Emulsion systems, particle size distributions, study

Quasi-elastic light scattering is an excellent technique for studying the formation and stability of submicrometer emulsions. Improvements in the methods of quasi-elastic light scattering data acquisition and analysis that enable full particle-size distribution studies of sub-micrometer emulsion systems are discussed. Using several oil/water emulsion systems as examples, we demonstrate the ability of these techniques to determine the effect of emulsifier concentration on the particle-size distribution produced by an inversion method of emulsification. Some of the benefits of obtaining the full distribution are also discussed. 

The aim of this paper is to describe the experimental and numerical techniques that, when combined, provide a procedure that enables full particle-size distribution studies of sub-micrometer emulsion systems. We then present distribution results for several oil/water emulsions to demonstrate the ability of these techniques to monitor the effect of processing variables (such as surfactant concentration) on the final emulsion. Finally, we discuss some of the problems of converting the intensity weighted distribution to a mass weighted distribution and suggest methods for minimizing or eliminating some of these problems. 

Using Quasi-Elastic Light Scattering To Study Particle Size Distributions in Submicrometer Emulsion Systems... 

Fundamental mixing studies on simple two-component systems have provided insight into the effect of mixing parameters on critical emulsion properties such as particle size distribution. For example, Nagata [81] has shown the distribution of sizes of the dispersed liquid phase as a function of agitator speeds. As we might expect, a normal distribution occurs at higher speeds. In a similar study, the effect of surface tension was determined for several liquid dispersed phases from benzene to paraffin oil [82],... 

In batch experiments, the solids were varied from 35 to 75% [10]. The primary surfactant was Aerosol A103 (disodium ethoxylated nonyl phenol half ester of sulfosuccinic acid) with HD as the cosurfactant. These were used in concentrations of 1 and 4 wt% on monomer, respectively. Two KPS concentrations, 1 and 2 wt% on water, were tried. The miniemulsions were produced by ultra-sonification. Parallel conventional emulsion polymerizations were conducted for comparison to the miniemulsion polymerizations (75 °Q. Coagulum-free latexes resulted from miniemulsion polymerizations up to 60% solids, while only 50% solids could be achieved for the cxmventional process. These differences were attributed to the resulting particle size distributions where the miniemulsion polymerizations produced latexes with larger particles, broader distributions and lower viscosities than their conventional counterparts. As in other studies, this difference in PSDs was explained by differing nucleation mechanisms. However, as in other studies, it was not possible to determine whether the nucleation in the miniemulsion systems was predominantly by radical entry into chxjplets. 

Phospholipid-stabilized intravenous emulsions have been widely used for parenteral nutrition and have also been introduced as drug carrier systems, especially for lipophilic compounds. The aim of the authors in the next papers we review here (50,51) was to consider in detail various methods, e.g.. PCS. nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), and small-angle x-ray diffraction studies (SAXS), to determine parameters related to the internal structure of the particles in a model intravenous emulsion stabilired by phospholipids. An emulsion with an extremely high fat load and a da.ssical emulsifier was chosen, PCS measurements were used to derive a particle size distribution and this was then used to calculate the total ot droplet surface area. The result indicated that there should be an excess of surfactants of 1.50%, Such an excess was not confirmed by either NMR or SAXS measurements and the dis-... 

The batch emulsion polymerization is commonly used in the laboratory to study the reaction mechanisms, to develop new latex products and to obtain kinetic data for the process development and the reactor scale-up. Most of the commercial latex products are manufactured by semibatch or continuous reaction systems due to the very exothermic nature of the free radical polymerization and the rather limited heat transfer capacity in large-scale reactors. One major difference among the above reported polymerization processes is the residence time distribution of the growing particles within the reactor. The broadness of the residence time distribution in decreasing order is continuous>semibatch>batch. As a consequence, the broadness of the resultant particle size distribution in decreasing order is continuous>semibatch>batch, and the rate of polymerization generally follows the trend batch>semibatch>continuous. Furthermore, the versatile semibatch and continuous emulsion polymerization processes offer the operational flexibility to produce latex products with controlled polymer composition and particle morphology. This may have an important influence on the application properties of latex products [270]. 

