Influence of confinement on thermodiffusion

Thermodiffusion of CO2 in Water by Nonequilibrium Molecular Dynamics Simulations

The components of a fluid mixture may segregate due to the Soret effect, a coupling phenomenon in which mass flux can be induced by a thermal gradient. In this work, we evaluate systematically the thermodiffusion of the CO₂-H₂O mixture, and the influence of the geothermal gradient on CO₂ segregation in deep saline aquifers in CO₂ storage. The eHeX method, a nonequilibrium molecular dynamics simulation approach, is judiciously selected to simulate the phenomenon. At 350 K, 400 bar, and CO₂ mole fraction of 0.02 (aquifer conditions), CO₂ accumulates on the cold side, and the thermal diffusion factor is close to 1 in a number of force fields. The lower the temperature, the higher is the separation and the thermal diffusion factor. In colder regions, water self-association is stronger, whereas the CO₂-H₂O cross-association and the CO₂-CO₂ interactions enhance at higher temperatures. Thermodiffusion and gravitational segregation have opposite effects on CO₂ segregation. At typical subsurface conditions, the Soret effect is more pronounced than gravity segregation, and CO₂ concentrates in the top (colder region). Our work sets the stage to model the effect of electrolytes on CO₂ segregation in subsurface aquifers and other areas of interest.

Use of Boundary-Driven Nonequilibrium Molecular Dynamics for Determining Transport Diffusivities of Multicomponent Mixtures in Nanoporous Materials

The boundary-driven molecular modeling strategy to evaluate mass transport coefficients of fluids in nanoconfined media is revisited and expanded to multicomponent mixtures. The method requires setting up a simulation with bulk fluid reservoirs upstream and downstream of a porous media. A fluid flow is induced by applying an external force at the periodic boundary between the upstream and downstream reservoirs. The relationship between the resulting flow and the density gradient of the adsorbed fluid at the entrance/exit of the porous media provides for a direct path for the calculation of the transport diffusivities. It is shown how the transport diffusivities found this way relate to the collective, Onsager, and self-diffusion coefficients, typically used in other contexts to describe fluid transport in porous media. Examples are provided by calculating the diffusion coefficients of a Lennard-Jones (LJ) fluid and mixtures of differently sized LJ particles in slit pores, a realistic model of methane in carbon-based slit pores, and binary mixtures of methane with hypothetical counterparts having different attractions to the solid. The method is seen to be robust and particularly suited for the study of study of transport of dense fluids and liquids in nanoconfined media.

Nanoconfined Fluids: Uniqueness of Water Compared to Other Liquids

Nanoconfinement can drastically change the behavior of liquids, puzzling us with counterintuitive properties. It is relevant in applications, including decontamination and crystallization control. However, it still lacks a systematic analysis for fluids with different bulk properties. Here we address this gap. We compare, by molecular dynamics simulations, three different liquids in a graphene slit pore: (1) A simple fluid, such as argon, described by a Lennard-Jones potential; (2) an anomalous fluid, such as a liquid metal, modeled with an isotropic core-softened potential; and (3) water, the prototypical anomalous liquid, with directional HBs. We study how the slit-pore width affects the structure, thermodynamics, and dynamics of the fluids. All the fluids show similar oscillating properties by changing the pore size. However, their free-energy minima are quite different in nature: (i) are energy-driven for the simple liquid; (ii) are entropy-driven for the isotropic core-softened potential; and (iii) have a changing nature for water. Indeed, for water, the monolayer minimum is entropy driven, at variance with the simple liquid, while the bilayer minimum is energy driven, at variance with the other anomalous liquid. Also, water has a large increase in diffusion for subnm slit pores, becoming faster than bulk. Instead, the other two fluids have diffusion oscillations much smaller than water, slowing down for decreasing slit-pore width. Our results, clarifying that water confined at the subnm scale behaves differently from other (simple or anomalous) fluids under similar confinement, are possibly relevant in nanopores applications, for example, in water purification from contaminants.

