A computational study of the porosity effects in silica monolithic columns

We report on a theoretical study of the influence of the through-pore porosity on the main chromatographic performance parameters (reduced theoretical plate height, flow resistance, and separation impedance) of silica monoliths. To investigate this problem devoid of any structural uncertainties, computer-generated structural mimics of the pore geometry of silica monolithic columns have been studied. The band broadening in these synthetic monoliths was determined using a commercial Computational Fluid Dynamics (CFD) software package. Three widely differing external porosities (epsilon = 0.38, epsilon = 0.60, and epsilon = 0.86) are considered and are compared on the basis of an identical intra-skeleton diffusivity (D-s = 5 x 10(-10) m(2)/s), internal porosity (epsilon(int) = 0.5), and for the same phase retention factor (k' = 1.25). Since the data are obtained for perfectly ordered structures, the calculated plate heights and separation impedances constitute the ultimate performance ever to be expected from a monolithic column. It is found that, if silica monoliths could be made perfectly homogeneous, domain size-based reduced plate heights as small as h(min) approximate to 0.8 (roughly independent of the porosity) and separation impedances as small as E-min approximate to 130 (epsilon = 0.60) and E-min approximate to 40 (F = 0.86) should be achievable with pure water as the working fluid. The data also show that, although the domain size is a much better reduction basis than the skeleton size, the former is still not capable of bringing the van Deernter curves of different porosity columns into perfect agreement in the C term dominated velocity range. It is found that, in this range, large porosity monoliths can be expected to yield smaller domain size-based reduced plate heights than small porosity monoliths.