Aqueous droplets

The terminal (highest calculated) settling velocity of the aqueous droplet in/through the hydrocarbon phase is ... 

Control of the particle size while retaining precise control over the release rate is enabled by compartmentalization of the sol-gel solution into droplets of definite size. This can be achieved by emulsification of the sol-gel solution by mixing it with a solution composed of a surfactant and a non-polar solvent (Figure 2.13). When an active molecule is located in the aqueous droplet of a W/O emulsion, encapsulation occurs as the silicon precursors polymerize to build an oxide cage around the active species. By changing the solvent-surfactant combination, the particle size can be varied from 10 nm to 100 pm as the size of the particles is controlled by the size of the emulsion droplet, which acts as a nano-reactor for the sol-gel reaction (Figure 2.13). 

Inverted W/O emulsion. The premixed emulsion was composed of aqueous droplets dispersed in an oil phase containing a lipophilic surfactant. The droplet mass fraction (/>, was set to 80%. This crude emulsion was sheared into the Couette-type cell [159] at constant shear rate V = 10000 s and a... 

At the air-water interface, water molecules are constantly evaporating and condensing in a closed container. In an open container, water molecules at the surface will desorb and diffuse into the gas phase. It is therefore important to determine the effect of a monomolecular film of amphiphiles at the interface. The measurement of the evaporation of water through monolayer films was found to be of considerable interest in the study of methods for controlling evaporation from great lakes. Many important atmospheric reactions involve interfacial interactions of gas molecules (oxygen and different pollutants) with aqueous droplets of clouds and fog as well as ocean surfaces. The presence of monolayer films would thus have an appreciable effect on such mass transfer reactions. 

Liquid surfactant membrane phase -Emulsion globule Internal aqueous droplets... 

Demulsification with electrostatic fields appears to be the most effective and economic way for breaking of W/0 emulsion in ELM processes 190, 91]. Electrostatic coalescence is a technique widely used to separate dispersed aqueous droplets from nonconducting oils. Since this type of technique is strictly a physical process, it is most suitable for breaking emulsion liquid membranes to recover the oil membrane phase for reuse. 

Freiberg, J. E., and S. E. Schwartz, Oxidation of S02 in Aqueous Droplets Mass-Transport Limitation in Laboratory Studies and the Ambient Atmosphere, Atmos. Environ., 15, 1145-1154(1981). 

Schweitzer, F., L. Magi, P. Mirabel, and C. George, Uptake Rate Measurements of Methanesulfonic Acid and Glyoxal by Aqueous Droplets, J. Phys. Chem. A, 102, 593-600 (1998). 

It is difficult to separate out the uptake and/or reactions of N03 with water and those of N2Os. As discussed in the next section, there is an abundance of evidence that N205 is taken up by aqueous droplets and surfaces in both the troposphere and stratosphere and hydrolyzes to form HNO,. However, it appears that N03 may also be taken up, and in this case, may act as a strong oxidant in solution (see, for example, Chameides, 1986a, 1986b and Pedersen, 1995). 

Chang, S. G., R. Toossi, and T. Novakov, The Importance of Soot Particles and Nitrous Acid in Oxidizing S02 in Atmospheric Aqueous Droplets, Atmos. Environ., 15, 1287-1292(1981). 

Pulsed gradient spin-echo (PGSE) NMR techniques have also been employed to study the structure of the oil phase [12]. This gives an idea of the mobility of each component in the HIPE, and showed that, for stable emulsions and HIPEs, the oil phase was indeed a reverse micellar solution which solubilises water. Further work using PGSE NMR has shown that water can diffuse between aqueous droplets in concentrated emulsions [101]. Presumably this involves solubilisation of the water molecules by the micellar oil phase. 

Kizling and coworker [21] suggested that salts in the aqueous phase stabilised w/o HIPEs by two means. First, the Ostwald ripening process is inhibited due to the decreased solubility of the aqueous solution in the continuous oil phase. Secondly, the attractive forces between adjacent aqueous droplets are lowered, as a result of the increase in refractive index of the aqueous phase towards that of the oil phase. When the refractive indices of the two phases are matched, the attractive forces are at a minimum and highly stable, transparent emulsions are formed. The attractive force, A, is given by ... 

Perona, M. J., The solubility of hydrophobic compounds in aqueous droplets , Atmospheric Environ., 26, 2549-2553 (1992). 

More recently an oil continuous microemulsion technique has been described,16 which allows the study of specific interactions between amino acid side chains and metal ions. Both the metal ion and amino acid are microencapsulated as aqueous droplets in a dispersed phase. The technique is of particular relevance to metalloprotein and metal-membrane interactions where the local dielectric constant can be considerably less than that of bulk water. 

Practical situations are not always so simple and one may encounter double emulsions, that is, emulsions that are oil-in-water-in oil (O/W/O) and water-in-oil-in-water (W/O/W). For example, O/W/O denotes a double emulsion, containing oil droplets dispersed in aqueous droplets that are in turn dispersed in a continuous oil phase. The double emulsion droplets can be quite large (tens of pm) and can contain many tens of droplets of the ultimate internal phase. Developments in and applications of double emulsions have been reviewed by Garti and Bisperink [52]. 

Figure 3.10 (a) Schematic of an inverse microemulsion in which two chemicals (A and B) dissolved in aqueous droplets react to produce a solid (C) whose size is controlled by the size of the droplet in which... 

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