Inverse micellar solutions

Water-in-fluorocarbon emulsions, stabilised with fluorinated nonionic surfactants, were investigated by small angle neutron scattering (SANS) spectroscopy [8,99]. The results indicated that the continuous oil phase comprised an inverse micellar solution, or water-in-oil microemulsion, with a water content of 5 to 10%. However, there was no evidence of a liquid crystalline layer at the w/o interface. A subsequent study using small angle x-ray scattering (SAXS) spectroscopy gave similar results [100]. 

Figure I. The solubility area of water in a hydrocarbon (CeH,) perUa(ethylene gly-c< ) dodecyl ether (C,t[EO]s) solution at 30°C. Key IM, inverse micellar solution.

Figure 2. The phase diagram water/benzene/pentafethylene glycol) dodecyl ether at 30°C. Key IM, inverse micellar solution LLC, lamellar liquid crystal and unmarked, aqueous micellar solution.

The two inverse micellar solutions will first briefly be described followed by an evaluation of the available experimental and theoretical information. 

A pentanol/potasslum oleate ratio of 15 that Is typical of the inverse micellar solution gives the corresponding value 1.02. Formally the two values are straddling the value v/a l = 1 in the correct directions, but it is obvious that they are extremely similar and the application of the zeroth order approachto these systems must be viewed with caution. The pronounced influence of a partition of cosurfactants between the Interface and the organic bulk is evident. 

Solubility of Sodium Chloride in an Inverse Micellar Solution of Pentanol... 

These inverse micelles will solubilize electrolytes in their aqueous core but the presence of the electrolytes also will influence the stability of the inverse micelle. A change in the stability of the inverse micelle will be reflected in modifications of the solubility region of the inverse micellar solution. This chapter will relate the changes in solubility areas from addition of electrolytes to the water to the structure of inverse micelles and other association complexes in the pentanol solution. 

The results showing augmentation of the surfactant alcohol ratio for maximum aqueous solubility with added electrolytes are not amenable to a similarly simple explanation, and the influence of the presence of electrolytes must be discussed against the relative stability of the inverse micelles and of the lyotropic liquid crystalline phase with which the inverse micellar solution is in equilibrium (7). 

A reduction of the stability of the liquid crystalline phase means a reduced region where it is stable and a corresponding increase of the region for the inverse micellar solution. The present results agree with these predictions, and it is justifiable to relate the changes in stability areas mainly to modifications of the potential distribution within the electric double layers. 

The changes in stability regions for inverse micellar solutions where added electrolytes appear were given a rational explanation using the associated structures determined in the inverse micelle solution with no electrolyte. 

The solubilisation of oil or water in a micellar solution of non-ionic surfactant, a) two-phase diagram (O - oil, W - water, 0, - oil in micellar solution, - water in inverse micellar solution, D - phase separation temperature region), b) interfacial tension as a function of T, according to Shinoda Friberg 1975... 

Quaternary phase diagram, L, - micellar solution, Lj - inverse micellar solution, M -micFoemulsion, after Friberg (1983)... 

Very early, the Swedish school attempted to determine the extent and shape of the region of existence of microemulsions in quaternary systems [76-78]. By examination of sections of the phase diagram at several levels of oil, Friberg and coworkers established a direct connection between the microemulsion areas and the inverse micellar solutions described by Ekwall [1]. Thus, prior to describing the phase diagrams of the quaternary systems, those of ternary systems made of water, sodium dodecylsulfate (SDS), and an alcohol are first presented here. 

Even though hydrotropes exhibit a resemblance to surfactants, a number of differences are obvious. The amount of hydrotrope needed to facilitate solubilization of the solute in water is usually much higher than that needed for surfactants. The reason for this is that the shorter carbon chain of the hydrotrope will result in a higher concentration for self-association, which is a requirement for solubilization (12). In the case of hydrotropes, the maximum solubilization will usually be higher than for surfactants. This might be explained by the fact that the micellar solution of a surfactant will be transformed into an inverse micellar solution via the formation of a lamellar liquid crystalline structure and the solubilization of the solute in water will be... 

