The investigation experimentally studies the optimal concentration of alumina nanoparticles in doped molten Hitec that maximizes its specific heat capacity. A simplified model of the interfacial area is developed to explain the optimal concentration. The specific heat capacities of pure Hitec and nano-Hitec fluid are measured using a differential scanning calorimeter (DSC), and the microstructures following solidification are observed using a scanning electron microscope (SEM). A novel sampling apparatus and process for preparing molten Hitec nanofluids were developed to prevent the precipitation of nanoparticles. An optimal concentration of 0.063 wt.% is identified as yielding the greatest enhancement of specific heat capacity of 19.9%. At a concentration of 2 wt.%, the detrimental effect of the dopant nanoparticles on the specific heat capacity is evident at all temperatures. The negative effect is more significant than that predicted by the thermal equilibrium model. The SEM images following the solidification of samples and the developed model reveal the uniform dispersion of nanoparticles with negligible agglomeration at concentrations of under 0.016 wt.%. The agglomeration becomes significant and the particle clusters seem to be inter-connected at high concentrations. Moreover, the optimal concentration is approximately the concentration at which the contributions of isolated particles and clusters of sizes from 0.2 to 0.6 mu M in the interfacial area to the specific heat capacity are equal. (C) 2013 Elsevier Ltd. All rights reserved.