Particle size distribution emulsion

Huang, X., Kakuda, Y., Cui, W. (2001). Hydrocolloids in emulsions particle size distribution and interfacial activity. Food Hydrocolloids, 15, 533-542. 

Analysis of a method of maximizing the usefiilness of smaH pilot units in achieving similitude is described in Reference 67. The pilot unit should be designed to produce fully developed large bubbles or slugs as rapidly as possible above the inlet. UsuaHy, the basic reaction conditions of feed composition, temperature, pressure, and catalyst activity are kept constant. Constant catalyst activity usuaHy requires use of the same particle size distribution and therefore constant minimum fluidization velocity which is usuaHy much less than the superficial gas velocity. Mass transport from the bubble by diffusion may be less than by convective exchange between the bubble and the surrounding emulsion phase. 

Copolymers with butadiene, ie, those containing at least 60 wt % butadiene, are an important family of mbbers. In addition to synthetic mbber, these compositions have extensive uses as paper coatings, water-based paints, and carpet backing. Because of unfavorable reaction kinetics in a mass system, these copolymers are made in an emulsion polymerization system, which favors chain propagation but not termination (199). The result is economically acceptable rates with desirable chain lengths. Usually such processes are mn batchwise in order to achieve satisfactory particle size distribution. 

Continuous emulsion copolymerization processes for vinyl acetate and vinyl acetate—ethylene copolymer have been reported (59—64). CycHc variations in the number of particles, conversion, and particle-size distribution have been studied. Control of these variations based on on-line measurements and the use of preformed latex seed particles has been discussed (61,62). 

If the secondary stream contains emulsifier it can function in three ways. When the emulsion feed is started quickly the added emulsifier can serve to lengthen the particle formation period and hence to broaden the particle size distribution. When the emulsion feed is started later and added in such a manner that the emulsifier is promptly adsorbed on existing particles, one can obtain quite narrow size distributions. If the emulsion feed is started later but added rapidly enough to generate free emulsifier in the reaction mixture a second population of particles can be formed, again yielding a broad size distribution. 

Particle Size Measurement. The best way to evaluate an emulsion s stability is probably to measure its particle size distribution. A number of methods are available for droplet size determination (see Sec. VIII.A). Optical microscopy, although a time-consuming technique, is a direct way of measuring droplets larger than 1 pm. Nowadays, laser lightscattering, diffraction, and transmission methods are becoming popular for routine determination of particle size [151, 152],... 

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). 

For parenteral products specific consideration needs to be included for tonicity adjustment, emulsion globule size, ease of resuspension and sedimentation rate, particle size and particle size distribution, viscosity and syringeability, and crystal form changes. Full consideration should be included of the proposed instructions for dilution or reconstitution of products and of compatibility with the proposed solvents or diluents. This should include a demonstration that the proposed storage temperature and extremes of concentration are suitable. 

Research on the modelling, optimization and control of emulsion polymerization (latex) reactors and processes has been expanding rapidly as the chemistry and physics of these systems become better understood, and as the demand for new and improved latex products increases. The objectives are usually to optimize production rates and/or to control product quality variables such as polymer particle size distribution (PSD), particle morphology, copolymer composition, molecular weights (MW s), long chain branching (LCB), crosslinking frequency and gel content. 

Particle Size Distribution Determination. To consider the full PSD, a population balance or age distribution analysis on particles must be employed. Table II gives a summary of recent work concerning the determination of PSD s in emulsion systems, using both the "monodispersed" approximation and the population balance approach. More details can be found in the literature sources cited in the Table. 

AKDs are waxy, water-insoluble solids with melting points around 50 °C, and ASAs are viscous water-insoluble liquids at room temperature. It is necessary to prepare them as stabilised emulsions by dispersion in a cationic polymer (normally cationic starch). Small amounts of retention aid and surfactants may also be present. Particle size distributions are around 1 fim, and addition levels around 0.1% (of pure AKD or ASA) by weight of dry fibre. This is an order of magnitude lower than the amount of rosin used in rosin-alum sizing (1-2%). Emulsions of AKD are more hydrolytically stable than ASA, and the latter must be emulsified on-site and used within a few hours. 

Small-Volume Parenterals Color, clarity of solutions, particulate matter, pH, sterility, endotoxins. Powders for injection solutions include clarity, color, reconstitution time and water content, pH, sterility, endotoxins/pyrogens, and particulate matter. Suspensions for injection should include additional particle size distribution, redispersability, and rheological properties. Emulsion for injection should include phase separation, viscosity, mean size, and distribution of dispersed globules. 

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).

Recently,it was reported that when maltodextrins were used as the encapsulating agent, increasing the dextrose equivalent by 10 could result in a three to six fold enhancement in shelf life (5). The reduction in emulsion size of feed emulsion also improved the shelf stability (Risch, S. J., University of Minnesota, personal communication, 1986). However, the influence of particle size distribution on the stability of encapsulated flavors has not been clearly addressed in the literature. 

ABS compositions with bimodal particle size distributions of the grafted rubber can be prepared by emulsion graft polymerization techniques. The preparation of ABS types by emulsion polymerization consists in brief of (13) ... 

Laser diffraction is the most commonly used instrumental method for determining the droplet size distribution of emulsions. The possibility of using laser diffraction for this purpose was realized many years ago (van der Hulst, 1957 Kerker, 1969 Bohren and Huffman, 1983). Nevertheless, it is only the rapid advances in electronic components and computers that have occurred during the past decade or so that has led to the development of commercial analytical instruments that are specifically designed for particle size characterization. These instruments are simple to use, generate precise data, and rapidly provide full particle size distributions. It is for this reason that they have largely replaced the more time-consuming and laborious optical and electron microscopy techniques. 

The major disadvantage of the laser diffraction and electrical pulse counting techniques is that they are only directly applicable to dilute emulsions or emulsions that can be diluted without disturbing the particle size distribution. However, many food emulsions are not dilute and cannot be diluted, either because dilution alters the particle size distribution or because the original sample is partially solid. For concentrated systems it is belter to use particle-sizing instruments based on alternative technologies, such as ultrasonic spectrometry or NMR (Dickinson and McClements, 1996). 

Figure D3.4.7 Change in cumulative particle size distribution of a 20% (w/v) oil-in-water emulsion stabilized by 2% (w/v) Tween 20 at the lower port (A) and upper port (B). (C) Change in mean droplet diameter and volume fraction of the emulsions as a function of time.

The characterization of emulsions by particle size distribution analysis has been facilitated in recent years by a range of new instruments. Most of these instruments employ laser light diffraction principles, and have replaced older spectrophotometric methods. 

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