Does the Sign of Charge Affect the Surface Affinity of Simple Ions?

G Hantal and M Klfma and L McFegan and J Kolafa and P Jedlovszky, JOURNAL OF PHYSICAL CHEMISTRY B, 127, 6205-6216 (2023).

DOI: 10.1021/acs.jpcb.3c02641

The role the charge sign of simple ions plays in determiningtheirsurface affinity in aqueous solutions is investigated by computersimulation methods. For this purpose, the free surface of aqueoussolutions of fictitious salts is simulated at finite concentrationboth with nonpolarizable point-charge and polarizable Gaussian-chargepotential models. The salts consist of monovalent cations and anionsthat are, apart from the sign of their charge, identical to each other.In particular, we consider the small Na+ and the largeI(-) ions together with their charge-inverted counterparts.In an attempt to avoid the interference even between the behaviorof cations and anions, we also simulate systems containing only oneof the above ions, and determine the free energy profile of theseions across the liquid-vapor interface of water at infinitedilution by potential of mean force (PMF) calculations. The obtainedresults reveal that, in the case of small ions, the anion is hydratedconsiderably stronger than the cation due to the close approach ofwater H atoms, bearing a positive fractional charge. As a consequence,the surface affinity of a small anion is even smaller than that ofits cationic counterpart. However, considering that small ions areeffectively repelled from the water surface, the importance of thisdifference is negligible. Further, a change in the hydration energytrends of the two oppositely charged ions is observed with their increasingsize. This change is largely attributed to the fact that, with increasingion size, the factor of 2 in the magnitude of the fractional chargeof the closely approaching water atoms (i.e., O around cations andH around anions) outweighs the closer approach of the H than the Oatom in the hydration energy. Thus, for large ions, being alreadysurface active themselves, the surface affinity of the anion is largerthan that of its positively charged counterpart. Further, such a differenceis seen even in the case when the sign of the surface potential favorsthe adsorption of cations.

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