Droplet interface

Since NFI3 is highly water-soluble and neutralization within the droplet occurs rapidly, " the rate-limiting step in acid neutralization is normally NH 3 transport to the air/droplet interface, which is dependent on [NH3]4 and particle surface area. At high [NH3J4, the rate of NH, uptake across the air/droplet interface is given by... 

A similar technique, the so-called spontaneous emulsification solvent diffusion method, is derived from the solvent injection method to prepare liposomes [161]. Kawashima et al. [162] used a mixed-solvent system of methylene chloride and acetone to prepare PLGA nanoparticles. The addition of the water-miscible solvent acetone results in nanoparticles in the submicrometer range this is not possible with only the water-immiscible organic solvent. The addition of acetone decreases the interfacial tension between the organic and the aqueous phase and, in addition, results in the perturbation of the droplet interface because of the rapid diffusion of acetone into the aqueous phase. 

Surface-active agents may be added during the processing of films (internal addition) or by surface treatment of the film (external addition). These tend to reduce the surface energy of the film/water droplet interface promoting a continuous film of water thus enhancing transparency. Examples include hydrophilic surfactants, such as sorbitol or glycerol fatty acid mono- or di-esters. 

In this model, two level-set functions (d, p) are defined to represent the droplet interface (d) and the moving particle surface (p), respectively. The free surface of the droplet is taken as the zero in the droplet level-set function 0> and the advection equation (Eq. (3)) of the droplet level-set function (droplet surface. The particle level-set function (4>p) is defined as the signed distance from any given point x in the Eulerian system to the particle surface ... 

The lifetime of double emulsions is considerably shortened by the rapid diffusion of the water-soluble small-molecule surfactants toward the droplet interface. 

On the basis of these results and predictions, a diffusion-adsorption model has been proposed to explain the results of water-to-droplet MT of FeCp-PrOH [53,97]. Assumptions of the model are as follows. Mass transfer of FeCp-X across the droplet/water interface competes with adsorption on the droplet interface. As illustrated in Figure 20, electrolysis of an FeCp-X/NB droplet renders distribution of the molecule to the water phase as FeCp-X+ at t = 0. At = , although FeCp-X transfers to the droplet interface with a diffusion-limited rate in water, redistribution of the molecule to the droplet interior competes with adsorption on the droplet/water interface. When the droplet surface is occupied by adsorbed FeCp-X to some extent at t = t", distribution of the compound to the droplet interior is assumed to be controlled by the fraction of the interfacial area adsorbed by FeCp-X (r/rx), where V is the amount of FeCp-X adsorbed on the droplet/water interface. 

In the pharmaceutical industry, it is common to immediately suspend a portion of sample in solutions of a small-molecule surfactant. The surfactant is expected to rapidly adsorb at incompletely covered droplet surfaces to prevent droplet coalescence between sample withdrawal and analysis of droplet size or concentration. However, the addition of small surfactant molecules can result in a displacement of the original emulsifier from the droplet interface and profoundly alter droplet-droplet interactions. Changes in system composition may therefore lead to greater errors than those generated by the lag between sample withdrawal and analysis (see Background Information, discussion ofOstwald ripening). 

To determine the adsorption kinetics, the effective age of the drop interface must be calculated. However, experimental data yield only interfacial tension values as a function of drop formation time. To determine the true age of the interface, both the fluid flow within the droplet and the dilation of the droplet interface must be interpreted using appropriate models. Miller et al. (1992) showed that the drop formation time, r op, can be converted into the effective age of the drop interface ... 

In the limiting case of quiescent small bubbles or droplets, the transfer coefficients vary inversely with average bubble or droplet diameter. For example, in heat transfer from a droplet interface to a gas, the minimum value is... 

