Characterization of bonded-phase silica gels with different pore diameters

Abstract

Three silica gels, manufactured in a similar fashion and with nominal pore diameters of 100, 200 and 250 Å, were cleaned of contaminants (< 100 ppm total), their surface was adjusted to ca. pH 4.5, and their particle sizes and particle size distributions were adjusted as closely as possible. Surface areas ( S BET); pore volumes ( V p(cum)) and median pore diameters ( D) were measured and compared. The silica gels were reacted with n-alkyldimethylchlorosilanes (C n H 2 n+1 )(CH 3) 2SiCl where n = 1,4,8 and 18. S BET, V p(cum), D and [silane] of the bonded silica gels were measured. It was found that the concentration of any one bonded silane increases as the pore diameter of the silica gel increases, and that the level of bonding decreases linearly as n increases. S BET and V p(cum) are reduced the greatest for a smaller diameter bonded silica gel, especially as n increases. The C 18-bonded silica gels were used to evaluate the effects of pore diameter on the high-performance liquid chromatography of small molecules (phenol, acetophenone, benzene) and large protein molecules (ribonuclease, insulin, lysozyme, serum albumin, ovalbumin). Capacity factor ( k′) and N/ m for small molecules slightly decreased (for non-polar molecules) as a function of the pore diameter and the pore volume of the bonded or bare silica gel (and increased as a function of the surface area of the bare silica gel). While resolution of small molecules decreased sharply as a function of both pore diameter and pore volume, the resolution of protein separation increased as the pore diameter and pore volume of the silica gel increased, the increase being the greatest for the higher molecular weight proteins.