Raman Spectroscopic Properties of Aqueous Chloride Salt Solutions: Chlorides of Alkalis, Alkaline Earths and First-Row Transition Metals

Abstract

Saline aqueous fluids play numerous key roles in many geologic processes, but how fluids flow and interact with rocks depends on their physical and chemical properties, which in turn depend on the types, concentrations, and speciation of solutes. Raman spectroscopy is a powerful tool to identify and characterize solutes in geologic fluids, for example by analysis of fluid inclusions in minerals, but our knowledge and understanding of the Raman spectroscopic properties of aqueous solutes is still limited. Here, we present a survey of the Raman spectroscopic properties of aqueous salt solutions at ambient temperature and pressure and over wide ranges of fluid composition, with a focus on chloride salts. Specifically, we analyzed solutions ranging from pure H2O up to salt saturation for 19 different chloride salts, mostly from group 1 (alkalis), group 2 (alkaline Earths), and the first-row transition metals. For each salt, we evaluated how increasing the concentration affects the Raman spectroscopic properties of the solvent H2O; and for many of the salts we also identify the characteristic Raman peaks that are distinct from those of H2O and thus reveal the formation of metal-ligand complexes in solution. Broadly, we find that the studied salts can be divided into two main categories: those that distort the OH-stretching region of the H2O spectrum but do not show evidence of ion pair formation; and those that show the emergence of new Raman peaks with increasing concentration indicative of ion pair formation. We find that the decisive factor that differentiates these two categories seems to be the ionic radius of the cation, with ion pair formation enabled when the ionic radius of the cation is less than about 100 pm. Moreover, we find that especially amongst the salts that do not show evidence for ion pair formation, the distortion of the OH-stretching band generally conforms to a remarkably simple relationship between Raman peak intensities and molal concentration of chloride. Our results thus allow for fast identification and quantification of some aqueous electrolyte solutes and species; provide insight into their speciation; and allow for calibration of a Raman spectroscopic proxy measurement for fluid salinity, and potentially pH.