Molecular insights: How counterions determine surfactant aggregation

The morphology of surfactant aggregates inherently determines their properties and functions. Hence, the modulation of morphology holds significant importance in their applications. Opting for an appropriate counterion emerges as an efficient and convenient method to shape the aggregate morphology of ionic surfactants. Both inorganic and organic counterions exert influence on surfactant aggregation by neutralizing the repulsive headgroup charges and synergistically adjusting the hydrophobic association among the alkyl tails. In this process, the charge property, polarizability, hydrophobicity, configuration, and functional groups of counterions are of pivotal influencing factors. A delicate adjustment of counterion structures can yield dramatic differences in the aggregation behavior of ionic surfactants. This review provides a comprehensive overview of the fundamental principles governing the impact of inorganic and organic counterions on the formation of diverse surfactant aggregates, encompassing spherical micelles, wormlike micelles, vesicles, coacervates, and other aggregates. It aims at providing insightful understanding on how to select counterions for surfactants to achieve desired aggregate structures and properties.