Mass transport properties of second-generation silica monoliths with mean mesopore size from 5 to 25 nm

The morphology of silica monoliths determines their mass transport properties. While eddy dispersion can be related to the size and structural heterogeneity of the macropores, longitudinal diffusion and trans-skeleton mass transfer resistance are influenced by the physical appearance of the mesopore space. We used two small analytes (uracil, naphthalene) and a large one (lysozyme) to characterize the column performance of a set of six second-generation analytical silica monoliths with systematically varied mean mesopore size from 5.5 to 25.7 nm. Within this set of sample columns, the mean macropore size was conserved at about 1.2 mu m. Longitudinal diffusion and trans-skeleton mass transfer resistance were derived from peak parking experiments. Both contributions to the overall plate height are affected by the structural hindrance in the mesopores, which increases with smaller mesopore size. For the weakly retained naphthalene, this effect is counteracted by surface diffusion, which increases with the surface area of the mesopore space. Additivity of individual plate height contributions allows for the determination of eddy dispersion by subtraction of the other terms from the overall plate height curves. Column performance for lysozyme is limited by mass transfer resistance, which increases strongly with smaller mesopore size until lysozyme becomes totally excluded from the smallest (5.5 nm) pores.