## 2 Nanofluid Thermal Conductivity
### Effects of Surfactant on Nanofluid Thermal Conductivity
It was suggested that finding the optimal pH and surfactant concentration can lead to increased thermal conductivity by altering the surface charges of the nanoparticles to prevent agglomeration. The thermal conductivity was measured as a function of surfactant concentration. It was observed that the thermal conductivity increased with surfactant concentration up to approximately 0.03 W% surfactant for both nanofluids. Above this concentration, the thermal conductivity decreased with the increase of surfactant concentration. It was suggested that this decrease was caused by the amount of surfactant on the surfaces of the nanoparticles allowing a narrow area for heat transfer. Using a KD2 Pro Thermal Property Analyzer, [PERSON] et al. [19] measured the thermal conductivity of Mg (OH)\({}_{2}\)-water nanofluid. The stability of the nanofluid was tested using CTAB, SDS, and OA as surfactants at volume fractions of 0.02%. It was observed that CTAB surfactant produced the most stable nanofluid. The nanofluid containing CTAB surfactant remained stable over a 30 days period, during which it was observed. Mg (OH)\({}_{2}\)-water nanofluid without surfactant only remained stable for 1 day. The thermal conductivity of the nanofluid without surfactant was not measured. However, it was suggested that due to the increase in stability of the nanofluid with the addition of surfactant, the thermal conductivity of Mg (OH)\({}_{2}\)-water nanofluid containing CTAB surfactant would be higher than that without. [PERSON] et al. [20] measured the thermal conductivity of Al\({}_{2}\)O\({}_{3}\)-water nanofluid containing SDS and polyvinylpyrrolidone (PVP) as surfactants. Measurements were obtained using a hot-disk thermal constant analyzer. It was observed that the thermal conductivity of the base liquid, water, decreased with increasing surfactant concentration, for both SDS and PVP. The thermal conductivity of water decreased more rapidly with the increased concentration of PVP surfactant, in comparison to the addition of SDS surfactant. It was suggested that this difference was caused by the longer alkyl chain in the PVP solution. It was observed that the thermal conductivity of Al\({}_{2}\)O\({}_{3}\)-water nanofluid increased with the surfactant concentration up to a maximum point, and then began to decrease. It was suggested that this decrease was caused by supersaturated adsorption leading to weaker heat transfer between the nanoparticles, and greater amounts of surfactant on the nanoparticle surfaces, allowing a narrower area for heat transfer. It was observed that both surfactants improved the stability of the nanofluid by forming a layer on the surfaces of the nanoparticles which prevented agglomeration. Using the transient hot-wire method, [PERSON] et al. [21] measured the thermal conductivities of Al\({}_{2}\)O\({}_{3}\)-therminol 66 nanofluid containing no surfactant and containing OA surfactant. Therminol 66 is usually applied in high temperature solar energy systems. It was observed that for the nanoparticle volume concentrations of \(\varphi=0.3\%\) and \(0.7\%\), the thermal conductivities of nanofluids containing surfactant were higher than those containing no surfactant, with a greater difference between the nanofluids with and without surfactant at \(\varphi=0.3\%\). However, for \(\varphi=1\%\), the thermal conductivity of the nanofluid containing no surfactant was higher than the thermal conductivity of the nanofluid containing surfactant. It was stated that the differences between the thermal conductivities of the nanofluids with and without surfactants were within the error of the measuring instrument for all nanoparticle volume concentrations tested. Therefore, it was suggested that the effect of surfactant on the thermal conductivity of nanofluids is negligible. It was stated that adding larger amounts of surfactant would not be beneficial in Al\({}_{2}\)O\({}_{3}\)-oil nanofluids, because it leads to a narrower heat transfer area, due to the amount of surfactant on the surfaces of the nanoparticles.