Emulsion droplet coalescence

Tokumitsu H, Ichikawa H, Fukumori Y (1999) Chitosan-gadopenteic acid complex nanoparticles for gadolinium neutron-capture therapy of cancer Preparation by novel emulsion-droplet coalescence technique and characterization. Pharm Res 16 1839-1835... 

In more realistic situations there is a certain probability of the emulsion droplets coalescing with the bulk oil phase or a part of the bulk oil becoming emulsified. The physics of such complex fiow conditions is not well understood at present. The starting point of describing such a fiow would be to treat it as a normal two-phase flow and use the concept of relative permeability and a model for the rheological properties of the emulsion phase. To account for the material exchange between the bulk phase and the emulsion phase, some form of droplet population balance model will be needed. 

In the final stage of emulsion-droplet coalescence the two droplets have made contact, the protecting surfactant films have fused, and the droplets are connected by a narrow neck. In this situation the neck can either grow spontaneously at all sizes or there is a critical neck size that has to be exceeded before growth occurs spontaneously. The driving force for the coalescence process is a decrease in surface area with a concomitant gain in surface free energy. One could then expect that the process is slower the smaller the surface tension. As the analysis presented below demonstrates, this intuitive expectation is erroneous (42). 

The Carboxyfluorescein concentration of the vesicle suspension Cv and after destroying the vesicles with the surfactant Triton X 100 C/ were calculated from the fluorescence intensity of the diluted aqueous solutions. The turbidity of the vesicle solution declined within seconds after detergent addition (vesicle busting), and we obtained then a clear, aqueous solution. Typical results of these measurements are summarized in Fig. 12. Due to Ihe self-quenching properties the destruction of the vesicles with a high inner Carboxyfluorescein concentration (0.05-0.2 mol/1) led to an increase of the Fluorophore concentration in the outer phase. This occurred if a large amount of emulsion droplets coalesced with the lower water phase, thus releasing their Carboxyfluorescein content. 

There appear to be two stages in the collapse of emulsions flocculation, in which some clustering of emulsion droplets takes place, and coalescence, in which the number of distinct droplets decreases (see Refs. 31-33). Coalescence rates very likely depend primarily on the film-film surface chemical repulsion and on the degree of irreversibility of film desorption, as discussed. However, if emulsions are centrifuged, a compressed polyhedral structure similar to that of foams results [32-34]—see Section XIV-8—and coalescence may now take on mechanisms more related to those operative in the thinning of foams. 

Liquid crystals stabilize in several ways. The lamellar stmcture leads to a strong reduction of the van der Waals forces during the coalescence step. The mathematical treatment of this problem is fairly complex (28). A diagram of the van der Waals potential (Fig. 15) illustrates the phenomenon (29). Without the Hquid crystalline phase, coalescence takes place over a thin Hquid film in a distance range, where the slope of the van der Waals potential is steep, ie, there is a large van der Waals force. With the Hquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the Hquid crystal is highly viscous, and two droplets separated by a viscous film of Hquid crystal with only a small compressive force exhibit stabiHty against coalescence. Finally, the network of Hquid crystalline leaflets (30) hinders the free mobiHty of the emulsion droplets. 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

Very finely disperse solids, which are adsorbed at the liquid/liquid interfaces, forming films of particles around the disperse globules. Certain powders can very effectively stabilize against coalescence. The solid s particle size must be very small compared with the emulsion droplet size and must exhibit an appropriate angle of contact at the three-phase (oil/water/solid) boundary [141]. 

Fig. 9 Schematic presentation of flocculation and coalescence of emulsion droplets. (From Ref. 144.)...

IB Ivanov, KB Danov, PA Kralchevsky. Flocculation and coalescence of micron-size emulsion droplets. Colloids Surfaces A Physicochem Eng Aspects 152 161-168, 1999. 

For parenteral emulsions, the formulation scientist must be particularly aware of changes in particle size distribution of the oil phase. Droplet coalescence results in increased droplet size. As a general rule, average droplet size should be less than 1 pm. Droplet sizes of more than 6 pm can cause blockage of capillaries (capillary emboli). 

In the preseparation chamber, the less dense oil droplets rise, collide, and fuse with adjacent droplets. According to Stoke s law, the larger the diameter of a particle, the faster is its rate of rise. Thus, as small droplets coalesce to form larger droplets, their upward vertical velocity increases. Coalescing tubes or plates are designed to enhance the separation of oil-water emulsions. The emulsion free water is directed away from the tubes or plates and enters the separation section. Some separators are built with an outlet zone for the discharge of clarified water. 

An emulsion is a dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

Fibrous bed coalescers generally have a fixed filter element constructed of fiberglass or other material that acts to coalesce (bring together) the oil droplets and to break emulsions. The coalesced oil droplets released from the filter are readily separated downstream by gravity. Coalescence in a fibrous bed coalescer involves three steps ... 

In this chapter, we review some results in the field of emulsion metastability, emphasizing the destruction of concentrated emulsions (droplet volume fraction

70%) through coalescence. The review concerning Oswald ripening (Section 5.2) is more concise, as this mechanism is fairly well understood and has been extensively documented in the literature. So far, the destruction of concentrated emulsions through coalescence is much less understood and has motivated many recent studies and developments that we summarize (Sections 5.3 to 5.6). 

Figure 6.8. Two-stage coalescence in double emulsions. Sequence of side view pictures, taken at a frequency of 5000 frames/s, of a water droplet coalescing on the globnle surface. Cfi = 30 CMC of SDS C = 2 wt% of SMO in dodecane. (Reproduced with permission from [18].)...

---