Adsorption of α-Alanine on Boehmite

Abstract

The adsorption of α-alanine on boehmite is effected at room temperature in aqueous solutions at different pH values (2.5, 6.0, 12.2). The different forms of α-alanine prevalent in the different media result in different modes of adsorption, different uptakes that generally increase with pH, and different responses of the pore structure of boehmite. The dominance of the zwitterionic form at pH 6.0 results in an adsorption mode that permits a three-dimensional array of crystalline α-alanine to be built on the surface of boehmite while allowing hydrogen bonding with surface hydroxyls through the carboxylate anion end, which acts as a proton acceptor. The formation of a well-crystallized α-alanine is less probable at pH 2.5, where the adsorption of the two prevailing forms, CH3CH(N+H3)COO−and CH3CH(N+H3)COOH, makes them unable to interact in a way that permits a three-dimensional framework of hydrogen bonding between COO−as proton acceptor and N+H3as proton donor. At pH 12.2 the prevailing CH3CH(NH2)COO−is adsorbed coordinately on Lewis acid sites represented by the surface Al ions. Such a mode of adsorption is more disperse than that occurring at lower pH values, causing an increase in the uptake and noncrystallinity of adsorbed alanine. The narrow pores where surface hydroxyl density is high are most affected by adsorption at pH 2.5 and 6.0, which facilitates building the three-dimensional order, due to limited space, of adsorbed alanine. In alkaline medium the adsorption takes place mainly in larger pores, which results in a lower loss of area than expected. The results are compared with those obtained previously using glycine as the adsorbate, and the differences are discussed in terms of the characteristics of hydration in aqueous media.