Selective adsorption of methyl esters of n-fatty acids at the silica/benzene and silica/carbon tetrachloride interface. Part 2.—Heats of adsorption

Abstract

The heats of adsorption of a series of n-fatty acid methyl esters at the silica/benzene and silica/tetrachloride interfaces have been determined calorimetrically. The heats of solution of the esters have also been determined in C₆H₆ and CCl₄. In all cases the heat of immersion increases as the solution concentration increases, until the limiting adsorption value is attained. Thereafter, the heat of immersion remains constant. The preferential molar heats of adsorption from both solutions are independent of the surface coverage of the adsorbate. The heats of adsorption at 25°C from the C₆H₆ solutions show no detectable dependence on the chain length of the ester. A similar lack of dependence on chain length is observed at 50°C, although there is a decrease in the value of the heats of immersion. This latter is in agreement with the hypothesis that there is a nett decrease in order of adsorbed layer as the adsorption temperature is increased. Essentially similar results were found with the CCl₄ solutions at 25°C, although the values of the preferential molar heats of adsorption were higher from this particular solvent. For both solvents removal of the H-bonded surface silanol groups by prior heating of the adsorbent caused no significant change in the measured heats of adsorption, which suggests that the adsorption centres are the isolated surface silanol groups of the adsorbent.