Polarisation of water under thermal fields: the effect of the molecular dipole and quadrupole moments Aidan Chapman and Fernando Bresme Imperial College London, UK Thermal gradients induce a wide range of powerful non-equilibrium coupling effects, such as the Ludwig-Soret and the Seebeck effects. These effects now have important applications in biomolecule sensing and nanoscale waste heat collectors, respectively. A more recently discovered coupling effect is thermopolarisation (TP) [1]. The application of thermal gradients gives rise to molecular orientation and subsequently electric fields, in polar fluids. Both water [1–3] and acetonitrile [4] have been shown to exhibit this effect in simulations. The importance of thermopolarisation has been discussed in applications such as optothermoelectrics, mechanisms for sonoluminescence, in the study of bioelectric effects, and the microwave drying of materials. In this work[5], we have used non-equilibrium molecular dynamics (NEMD) simulations to investigate thermopolarisation for four popular rigid water models (OPC, TIP3P, TIP4P/2005 and SPC/E). The effect is quantified by the thermopolarisation coefficient, , which is the ratio between the induced electric field and the applied thermal gradient. Near ambient temperatures, this coefficient has a magnitude on the order of 0.1-1 mV/K [2,5] for all four models. Below an inversion temperature, the coefficient is positive[3,5], but above which the coefficient changes sign and the field changes direction. We have precisely determined the temperatures of inversion for each model by applying thermal gradients that cover the temperatures of inversion. Clear lines of inversion can be seen on the density-temperature phase diagrams of all four water models investigated, due to changes in the inversion temperature with pressure. The electrostatic potential in water can be expanded into multipole moments, with the dipolar and quadrupolar contributions dominating. This has allowed us to define an alternative criterion for the inversion in as the point where the dipolar and quadrupolar contributions to the field are equal and opposite. Furthermore, considering just these two terms, we have derived an expression for the thermopolarisation coefficient in terms of the molecular multipole moments, the average molecular orientation, and the thermal expansion coefficient. References 1. F. Bresme, A. Lervik, D. Bedeaux, and S. Kjelstrup, Physical Review Letters 101 , 020602 (2008). 2. Iriarte-Carretero, M. A. Gonzalez, J. Armstrong, F. Fernandez-Alonso, and F. Bresme, Physical Chemistry Chemical Physics 18 , 19894 (2016). 3. J. Armstrong and F. Bresme, Physical Review E 92 , 060103 (2015).
4. O. R. Gittus, P. Albella, and F. Bresme, The Journal of Chemical Physics 153 , 204503 (2020). 5. A. Chapman and F. Bresme, Physical Chemistry Chemical Physics 24 , 14924-14936 (2022).
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