Basic properties 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.
Surfactants adsorbed on air/water surfaces form monomolecular membranes as well as typical aggregate structures formed in aqueous surfactant solutions (b). Parts of spherical micelles and vesicles were excised to reveal the internal structure. For better presentation, the dimensions of the monomolecular film and the individual aggregates are not given in the true scale of 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). The types of aggregates formed by surfactants are varied, depending on the structure and concentration of the surfactant, as well as the external conditions. Typical examples include spherical, rod-shaped, disk-shaped and worm-shaped micelles;
monolayer or multilayer vesicles; layered, hexagonal and cubic liquid crystals; and gels containing three-dimensional networks. 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), respectively. (b) Model of anionic surfactant-assisted dispersion of single-walled carbon nanotubes (left) and fluorescence spectra of the dispersion (right). (c) Alkyl glycoside emulsified toluene-water system, the aqueous phase is stained green and toluene is stained red[22]. (d) Extra virgin olive oil foam stabilized by octadecyl sucrose ester. (e) Pt-Ru nanoparticles synergistically formed by Pluronic F127 as a soft template and silica nanoparticles as a hard template
