Theories of microemulsion formation

Three key theories to explain microemulsion formation of have been proposed the mixed film, solubilization and thermodynamic theories. As described, these theories are not mutually exclusive, as elements from each can contribute to an understanding of microemulsion formation and stability. 

---

Before describing how microemulsion nature and structure are determined by the structure and chain length of surfactant and cosurfactant, it is necessary first to briefly review the theories of microemulsion formation and stability. These theories will highlight the important factors required for microemulsion formation. This constitutes the first part of this review. The second part describes the factors that determine whether a w/o or o/w microemulsion is formed. This is then... 

This potential force occurs in microstructured fluids like microemulsions, in cubic phases, in vesicle suspensions and in lamellar phases, anywhere where an elastic or fluid boundary exists. Real spontaneous fluctuations in curvature exist, and in liposomes they can be visualised in video-enhtuiced microscopy [59]. Such membrane fluctuations have been invoked as a mechanism to account for the existence of oil- or water-swollen lamellar phases. Depending on the natural mean curvature of the monolayers boimding an oil region - set by a mixture of surfactant and alcohol at zero -these swollen periodic phases can have oil regions up to 5000A thick With large fluctuations the monolayers are supposed to be stabilised by steric hindrance. Such fluctuations and consequent steric hindrance play some role in these systems and in a complete theory of microemulsion formation. 

According to the thermodynamic theory of microemulsion formation, the total interfacial tension of the mixed film of surfactant and cosurfactant must approach zero. The total interfacial tension is given by the following equation. 

The status of the systems commonly referred to as microemulsions among surface and colloid chemists is still somewhat uncertain. Various experimental approaches have been used in an attempt to ascertain the details of their structural and thermodynamic characteristics. As a result, new theories of the formation and stability of these interesting but quite complex systems are appearing. Although a great deal has been learned about microemulsions, there is much more to be learned about the requirements for their preparation and the relationships among the chemical structure of the oil phase, the composition of the aqueous phase, and the structures of the surfactant and the cosurfactant, where needed. 

The effectiveness of the method is most probably based on the fact that alkyl hypochlorite is formed at the oil/water interface where the cosurfactant alcohol resides. The oxidation that follows takes place either inside or on the surface of oil droplet. The rate of the reaction can result from a large hydrocarbon/water contact area permitting interaction between oil-soluble sulfide with interfacial cosurfactant that served as an intermediary. An extension ofthis procedure to mustard deactivation has also been proposed [20b]. Such systems could be also applied to the degradation of several environmentally contaminating materials The formation of microemulsions, micelles and vesicles is promoted by unfavourable interactions at the end sections of simple bilayer membranes. There is no simple theory of solute-solvent interactions. However, the formation of... 

Several theories have been proposed to account for the thermodynamic stability of microemulsions. The most recent theories showed that the driving force for microemulsion formation is the ultralow interfacial tension (in the region of 10 4-10 2 mN m 1). This means that the energy required for formation of the interface (the large number of small droplets) A Ay is compensated by the entropy of dispersion —TAS, which means that the free energy of formation of microemulsions AG is zero or negative. 

Formation and Structure of Middle Phase Microemulsion. The 1 - m - u transitions of the microemulsion phase as a function of various parameters are shown in Figure 4. Chan and Shah (31) compared the phenomenon of the formation of middle phase microemulsion with that of the coacervation of micelles from the aqueous phase. They concluded that the repulsive forces between the micelles decreases due to the neutralization of surface charge of micelles by counterions. The reduction in repulsive forces enhances the aggregation of micelles as the attractive forces between the micelles become predominant. This theory was verified by measuring the surface charge density of the equilibrated oil droplets in the middle phase (9). 

The formulation of microemulsions or micellar solutions, like that of conventional macroemulsions, is still an art. In spite of exact theories that have explained the formation of microemulsions and their thermodynamic stabihty, the science of microemulsion formulation has not advanced to a point where an accurate prediction can be made as to what might happen when the various components are mixed. The very much higher ratio of emulsifier to disperse phase which differentiates microemulsions from macroemulsions appears at a first sight that the appHcation of various techniques for formulation to be less critical. However, in the final stages of the formulation it can be realised immediately that the requirements are critical due to the greater number of parameters involved. 

