Physicochemistry of Pure Lead(II) Soaps: Crystal Structures, Solid and Liquid Mesophases, and Glass Phases – Crystallographic, Calorimetric, and Pair Distribution Function Analysis

Abstract

A complete characterization of pure lead(II) soaps is crucial to their identification in the degradation of paintings. We present here our study on the physicochemical behavior and structure of the members of the lead(II) alkanoates series, from acetate to octadecanoate (stearate), in all the phases that they present: from the completely ordered phase (crystal) to the liquid phase. These soaps present two polymorphic structures in the crystal phase, now solved for all the compounds, one intermediate solid phase and a liquid crystal, prior to the isotropic liquid. In some members, different glass states (from different phases) are also found, with interesting photophysical properties. The compounds have been thoroughly analyzed as a function of temperature mainly by X-ray diffraction (powder and single crystal), differential scanning calorimetry and high-energy X-ray total scattering (for pair distribution function or PDF analysis), and other techniques like polarizing light microscopy, infrared and UV-Vis spectroscopy, and nuclear magnetic resonance. The use of new techniques that give information on the short-range order (like PDF analysis or X-ray absorption) for the characterization of the disordered phases can help in the identification of lead(II) soaps and other compounds, in general, in the initial stages of formation prior to crystallization.