Molecular dynamics study on vibration-mode matching in surfactant-mediated thermal transport at solid–liquid interfaces

Surfactants have attracted attention as a means of enhancing thermal transport across solid–liquid interfaces. In the present study, non-equilibrium molecular dynamics simulation was used to study the effect of surfactants on interfacial thermal transport at solid–liquid interfaces, from the viewpoint of vibration-mode matching. The solid atom, surfactant molecule, and solvent molecule were all represented by a single atom. The vibrational characteristics of surfactant molecules were altered by changing surfactant mass m_srf, surfactant concentration c_srf, and the interaction strength between solid atoms and surfactant molecules, ε_sld–srf. For given values of c_srf and ε_sld–srf, the interfacial thermal resistance (ITR) between the solid and surfactant solution exhibited a minimum as a function of m_srf. This minimum was found to result from the mutual interference of interparticle heat transfer among atoms in the solid surface layer, and surfactant and solvent molecules in the first and second adsorption liquid layers. The amount of interparticle heat transfer was only partly correlated with the traditionally used overlap of vibrational density of states and with the matching of the characteristic frequencies associated with the spring constant of potential of mean force, proposed here. From this result, we conclude that ITR at solid–liquid interfaces can be minimized by optimizing the vibrational characteristics of surfactant molecules, but the theory of vibration-mode matching should be refined in order to fully identify the condition under which the best vibrational matching occurs between solid, surfactant, and solvent.