Colloidal Interactions in Aquatic Environments: Effect of Charge Heterogeneity and Charge Asymmetry

Abstract

The classical theory of colloids and surface science has universally been applied in modeling and calculations involving solid-liquid interfaces encountered in natural and engineered environments. However, several discrepancies between the observed behavior of charged solid-liquid interfaces and predictions by classical theory have been reported in the past decades. The hypothesis that the mean-field, pseudo-one-component approximation adopted within the framework of the classical theory is responsible for the differences observed is tested in this work via the application of modeling and experimental techniques at a molecular level. Silica and silicon nitride are selected as model charged solid surfaces, and mixtures of symmetric and asymmetric indifferent and non-indifferent electrolytes are used as liquid phases. Canonical Monte Carlo simulations (CMC) of the electrical double layer (EDL) structure of a discretely charged planar silica surface, embedded in solutions of indifferent electrolytes, reveal the presence of a size exclusion effect that is enhanced at larger values of surface charge densities. That effect translates into an unexpected behavior of the interaction forces between a charged planar surface and a spherical particle. CMC simulations of the electrostatic interactions and calculations of the EDL force between a spherical particle and a planar surface, similarly charged, reveal the presence of two attractive force components: a depletion effect almost at contact and a long-range attractive force of electrostatic origin due to ion-ion correlation effects. Those two-force components result from the consideration of discreteness of charge in the interaction of solid-liquid interfaces, and they contradict the classical theory predictions of electrostatic repulsive interaction between similarly charged surfaces. Direct interaction force measurements between a charged planar surface and a colloidal particle, performed by atomic force microscopy (AFM), reveal that, when indifferent and non-indifferent electrolytes are present in solution, surface charge modification occurs in addition to the effects on the EDL behavior reported for indifferent electrolytes. Non-uniformity and even heterogeneity of surface charge are detected due to the action of non-indifferent, asymmetric electrolytes. The phenomena observed explain the differences between the classical theory predictions and the experimental observations reported in the open literature, validating the hypothesis of this work.