Effective charges and zeta potentials of oil in water microemulsions in the presence of Hofmeister salts

Abstract

We present a theory which allows us to calculate the effective charge and zeta potential of oil droplets in microemulsions containing Hofmeister salts. A modified Poisson-Boltzmann equation is used to account for the surface and ion polarizations and hydrophobic and dispersion interactions. The ions are classified as kosmotropes and chaotropes according to their Jones-Dole viscosity B coefficient. Kosmotropes stay hydrated and do not enter into the oil phase, while chaotropes can adsorb to the oil-water interface. The effective interaction potentials between ions and oil-water interface are parametrized so as to accurately account for the excess interfacial tension.

How the Influence of Different Salts on Interfacial Properties of Surfactant–Oil–Water Systems at Optimum Formulation Matches the Hofmeister Series Ranking

“…In this sense, the ammonium ion can interact closely with the R‐sulfonate considered chaotropic, according to Collins' concept of matching water affinities (Collins, ; Kunz, ). As a consequence, a chaotropic salt can reduce the effects of head group repulsion, favoring the decrease in interfacial curvature and promoting microemulsion formation (Chen et al, ; Leontidis, ; Liu et al, ; Santos et al, ). This phenomenology is related to the phenomena of “hydration force,” which influences the agglomeration and interfacial activity of surfactant molecules due to the reduction of the hydration repulsion effects (Evans and Ninham, ; Israelachvili and Wennerstrom, ; Kralchevsky et al, ).…”

Chemometric study of kosmotropic and chaotropic ion properties related to Hofmeister effects

“…They are different effects of ions of the same valence on various properties of aggregates, such as critical micellar concentration, the aggregate size and shape, and phase stability. Specific ion effects play an important role in many biological and physicochemical processes, as for example in salt solubility, electrolyte activities, changes in surface tension of water in the presence of electrolytes, pH values, values of zeta potentials, buffer activities, microemulsion structures, aggregation of colloidal particles, cloud points of polymers and surfactant solutions, enzyme activities, ion binding to micelles, proteins and membranes, ion transport across biological membranes, and bacterial growth . Biological activity of cells is also connected with the Hofmeister effects .…”

“…Generally, Hofmeister effects observed in solution are related to the size of ions, their hydration, electrostatic interactions, ionic dispersion forces, thermal movements, fluctuations, and effect of ions on the water structure at the interface 5,7,[9][10][11][12] . Dissolved substances that enhance the ordering of water molecules are called the kosmotropic ones (water-structure makers, from Greek κοσμος-order) and the substances that destroy the ordering of water molecules are called the chaotropic ones (water-structure breakers, from Greek χαος-disorder) 13 .…”

“…Specific ion effects play an important role in many biological and physicochemical processes, as for example in salt solubility, electrolyte activities, changes in surface tension of water in the presence of electrolytes, pH values, values of zeta potentials, buffer activities, microemulsion structures, aggregation of colloidal particles, cloud points of polymers and surfactant solutions, enzyme activities, ion binding to micelles, proteins and membranes, ion transport across biological membranes, and bacterial growth 2-8 . Biological activity of cells is also connected with the Hofmeister effects 3 .Generally, Hofmeister effects observed in solution are related to the size of ions, their hydration, electrostatic interactions, ionic dispersion forces, thermal movements, fluctuations, and effect of ions on the water structure at the interface 5,7,[9][10][11][12] . Dissolved substances that enhance the ordering of water molecules are called the kosmotropic ones (water-structure makers, from Greek κοσμος-order) and the substances that destroy the ordering of water molecules are called the chaotropic ones (water-structure breakers, from Greek χαος-disorder) 13 .Although specific ion effects have been known for more than a century, the mechanism of these universal effects has not been explained yet.…”

Theoretical investigation of a polarizable colloid in the salt medium

“…In order to test the range of validity of the proposed approach, we test its predictions against MC simulation data for systems bearing different salt concentrations and colloidal surface charges. Moreover, we compare our extended theory with one recent theory which has been presented recently for surface polarizations problems 30 .…”