Dispersion emulsion droplets

FIG. 14. Transmission electron micrograph of Voltaren Emulgel the interface hetween the continuous hydrogel and the dispersed emulsion droplets consists of multiple bilayers of hydrated surfactant molecules, bar 500 nm. From Miiller-Goymann, C., and Schutze, W., Mehrschichtige Phasengrenzen in Emulsionen, Dtsch. Apoth. Ztg., 130 561-562 (1990). 

A dispersed emulsion droplet (electrostatically stabilized) can be thought of as a charged center with a pliable cover of compensating charge due to orientation of the water molecules (or counterions) around the droplet. When exposed to an electric field, the droplet will move according to the surface charge. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

The degree of emulsification in such materials can also be estimated by the measurement of ultrasound velocity in conjunction with attenuation [4]. It is possible to determine factors such as the degree of creaming (or settling ) of a sample, i.e. the movement of solid particles/fat droplets to the surface (or to the base) [5], Such information gives details, for example, of the long-term stability of fruit juices and the stability of emulsions such as mayonnaise. The combination of velocity and attenuation measurements shows promise as a method for the analysis of edible fats and oils [6], and for the determination of the extent of crystallization and melting in dispersed emulsion droplets [7]. 

Chakraborty et al. [108] studied separation of toluene from //-heptane using ELMs and examined photomicrographs of the emulsions prepared using 4000 and 8000 rpm immediately after preparation. In both cases, the diameters of the dispersed emulsion droplets were in the 10-120 pm range and those of the internal oil droplets were in the 0.001-0.12 pm range. 

For molecules the dispersion force varies inversely with the sixth power of the intermolecular distance. For dispersed emulsion droplets, the dispersion forces can be approximated by adding up the attractions between all interdroplet pairs of molecules. When added this way the dispersion... 

The most common types of emulsions consist of only two Hquids, water and an oil. An o/w emulsion consists of oil droplets dispersed in a continuous aqueous phase, and a w/o emulsion consists of water droplets dispersed in oil (Fig. 1). Occasionally inversion takes place an o/w emulsion changes into w/o emulsion and vice versa. More complex emulsions such as double emulsions are formed because the water droplets in a continuous oil phase themselves contain dispersed oil droplets (Fig. 2). Such oil-in-water-in-oil emulsions are noted as o/w/o. In the same manner a w/o/w emulsion may be formed, which finds use as a system for slow deHvery, extraction, etc (6,7). 

Thus, in the relatively simple case of oil in water emulsions, where a surface active agent such as a soap is used as the emulsifying agent, it is known that the soap adsorbed on the surface of the oil particles decreases the interfacial tension, thus stabilizing the emulsion. The adsorbed soap ions also give a net electrostatic charge to the dispersed oil droplets, serving to repel other oil droplets, with the net effect that flocculation is hindered (and stability is increased). It is even possible to measure the amount of adsorbed soap ions and to calculate the values of the surface potential. 

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. 

A Malvern Mastersizer (Malvern Instruments Ltd, Malvern, UK) with optical parameters defined by the manufacturer s presentation code 0505 was used to determine the droplet size distribution. The measurement was made in triplicate at room temperature. Water was used to disperse the emulsion droplets. 

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]. 

Colloidal suspensions, emulsions and solid dispersions are produced by means of colloid mills or dispersion mills. Droplets or particles of sizes less than 1 (im may be formed, and solids suspensions consisting of discrete solid particles are obtainable with feed material of approximately 100-mesh or 50 p,m in size. 

A high internal phase liquid-liquid emulsion (HIPE) is one where the internal or dispersed phase droplets occupy >74% of the total volume of the emulsion. At this point the droplets contact each other and beyond this volume % the droplets are forced into distorted polyhedra. If for example styrene and divinylbenzene are employed as the continuous phase and water droplets dispersed in this oil phase using a suitable surfactant to form a HIPE, the comonomers can be polymerized to form a poly(styrene-divinylbenzene) polyHIPE. Typically the water droplets are... 

We first consider emulsion droplets submitted to attractive interactions of the order of ks T. Reversible flocculation may be simply produced by adding excess surfactant in the continuous phase of emulsions. As already mentioned in Chapter 2, micelles may induce an attractive depletion interaction between the dispersed droplets. For equal spheres of radius a at center-to-center separation r, the depletion... 

The general method of procedure was to disperse a known volume pf parafl n in water with the aid of the soap. The average diameter of the kerosene emulsion droplets was determined by counting with the aid of a microscope and hemacytometer, from which the total interfacial area could be calculated. 

If the organic solution of diacid chloride is recast as a dispersed phase in an emulsion with the aqueous solution of diamine as the continuous phase, the polymer membrane forms around the dispersed phase droplets, effectively making polyamide shell capsules around the organic phase. Of course, the relative volumes could be reversed so that the aqueous phase was encapsulated if so desired. 

Because the size of the emulsion droplets dictates the diameter of the resulting capsules, it is possible to use miniemulsions to make nanocapsules. To cite a recent example, Carlos Co and his group developed relatively monodisperse 200-nm capsules by interfacial free-radical polymerization (Scott et al. 2005). Dibutyl maleate in hexadecane was dispersed in a miniemulsion of poly(ethylene glycol)-1000 (PEG-1000) divinyl ether in an aqueous phase. They generated the miniemulsion by sonication and used an interfacially active initiator, 2,2 -azobis(A-octyl-2-methyl-propionamidine) dihydrochloride, to initiate the reaction, coupled with UV irradiation. 

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