Size- and Concentration-Dependent Adsorption of Polydisperse Dispersant Oligomers to TiO₂ Nanoparticles and the Effect of Adding Poor Solvents

Polyhydroxystearic acid (PHSA) is widely used as an oil-soluble dispersant for metal oxide particles in cosmetics. PHSA is essentially polydisperse. In this study, we investigated the size-dependent adsorption of PHSA oligomers to titanium dioxide nanoparticles (TiO₂NPs) by gel permeation chromatography. In dispersions with a medium of diethyl sebacate, a good solvent for PHSA, the total adsorbed mass of PHSA increased and became saturated as the PHSA concentration increased. Above the saturation point of the adsorbed mass, small PHSA oligomers increasingly adsorbed with increasing PHSA concentration, while the adsorption of large oligomers decreased. This can be explained by the entropic penalty related to the conformational constraint in the adsorbed layer and the advantage of the total gain of the PHSA-TiO₂NP interaction for the adsorption of small PHSA oligomers. In contrast, the addition of cyclic silicones, acting as poor solvents for PHSA, to the above dispersions resulted in enhanced adsorption of large PHSA oligomers, while there was no change in the adsorption of small oligomers. The enhanced adsorption of large oligomers was more apparent when cyclic silicones were added to the dispersions with high PHSA concentrations. We propose that cyclic silicones are not favorable for large PHSA oligomers and the poor solvency of the mixed solvent causes shrinking of the adsorbed PHSA oligomers on the TiO₂NPs, which generates additional space for adsorption. The effect of the ring size of the cyclic silicone was also systematically investigated.