Surfactant-cosurfactant interaction

The overall rigidity of a liquid crystal can be altered by two effects. One is the disorder introduced by an increase in temperature and the corresponding increase in molecular motion that debilitates directional forces such as polar interactions. The other is the weakening of polar interactions (nonionic hydrophilic instead of ionic, or the introduction of alcohol cosurfactant) or the promotion of disparity in the molecular size and shape, by introducing double bonds, random tail length, tail branching, surfactant-cosurfactant mixtures, etc. All these alterations have been used to make microemulsions. 

One of the most common structures encountered in microemulsions consists of water or oil droplets dispersed in a continuous phase of oil or water, respectively. The type of dispersion results from the preferred curvature Co of the surfactant layer, which is by convention positive for oil-in-water (O/W) systems and negative for water-in-oil (W/O) systems. Co can be varied by adjusting the surfactant/cosurfactant ratio, which allows swelling of the droplets until a maximum is reached. When the systems become more concentrated, the micellar swelling is mostly limited by attractive interparticle interactions, as observed, for example, for microemulsions close to a critical point. 

Extensive studies have been reported by Kunieda s group regarding the formation of worm-like micelles and micellar transient networks in water-surfactant-cosurfactant systems. However, for applications, it is also relevant to know the effect of additives on systems containing worm-hke micelles. It is reported that oils induce a rod-sphere transition in surfactant micellar solutions, leading to a reduction in viscosity [32]. Kunieda s group studied the solubilization of different oils in wormlike micellar solutions [19, 33]. The amount of solubilized oil, its location within the micelle, and its effect on micellar shape and size demonstrated to strongly depend on the nature of the oil and its interactions with the surfactants. 

The use of reversed micelles in the selective recovery and concentration of low and high molecular weight bioproducts from dilute aqueous streams appears to be a promising new avenue for innovative research and applications. To date, it has been shown that electrostatic interactions between the charged solute residues and the surfactant headgroups, as well as hydrophobic effects, can play a significant role in determining the selectivity of this process for one protein over another. Moreover, there appears to be some latitude in the selection of surfactants and cosurfactants that enables enhancements in selectivity to be made over and above those already inherent in the process. 

Structural entitles In water-ln-oll (W/0) mlcroemul-slon at low water content are reviewed. These structures Include monomers of surfactant associated with a few water and cosurfactant molecules. These small aggregates are stable In a non-polar environment In spite of their polar character and provide an Interesting case of unusual Interaction between polymers and mlcroemulslon structures. Examples are provided of cases when this Interaction Is Important for stability of mlcroemulslons with added organic or Inorganic polymers. 

The decrease in Tg of nonionic microemulsion systems is due to the absence of electrostatic interactions between the surfactant and polystyrene. The nonionic surfactant behaves as a low molecular weight additive (i.e. plasticizer) which then lowers the Tg. In order to determine whether the cosurfactant 2-pentanol has any effect on Tg, a nonionic system was prepared containing 2-pentanol. The Tg of the polymerized solid was then determined. The Tg remained the same as in the nonionic system containing no cosurfactant. 

Polymerization of styrene in microemulsions has produced porous solid materials with interesting morphology and thermal properties. The morphology, porosity and thermal properties are affected by the type and concentration of surfactant and cosurfactant. The polymers obtained from anionic microemulsions exhibit Tg higher than normal polystyrene, whereas the polymers from nonionic microemulsions exhibit a lower Tg. This is due to the role of electrostatic interactions between the SDS ions and polystyrene. Transport properties of the polymers obtained from microemulsions were also determine. Gas phase permeability and diffusion coefficients of different gases in the polymers are reported. The polymers exhibit some ionic conductivity. 

It should be noted that high concentrations of ionic species can alter the phase stability of microemulsions based upon ionic surfactant systems. Nonionic surfactant systems are much less susceptible to this effect. The curvature of the interfacial film of the microemulsion droplet is determined by a balance between the electrostatic interactions of the head groups and repulsive interactions of the surfactant tail group. Addition of ionic solutes can upset this delicate balance and induce phase separation. By changing the structure of the surfactant or through the addition of cosurfactants one can restore this balance and thus allow the dissolution of high concentrations of ionic species. 

Most single-chain surfactants do not lower the oil-water interfacial tension sufficiently to form microemulsions nor are they of the correct molecular structure, and short- to medium-chain length alcohols are necessary as cosurfactants. The cosurfactant also ensures that the interfacial film is flexible enough to deform readily around each droplet as their intercalation between the primary surfactant molecules decreases both the polar head group interactions and the hydrocarbon chain interactions. Medium-chain alcohols such as pentanol and hexanol have been used by many investigators as they are particularly effective... 

In this article we evaluate interactions in a system stabilized with an ionic surfactant and with a carboxylic acid as the cosurfactant. Such a system is distinguished from the common soap/alcohol stabilizer combinations by the fact that the soap/acid system does not require a minimum water concentration to dissolve the soap. 

The transport properties of microemulsions are of great interest both for the information they provide about the physical properties of the systems, and in industrial applications of these materials. The transport of matter or energy through oil in water (0/W) microemulsions is determined both by the volume fraction and geometry of the oil and emulsifier microdroplets (the structure effect") and by possible modifications in the transport properties of the continuous water phase by its interaction with the hydrophilic groups in the surfactant and cosurfactant that stabilize the microemulsion (the "hydration effect"). Through the use of appropriate mixture theories, these two effects can in part be separated. 

The two surfactant molecules should adsorb simultaneously and they should not interact with each other, otherwise they lower their respective activities. Thus, the surfactant and cosurfactant molecules should vary in nature, one predominantly water-soluble (e.g., an anionic surfactant) and the other predominantly oil-soluble (e.g., a medium-chain alcohol). 

Recently, MEEKC has been applied for the determination of catechins. In a similar fashion to MEKC, MEEKC uses a pseudo-stationary phase for separation of analytes. Instead of having micelles, surfactant-stabilized oil droplets in microemulsion solution serve as the pseudo-stationary phase for the partitioning of analytes. The formation of a microemulsion involves an organic solvent, SDS as a surfactant, and an alcohol as a cosurfactant. Pomponio et al. first developed a MEEKC method for the analysis of six catechins (C, EC, GC, ECG, EGC, and EGCG). This involved the use of heptane (1.36%, w/v), SDS (2.31%, w/v), and 1-butanol (9.72%, w/v), which serve, respectively, for the three functions mentioned above. The medium was a 50-mM phosphate buffer (86.61%, w/v) at pH 2.5. Electro-osmotic force (EOF) was almost absent, and analytes, due to their interactions with SDS-coated oil droplets. 

---