Role of Counterion to Form a Stable Emulsion

Abstract

An emulsion is a mixture of two immiscible liquids such as oil and water, in which one is dispersed in the other as droplets. Generally, surfactants are used to form a stable emulsion. Surfactants, having both hydrophobic carbon chains and hydrophilic polar headgroups, position themselves on the interface between oil and water to stabilize an emulsion. Do you kwon that counterions, a part of surfactants, play an important role in stabilizing an emulsion? The counterions affect the properties of emulsion, such as the phase behavior and the shape. "Screening effect" is one of the important roles of counterions; electrostatic repulsions between polar headgroups must be screened to a certain extent by the counterions in order to form a stable emulsion. Here, these counterions' roles are reviewed by taking surfactant AOT-based W/O （water in oil） emulsions for instance.