Droplet Size from Interfacial Area

Calculation of Droplet Size from Interfacial Area 

If it is assumed that all surfactant and cosurfactant molecules are adsorbed at the interface, it is possible to calculate the total interfacial area of the microemulsion from a knowledge of the area occupied by the surfactant and cosurfactant molecules. 

Total interfacial area = Total number of surfactant molecules (n ) x area per surfactant molecule (A ) -H total number of cosurfactant molecules x area per cosurfactant molecule 

The total interfacial area A per kilogram of microemulsion is given by the expression. 

Dynamic Light Scattering (Photon Correlation Spectroscopy PCS) 

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Flow Reactors Fast reactions and those in the gas phase are generally done in tubular flow reaclors, just as they are often done on the commercial scale. Some heterogeneous reactors are shown in Fig. 23-29 the item in Fig. 23-29g is suited to liquid/liquid as well as gas/liquid. Stirred tanks, bubble and packed towers, and other commercial types are also used. The operadon of such units can sometimes be predicted from independent data of chemical and mass transfer rates, correlations of interfacial areas, droplet sizes, and other data. 

These share certain features such as relatively low efficiency and low cost relative to most process equipment. The energy required to produce the increase in area is typically less than 0.1 percent of the total energy consumption. This is because atomization is a secondary process resulting from high interfacial shear or turbulence. As droplet sizes decrease, this efficiency drops lower. 

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]. 

The larger interfacial area will have a significant total free energy as is also shown in the figure. If the interfacial tension is 35 mN/m, then by the time the droplet size is r=0.64 pm, the total energy will have increased from 0.05 J to 2.6 x 104 J. This 26 kj of energy had to be added to the system to achieve the emulsification. If this amount of energy cannot be provided, say... 

The gas-side mass-transfer coefficients kefl and ko increase with liquid feed rate or with gas velocity at each given position in the venturi scrubber and decrease at constant liquid rate and gas velocity with increasing distance from the point of liquid injection (J7, VI1). The values ofkifl generally increase with increasing liquid flow rate or gas velocity (often referred to as the velocity at the throat). However, ki,a will sometimes exhibit a maximum when the gas velocity increases the explanation is that, at higher gas velocities, an increase in turbulence in the throat of the venturi results in the formation of droplets smaller than the thin filaments first formed at lower gas velocities. Internal circulation is reduced in these smaller droplets, and there is also a reduction in the size of the zone of intense turbulence. These two phenomena lead to a maximum for the values of/cL. as found experimentally by Kuznetsov and Oratovskii (K15) and Virkar and Sharma (VI1). The values of the effective interfacial area a increase with both gas and liquid flow rates. 

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