Efficient Dispersion of TiO2 Nanoparticles in Silicone Oil Governed by Coupled Competitive and Cooperative Adsorption of Dispersants

Efficient dispersion of TiO2 nanoparticles (TiO2NPs) in non-aqueous silicone oil is essential for applications such as sunscreen formulations, in which performance is highly sensitive to dispersion quality. In this study, we propose a strategy for enhancing dispersion efficiency while minimizing polymer usage, which can impart undesirable tackiness, by combining an amino-modified poly(dimethylsiloxane) (AEAPS) with oleic acid (Ol). Rheological measurements revealed that AEAPS alone stabilizes TiO2NP dispersions only at sufficiently high concentrations, whereas Ol alone is ineffective. In contrast, their combination markedly improves dispersibility even at low AEAPS concentrations, requiring only 25% of the AEAPS amount otherwise required. Adsorption analysis indicated a transition from complementary to competitive adsorption as surface coverage increases. At low AEAPS coverage, Ol adsorbs noncompetitively and increases the overall surface coverage. At higher AEAPS coverage, Ol preferentially displaces the lower-molecular-weight fractions of AEAPS, while higher-molecular-weight species remain adsorbed. Mapping the apparent surface coverage and layer composition identifies a well-dispersed regime characterized by high overall coverage and a minimum AEAPS fraction, highlighting the essential role of AEAPS-mediated stabilization. In this context, Ol passivates uncovered TiO2 surface sites that would otherwise promote interparticle attraction through hydrogen bonding and/or polymer bridging, while potentially reinforcing the steric repulsion provided by the expanded loops and tails of adsorbed AEAPS. Notably, the apparent surface coverage can exceed unity depending on composition, indicating formation of a mixed adsorption layer that cannot be described by a simple additive model. This behavior is attributed to partial association between the AEAPS amine groups and the carboxylic acid of Ol, which increases adsorption density. Consistent with this interpretation, the adsorption preference of Ol increases with AEAPS concentration, suggesting that AEAPS provides additional anchoring sites for Ol. These results demonstrate that cooperative adsorption between polymeric dispersants and small-molecule additives can yield synergistic stabilization beyond conventional limits, providing a design principle for high-efficiency dispersant systems in non-aqueous suspensions.