Inversion, multiple emulsions

Electrostatic and non-electrostatic biopolymer complexes can also be used as effective steric stabilizers of double (multiple) emulsions. In this type of emulsion, the droplets of one liquid are dispersed within larger droplets of a second immiscible liquid (the dispersion medium for the smaller droplets of the first liquid). In practice, it is found that the so-called direct water-in-oil-in-water (W/O/W) double emulsions are more common than inverse oil-in-water-in-oil (O/W/O) emulsions (Grigoriev and Miller, 2009). In a specific example, some W/O/W double emulsions with polyglycerol polyricinoleate (PGPR) as the primary emulsifier and WPI-polysaccharide complexes as the secondary emulsifying agent were found to be efficient storage carriers for sustained release of entrapped vitamin Bi (Benichou et al., 2002). 

Gohla, S.H. and Nielsen, J. (1995) Partial phase solu-inversion technology (PPSIT). A novel process to manufacture long term stable multiple emulsions by an in situ one step procedure. Seife Ole Fette Wachse, 121(10), 707-10. 

Related to particle sizing, Molau and Kesskula described the concept of type I and II occlusion [5]. The prepolymer is viscous and has a retarding effect on the phase inversion. In most cases multiple emulsions are formed after the phase inversion point. If the agitation is not extremely high these multiple emulsions survive the further copolymerization and give SAN occlusions in the rubber particles. These occlusions are called type I. Type II occlusions are formed when monomer dissolved in the rubber phase is copolymerized. Because SAN is not compatible with the rubber, separation occurs within the rubber particle, giving type II occlusions. 

The exact mechanism of inversion remains unclear, although obviously some processes of coalescence and dispersion are involved. In the region of the inversion point multiple emulsions may be encountered. The process is also not always exactly reversible. That is, hysteresis may occur if the inversion point is approached from different sides of the composition scale. Figure 18 shows the irreversible inversion of a diluted bitumen-in-water emulsion brought about by the application of shear (60). 

Figure 2. Change in droplet diameter of multiple emulsions as a function of the concentration of secondary emulsifier (il) and of the calculated weighted or apparent HLB of the surfactant system. Hatched regions represent boundaries for inversion. Reprinted with permission from Ref. 28. Copyright 1979, Academic Press.

This refers to the process when an exchange occurs between the disperse phase and the medium. For example, an O/W emulsion may, with time or change of conditions, invert to a W/O emulsion. In many cases phase inversion passes through a transition state whereby multiple emulsions are produced. 

It was mentioned earlier that the B and C regions often exhibit multiple emulsions, which is actually the simultaneous occurrence of both emulsion types. There ts some evidence that multiple emulsions start occurring before the catastrophic inversion takes place, and some researchers have proposed a competitive kinetic model in which one of the emulsions could be more stable and thus would prevail (87,89,110). This is consistent with the fact that the variables susceptible to influence the breaking-coalescence mechanisms do influence the location of the inversion tine and hysteresis region. 

The other two branches of the standard inversion line are essentially vertical, and are located typically at 30% water on the negative SAD side of optimum formulation, and at 70% water on the positive side. When the water content is low, the emulsion is always W/0, regardless of the formulation. Similarly, when the oil content is low, an 0/W can be expected, whatever the formulation. In these extreme WOR regions, the phase which is present in larger volume becomes the external phase of the emulsion. It may be said that the composition dominates. However, a closer look at the conductivity value indicates the presence of multiple emulsions in the B" and zones, i.e., where the composition effects dominate over the normal formulation trend. These B" and regions have been called abnormal in opposition to the other ones which are labeled normal because they follow the Bancroft rule and the wedge theory (172). 

When the dynamic process is pushed too far, it finally results in inversion, and the emulsion type changes, often with the production of a multiple emulsion as an intermediate situation (207, 208). 

In principle, multiple emulsions can be prepared by any of the many methods for the preparation of conventional emulsion systems, including sonication, agitation, and phase inversion. Great care must be exercised in the preparation of the final system, however, because vigorous treatments normally employed for the preparation of primary emulsions will often break that system if used in secondary emulsion formation, resulting in loss of the identity of the primary phase. 

