Emulsions spontaneous formation

N. Shahidzadeh, D. Boim, and J. Meunier A New Mechanism of Spontaneous Emulsification Relation to Surfactant Properties. Europhys. Lett 40, 459 (1997). R.W. Greiner and D.F. Evans Spontaneous Formation of a Water-Continuous Emulsion from a W/O Microemulsion. Langmuir 6, 1793 (1990). 

H. Kunieda, Y. Fukui, H. Uchiyama, and C. Solans Spontaneous Formation of Highly Concentrated Water-in-Oil Emulsions (Gel-Emulsions). Langmuir 12,2136 (1996). 

K. Ozawa, C. Solans, and H. Kunieda Spontaneous Formation of Highly Concentrated Oil-in-Water Emulsions. J. Colloid Interface Sci. 188, 275 (1997). 

FIGURE 7.4 Intuitive explanation of the spontaneous formation of emulsion droplets from a SEDDS. 

Any surfactant adsorption will lower the oil-water interfacial tension, but these calculations show that effective oil recovery depends on virtually eliminating y. That microemulsion formulations are pertinent to this may be seen by reexamining Figure 8.11. Whether we look at microemulsions from the emulsion or the micellar perspective, we conclude that the oil-water interfacial free energy must be very low in these systems. From the emulsion perspective, we are led to this conclusion from the spontaneous formation and stability of microemulsions. From a micellar point of view, a pseudophase is close to an embryo phase and, as such, has no meaningful y value. 

It was observed that the titration of a coarse emulsion by a coemulsifier (a macromonomer) leads in some cases to the formation of a transparent microemulsion. Transition from opaque emulsion to transparent solution is spontaneous and well defined. Zero or very low interfacial tension obtained during the redistribution of coemeulsifier plays a major role in the spontaneous formation of microemulsions. Microemulsion formation involves first a large increase in the interface (e.g., a droplet of radius 120 nm will disperse ca. 1800 microdroplets of radius 10 nm - a 12-fold increase in the interfacial area), and second the formation of a mixed emulsifier /coemulsifier film at the oil/water interface, which is responsible for a very low interfacial tension. 

The most significant difference between emulsions (opaque) and microemulsions (transparent) lies in the fact that stirring or increasing the emulsifier concentration usually improves the stability of coarse emulsion. This is not the case with microemulsions - their formation depends on specific interactions among the constituent molecules. If these interactions are not realized, neither intensive stirring nor increasing the emulsifier concentration will produce a microemulsion. On the other hand, once the conditions are right, spontaneous formation occurs and little mechanical work is required [26]. 

Greiner, R.W. and Evans, D.F. (1990) Spontaneous formation of a water-continuous emulsion from a w/o... 

It has also been shown from thermodynamic consideration (Equation 3), that if the interfacial tension is very low, the thermodynamically stable emulsions can be formed. Previous investigators (20,45,47,48) have calculated that for a situation likely to occur in microemulsion formation, the interfacial tensions would need to be in the order of 10 to 10 5 dynes/cm for thermodynamic stabilization and for spontaneous formation of microemulsions. 

The thermodynamically stable micro- or mini-emulsions can solubilize more oil and still remain isotropic, even with a minimal increase in the droplet size. The formation of thermodynamically stable mini- or micro-emulsions depends on the specific interactions among the constituent molecules. If these interactions are not realized, neither intensive homogenization nor excessive emulsifier will produce the thermodynamically stable emulsions. Once the preparation condition is right, spontaneous formation of the thermodynamically stable emulsion product occurs and only minimal mixing is required [7]. The properties of thermodynamically stable micellar solutions are time independent. These micellar solutions are independent of the order of mixing, and they will return to the original state when subject to a small disturbance which is subsequently... 

