Dependencies of the Adhesion Forces between TiO2 Nanoparticles on Size and Ambient Humidity

We study the variation of the adhesion forces between wet TiO2 nanoparticles as a function of their size and the ambient relative humidity. Combining all-atom molecular dynamics and capillary simulations we demonstrate that the linear scaling of the interparticle forces with the particle diameter, well established for microscopic and macroscopic particles, can be extended down to diameters of a few nm. At this size scale, however, the molecular nature of the water adsorbates dictates the adhesion forces both via solvation effects and influencing parameters of analytical capillary models such as the equilibrium particle–particle separation distance and the water/particle contact angle. Moreover, the water surface tension becomes considerably larger than the macroscopic bulk value due to combined effects of thin-film confinement and tight curvature, in a way that strongly depends on humidity and particle size. Taking these effects into proper account, classical capillary equations can be used to predict the interparticle forces even of the smallest particles considered here (4 nm), although the circular approximation fails to reproduce the distance at which the water meniscus breaks. Finally, the transition between the dominating effects at the nanoscopic scale and conventional capillary theory valid at microscopic size scales can be only rationalized if the presence of roughness asperities on the surface of the large particles is explicitly taken into account.