Effect of the surface coverage of endcapped C18-silica on the excess adsorption isotherms of commonly used organic solvents from water in reversed phase liquid chromatography

The excess adsorption isotherms of methanol, ethanol, 2-propanol, acetonitrile, and tetrahydrofuran from water were measured on five different silica-based packing materials by the minor disturbance method. These materials were prepared with the same lot of 5-microm particles (average pore size 90 A), all endcapped with trimethylchlorosilane (TMS), and bonded to octadecyl chains with different surface coverages (0, 0.42, 1.01, 2.03, and 3.15 micromol/m2). The relative adsorption of one eluent by respect to a second one informs on the heterogeneity of the material (alkyl-bonded and bare silica regions) and on the accessibility of the unreacted silanol groups to the mobile phase. It is shown that the total surface area of the adsorbent decreases with increasing degree of surface coverage with octadecyl chains and that the relative surface area of the regions occupied by accessible silanol groups to the regions occupied by alkyl-bonded groups decreases. For the five columns, an average of 10% of the adsorbent surface area is covered of bare silica accessible to the liquid phase, with a minimum of 5% with tetrahydrofuran and a maximum of 12% with ethanol or 2-propanol. Increasing the surface coverage by the C18 chains causes a significant increase of the attraction potential of the hydrophobic surface toward the organic solvent. This result is confirmed by the increase of the number of adsorbate monolayers with increasing bonding density of the octadecyl chains. This number is twice larger for the 315C18 column than for the C1 column.