Droplet dimension

It seems that coalescence process is very delayed by the emulsifier. In order to investigate the flocculation process, a study through an optical microscopy of the droplets and flocks has been employed. Unfortunately, a little difference in the contrast (brown against pale yellow) between droplets and continuous phase has not permitted a good determination of mean droplets dimensions. Probably, mean diameter should be few microns (i-5). 

Equations 1.2 and 1.5 both show a decrease of the droplets dimension by increasing the solution conductivity. These relationships are particularly relevant because in solution, when different electrolytes are present, the conductivity K may be obtained as the sum of the conductivities due to the different species and is proportional to the ion concentration ... 

This relationship showed all factors that can influence the droplet dimensions, and consequently the effectiveness of ESI. 

Hessel [79] has proposed a mechanism of spontaneous emulsification which involves a two-step process (1) the monomer phase swells the bilayers of the unilamellar vesicl which results in microemulsion droplets budding off the bilayer, and (2) the microemulsion droplets swell to miniemulsion droplet dimensions (50-400 nm). Diffusion of the monomer through the aqueous phase is... 

In distinction from macroemulsions, where the kinetic stability is the manifestation of droplet-droplet hydrodynamic interaction and droplet deformation, in miniemulsions the kinetic stability is the manifestation of the interplay between surface forces and Brownian movement (23). As the molecular forces of attraction decrease linearly with decreasing droplet dimension, namely, approximately 10 times at the transition from macroemulsions to miniemulsions, the potential minimum of droplet-droplet interaction (secondary minimum) decreases, and for miniemulsions this depth can be evaluated as 1—5 kT (12, 37). At this low energy. Brownian movement causes droplet doublet disaggregation after a short time (the doublet fragmentation time,7j). If this time is shorter than the lifetime of the thin film, rapid decrease in the total droplet concentration (t.d.c.) is prevented (restricted by the coalescence time, r ), i.e., stability is achieved due to this kinetic mechanism (23). 

Figure 6 Dependence of doublet lifetime on the Stem potential for different electrol)fte concentrations and droplet dimensions. Numbers near curves correspond to droplet radius. (1) Curves 1 -4 without accoxmt for retardation of molecular forces of attraction, T = exylkT (2) curves 1 =4 with accoimt for retardation. (From Ref. 26.)...

We exclude from consideration a special case of extremely dilute emulsions. Comparing Fig. 6 with the results of calculations according to Eq. (51) one concludes that condition (49) is mainly satisfied. It can be violated if simultaneously the droplet volume fiaction and the droplet dimension are very small. This occurs if < 10 and a < (0.2—0.3) pm. Discussing this case we exclude from consideration flie situation when a < 0.1 pm, corresponding to microemulsions and [Pg.86]

The main simplification in all existing models for emulsion dynamics (23, 39) is the neglect of the coalescence time dependence on droplet dimensions. This simplification is not justified and decreases very much the value of the prediction, which can now be made with use of the PBE. For elimination of this unjustified simplification it is necessary to determine the coalescence time for emulsion films between droplets of different dimensions i and j, namely, Tpy, similar to flie existing analytical expressions for the doublet-fragmentation time, (12). The determination of a large set of values by means of a comparison of ex-... 

EDM with experiments using l.d.c. emulsions and s.d.e. may result in (1) the quantification of emulsion film stability, namely, the establishment of the coalescence time dependence on the physicochemical specificity of the adsorption layer of a surfactant (polymer), its structure, and the droplet dimensions. This quantification can form a basis for the optimization of emul-sifier and demulsiner selection and synthesis for emulsion technology applications, instead of the current empirical level applied in this area and (2) the elaboration of a commercial device for coalescencetime measurement, which in combination with EDM will represent a useful approach to the optimization of emulsion technology with respect to stabilization and destabilizatioa... 

TABLE 1 Conductometricaily Estimated Water Pool Size Overall Droplet Dimension (R ), Surfactant Aggregation Number (A,), and Cosurfactant Aggregation Number (A,)per Droplet tor a Number of W O Microemulsions at 293 K at Constant Weight Fraction of Water = 0.2 and Surfactant Cosurfactant Weight Ratio = 0.5... 

The most spectacular effect is that of zero interfacial tension. Such an effect would lead, in the case of a two-component system and in the absence of other constraints, to a total molecular dispersion of the two constituents. For a pseudo-ternary system, e.g., water/oil plus some surfactant system located on the interface, this effect may produce an unusual dispersion state in which droplet dimensions are of the order of 100 A. Such transparent and fluid systems are known as microemulsions. Over the past few years, these systems... 

The droplet diameter ranged from 4 to 11 jm, with a predominant population around 7 pm. As expected, the droplet density grows along with amount of liquid crystal, while the droplet dimension is not significantly affected. 

In addition to the understanding of droplet interfacial properties, the dilntion method can as well shed light on the structural aspects of the system viz, droplets dimension, their population, amphiphile compositions at the interface, etc. Snch information has been found to corroborate with results of DLS, SANS, SAXS, NMR, and other sophisticated techniques [25,26,44 and references therein]. The rationale behind such analysis along with typical results is presented in what follows. 

W/o microemulsions have been used for many years as microreactors for the synthesis of ultrafine metallic particles [78, 79]. Since the pioneer works of Boutonnet et al. [80], who studied the production of colloidal Pt, Pd, Rh, and Ir particles by hydrogen or hydrazine (N2H4) reduction in w/o microemulsions, many studies have been made on the synthesis of this type of material. A reverse micelle (microemulsion) method, as a kind of soft technique, is a suitable way for obtaining the uniform and size controllable nanoparticles. The droplet dimension was modulated by various parameters, in particular W [81]. Some studies indicated that with the assistant of cosurfactant, the size of nanoparticles prepared in quaternary reverse micelle system is more controllable [82]. For example, compared with the anionic (AOT) ternary reverse micelle system, the droplet dimension of the quaternary cationic (cetyltrimethyl-ammonium bromide, CTAB) reverse micelles can be elaborately adjusted by changing W with the additional modulation of cosurfactant at the interface of water and oil. The microstmcture and djmamic exchange process are dominated by the influence of cosuifactant on the curvature radius and interface rigidity of the droplets in the quaternary reverse miceUe [82]. 

Microfluidic generation of polymer particles starts from the emulsiflcation of liquid monomers, oligomers, or polymers. This step is followed by polymerization, cross-linking, or physical gelation of the molecules compartmentalized in droplets. Dimensions of polymer microbeads are predetermined by the dimensions of precursor droplets, which are in turn controlled by the geometry and dimensions of microchannels in MF droplet generator, the mechanism of droplet formation, the macroscopic properties of the droplet and continuous phases, and the relative volumetric flow rates of the continuous and droplet phases. 

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