Droplets interdroplet interaction

The alternative NMR approach that has provided information on microemuisions is relaxation. However, on the whole, relaxation has been less informative than anticipated from earlier studies of micellar solutions and has provided little unique information on microemulsion structure, although in the case of droplet structures it is probably the most reliable way of deducing any changes in droplet size and shape, particularly for concentrated systems. The reason for this is that NMR relaxation probes the rotational diffusion of droplets, which is relatively insensitive to interdroplet interactions. This is in contrast to, for example, translational collective and self-diffusion and viscosity which depend strongly on interactions. Furthermore, NMR relaxation is a useful technique for characterizing the local properties of the surfactant film. 

Finally, relaxation is a reliable technique for monitoring changes in droplet size and shape, since it is to a good approximation independent of interdroplet interactions up to high droplet volume fractions. The reason for this is that relaxation arises from droplet reorientation and intradroplet surfactant diffusion, processes only weakly dependent on interdroplet interactions. 

Stability of a macroemulsion is an important factor as this determines its extent of usability for particle preparation or various other applications. Instability is basically coalescence of the dispersed phase droplets or Ostwald ripening (growth of large droplets at the expense of much smaller ones). When this process goes on, the emulsion eventually breaks into two layers. Other processes related to stability but considered less important [3] are (a) creaming or sedimentation, the rate of which is dependent on the difference in density between the continuous and dispersed phases, droplet size, viscosity of the continuous phase and interdroplet interaction and (b) flocculation, dependent on colloidal interactions between the droplets [8, 12]. Several factors determine the stability of macroemulsions these are discussed here in brief. This discussion is largely derived from Rosen [3] and some subsequent investigations [e.g. 6, 7, 13-15]. 

Two factors have been considered [245] to control the particle size in reverse microemulsions. One of them is the number of microemulsion droplets when the number is large, the average content of the reactants per droplet becomes low, causing the formation of a large number of nuclei in the system and finally, particles of small size. Another factor for small size is reduced interdroplet interaction and exchange of materials. This may take place due to steric hindrance offered by surfactant films, or their low deformability or strong attachment to droplets. A general experience, of course, is that the two-emulsion method yields smaller particles, especially in case of metals [242]. 

The unique density dependence of fluid properties makes supercritical fluids attractive as solvents for colloids including microemulsions, emulsions, and latexes, as discussed in recent reviews[l-4]. The first generation of research involving colloids in supercritical fluids addressed water-in-alkane microemulsions, for fluids such as ethane and propane[2, 5]. The effect of pressure on the droplet size, interdroplet interactions[2] and partitioning of the surfactant between phases was determined experimentally[5] and with a lattice fluid self-consistent field theory[6]. The theory was also used to understand how grafted chains provide steric stabilization of emulsions and latexes. 

M.J. Hou, M. Kim, and D.O. Shah 1988 A light scattering study on the droplet size and interdroplet interaction in microemulsions of AOT-oil-water system, /. Colloid Interf. Sci. 123, 398-412. 

In Figure 10.16 the average film radius vs. interdroplet distance is shown for emulsions with size a = 0.4 i.m, y = 0.1 mNm", /1h = 5 X 10 J and Po = 100 mV. Expressions (10.3), (10.9), (10.10), (10.29) and (10.62) were used to derive lF (r z). The full (upper) curve is for an electrolyte concentration equal to 0.01 M monovalent electrolyte, while the dotted one (lower) is for 0.005 m. In both cases the interaction energy is lowest in the absence of any deformation (r = 0). Still, a well defined peak is observed at small distances which is due to the reasons discussed above. It can also be seen that the film radius seems to have non-zero values at separations where the interdroplet interactions vanish. This is due to the fact that the surface of a flexible droplet will fluctuate even if there is no second droplet to interact with. If one examines expressions (10.29) and (10.31) it becomes evident that they do not depend on the droplet positions and therefore do not decay with the distance. This reflects in a simple way the fact that due to fluctuations the droplet surface could be greater than that of a sphere with the same volume (see ref. 66 for more detailed discussion). 

The objective of this paper is to illustrate the efficacy of inferring the interdroplet forces in a concentrated protein stabilized oil-in-water emulsion from the knowledge of the equilibrium profile of continuous phase liquid holdup (or, dispersed phase faction) when the emulsion is subjected to a centrifugal force field. This is accomplished by demonstrating the sensitivity of continuous phase liquid holdup profile for concentrated oil-in-water emulsions of different interdroplet forces. A Mef discussion of the structure of concentrated oil-in-water emulsion is presented in the next section. A model for centrifugal stability of concentrated emulsion is presented in the subsequent section. This is followed by the simulation of continuous phase liquid holdup profiles for concentrated oil-in-water emulsions for different centrifugal accelerations, protein concentrations, droplet sizes, pH, ionic strengths and the nature of protein-solvent interactions. 

Figure 3.6 Effect of Ca2+ content on predicted values of osmotic pressure (H, , left axis) of caseinate nanoparticles in emulsion continuous phase and the free energy of the depletion interaction (AGdep, , right axis) between a pair of emulsion droplets (

Shear-induced crystallization had a much greater effect in bulk systems than emulsified systems (Fig. 6) and resulted in an accelerated rate of crystallization. Prior to, and during, the initial stages of crystallization, intradroplet fat is protected from interdroplet crystallization by the spherical shape and pressure of the droplet and is not directly available to the shear field, i.e., no protruding crystals. This observation is consistent with microstructure work where limited destabilization was observed in droplets with no visible crystals. Initially, droplet interfaces in the PSCO system showed that the crystallized fat was not available at the surface, limiting the occurrence of crystal-induced flocculation and coalescence. Droplets remained stable until their interfaces were disturbed by the shear fleld or crystal interaction. 

The results show a transition from predominantly viscous to predominantly elastic response as

elastic interaction that occurs at and above this critical this volume fraction, the interdroplet distance is comparable to twice the thickness of PHS chains, resulting in their interpenetration and/or compression. As tp exceeds 0.7, the storage modulus increases very sharply with any further increase in repulsion between the water droplets. 

A large disparity exists between knowledge concerning kinetic stability and thermodynamic stability. The main attention has been paid to kinetic stability for both macroemulsions (16-22) and miniemulsions (23-30). As a result, the droplet-droplet interaction and the collective processes in dilute emulsions are quantified (38, 39) and important experimental investigations are made (27, 28, 40). Some models are elaborated for the entire process of coalescence in concentrated emulsions as well (41, 42). Given thermodynamic stability, a thin interdroplet film can be metastable. 

---