Origin of Hydrophilic Surface Functionalization-Induced Thermal Conductance Enhancement across Solid-Water Interfaces

Thermal transport across solid-water interfaces is critical for a wide range of applications such as solar thermal evaporation, nanoparticle-assisted hyperthermia therapeutics, and nanofluids. Surface functionalization using self-assembled monolayers (SAMs) to change the hydrophilicity of the solid surface is a common strategy to improve the thermal conductance of solid-water interfaces. Although it is known that hydrophilic interfaces increase the interfacial bonding, how it impacts the molecular level energy transport across the interface is still not clear. In this paper, we perform molecular dynamics simulations to calculate the thermal conductance of differently functionalized gold (Au)-water interfaces. Combining the heat flux decomposition to different interatomic interactions across interfaces and analyses of water structures close to the functionalized surfaces, we found that there is a collaborative effect from the electrostatic interactions and the Lennard-Jones (L-J) interactions (especially the repulsive part). The electrostatic interactions, which are between the polar functional groups of SAMs and water, will attract water molecules closer to the SAM surface, leading both the electrostatic and L-J interactions to have larger effective forces across the interfaces. This increases the power exchanged between solid and water atoms, enhancing the thermal energy transport. The results from this work will provide new insights to the understanding of thermal transport across solid-water interfaces.