For the purposes of enhancing the heat-transfer utility of the heat pipe in a solar collector, the present work attempted to improve nanoparticle dispersion stability by means of a chemical reformation process wherein nanofluid is formulated with hydroxyl radicals combined with oxidized multi-walled CNTs (MWCNTs). Experiments entailing measurements of thermal conductivity and viscosity in distilled water as functions of temperature were carried out to determine the best nanoparticle mixture ratio. The thermal conductivity increased with the increasing volumetric ratio of the oxidized MWCNTs and with increasing temperature. The viscosity also increased, slowly, until its concentration reached 0.01%, and then steeply increased, and was lower at high temperature. (C) 2013 Elsevier Ltd. All rights reserved.