Preparation and characterization of stable methyl myristate−in−water nanoemulsions as advanced working fluids for cooling systems

Abstract

Phase change material emulsions (PCME) have gained increasing scientific interest due to their potential to enhance the storage capability of thermal facilities. Herein we present the design and characterization of oil−in−water (O/W) nanoemulsions by employing a dispersed phase mixture (2–12 wt%) enriched in methyl myristate as phase change material. The emulsifier and dispersed phase compositions were optimized based on dynamic light scattering and calorimetric analyses. A two−surfactant formulation composed of sodium dodecyl sulfate and BrijTM S2 (20:49 in weight) was selected to produce stable colloidal dispersions of a methyl stearate:n–hexadecane:methyl myristate mixture (at a mass proportion of 1:3:36) in water. No phase separation or significant growth in emulsified droplet size was detected under storage conditions or when the slurries were subjected to different heating−cooling cycles. The melting/crystallization transitions, rheological behavior, thermal conductivity and density of optimized nanoemulsions were experimentally investigated in order to further understand how the concentration and physical state of suspended droplets may influence those thermal and physical properties. According to differential scanning calorimetry studies, slurries showed moderate subcooling degrees (∼3 °C), even though their solid−liquid transitions extended over a slightly wider range of temperatures than the same mixture used as the dispersed phase but in bulk−form. The shear−thinning character observed for developed nanoemulsions at low temperatures disappeared with the melting of suspended droplets. Considering an operating temperature interval of 15 °C around melting−crystallization phase changes, the 12 wt% optimized suspension presented a storage capacity 18 % higher than that of water under the same conditions. Furthermore, thermal reliability tests verified that phase change characteristics did not significantly changed after 8 months of storage and throughout 500 thermal cycles.