Interactions of L-alanine with alumina as studied by vibrational spectroscopy

The interactions of L-alanine with gamma- and cc-alumina have been investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). L-Alanine/alumina samples were dried from aqueous suspensions, at 36.5 degrees C, with two amino acid concentrations (0.4 and 0.8 mmol g(-1)) and at different pH values (1, 6, and 13). The vibrational spectra proved that the nature Of L-alanine interactions with both aluminas is the same (hydrogen bonding), although the groups involved depend on the L-alanine form and on alumina surface groups, both controlled by the pH. For samples prepared at pH 1, cationic L-alanine [CH3CH(NH3+)COOH] displaces physisorbed water from alumina, and strong hydrogen bonds are established between the carbonyl groups of alanine, as electron donors, and the surface Al-OH2+ groups of alumina. This occurs at the expense of alanine dimer dissociation and breaking of intramolecular bonds. When samples are prepared at pH 6, the interacting groups are Al-OH2+ + and the carboxylate groups of zwitterionic L-alanine [CH3CH(NH3+)COO-. The affinity of L-alanine toward alumina decreases, as the strong NH3+center dot center dot center dot-OOC intermolecular hydrogen bonds prevail over the interactions with alumina. Thus, for a load of 0.8 mmol g(-1), phase segregation is observed. On alpha-alumina, crystal deposition is even observed for a load of 0.4 mmol g(-1). At pH 13, the carboxylate groups of anionic L-alanine [CH3CH(NH2)COO- are not affected by alumina. Instead, hydrogen bond interactions occur between NH2 and the Al-OH surface groups of the substrate. Complementary N-2 adsorption-desorption isotherms showed that adsorption Of L-alanine occurs onto the alumina pore network for samples prepared at pH I and 13, whereas at pH 6 the amino acid/alumina interactions are not strong enough to promote adsorption. The mesoporous structure and the high specific surface area of gamma-alumina make it a more efficient substrate for adsorption Of L-alanine. For each alumina, however, it is the nature of the specific interactions and not the porosity of the substrate that determines the adsorption process.