Iso-electric point of fluid

TL;DR

This paper investigates the origins of surface charge density at the solid-liquid interface near the iso-electric point using a potential trap model, revealing how water dissociation and trapped ions influence zeta potential and surface charge.

Contribution

It introduces a potential trap model to analyze surface charge density sources at various pH levels, providing new insights into the mechanisms behind the iso-electric point in water-silica systems.

Findings

Water dissociation dominates surface charge below pH 8.

Bulk ions trapped at the interface dominate above pH 9.

Salt concentration affects zeta potential and surface charge density.

Abstract

Iso-electric point(IEP) is the PH, at which the $\zeta$ potential is measured to be zero. The occurrence of IEP has been understood due to the neutralization of surface charge density (SCD) at the solid-liquid interface. In this work, we use the potential trap model to study the sources of the surface charge density at verious PC and PH, by taking the water-silica system as an example. It is revealed that in the case of $PH<8$, the SCD is mainly originated from the dissociation of water molecules. And the bulk ions trapped at the interface can dominate the SCD when $PH>9$. Due to the mass action law, the dissociation of water molecules is suppressed at the PH close to IEP, leading to a zero surface charge density. In this way, zero $\zeta$ potential is obtained at the IEP. It has also been obtained that the increase of the salt concentration in the water can decrease the $\zeta$ potential, but increase the surface charge density.