Droplets emulsions

Since emulsion droplets are not rigid spheres, the coefficient of 0 is around 3-6 for many emulsion systems [3-5], More concentrated emulsions are non-Newtonian depends on shear rate and are thixotropic (ri decreasing with... 

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. 

Surfactants provide temporary emulsion droplet stabilization of monomer droplets in tire two-phase reaction mixture obtained in emulsion polymerization. A cartoon of tliis process is given in figure C2.3.11. There we see tliat a reservoir of polymerizable monomer exists in a relatively large droplet (of tire order of tire size of tire wavelengtli of light or larger) kinetically stabilized by surfactant. 

Figure C2.3.11 Key surfactant stmctures (not to scale) in emulsion polymerization micelles containing monomer and oligomer, growing polymer particle stabilized by surfactant and an emulsion droplet of monomer (reservoir) also coated with surfactant. Adapted from figure 4-1 in [67],...

Fig. 2. Aerosol emulsion droplets containing propellant (a) in the internal phase with subsequent formation of aerosol foam and (b) in the external phase...

Much commercial equipment is available for emulsification (Fig. 3) and has been well described (9). The following discusses the relations between energy input and emulsion droplet size. 

Polymers have so far been used comparatively less than the common surfactants to stabilize emulsions in spite of the fact that excellent stabilization by them can be achieved (18—20). AppHcation probably has been limited because the adsorption of polymers to emulsion droplets has displayed some intricate phenomena small changes in polymer stmcture or in solvent properties may lead to drastic changes in adsorption. 

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 final factor influencing the stabiHty of these three-phase emulsions is probably the most important one. Small changes in emulsifier concentration lead to drastic changes in the amounts of the three phases. As an example, consider the points A to C in Figure 16. At point A, with 2% emulsifier, 49% water, and 49% aqueous phase, 50% oil and 50% aqueous phase are the only phases present. At point B the emulsifier concentration has been increased to 4%. Now the oil phase constitutes 47% of the total and the aqueous phase is reduced to 29% the remaining 24% is a Hquid crystalline phase. The importance of these numbers is best perceived by a calculation of thickness of the protective layer of the emulsifier (point A) and of the Hquid crystal (point B). The added surfactant, which at 2% would add a protective film of only 0.07 p.m to emulsion droplets of 5 p.m if all of it were adsorbed, has now been transformed to 24% of a viscous phase. This phase would form a very viscous film 0.85 p.m thick. The protective coating is more than 10 times thicker than one from the surfactant alone because the thick viscous film contains only 7% emulsifier the rest is 75% water and 18% oil. At point C, the aqueous phase has now disappeared, and the entire emulsion consists of 42.3% oil and 57.5% Hquid crystalline phase. The stabilizing phase is now the principal part of the emulsion. 

Figure 18 is drawa for particles of 5 nm radius actiag oa a flat iaterface of lO nm. The total force is repulsive evea for coatact angles of 60° for 90° angles the repulsion is extremely strong. Hence, the stabilization is exceUent, beariag ia mind that thousands of such particles are adsorbed onto one emulsion droplet. 

Fig. 18. The repulsion force from adsorbed particles is greater than the van der Waals force between flocculated emulsion droplets under certain...

The responses chosen all relate to important foam properties. We believed that yi, the emulsion droplet size, determines y2, the cell size in the resultant foam, and we wished to determine whether this is true over this range of formulations. The foam pore size ys should determine the wetting rate y7, so these responses could be correlated, and yg, the BET surface area, should be related to these as well. The density y and density uniformity ys are critical to target performance as described above, and ys, the compressive modulus, is an important measure of the mechanical properties of the foam. 

Desulfurization processes are absolutely necessary for producing clean fuels. Possible strategies to realize ultradeep suffiirization currently include adsorption, extraction, oxidation, and bioprocesses. Oxidative desulfurization (ODS) combined with extraction is considered one of the most promising of these processes [13]. Ultradeep desulfurization of diesel by selective oxidation with amphiphilic catalyst assembled in emulsion droplets has given results where the sulfur level of desulfurized diesel can be lowered from 500 ppm to about 0.1 ppm without changing the properties of the diesel [12]. 

