Structure of Clays and Polymer–Clay Composites Studied by X-ray Absorption Spectroscopies

Abstract

A wide range of spectroscopic techniques employ higher-energy electromagnetic radiation, ranging from vacuum UV (≈10−40 eV, 125−31 nm), including soft X-rays (40−1500 eV, 31−0.8 nm), and going to hard X-rays (1500−105 eV, 0.8−0.01 nm) for elucidating molecular structures of chemical and biological interest. A typical X-ray absorption (XAS) spectrum has a large absorption near the edge followed by serial oscillations that gradually fade away. This set of oscillations extends over a wide energy range and can be divided into two regions: the absorption near the edge is called XANES (X-ray absorption near-edge structure) and the second region is the so-called EXAFS (extended X-ray absorption fine structure). The XAS data enables the determination of crystallographic parameters and also the signal intensity con‐ tains information of the oxidation state and the chemical bond in the solid. For in‐ stance, theoretical calculations were essential to verify the differences between the oxygen and silicon sites in clays. Experimental and theoretical EXAFS studies of clays with Cu(II) show that Cu(II) has interchangeable octahedral, tetragonal, and square planar coordinations in the clay interlayer, depending on Cu(II) loading and degree of hydration. XANES data of intercalated poly(aniline) show new bands at 398.8 eV and 405−406 eV, which were assigned to new chromophoric segments formed within the galleries of the Montmorillonite clay. Hence, in this chapter, this amazing new area will be reviewed concerning the state-of-the-art results of charac‐ terization of their structural features. Previous and new results of the X-ray absorp‐ tion spectroscopy of clays and polymer–clay materials obtained by our group will be considered. The main goal of this work is to contribute to the rationalization of some important results obtained in the open area of clays and clay materials charac‐ terization.