tween glycine and boehmite. We identified the presence of The adsorption of a-alanine on boehmite is effected at room glycine adsorbed on the surface of boehmite at pH 6.0 in temperature in aqueous solutions at different pH values (2.5, 6.0, both zwitterionic (a) and noncharged (b) forms. The inter-12.2). The different forms of a-alanine prevalent in the different action was seen to proceed via hydrogen bonding in which media result in different modes of adsorption, different uptakes the surface hydroxyls are the proton acceptors. At pH 2.5 that generally increase with pH, and different responses of the the b form was identified on the surface of boehmite, and pore structure of boehmite. The dominance of the zwitterionic at pH 12.2 the glycinate anion interacted coordinately form at pH 6.0 results in an adsorption mode that permits a threethrough the lone pair of electrons on the NH 2 group.
dimensional array of crystalline a-alanine to be built on the surface a-Alanine resembles glycine as an a-amino acid; their of boehmite while allowing hydrogen bonding with surface hydroxyls through the carboxylate anion end, which acts as a proton pK a and pK b are practically the same, and consequently acceptor. The formation of a well-crystallized a-alanine is less both are characterized by nearly the same iep (Ç6.0). The probable at pH 2.5, where the adsorption of the two prevailing replacement of one hydrogen of the methylene group in forms, CH 3 CH(N / H 3 )COO 0 and CH 3 CH(N / H 3 )COOH, makes glycine with a methyl group in a-alanine appears to increase them unable to interact in a way that permits a three-dimensional molar volume. Furthermore, the absence of counterions in framework of hydrogen bonding between COO 0 as proton acaqueous solutions of amino acids causes the molecules to ceptor and N / H 3 as proton donor. At pH 12.2 the prevailing attract the water molecules nearby and the charged ends CH 3 CH(NH 2 )COO 0 is adsorbed coordinately on Lewis acid sites to neutralize their charges. This electrostriction leads to a represented by the surface Al ions. Such a mode of adsorption is decrease in the partial molar volume of the acid upon dissomore disperse than that occurring at lower pH values, causing an lution, which implies that electrostriction in the liquid has increase in the uptake and noncrystallinity of adsorbed alanine.
an even greater effect than close packing in the solid state.
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 electrostriction volumes per mole for glycine and athe three-dimensional order, due to limited space, of adsorbed alanine are 13.5 and 12.7 cm 3 , respectively (4). Although alanine. In alkaline medium the adsorption takes place mainly in the difference is not large, it indicates a difference in the larger pores, which results in a lower loss of area than expected. ordering of the water molecules around the charged ends of The results are compared with those obtained previously using the two acids. This difference in mode of hydration may glycine as the adsorbate, and the differences are discussed in terms cause the two acids to differ in their reactivity toward adsorpof the characteristics of hydration in aqueous media. ᭧ 1997 Academic tion on boehmite.
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In the present article we describe our study of the interaction between a-alanine and boehmite through the characterization of boehmite impregnated in aqueous a-alanine solu-