Mixed emulsions

In a well-mixed emulsion reactor A in a continuous aqueous phase reacts with B in an organic drop phase in the reaction A + B products. The reaction is very fast and is limited by the diffusion of A into the organic phase. The feeds are 1 liter/min of 9 molar A and 1 liter/min of 2 molar B. The stirring is such that the drops are monodisperse at a diameter of 200/fm. 

The reaction A + B products occurs in a well-mixed emulsion reactor with B entering in the continuous aqueous phase and A in the organic phase. Reaction occurs by A transferring from the organic phase into the aqueous phase, where reaction occurs. The organic phase forms drops 0.1 cm in diameter, and the flow rate of the organic phase is 1 liter/min at Cao ... 

In order to conveniently distinguish between these two cases, they are simply referred to here as (i) bilayer emulsions and (ii) mixed emulsions (Jourdain et al., 2008, 2009 Dickinson, 2008a). 

Especially troublesome is bridging flocculation. It is therefore much more convenient to prepare emulsions with protein and polysaccharide components both present together in the aqueous medium before homogenization (Dickinson et al., 1998 Garti et al., 1999 Dickinson, 2008a). Moreover, in a direct comparison between the two techniques (Jourdain et al., 2008), it has been demonstrated that the experimentally more straightforward mixed emulsion approach can actually produce a better level of stability than the bilayer approach. 

Therefore, two contributory factors may provide an explanation for more effective electrostatic / steric stabilization of the so-called mixed emulsions in comparison with the sequentially assembled biopolymer interfaces of the bilayer emulsions firstly, a greater hydrophilicity of the adsorbed protein-polysaccharide complexes, caused by the larger net negative charge, and, secondly, a more bulky architecture of the normal complexes as compared to the interface complexes. 

Table 7.3 Relationship between molecular parameters (A2, p) of sodium caseinate (0.5 wt%) + dextran sulfate complexes at pH = 6.0 formed in the bulk and at the interface of a protein foam, and the corresponding properties (J43, Q of the bilayer and mixed emulsions (20 vol% oil, 0.5 wt% sodium caseinate) containing 0.1 or 1.0 wt% dextran sulfate (Jourdain et aL, 2008 Semenova et al., 2009).

Figure 7.22 Microstructure of acidified mixed emulsions (20 vol% oil, 0.5 wt% sodium caseinate) containing different concentrations of dextran sulfate (DS). Samples were prepared at pH = 6 in 20 mM imidazole buffer and acidified to pH = 2 by addition of HCl. Emulsions were diluted 1 10 in 20 mM imidazole buffer before visualization by differential interference contrast microscopy (A) no added DS (B) 0.1 wt% DS (C) 0.5 wt% DS (D) 1 wt% DS. Particle-size distributions of the diluted emulsions determined by light-scattering (Mastersizer) are superimposed on the micrographs, with horizontal axial labels indicating the particle diameter (in pm). Reproduced with permission from Jourdain et al. (2008).

Once the silver is added, pour the emulsion into a very dark brown glass bottle, or better yet search your local antique shop for the less common black glass bottle. Cork the bottle and shake it vigorously for a couple of minutes. Set the bottle aside, undisturbed for at least a day. Never allow the emulsion to be shaken again, as this will stir up any precipitates and produce millions of fine bubbles, both of which will make it impossible to make an even coating. The mixed emulsion has a remarkable shelf life if kept in a cool, dark place. 

A typical starting formulation for a two-component epoxy-polyamide emulsion is shown in Table 14.3. This formulation can be used as either a coating or an adhesive. In either application, once the mixed emulsion is applied to the substrate, the water must be evaporated. In the case of the adhesive application, this must be completed before nonporous... 

Preparation of Freund s Emulsions. For maximum efficiency, it is necessary to obtain a stable, water-in-oil emulsion. Several ways of pre-. paring such emulsions have been described, but there is no doubt that the simplest and most efficient, at least for the relatively small volumes that most people require, is the double-hub connector method described here. It may occasionally be difficult to persuade the phases to combine as water in oil rather than oil in water or mixed emulsions. Cooling the separate phases before mixing may help, but an infallible way of overcoming the problem is to use 2-4 volumes of oily adjuvant to 1 volume of aqueous... 

