Adsorption of Natural 3-Phenylpropenic Acids on Cerium Dioxide Surface

Resumo

It has been shown that adsorption of natural antioxidants, such as 3-phenylpropenic (cinnamic), 3-(4-hydroxyphenyl)propenic (coumaric), and 3-(3-methoxy-4-hydroxyphenyl)propenic (ferulic) acids, on the surface of cerium dioxide is characterized by S-type isotherms, while adsorption of 3-(3,4-dihydroxyphenyl)propenic (caffeic) acid is described by an H-type isotherm. A linear correlation between adsorption values and hydrophobicity parameter logP has been found in initial regions of the S-type isotherms, with such correlation being typical for parallel orientation of adsorbed molecules relative to an adsorbent surface. The maximum adsorption values (≈2.9 × 10^–4 mol/g), as well as dissociation constants (pK_COOH = 4.4–4.6), are almost identical for all acids, thereby suggesting that adsorbate molecules are predominantly bound to the adsorbent surface via carboxyl groups. The presence of hydroxyl groups in molecules of coumaric, ferulic, and caffeic acids widens the рН range of their adsorption as compared with cinnamic acid and promotes their oxidation on the cerium dioxide surface.