Hydrogen bonding interactions of para-substituted phenols in solution and at a solid-solution interface.

The adsorption characteristics at alumina-silica-solution interfaces have been studied for solutions consisting of phenol dissolved in different solvents and for a series of p-substituted phenols dissolved in benzene. Characterisation of the adsorbent surfaces has been attempted by low temperature nitrogen adsorption, vapour phase adsorption of benzene and dehydration experiments. Molecular area requirements of the adsorptives have been determined from the adsorption isotherms and these indicate that the phenols are perpendicularly orientated to the adsorbent surfaces. By application of a thermodynamic equation derived by Everett on the basis of an ideal adsorption system, the adsorption affinities of the phenols for the silica surface have been calculated and are found to be related, on the one hand, logarithmically to the Hammett p-substituent parameter and on the other, to the solvent parameter ET when considering the adsorption of phenol itself. The virtual independence of the affinity of the p-substituted phenols for alumina has been attributed to the much stronger bonding of the phenols to the alumina surface than to the silica surface. Calculation of surface area values from Everett's equation using the experimentally determined molecular area requirements have shown that deviations from ideality arise principally from interactions in the adsorbed phase. Hydrogen bonding in the solution phase has been studied for phenol bonding to ether and alcohol type donors in non polar solvents. The logarithm of the association constant has been correlated with the solvent parameter ET, the change in standard free energy on association, (-?G0), increasing with increase in donor reactivity. The enthalpy change on association, ?H0, has been determined from the temperature coefficient of the association constant and found to be virtually constant for the complexes studied. Variations in ?G0 have been considered to arise principally from changes in the negative entropy of association, (-?S0), which increase as the steric complexity of the donor molecule increases, this trend being as predicted theoretically.