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, stable solutions. Typical examples are rare-earth complexes and fullerene C60 . When the size of the object insoluble in water is large, surfactants can not assist its
complete dissolution, but only coated in its surface, play a dispersive, stabilizing effect, typical of one-dimensional, surface hydrophobic carbon nanotubes. When we look at the above process from the energy point of view, we can find that this is actually the surfactant to reduce the interfacial energy of the solid/liquid interface process. 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. Take the system in Fig. 3c as an
example, water (green) and toluene (red) are immiscible with each other. When the surfactant alkyl glycoside is 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, so that the toluene can 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, and other materials. 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 [24]. The mesoporous metal nanoparticles formed by this method have a high specific surface area and are an excellent electrocatalyst. The micelles formed by surfactants 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 crystals have the fluidity of liquid and the order of crystal, and the materials synthesized with liquid crystals as templates often have good structural controllability and plasticity, which has gained wide attention in the fields of optics, life sciences, materials science, and cosmetic science.
