Particle size, emulsions concentration

The oil-rich layer eventually coagulated to form an oil layer this type of separation was described as mixed-particle-size emulsion by Tadros and Vincent (1 ). The addition of sodium chloride to the water used in making the emulsion gave faster emulsion separation especially for high NaOH (5 x 10 or greater) concentration emulsions. When the NaCl content exceeded 2.0%, no homogeneous emulsion could be obtained as separation occurred immediately. 

It was demonstrated that particle size and concentration are parameters to which this technique is sensitive. Clearly, with suitable calibration techniques these techniques can be used in industrial reactors and processes as an example, of the differences found a batch reaction, Figure 24 shows the initial and final spectra obtained through a seeded polystyrene emulsion reaction. Note the dramatic changes in the reflectance spectra. 

Poljmierisation to high solids content has been described in the patent literature (12). However for emulsion polymers a serious problem is one of mechanical stability during polymerisation. Palmgren (22) has studied the mechanical stability of monodisperse PVC latices as a function of particle size, emulsifier concentration, electrolyte concentration and residual monomer content. The mechanical stability of PVC latices of different particle size containing different surface concentrations of SDS emulsifier is shown in Fig 4. At a given emulsifier coverage on the latex, these data indicate that the latex stability decreases with increasing particle size. Latex... 

Disperse systems occur in a variety of industrial processes. In many of these processes, the mean particle size, the particle size distribution, and the particle concentration are important criteria for the process flow and the quality of the produced products. In emulsions, particle size and concentration have a significant impact on the stabihty of the emulsion. In spray drying processes, the particle size defines the ratio of surface area to volume of the particles and therefore the expiration of the drying process and the properties of the resulting particles. Also in dedusting and filtration processes, the particle size and concentration have a significant impact. 

The compounding technique for latex differs from that of dry mbber and is fundamentally simpler. A critical factor of colloidal stabiUty makes necessary that each ingredient is of optimum particle size, pH, and concentration when added as an aqueous dispersion to the latex. Rubber latex is a colloidal aqueous emulsion of an elastomer and natural mbber latex is the milky exudation of certain trees and plants that of greatest commercial importance is the... 

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. 

Models for emulsion polymerization reactors vary greatly in their complexity. The level of sophistication needed depends upon the intended use of the model. One could distinguish between two levels of complexity. The first type of model simply involves reactor material and energy balances, and is used to predict the temperature, pressure and monomer concentrations in the reactor. Second level models cannot only predict the above quantities but also polymer properties such as particle size, molecular weight distribution (MWD) and branching frequency. In latex reactor systems, the level one balances are strongly coupled with the particle population balances, thereby making approximate level one models of limited value (1). 

Hayashi et al., 1989], involving the addition of monomer and initiator to a previously prepared emulsion of polymer particles, is especially useful for this purpose since it allows the variation of certain reaction parameters while holding N constant. Thus, h in seeded styrene polymerization drops from 0.5 to 0.2 when the initiator concentration decreases from 10-2 to 1CT5 M. At sufficiently low Ru the rate of radical absorption is not sufficiently high to counterbalance the rate of desorption. One also observes that above a particular initiation rate ([I] = lO-2 M in this case), the system maintains case 2 behavior with h constant at 0.5 and Rp independent of Ri. A change in Ri simply results in an increased rate of alternation of activity and inactivity in each polymer particle. Similar experiments show that h drops below 0.5 for styrene when the particle size becomes sufficiently small. The extent of radical desorption increases with decreasing particle size since the travel distance for radical diffusion from a particle decreases. 

When the polymerization has proceeded to such an extent that all of the monomer droplets have vanished, which occurs after 60-80% conversion, all of the residual monomer is located in the latex particles. The monomer concentration in the particles now declines as polymerization proceeds further, i.e., in this final period the reaction is first order. At the end of the polymerization, the emulsion consists of polymer particles with a size distribution between 50 and 150 pm, which is larger than the original micelles, but smaller than the original monomer droplets. The changes of surface tension and overall rate of polymerization with conversion are schematically shown in Fig. 2.2. 

The self-emulsifying behaviour of a binary nonlonlc surfactant vegetable oil mixture has been shown to be dependant on both temperature and surfactant concentration. The quality of the resulting emulsions as assessed by particle size analysis showed that manipulation of these parameters can result In emulsion formulations of controlled droplet size and hence surface area. Such considerations are Important when the partition of lipophilic drugs Into aqueous phases and drug release rates are considered. 

Acetonitrile was the most efficient of all dipolar aprotic solvent for the formation of monodisperse particles. Figure 1.2.6 shows the relation between solubility curves of metal alkoxide and particle size in octanol/acetonitrile solution. The solubility of metal alkoxide in octanol gradually decreased with increasing acetonitrile concentration. Metal alkoxide was precipitated to form an emulsion when the acetonitrile was added at more than 40 vol%. The particle size increased with increasing... 

These stabilizers are added to the formulation in order to stabilize the emulsion formed during particle preparation. These stabilizers, however, can also influence the properties of the particles formed. The type and concentration of the stabilizer selected may affect the particle size. Being present at the boundary layer between the water phase and the organic phase during particle formation, the stabilizer can also be incorporated on the particle surface, modifying particle properties such as particle zeta potential and mucoadhesion (203). Other polymers have also been evaluated as stabilizers in earlier studies such as cellulosic derivatives methylcellu-lose (MC), hydroxyethylcellulose ( ), hydroxypropylcellulose (HPC), and hydroxypropylmethylcellulose (HPMC), as well as gelatin type A and B, carbomer and poloxamer (203). 

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).

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