An ionic surfactant molecule typically contains a charged headgroup and a hydrophobic tail. This amphiphilic structure drives surfactant molecules to self-assemble into aggregates in solution and adsorb at different interfaces. The driving force primarily arises from the balance between the electrostatic repulsion of the same charged headgroups and the hydrophobic association of the tails. In aqueous solutions, surfactants spontaneously self-assemble into a diverse array of microstructures [[1], [2], [3], [4], [5]]. Numerous endeavours have been undertaken to predict the microstructures of surfactant aggregates. Generally, the preferred structure is governed by the critical packing parameter (CPP) proposed by Israelachvili et al. [6]. The CPP is defined as CPP = V0/al0, where V0 and l0 represent the volume and length of the surfactant hydrophobic tail, and a denotes the area of the surfactant headgroup in the aggregate. The aggregate morphologies can be speculated based on the CPP value: when 0 < CPP ≤1/3, only spherical micelles are present; when 1/3 < CPP ≤1/2, rod-like or wormlike micelles predominate; when 1/2 < CPP ≤1, vesicles form; and when 1 < CPP, coacervates emerge. Typically, the electrostatic binding of counterions with the headgroups weakens the electrical repulsion between the ionic headgroups, facilitating surfactant aggregation. Consequently, an increase in counterion concentrations generally reduces headgroup areas [7], leading to an increase in CPP and the potential transition in aggregate morphology from spherical micelles to wormlike micelles or vesicles [[8], [9], [10], [11], [12], [13]]. The electrical screening effect is non-specific and exists universally between any ionic surfactants and their counterions. However, in addition to non-specific electrostatic interaction, the interactions between surfactants and their counterions also encompass complexation, hydrophobic interaction, hydrogen bonds, π-π stacking and others, depending on the nature and structure of the counterions. These interactions take place not only in the surfactant headgroups but also in the transition region from the headgroups to the hydrophobic cores (palisade layers), or even within the hydrophobic cores. Therefore, the effect of counterions on CPP is not confined to alter headgroup area but also involves adjustment to alky tail volume. Choosing an appropriate counterion can effectively modulate the CPP, giving rise to a rich array of aggregates, including spherical micelles, rod-like micelles [14,15], wormlike micelles [16,17], vesicles [18], coacervates [19], gels [20], liquid crystal [21], and so on, which subsequently influence their solubilization efficiency, rheological properties, flow characteristics, spreading behavior, and encapsulation efficacy. These attributes play a critical role in their utilization across various sectors, including cosmetics, enhanced oil recovery, pharmaceutical formulation, and agricultural agrochemicals [[22], [23], [24]]. This review emphasizes the strategic selection of counterions to modulate surfactant aggregate formations, presenting a viable and efficient strategy for the design of novel surfactants. By varying the counterions, the aggregate structures of surfactants can shift from micelles to a variety of other forms. The structural changes in surfactant aggregates induced by counterions can be effectively analyzed using scattering techniques (SAS), such as small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), in conjunction with Cryo-TEM [[25], [26], [27]]. Cryo-TEM enables direct visualization of surfactant aggregates at both nanoscale and microscale resolutions. SAS techniques allow for in situ investigation of structural organization and interactions, providing insights into self-assembly structures ranging from a few nanometers to several hundred nanometers with sub-nanometer resolution. A notable recent advancement involves leveraging SAS to monitor dynamic changes in evolving systems. By employing these methods, researchers can explore how various counterions affect the formation and properties of surfactant aggregates.

Counterions are generally classified into inorganic and organic categories, and they interact with surfactants in different regions and via distinct mechanisms. The simple inorganic counterions can be conceptualized as hard spheres, primarily interacting with surfactant headgroups through electrostatic attraction in the electrical double layer [[28], [29], [30], [31]]. Compared with inorganic counterions, organic counterions exhibit greater structural diversity, potentially introducing additional hydrophobic interactions, hydrogen bonding, and π-π stacking, contingent upon the characteristics and structures of both counterions and surfactants. Although the effects of counterions on the assembly of surfactants have been well reviewed previously [[28], [29], [30], [31], [32], [33], [34], [35], [36]], the evolving landscape of surfactant development has unveiled novel phenomena, offering fresh insights into the influence of counterions on surfactant aggregation. Thus, revisiting this topic with a comprehensive comparison of different counterions and surfactants is warranted. This review aims to refine the fundamental principles guiding the selection of counterions to achieve specific intriguing surfactant aggregates. Its scope will encompass the following key sections: (1) promoting monomer association into spherical micelles, i.e., reducing the critical micelle concentration (CMC) through counterion addition; (2) employing counterions to induce wormlike micelles; (3) fabricating biomimetic vesicles; and (4) constructing coacervates and other kinds of surfactant aggregates. An in-depth comprehension of counterion effects is of great importance for advancing fundamental knowledge and enhancing the practical applications of surfactants in areas such as foaming, detergency, emulsification, flotation and beyond. By presenting a comprehensive understanding, this review endeavours to aid in the judicious selection of counterions for various surfactant applications.