How Chemistry, Nanoscale Roughness, and the Direction of Heat Flow Affect Thermal Conductance of Solid–Water Interfaces

We quantify the Kapitza thermal conductance of solid–liquid interfaces between self-assembled monolayers (SAMs) and liquid water using nonequilibrium molecular dynamics simulations. We focus on understanding how surface chemistry, nanoscale roughness, and the direction of heat flow affect interfacial thermal conductance. In agreement with calculations by Shenogina et al. (Phys. Rev. Lett., 2009, 102, 156101) for SAMs with homogeneous headgroup chemistries, we find that for mixed −CF3/–OH SAMs, thermal conductance increases roughly linearly with the fraction of −OH groups on the surface. Increasing nanoscale roughness increases solid–water contact area, and therefore the apparent thermal conductance. However, the inherent thermal conductance, which accounts for the increased contact area, shows only small and subtle variations. These variations are consistent with expectations based on recent work on the effects of nanoscale roughness on interfacial tension (Mittal and Hummer, Faraday Disc., 2010, 146, 341). Finally, we find that SAM–water interfaces show thermal rectification. Thermal conductance is larger when heat flows from the ordered SAM phase to the disordered liquid water phase, and the magnitude of rectification increases with surface hydrophilicity.