Ionic Rearrangements Lead to Like-Charge Attraction in a Nanoslit

Like-charge attraction has been observed in processes like protein aggregation, colloidal stability, and surface wettability. That this attractive force between two like-charged objects at close separation arises from ionic electrostatic correlations was elucidated decades ago. However, how those correlations (and other energetic components of the electrochemical potential) ultimately translate into a drop in the force between two like-charged objects has not been fully explored. In order to gain a deeper insight into this process, we systematically analyze like-charge attraction between two parallel walls over a wide range of wall surface charge densities and electrolyte properties (ion concentration, valence, and size). We do this with over 70,000 classical density functional theory calculations of primitive model charged, hard sphere electrolytes. We find that, as the slit narrows, the ions undergo substantial rearrangements as their electrochemical potential's energetic components grow and decline nonmonotonically. This initially leads to relatively high co-ion concentration, but later causes the co-ions to be expelled from the slit. In turn, counterions shift to the middle of the pore, decreasing their edge concentrations, causing a decrease in the disjoining pressure. Our approach also provides more clarity about the extent to which charge inversion and like-charge attraction are related.