Interaction of Monovalent Ions with Hydrophobic and Hydrophilic Colloids: Charge Inversion and Ionic Specificity

Abstract

Here we study experimentally and by simulations the interaction of monovalent organic and inorganic anions with hydrophobic and hydrophilic colloids. In the case of hydrophobic colloids, our experiments show that charge inversion is induced by chaotropic inorganic monovalent ions but it is not induced by kosmotropic inorganic anions. For organic anions, giant charge inversion is observed at very low electrolyte concentrations. In addition, charge inversion disappears for both organic and inorganic ions when turning to hydrophilic colloids. These results provide an experimental evidence for the hydrophobic effect as the driving force for both ion specific effects and charge inversion. In the case of organic anions, our molecular dynamics (MD) simulations with full atomic detail show explicitly how the large adsorption free energies found for hydrophobic colloids are transformed into large repulsive barriers for hydrophilic colloids. Simulations confirm that solvation free energy (and hence the hydrophobic effect) is responsible for the build up of a Stern layer of adsorbed ions and charge inversion in hydrophobic colloids and it is also the mechanism preventing charge inversion in hydrophilic colloids. Overall, our experimental and simulation results suggest that the interaction of monovalent ions with interfaces is dominated by solvation thermodynamics, that is, the chaotropic/kosmotropic character of ions and the hydrophobic/hydrophilic character of surfaces.

The Chaotropic Effect as an Assembly Motif in Chemistry

“…Classifications of chaotropic ions as hydrophobic, occasionally found in the literature,109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119 are unfortunate because they may lead to incorrect assignments in terms of the underlying driving force for associative processes. Vice versa, hydrophobic ions do actually exist, such as BPh 4 − , (C 2 F 5 ) 3 PF 3 − , and AsPh 4 + ,80, 120, 121, 122 and it is also important not to label these as being superchaotropic47, 51, 52, 123 when sequences of ionic properties, scales for ionic solvation, or reasons for aqueous assembly processes are being developed. Superchaotropic and hydrophobic ions are neighbors on the continuum solvation scale in Figure 1 b (because they are both large and highly polarizable), but they would differ in terms of their surface charge density and dominant water‐solvation pattern (Figure 1 a).…”

Cation-Size-Dependent DNA Adsorption Kinetics and Packing Density on Gold Nanoparticles: An Opposite Trend

“…charge screening) instead of hydrophobic for describing the Hofmeister series. 18,19 {Merk, 2014 #12163}{Pfeiffer, 2014 #12162}…”

Experimental demonstration of charge inversion in a protein channel in the presence of monovalent cations

“…On the contrary, chaotropic cations like K + are less hydrophilic [14] and display a lower affinity towards the protein. Interestingly, hydrophobic/hydrophilic effects have been reported as the driving force behind charge inversion found in colloids in presence of monovalent ions [15].…”