Specific Ion Effects on Particle Aggregation Induced by Monovalent Salts within the Hofmeister Series

Abstract

Ion specific effects of monovalent salts on charging and aggregation for two types of polystyrene latex particles were investigated by electrophoresis and time-resolved light scattering. The chemical composition of the electrolytes was systematically varied in the experiments. Accordingly, NaH2PO4, NaF, NaCl, NaBr, NaNO3, and NaSCN were used to vary the anions and N(CH3)4Cl, NH4Cl, CsCl, KCl, NaCl, and LiCl for the cations. The salt concentration dependence of the electrophoretic mobilities indicates that the surface charge was screened by the counterions when their concentrations increased. For the SCN(-) ions, adsorption on positively charged particles leads to charge reversal. The aggregation rates are small at low electrolyte concentrations indicating stable dispersions under these conditions, and they increase with the salt concentration. When viscosity corrections are taken into account, no ion specific effects in the fast aggregation regime can be established. The slow and fast aggregation regimes are separated by the critical coagulation concentration (CCC). Within the experimental error, the CCCs are the same in systems containing different co-ions but the same counterions, with the exception of ammonium salts. However, the variation of counterions leads to different CCC values due to specific interaction of the counterions with the surface. These values follow the Hofmeister series for negatively charged sulfate latex particles, while the reversed order was observed for positively charged amidine latex. Comparison between experimental CCCs and those calculated by the theory of Derjaguin, Landau, Verwey, and Owerbeek reveals that variations in the surface charge due to ionic adsorption are mainly responsible for the ion specific effects in the aggregation process.

Effect of Ionic Compounds of Different Valences on the Stability of Titanium Oxide Colloids

“…However, large number of experimental literature data shows that the CCC is sensitive to the type or chemical composition of coions and counterions present in the systems [37][38][39][40][41]. Such a deviation from the theoretical prediction was explained by the different affinity of the ions and thus, different extent of adsorption of the surrounding anions and cations to the surfaces.…”

“…Colloidal stability of titania spheres of 200 nm in radius was investigated in time-resolved dynamic light scattering measurements [57], where the stability ratio values were determined to express the rate of aggregation in the dispersions (Figure 2). In the calculation of the stability ratios, the aggregation rates are normalized to the one measured in unstable dispersions, where the aggregation is controlled only by the diffusion of the particles [40,[60][61][62][63]. Therefore, note that stability ratio values close to one indicate unstable dispersions, while higher values refer to more stable samples.…”

“…These results clearly indicated that adsorption of ions led to different surface charge and hence, to different CCIS. The SCN − < NO 3 − < Cl − sequence followed the indirect Hofmeister series for positively charged hydrophobic particles [37,38,40,42,69], however, the position of the H 2 PO 4 − is atypical, since its presence should lead to the highest CCIS (Scheme 1). Note that the numerical value of the CCIS and CCC are the same for monovalent salts.…”

On Modulating Interfacial Structure towards Improved Anti-Icing Performance

“…While anions have the stronger impact in the behaviors listed in Figure 19A, counter-ion solvation also leads to differentials in biomacromolecular adsorption ( Figure 19B) which can be expressed in terms of adsorption energies, conformational changes, and adhesive properties [175][176][177][178][179]. The effect on coagulation of polystyrene latex particles for both anions and cations [180] is shown in Figure 19C. Control of type and distribution of ions is a parameter that could be additionally exploited to disrupt the interfacial water structures as shown for the case of polyelectrolyte brushes (Figure 15).…”

Cation-Specific Effects on the Self-Assembly of Collagen Molecules Mediated by Acetate on Mica Surface Observed with Atomic Force Microscopy

“…In 1920s, the order of efficiency was followed by some cations as salting-out agents [5]; the order was expressed as K + > Na + > ⋅ ⋅ ⋅ >Mg 2+ > Ca 2+ after more than eighty years [6,7]. The numbers of papers involved in Hofmeister series related to various fields especially biomedicine are large and still keep growing every year [8][9][10].…”