Temperature Independent Adsorbate Structure of 4-Octyl-, 4-Decyl-, and 4-Dodecylbenzenesulfonates at the Al₂O₃/Water Interface

In situ Fourier transform infrared (FTIR)-attenuated total reflection (ATR) spectroscopy and linear dichroism (LD) analysis were used to characterize the Gibbs' surface excess (Γ) and molecular orientation (ϑ, mean tilt from the interface normal) of sodium 4-n-octylbenzenesulfonate (SOBS), sodium 4-n-decylbenzenesulfonate (SDeBS), and purified sodium 4-n-dodecylbenzenesulfonate (SDoBS) adsorbed at the Al₂O₃/water interface. The Γ values were confirmed by the solution depletion method. The ϑ values were determined separately for the phenyl rings, alkyl chains, and SO₃^- head groups. Thin films (150−220 nm) of Al₂O₃ on the surface of ZnSe IR internal reflection elements allowed in situ IR ATR adsorption measurements on a model hydrophilic solid from the near IR to 950 cm^-1. Both Γ and ϑ for SOBS and SDeBS were insensitive to ionic strength, but were slightly sensitive to pH and temperature. The crystal structure of SOBS·¹/₂H₂O was determined by single-crystal X-ray diffraction and compared with polarized single-crystal transmission and IR ATR spectroscopy to assist in assignment of IR bands at 1010, 1036, and 1125 cm^-1 to modes with transition moments parallel to the 1,4-axis of the phenyl ring (“axial” bands). Temperature-dependent transmission IR spectra of slurries of SOBS and SDeBS were obtained; crystalline hemihydrates of SOBS, SDeBS, and SDoBS were identified, along with monohydrates of SOBS and SDeBS. The Γ, ϑ, and band positions in temperature-dependent IR ATR showed no phase changes associated with the onset of alkyl chain motion at temperatures as much as 40 and 45 °C below the Krafft temperature, in the case of SDeBs and SDoBs, respectively; the IR spectra indicated that the environments of both the polar and nonpolar moieties of the surfactants closely resembled those in micelles at all temperatures examined. The IR bands associated with the crystalline surfactants were not observed in adsorbed layers of these surfactants. Absorptivities of the ν_as(SO₃^-) bands and ν(CH₂) bands were shown to be reliable for the determination of Γ in in situ IR ATR, but the absorptivities of the axial bands decreased upon adsorption, making them unreliable for this purpose.