Ionic Surfactant Systems

Although the thermodynamic theory of microemulsions still has some way to go to be more or less complete, a number of generalizations can be made regarding ionic surfactant microemulsions  

A cosurfactant is always required to form W/O microemulsions using ionic surfactants, not only to lower the interfacial tension a but also to reduce head group crowding due to the type 2 curvature. 

O/W microemulsions require less cosurfactant than W/O systems, aU other things being equal (OTBE). 

Electrical double-layer effects favor O/W systems. As a result, the addition of electrolyte will tend to push the same surfactant/cosurfactant system toward the formation of W/O microemulsions. 

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Sole, I., Maestro, A., Gonzalez, C., Solans, C. and Gutierrez, J.M. (2006) Optimization of nano-emulsion preparation by low-energy methods in an ionic surfactant system. Langmuir, 22 (20), 8326-8332. 

In the case where no excess salt is added to the binary ionic surfactants system, assuming that the nature of the diffuse electrical doyble layer of the surface is similar to that of the micelle, the following equation can be obtained... 

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. 

Tiemessen, H.L.G.M. Non-ionic Surfactant Systems for Transdermal Drug Delivery, Thesis Leiden University The Netherlands, 1989. 

A detailed derivation of Eq. (3) may be found elsewhere. In the presence of excess inorganic electrolyte in the univalent ionic surfactant system, the factor 2 in Eq. (3) can be reduced to 1 by thermodynamic modification. 

Stacking the isothermal Gibbs triangles on top of each other results in a phase prism (see Fig. 1.3(a)), which represents the temperature-dependent phase behaviour of ternary water-oil-non-ionic surfactant systems. As discussed above, non-ionic surfactants mainly dissolve in the aqueous phase at low temperatures (2). Increasing the temperature one observes that this surfactant-rich water phase splits into two phases (a) and (c) at the temperature T of the lower critical endpoint cepp, i.e. the three-phase body appears. Subsequently, the lower water-rich phase (a) moves towards the water corner, while the surfactant-rich middle phase (c) moves towards the oil corner of the phase prism. At the temperature Tu of the upper critical endpoint cepa a surfactant-rich oil phase is formed by the combination of the two phases (c) and (b) and the three-phase body disappears. Each point in such a phase prism is unambiguously defined by the temperature T and two composition variables. It has proved useful [6] to choose the mass fraction of the oil in the... 

Strey, R. (1996) Water-non-ionic surfactant systems, and the effect of additives. Ber. Bunsenges. Phys. Chem., 100, 182. 

In this equation, hs is the hard sphere volume fraction which is about 14% larger in o/w-droplet microemulsions of non-ionic surfactant than the dispersed volume fraction. This is caused by the water penetration in the surfactant layer [64]. S(q) approaches unity for q values smaller than the minimum of I(q). This behaviour occurs even for fairly high volume fractions in non-ionic surfactant systems (see for example Fig. 8 in Ref. [64]). Seeing that the value of the radius is fixed by the position of the minimum of I(q), the approximation of S(q) 1 in Eq. (2.12) does not lead to a significant error in the determination of Rq if the low q part of the experimental curve is not taken into... 

Formulation essentially relates to the content of the systems and generally not to the way it is attained if thermodynamically stable systems are considered. The simplest microemulsion system would contain an organic oil phase (O), an aqueous phase generally referred to as water (W), and a surfactant (S) at a given temperature (T) and pressure (p). This means that at least five variables are required to describe the system. In practice, the situation is much more complicated. Water always contains electrolytes. Moreover, oils as well as nearly all commercial surfactants are mixtures. In most cases, particularly with ionic surfactant systems, a co-surfactant (e.g. an alcohol (A)) is added, among other functions, to reduce the rigidity of the surfactant layer and thus to prevent the formation of gel-like mesophases. 

The salinity effect of different salts, particularly divalent cation salts, is expressed through the term bS in the correlation for non-ionic surfactants of the polyethoxylated phenol or alcohol type. No information is available yet on the salinity effect on other non-ionics such as alkyl-polyglucosides. The salinity effect on ionic surfactant systems is a more complex issue because the surfactant itself is also a (more or less) dissociated electrolyte. Its degree of dissociation is paramount as far as its hydrophilicity is concerned. For instance sodium salts of alkyl sulphonic acids are essentially completely dissociated, hence they act as the sulphonate ion, and it is essentially the same with the salt of potassium or ammonium. The presence of multivalent anions produces an interference with the monovalent anionic surfactant ion, such as an alkyl benzene sulphonate, but it is essentially an ideal mixing rule. 

In ionic surfactant systems, the partition coefficient between excess phases is often found to be unity, hence an optimum formulation is defined by SAD = 0. With polyethoxylated surfactants, the CMC in water is often extremely low, whereas the monomer solubility in many oils is high. Consequently, the assumption of unit activity coefficient is not valid anymore and the partition coefficient between excess phases is not unity. In such cases, the partition coefficient value at optimum formulation is taken as a reference, and the deviation from this reference, the so-called hydrophilic-lipophilic deviation (HLD) is defined by dividing by RT to make the yardstick dimensionless [34]. 

