Properties and Functions of Surfactants
Although surfactants have different structures, they have the same basic properties – adsorption and aggregation. When dissolved in water, surfactants are easily adsorbed (enriched) on air/water surfaces, forming neatly arranged monolayers (Figure 2(a)). In addition to the air/water surface, surfactants can also enrich at the oil/water interface and reduce the interfacial tension, changing the structure and properties of the oil/water interface film. With the aid of surfactants, oil and water can form emulsions, which are widely used in daily life and industrial and agricultural production. They can also be adsorbed on solid surfaces to improve the wettability of solid substrates. The reduction of surface/interfacial tension by adsorption is one of the basic properties of surfactants.
Schematic diagram of surfactants adsorbed on air/water surfaces to form monomolecular films (a) and typical aggregate structures formed in aqueous solutions of surfactants (b). In Fig. (b), a part of the spherical micelles and vesicles is excised to reveal the internal structure. For better presentation, the monomolecular membrane and the individual aggregate dimensions are not given in true scale for the same surfactant.
Excess surfactants tend to aggregate in the bulk phase when they reach saturation adsorption on air/water surfaces. Since most surfactants first aggregate into micelles, this concentration is called the critical micellar concentration (cmc). There are various types of aggregates formed by
surfactants, which are closely related to the structure and concentration of surfactants as well as the external conditions. Typical examples include spherical, rod-shaped, disk-shaped and worm-shaped micelles; monolayered or multilayered vesicles; layered, hexagonal and cubic liquid crystals; and gels containing three-dimensional networks, etc. Some of these types are illustrated in Fig. 2(b). Such aggregation processes, because they are spontaneous, are also called self-assembly. These structures are important research objects in the field of soft matter and nanotechnology; at the same time, because this spontaneous transition from disorder to order is contrary to the law of entropy increase, it is of high scientific research value.
Typical functions of surfactants, taking hydrocarbon surfactants as an example. (a) Solubilization. The solubilization of rare earth complexes in amphoteric surfactant worm-like micelles (i)[19] and fullerene C60 in block copolymer micelles (ii)[20] , respectively. (b) Anionic surfactant-assisted dispersion of a model single-walled carbon nanotube (left) and fluorescence spectrum of the dispersion (right) [21] . (c) Alkyl glycoside emulsified toluene-water system, the aqueous phase is stained green and toluene is stained red [22]. (d) Octadecyl sucrose ester stabilized extra virgin olive oil foam [23]. (e) Pt-Ru nanoparticles synergistically formed by Pluronic F127 as a soft template and silica nanoparticles as a hard template [24]
2. Typical functions of surfactants
The basic properties of surfactants for easy adsorption and self-aggregation give rise to a wide variety of functions. The hydrophobic tail chains of surfactants can be inserted into the oil to reduce the interfacial tension between oil and water, and then further solubilized with the aid of mechanical agitation to form solubilized micelles or emulsions. This is the mechanism of decontamination. Washing plays an important role in many applications of surfactants. In daily life, surfactants are widely found in various kinds of detergents, such as washing powder, detergent, shampoo, and are the core ingredients of these products. In agriculture and industry, from vehicle cleaning to tertiary oil recovery auxiliaries, this property of surfactants is utilized without fail. In basic research, the same principle is used to dissolve structurally complex insoluble substances in water to form homogeneous, stable solutions. Typical examples are rare earth complexes [19], fullerene C60 [20], etc. (Fig. 3(a)). When the size of the water-insoluble guest is large, surfactants can not assist in its complete dissolution, but can only be coated on its surface, playing a dispersive, stabilizing role, typical of one-dimensional, surface-hydrophobic carbon nanotubes (Figure 3(b)) [21]. When we look at the above process from the energy point of view, we can find that it is actually a process of surfactant lowering the interfacial energy at the solid/liquid interface. Similarly, the amphiphilic nature of surfactants ensures their adsorption at the liquid-liquid interface, resulting in the lowering of the liquid-liquid interfacial energy, which is typically applied to emulsification. Take the system in Fig. 3c as an example, water (green) and toluene (red) are immiscible with each other, and when the surfactant alkyl glycosides are added to the mixture, the interfacial energy between toluene and water decreases, which means that the system can be stabilized even if the area of the liquid/liquid interface between water and toluene is increased, and therefore, toluene can be present in the aqueous phase in the form of small-sized droplets, forming a stable emulsion [22]. Similarly, when insoluble gases are dispersed (wrapped) by liquids, foam systems can be formed, and the reduction of interfacial energy (surface tension) at the gas/liquid interface by surfactants has a positive effect on the enhancement of foam stability. Thanks to the development of surfactant science, the foaming field has been expanded from the water phase to the oil phase, and the foaming performance and foam stability have also reached a high level. A recent work (Figure 3(d)) demonstrated the good foaming properties and high temperature stability of octadecyl sucrose esters in the foaming system of extra virgin olive oil, which is of great value in food science [23].
