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

Abstract

The surface charge properties and aggregation behavior of positively charged Mg−Al−NO 3 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 (σ eff ) 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 σ eff and the ionic valences of salts (z i ) is theoretically analyzed, which can accurately describe the dependence of CCC on the σ eff and z i 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 (E sc ) of anions within the Stern layer is developed. Especially, an empirical relationship of E sc with the characteristic physical parameters of anions is proposed. Our model can accurately predict the CCISs of at least monovalent anions and divalent anions (CO 3 2− and SO 4 2− ), 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.

Nanocomposite-based dual enzyme system for broad-spectrum scavenging of reactive oxygen species

“…Ionic strength is an important parameter in potential applications of antioxidant nanoparticles, since the presence of high level of electrolytes is one of the most frequent reasons for particle aggregation for materials including LDHs 32,44 . This may lead to significant loss in the ROS-scavenging activity.…”

“…Therefore, the resistance against salt-induced aggregation was studied in electrophoretic and DLS measurements. In general, the colloidal stability of the CASCADE particles in salt solutions of different concentrations can be described by the DLVO theory 40,41,44 . Accordingly, stability ratios decreased by increasing the ionic strength (Fig.…”

Influence of adsorption of ionic liquid constituents on the stability of layered double hydroxide colloids

“…If one considers the indirect Hofmeister series for positively charged hydrophobic LDH surfaces, these values should be higher than for the anion, as it was observed in the presence of standard inorganic electrolytes indicating the effect of the IL cation. 37,48 Nevertheless, the lowest CCC was measured for this ion. Such a discrepancy from the traditional series is probably due to the different counterion affinity to the oppositely charged particles and the different extent of IL ion pair formation on the surface.…”

“…Previous studies [30][31][32][33][34] on other particle-IL systems pointed out that ion specific effects, similar to inorganic electrolytes, [35][36][37] are important in the presence of IL constituents since they govern the adsorption of ions from solutions. Moreover, IL cations and anions can be arranged in sequences based on their adsorption affinity to a given surface.…”

“…45 In addition, surface adsorption of cations followed the above order and results of calculations revealed that the hydration of both the surface and the ions played a major role in the adsorption mechanism. 46 Particle aggregation studies 31,[34][35][36][37]47,48 shed light on the fact that critical coagulation concentration (CCC) data determined with the same particles in different electrolyte solutions can be ordered in the Hofmeister series too. However, the DLVO (Derjaguin, Landau, Vewey and Overbeek) theory, 49 which describes the surface forces in the presence of salt constituents, failed to predict the dependence of such forces on the type of salts present and consequently, the trends in CCC values measured in various salt solutions.…”