Volume fractions, emulsions, effect water

Abstract Pressure effect on the structure of a 3-component micro-emulsion AOT/water/n-decane was investigated by means of small-angle neutron scattering. The measured pressure range was from 0.1 (1 atm) to 83.5 MPa. The present sample was at the composition of the molar ratio of [water]/[AOT] = 40.8 and the volume fraction of both water and AOT against the whole volume 0.6. 

Table 1. Effect of the HLB of the surfactant on the formation and stability of concentrated emulsions. The concentrated emulsion contains styrene and water as the two phases and the volume fraction of the dispersed phase is 0.9. The concentrated emulsion was prepared at room temperature and its stability test was conducted by heating the emulsion at 50 °C for 3 h and 24 h, respectively...

At lower Na+ concentration, the high negative charge (zeta potentials of salt-free emulsion droplets are 115 mv and above (5)) immobilizes a water layer around the droplets, thus effectively increasing the apparent volume fraction. Addition of Na+ reduces the net charge and some of the immobilized water is released giving a lower apparent volume fraction and lower apparent viscosity. 

Assuming that the W/O emulsion behaves as a near hard-sphere dispersion, it is possible to apply the Dougherty-Krieger equation [7, 8] to obtain the effective volume fraction, 4>. The assumption that the W/O emulsion behaves as a near hard sphere dispersion is reasonable as the water droplets are stabihsed with a block copolymer with relatively short PHS chains (of the order of lOnm and less). Any lateral displacement of the polymer will be opposed by the high Gibbs elasticity of the adsorbed polymer layer, and the droplets will maintain their spherical shape up to high volume fractions. 

Unfortunately, no such simple solution is available for the ESA effect at the high frequencies at which it is ciu-rently used. Goetz and El-Aasser (24) attempted to compare the electroacoustic and electrophoretic behavior of concentrated miniemulsion systems of toluene in water, stabilized by cetyl alcohol and sodium laiuyl sulfate. They concluded that the simple correction which works well for CVP does not produce a similar reconciliation in the case of the ESA effect. Their conclusions are, however, suspect because of uncertainties arising from the dilution of the emulsion system this can so easily lead to changes in siuface properties, no matter how carefully it is done. Texter s results (34) referred to above are perhaps more definitive in this case. He showed that in the range from 2 to 5% by volume where his particles showed a nonlinear dependence of the ESA signal on volume fraction, the Levine and Neale model (3 8) was unable to account for the nonlinearity. His particles were, however, nonspherical and that may at least partially explain the discrepancy. 

Figure 12 Water droplets in model oils with asphaltene and resin fractions extracted from a crade oil normalized cumulative volume showing the effect of asphaltene and resin content. Key emulsion 1 - high asphaltene, no resin 2 -lowasph., no resin 3 - high asph., high resin 4 - high asph., low resin.

Figure 8.19(a) The effect of temperature on the types of emulsions, and the volume fractions of oil, cream, and water phases 1 day after agitation. (O) Drained phase-cream boundary ( ) coalesced phase-cream boundary, (b) Schematic diagram of water, surfactant, and oil phases after the complete phase separation of the system. Surfactant is lipophilic at higher temperature. From Shinoda and Sagitani [114] with permission. 

Even simply for the purpose of classifying rheological behaviour, it is convenient to distinguish between flocculated and non-flocculated systems. This is because, firstly, the theoretical position is much more well developed as a function of oil volume fraction for dispersed (non-flocculated) systems than for flocculated ones, an4 secondly, the experimental behaviour of many emulsion systems can be interpreted most effectively at the mechanistic level from a detailed consideration of the type and extent of flocculation. Much of the experimental work published recently on emulsion rheology has been concerned with the role of water-soluble polymers in controlling the structure and stability of flocculated systems. Of particular importance in such systems is the viscoelasticity of the polymer-containing aqueous continuous phase and the nature of the interaction between polymer and emulsion droplets. 

When the effect of 0 on the emulsion stability is considered, one must distinguish between the effect of the volume fraction on the ease of emulsification and on the subsequent emulsion stability. The volume fractions of oil and water have a considerable effect on the ease of emulsification. For example, it is very difficult to prepare an OAV emulsion at an oil-to-water volume ratio of 9 1, even if an appropriate surfactant is selected. As a rule, the resulting system has an O/W/O type, which quickly converts into O/W plus excess oil after the mixer has been turned off. However, if the mixing cycle is repeated several times, or the oil is added not as a single batch but gradually, one can obtain a very concentrated and stable OAV macroemulsion. 

The predicted volume fraction dependence has not been observed for hydro-carbon-in-water emulsions. Instead, the experimentally determined ripening rates are found to be independent of volume fraction for 0.01 0.3, in clear disagreement with Enomoto s theory (Figure 9.6). Taylor s studies were conducted at a fairly high surfactant concentration (5%). It has been suggested that emulsion droplets may have been effectively screened from one another by micelles present in the solution. ... 

Operation in biphasic mixtures using water-immiscible solvents introduces a linked equilibrium in the partition of educt and product and possible transport limitations at the interface, which have to be considered. Besides, enzyme deactivation at the interface and possible effects of the residual solvent solubility in aqueous buffers on enzyme stability have to be checked. Table 3 summarizes some data on stability of ADHs dissolved in aqueous buffers in a biphasic mixture with organic solvents [48]. Two different reactor concepts for continuous operation and enzyme catalysis in homogeneous phase have been studied—a bimembrane reactor [13,14] and an emulsion reactor [49]—which are discussed below with regard to reaction engineering. Using water-inuniscible solvents one can make use of the fact that NAD(P)/NAD(P)H are charged molecules and practically insoluble in apolar solvents. The coenzyme introduced in the reaction is therefore confined and physically immobilized with the enzymes in the aqueous phase. This facilitates efficient use of the coenzyme, especially if the volume fraction of the aqueous phase is kept low [13]. 

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