The structure of adsorbed sodium dodecyl sulfate (SDS) on Al2O3 was observed by infrared attenuated total reflection (IR ATR) spectroscopy as a function of temperature, and the band positions (nu), widths (Delta nu), and relative absorptivities (epsilon) were compared with those of the various crystalline phases and the H-alpha hexagonal and L-alpha lamellar liquid crystalline (LC) phases of SDS. The structures of both bilayers and monolayers of SDS adsorbed onto Al2O3 were most similar to that of SDS micelles; neither nu nor Delta nu of adsorbed SDS IR ATR bands were similar to those of crystalline phases of SDS. SR ATR linear dichroism results indicated that the terminal methyl groups were not in the trans conformation expected for crystalline SDS in either monolayers or bilayers of adsorbed SDS. Furthermore, ATR spectra of adsorbed SDS were dissimilar from both the H-alpha and L-alpha LC SDS phases; the epsilon(2422)/epsilon(2854) ratio was much higher than that in the LC phases. A loss of gauche chain conformers was indicated by a bathochromic shift in the 2922 cm(-1) CH2 band upon adsorption (similar to 2 cm(-1)), but further ordering did not occur down to 4 degrees C, which is well below the Krafft and critical micelle temperatures (12-15 degrees C) for SDS in water. The previous, necessary assumption that epsilon did not change significantly upon adsorption was shown to be valid for the CH, bands down to 4 degrees C. The narrowed bands and increased epsilon indicative of purely all-trans chain conformation were absent, but the loss of gauche conformers upon adsorption reinforced the all-trans chain model widely used to interpret linear dichroism results for adsorbates.