Ion specificity in NaCl solution confined in silicon nanochannels

Abstract

Ion specificity of Na+ and Cl− ions for NaCl solution confined in silicon nanochannels is investigated with molecular dynamics (MD) simulations. The MD simulation results demonstrate that ion specificity for Na+ and Cl− ions exhibits clearly in nanochannels with high surface charge density. The two types of ions show different density distributions perpendicular to the channel surface due to the ion specificity when they act as counterions near negatively and positively charged surfaces, respectively. Both the two counterion distributions cannot be predicted by Poisson-Boltzmann equation within 0.75 nm near the surface. In addition, the ion specificity is also demonstrated through affecting the water density distributions. In the nanochannel with negatively charged surfaces, the presence of the Na+ ions reduces the number of water peaks in water density distribution profile. In comparison, when the Cl− ions act as counterions near positively charged surfaces, they do not affect the number of the water peaks. Besides the influence on the water density distribution, ion specificity also exhibits through affecting the water molecule orientation in the adsorbed layer. It is found that Cl− ions make the water molecules in the adsorbed layer align more orderly than Na+ ions do when the two types of ions act as the counterions near the positively and negatively charged surfaces with the same surface charge density.