Specific Ion Effects on the Colloidal Stability of Layered Double Hydroxide Single-layer Nanosheets.

The surface charge properties and aggregation behavior of positively charged Mg-Al-NO3 layered double hydroxide (LDH) single-layer nanosheets dispersed in water were investigated in the presence of K+ salts with different mono-, di-, and trivalent anions, using electrophoresis and dynamic light scattering techniques. An increase in the salt concentration can significantly decrease the effective surface charge density (σeff) of LDHs, leading to the aggregation of nanosheets. The critical coagulation concentration (CCC) or ionic strength (CCIS) of salts for nanosheets significantly decreases with an increase in the valence of anions. Specific ion effects, with a partially reverse Hofmeister series, are observed. On the basis of the Stern model and the DLVO theory, the relationship of CCC with σeff and the ionic valences of salts (zi) is theoretically analyzed, which can accurately describe the dependence of CCC on the σeff and zi but cannot explain the origin of specific ion effects. To explore the origin of specific ion effects, a correlation between CCIS and the specific adsorption energy (Esc) of anions within the Stern layer is developed. Especially, an empirical relationship of Esc with the characteristic physical parameters of anions is proposed. Our model can accurately predict the CCISs of at least monovalent anions and divalent anions (CO32- and SO42-), demonstrating that the specific ion effects observed can be attributed to the differences in ionic size, polarizability, and hydration free energy (or the formation capacity of anion-cation pairs) of different anions. This work not only deepens the understanding of specific ion effects on the colloidal stability but also provides useful information for the potential applications of LDH single-layer nanosheets.