## Diffusion

In addition to the roles of solubility and distribution ratios during the equilibrium portion of an extraction, diffusion is the largest factor influencing the extraction of solutes. Diffusion is that spontaneous, irreversible process by which a compound moves from an area of high concentration to an area of lower concentration, resulting in a concentration equilibrium within a single phase. More rigorously, the diffusional flow, J, of a compound is defined as the mass of the material of interest passing through a reference surface during a specified time, and laws of diffusion can correlate this diffusional flow with the concentration gradient responsible for the flow. If the rate of mass flow per unit area, or the diffusion flow, J, is in gcm_2s_1 and concentration is in molcm~3, Fick's first law of diffusion provides a correlation with the concentration gradient such that J =— D(Ac/Ax), where D is the diffusion coefficient (given in cm2s_1) and Ac/Ax is the concentration gradient (in g cm4, concentration c is in g cm~3 and area x is in cm2). Thus, Fick's first law defines a diffusion coefficient that is independent of solute concentration and is unique to every solute-solvent pair at constant temperature. Generally, this diffusion coefficient, D, is in the range 10~5-10~6 cm2 s_1 in liquid solutions, whether aqueous or organic. When a steady state cannot be assumed, the concentration change with time must be considered, leading to Fick's second law of diffusion, Ac/At = D(A2c/Ax2) = DV2moic, where t is time (s) and Vmol is molar volume. Thus in non-steady-state conditions the temporal rate of concentration change is proportional to the spatial rate of concentration change in the direction of the concentration gradient.

## 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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