Effective thermal conductivity of microemulsions consisting of water micelles in n-decane

Microemulsions consist of a liquid dispersion medium and a dispersed phase of colloidal liquid micelles in the nanometer scale. Despite its technological relevance, the effective thermal conductivity of mi-croemulsions has scarcely been investigated both experimentally and theoretically. To elucidate the in-fluence of the concentration of the dispersed micelles and their morphology on the effective thermal conductivity of reverse microemulsions consisting of a n-decane-rich continuous phase and a dispersed surfactant-stabilized phase with water-free or water-swollen micelles, a steady-state parallel-plate instru-ment was used between 298 and 318 K at water volume fractions from 0 to 0.47. The measured effective thermal conductivities increase with increasing volume fraction of the dispersed water micelles, which is by trend accompanied by increasing effective viscosities and decreasing translational particle diffusion co-efficients, as obtained by capillary viscometry and dynamic light scattering. The Hamilton-Crosser model and an analytical thermal-resistance model, which are often employed for dispersions of solid particles in liquids, describe the effective thermal conductivity of microemulsions well in case of virtually spheri-cal micelles at low water loading. Distinct underestimations by both models assuming spherical particles appear at higher water contents and temperatures beyond the percolation at about 308 K, associated with micelle clusters up to a bicontinuous microemulsion structure. Especially for these states, it could be found that the effective thermal conductivity can be well represented by the average of a serial and a parallel resistance model for thermal conduction.(c) 2022 Elsevier Ltd. All rights reserved.