Inside the surfactant solution, the surfactant can be used as an excellent soft template after the formation of micelles, which not only forms a homogeneous and stable structure, but also can be easily removed, and plays an important role in the synthesis of inorganic semiconductor quantum dots, silicon nanoparticles, molecular sieves and other materials. Interestingly, surfactant soft templates can also be used in synergy with hard templates such as silicon nanoparticles, e.g., in the preparation of hollow and mesoporous noble metal materials, surfactants can be loaded on the surface of hard templates together with noble metal salts to provide mesoporous templates for the subsequent formation of noble metal particles [24]. The mesoporous metal nanoparticles formed by this method have a high specific surface area and are an excellent electrocatalyst (Figure 3(e)). The micelles formed by surfactants tend to be non-polar inside, and when a small amount of non-polar components are added to the surfactant solution, they can be encapsulated by the micelles to form a thermodynamically stable microemulsion system. In addition to the high stability, microemulsions also have the property of optical transparency, therefore, microemulsions have an irreplaceable role in the loading of oil-soluble drugs and the development of colloidal optics [25]. Different types of liquid crystals, called solvated liquid crystals, can be formed at high surfactant concentrations. As a soft material with long-range ordered structure, liquid crystals have both liquid fluidity and crystalline order, and the materials synthesized with liquid crystals as templates tend to be structurally controllable and malleable, which has gained wide attention in the fields of optics, life sciences, materials science, and cosmetic science, etc. [26].
The applications of surfactants in living systems are also being widely explored. The anionic surfactant sodium dodecyl sulfate (SDS) is an auxiliary component of gels for protein isolation; the use of surfactants as drug carriers, polymerized phenolic surfactants as bioadhesives, environmentally responsive surfactants for the preparation of smart soft materials, and surface-active proteins for assisting genetic engineering are perfectly merging the ancient surfactants with the “Century of the Biology”. The research on ancient surfactants and the “century of biology” is being perfectly integrated. In recent years, surfactants have also been used in the fields of flexible electronic devices, fuel cells, high-efficiency proton exchange membranes, energy saving and pollution reduction [27]. In addition, some surfactants also have various functions such as sterilization and antistatic. The diversification of surfactant properties has led to a wide range of applications. In addition to daily chemicals, washing, cosmetics, petroleum additives and other fields, surfactants in pesticide emulsions, mineral flotation, textiles and other industries is also very common, and thus has the reputation of “industrial monosodium glutamate”.
It should be noted that sometimes the demand for surfactant properties is diametrically opposed. As far as emulsion is concerned, some systems need to add surfactants to enhance their stability, at this time, surfactants are called emulsifiers; some systems rely on surfactants to break the emulsion, at this time, surfactants are also known as emulsion breakers; in the foam system, surfactants can be used to enhance the foaming performance (foaming agent) can be applied to the defoaming process (defoamer); some surfactants have good biocompatibility, and are used to enhance the foam performance of the foam system (foamer). Some surfactants have good biocompatibility and are used in food, cosmetics and other fields, while some surfactants have strong bactericidal effect and can be realized to be used in antiseptic system. In short, the needs of people’s daily life and industrial and agricultural production are diversified, and should be analyzed on a case-by-case basis when choosing specific surfactants.
At the same time, in addition to the surfactants can play an important role in the case of a single component, different surfactant molecules can produce synergistic effect, resulting in 1 + 1 > 2 effect. Typical examples are anionic/cationic surfactant complex system [28], hydrocarbon/fluorocarbon surfactant complex system [29], and anionic/nonionic surfactant complex system [30]. In addition, surfactants can also play a more important role together with biomolecules, natural products, synthetic polymers, clays, etc. For example, alveolar surface-active substances play a physiological role by lowering alveolar tension with the synergistic effect of lipoproteins.
