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Adsorption of poly(hydroxystearic acid) to TiO2 nanoparticles, studied using gel permeation chromatography

Naden, Benjamin J., Kessell, Lorna M., Luckham, Paul F., Tadros, Tharwat F.
Colloids and Surfaces A: Physicochemical and Engineering Aspects 2015 v.478 pp. 36-44
acid value, adsorption, gel chromatography, molecular weight, nanoparticles, polymers, solvents, sorption isotherms, stearic acid, sunscreens, thermodynamics, titanium dioxide, titration, ultraviolet radiation
TiO2 nanoparticles are becoming increasingly important as sunscreens due to their propensity to absorb UV light. It is crucial that the particles are nanoscale so that scattering of visible light is minimal and UV attenuation is optimised, thus stabilisation is essential to prevent particles from aggregating. In this study we investigate the adsorption of dispersant based on stearic acid to TiO2 nanoparticles by adsorption isotherm determination. The methods used to determine dispersant concentration (acid value titration and gel permeation chromatography, GPC) provided information regarding the chemical nature of the polymers and insights into the way the molecules interacted with particle surfaces in different solvent conditions. It was observed that the smaller dispersant molecules adsorb preferentially, contrary to the behaviour of adsorbing homopolymers that display a direct relationship between molecular weight and strength of adsorption. Adsorption in this case is a result of single point adsorption that occurs via interaction of the carboxylate group of the dispersant to the TiO2 surface, the thermodynamics of which favours low molecular weight components. This is clearly demonstrated by GPC which was able to resolve and quantify the non-adsorbed molecules by size. Discrepancies between GPC and acid value measurement for dispersant concentration reinforce this observation.