Non-thermal effect of microwave in supercritical water: A molecular dynamics simulation study
Introduction
Supercritical water acted a very significant medium has been employed to develop new technologies contributed to environmental friendly and energy renewable [1], [2], [3], [4], [5], [6], [7]; and supercritical water oxidation (SCWO), supercritical water biomass gasification (SCBG) and hydrolysis of polymers in supercritical water (HPSCW) for composites/plastics recycling are mainly three applications for supercritical water technology [7]. The major reason for these wide applications is that the SCW has numerous unusual properties different from bulk water because of the special gas–liquid state, such as high diffusivity, high thermal conductivity, low viscosity and low dielectric constant. Numerous experiments and simulations [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] were carried out aimed at elucidating microscopic structure and physiochemical properties of SCW. Note that the hydrogen bond network in SCW is weaker than that in bulk water and the tetrahedral structure of bulk water is no longer present in SCW [21]. In supercritical water, hydrogen bonding plays an important role, which can affect the structure and observable properties of water.
Meanwhile, microwave irradiation as an alternative heat source have been applied in a variety of physiochemical purposes such as heating, synthesis of catalysts, separation processes and the enhancement of reaction rates [22], [23], [24]. It also has been introduced into the extraction of Chinese herbal medicine, environmental protection and bio-energy [25], [26], [27], [28], [29], [30]. The influences of microwave field on molecular systems provide a significant insight into their microscopic interactions with field [31]. A series of investigations [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46] have been conducted to evaluate the microwave irradiation effect on molecular liquids and materials; in particular, the non-thermal effects have been discussed. Recent years, further studies have shown the potential advantages that the chemical properties that supercritical and near critical fluids in conjunction with microwave irradiation were applied in green chemistry [47], [48], [49]. Combining supercritical water as a solvent with microwave heating offers an easy and effective method for chemical reactions. Specially, an instance of this have analyzed the interaction between glucose gasification process in supercritical water and the external microwaves, where external electric field improved the yields of hydrogen gas [50]. Although the properties of liquids and fluids under external electromagnetic field have been investigated, the mechanism of microwave non-thermal effect on the structure and properties of SCW is not fully understood.
Since the high temperature and pressure, it is considerable difficulty to measure the structure and properties of SCW by direct experiment. And the microwave non-thermal effect is also hard observed in the ambient laboratory conditions. Molecular dynamics is an efficient simulation methodology to study the microwave non-thermal effect on a variety of molecular systems at all kinds of fastidious condition. It simulated at the atomistic level, which affords both structural and time-dependent information on the basis of the intermolecular interactions [51], [52]. So we utilize the molecular dynamics simulation to investigate the non-thermal and field-induced effect on the SCW. The purpose of this paper is to reveal the conformation transition and dynamics properties of the SCW under the microwave irradiation. We study the radial distribution functions, tetrahedral structure, diffusion coefficient, dielectric constant, hydrogen bonding and energy distribution. By analysis of these simulated results, the molecular structure, mobility, dielectric response and the hydrogen bonding dynamics can be detected.
Section snippets
Simulation details
All simulations were performed using a modified version of the GROMACS 4.5.5 [53] simulation package with OPLS-AA [54] force field. Since various water properties of SPC/E model under ambient conditions are closer to that of liquid water [55], [56] and the experimental critical properties of SPC/E have a agreement with the results of liquid water at supercritical condition [57], [58]. For comparison, the flexible water model of SPC was used for all simulations. Both SPC and SPC/E potential
Radial distribution functions
In order to investigate the structural information of SCW under the microwave field, the radial distribution functions (RDFs) of O–O, O–H and H–H are obtained in Fig. 1. And a smaller subset of RDFs is given in each of profiles.
In those figures, a comparison of SPC and SPC/E models exhibits similar features with slight differences. A more obvious reduction in the height of the first and the second peaks of SPC model is identified than SPC/E. The O–O RDFs are plotted in Fig. 1(a and b). The
Conclusions
Molecular dynamics simulations have been performed to investigate the microwave field-dependence structural and dynamics properties in supercritical water, using the SPC and SPC/E water models. It is worth noting that the microwave field intensity has a significant effect on the molecular microscopic structure, and field-induced the tetrahedral structure of water was reconstructed. The dielectric constants of SCW for both models are sensitive in response to field intensity. Specifically, a
CRediT authorship contribution statement
Ying Hu: Methodology, Software, Investigation, Writing - original draft, Writing - review & editing. Guozhu Jia: Conceptualization, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work has been supported by the Major Cultivation Project of Education Department in Sichuan Province, China (18CZ0006).
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