Fourier-Transform Infrared Study of Ethylene Glycol Monoethyl Ether Adsorbed on Montmorillonite: Implications for Surface Area Measurements of Clays

Infrared spectra of 2-ethoxyethanol (ethylene glycol monoethyl ether, EGME) adsorbed on Camontmorillonite were obtained using diffuse-reflectance infrared-Fourier-transform (DRIFT) and attenuated total-reflectance (ATR) spectroscopy. The molecular conformation of adsorbed EGME shows predominantly a gaucheconfiguration for the CH2-CH2linkage as originally present in the liquid. The orientation of the adsorbed EGME molecule in the interlamellar region of the clay was deduced from measurements of the polarized ATR spectra. The planar zig-zag skeleton of the molecule appears to have its long axis tilted at an angle of 44° to the clay surface; the molecule is in van der Waals contact with both adjacent surfaces. Further, the plane of the skeleton of the molecule is tilted at an angle of 69° to the clay surface. This orientation leads to estimates of the d-value of the clay-EGME complex and of the surface area occupied by each molecule. These values are in excellent agreement with independent measurements of both parameters. The full surface coverage on montmorillonite is represented by a single layer of tilted EGME molecules shared by two adjacent clay sheets; each molecule occupies a total surface area consisting of one portion (25.4 Å2) on the lower surface of the interlayer plus another portion (14.8 Å2) on the upper surface. The implication of this model for materials such as kaolinite, where adsorption takes place on the external surface only, is that on these materials each EGME molecule occupies 20 Å2at full surface coverage.