Specific ion effects on particle aggregation induced by monovalent salts within the Hofmeister series.

Tamas Oncsik,Istvan Szilagyi,Gregor Trefalt,Michal Borkovec

doi:10.1021/acs.langmuir.5b00225

Tamas Oncsik, Istvan Szilagyi + Show 2 more

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https://doi.org/10.1021/acs.langmuir.5b00225

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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.

Highlights

While the Hofmeister series has been known for more than a century, the molecular mechanisms governing this characteristic sequence in the ionic specificities are still not fully clarified.[1−3] The Hofmeister series was discovered in protein precipitation experiments, and this series classifies ions according to their increasing stabilization power of protein solutions, namely, This sequence states that solutions of negatively charged proteins remain stable even at high salt concentrations in solutions containing the ions appearing in the right, while they precipitate already at lower salt concentrations containing the ions appearing on the left
We focus on the relation between the Hofmeister series and colloidal particle aggregation.[14−27] This question has been pursued for quite some time, whereby that community rather used the term lyotropic series
Surface charge and aggregation of anionic and cationic polystyrene latex particles were investigated in the presence of various monovalent electrolytes by electrophoresis and timeresolved light scattering

Summary

■ INTRODUCTION

While the Hofmeister series has been known for more than a century, the molecular mechanisms governing this characteristic sequence in the ionic specificities are still not fully clarified.[1−3] The Hofmeister series was discovered in protein precipitation experiments, and this series classifies ions according to their increasing stabilization power of protein solutions, namely, This sequence states that solutions of negatively charged proteins remain stable even at high salt concentrations in solutions containing the ions appearing in the right, while they precipitate already at lower salt concentrations containing the ions appearing on the left. ± 0.2) × 10−18 m3/s and for the amidine latex (3.0 ± 0.2) × 10−18 m3/ s These aggregation rate coefficients agree within experimental error with our previous measurements with time-resolved SLS for the same particles at pH 5.0 in 1.0 M KCl.[53] The hydrodynamic factors α were for the sulfate latex 1.37 ± 0.02 and for the amidine 1.31 ± 0.02. These values compare reasonably well with the theoretical value of 1.39, which can be calculated from low Reynolds number hydrodynamics.[56]. Absolute rate coefficients were obtained by dividing the apparent rate coefficient by the apparent rate coefficient in 1.0 M KCl at the same particle concentration and by multiplying this ratio by the absolute aggregation rate coefficient in 1.0 M KCl

■ RESULTS

■ CONCLUSION

■ REFERENCES