A study of the sorption characteristics at the solution-solid interface, and the hydrogen bonding characteristics of a series of para-substituted phenols

Abstract

A characterisation of the activated alumina used in the sorption studies has been attempted from results obtained by X-ray diffraction techniques, low temperature nitrogen sorptions and sorptions from the liquid phase. The sorption characteristics at an alumina-dioxan interface of a series of 4-substituted phenols (phenol, 4-methyl-, 4-t-butyl-, 4-chloro-, 4-nitro-, and 4-cyano-phenols) have been determined at 35 C.; solubilities in dioxan measured at 35 C.; experimental adsorption isotherms constructed and sorption saturation values estimated. In order to compare the sorptive affinity of the alumina surface for the phenols an index of sorption has been defined as "the number of moles of phenol sorbed at constant relative mole fraction of phenol in the mobile phase", and it has been demonstrated that this index can be related logarithmically to the Hammett sigma constant for the appropriate 4-substituent.The effects of 4-substitution on properties of the OH bond which reflect its hydrogen bonding tendency have been examined, such as association to dioxan in solution. Absorbance measurements in the near ultra-violet have enabled association constants in solution for the process substituted phenol + dioxin ↔ complex to be determined at 25 C. and 55 C. Comparisons have been made with other relevant characteristics of the OH bond, for example acidity, polarity and near infra red stretching frequency, again showing (as with the hydrogen bonding association constants) good correlation with the Hammett structural parameter. The results obtained suggest that in the sorption of 4-substituted phenols from dioxan solution onto an activated alumina the forces on the surface are the dominant ones, since the phenols are not adsorbed as complexes but as discreet molecules. Interaction with the surface appears to be through the mechanism of hydrogen bonding and the phenol molecules probably assume a vertical orientation on localised sorption sites.