Droplet sizes in emulsions

As the water drop provides only a limited amount of surfactant, the interfacial tension passes through a minimum when the flux of molecules adsorbing at the interface is equal to the flux of molecules desorbing into the external phase. Note that under these dynamic conditions, the interfacial tension can reach values which are well below the equilibrium values. This can be relevant for technologic processes, like the control of the droplet size in emulsions. 

Using the HLB system for the characterisation of surfactants, a minimum interfacial tension is observed when the HLBr is reached. Fig. 6.7 shows, as an example, the interfacial tension and the droplet size in emulsions of decane and sunflower oil, respectively, as a function of the HLB. For decane, the minimum y value and the minimum size of emulsion droplets are observed in the region of HLB 9 and for sunflower oil around 11, which corresponds to the required HLB values. Apparently, one of the reasons leading to an increase in emulsion stability when reaching HLBtp is the increase of emulsion dispersity under conditions of the maximum decrease in interfacial energy. 

Equation (9.21) has ben used to determine droplet sizes in emulsions and disperse phases using the PGSE NMR method [132, 133]. 

Other important characterization techniques include electrophoresis measurements of droplets [11, 12] (see Section XIV-3C), infrared absorption of the constituent species [13], and light or x-ray scattering. NMR self-diffusion measurements can be used to determine droplet sizes in W/0 emulsions [14]. 

Ultrasonically assisted extraction is also widely used for the isolation of effective medical components and bioactive principles from plant material [195]. The most common application of low-intensity ultrasound is as an analytical technique for providing information about the physico-chemical properties of foods, such as in the analysis of edible fats and oils (oil composition, oil content, droplet size of emulsions, and solid fat content) [171,218]. Ultrasonic techniques are also used for fluids characterisation [219]. 

J du Plessis, LR Tiedt, AF Kotze, CJ van Wyk, C Ackerman. A transmission electron microscope method for determination of droplet size in parenteral fat emulsions using negative staining. Int J Pharm 46, 1988. 

Sensory Analysis. A paired comparison test was run to determine if the difference in oil droplet size in the emulsion changed the perceived intensity of the orange flavor. The coarsest emulsion (3.87 pM) and the Microfluidized sample (0.90 pM) from the third set of spray dried samples were compared. The solutions were prepared using 200 ppm flavor in a 10% (w/v) sucrose solution with 0.30% of a 50% citric acid solution added. The amount of each powder required to attain 200 ppm orange oil was calculated on the basis of percent oil in each powder (determined by Clevenger analysis). A pair of samples at approximately 10 C was given to each of 24 untrained panelists. The samples were coded with random numbers. Half the panelists were asked to taste the coarsest sample first while while the other half tasted the Microfluidized sample first. This was done to determine whether or not adaptation was a factor. The panelists were asked to indicate which sample had the most intense orange flavor. 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

A major question to address in the future is how structure influences the dynamics of digestion and the signaling processes involved. Armand et al. (1996, 1999) investigated the digestion and lipid absorption from emulsions with different droplet sizes in humans. Healthy subjects received intragastrioally a coarse (10 pm) and a fine (0.7 pm) lipid emulsion of identical composition in random order. Gastric and duodenal aspirates as well as triglyceride appearance in the blood were analyzed. They found an increase in droplet size in the stomach however, the fine emulsion retained droplets... 

H., Jaussan, V., and Lairon, D. (1999). Digestion and absorption of 2 fat emulsions with different droplet sizes in the human digestive tract. Am. J. Clin. Nutr. 70, 1096-1106. 

Formation of spray in small drops (i.e. 30 microns, with a narrow size distribution) using a rotary disc or nozzle, to enhance the product-air interface. The preparation of the feed (dry matter content, composition, temperature, mixing) must facilitate pumping and spraying with minimal modification (partition of constituents, droplet coalescence in emulsions). 

If the monomer droplet size in a conventional emulsion polymerization can be reduced sufficiently (see below), the loci of polymerization become the monomer droplets. This system is referred to as a miniemulsion polymerization and will be discussed in detail below. The particle diameter will range from 50 to 500 nm. 

It should be noted that the droplet sizes in Hallworth s emulsions are considerably greater than those investigated by Davis and Smith. The importance of the two possible routes of degradation of the emulsions, coalescence or molecular diffusion, may be dependent upon the droplet size and size distribution. Also an interfacial coherent film may reduce the demulsification by either mechanism, i.e. by reducing the rate of coalescence or by presenting an interfacial barrier to... 

Particle Size. After determining, with bottle tests, which systems easily produced stable oil-in-water emulsions, the droplet size distributions for the oil-in-water emulsions were determined with a Model TA II Coulter Counter. The quantitative results obtained with the Coulter Counter were verified by qualitative observations with an optical microscope. The droplet size distributions for several oil-in-water emulsions are given in Figure 5. A qualitative correlation between droplet size and emulsion stability was observed. The smaller the median droplet size, the more stable was the emulsion. The pore size distribution for a 300-md Berea sandstone core is given for comparison. 

The above mean is also referred to as the mean length diameter, dy, because it represents the sum of the length of the droplets divided by the total number of droplets. It is also possible to express the mean droplet size in a number of other ways (Table 2). Each of these mean sizes has dimensions of length (meters), but stresses a different physical aspect of the distribution, e.g., the average length, surface area, or volume. For example, the volume-surface mean diameter is related to the surface area of droplets exposed to the continuous phase per unit volume of emulsion, As ... 

Average Droplet Size and Droplet Size Distribution, All practical emulsions show some form of droplet size distribution with an average value representing this size distribution. The average droplet size and the droplet size distribution affect the rheology of emulsion (discussed in Chapter 4). Droplet size in relation to pore throat size affects the flow of fluids in porous media, as discussed previously (Figure 3). 

Soo and Radke (11) also studied the effect of average droplet size of emulsion on the flow behavior in porous media. The droplet size distribution of the emulsions that were prepared with surfactants and NaOH in a blender are shown in Figure 12. These droplet size distributions were found to be log-normal distributions. Others (9, 27) have also observed that the size of emulsion droplets was log-normally distributed. Soo and Radke (11) conducted experiments with emulsions having different average mean diameter in fine Ottawa water-wet sand packs. Their results of the reduced permeability, k/ko, and reduced effluent volume concentration as a function of the pore volume of oil (in the emulsion) injected are shown in Figure 13. All emulsions were of 0.5% quality, and the initial permeability, ko, was 1170 mD (millidarcies). The lines in the figure represent results of flow theory (12,13) based on deep-bed filtration principles. 

A portion of the water in an emulsion can be dispersed within the oil droplets. This portion of the total water should be treated as oil when estimating emulsion viscosity. Generally, added water is present in the continuous phase. If the crude oil contains water prior to emulsion formation, this water may be present in either the continuous (water) phase or the dispersed (oil) phase after emulsion formation, depending primarily on the water droplet size in the crude oil. In order to predict how much of the water in the crude oil will be freed into the continuous phase, emulsion preparation experiments with the actual crude oil to be used are necessary. 

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