FTIR-ATR study of the co-adsorption of benzophenone with sodium dodecyl sulfate and sodium dodecylbenzenesulfonate using Al 2O 3-coated optics

Abstract

The adsorption of sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate (SLABS), and the co-adsorption of those surfactants with a “probe” species — benzophenone (BPN) — at the Al 2O 3/aqueous solution interface were examined via polarized Fourier transform infrared (FTIR) attenuated total reflection (ATR) spectroscopy. The internal reflection element (IRE) was ZnSe, coated with 0.1–0.25 μm of Al 2O 3 deposited by r.f. sputtering. This technique allows in situ determination of adsorption isotherms and adsorbate structures from IR-ATR spectra of the surfactants. Spectra of the surfactant head groups may be analyzed at energies down to about 900 cm −1. The interface remains in contact with surfactant solution while the spectra are obtained. Solvation-induced band shifts for BPN co-absorbed with SDS or with SLABS, compared with band positions in various solvents, indicated that BPN resided in the alkyl region of the SDS or SLABS surface layer. The extent of co-adsorption was a function of pH and ionic strenght. The presence of the BPN reduced the adsorption density of both SDS and SLABS. Orientation of the probe molecules within the adsorbed SDS layer was determined from the linear dichroic ratios exhibited by polarized ATR spectra. The orientation of the BPN molecules changed significantly with solution conditions, but generally the long axis of the BPN molecules was normal to long axes of the surfactant alkyl chains. The surfactant alkyl chains were found to be inclined to the surface normal at angles of 42–50, and the phenyl rings of SLABS were inclined at 38–45, depending upon the conditions. The implications of these results for the IR spectral analysis of adsorbate structures are discussed.