Adsorptive interaction of certain beta-lactam antibiotics in aqueous solution: Interpretation by frontier orbital theory

The adsorption of certain beta-lactam antibiotics such as 7-aminocephalosporanic acid, 7-aminodeacetoxy cephalosporanic acid, cephalexin, cefadroxyl, cephalosporin-C, and 6-aminopenicillanic acid in aqueous solution is studied using two different types of polymeric resins and activated carbon as the adsorbents. Adsorption affinity expresses as the slope of the linear region of the isotherm for a solute is found to be different for different adsorbents, and this difference can be interpreted from sorbent surface chemistry and morphological structure. The adsorptive interaction on the polymeric resins and activated carbon was computed based on the Frontier Orbital Theory. Electronic states of the adsorbent and adsorbate were calculated using the semiempirical molecular orbital(MO) method from which the characteristic energy of adsorption in aqueous solution was estimated. Adsorption affinity was correlated by the ratio of characteristic energy to that of the reference adsorbate. It was found that charge transfer interaction plays an important role in the adsorption of beta-lactams in aqueous solution. The experimentally measured enthalpy of adsorption was also correlated by the ratio of the characteristic energy to that of the reference adsorbate. The enthalpy of adsorption seems to correlate well with the adsorptive interaction energy computed from molecular orbital theory.