High-internal-phase ratio emulsions

These concentrated emulsions have been referred to by a number of different names in the literature, including high internal phase ratio emulsions (HIPREs) [1,3-7], gel-emulsions [8-14] and hydrocarbon gels [15,16], In this review, the term HIPE will be used throughout. 

As a matter of fact, the internal phase content is the most imponanc variable as far as the viscosity of high internal phase ratio emulsion is concerned, and provided that the formulation ensures a proper stability. 

It is important to remaiic that in both cases the drop size decreases considerably when approaching the inversion line by augmentation of the internal phase content in any A region (a path indicated as a black arrow). This effect seems to be due to a considerable improvement in stirring efficiency in the viscous high-internal-phase-ratio emulsions located in these zones. So far it is not known whether this fact is absolutely general, but it may be said that it is a quite common circumstance, and this is why Fig. 19 indicates the presence of a small drop size strip in the vicinity of the vertical branches of the inversion line. 

It may be said that the effects of the physical variables are well documented, though not completely elucidated for high internal phase ratio emulsions, which are very viscous and nonNewtonian. On the contrary, the basic formulation effects have been uncovered in the past 20 years, but many complex formulation-related phenomena still remain unclear. 

Anionic surfactants are usually less expensive and they perform in a similar way. However, this type of surfactant frequently contains a sulfur atom and a sodium cation, which are forbidden for the combustion application of the emulsion, at a first glance, cationic surfactants are also ruled out due to their cost, unless they can be used in very small proportions, which is not the convenient situation for a high internal-phase ratio emulsion, because this tends to shrink the A region width. 

Lissant, K.J., The geometry of high-internal-phase-ratio emulsions, J. Colloid Interface Sci., 22, 462, 1966. 

Pal, R. (1999) Yield stress and viscoelastic properties of high internal phase ratio emulsions. Colloid Polym. Sci., 277 (6), 583-588. 

Since the continuous and dispersed phase have generally different densities, there is a neat Archimedes pull on the dispersed phase drops that drives a separation process called. rcrr/ing. This separation tends to gather the drops in a region that becomes a high internal phase ratio emulsion, sometimes referred to as a cream. 

On the other hand, the slow shear mixing of high internal phase ratio emulsions located in the shaded zones of Fig. 3c has been found to be very efficient in producing exttemely small droplets, irrespective of the surfactant concentration and stirring energy. There is thus another minimum drop size (shaded) strip located in each of the A regions, near and parallel to the vertical branch of the inversion line [3,50,51]. 

Viscosity is an important physical property of emulsions in terms of emulsion formation and stability (1, 4). Lissant (1 ) has described several stages of geometrical droplet rearrangement and viscosity changes as emulsions form. As the amount of internal phase introduced into an emulsion system increases, the more closely crowded the droplets become. This crowding of droplets reduces their motion and tendency to settle while imparting a "creamed" appearance to the system. The apparent viscosity continues to increase, and non-Newtonian behavior becomes more marked. Emulsions of high internal-phase ratio are actually in a "super-creamed" state. 

The composition effect is not sinqiler. It has been found with low-viscosity oil/water emulsions that the drop size tends first to increase and then to decrease as the intemai phase ratio increases (93). lliis results in isodrop size contours as indicated in Fig. 19. On the other hand, it seems that with high-viscosity oil the drop size decreases steadily when the internal phase ratio increases-... 

On the other hand, transitional inversion is used as well to attain extremely small drop emulsions, som imes referred to as miniemulsions or gel emulsions, because of the high viscosity resulting frtnn the exceedingly small drop size, even at a low internal phase ratio (111-115). 

Fig. 2. Emulsion-templated crosslinked poly(acrylamide) materials synthesized by polymerization of a high internal phase C02-in-water emulsion (C/W HIPE). (a) SEM image of sectioned material, (b) Confocal image of same material, obtained by filling the pore structure with a solution of fluorescent dye. As such (a) shows the walls of the material while (b) show the holes formed by templating the SCCO2 emulsion droplets. Both images = 230 iJim X 230 jjim. Ratio of C02/aqueous phase = 80 20 (v/v). Pore volume = 3.9 cm /g. Average pore diameter = 3.9 jjim. Reprinted with permission copyright 2001 WILEY VCH Verlag GmbH Co. [28].

In order to produce highly porous materials, a certain class of emulsion, known as high internal phase emulsion, or HIPE, is used. HIPEs are defined as having an internal, or droplet, volume phase ratio, (f>, of 0.74 or greater. A volume fraction of 0.74 represents the maximum volume ratio at which the droplet phase will pack as uniform non-deformable spheres. Values of 0 up to 0.99 can be observed, indicating that the droplet phase in a HIPE is either non-uniform or that the droplets are deformed into polyhedral ones. ... 

The viscosity of emulsions in the A+, Pc regions far from SAD = 0 can be high with respect to their external phase. However, close to SAD = 0 the emulsion viscosity can be extremely low, probably because of the low interfacial tension, which allows easy deformation of droplets near the A /A boundary. Abnormal emulsions have low internal phase ratio and exhibit viscosities similar to their external phase. However, real systems can show large deviations from the schematic WOR map. 

The emulsion relative viscosity increases in the A regions in the direction of higher internal phase ratio (at constant formulation), so that the viscosity maximum is located near the vertical branches of inversion line (see Fig. 3c). This high viscosity, which is due to a high internal phase content, is... 

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