Specific ion effects on adsorption at the solid/electrolyte interface: A probe into the concentration limit

Adsorption of organic acid at the mineral oxide-electrolyte interface has been explored. The adsorption of 2,4-dihydroxybenzoic acid onto α-alumina illustrates that specific ion effects show up at very low salt concentration (<0.05 mM). These surprising Hofmeister effects occur at salt concentrations an order of magnitude lower than in a previous study (J. Colloid Interface Sci. 2010, 344, 482). Salts enhance adsorption and specifically at ≤]0.05 mM. With increasing concentration of ion, the adsorption density decreases. The results are accounted for by incorporating the ion size and dispersion forces in the theoretical modeling based on ab initio calculations of polarizabilities. The order appears to be governed by ion size, determining the maximum concentration that ions can attain near the surface due to close packing.