The inverse micellar solution consisted of 8% water and 92% Laureth 4 (or Brij 30), poly(4)-oxyethylene lauryl ether. No oil phase was required for this synthesis. The mixtures were aged for 24h. The silver particles were stable in air, so special atmospheric precautions were unnecessary. The average hydrodynamic radii of the particles were 25, 17 and 23 nm for procedures (i), (ii) and (iii) respectively. 

It is obvious that the combination water and surfactant with their micellization and solubiliztion is unable to explain the phenomenon of microemulsions. For an explanation the role of the cosurfactant must be understood. The next section will display the relationship with cosurfactant inverse micellar solutions and water-in-oil, W/0, microemulsions. 

The relations between micellar solutions and microemulsions has been reviewed for microemulsion systems with ionic surfactants. The W/0 microemulsions are a direct continuation of the cosurfactant inverse micellar solution. At low water content no surfactant association takes place the surfactant molecules form small aggregates with a few water and cosurfactant molecules. The W/0 microemulsions are thermodynamically stable. 

The reaction was studied both in aqueous micellar solutions and in oil inverse-micellar solutions in order to investigate the effect of charged interfaces on metal ion reactivity. 

A typical phase diagram of a ternary system of water, ionic surfactant and long-chain alcohol (co-surfactant) is shown in Figure 15.5. The aqueous micellar solution A solubilizes some alcohol (spherical normal micelles), whereas the alcohol solution dissolves huge amounts of water, forming inverse micelles, B. These two phases are not in equilibrium, but are separated by a third region, namely the lamellar liquid crystalline phase. These lamellar structures and their equilibrium with the aqueous micellar solution (A) and the inverse micellar solution (B) are the essential elements for both microemulsion and emulsion stability [3]. 

The microemulsion may be related to the micellar solutions A and B shown in Figure 15.4. A W/O microemulsion is obtained by adding a hydrocarbon to the inverse micellar solution B, whereas an O/W microemulsion emanates from the aqueous micellar solution A. These microemulsion regions are in equilibrium with the lamellar liquid crystalline stmcture. To maximize the microemulsion region, the lamellar phase has to be destabilized, as for example by the addition of a relatively short chain alcohol such as pentanol. In contrast, for a macroemulsion, with its large radius, the parallel packing of the surfactant/co-surfactant is optimal and hence the co-surfactant should be of chain length similar to that of the surfactant. 

The essential feature of importance for the microemulsion is the fact that the inverse micellar solution and the aqueous solution of normal micelles are not in mutual equilibrium except for extremely low-surfactant concentrations. For higher-surfactant concentrations, the equilibrium is with the liquid crystalline phase. As a consequence, the transition from the normal micelles to inverse micelles (Figure 1.2) does not happen directly, but through a lamellar liquid crystal (Figure 1.3). 

W/0 microemulsions stabilized by an ionic surfactant also employ a less hydrophilic amphiphile, which is known as the cosurfactant. The original cosurfactants were alcohols [5] and Gillberg realized early on [1] that W/O miCToemul-sions were obtained simply by adding a hydrocarbon to EkwaU s inverse micellar solution (Figure 1.4). Addition of the hydrocarbon does not imply significant... 

FIGURE 1.4 Addition of hydrocarbon to the inverse micellar solution (solid line) (Figure 1.1) gives a W/O microemulsion (hatched line). 

Structure changes [6] and the W/0 tnicroemulsions were hence described as inverse micellar solutions. The approach was initially not received well by Schulman s successors [7], and it is remarkable that Schulman s initial publication on the concept described these miaoemulsions as colloid solutions. The term microemulsion was coined much later [8]. 

Figure 4.26 Sequence of phases observed on increasing solvent content, in a binary amphiphile-solvent system, representing a hypothetical phase diagram where phase transitions are controlled by solvent content only. Here a, b, c and d indicate intermediate phases (for example the bicontinuous cubic structure shown in Fig. 4.25d), L2 denotes the inverse micellar solution, Hn is the inverse hexagonal phase, L is the lamellar phase. Hi is the normal hexagonal phase and Li is the normal micellar phase. In practice, the full sequence of phases is rarely observed, and in reality the phase transitions depend on temperature as well as concentration...

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