The computational fluid dynamics investigations listed here are all based on the so-called volume-of-fluid method (VOF) used to follow the dynamics of the disperse/ continuous phase interface. The VOF method is a technique that represents the interface between two fluids defining an F function. This function is chosen with a value of unity at any cell occupied by disperse phase and zero elsewhere. A unit value of F corresponds to a cell full of disperse phase, whereas a zero value indicates that the cell contains only continuous phase. Cells with F values between zero and one contain the liquid/liquid interface. In addition to the above continuity and Navier-Stokes equation solved by the finite-volume method, an equation governing the time dependence of the F function therefore has to be solved. A constant value of the interfacial tension is implemented in the summarized algorithm, however, the diffusion of emulsifier from continuous phase toward the droplet interface and its adsorption remains still an important issue and challenge in the computational fluid-dynamic framework. 

Fig. 14.3 Analytical characterization of an emulsion formulation of GLA (EAS) indicates the localization of the TLR4 agonist, (a) Aqueous phase extraction followed by HPLC indicates that GLA is found in the oil phase of the emulsion, (b) A decrease in zeta potential due to incorporation of GLA in the emulsion indicates that the TLR4 agonist associates with the oil droplet interface. ES represents the emulsion alone. Reproduced with permission from Anderson et al. (2010) Coll Surf B 75, 123-132...

Oil-acrylate hybrid-emulsions were formed using the fatty-acid hydroperoxides as initiator system for the miniemulsion polymerization of acrylate. The initiation took place at the droplet interface and resulted in the formation of triglycide modified polyacrylate molecules which act as compatibili-... 

In Eq. (1) R is the radius of the homogeneously formed sphere, 6 is the wetting angle, yi2, y 13, and y23 are the specific free surface energies at the solution substrate, solution droplet and substrate]droplet interface boundaries, respectively, k is the specific free line energy (or line tension) at the droplet periphery and could be either zero or a positive, or a negative quantity [v, vii-x]. 

The crosslinking of starch at the droplet interface in inverse miniemulsion leads to the formation of hydrogels. The formulation process for the preparation of crosslinked starch capsules in inverse miniemulsion is schematically shown in Fig. 10. The influence of different parameters such as the amount of starch, surfactant P(E/B-fe-EO), and crosslinker (2,4-toluene diisocyanate, TDI) on the capsule size and stability of the system were studied. The obtained capsules were in a size range of 320-920 nm. Higher amounts of starch and surfactant result in a smaller capsule size. The TEM images of crosslinked starch capsules prepared with different amount of crosslinker (TDI) are presented in Fig. 11. The nanocapsules can be employed as nanocontainers for the encapsulation of dsDNA molecules with different lengths [114] and for the encapsulation of magnetite nanoparticles. 

When enzymes are added to a mixture of two incompatible aqueous polymer solutions, they often partition to one of the phases (96, 97). Mixing produces a fine emulsion with droplets of one of the phases distributed in the other continuous phase. The enzyme that partitions into a droplet may be regarded as being temporarily immobilized (98). Mass transfer across the droplet interface is facilitated compared to traditional immobilized systems. Such systems are... 

Efforts to visualize the structure formation reaction by means of electron micrographs delivered an interesting observation While at the start of the reaction the protein seems to be enriched at fat droplet interfaces, a reduction in protein concentration around fat droplets seems to have taken place (Figure 19.17). The proteins seem to approach a more stable situation assembling a network within the continuous phase, rather than staying at the interface, which seems to explain the viscosity increase. 

Zhang, B., McDonald, C. and Li, L. (2004) Combining liquid chromatography with MALDI mass spectrometry using a heated droplet interface. Anal. Chem. 76, 992-1001. 

Solid Fat. The consistency and the emulsion stability of margarine and most other table spreads depends on crystallized fat. Freeze-fracture electron microscopy of deoiled margarine shows the crystalline nature of the water droplet interface as well as a continuous fat matrix that appears to be an interconnected network... 

Unsteady-state mathematical model based on the advancing front model of Ho et al. [3] considers a reaction front to exist within the emulsion globule and assumes instantaneous and irreversible reaction between the solute and the internal reagent at the membrane-internal droplet interface. 

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