In some cases under the conditions similar to those corresponding to the formation of lyophilic colloidal systems, a spontaneous formation of emulsions, the so-called self-emulsification, may take place. This is possible e.g. when two substances, each of which is soluble in one of the contacting phases, react at the interface to form a highly surface active compound. The adsorption of the formed substance under such highly non-equilibrium conditions may lead to a sharp decrease in the surface tension and spontaneous dispersion (see, Chapter III, 3), as was shown by A.A. Zhukhovitsky [42,43], After the surface active substance has formed, its adsorption decreases as the system reaches equilibrium conditions. The surface tension may then again rise above the critical value, acr. Similar process of emulsification, which is an effective method for preparation of stable emulsions, may take place if a surfactant soluble in both dispersion medium and dispersed liquid is present. If solution of such a surfactant in the dispersion medium is intensively mixed with pure dispersion medium, the transfer of surfactant across the low surface tension interface occurs (Fig. VIII-10). This causes turbulization of interface... 

Microemulsions can be prepared by controlled addition of lower alkanols (butanol, pentanol, and hexanol) to milky emulsions to produce transparent solutions comprising dispersions of either water-in-oil (w/o) or oil-in-water (o/w) in nanometer or colloidal dispersions (-100 nm). The alkanols (called cosurfactants) lower the interfacial tension between oil and water sufficiently for almost spontaneous formation. The miscibility of oU, water, and amphiphile (surfactant plus cosurfactant) depends on the overall composition, which is systan specific. Phase inversion method is further divided into two types ... 

Figure 8 Schematic change in spontaneous curvature of surfactant layers in the process of spontaneous formation of highly concentrated W/O emulsions (from Ref [58], with permission).

Kunieda, H., Fukui, Y., Uchiyama. H., and Solans, C. (1996) Spontaneous formation of highly concentrated water-in-oil emulsions (gel-emulsions). Langmuir, 12, 2136-2140. 

The importance of the properties of the interfacial region for spontaneous emulsification was first demonstrated by Gad (4), who observed that when a solution of lauric acid in oil was carefully placed on an aqueous alkaline solution, an emulsion spontaneously formed at the interface. The reason for this spontaneous emulsification is the formation of a mixed film of lauric acid and sodium laurate (produced by partial neutralization of the acid by alkali) which produces an ultra-low interfacial tension. 

Microemulsions are stable emulsions of hydrocarbons and water in the presence of surfactants and co-surfactants. They are characterized by spontaneous formation, ultra-low interfacial tension, and thermodynamic stability. 

Prepolymer ionomers with isocyanate end-groups and of sufficiently low molecular weight can be mixed with water in the presence of hydrophobic organic solvents, such as methylene chloride or toluene, or, in the case of sufficiently low viscosities, even without solvent. The prepolymers form emulsions, which in turn form polymer dispersion after chain extension by reaction of the isocyanate. Such a process requires powerful high-shear mixing equipment in the presence or absence of solvents. Solutions of ionomers in hydrophilic solvents form emulsions spontaneously in the presence of water. The formation of the emulsion must be... 

Gel emulsions can be prepared by the usual preparation method for W/O emulsions, dissolving a suitable emulsifier in the component that will constitute the continuous phase followed by addition of the component which will constitute the dispersed phase, with continuous stirring. However, they can be prepared according to several other methods that have been proposed specifically for highly concentrated emulsions.In the following, attention will be focused on the multiple emulsion method and spontaneous formation method, - because they constitute interesting novel methods of preparation. 

In the spontaneous formation method, emulsification is achieved by a rapid temperature change of a micellar solution or microemulsion without the need for mechanical stirring.It should be noted that for W/0 gel emulsions the system is below the HLB temperature at the start and emulsification takes place when an O/W microemulsion is rapidly heated from temperatures below to above the HLB temperature. Formation of O/W gel emulsions, by this method, is achieved by quickly cooling a water-in-oil microemulsion from a temperature higher than the HLB temperature of the system to a temperature below it, at which two phases, a micellar solution phase and an oil phase, appear. The emulsification process for a gel emulsion of the W/0 type is depicted in Figure 11.13. 

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