSTRACT FROM AUTHOR]

Graphical abstract Relation between bubble and nanoparticle sizes at different microwave power with and without anti-solvent. Highlights • A synergistic effect of microwave and anti-solvent on nanoparticle formation was studied. • In situ measurements of bubble and nanoparticle were carried out and reported. • Simultaneous suppression of superheating and rapid and finer nanoparticle formation was achieved. Abstract Microwave-assisted nanoparticle formation has been subject of interest of many researchers in producing nano-structured crystals and in this regard, previous studies have indicated that higher microwave power generally is desirable to have faster nanoparticle growth. Nevertheless, it has been acknowledged that high irradiation power especially at longer duration may cause superheating which degrades the product quality. Consequently, efforts should be dedicated in finding trade-off between preventing superheating and also ensuring both rapid particle growth and fine nanoparticle formation. This study was, therefore, carried out to propose and test a synergistic effect of microwave and addition of anti-solvent (ethylene glycol) in addressing the issues above. From the results and findings and comparisons with our previous studies on microwave assisted nanoparticle formation, adding anti-solvent is able to suppress superheating and swift bubble growth and consequently stable nanoparticle formation could be achieved. Moreover, it is also evident that a combined effect of microwave and anti-solvent results in not only stable but also finer nanoparticle formation. It is envisaged that the outcomes from this work may further contribute to fundamental aspects microwave assisted nanoparticle synthesis and its commercial applications. [ABSTRACT FROM AUTHOR]

Water behavior during and after microwave irradiation is important phenomena for industrial applications. Recently, we reported a non-thermal effect of microwave on water surface tension. The bubble formation during microwave irradiation was suggested as one of the underpinning causes. This study examined the possibility by monitoring bubble formation during and after microwave irradiation. Moreover, the influence of NaCl on surface tension and bubble formation were also investigated. It was found that the presence of NaCl strongly increased both bubble formation and surface tension effect. The results indicated that bubble formation played an important role in surface tension reduction. The phenomena can provide useful pathways to improve the efficiency of chemical processes with microwave. [ABSTRACT FROM AUTHOR]

ABSTRACT A Belousov-Zhabotinsky reaction in a pendant drop under microwave radiation was observed under nonstirring conditions. The period decreased as the irradiation power and time were increased because the droplet temperature increased. Moreover, the color change of the solution during the oscillatory reaction depended on the irradiation power and time although the droplet temperature returned to room temperature quickly. Irradiation at higher powers or longer times suspended the oscillatory reaction. However, for moderate irradiation powers or times, the oscillatory reaction stopped after irradiation because the concentration distribution of the reagents became homogeneous. After a time, the oscillatory reaction restarted because a concentration gradient developed again. Accordingly, the effect of the microwave irradiation history continued because the oscillatory reaction stopped even after the irradiation was stopped. This is because the rotation of the polar molecule caused by the irradiation produced a flat concentration distribution for the reagents through increased molecular diffusion. [ABSTRACT FROM AUTHOR]

Abstract: An improved reaction rate of biodiesel fuel formation was observed under microwave irradiation. Such irradiation is effective for not only heating during the reaction but also preheating. The advantages observed in this study suggest that a continuous process, that is, the introduction of reactant preheated by microwave irradiation into reactor will be feasible. The reasons for the improved reaction rate are explained through the use of optimized structures, dipole moments, IR spectra and activation energies obtained using molecular orbital calculations. Planar triolein formed by microwave irradiation has higher reactivity, a lower dipole moment, a lower activation energy and stronger vibration around the carboxyl carbon, and is more reactive than triolein with a higher dipole moment. [Copyright &y& Elsevier]

Bubble formation around nucleated particle is inevitable when quick heating of microwave is employed in nanoparticle synthesis. Since these bubbles may cause bumping behavior, the bubble growth must be possibly suppressed to ensure stable microwave‐assisted operation. In this study, sequential and simultaneous ultrasound treatment and microwave irradiations on nanoparticle synthesis are proposed and investigated to assess the benefit of ultrasound in preventing superheating behavior and producing finer particles. Experimental results indicate that the inclusions of ultrasound treatment result in finer particle sizes and suppression of bumping behavior. Particularly, consecutive ultrasound treatment and microwave irradiation are recommendable due to its practicality and generating smallest bubbles and final particle sizes. This technique can therefore be expected as an innovative way to enhance industrial applications of quick thermal response induced by microwave. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The present work investigates the influence of heating methods on the interfacial tension of the decane-water interface in the presence of the common industrial surfactant, Triton X-100. The work considers heating by conventional methods, such as a waterbath, in contrast to microwave irradiation. Use of the holm interface allowed the measurement of interfacial tension even during irradiation. The interfacial tension was found to increase as the samples were heated from 25 to 65 °C by the two methods. However, microwaves were found to elicit changes to the response of interfacial tension to changing temperature, resulting in a much more dramatic increase in interfacial tension. The impact of microwaves can be attributed to different networks of hydrogen bonding between surfactant and water at the interface and in the bulk. [ABSTRACT FROM AUTHOR]

Inthis study, microwave irradiation was applied to hanging dropletsof both water and ethylene glycol. Once the irradiation had ceasedand the droplet was allowed to return to its original temperature,it was found that the surface tension of ethylene glycol returnedto its original value. In contrast, the water surface tension remainedwell below its original value for an extended period of time. Similarobservations have been reported for magnetically treated water, butthis is the first time that such a lasting effect has been reportedfor microwave irradiation. The effect can be attributed to the uniquehydrogen bonds of interfacial water molecules. While the irradiationintensities used in this study are well above those in household devices,there is certainly the potential to apply the methodology to industrialapplications where the manipulation of surface tension is requiredwithout the use of chemical addition. [ABSTRACT FROM AUTHOR]

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. [Display omitted] [ABSTRACT FROM AUTHOR]