Droplet diameter/size emulsions

Droplet diameters of emulsions stored for 12 months show smaller dmean values than droplets of fresh emulsions in the case of acrylate variation (Fig. 5). For SE47-01-05-17 and SE47-10-05-17 the difference in dmean is approx. 1 p.m, whereas emulsions with concentrations between 0.3 and 0.9 wt. % acrylate have dmsm values which are 0.5 p.m smaller compared to dmsm from fresh emulsions. Droplet size distributions of aged emulsions are less broad than... 

The largest portion of the monomer (>95%) is dispersed as monomer droplets whose size depends on the stirring rate. The monomer droplets are stabilized by surfactant molecules absorbed on their surfaces. Monomer droplets have diameters in the range 1-100 pm (103-105 nm). Thus, in a typical emulsion polymerization system, the monomer droplets are much larger than the monomer-containing micelles. Consequently, while the concentration of micelles is 1019-1021 the concentration of monomer droplets is at most 1012-1014 L 1. A further difference between micelles and monomer droplets is that the total surface area of the micelles is larger than that of the droplets by more than two orders of magnitude. The size, shape, and concentration of each of the various types of particles in the... 

In a separate experimental study, Aronson and Petko [90] also observed an increase in yield value with increasing salt concentration, for w/o HIPEs. However, the interfacial tension of the emulsions was seen to decrease with addition of a number of electrolytes to the aqueous phase, in contrast to observations made by Pons et al. The reason for this discrepancy is not clear, but may be due to different interactions between the different surfactants and salts used by each group. It is hinted that the increase in yield value on addition of salt, as observed by Aronson et al., is due to a decrease in average droplet size however, this was not examined extensively, and determination of droplet diameters was by optical microscopy only. 

As mentioned previously, Bibette [95] has developed a very elegant method for the purification of coarse, polydisperse emulsions to produce monodisperse systems. This technique is based on the attractive depletion interaction between dispersed phase droplets, caused by an excess of surfactant micelles in the continuous phase. A phase separation occurs under gravity, between a cream layer and a dilute phase since the extent of the separation increases with increasing droplet diameter, a separation based on size occurs. By repeating this process, emulsions of very narrow size distribution can be produced. 

Another factor of concern le the population density of droplets as coalescence occurs and the majority of droplets reach critical size and fall out of the emulsion. The purpose of applying electrical coalescence to the emulsion was to increase the droplet diameter and to cause them to quickly plunge out of the emulsion by Stoke a law. As more and more water droplets fall out, a stage is reached when very few droplets remain which are a great distance apart. 

The absorption of ozone from the gas occurred simultaneously with the reaction of the PAH inside the oil droplets. In order to prove that the mass transfer rates of ozone were not limiting in this case, the mass transfer gas/water was optimized and the influence of the mass transfer water/oil was studied by ozonating various oil/water-emulsions with defined oil droplet size distributions. No influence of the mean droplet diameter (1.2 15 pm) on the reaction rate of PAH was observed, consequently the chemical reaction was not controlled by mass transfer at the water/oil interface or diffusion inside the oil droplets. Therefore, a microkinetic description was possible by a first order reaction with regard to the PAH concentration (Kornmuller et al., 1997 a). The effects of pH variation and addition of scavengers indicated a selective direct reaction mechanism of PAH inside the oil droplets... 

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.

In the situation where V2DA is of the same order or larger than the distance between any diffusional barriers in the system, so-called restricted diffusion is observed. In a W/O emulsion, for example, the water molecules are restricted in the extent of their diffusion by the presence of the boundaries of the water droplets. The extent of the restriction of the diffusion of the water molecules is reflected in the ratio R = E /E. An expression for the echo attenuation R-factor as a function of droplet diameter has been derived by Murday and Cotts for uniform spherical droplet sizes [7] ... 

In (5) D00 is the median diameter and a is the standard deviation of the distribution. By fitting the experimental R-values, the parameters D0 0 and a can be determined and hence the size distribution of the droplets in the emulsion can be obtained. For microbiological safety aspects Dj 3 is more important. D3>3 is the volume weighted mean droplet diameter and a is the standard deviation of the logarithm of the droplet diameter. The parameter D3 3 is related to the parameter D00 according to ... 

In contrast to the conventional emulsions or macroemulsions described earlier are the disperse systems currently termeraiicroemulsions. The term was Lrst introduced by Schulman in 1959 to describe a visually transparent or translucent thermodynamically stable system, with much smaller droplet diameter (6-80 nm) than conventional emulsions. In addition to the aqueous phase, oily phase, and surfactant, they have a high proportion of a cosurfactant, such as an alkanol of 4-8 carbons or a nonionic surfactant. Whereas microemulsions have found applications in oral use (as described in the next chapter), parenteral use of microemulsions has been less common owing to toxicity concerns (e.g., hemolysis) arising from the high surfactant and cosolvent levels. In one example, microemulsions composed of PEG/ethanol/water/medium-chain triglycerides/Solutol HS15/soy phosphatidylcholine have been safely infused into rats at up to 0.5 mL/kg. On dilution into water, the microemulsion forms a o/w emulsion of 60-190 nm droplet size (Man Corswant et al., 1998). 

A peculiar advantage of membrane emulsification is that both droplet sizes and size distributions may be carefully and easily controlled by choosing suitable membranes and focusing on some fundamental process parameters reported below. Membrane emulsification is also an efficient process, since the energy-density requirement (energy input per cubic meter of emulsion produced, in the range of 104-106 J m-3) is low with respect to other conventional mechanical methods (106-108 J m-3), especially for emulsions with droplet diameters smaller than 1 (4m [1]. The lower energy density requirement also improves the quality and functionality... 

The size distribution curve of the droplets in the emulsions varied little if soaps of sodium, potassium, or caesium were used. As a rule the curves had a marked peak, at a diameter of about 2fi, which apparently depended little on the manner of preparation of the emulsion, and slightly on the nature of the oil. Emulsions of water in oil, stabilized by magnesium or aluminium soaps, had a similar distribution of sizes among the droplets. 

The aim of this first section is to describe the rupturing mechanisms and the mechanical conditions that have to be fulfilled to obtain monodisperse emulsions. A simple strategy consists of submitting monodisperse and dilute emulsions to a controlled shear step and of following the kinetic evolution of the droplet diameter. It will be demonstrated that the observed behavior can be generalized to more concentrated systems. The most relevant parameters that govern the final size will be listed. The final drop size is mainly determined by the amplitude of the applied stress and is only slightly affected by the viscosity ratio p. This last parameter influences the distribution width and appears to be relevant to control the final monodispersity. 

This relationship is particularly useful as it allows one to calculate the total surface area of droplets in an emulsion, an important parameter that can be used to estimate the emulsifier concentration required to produce a kineticaUy stable emulsion. An appreciation of the various types of mean droplet diameter is also important because different experimental techniques used to measure droplet sizes are sensitive to different mean values (24). Consequently, it is always important to be clear about which mean diameter has been determined in an experiment when using or quoting droplet size data. 

Droplet growth rates and viscosity decline rates both are exponential processes, following a straight line on a semi-log plot (log x or log vs. time), where is the mean droplet diameter. Emulsion failure is also associated with a certain minimum viscosity, depending on water content, crude-oil content, temperature, etc. Viscosity and mean droplet size may be projected to estimate the time remaining before emulsion failure. The ultimate droplet size and viscosity should be determined experimentally for the same formulation in a pilot-plant pipe loop. 

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