Phase inversion of emulsions

The phase inversion of emulsions can be one of two types (i) transitional inversion, which is induced by changing facers which affect the HLB of the system (e.g., temperature and/or electrolyte concentration) and (ii) catastrophic inversion, which is induced by increasing the volume fraction of the disperse phase. 

Phase Inversion The phase inversion of brine/oil/surfactant systems was established routinely by measuring solution conductivity employing a Jenway FWA 1 meter and cell. The process identifies the range over which a large decrease in conductivity occurs as the sytem under test is converted from an oil in water emulsion to a water in oil emulsion. Phase... 

D.J. Miller, T. Henning, and W. Grtibein Phase Inversion of W/O Emulsions by Adding Hydrophilic Surfactant—a Technique for Making Cosmetics Products. ... 

S. Sajjadi, F. Jahanzad, and M. Yianneskis Catastrophic Phase Inversion of Abnormal Emulsions in the Vicinity of the Locus of Transitional Inversion. Colloid and Surfaces A Physicochem. Eng. Aspects 240,149 (2004). 

Microscopic examination revealed that phase inversion of the emulsions occurred at a volume fraction of ca. 0.5 and that the average size of the dispersed droplets was about 5 microns. 

Phase inversion of the oil-in-water emulsion of cream by shearing. 

Development of W/O/W Emulsion during Phase Inversion. Sherman et al. (2lQ and Dokic et al.(25) emphasized that the development of a W/O/W type dispersion precedes the thermal induced phase inversion of 0/W emulsions. This suggest that the state of multiple structure in emulsions may be generalized as one of the mesophase between 0/W and W/0 emulsions, and also that there is a possibility of more simplifying the method for preparing W/O/W emulsions. 

Figure 6. Development of the W/O/W-type dispersion during phase inversion of W/0 emulsions. "Reproduced with permission from Ref. 21. Copyright 1983, Academic Press. "...

Phase inversion is a commonly observed phenomenon in which the continuous phase abruptly becomes the dispersed phase and vice versa (see Pacek et al. and Pacek, Nienow, and Moore Systematic studies on the effect of surfactant concentration and mixing on phase inversion and emulsion drop size have been carried out by Brooks and Richmond. Fig. 5 shows schematically the steps occurring during phase inversion. Although conflicting information exists on the subject, the following conclusions can be made ... 

Miller, D.J., Henning, T. and Grunbein, W. (2001) Phase inversion of W/O emulsions by adding hydrophilic surfactant - a technique for making cosmetic products. Colloid Surf. A, 183, 681. 

Lehnert S, Tarabishi H, Leuenberger H. Investigation of thermal phase inversion in emulsions. Colloids Surfaces A Physicochem Eng Aspects 1994 91 227-235. 

Figure A. Formation of rubber particles by phase Inversion of polymeric oil-in-oil emulsion. Phase contrast micrograph (rubber phase is light) according to G. E. Molare and H. Keskkula.

Figure 7.11. Light-responsive and reversible inversion of emulsion (dodecane/ water+NaNOs). The conductivity measurements indicate the type of continuous phase (conducting water vs. insulating oil) in samples maintained under gentle agitation (stirring bar). The emulsifier contains an azobenzene-modified polyacrylate (n = 5, x=3% in Fig. 7.1) and a temperature-responsive surfactant (C12E4) that in absence of polymer would stabilize inverse emulsion above 24°C. (a) Temperature sweep of the same sample exposed to UV or blue light, (b) Switches of the wavelength of exposure between UV and blue lights at fixed temperature (25°C) at times pointed by arrows.

GEJ Vaessen. Predicting catastrophic phase inversion in emulsions. PhD dissertation, Eindhoven University of Technology, Netherlands, 1996. 

Hence, the rate of film thinning in System II is much greater than that in System I. Therefore, the location of the siufactant has a dramatic effect on the thinning rate and, thereby, on the drop lifetime. Note also that the interfacial tension in both systems is the same. Henee, the mere phase inversion of an emulsion, from Liquid 1-in-Liquid 2 to Liquid 2-in-Liquid 1 (Fig. 15), could change the emulsion lifetime by orders of magnitude. As diseussed in Sec. V, the situation with interaction in the Taylor regime (between spherical, nondeformed drops) is similar. These facts are closely related to the explanation of the Bancroft rule for the stability of emulsions (see Sec. V) and the process of chemical demulsifrcation (1). 

Salager, J. L., Phase inversion and emulsion inversion on the basics of catastrophe theory, in Encyclopedia of Emulsion Technology, Vol. 3, Becher, P. (Ed.), Marcel Dekker, New York, 1983, pp. 79- 134. 

FORMATION OF RUBBER PARTICLES BY PHASE INVERSION OF A POLYMERIC OIL-IN-OIL EMULSION... 

Discontinuities in electrical conductivity values demonstrate the existence of a phase inversion process in macroemulsions. For 1 1 (aqueous oil) emulsions at 25°C, phase inversion of O/W to W/0 occurs around optimal salinity (1.5% NaCl). 

P. Dokic, P. Sherman, Study on thermal induced phase inversion of concentrated O/W emulsions stabilized by various tween emulsifiers, Colloid Polym. Sci. 1980, 258, 1159. 

Molau and Keskkula [351] were among the first to study the mechanism of particle formation in rubber containing polymers. They showed that phase separation occurs between the rubber and a vinyl polymer during the polymerization of solutions of rubber in vinyl monomers which is followed by formation of an oil-in-oil emulsion. During phase inversion of the emulsion, rubber solution droplets are formed which change into solid rubber particles in the final polymer. Structural investigations by phase contrast optical microscopy, shown in this chapter (Section 5.3), reveal dispersed particle size and distribution. Ugelstad and Mork [352] reported on new diffusion methods for the preparation of emulsions and polymer dispersions where the size and distribution of the latex particles were monitored by very simple optical, SEM and TEM methods. A microemulsion polymerization has been reported for the first time [353] with... 

The phase inversion that occurs on heating an emulsion is dearly demonstrated in a study of the phase behaviour of emulsions as a function of temperature. This is illustrated schematically in Figure 9.2 by what happens when the temperature is increased [13, 14]. 

A clear demonstration of the phase inversion that occurs on heating an emulsion is illustrated from a study of the phase behavior of emulsions as a function of temperature. This is illustrated in Fig. 2.10 which shows schematically what happens when the temperature is increased [13,14]. At low temperature, over the Winsor 1 region, 0/W macroemulsions can be formed and are quite stable. On increasing the temperature, the 0/W emulsion stability decreases and the macroemulsion finally resolves when the system reaches the Winsor 111 phase region (both 0/W and W/0 emulsions are unstable). At higher temperature, over the Winsor 11 region, W/0 emulsions become stable. 

Inversion of emulsion type is rare in practice although localized inversion may occur through interaction of the Components of the systems with packaging materials. This type of instability is dealt with later in relation to phase volume and emulsifier type. 

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