Adsorption of Natural 3-Phenylpropenic Acids on Highly Dispersed Alumina Surface

It has been shown that adsorption of 3-phenylpropenic (cinnamic), 3-(4-hydroxyphenyl)propenic (coumaric), and 3-(3-methoxy-4-hydroxyphenyl)propenic (ferulic) acids on the surface of highly dispersed alumina is characterized by L-type isotherms according to the Giles classification. The maximum adsorption values (~3.3 × 10 –4 mol/g), as well as dissociation constants (p K COOH = 4.5 ± 0.1), are almost equal for all acids, thereby suggesting that their molecules are identically bound to the adsorbent surface via carboxyl groups. The pH dependences of the adsorption of cinnamic, coumaric, and ferulic acids are described by dome-shaped curves, and the positions of their maxima in the pH scale coincide with the thermodynamic constants of carboxyl group dissociation. A linear correlation found between the adsorption values of 3-phenylpropenic acids and their hydrophobicity parameters log P indicates a noticeable contribution of hydrophobic interactions to the adsorption of the acids on the surface of highly dispersed alumina.