Solvent and nonlinear effects on the charge renormalization of nanoparticles within a molecular electrolyte model

A general analysis of the effect of the molecular structure of a polar solvent on the effective interactions among suspended charged nanoparticles (macroions up to 30 nm in size) is performed using a simple molecular model for the solvent in the supporting electrolyte. The solvent molecules are modeled as small rods with end point charges of opposite sign and equal magnitude, whereas the small ions are assumed to be point-like. We compare the renormalized charges of the effective pair potentials (EPPs) among the spherical nanoparticles, obtained after contracting the supporting electrolyte, with those obtained from a similar model, which does not include the solvent molecules. The parameters of both models have been adjusted to give the same screening length. The comparison shows that the renormalized charges are overestimated when the molecular structure of the solvent is neglected. This is in agreement with the image charge effect induced by the different permittivities inside and outside the nanoparticles for the model with explicit solvent molecules; an effect that is missing in the model without solvent molecules. A new numerical method allows us to explore macroion diameters much larger than the solvent molecular size.