Sephadex^® & Darcy’s Law

Sephadex^® G-10, G-25, and G-50 can be assumed to behave as rigid spheres in SEC and therefore obey Darcy’s Law. This Law describes a general relationship for flow in porous media:

U = K × ΔP × L^-1 Equation (1)

U = linear flow rate expressed in cm/h
ΔP = pressure drop over the packed bed expressed in cm water
L = bed height expressed in cm
K = constant of proportionality depending on the properties of the bed material and the buffer Assuming a buffer with viscosity of 1 cP: U = K_o × ΔP × L^-1 Equation (2)
Ko = the “specific permeability” depending on the particle size of the medium and the water regain

Note that flow is proportional to the pressure drop over the bed and, assuming a constant pressure head, inversely proportional to the bed height. In practice this means that the pressure/flow considerations that must be made when using other SEC media do not apply to Sephadex^® and that a doubling of flow rate leads to a doubling in column pressure. To a good approximation, flow rate is independent of the column diameter.

Flow at viscosities greater than 1 cP can be obtained by using the relationship: flow rate is inversely proportional to viscosity. High buffer viscosities can be compensated for by increasing the operating pressure to maintain a high flow rate.

Theoretical flow (not maximum) can be calculated from equation (2) by inserting values for ΔP and L. Specific permeabilities (K) are given in Table A2.1.