Heat and mass transfer of water at nanoscale solid-liquid interfaces

A better physical understanding of heat and mass transfer of water at nanoscale solid interfaces is essential for the rational design of novel nanoconstructs for clean water and energy as well as for biomedical applications. Both nanoscale transfer phenomena are strongly influenced by solid-liquid nonbonded interactions occurring at the interface. First, classical Molecular Dynamics (MD) is used for investigating water transport in the proximity of several inorganic and biological solid surfaces, according to different surface functionalizations (i.e. hydrophobic/hydrophilic) and physical conditions. Results show that the self-diffusion coefficient D of water in nanoconfined geometries is reduced respect to bulk conditions. In fact, D scales with the dimensionless parameter θ, i.e. the ratio between the volume of confined water, which is defined by the solvent accessible surface and a characteristic length of confinement δ depending on surface chemistry, and the total one. The D(θ) relationship is then interpreted within the thermodynamics of supercooled water. Second, water diffusion in nanoconstructs also plays a fundamental role in nanoscale heat transfer phenomena. Non-equilibrium MD simulations are used to investigate the characteristic solid-liquid thermal boundary resistance of solvated nanoparticles with different degree of hydrophobicity, curvature or surface pegylation, where modeling guidelines are needed in order to optimize the design of nanofluids for novel coolants, solar collectors or ablation therapies. Results show that solid-liquid thermal boundary transmittance is proportional to the hydrophilicity of the nanoparticle surface. Once a theoretical framework for the transport properties of nanoconfined water is established, the obtained scaling laws are applied to engineering and biomedical applications. Atomistic simulations are used for investigating the critical limitations of zeolite-based materials for filtering or thermal storage purposes, namely the limited water flux within the subnanometer pores and the low thermal transmittance, respectively. Infiltration isotherms of water in defective silicalite-I membranes are evaluated by MD simulations, and the water transport within the nanopores is interpreted in terms of solvent-structure and solvent-solvent nonbonded interaction energies. Large networks of carbon nanofillers, instead, may be introduced for enhancing the thermal transmittance of zeolite-based composite materials: non-equilibrium MD simulations show that CNTs with short overlap length and a few bonded interlinks already present a remarkable enhancement in the overall transmittance of the nanoconstructs, which also prove the importance of solid-solid interfaces for optimizing heat transfer at the nanoscale. Finally, water self-diffusivity has also a strong influence on the performances of contrast agents for Magnetic Resonance Imaging (MRI). In fact, lower mobility of water molecules close to MRI contrast agents enhances their longitudinal and transverse relaxivities. Here, MD simulations and the D(θ) relationship are shown to accurately predict the relaxometric responses of Gd(DOTA) or SPIOn MRI contrast agents confined within hydrated nanopores, proving that the D(θ) scaling law can help in tailoring nanostructures with precise modulation of water mobility.