Temperature Dependent Structure of Adsorbed Sodium Dodecyl Sulfate at the Al2O3/Water Interface

Abstract

The structure of adsorbed sodium dodecyl sulfate (SDS) on Al 2 O 3 was observed by infrared attenuated total reflection (IR ATR) spectroscopy as a function of temperature, and the band positions (v), widths (Δv), and relative absorptivities (E) were compared with those of the various crystalline phases and the H α hexagonal and L α lamellar liquid crystalline (LC) phases of SDS. The structures of both bilayers and monolayers of SDS adsorbed onto Al 2 O 3 were most similar to that of SDS micelles; neither v nor Δv of adsorbed SDS IR ATR bands were similar to those of crystalline phases of SDS. IR 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 α and L α LC SDS phases; the e 2922 /e 2854 ratio was much higher than that in the LC phases. A loss ofgauche chain conformers was indicated by a bathochromic shift in the 2922 cm -1 CH2 band upon adsorption (∼2 cm -1 ), but further ordering did not occur down to 4 °C, which is well below the Kraft and critical micelle temperatures (12-15 °C ) for SDS in water. The previous, necessary assumption that e did not change significantly upon adsorption was shown to be valid for the CH X , bands down to 4 °C. The narrowed bands and increased e 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.