The effect of pH on particle agglomeration and optical properties of nanoparticle suspensions

Abstract

Nanoparticle suspensions (NPSs) are used extensively to improve the thermal efficiency of different engineering systems because of their unique thermo-optical properties. NPSs have also been uses in coatings as they improve or alter the appearance of objects, where radiative and optical properties play a significant role. In all these applications, pH of a suspension has a significant impact on the particle agglomeration behaviour, and consequently on optical properties and then on radiative heat transfer. In the present study, the effect of pH on the agglomeration of TiO2 NPSs is investigated. For this purpose, both the experimental and theoretical studies were carried out. In the experiments, the particle size distribution and the average (effective) particle size for the NPSs in different conditions (the pH and particle volume fraction) were measured using the dynamic light scattering (DLS) technique. The effects of different particle agglomerates under different pH values on the dependent/independent scattering are explored and the corresponding regimes are demarked for different conditions. The effects of particle agglomeration, particle size distribution and their contributions to the scattering coefficients are obtained using the UV–Vis-NIR spectroscopy technique. The results show that pH may have significant effect on the optical properties involving NPSs. Adjusting the pH value based on the isoelectric point of the nanoparticle is an efficient method when specific radiative properties are required for specific applications. Different regimes of the dependent/independent scattering can be obtained by changing the pH of a particular nanoparticle suspension.