Multiple emulsions droplet size

The effect of the WPI/xanthan or WPI/modified pectin (MP) (Garti and Wicker, 2005) ratio on stability of W/O/W multiple emulsions was examined (Tables 5.4 and 5.5) Addition of hydrocolloids caused significant improvement, mainly to the multiple emulsion droplet sizes. The droplets were smaller in comparison to droplets of multiple emulsions prepared solely with WPI. Multiple emulsion droplets stabilized with WPI and pectins are one-third the size... 

On the other hand, in multiple emulsions stabilized with WPI alone, the droplet size decreased with time from 14.6 to 10.5 pm after 20 days, probably due to internal aqueous phase explosion. As the ratio WPI/MP decreases (e.g., to a 4 to 0.1 wt ratio), the multiple emulsion droplet size can increase from 4.7 to 7.2 pm after 20 days. Lack of exhibited hydrocolloid is attributed to the weakness of the emulsifier film adsorbed onto the external oil-water interface, as reported in the case of multiple emulsions stabilized with pure WPI (Uruakpa and Arntfield, 2005). 

The characteristics of the microstructure formed (such as emulsion droplet size) are dependent on the type of microstructure, type of deformation (shear, extension, or combined), and deformation rate as well as the TSVs. In order to maximize the fluid microstructure/flow field interactions, the flow field must be uniform which requires the generation of the flow field over a small processing volume. There are several types of equipment such as multiple expansion contraction static mixer (MECSM) or its dynamic... 

Florence and Whitehill [38] distinguished between three types of multiple emulsions (W/O/W) that were prepared using isopropyl myristate as the oil phase, 5 % Span 80 to prepare the primary W/0 emulsion and various surfactants to prepare the secondary emulsion (a) Brij 30 (polyoxyethylene 4 Lauryl ether) 2%. (b) Triton X-165 (polyoxyethylene 16.5 nonyl phenyl ether (2%). (c) 3 1 Span 80 Tween 80 mixtures. A schematic picture of the three structures is shown in Fig. 1.34. The most common structure is that represented by (b) whereby the large size multiple emulsion droplets (10-100 pm) contain water droplets 1 pm. A schematic representation of some breakdown pathways that may occur in W/O/W multiple emulsions is shown in Fig. 1.35. 

Several methods can be applied for characterization of multiple emulsions (i) Droplet size analysis The droplet size of the primary emulsion (internal droplets of the multiple emulsion, are usually in the region 0.5-2 pm, with an average of 0.5-1.0pm. The multiple emulsion droplets cover a wide range of sizes, usually... 

The electrostatic stabilization mechanism is well documented for simple OfW emulsions (Myers, 1998a). Multiple emulsion droplets are much larger in size, and therefore the repulsive electrostatic forces are less pronounced (Figure 5.5). 

Emulsions can behave like microcapsules, microspheres, or mesophasic lyotropic liquid crystals, since the interfacial film is rather thick and, in most cases, multilayered. Increasing the xanthan content in the WPI/xanthan system to 1 wt% in the external aqueous phase decreases droplet diameter from 11.8 to 3.1 pm (Figure 5.17), but no significant improvement in droplet size occurs between ratios of 4 to 0.5 and 4 to 1. It was also found that by increasing the protein-to-polysaccharide ratio, the multiple emulsion droplets were smaller and stability was increased. 

Figure 6.9 W/O/W emulsion prepared by SPG membrane emulsification for trans-catheter arterial injection chemotherapy of hepatocellular carcinoma (HCC) (a) Microscopic view of multiple emulsion droplets. Internal water droplets containing anticancer drug are visible as black dots, (b) Particle size distribution of oil droplets immediately after preparation and 40 days after preparation (Nakashima et al., 2000 Higashi et al., 1995).

Multiple emulsions made of low-molecular-weight emulsifiers (the so-called monomeric emulsifiers) are mostly unstable thermodynamically. This is mainly because in the second stage of the emulsification severe homogenization or shear are not recommended, and as a result large droplets are obtained. During years of research attempts have been made to find proper and more suitable combinations of emulsifiers to reduce droplets sizes and to improve the emulsion stability. Aggregation, flocculation, and coalescence (occurring in the inner phase and between the multiple-emulsion droplets) lead to rupture of droplets and separation of the phases and thus are major factors behind the instability of the emulsions. 

Liquid-to-liquid emulsification is a critical step in the multiple emulsion microencapsulation process (W/OAV or O/W/O). It was found that the size of these droplets decreases with increasing homogenization intensity and duration. The emulsion droplet size depends, as expected, on viscosity, total volume size, and the volume ratio of the continuous phase to the dispersed phase in the rotor/stator design being investigated. All these physical parameters influence the structure of the microspheres obtained by this technique. 

Droplet Size Droplet size was determined on a Van tan Coulter counter (Model 4000, Zymo Instruments, India) after diluting the multiple emulsion 100 times with an external phase. The droplet size ranged from < 1.0 to 1.93 pm. Table 9.1 shows the droplet size and the polydispersity of the multiple emulsion formulations. The particle sizes of 6-MP loaded, coated, and uncoated multiple emulsions was nearly the same. The mean diameter of PEG-coated multiple emulsions was found to decrease with time, and it reached its minimum size after 6 to 8 hours. The size was reduced by nearly 15%, and the polydispersity was decreased from 0.18 to 0.09. Kim et al. (1995) found the separation of the aggregated multiple emulsion droplets to occur over several days of equilibration. In our case, it was few hours because of steric stabilization of individual particles surface coated (Vanderhoff and El Asser, 1988). 

For the reasons described above, the droplet size distribution of the same emulsion measured on different laser diffraction instruments can be significantly different, depending on the precise design of the optical system and the mathematical theory used to interpret the diffraction pattern. It should be noted, however, that the most common source of error in particle size analysis is incorrect operation of the instrument by the user. Common sources of user error are introduction of air bubbles into the sample, use of the wrong refractive index, insufficient dilution of emulsion to prevent multiple scattering. and use of an unclean optical system. 

Inversion of multiple w/o/w emulsions to o/w emulsions has been found to occur (28) only when the oil droplet size is reduced below a critical size or if the HLB of the emulsifiers approaches the required HLB of the oil phase. When these droplets were reduced in size below about 5 pm they no longer could accommodate an inner aqueous phase. Droplet size reduces with increasing concentrations of secondary surfactant (Fig. 2) which might, as Magdassi and co workers (28) point out, explain the results of Matsumoto et al (27). 

To measure the droplet size distribution of the resulting multiple emulsion (with diameters >5 pm), optical microscopy combined with image analysis can be used. An alternative method to measure droplet size distribution is to use light diffraction and then to apply Fraunhofer s diflraction theory. Details of this method are provided in Chapter 19, but basically a laser beam that has been enlarged and... 

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