This new theory of the non-equilibrium thermodynamics of multiphase polymer systems offers a better explanation of the conductivity breakthrough in polymer blends than the percolation theory, and the mesoscopic metal concept explains conductivity on the molecular level better than the exciton model based on semiconductors. It can also be used to explain other complex phenomena, such as the improvement in the impact strength of polymers due to dispersion of rubber particles, the increase in the viscosity of filled systems, or the formation of gels in colloids or microemulsions. It is thus possible to draw valuable conclusions and make forecasts for the industrial application of such systems. 

In most ionic surfactant studies concerning the formation of microemulsions, it is assumed that the brine is a pseudocomponent, one used solely to maintain the mass balance in the system. Recent evidence indicates that this may not be the case with some W/O microemulsions, in which the aqueous core of the W/O structure exhibits a lower salinity and may sometimes even be salt-free. Such segregation has been explained by negative adsorption of the coions according to the Gouy-Chapman double layer theory [136]. 

Several theories of surfactant phase are available. Following Scriven (1976), this phase is assumed to be bicontinuous in oil and water, and the interface is assumed to have zero mean curvature, hence the pressure difference between oil and water is zero. Talmon and Prager (1978, 1982) divided up the medium into random polyhedra. The flat walls ensure no pressure difference between oil and water. They placed oil and water randomly into the polyhedra so that both oil and water were continuous when sufficient amounts of both phases were present. As in the earlier models of oil-in-water microemulsions, this randomness gave rise to an increased entropy which overcame the increased surface energy to yield a negative free energy of formation, reached only when the interfadal tension is ultralow. Such structures can form spontaneously. This random structure is characterized by a length scale. This led Jouffrey et al. (1982) to postulate that... 

The thermodynamics of microemulsion discussed in the beginning of the chapter has accounted for the basic conditions required for the formation and stability of reverse micellar systems. The energetics of formation in terms of Gibbs free energy, enthalpy, and entropy need to be quantified with reference to the system composition and the droplet structures. For the formation of w/o systan, a simple method called dilution method can exfiact energetic information for many combinations along with the understanding of their structural features. The method has been amply studied and presented in literature [4,27-32]. We, herein, introduce and present the method with basic theory and examples. 

The effect of alcohol on the dynamic properties of micellar systems has been considered as a first approach toward the understanding of microemulsion systems. In mixed alcohol + surfactant micelles, the theory predicts the existence of three relaxation processes, which have been experimentally observed using chemical relaxation techniques a slow process associated with the formation/breakdown of mixed micelles and two fast processes associated with the exchange of the surfactant and alcohol, respectively, between the mixed micelles and the bulk aqueous phase. With g representing a mixed micelle with a alcohol (A) molecules and s surfactant (S) molecules, these two exchange reactions can be written in the form... 

It must be pointed out that formation and stabihzation of nanoparticles in reversed micelles are the result of a delicate equilibrium among many factors. In addition, lacking a general theory enabling the selection a priori of the optimal conditions for the synthesis of nanoparticles of a given material with the wanted properties, stable nanoparticles containing w/o microemulsions can be achieved only in some system-specific and experimentally selected conditions. 

Lagues et al. [17] found that the percolation theory for hard spheres could be used to describe dramatic increases in electrical conductivity in reverse microemulsions as the volume fraction of water was increased. They also showed how certain scaling theoretical tools were applicable to the analysis of such percolation phenomena. Cazabat et al. [18] also examined percolation in reverse microemulsions with increasing disperse phase volume fraction. They reasoned the percolation came about as a result of formation of clusters of reverse microemulsion droplets. They envisioned increased transport as arising from a transformation of linear droplet clusters to tubular microstructures, to form wormlike reverse microemulsion tubules. 

The first part of the book discusses formation and characterization of the microemulsions aspect of polymer association structures in water-in-oil, middle-phase, and oil-in-water systems. Polymerization in microemulsions is covered by a review chapter and a chapter on preparation of polymers. The second part of the book discusses the liquid crystalline phase of polymer association structures. Discussed are meso-phase formation of a polypeptide, cellulose, and its derivatives in various solvents, emphasizing theory, novel systems, characterization, and properties. Applications such as fibers and polymer formation are described. The third part of the book treats polymer association structures other than microemulsions and liquid crystals such as polymer-polymer and polymer-surfactant, microemulsion, or rigid sphere interactions. 

---