Ion‐Specific Aggregation of Hydrophobic Particles

Abstract

This work shows that colloidal stability and aggregation kinetics of hydrophobic polystyrene (PS) nanospheres are extremely sensitive to the nature of the salt used to coagulate them. Three PS latices and four aggregating electrolytes, which all share the same cation (Na(+)) but have various anions located at different positions in the classical Hofmeister series depending on their kosmotropic or chaotropic character, are used. The present study focuses on analyzing different aggregating parameters, such as critical coagulation concentrations (CCC), cluster size distributions (CSD), initial kinetic constants K(11), and fractal dimensions of the aggregates d(f). While aggregation induced by SO(4)(2-) and Cl(-) behaved according to the predictions of the classical Derjaguin-Landau-Verwey-Overbeek theory, important discrepancies are found with NO(3)(-), which become dramatic when using SCN(-). These discrepancies among the anions were far more significant when they acted as counterions rather than as co-ions. While SO(4)(2-) and Cl(-) trigger fast diffusion-limited aggregation, SCN(-) gives rise to a stationary cluster size distribution in a few aggregation times when working with cationic PS particles. Clear differences are found among all analyzed parameters (CCC, CSD, K(11), and d(f)), and the experimental findings show that particles aggregate in potential wells whose depth is controlled by the chaotropic character of the anion. This paper presents new experimental evidence that may help to understand the microscopic origin of Hofmeister effects, as the observations are consistent with appealing theoretical models developed in the last few years.

Ion specific effects on the stability of layered double hydroxide colloids

“…3A). 20,21 The electrophoretic mobilities also decreased at the same concentration in the Cl À 4 NO 3 À 4 SCN À order indicating specific adsorption of the ions. The mobilities were positive at low concentrations due to the structural charge of the particles and passed through a maximum, which can be predicted by the standard electrokinetic model.…”

“…18,20,21,27 The situation in the HCO 3 À case is more complicated. A similar tendency was observed in other systems containing these anions and positively charged colloidal particles.…”

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.…”

Observation of the Unusual Aggregation Kinetics of Colloidal Minerals in Acidic Solutions

“…[10][11][12] Specific ion effects occur everywhere such as surface tension, 13 zeta potential, 14 protein precipitation, 15,16 enzyme activity, 17,18 osmotic behaviour, 19 polymer aggregation [20][21][22] and colloidal stability. [23][24][25] With use of differential scanning calorimetry, Thromann 22 demonstrated that five potassium salts alter the stability of temperature responsive polymers in accordance to Hofmeister series. The dynamic light scattering (DLS) technique proves to be a powerful tool for the investigation of aggregation kinetics.…”