Surfactant molecules in aqueous solution

There are fundamental reasons why to study the effect of pressure on surfactant molecules in solution. While an increase in hydrostatic pressure is similar to a decrease in temperature, it is often not fully appreciated that through a change in pressure one solely changes the space available to the molecules under investigation, whereas through a change in temperature one varies in fact two parameters the energy of the molecules and the space available to them. Therefore, different surfactant molecules will respond to external pressure differently when the intermolecular interactions are varied. This also applies to the same anionic surfactants with different counter cations. Consequently, with the addition of a pressure parameter, new information can be obtained on the structure and dynamics of the surfactant molecules in aqueous solution. 

In Fig. 1, the typical self-assembly process of surfactant molecules in aqueous solution to form aggregates called micelles is illustrated. At very low concentrations, the solution is a simple dispersion of individual... 

Fig. 25. Schematic representation of molecular adsorption processes at a nanocapsule membrane. A certain fraction Pads of the triglyceride molecules (hexagons) as well as of surfactant molecules in aqueous solution (circles) is adsorbed by tbe inner and outer membrane surface. They undergo a continuous exchange process with another fraction Pdes of the molecules in the bulk medium. This exchange is characterized by average residence times in the adsorbed and the desorbed state Tads and Tdes- A third fraction P ee of the bulk phase is excluded from this exchange process within the experimental time frame by the Umited self-diffusion. All numbers are discussed in Section 4.4.

Following the concept of Tanford (1980), we find interactions of surfactant molecules in aqueous solution encouraging the formation of micelles and adsorption layers at a liquid... 

This approach is based on the use of the unique physical properties of surfactant molecules in aqueous solutions, which n es it possible to electropolymerize thiophene derivatives and to study the influence of micelles on the electropolymerization process and on the structures and properties of die resulting polymer films. 

Wormlike micelles are elongated and semiflexible aggregates resulting from the self-assembly of surfactant molecules in aqueous solutions. Wormlike micellar solutions have received considerable attention during the past few decades because of their remarkable structural and rheological properties. 

Disperse systems can also be classified on the basis of their aggregation behavior as molecular or micellar (association) systems. Molecular dispersions are composed of single macromolecules distributed uniformly within the medium, e.g., protein and polymer solutions. In micellar systems, the units of the dispersed phase consist of several molecules, which arrange themselves to form aggregates, such as surfactant micelles in aqueous solutions. 

Nonionic surfactants dissolve in aqueous solutions through hydrogen bonding between the water molecules and the oxyethylenic portion of the surfactant. These interactions are weak but enough in number to maintain the molecule in solution up to the cloud point temperature, at which the surfactant separates as a different phase (4). Figure 3 shows that electrolytes like calcium chloride, potassium chloride, or sodium chloride reduce the cloud point of Triton X-100. Hydrochloric acid instead promoted a salting-in effect similar to that observed for ethanol. 

Amphiphilic molecules in aqueous solution have a tendency to seek out the surface and to orientate themselves in such a way as to remove the hydrophobic group from the aqueous environment and hence achieve a minimum free energy state (see Fig. 6.1). A consequence of the intrusion of surfactant molecules into the surface or interfacial layer is that some of the water molecules are effectively replaced by hydrocarbon or other nonpolar groups. Since the forces of inter-molecular attraction between water molecules and nonpolar groups are less than those... 

Micellar media are formed from tensioactive molecules in aqueous solution. Mi-cellization is a manifestation of the strong self-association of water and water-like solvents [95]. Micelles are known to increase the solubilization of weakly polar substances in water and, as a consequence, their presence determines the magnitude of hydrophobic interactions. Micelles aggregate spontaneously in aqueous solution beyond a critical concentration which is a function of pressure [96]. As a result, pressure may induce an extra kinetic effect on the rate of organic reactions carried out in aqueous micellar systems. Representative ionic micelles are sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB). Recent examples demonstrate the beneficial effect of the presence of surfactants in Lewis acid-catalyzed reactions, a kind of biactivation [97]. 

In concentrated solutions, surfactants and amphiphilic polymers tend to form mesophases such as nematic, lamellar, hexagonal or cubic structures. The characterization of these phases is very important for various technical applications. At present, phase diagrams of amphiphilic molecules in aqueous solution cannot be calculated by using the atomic-level MD technique. In order to improve this situation, some years ago a new simulation technique was introduced (11 -13),... 