The linear temperature term in Eqs. (3.24) and (3.25) is only an approximation, which maybe refined by using a Van t Hoff-type expression deduced from Eq. (3.22), particularly for the polyethoxylated non-ionic surfactant systems, for which the variation has been found to be significantly non-linear with temperature. 

In order to separate the phases of a surfactant-based system for product isolation and, where necessary, for catalyst recovery the appropriate phase region of the phase diagram has to be chosen first. Within this region the phase composition and the kinetics of phase separation are essential questions. Near to the phase boundaries the composition of the phases is rather similar and separation of the components will often be incomplete. The phase separation often takes a long time because of low interfacial tension and high stability of the emulsified two-phase system. The kinetics of phase separation depends sensitively on the temperature of the system, especially on the temperature distance to phase boundaries. Figure 5.15 shows a plot of separation times for a water-oil-non-ionic surfactant system as a function of the temperature. 

Figure 5.15 Section of phase prism and separation times of water, oil, non-ionic surfactant system as function of temperature.

In order to check this theory predictive calculations for some non-ionic and ionic surfactant system were carried out. They showed good agreement with experimental results. 

One limitation of the HLB concept is its failure to account for variations in system conditions from that at which the HLB is measured (e.g., temperature, electrolyte concentration). For example, increasing temperature decreases the water solubility of a nonionic surfactant, ultimately causing phase separation above the cloud point, an effect not captured in a temperature-independent HLB value. When both water and oil are present, the temperature at which a surfactant transitions from being water soluble to oil soluble is known as the phase inversion temperature (PIT). Below the PIT, nonionic surfactants are water soluble, while above the PIT. they are oil soluble. Thus, from Bancroft s rule, a nonionic surfactant will form an 0/W emulsion below its PIT and a W/0 emulsion above its PIT. Likewise, increasing salt concentrations reduces the water solubility of ionic surfactant systems. At elevated salt concentrations, ionic surfactants will eventually partition into the oil phase. This is illustrated in Fig. 13. which shows aqueous micelles at lower salt concentrations and oil-phase inverse micelles at higher salt concentrations. Increasing the system temperature will likewise cause this same transition for nonionic surfactant systems. 

Other ionic surfactant systems were also studied, particularly isomerized a-olefin sulfonates [69] and other branched surfactants [70], polyallq l ones such as dioleyl phosphates [71], and zwitterionic surfactants [72]. 

In an ionic surfactant system there is an inhomogeneous distribution of counterions, with a higher concentration close to the charged microscopic interfaces than at large distances from them. A number of NMR parameters give insight into the counterion distribution as well as into specific ionic interactions. Following is a partial list of possible approaches. 

The third effect is much more important with ionic surfactant systems than with nonionic ones, since it is linked with the shield effect produced by electrolyte ions with respect to the ionic group-water Acw interaction. 

In many instances the SOW ideal ternary case is not sufficient to describe the behavior of an actual system. It has been noted that the addition of alcohol as a disordering agent is often required to avoid viscous or sol id-like mesophases. particularly to produce microemulsions with ionic surfactant systems (108). According to the corre)ations for optimum formulation, the alcohol effect can also be that of a cosurfactant that modifies the overall balance of affinity through the flA) and 0(i4) terms. The use of two surfactants i.s also often recommended to attain a better emulsion stability, a statement that. should not be taken for granted in all cases, although it could prove correct in some ca.ses. Hence, it is often... 

The investigation technique is the unidimensional formulation scan as in the phase behavior. studies discussed in the previous chapter. For the sake of simplicity, the scanned variable is often taken as the salinity for ionic surfactant systems, and as surfactant EON or temperature for nonionic systems, but it should be well understood that other formulation variables would produce exactly the. same effects. In the reasoning, the formulation will be referred to a.s SAD, the deviation from optimum formulation, whatever the variable used to produce the scan. 

For ionic surfactant systems the definition of SAD as a function of flic formulation variables is... 

Research on microemulsions was a major topic in his scientific activity, since the earlier work under Prof. Shinoda s supervision [1, 2], through his entire scientific career. First the attention was focused to find the conditions to produce three-phase equilibria (balanced conditions) in both ionic [9-12] and nonionic [13-17] surfactant systems. In this context it was shown that the effect of temperature in ionic surfactant systems is opposite to that in polyoxyethylene-type nonionic surfactants [10] and that both types of surfactant systems display similarities in phase behavior [18]. The most detailed phase equilibria of a water/ nonionic surfactant/ahphatic hydrocarbon system around the HLB temperature (Figure 2) was reported in 1982 [16]. 

A remarkable contribution in recent years was to have shown for the first time the formation of highly viscoelastic worm-like micelles (Figure 12) in mixed nonionic surfactant systems [110]. This finding allowed to clarify the relation between packing constraints of hydrophobic chains and micellar growth because the complex interactions between counterions (present in ionic surfactant systems) and headgroups had not to be taken into consideration. 

He showed the formation of viscoelastic worm-like micelles in various nonionic and ionic surfactant systems and described the evolution of micellar growth namely by rheology and small-angle X ray scattering [110, 112, 118, 133, 137, 144—155], Zero-shear viscosities 3 x 10 times that of water was reported for certain systems [118],... 

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