Typical Functions of Surfactants
The basic properties of surfactants, namely adsorption and self-aggregation, give rise to a wide variety of functions. The hydrophobic tail chain of a surfactant can be inserted into an oil contamination to lower the oil/water interfacial tension, and then further solubilized with the aid of mechanical agitation, etc., 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 and stable solutions. Typical examples are rare-earth complexes and fullerenes. When the size of water-insoluble objects is large, surfactants can not assist in their complete dissolution, but can only be coated on their surfaces to play a dispersing and stabilizing role, typical examples are one-dimensional, surface-hydrophobic carbon nanotubes (Fig. When we look at the above process from the energy point of view, it can be found that this is in fact a surfactant to reduce the interfacial energy of the solid/liquid interface. Similarly, the amphiphilic nature of surfactants ensures their adsorption at the liquid-liquid interface, resulting in the reduction of the liquid-liquid interfacial energy, the most typical application of which is emulsification. The most typical application is emulsification. Water (green) and toluene (red) are immiscible, but 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 increases, and toluene can therefore be present in the aqueous phase in the form of small-sized droplets, forming a stable emulsion. Similarly, when the insoluble gas is dispersed by the liquid (package), can form a foam system, surfactants on the gas/liquid interface interfacial energy (surface tension) to reduce the enhancement of foam stability has a positive effect. 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 has demonstrated the good foaming properties and high temperature stability of octadecyl sucrose esters in foaming systems for extra virgin olive oil, which has important applications in food science.
Inside the surfactant solution, the surfactants form micelles that can serve as an excellent class of soft templates, which are not only homogeneous and stable, but also easy to remove, and play an important role in the synthesis of inorganic semiconductor quantum dots, silicon nanoparticles, molecular sieves, etc. Interestingly, octadecyl sucrose ester has been shown to have good foaming properties and high temperature stability in foaming systems of extra virgin olive oil. Interestingly, surfactant soft templates can also be used in synergy with hard templates such as silicon
nanoparticles. For example, 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. The mesoporous metal nanoparticles formed by this method have a high specific surface area, and they are a kind of excellent electrocatalysts. The micelles formed by the surfactant are often non-polar, 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 fields of loading oil-soluble drugs and developing colloidal optics. At higher surfactant concentrations, different types of liquid crystals, called solvated liquid crystals, can be formed. As a soft material with long-range ordered structure, liquid crystal has both liquid fluidity and crystalline order, and the materials synthesized with liquid crystal as template often have good structural controllability and plasticity, and thus have gained wide attention in the fields of optics, life sciences, materials science and cosmetic science.
The applications of surfactants in living systems are also being widely explored. Anionic surfactant sodium dodecyl sulfate (SDS) is an auxiliary ingredient in gels for protein isolation; the use of surfactants as drug carriers, polymerized phenolic surfactants as bio-adhesives, environmentally-responsive surfactants for the preparation of smart soft materials, and the use of surface-active proteins to assist in genetic engineering are perfectly integrating the ancient surfactants with the “Century of 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
