Faster dewetting of water from C8- than from C18-bonded silica particles used in reversed-phase liquid chromatography: Solving the paradox

Abstract

For comparable surface coverage of alkyl-bonded chains (∼3 μmol/m2), the dewetting of 100% aqueous mobile phases from the mesopores of octyl(C8)-bonded silica particles is found 70 times faster than that from the same but octadecyl(C18)-bonded silica particles. This observation was made in this work for both fully porous (5 μm Symmetry) and superficially porous (2.7 μm CORTECS) particles. This experimental result is paradoxical because (1) the average pore size of C8-bonded materials is 10–15 Å larger than that of C18-bonded materials for the same unbounded silica gel and (2) the contact angle of water measured on smooth and planar C8-bonded surface is about 6° smaller than that on the same but C18-bonded surface (104° versus 110°). The equilibrium Laplace pressure is then expected to be smaller and the kinetics of water dewetting to be slower for silica-C8 than for silica-C18 stationary phases used in RPLC. The solution to this riddle is investigated based on (1) the calculation of the dewetting time assuming that the pores are monosized and the process is driven by the Laplace pressure, (2) the measurement of the advancing and receding contact angles of three different C18- and C8-bonded silica gels (4 μm NovaPak, 5 μm Symmetry, and 2.7 μm CORTECS) from the water porograms measured in a range of water pressure from normal pressure to 500 bar, and (3) on the calculation of the pore connectivity for both C8 and C18-bonded silica. First, the experimental results show that the observed dewetting times are of the order of minutes or even hours instead of millisecond as predicted by the dewetting model. Secondly, the advancing and receding contact angles of water onto the C8-bonded silicas are found larger (by an average of +7° and +2°, respectively) than those measured for the same but C18-bonded silica (average of 112° and 92°). Finally, the calculated pore connectivity is decreasing by about 30% for 90 Å unbounded silica materials from C8 to C18-bonded RPLC phases. Overall, the observed and much faster dewetting of water from C8 column than that from C18 column is primarily explained by a higher internal pore connectivity due to the thinner thickness of the alkyl-bonded layer (7 Å versus 15 Å) and, to a lesser extent, by a higher extrusion Laplace pressure of water (≃+10 bar).