Separation Speed, Friction Coefficients, and Viscosity

In Chapter 3 we found that all relative transport processes, whether induced by external fields or diffusion, proceed at a rate inversely proportional to the friction coefficient /. Since virtually all separation methods require a certain level of completion of transport, or a certain number of transport steps, the time scale of the separation is linked to the time scale of the required transport both ultimately hinge on the magnitude of /. This conclusion is valid whether one is using methods such as chromatography where the transport processes must maintain the distribution of components between phases at a point near equilibrium, or electrophoresis where transport proceeds only fractionally to equilibrium. 

By learning more about the nature of /, we can discover methods for changing solvent properties and other parameters of the system so that / can be minimized. This is an important consideration in the design of optimal high-speed separation systems. 

Fundamental to the description of the friction coefficient in liquids is Stokes law, which shows that / is proportional to the radius a of spherical particles undergoing transport and proportional to the viscosity 17 of the medium [24,25] 

This equation provides the important link between / and 17 noted in the last section. 

Avogadro s number J( appears in Eq. 4.42 because / is for one mole, as noted in Chapter 3. This equation leads to 2 x 1015 g/smol for small organic molecules in water. Because of a larger radius a, / may be one or two orders of magnitude greater than this for macromolecules. 

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