Temperature and salinity effects on the Raman OH-stretching vibration bands of water: starting point to know hydrate occupancy and unreacted water in the gas hydrates.

Properties of gas hydrates can be in situ detected by Raman spectroscopy in order to determine structure, evolution, composition, cage occupancies and vibrational spectra of the guest molecules. Temperature, pressure, dissolved ions and cooperative effects related to hydrogen bonds and solute solvation promote structural changes on water molecules. In order to know the effects of different parameters on water OH-stretching vibrations, we present an experimental Raman study on how the addition of sodium chloride to liquid water under different temperature conditions influences the water hydrogen bonds. To this purpose, Raman spectra of ultrapure water and of NaCl water solutions were acquired under different temperature conditions. Experiments were performed with an excitation wavelength of 532 nm, a total acquisition time of 300 seconds at 20 and -20 °C respectively. The obtained preliminary results demonstrated that salinity and temperature affected OH-stretching vibrations and that the results can be correlated with the NaCl concentrations. These obtained results represent the starting point in order to obtain data about hydrate occupancy and unreacted water in the gas hydrate mass in different experimental conditions.