Structural and electronic evolution of the As(OH)<SUB>3</SUB> molecule in high temperature aqueous solutions: An x-ray absorption investigation

International audienceThe geometrical and electronic structure of the arsenious acid molecule As(OH)3 in aqueous solutions has been investigated by x-ray absorption spectroscopy (XAS) within extended x-ray absorption spectroscopy (EXAFS) and x-ray absorption near edge structure (XANES), using realistic first-principle calculations in the latter case. This investigation was performed on aqueous solutions of arsenious acid from ambient to supercritical conditions (P=250 and 600 bars, T⩽500 °C) using a new optical cell. The analysis of the XAS spectra is consistent with (1) a constant As-O distance, (2) an opening of the O-As-O angles within the C3V pyramidal structure in the range 30-200 °C. This structural evolution comes along with a small decrease of the partial charges of the atoms in the As(OH)3 molecule. The explanation invoked for both structural and electronic modifications observed is the weakening of the interactions, through hydrogen bonds, between the As(OH)3 complex and water molecules. This is a fingerprint of the similar weakening of hydrogen bonding interactions in the solvent itself