Surfactants are amphiphilic organic compounds with both a hydrophobic (usually alkyl chain) tail and a hydrophilic head (either a charged or noncharged water-soluble moiety) present on the same molecule (Fig. 1a) [1]. They play an important role in nearly every aspect of our lives from the moment we are born [2]. With the first successful breath at birth, pulmonary surfactants in the lungs thin out the alveolar membrane and increase the surface of the alveoli for gas exchange [3,4]. They act as powerful cleaners for washing dishes, laundry, cleansing our faces, and so on [5]. They are used in an array of cleaning products for their ability to lower water's surface tension, in essence making the molecules more “slippery”, preventing them from sticking together, and promoting interaction with oil and grease. They are key additives in lubricants [6], inks [7], anti-fogging liquids [8], herbicides [9], adhesives [10], emulsifiers [11,12], and fabric softeners [13]. From a basic research point of view, they are frequently adopted as templates in creating nanoparticles [14] and mesoporous materials [15], micellar catalysts in organic synthesis [16], smart carrier vehicles for drug delivery [17], and even used as models to understand mechanisms of self-assembly fundamentally [18], resulting from their polymorphic ordered topologies when dispersed in water.

When increasing surfactant concentration in water, these amphiphilic molecules tend to “physically” dimerize, trimerize, oligomerize, and finally “self-polymerize” into well-organized micellar aggregates, owing to van der Waals interactions between their alkyl tails as well as the hydrophobic effect. Concurrently, the interface between water and oil is also steadily occupied by the surfactant tails until a complete monolayer is formed [19]. The concentration of surfactants (Cs) above which micelles form and all additional surfactants added to the system aggregate into micelles is identified as the critical micelle concentration (CMC) [20].

Surfactants are able to organize themselves into various micellar structures: wormlike, vesicular, disk-shaped, or spherical, dependent on the molecular structure of the surfactant as well as the bulk aqueous environment [1]. Using the principle of the “critical packing” parameter p developed by Israelachvili [21], it is possible to predict the shape of self-assembled aggregates based on the effective volume and maximum length of the hydrophobic tail (represented by “v” and “l”, respectively) as well as the effective surface area per molecule (“a0”) of the surfactants that make up the aggregate, or the area per molecule at the interface between the surfactants and water: p = v/a0l. When the ratio is less than one-third, spherical aggregates tend to form, while wormlike micelles are formed with ratios between one-third and one-half, and lamellar structures are observed when the ratio equals one (Fig. 1b). p is primarily influenced by the structure of the surfactant molecule and is also affected by the solution environment: it arises from an intricate interplay between surfactant chemical architecture and external factors, such as concentration of surfactant, and solution pH, temperature, salinity, etc. [22].

Traditional organic molecule synthesis has relied heavily on kinetic control of reactions, resulting in irreversible covalent bonds being formed between the starting materials. In this case, the goal is typically to pursue an energetically more advantageous pathway to form a specific product; for instance, A progresses to C instead of B (Fig. 1c, left). The irreversible nature of the reaction ensures that once the specific product is generated, it cannot be converted into another compound or reconverted back to the starting materials.

In contrast to the classical covalent chemistry described above, dynamic covalent chemistry (DCC) [[23], [24], [25]], in which reversible chemical reactions are conducted under conditions of equilibrium control, has seen a surge of interest in chemistry and materials science due to its unique characteristics, which may find applications in numerous fields. The reactions' reversibility [26] enables the prospect of “proof-reading” and “error checking” of the resulting dynamic combinatorial library of interconverting components, generating the most thermodynamically stable product under specific conditions [27]. That is to say, when DCC is at play (Fig. 1c, right), it is the relative stability of the products (i.e., thermodynamic parameters) that governs the distribution of products instead of each pathway's energy barriers (i.e., kinetic parameters). The reaction outcome therefore mainly depends on and is also affected by the conditions of reaction [28], such as concentration, pressure, temperature, catalyst, as well as external factors, like light and pH. Time also plays a crucial role since the kinetic parameters dictate how long an equilibrium takes to establish. In these reactions, covalent bonds are constantly formed and disrupted, resulting in a dynamic equilibrium that allows the reaction outcome to alternate between different molecular structures. A significant proportion of reversible covalent bonds falls under the category of dynamic polar reactions, characterized by the formation of charged reactive intermediates during the exchange process. These dynamic covalent reactions can be classified based on the primary bond type established or broken during the exchange, encompassing CN bonds (imines, Schiff bases, enamines), CC bonds (aldol reaction, alkyne/olefin metathesis), CO bonds (nucleophilic additions to CO bonds, ester exchange), CS bonds (thioester exchange, thiazolium-Michael additions), SS bonds (disulfide chemistry), and BO bonds (boronic esters). [29] For example, imine exchange reactions involve the dynamic interchange of imine functional groups, which consists of a double bond between a carbon and a nitrogen atom. These reactions play a crucial role in dynamic combinatorial chemistry, enabling the generation and screening of diverse libraries of compounds to identify specific properties (Fig. 1d). In addition, orthogonal dynamic covalent bonds have found utility in the construction of orthogonal dynamic combinatorial libraries (DCLs), wherein two or more reversible covalent bonds are activated under distinct conditions. This orthogonal combination of diverse dynamic covalent bonds empowers the creation of a wider spectrum of covalent self-assembled structures, serving as a foundational framework for investigating novel functions and properties. [30] Besides, covalent bond formation and breaking usually displays slow kinetics. These characteristics make DCC crucial in creating complex individual molecular architectures and sophisticated self-assembled nanostructures.

Upon incorporating dynamic covalent bonds into surfactant molecular architectures, combinatorial libraries of versatile surfactants with varied functionalities can be readily obtained through a very simple, handy, and cost-effective strategy with minimum, or even without any, effort in organic synthesis, which is a challenging task for those who are not synthetic chemists by training. Indeed, the multidisciplinary research field of dynamic covalent surfactant (DCS) is now emerging, arousing interest from a range of areas, including surfactant and colloid science, supramolecular chemistry, self-assembly, smart materials, drug delivery, and nanotechnology.

In addition to reducing the burden required to synthesize surfactants, the incorporation of dynamic covalent chemistry into surfactants is revolutionizing the field by imparting them with unprecedented responsiveness to changes in the surrounding environment, a route towards elaborate materials with on-demand functionalities. DCS design relies on tailoring two precursors to react into thermodynamically stable structures with desired properties (such as the hydrophilic-lipophilic balance (HLB) and “critical stacking” parameter (p)) under specific environmental conditions. These smart surfactants, like traditional ones, can form specific-shaped micelles or stabilize emulsions on demand. What sets them apart is their remarkable reversibility and responsiveness to external stimuli like pH, temperature, or chemical triggers. Upon environmental shifts, they readily revert to their precursor state, allowing disassembly/reassembly into different micelle shapes or resulting in emulsion breakdown (Scheme 1). This dynamic behavior enables the manipulation of the system at will by simply tuning the environment, instead of being confined to the static functionality of traditional surfactants. “Smart” materials, which dynamically alter their structures and functionalities based on environmental stimuli, especially those capable of switching between an ‘on’ and ‘off’ state, are indeed a highly topical research field [1]. The ability of DCS to dynamically adapt ensures efficient performance, stability, and tunability—making them ideal building blocks for smart materials, drug delivery systems, and other cutting-edge technologies. Dynamic covalent chemistry is thus paving the way for the design of highly advanced surfactants which can meet the evolving demands of modern applications.