Multiple emulsions reportedly have been prepared conveniently by the phase inversion technique mentioned earlier however, such systems will generally have a limited persistence. It requires a very judicious choice of surfactant or surfactant combinations to produce a system that has useful characteristics of formation and stability. A general procedure for the preparation of a W1/0/W2 multiple emulsion, illustrated in Figure 11.14, may involve the formation of a primary emulsion of water-in-oil using a surfactant suitable for the... 

The values of Rp varied with the 1.0 power of emulsifier concentration and the 2.0 power of initiator concentration. The values of N varied with the 3.7 power of emulsifier concentration and 0.3 power of initiator concentration. These variations were unusual, and transmission electron microscopic examination of the inverse emulsions and latexes showed that some appeared to be multiple emulsions, i.e., possibly 10-50nm droplets of oil dispersed in 100-300nm droplets of water dispersed in the continuous o-xylene phase. 

Emulsions are dispersions of two immiscible fluids, such as water and oil. Simple emulsions are of two different kinds direct emulsions (OAV) are dispersions of oil into water, whereas inverse emulsions (W/O) are dispersions of water into an oil continuous phase. Multiple emulsions of water in oil in water WilOIW are direct emulsions (oil in where the dispersed oil phase... 

To prepare stable multiple emulsions that can be studied over a long time, we use amphiphiUc copolymers instead of small molecular surfactants as the globule interface stabilizers (Sela et al., 1994 Garti, 1998 Benichou et al. 2004 Michaut et al. 2003, 2004a). However, the interface of inverse droplets is stabilized either with a small tensioactive molecule or with a polymeric surfactant. 

In the inverse emulsion, the mean square displacement of the droplets is proportional to time ((Ar (t)) = 6Dt, where D is the diffusion coefficient) over the entire time scale, which conforms with Brownian motion. In multiple emulsions, the dynamics of the droplets inside the globule are XMhdiffusive oc with a < 1) over almost three decades of time (from t < 1ms to 0.1 ms) before returning to the Brownian motion at larger time scales with a smaller diffusion coefficient than that of the inverse emulsion (Figure 2.4). 

Figure 2.4 Mean square displacement of inverse droplets In an inverse emulsion dashed line), in globules of a multiple emulsion continuous line). Straight hues, of slopes 1 and 3/4, are guides for the eye.

From these structural studies we can conclude that when a small molecular surfactant is used to stabilize inverse droplets inside the globule phase, the inverse droplets are submitted to attractive interaction toward the droplets inner interface, and slowly diffuse along the surface of the globules. A single droplet thus spends an increased amount of time close to the globules interface. This phenomenon appears to decrease the stability of multiple emulsions. 

We will consider first the dynamical rheological properties of concentrated inverse emulsions (stabilized by Span 80 in dodecane) for various volume fractions ( ). The experimental results are given in the form of the plot G Rd/( in relation to ( ) (Figure 2.8, hollow symbols). From the value of the interfacial tension, y = 3.5mN-m measured by tensiometry, and using 3, we calculate values for a and ( )c equal to 1 and 0.66, respectively. We will therefore use this measured value of a in the subsequent analysis of concentrated multiple emulsions. 

The values of G Rd/( for inverse emulsions and for multiple emulsions superimpose (Figure 2.8). Thus the interfaces between the droplets and... 

Figure 2.8 Mason s plot of the rescaled elastic modulus of concentrated emulsions as a function of the volume fraction inverse dodecane in water emulsion stabilized with Span 80 mnltiple emulsion WIOIW stabilized by Span 80 and modified polyacrylic acid 10C12. The straight line is a linear fit to the multiple emulsions data.

The use of an amphiphilic copolymer as a surfactant allows for an increase of the lifetime of multiple emulsions. The structural properties emulsions should thus be precisely studied. We performed direct confocal naicroscopy observations as well as light-scattering experiments in order to investigate the dynamics of the inverse droplets inside oil globules. Depending on the emulsion formulation, we showed that droplets are either unifomaly distributed inside direct globules, or they are depleted toward the inner surface of the globules. These structural differences appear to play a key role in the destabilization and release processes. 

Moreover, we used rheology to determine the interface composition of inverse droplets, which is difficult to measure otherwise. The elastic modulus of a concentrated emulsion is directly linked to the interfacial tension of the droplets interfaces. We showed that the measurement of the elastic modulus of a concentrated multiple emulsion leads to the value of the interfacial tension of the droplets interfaces. We were thus able to access the composition of these interfaces in situ. 

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