Calorimetric heats of adsorption at the silica/isooctane interface

Abstract

The heats of immersion of a well-characterized mesoporous silica gel (Davisil 62) in solutions of six organic compounds (2-methylnaphthalene, benzothiophene, 2-methylbenzothiophene, pyridine, isoquinoline, and quinaldine) in isooctane (2,2,4-trimethylpentane) have been measured as a function of surface coverage by batch solution microcalorimetry. Adsorption isotherms at the silica/isooctane interface were obtained by conventional surface excess methods. The heat of adsorption of gaseous pyridine on silica was derived by correcting the heat of adsorption from the liquid for the bulk heats of mixing and phase change and the measured heats of wetting of silica covered with varioius preadsorbed amounts of pyridine in pyridine/isooctane solution. Connection was then made to the gas-phase adsorption data available for pyridine by the use of a rigorous thermodynamioc cycle. All of the compounds studied appeared to have site-limited adsorption (∼ 1 adsorbate per 2 surface silanol groups). 2-Methylnaphthalene and its sulfur aromatic analogs had very similar isotherms and low heats of adsorption from isooctane, in agreement with chromatographic experience. The one- and two-ring N-bases had high adsorption heats from isooctane which varied strongly with coverage (pyridine) or with adsorbate size (pyridine vs two-ring). It is argued, based upon the heats and entropies of adsorption, that the optimum hydrogen bond is the plane of the silica surface and that the observed large adsorbate steric effect reflected the geometry sensitivity of the hydrogen bond.