Polarisation of water under thermal fields: the effect of the molecular dipole and quadrupole moments.

The investigation of the behaviour of water under thermal fields is important to understand thermoelectricity of solutions, aqueous suspensions, bioelectric effects or the properties of wet materials under spatially inhomogeneous temperature conditions. Here we discuss the response of bulk water to external thermal fields using non-equilibrium molecular dynamics simulations, and five widely used forcefields: TIP4P/2005, TIP4P/2005f, OPC, SPC/E and TIP3P. These models all show the thermal polarisation (TP) effect in bulk water, namely the build-up of an electrostatic field induced by the temperature gradient. The strength of this effect is ∼0.1-1 mV K-1 at near-standard conditions for all forcefields, supporting the generality of TP. Moreover, all the models predict a temperature inversion of the polarisation field, although the inversion temperatures vary significantly across different models. We rationalise this result by deriving theoretical equations that describe the temperature inversion as a balance of the isobaric thermal expansion, dipole orientation in the thermal field and the ratio of the molecular dipole/quadrupole moments. Lower ratios lead to higher inversion temperatures. Based on our results, we conclude that the accuracy of the forcefields describing the TP effect decreases as, TIP4P/2005 ∼ TIP4P/2005f ∼ OPC > SPC/E > TIP3P. At coexistence conditions, the inversion temperature is expected to be around 400 K. Furthermore, we establish a correlation between the TP inversion temperature and the temperature corresponding to the minimum of the liquid-vapour surface potential of water

Thermophysical properties of water using reactive force fields.

The widescale importance and rich phenomenology of water continue to motivate the development of computational models. ReaxFF force fields incorporate many characteristics desirable for modeling aqueous systems: molecular flexibility, polarization, and chemical reactivity (bond formation and breaking). However, their ability to model the general properties of water has not been evaluated in detail. We present comprehensive benchmarks of the thermophysical properties of water for two ReaxFF models, the water-2017 and CHON-2017_weak force fields. These include structural, electrostatic, vibrational, thermodynamic, coexistence, and transport properties at ambient conditions (300 K and 0.997 g cm-3) and along the standard pressure (1 bar) isobar. Overall, CHON-2017_weak predicts more accurate thermophysical properties than the water-2017 force field. Based on our results, we recommend potential avenues for improvement: the dipole moment to quadrupole moment ratio, the self-diffusion coefficient, especially for water-2017, and the gas phase vibrational frequencies with the aim to improve the vibrational properties of liquid water

Mass dipole contribution to the isotopic Soret effect in molecular mixtures

Temperature gradients induce mass separation in mixtures in a process called thermal diffusion and are quantified by the Soret coefficient ST. Thermal diffusion in fluid mixtures has been interpreted recently in terms of the so-called (pseudo-)isotopic Soret effect but only considering the mass and moment of inertia differences of the molecules. We demonstrate that the first moment of the molecular mass distribution, the mass dipole, contributes significantly to the isotopic Soret effect. To probe this physical effect, we investigate fluid mixtures consisting of rigid linear molecules that differ only by the first moment of their mass distributions. We demonstrate that such mixtures have non-zero Soret coefficients in contrast with ST = 0 predicted by current formulations. For the isotopic mixtures investigated in this work, the dependence of ST on the mass dipole arises mainly through the thermal diffusion coefficient DT. In turn, DT is correlated with the dependence of the molecular librational modes on the mass dipole. We examine the interplay of the mass dipole and the moment of inertia in defining the isotopic Soret effect and propose empirical equations that include the mass dipole contribution

Thermal conductance of the water-gold interface: the impact of the treatment of surface polarization in non-equilibrium molecular simulations.

Interfacial thermal conductance (ITC) quantifies heat transport across material-fluid interfaces. It is a property of crucial importance to study heat transfer processes at both macro- and nanoscales. Therefore, it is essential to accurately model the specific interactions between solids and liquids. Here, we investigate the thermal conductance of gold-water interfaces using polarizable and non-polarizable models. Both models have been fitted to reproduce the interfacial tension of the gold-water interface, but they predict significantly different ITCs. We demonstrate that the treatment of polarization using Drude-like models, widely employed in molecular simulations, leads to a coupling of the solid and liquid vibrational modes that give rise to a significant overestimation of the ITCs. We analyze the dependence of the vibrational coupling with the mass of the Drude particle and propose a solution to the artificial enhancement of the ITC, preserving at the same time the polarization response of the solid. Based on our calculations, we estimate ITCs of 200 MW/(m2 K) for the water-gold interface. This magnitude is comparable to that reported recently for gold-water interfaces [279 ± 16 MW/(m2 K)] using atomic fluctuating charges to account for the polarization contribution

Orientation of Janus particles under thermal fields: The role of internal mass anisotropy.

Janus particles (JPs) are a special kind of colloids that incorporate two hemispheres with distinct physical properties. These particles feature a complex phase behavior, and they can be propelled with light by heating them anisotropically when one of the hemispheres is metallic. It has been shown that JPs can be oriented by a homogeneous thermal field. We show using multiscale simulations and theory that the internal mass gradient of the JPs can enhance and even reverse the relative orientation of the particle with the thermal field. This effect is due to a coupling of the internal anisotropy of the particle with the heat flux. Our results help rationalize previous experimental observations and open a route to control the behavior of JPs by exploiting the synergy of particle-fluid interactions and particle internal mass composition

Molecular dynamics computations of brine-CO2 interfacial tensions and brine-CO2-quartz contact angles and their effects on structural and residual trapping mechanisms in carbon geo-sequestration

In the context of carbon geo-sequestration projects, brine–CO2 interfacial tension γ and brine–CO2–rock surface water contact angles θ directly impact structural and residual trapping capacities. While γ is fairly well understood there is still large uncertainty associated with θ. We present here an investigation of γ and θ using a molecular approach based on molecular dynamics computer simulations. We consider a system consisting of CO2/water/NaCl and an α-quartz surface, covering a brine salinity range between 0 and 4 molal. The simulation models accurately reproduce the dependence of γ on pressure below the CO2 saturation pressure at 300 K, and over predict γ by ~20% at higher pressures. In addition, in agreement with experimental observations, the simulations predict that γ increases slightly with temperature or salinity. We also demonstrate that for non-hydroxylated quartz surfaces, θ strongly increases with pressure at subcritical and supercritical conditions. An increase in temperature significantly reduces the contact angle, especially at low-intermediate pressures (1–10 MPa), this effect is mitigated at higher pressures, 20 MPa. We also found that θ only weakly depends on salinity for the systems investigated in this work

Plastic deformation and twinning mechanisms in magnesian calcites: a non-equilibrium computer simulation study

Deformation twinning provides a mechanism for energy dissipation in crystalline structures, with important implications on the mechanical response of carbonate biogenic materials. Carbonate crystals can incorporate magnesium, e.g. in the sea, modifying their elastic response significantly. We present a full atom computational investigation of the dependence of the twinning response of calcite with magnesium content, covering compositions compatible with three main structures, calcite, dolomite and magnesite. We find, in agreement with experiments that the incorporation of magnesium disfavors twinning as a dissipation mechanism in ordered structures (dolomite, magnesite), however the response is strongly dependent on the arrangement of the magnesium ions in the crystal structure. We show that structures with a high content of magnesium (>33%) in a disordered arrangement, lead to plastic response before twinning or fracturing. We demonstrate that the position of the magnesium ions plays a key role in the determination of the crystal deformation mode. This observation is correlated with the formation of percolation clusters of magnesium in magnesian calcites