Heat transfer and dispersion in nanoporous materials filled with fluids: impact of interfaces and triple lines

Abstract

With their large internal surface area and severely confining porosity, nanoporous materials are at the forefront in the mitigation strategy to the environmental and energy crisis with key applications in adsorption, catalysis, insulation, etc. Yet, despite intense research on the behavior of nanoconfined fluids, heat generation, transfer, and dispersion involved in adsorption-based processes remain a blind spot. First, thermal effects such as heat release upon fluid adsorption, transport and reaction are well-known in the field, but the molecular phenomena ruling the overall, i.e. effective, heat conductivity in fluid-filled nanoporous solids remain to be fully explored. In particular, the impact of large fluid/solid interfaces and of the contact line between gas, liquid, and solid phases has received little attention. Second, while thermal transport within the nanoconfined fluid at rest or in stationary motion has been considered, thermal processes in out-of-equilibrium situations corresponding to fluid penetration in a nanoporous material has raised little interest. Better understanding such issues using a robust microscopic framework would allow predicting and rationalizing at early stages any energy transfer or dissipation issues (i.e. to anticipate critical aspects at an advanced state in technological development). From a fundamental viewpoint, unveiling the molecular phenomena involved in such problems (e.g. heat release, conductivity, dispersion) could pave the way for the design of novel adsorption or separation methods in which heat transfer is harnessed to control or stimulate processes. Here, we propose a joint experimental/ theoretical approach to investigate thermal processes at play when fluids are confined in nanoporous solids. Using simple systems made up of water in silica nanoporous materials with various hydrophilicity/hydrophobicity, the fluid pressure will be varied to cover situations from a confined gas or liquid (monophase) to gas/liquid coexistence (multiphase) as encountered in chemical engineering applications. First, by treating situations at equilibrium (no motion), we will assess the role of solid/gas, solid/liquid, and gas/liquid interfaces and of solid/liquid/gas lines on the effective heat conductivity in these complex fluid/solid systems. Second, by considering out-of-equilibrium situations in which the monophasic or multiphasic fluid is in motion with respect to the solid (i.e. fluid flow or emptying/filling), we will unravel the mechanisms through which heat is generated, transferred and dispersed. In more detail, model porous materials including all-silica zeolites (~nm pore) and mesoporous silicas (a few nm pore) will be used to probe pore size effects - in particular, these families allow probing different situations as pore filling is reversible, continuous in zeolite (Langmuir-like adsorption) and irreversible, discontinuous in mesoporous silica (adsorption followed by capillary condensation). Moreover, for the mesoporous silicas, we will use different surface terminations (from hydroxylation to organic-grafting) to consider the impact of hydrophilicity/hydrophobicity on the thermal processes under study. In addition to combining experiment/theory, a strength of this project at the crossroad between materials science, chemical engineering and chemical physics is its multiscale nature. While molecular simulation will unravel the basic microscopic mechanisms responsible for heat generation, conduction and dispersion, experiments will be used to rationalize and predict the macroscopic behavior in combination with Lattice Boltzmann calculations and formal chemical engineering approaches. (AU)