Diffusion of drugs in solution

Diffusion is the process by which a concentration difference is reduced by a spontaneous flow of matter. Consider the simplest case of a solution containing a single solute. The solute will spontaneously diffuse from a region of high concentration to one of low concentration. Strictly speaking, the driving force for diffusion is the gradient of chemical potential, but it is more usual to think of the diffusion of solutes in terms of the gradient of their concentration. Imagine the solution to be divided into volume elements. Although no individual solute particle in a particular volume element shows a preference for motion in any particular direction, a definite fraction of the molecules in this element may be considered to be moving in, say, the X direction. In an adjacent volume element, the same fraction may be moving in the reverse direction. If the concentration in the first volume element is greater than that in the second, the overall effect is that more particles are leaving the first element for the second and hence there is a net flow of solute in the x direction, the direction of decreasing concentration. The expression which relates the flow of material to the concentration gradient (dc/dx) is referred to as Fick's first law:

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