We report an extensive analysis of the non-equilibrium response of alkali halide aqueous solutions (Na(+)/K(+)-Cl(-)) to thermal gradients using state of the art non-equilibrium molecular dynamics simulations and thermal diffusion forced Rayleigh scattering experiments. The coupling between the thermal gradient and the resulting ionic salt mass flux is quantified through the Soret coefficient. We find the Soret coefficient is of the order of 10(-3) K(-1) for a wide range of concentrations. These relatively simple solutions feature a very rich behavior. The Soret coefficient decreases with concentration at high temperatures (higher than T ∼ 315 K), whereas it increases at lower temperatures. In agreement with previous experiments, we find evidence for sign inversion in the Soret coefficient of NaCl and KCl solutions. We use an atomistic non-equilibrium molecular dynamics approach to compute the Soret coefficients in a wide range of conditions and to attain further microscopic insight on the heat transport mechanism and the behavior of the Soret coefficient in aqueous solutions. The models employed in this work reproduce the magnitude of the Soret coefficient, and the general dependence of this coefficient with temperature and salt concentration. We use the computer simulations as a microscopic approach to establish a correlation between the sign and magnitude of the Soret coefficients and ionic solvation and hydrogen bond structure of the solutions. Finally, we report an analysis of heat transport in ionic solution by quantifying the solution thermal conductivity as a function of concentration. The simulations accurately reproduce the decrease of the thermal conductivity with increasing salt concentration that is observed in experiments. An explanation of this behavior is provided.

We investigate the response of molecular fluids to temperature gradients. Using non equilibrium molecular dynamics computer simulations we show that non polar diatomic fluids adopt a preferred orientation as a response to a temperature gradient. We find that the magnitude of this thermomolecular orientation effect is proportional to the strength of the temperature gradient and the degree of molecular anisotropy, as defined by the different size or mass of the molecular atomic sites. We show that the preferred orientation of the molecules follows the same trends observed in the Soret effect of binary mixtures. We argue this is a general effect that should be observed in a wide range of length scales.Thermal gradients are responsible for a wide range of non equilibrium effects, electron transport (thermoelectricity) [1], mass transport in suspensions (thermophoresis) [2][3][4][5][6], mass separation in liquid mixtures [7][8][9] and nucleation and growth of colloidal crystals [10]. Recently it has been shown that temperature gradients can induce orientation in polar fluids, a physical effect that is supported by Non Equilibrium Thermodynamics Theory (NET) and that can be explained in terms of the coupling of a polarization field and a temperature gradient [11]. The polarization of the fluid results in sizable electrostatic fields whose strength scales linearly with the temperature gradient.In this Letter we show that temperature gradients can also induce the molecular orientation of non polar fluids. We call this effect thermomolecular orientation (TMO). The physical origin of this effect cannot be discussed in terms of the polarization field / heat flux coupling, although it is important to note that there is not a principle within non equilibrium thermodynamics theory that precludes observing molecular orientation in non polar fluids. To investigate this hypothesis we use boundary driven non equilibrium molecular dynamics (NEMD) simulations. NEMD has been successfully employed before to investigate thermodynamic and transport properties of atomic and molecular fluids as well as simple ionic liquids [12][13][14][15]. In this work we focus on diatomic fluids. The advantage of using such a model is that it provides simplicity, the necessary "flexibility" to change the molecular anisotropy in a controlled way, and as we will see below, it is possible to establish a direct connection with binary mixtures, making it possible to investigate correlations between TMO and the Soret effect [16,17].We have investigated the TMO effect using a diatomic molecule consisting of two tangent spheres of diameters σ 1 , σ 2 , masses m 1 , m 2 and bond length σ = (σ 1 + σ 2 )/2. The bond length was kept constant using a rigid bond through the Rattle algorithm [18]. The interaction between the particles is completely repulsive, U ij (r) = 4ε (σ ij /r) 12 , where ε is the interaction strength, which we use to define the reduced temperature, T * = k B T /ε, and σ ij = (σ i + σ j )/2 is defined in terms of the diameters of sites i and j. ...

We report an extensive non equilibrium molecular dynamics investigation of the thermal conductivity of water using two of the most accurate rigid non polarizable empirical models available, SPC/E and TIP4P/2005. Our study covers liquid and supercritical states. Both models predict the anomalous increase of the thermal conductivity with temperature and the thermal conductivity maximum, hence confirming their ability to reproduce the complex anomalous behaviour of water. The performance of the models strongly depends on the thermodynamic state investigated, and best agreement with experiment is obtained for states close to the liquid coexistence line and at high densities and temperatures. Considering the simplicity of these two models the overall agreement with experiments is remarkable. Our results show that explicit polarizability and molecular flexibility are not needed to reproduce the anomalous heat conduction of water.

We report non-equilibrium molecular dynamics simulations (NEMD) of water under temperature gradients using a modified version of the central force model (MCFM). This model is very accurate in predicting the equation of state of water for a wide range of pressures and temperatures. We investigate the polarization response of water to thermal gradients, an effect that has been recently predicted using Non-Equilibrium Thermodynamics (NET) theory and computer simulations, as a function of the thermal gradient strength. We find that the polarization of the liquid varies linearly with the gradient strength, which indicates that the ratio of phenomenological coefficients regulating the coupling between the polarization response and the heat flux is independent of the gradient strength investigated. This notion supports the NET theoretical predictions. The coupling effect leading to the liquid polarization is fairly strong, leading to polarization fields of ~10(3-6) V m(-1) for gradients of ~10(5-8) K m(-1), hence confirming earlier estimates. Finally we employ our NEMD approach to investigate the microscopic mechanism of heat transfer in water. The image emerging from the computation and analysis of the internal energy fluxes is that the transfer of energy is dominated by intermolecular interactions. For the MCFM model, we find that the contribution from hydrogen and oxygen is different, with the hydrogen contribution being larger than that of oxygen.