Raman spectroscopic study of a H2O + Na2SO4 solution at 21– 600 °C and 0.1 MPa to 1.1 GPa: Relative differential ν1-SO42- Raman scattering cross sections and evidence of the liquid–liquid transition

Relative differential Raman scattering cross sections of the ν1-SO42- mode in a H2O + 1.54 m Na2SO4 solution were obtained from in situ experiments at temperatures from 21 to 600 °C and pressures from 0.1 MPa to about 1.1 GPa using a hydrothermal diamond-anvil cell. Additionally, frequency and width of the most intense ν1-SO42- Raman line were determined for the same solution as a function of temperature and pressure.The relative differential ν1-SO42- Raman scattering cross section showed no significant dependence on temperature along the liquid–vapor curve, but a noticeable increase with pressure along the two isochores. In all three cases, however, the changes in the empirically determined relative differential cross section with temperature were smaller than predicted from Bose–Einstein statistics. The implications for quantitative Raman spectroscopy on samples at elevated temperature and pressure are discussed for natural aqueous inclusions and measurements using optical cells.The determined shifts in the wavenumber ω   of the ν1-SO42- Raman line of unassociated sulfate ions and solvent-separated ion pairs with pressure and temperature can be used to obtain good estimates of the pressure in inclusions or hydrothermal diamond-anvil cells. Furthermore, the liquid–liquid transition in the solution was evident from a pronounced decrease in the (∂ω/∂P)T slope at 200 MPa and an increase in the broadening of the ν1-SO42- line at the same pressure along the 21 °C isotherm. In contrast, the transition was reflected with much less certainty in the change of band parameter in the OH stretching region with pressure at the same temperature. Thus, Raman spectroscopy on an appropriate solute species can be used as a sensitive technique to trace the liquid–liquid transition in aqueous solutions in P–T–X space.