From Pore Scale to Column Scale Dispersion in Capillary Silica Monoliths

We study time and length scales of eddy dispersion in 100 μm i.d. capillary silica monoliths. First, the monolith’s macropore morphology was visualized over complete column cross sections by confocal laser scanning microscopy, revealing a wall region with large voids (with a lateral dimension of up to ∼15 μm) and a homogeneous core region. A bulk segment from the core region was then physically reconstructed to receive a 60 × 12 × 12 μm matrix consisting of ∼3 × 108 cubic voxels of 30 nm edge length for direct numerical simulations of fluid flow by the lattice-Boltzmann method and convective-diffusive mass transport by a random-walk particle-tracking technique on a high-performance computing platform. Pore-scale dispersion was analyzed in detail using the generalized Giddings equation. Eddy dispersion contributions originating in the bulk macropore heterogeneity were quantified and correlated with structural features of the monolith. To complement the simulation results, column scale dispersion was investigated by analysis of chromatographic plate heights. We found a much smaller bulk dispersion than generally assumed for silica monoliths (plate heights of ∼2 μm over a wide velocity range), promising excellent separation efficiency also at high flow velocities (∼1 cm/s). This potential is not realized by the capillary monolith in chromatographic practice because of the wall defect.