It should be noted that the probability for the continuous particle nucleation throughout the emulsion polymerization increases with increasing surfactant concentration. For constant monomer weight, the higher the surfactant concentration, the smaller the latex particles produced in the emulsion polymerization system. In addition, the longer the particle nucleation period, the broader the residence time distribution of particle nuclei within the reactor (i.e., the broader the resultant particle size distribution). These rules of thumb, based on a large number of fundamental studies on nucleation and growth of particle nuclei, have been widely used in industry to effectively use surfactant to stabilize various latex products with balanced performance properties. 

A number of studies endeavored to experimentally determine the values of the desorption rate constant. It is also interesting to note that Lee and Poehlein [46,48,49] modified the approach of Ugelstad et al. [8,9] and applied it to emulsion polymerization carried out in a single continuous stirred tank reactor (CSTR) system. The resultant latex particle size distribution data were then used to determine the value of A des. The k data obtained from other literature are summarized in Table 4.4. Significant variations in the values of k isi for the emulsion polymerizations of styrene at 60 °C are observed. 

Monomer compositional drifts may also occur due to preferential solution of the styrene in the mbber phase or solution of the acrylonitrile in the aqueous phase (72). In emulsion systems, mbber particle size may also influence graft stmcture so that the number of graft chains per unit of mbber particle surface area tends to remain constant (73). Factors affecting the distribution (eg, core-sheU vs "wart-like" morphologies) of the grafted copolymer on the mbber particle surface have been studied in emulsion systems (74). Effects due to preferential solvation of the initiator by the polybutadiene have been described (75,76). 

With the advent of advanced characterization techniques such as multiple detector liquid exclusion chromatography and - C Fourier transform nuclear magnetic resonance spectroscopy, the study of structure/property relationships in polymers has become technically feasible (l -(5). Understanding the relationship between structure and properties alone does not always allow for the solution of problems encountered in commercial polymer synthesis. Certain processes, of which emulsion polymerization is one, are controlled by variables which exert a large influence on polymer infrastructure (sequence distribution, tacticity, branching, enchainment) and hence properties. In addition, because the emulsion polymerization takes place in an heterophase system and because the product is an aqueous dispersion, it is important to understand which performance characteristics are influended by the colloidal state, (i.e., particle size and size distribution) and which by the polymer infrastructure. 

Experimental studies show, however, that these limiting approximations must be used with caution. For example, with some emulsion polymerization systems the mean number of radicals per particle may run from one-half to several depending on the size of the particle (I). Assuming that the polymerization process is stationary with known rates of radical arrival and termination, Stockmayer (6) and O Toole (3) have shown how to calculate not only the mean number of radicals but the entire number distribution as well. Until now, no methods of the same generality seem to exist for calculating the polymer size distribution. 

A study was made of the impact of incorporation of a small amount of carboxylic monomers (acrylic acid or methacrylic acid) into the latex particles in the limited flocculation process, often encountered in the semi-batch surfactant-free emulsion polymerisation of pure butyl acrylate. The possibility of producing carboxylated polybutyl acrylate latices with a smaller particle size was evaluated. The resultant latex was characterised to gain a better understanding of the effect of the surfactant-free technique on their physical properties, e.g. zeta potential, distribution of acrylic acid or methacrylic acid in the particles, and stability towards the added salt, compared with the conventional emulsion polymerisation system stabiUsed by surfactants. 35 refs. 

Latex particle size is a key parameter for emulsion polymerization, with strong impact on the final properties of the polymers synthesized and, in general, on the polymerization kinetics in heterogeneous systems. An experimental methodology which combines a continuous-flow system and an ESR time sweep experiment was extended by Parker et al. [144] to study the effects of varying latex particle size on polymerization kinetics andradical distribution in the case of MMA emulsion... 

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