Emulsions internal phase ratio

Low intensity wet drum magnetic separators, 15 442-449 Low 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. 

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. 

Figure 26 compares the conversion as a function of time in concentrated emulsion and bulk polymerization and shows that polymerization proceeds much faster in a concentrated emulsion. The concentrated emulsion has an internal phase ratio of 0.93 and a molar ratio of MAA/styrene of 0.036. The molecular weight distributions of the polymers generated by both processes are presented in Fig. 27, which shows that concentrated emulsion polymerization leads to molecular weights an order of magnitude higher. Since the copolymer composition changes with conversion, the GPC curves were recorded at the same conversion. 

Fig. 29. Proton NMR spectrum of the copolymer latex (in CDC13) for a mole feed ratio of methacrylic acid to styrene of 0.12, internal phase ratio 0.93, AIBN 0.3 g, SDS 0.4 g and water 3 ml, polymerized first for 12 h at 40 °C. After this polymerization, additional water (twice as much as the weight of the emulsion) was added into the tubes, then polymerization continued for 2 days...

The maximum internal phase ratio that can be attained without deforming the drop spherical shape depends on the drop size distribution. For monodispersed rigid spheres, the most dense tessellation is the hexagonal packing at about 74% of internal phase. As an example, for randomly settled monodispersed spheres it could be 65%. For very poly dispersed emulsions it might be higher than 90% (16). 

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. 

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-... 

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. 

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). 

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. 

Many empirical formulas have been proposed to render the effect of internal phase ratio on the emulsion viscosity, but they are only valid in specific cases. Pal and Rhodes (84, 85) proposed and used a semi-empirical equation, that makes use of experimental data O qq as the internal phase fraction O at which the relative viscosity qj. = 100. This experimental value must be attained in the same formulation and emulsification conditions, particularly stirring characteristics, which is maybe why it significantly embodies the overall effects of all remaining variables ... 

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. 

The most spectacular viscosity reduction effect is with bimodal emulsions, which exhibit a distribution curve with two maxima, in most cases as the result of mixing two emulsions. These biemulsions are frequently found to be le.ss viscous than their base emulsions whenever the difference in average size or mode. separation is large enough. Most of the bimodal dispei ion studies have been carried out on solid suspensions instead of emulsions (S0-S2) however, the results seems to be directly applicable to emulsions. Figure 9 indicates such a case attained by mixing emulsions with identical internal phase ratio but different sizes, with a mode separation mea-sured as the cone.sponding diameter ratio is 3 (53),... 

Emulsifier is a general term that refers to chemical species that occupy the interfacial region between the droplet and the continuous phase. Emulsifiers aid in the formation and stabilization of emulsions. Emulsifiers are amphiphilic molecules, containing both hydrophilic and lipophilic groups, which provide the molecule with some affinity for both the disperse phase and the continuous phase. Emulsion stabilizers are polymeric molecules of higher molecular weight which form a protective steric layer around the dispersed droplets and also have some affinity for both phases. The dispersed, or discontinuous, phase is also referred to as the internal phase, whereas the continuous phase is also referred to as the external phase. Emulsions with an internal phase to total volume ratio of <0.3 are called low internal phase ratio emulsions. The external phase is usually the phase having... 

FIGURE 15.2. Because emulsion droplets can distort, emulsions of >0.95 internal phase ratio are possible. Close-packed spheres reach a maximum of only 0.74 internal phase ratio. 

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. 

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