Effect of cations on van der Waals interactions between particles in aqueous alkali nitrate electrolytes
The van der Waals interaction has been extensively studied for colloidal forces and resultant emergent phenomena such as colloidal stability, aggregation, and suspension rheology, but the effect of electrolytes on this interaction, especially at intermediate and high electrolyte concentrations, remains incompletely understood. We have extended the Lifshitz theory for van der Waals interactions in pure water to alkali nitrate solutions at arbitrary concentrations by developing a dielectric response model for alkali nitrate solutions that is based on electronic structure calculations of the molecular constituents. Due to their importance in catalysis, ceramics, and coating technologies, the Hamaker constants for rutile, boehmite, and alumina nanoparticles suspended in alkali nitrate solutions are calculated as a function of salt concentration. Contrary to prevailing assumptions, increasing the concentration of sodium (Na), potassium (K), and rubidium (Rb) nitrate solutions causes appreciable increases of the Hamaker constants relative to pure water instead of decreases, whereas cesium nitrate (CsNO3) has almost no effect on the Hamaker constant. We discussed the influence of the solution molar volume, the polarizability of the dissolved ions, and optical properties of the interacting particles in the context of previously published work. Our study indicates a non-vanishing role of van der Waals interactions on colloidal stability at intermediate and high electrolyte concentrations, leading to physical insights on emergent phenomena associated with nanoparticles.