Improving the supercooling degree of titanium dioxide nanofluids with sodium dodecylsulfate

• Effect of SDS on reducing supercooling degree of TiO2/H2O nanofluid is found.
• The effect of SDS depends on SDS-to-TiO2 mass ratio and SDS concentration.
• The greatest effect of SDS is achieved when SDS adsorbed on TiO2 reaches saturation.

The solidification processes of titanium (TiO2) nanofluids and deionized water (DW) were measured by differential scanning calorimetry to explore the effect of sodium dodecylsulfate (SDS) surfactants on the supercooling degree of TiO2 nanofluids. The supercooling degrees of TiO2 nanofluids without surfactants were approximately 11.5% lower than that of DW, and the values did not change significantly with nanoparticle concentration. However, the addition of SDS surfactants could reduce the supercooling degree of TiO2 nanofluids. With increasing surfactant-to-nanoparticle mass ratio and SDS concentration, the reduction in the supercooling degrees of TiO2 nanofluids increased to a maximum value of approximately 30.6%. These phenomena indicated that the surfactants served an important function in enhancing heterogeneous nucleation in TiO2 nanofluids. The theoretical analysis of heterogeneous nucleation associated with surfactants revealed that the surfactants reduced the free energy change required for nucleation in TiO2 nanofluids by changing the contact angle of nanoparticles. The supercooling degree of TiO2 nanofluids was found to be closely related to the adsorption density of SDS, that is, large adsorption densities resulted in low supercooling degrees. When the saturation adsorption density of SDS on TiO2 nanoparticles was reached, the reduction in the supercooling degree of TiO2 nanofluids caused by surfactants was at its maximum.