Sephadex and Darcy’s Law

Appendix 2, Extracted from Size Exclusion Chromatography Principles and Methods (PDF)
GE Healthcare, 2014

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 (see Appendix 5)
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 = Ko × 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.

Table A2.1. Specific permeabilities of Sephadex

Sephadex type Permeability K
Sephadex G-10 19
Sephadex G-25 Superfine 9
Sephadex G-25 Fine 30
Sephadex G-25 Medium 80
Sephadex G-25 Coarse 290
Sephadex G-50 Fine 36