The mix emulsion is subsequently foamed in the continuous ice cream freezer, creating a dispersed phase of air bubbles, and is concomitantly frozen, forming another dispersed phase of ice crystals. Air bubbles and ice crystals are usually in... 

The models show considerably different A or when s 1 sec". The bubble flow model (BFM) is for perfectly mixed emulsion, but a A or only slightly larger is obtained for BFM when the emulsicm is vertically unmixed. For the bubbling bed model (BBM)A or is fairly sensitive to different assumptions iorf an/w of about unity is recommended (FI 1, K24) to fit the reaction data available. The valued = 0.35 is taken from Rowe (D5) for 75-jU,m-diam. spherical particles. 

Species mass balances were developed for the two phases and solved ana-l3dically for first order reactions. Thus, for the case of a completely mixed emulsion phase the concentration of a reactant leaving this phase is given by ... 

Emulsification and coalescence are related to spontaneously formed unstable W/0 emulsion (Castor et al., 1981b) or mixed emulsion. Isolated oil droplets are emulsified after contacting with alkaline solution. The emulsified droplets coalesce with each other to become larger droplets while they move in the... 

The analysis was extended to predict mean drop size evolution for mixed emulsions consisting initially of some drops of pure decane and some of pure squalane. Its predictions based on interfacially controlled transport were in better agreement with the experimental results of Binks et al. than were those of the authors model, which was based on diffusion-controlled transport. 

An ideal emulsion or suspension is difficult to physically, chemically or kinetically realize. Mixed mechanisms exist. Nonetheless categorizations can still be made such as the two styrene and two acrylamide examples discussed in the previous paragraph. These are distinguishable by the primary nucleation mechanism and the primary phase of initiator and monomer solubility. However, the possibility of having mixed emulsion and suspension mechanisms, which are largely indistinguishable, also exists. Consider two scenarios ... 

Typical examples of the mixed emulsion-spinning of polyvinyl chloride, polyethylene, poly-vinylidene chloride, and polyvinyl acetate, and some properties of the mixed fibers are shown in Table 4.26. The measurement of the fiber properties is carried out after hot-drawing in dry air at 180°C, heat treatment for 100 sec at 250°C, and acetalization for 40 min at 70°C without tension. The draw ratios shown in the table are the highest possible ratios under the given experimental conditions. In most cases, there is a maximum possible draw ratio at a certain mixing ratio. However, only the results of experiments at mixing ratios of 1 3 and 1 1 are shown in the table. 

Next, what has been called mixed emulsions and multiple emulsions will be examined. Actually, these kinds of emulsion can be found during the simple W/0 emulsion process of fabrication, during the trans portation of complex fluids, in crude oil production, and also during the evolution of simple emulsions. Therefore, it appears to be very important to detect them and the DSC test is very efficient as it will be seen in the following. 

Mixed emulsions are so called because they are obtained from the mixing of two simple emulsions that differ by the compositions of the dispersed phases. The mixing is done gently in order to avoid coalescence at the very maximum. The resulting emulsion has flie particularity of containing droplets that are different in composition and close togeflier (Fig. 14). Should the medium wherein they are dispersed be permeable to their components then mass transfers between... 

Figure 15 Cooling thermogram of a mixed emulsion (a) t=A min (b) t=21 min (c) t=61 min (d) f=82 min. I solidification of pure water II solidification of water plus urea droplets. (Courtesy of A. Gauthier, UTC, France.)...

Let us see what has been deduced from DSC tests performed on mixed emulsions prepared from a W/0 emulsion (emulsion I) and a (water plus urea)-in-oil emulsion (emulsion n), the compositions of which are ... 

The traditional testing of demulsifiers is to undertake bottle-shake tests. In these tests one has a pre-mixed emulsion and the chemieal under study is applied. After this the bottle is gently shaken in order to distribute the chemieal evenly into the emulsified system. The efficiency of the chemical applied is read from the resolution of the dis-... 

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