The reason for this behaviour is that the surfactant molecule contains two structurally distinct parts, one of which is hydrophilic while the other is hydrophobic. Oil-soluble surfactants have an oleophilic and an oleophobic part. In the great majority of surfactants, the hydrophobic part is a hydrocarbon chain, which usually has an average length of 12 to 18 carbon atoms and may include an aromatic ring. A single molecule in aqueous solution seeks the surface, because its hydrocarbon tail is repelled by the water, and it tends to remain there, with the hydrophobic part above the surface and the hydrophilic part below, i.e. in the water phase. Further molecules seek the surface or the walls of the vessel until both are full, at which point further additions result in the formation of micelles, which are clusters of molecules arranged with the hydrophobic parts towards the centre and the hydrophilic parts on the outside. 

The dual nature or amphiphilicity of surfactant molecules provides a thermodynamic driving force for adsorption and aggregation of surfactant molecules. In aqueous media the hydrophobic sections of surfactant molecules are attracted to the hydrophobic section of adjacent surfactant molecules. The association of adjacent hydrophobic sections of surfactant molecules reduces the less favorable interactions between water molecules and individual hydrophobic sections of surfactant, thereby reducing system free energy. The effects of association between adjacent hydrophobic sections of surfactant molecules are enhanced in aggregate structures such as adsorbed layers of surfactant and solution micelles. 

An important factor controlling the mode of association of amphiphilic molecules in aqueous solution is the structure of the hydrophobic moiety. In typical surfactants the hydrophobic region is composed of a flexible hydrocarbon chain which can intertwine during the micellization process to form approximately spheroidal aggregates. Association commences at a critical concentration (the critical micelle concentration, cmc) and the micelles, which are generally composed of between 30-100 monomers, are of a narrow size distribution. In contrast, rigid planar aromatic molecules, such as the cationic dyes and the purine and pyrimidine bases of nucleosides can associate by a stacking process. 

Fig. 6 Bubble diagram depicting maximum measured surface tension depression for organic surfactant species in aqueous solution (from Tables 2, 3, 4 and 7) as a function of average carbon oxidation state (OSc) in the molecule and number of carbon atoms (iic) [296]. The size of the bubble is proportional to the maximum measured departure from the surface tension of water (72 mN m ) for each species. Also shown are typical values of (OSc) and c for atmospheric organic aerosol material as classified from Aerodyne Aerosol Mass Spectrometer measurements. HOA indicates hydrocarbon-like organic aerosol, BBOA is biomass buming aerosol, SV-OOA is semivolatile oxidized organic aerosol, and LV-OOA is low-volatility oxidized organic aerosol...

For most pure solutes, solubility is a more-or-less yes or no question. Under a given set of conditions of solvent and temperature, and sometimes pressure, the solute has a specific solubility limit which, when passed, results in the formation of crystals or at least a distinct separate phase that can hypothetically be separated from the solvent or supernatant liquid by physical means. While crystalline hydrates may be separated from water solutions, they will normally have specific compositions that make them unique and subject to characterization by chemical analysis, for example. Surfactants and other amphiphiles, on the other hand, can exhibit a number of intermediate or mesophases in going from a dilute solution of individual or independent molecules to crystalline hydrates or anhydrous structures. A hypothetical spectrum of surfactant mesophases in aqueous solution is given in Figure 4.2. 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

Ahphatic amine oxides behave as typical surfactants in aqueous solutions. Below the critical micelle concentration (CMC), dimethyl dodecyl amine oxide exists as single molecules. Above this concentration micellar (spherical) aggregates predorninate in solution. Ahphatic amine oxides are similar to other typical nonionic surfactants in that their CMC decreases with increasing temperature. 

It is well known, that in aqueous solutions the water molecules, which are in the inner coordination sphere of the complex, quench the lanthanide (Ln) luminescence in result of vibrations of the OH-groups (OH-oscillators). The use of D O instead of H O, the freezing of solution as well as the introduction of a second ligand to obtain a mixed-ligand complex leads to either partial or complete elimination of the H O influence. The same effect may be achieved by water molecules replacement from the inner and outer coordination sphere at the addition of organic solvents or when the molecule of Ln complex is introduced into the micelle of the surfactant. 

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