AN INVESTIGATION OF THE SURFACE-PROPERTIES OF ANODIZED ALUMINUM BY INVERSE GAS-CHROMATOGRAPHY

Inverse gas chromatography was used to characterize the oxide surface of anodized aluminum samples which had been conditioned at two differe nt temperatures. n-Alkanes, benzene, their perfluorinated analogues, a nd methylene chloride were used as probe molecules. A comparison of th e enthalpies of adsorption of the alkanes and perfluoroalkanes indicat ed that the surfaces of the anodized aluminum are comparable to those of aluminum which had been chemically treated in acidic or neutral sol ution. DELTAH(sp) and DELTAS(sp) values for C6H6, C6F6, and CH2Cl2, wh ich can interact with the surface through nondispersive interactions, were determined from a study of the temperature dependence of the DELT AG(sp) values (free energy of specific, or acid-base, interactions). I t appears that these values are strongly affected by the chemical envi ronment created by the pore structure which results from electrochemic al anodization. All three probes exhibited more exothermic nondispersi ve interactions for the anodized surfaces than for the chemically trea ted surfaces. Conditioning the anodic oxide at a higher temperature ma de these interactions stronger for the acidic benzene and methylene ch loride probe molecules, but diminished the interactions with the sligh tly basic perfluorobenzene probe molecule. In contrast to chemically t reated samples, a significant portion of the total enthalpy of adsorpt ion on the anodic oxide could be attributed to acid-base interactions. The entropies of nondispersive interactions suggest that the probe mo lecules are more ordered on the surfaces of the anodized samples than on the surfaces of the chemically treated samples.