Diffusion in Liquids

In liquid systems, with small or medium-sized molecules in dilute solution, diffusion is highly dependent on viscosity, y, and consequently on temperature, T. Assuming a spherical particle, diffusion in liquids can be expressed by the Stokes-Einstein equation, D = (10^7T/yVm,l).

Figure 4 Schematic diagram of the role of interfacial diffusion in liquid-liquid extraction. Each stagnant layer is about 10~2-10~4cm. In this depiction, the molecules diffusing through the liquid-liquid interface contain a moiety (x) with an affinity toward phase 2 and a moiety (—) with an affinity toward phase 1.

Figure 4 Schematic diagram of the role of interfacial diffusion in liquid-liquid extraction. Each stagnant layer is about 10~2-10~4cm. In this depiction, the molecules diffusing through the liquid-liquid interface contain a moiety (x) with an affinity toward phase 2 and a moiety (—) with an affinity toward phase 1.

For the purposes of extraction, the rate of diffusion across the liquid-liquid boundary layer is of primary importance. This diffusion rate is dependent on solute shape and size and on solvent viscosity. Agitation or turbulence at the liquid-liquid interface can enhance the rate of diffusion across the phase boundary, but there is a practical limit to the degree of agitation in an extraction mixture. Figure 4 depicts the liquid-liquid system, including the stagnant films on either side of the phase boundary. In practical extraction examples the bulk phases are adequately stirred so that diffusion in the bulk phases can be neglected. However, the interfacial stagnant layers are about 10~2-10~4cm (compared with diffusion coefficients in the range 10~5-10~6 cm2 s-1) and must be considered as controlling the overall extraction kinetics. Moderate shaking or agitation can reduce the thickness of the stagnant, or stationary, films. If agitation is too vigorous, solutes in the mixture are given a high translational motion without an increase in the rate of solute movement to the phase interface. As phase dispersion increases, the relative velocity of the two phases decreases, until the limiting case of an emulsion (in which relative velocity becomes zero) is reached.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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