Taming the thermodiffusion of alkali halide solutions in silica nanopores

Thermal fields give rise to thermal coupling phenomena, such as mass and charge fluxes, which are useful in energy recovery applications and nanofluidic devices for pumping, mixing or desalination. Here we use state of the art non-equilibrium molecular simulations to quantify the thermodiffusion of alkali halide solutions, LiCl and NaCl, confined in silica nanopores, targeting diameters of the order of those found in mesoporous silica nanostructures. We show that nanoconfinement modifies the thermodiffusion behaviour of the solution. Under confinement conditions, the solutions become more thermophilic, with a preference to accumulate at hot sources, or thermoneutral, with the thermodiffusion being inhibited. Our work highlights the importance of nanoconfinement in thermodiffusion and outlines strategies to tune mass transport at the nanoscale, using thermal fields.

Configurational contribution to the Soret effect of a protein ligand system : An investigation with density-of-states simulations

Many of the biological functions of proteins are closely associated with their ability to bind ligands and change conformations in response to changing conditions. Since binding state and conformation of a protein affect its response to a temperature gradient, they may be probed with thermophoresis. In recent years, thermophoretic techniques to investigate biomolecular interactions, quantify ligand binding, and probe conformational changes have become established. To develop a better understanding of the mechanisms underlying the thermophoretic behavior of proteins and ligands, we employ a simple, off-lattice model for a protein and ligand in explicit solvent. To investigate the partitioning of the particles in a temperature gradient, we perform Wang-Landau-type simulations in a divided simulation box and construct the density of states over a two-dimensional state space. This method gives us access to the entropy and energy of the divided system and allows us to estimate the configurational contribution to the Soret coefficient. In this work, we focus on dilute solutions of hydrophobic proteins and investigate the effect of ligand binding on their thermophoretic behavior. We find that our simple model captures important aspects of protein-ligand interactions and allows us to relate the binding energy to the change in Soret coefficient upon ligand binding.

Thermodiffusion in multicomponent n-alkane mixtures

Compositional grading within a mixture has a strong impact on the evaluation of the pre-exploitation distribution of hydrocarbons in underground layers and sediments. Thermodiffusion, which leads to a partial diffusive separation of species in a mixture due to the geothermal gradient, is thought to play an important role in determining the distribution of species in a reservoir. However, despite recent progress, thermodiffusion is still difficult to measure and model in multicomponent mixtures. In this work, we report on experimental investigations of the thermodiffusion of multicomponent n-alkane mixtures at pressure above 30 MPa. The experiments have been conducted in space onboard the Shi Jian 10 spacecraft so as to isolate the studied phenomena from convection. For the two exploitable cells, containing a ternary liquid mixture and a condensate gas, measurements have shown that the lightest and heaviest species had a tendency to migrate, relatively to the rest of the species, to the hot and cold region, respectively. These trends have been confirmed by molecular dynamics simulations. The measured condensate gas data have been used to quantify the influence of thermodiffusion on the initial fluid distribution of an idealised one dimension reservoir. The results obtained indicate that thermodiffusion tends to noticeably counteract the influence of gravitational segregation on the vertical distribution of species, which could result in an unstable fluid column. This confirms that, in oil and gas reservoirs, the availability of thermodiffusion data for multicomponent mixtures is crucial for a correct evaluation of the initial state fluid distribution.

Sorting particles with nanoscale thermophoretic devices: how efficient is it?

We investigate particle separation driven by thermal gradients across solid state nanopores using a combined molecular dynamics simulation, non-equilibrium thermodynamics theory and a kinetic model approach. The thermophoretic device, a thermal nanopump, exploits thermal gradients to sort particles of different mass, which accumulate preferentially in hot or cold reservoirs. We show that the large amount of energy dissipated by the thermal nanopump during the transport process leads in general to very low efficiencies, 0.01-0.15%. We find that the nanopump thermal conductivity and structure plays a crucial role in determining the efficiency and a route to enhance it. Doubling the pore radius, from 0.5-1 nm radius, leads to a large increase in the mass diffusion and to a 20 fold increase in the efficiency. Addition of nanoscale defects, without modification of the nanopore structure, leads to a large reduction of the nanopump thermal conductivity and to a large enhancement of the thermodynamic efficiency. We find that nanopumps with nanoscale defects are >3 times more efficient than those without defects. Finally, we identify the microscopic variables responsible for the enhancement of thermally induced transport across nanopores and discuss strategies to tune these variable in order to regulate transport efficiency.