Alkali Halide Solutions under Thermal Gradients: Soret Coefficients and Heat Transfer Mechanisms

2015

Phys. Rev. E

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Temperature gradients polarize water, a nonequilibrium effect that may result in significant electrostatic fields for strong thermal gradients. Using nonequilibrium molecular dynamics simulations, we show that the thermal polarization features a significant dependence with temperature that ultimately leads to an inversion phenomenon, whereby the polarization field reverses its sign at a specific temperature. Temperature inversion effects have been reported before in the Soret coefficient of aqueous solutions, where the solution changes from thermophobic to thermophilic at specific temperatures. We show that a similar inversion behavior is observed in pure water. Microscopically, the inversion is the result of a balance of dipolar and quadrupolar contributions and the strong temperature dependence of the quadrupolar contribution, which is determined by the thermal expansion of the liquid. where ε r is the fluid dielectric constant, ∇T the thermal gradi-56 ent, T the temperature, and L pq /L pp the ratio of phenomeno-57 logical coefficients, where L pq measures the TP coupling. 58 Using computer simulations and different water models, we 59 found that the TP field changes linearly with the thermal gra-60 dient, as predicted by Eq. (1) [22][23][24]. We show in this Rapid 61 Communication that the TP field features a strong temperature 62 dependence and reverses sign at a specific temperature. 63 Further, we provide a microscopic interpretation of the TP field 64 inversion, which advances our microscopic understanding of 65 TP effect, beyond the phenomenological treatment of NET. 66We have performed nonequilibrium molecular dynamics 67 (NEMD) simulations (see, e.g., Ref.[22]). Two geometrical 68 regions were defined, in which the temperature is controlled 69 by rescaling velocities every time step. The molecules outside 70 the thermostatting regions move according to Newtonian 71 dynamics. We computed the temperature profile and the TP 72 field locally by dividing the simulation box into bins of width 73 0.7Å. For analysis purposes we discarded the bins next to 74 the thermostats (typically 10). All the computations were per-75 formed with the extended simple point charge (SPC/E) model 76 of water [25], and the electrostatic interactions and TP fields 77 were computed using the three-dimensional particle-particle 78 particle-mesh (PPPM) Ewald summation method [26] with 79 "tin-foil" boundary conditions [24]. Our longest simulation 80 involved a 126 ns trajectory. To ensure the system reached the 81 stationary state, we discarded the first 2 ns of the trajectory. 82 The total electric field was computed on the fly every ten 83 steps, while the calculations of the dipolar and quadrupolar 84 contributions were performed externally. The trajectories were 85 integrated using the velocity-Verlet algorithm with a time step 86 of 2 fs using the parallel code LAMMPS [27]. For each of 87 these systems investigated in this work, the density required 88 in order to achieve a given pressure P was determined via an 89 NP T s...

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confidence: 88%

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confidence: 97%

Temperature inversion of the thermal polarization of water

Armstrong

2015

Phys. Rev. E

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“…Work in our research group on TMO has spanned a range of systems, from non-polar dumbbell molecules [2,3] to a range of both rigid and flexible water molecules [4][5][6][7] as well as continuing work on some more complex systems [8]. In this paper, we report on our findings regarding TMO in rigid, dipolar dumbbells consisting of positively and negatively charged Lennard-Jones spheres bound together.…”

Section: Introductionmentioning

confidence: 95%

Thermo-molecular orientation effects in fluids of dipolar dumbbells

Daub

Åstrand

Phys. Chem. Chem. Phys.

2014

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We use molecular dynamics simulations in applied thermal gradients to study thermomolecular orientation (TMO) of size-asymmetric dipolar dumbbells with different molecular dipole moments.We find that the direction of the TMO is the same as in apolar dumbbells of the same size, i.e. the smaller atom in the dumbbell tends to orient towards the colder temperature. The ratio of the electrical polarization to the magnitude of the thermal gradient does not vary much with the magnitude of the molecular dipole moment. We also investigate a novel second order TMO that persists even in size-symmetric dipolar dumbbells where molecules have a slight tendency to orient perpendicular to the gradient except very close to the hot region, where (anti-)parallel orientations are preferred. Finally, we investigate rotational correlation functions and characteristic rotational times in these systems in an attempt to model possible spectroscopic signatures of TMO in experiments. Although we cannot detect any difference in integrated rotational times between equilibrium simulations and simulations in a thermal gradient, more careful modelling of the anisotropic rotational dynamics in the thermal gradient may be more successful.

“…In alkali halide solutions when s T < 0 for T < T 0 the solution becomes thermophilic (the solute migrates towards the hot region), while at T > T 0 the solution becomes thermophobic, s T > 0, and the solute migrates towards cold regions. Sign changes of the Soret coefficient have been observed in a variety of systems: alkali halide solutions, 12 mixtures of organic liquids, such as benzene-cyclohexane 15 and aqueous protein solutions 16 and ternary mixtures of poly(ethylene oxide), ethanol and water. 14 Non-equilibrium thermodynamics 1,17,18 provides a general theoretical framework to rationalize thermal coupling effects such as the LS effect.…”

Section: Introductionmentioning

confidence: 99%

The role of ion–water interactions in determining the Soret coefficient of LiCl aqueous solutions

Lecce

Albrecht

Phys. Chem. Chem. Phys.

2017

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The application of a thermal gradient to an aqueous electrolyte solution induces the Soret effect, and the salt migrates towards hot (thermophilic) or cold regions (thermophobic). Experimental studies of LiCl reported changes in the sign of the Soret coefficient as well as a minimum in this coefficient at specific salt concentrations and temperatures. At the minimum the thermodiffusive response of the solution is enhanced significantly. We have performed non-equilibrium molecular dynamics simulations of LiCl solutions to quantify the dependence of the sign change and minimum of the Soret coefficient with salt concentration and temperature. We find that the ion mass plays a secondary role in determining the magnitude of the Soret coefficient, while the diameter of the cation has a significant impact on the coefficient and on the observation of the minimum. Our simulations show that the ordering of water around Li + plays a key role in determining the Soret coefficient of LiCl salts.