Interaction between macroparticles in aqueous electrolytes

We report results of theoretical calculations for the interaction between two isolated, structureless spherical macroparticles immersed in aqueous electrolytes comprising waterlike molecules (hard spheres embedded with point dipoles and tetrahedral quadrupoles), 1:1 cations and anions (the diameter of cations is equal to that of anions). The reference hypernetted‐chain (RHNC) theory with hard‐sphere bridge functions is employed in the calculations. The fluid structure and the potential of mean force scaled by d_S/(πd_M) (d_S and d_M denote the solvent and macroparticle diameters, respectively) converge to limiting behaviors with increasing d_M as long as the surface charge density of the macroparticle is kept constant. The qualitative aspects of the conclusions are not altered even when d_M is set at 10d_S. The interaction between neutral macroparticles in pure water is characterized by strong, short‐range attraction. However, a relatively minor, short‐range repulsive component is added to the interaction when ions are included in water. On the other hand, the presence of apolar particles in water at a trace concentration leads to considerable enhancement of the attraction. Effects of ionic sizes on the interaction between charged macroparticles are substantially large and opposite to those which would be observed using the primitive model. When the size of counterions is sufficiently large and the ionic concentration is sufficiently high, there is a regime where the interaction between highly like‐charged macroparticles is strongly attractive.