Hollow and porous polymer capsules of micrometer size have been fabricated by using emulsion polymerization or through interfacial polymerization strategies [79,83-84, 88-90], Micron-size, hollow cross-linked polymer capsules were prepared by suspension polymerization of emulsion droplets with polystyrene dissolved in an aqueous solution of poly(vinyl alcohol) [88], while latex capsules with a multihollow structure were processed by seeded emulsion polymerization [89], Ceramic hollow capsules have also been prepared by emulsion/phase-separation procedures [14,91-96] For example, hollow silica capsules with diameters of 1-100 micrometers were obtained by interfacial reactions conducted in oil/water emulsions [91],... 

Furthermore, should free radicals be present, the vinyl groups would much more rapidly polymerise depleting the emulsion droplets of monomer, providing the control required for a particular particle size. The composition of the solution thus determines not only the phase behaviour, but the rate of polymerisation and the particle size. If, the organism has in its genetic code, the abihty to synthesise the monomer, it presumably has... 

Irrespective of the physical form of the carotenoid in the plant tissue it needs to be dissolved directly into the bulk lipid phase (emulsion) and then into the mixed micelles formed from the emulsion droplets by the action of lipases and bile. Alternatively it can dissolve directly into the mixed micelles. The micelles then diffuse through the unstirred water layer covering the brush border of the enterocytes and dissociate, and the components are then absorbed. Although lipid absorption at this point is essentially complete, bile salts and sterols (cholesterol) may not be fully absorbed and are not wholly recovered more distally, some being lost into the large intestine. It is not known whether carotenoids incorporated into mixed micelles are fully or only partially absorbed. 

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. 

K. G. Hollingsworth, M. L. Johns 2003, (Measurement of emulsion droplet sizes using PFG NMR and regularization methods),/. Colloid Interface Sci. 258, 383. 

M. L. Johns, L. F. Gladden 2002, (Sizing of emulsion droplets under flow using flow-compensating NMR-PFG techniques),/. Mag. Reson. 154, 142. 

Phase dilution test Place two emulsion droplets on a glass slide and add a droplet of one component to each emulsion droplet, stir, and observe under a microscope. This test is based on the principle that an emulsion can only be diluted with the liquid that constitutes the continuous phase. 

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. 

Izquierdo, P., Wiechers, J.W., Escribano, E., Garcia-Celma, M.J., Tadros, T.F., Esquena, J., Federen, J.C. and Solans, C. (2007) A study on the influence of emulsion droplet size on the skin penetration of tetracaine. Skin Pharmacology and Physiology, 20, 263—270. 

Prior to the addition of the silica precursor (TEOS), the acidic copolymer solution appears transparent and the SANS data shows that the copolymer forms spherical micelles of size 7.1 nm (figure 1-a). After the addition of TEOS, the solution becomes immediately turbid. Most probably, it is because TEOS is hydrophobic and forms an emulsion droplets under stirring when added to the solution [3], Then, the opacity increases with time (figure 1-b), until a thick white precipitate forms after about 23 minutes (figure 1-c). 

PHEMA solubility decreases with increasing ion concentration. As a result, Mikos et al. used salt solutions of varying ionic strength to dilute the reaction mixtures (Liu et al., 2000). It was noted that increasing the ion content of the aqueous solution to 0.7M, interconnected macropores were obtained at 60 vol% water. Surfactants may also be used to control the network pore structure. However, not much work has been done in this area, since surfactants typically work to reduce the surface repulsions between the two phases and form a uniform emulsion. These smaller emulsion droplets when gelled will create a network with an even smaller porous structure. Yet, this is still a promising area of exploration, since it may be possible to form alternate phase structures such as bicontinuous phases, which would be ideal for cellular invasion. 

Figure 5.16 Formation of emulsion droplets, (a) Aqueous MPS solution after acid-catalysed hydrolysis and condensation, (b) Micrometre-sized emulsion droplets are rapidly formed upon addition of the base catalyst triethanolamine. (Reproduced from ref. 28, with permission.)... 

The key to the successful preparation of this new composite is the identification of a surfactant, PE-b-PEG, that is capable of stabilizing the emulsion and promoting the dissolution of the PE. Then submicrometre particles of low-density PE silica and high-density PE silica are synthesized by carrying out a silica sol-gel polycondensation process within emulsion droplets of TEOS-dissolved PE, at elevated temperatures (78 and 130°C for low- and high-density PE, respectively). 

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