Assessment emulsion coalescence

Assessment and Prediction of Emulsion Coalescence Using Rheological Techniques I 459... 

To break up a drop into smaller ones, it must be strongly deformed and this deformation increases p [87]. Surfactants play major roles in the formation of emulsions By lowering the interfacial tension, p is reduced and hence the stress needed to break up a drop is reduced (2,3) and surfactants prevent coalescence of newly formed drops. To describe emulsion formation one has to consider two main factors hydrodynamics and interfacial science. To assess emulsion formation, one usually measures the droplet size distribution using for example laser diffraction techniques. A useful average diameter d is. 

Assessment of the stability of an emulsion against coalescence involves droplet counting218. The most unequivocal method (but one which is rather laborious) is to introduce a suitably diluted sample of the emulsion into a haemocytometer cell and count the microscopically visible particles manually. 

The physical instability of emulsions involves creaming, flocculation, coalescence, or breaking, whereas the chemical instability can be a result of hydrolysis of the stabilizing moieties. In order to assess the stability of the emulsion, heating and freezing cycles and centrifugation and steam sterilization can be employed. 

Any pair of emulsifying agents that fall at opposite ends of the HLB scale - for example. Tween 80 (sorbitan monooleate with 20 mol EO, HLB = 15) and Span 80 (sorbitan monooleate, HLB = 5) - can be taken and used in various proportions to cover a wide range of HLB numbers. The emulsions should be prepared in the same fashion, with a few percent of the emulsifying blend. The stability of the emulsions can then be assessed at each HLB number, either from the rate of coalescence or qualitatively by measuring the rate of oil separation. In this way it should be possible to determine the optimum HLB number for a given oil. Subsequently, having found the most effective HLB value, various other surfactant pairs can be compared at this HLB value to identify the most effective pair. 

The effects of interfacial tension, interfacial charge and interfacial viscosity on coalescence, and emulsion stability for crude oil emulsions in alkaline solutions have been assessed. It was observed that the NaOH concentration which yields higher interfacial shear viscosity also results in higher emulsion stability. 

The assessment of hydrodynamic and molecular interactions of drops can be made in the same manner as previously described for emulsions in Part V. Upon approach of the drops to each other under the action of turbulent pulsations up to distances smaller than Ao, they are subject to significant resistance from the environment, and the force of moleetilar attraction leads to collision and coalescence of the drops. If the basic mechanism of drop coagulation is that of turbulent diffusion, the coefficient of turbulent diffusion depends on the coefficient of hydrodynamic resistance [see Eqs. (11.70), (11.72), and (11.74)] and hence on the relative distance between the approaching drops ... 

In many cases the creaming or sedimentation occurs simultaneously with coalescence and is related to emul sion stability. In the next section, we will briefly con sider the assessment of emulsion shelf-life by NMR. 

This chapter has covered different physical phenomena and processes, ranging from bulk-fluid dynamics to microscopic interdroplet surface chemistry. All of these topics play a role in the electrostatic separation of W/O emulsions and the development and construction of an optimal, compact electrostatic coalescer. In some areas, such as turbulent droplet break-up, the understanding is well developed. In other fields there are still many questions to be answered. It is interesting to note that various authors have performed experimental assessments of W/O emulsion separation by using electrostatic fields. There is agreement on some as-... 

As noted above, because of the possible instabihty of the primary emulsion, great care must be taken in the choice of the secondary dispersion method. Excessive mechanical agitation such as in colloid mills, high-speed mixers, and sonication could result in coalescence of the primary emulsion and the production of a simple emulsion. The evaluation of the yield of filled secondary emulsion drops, therefore, is very important in assessing the value of different preparation methods and surfactant combinations. 

Following the same strategy, the formation and stability of W/C emulsions was assessed in the presence of a PEO-fc-PFOA copolymer (M p o = 2000 g/mol and M pfoa 31,000 g/mol) [40], Note that the hydrophilic/C02-philic balance (HCB) was lower, and the Af of the copolymer was higher compared to the previous study. The observation of a W/C emulsion was reported from 25°C and 143 bar while flocculation or coalescence of dispersed water phase slowly occurred upon stopping of the stirrer. Notably, the formation of a W/C emulsion was reported at the cloud point pressure of the copolymer in a binary polymer/C02 mixture. 

For quantitative assessment of emulsion stability after dilution of the EC, it is necessary to measure the coalescence rate. This could be done by measuring the droplet number as a function of time, using, for example, a Coulter counter. As an illustration, Figure 14.5 shows the results obtained using the model xylene EC. 

To characterize emulsion systems, it is necessary to obtain fundamental information on the liquid/liquid interface (e.g. interfacial tension and interfacial rheology) and properties of the bulk emulsion system, such as droplet size distribution, flocculation, coalescence, phase inversion and rheology. The information obtained, if analyzed carefully, can be used for the assessment and (in some cases) prediction of the long-term physical stability of the emulsion. 

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