How Much H2O Is There in the Ocean? The Structure of Water in Sea Water

Abstract

AbstractWe report on laboratory and field Raman spectroscopy experiments on the structure of water in sea water and describe this as a function of temperature and pressure. The Raman spectrum of water/seawater is dominated by the water stretching mode band with a frequency shift in the range 2,800–3,800 cm−1. Here the hydrogen bonded (HB) clusters of several water molecules and nonhydrogen bonded (nHB, singlet H2O) forms are revealed with an isoskedastic point, similar to the isosbestic point for pH sensitive dyes. This band can be deconvolved into five Gaussian peaks, each with a specific spectroscopic assignment. We find that within the temperature range of 0–40 °C the vastly greater mass and volume of ocean water (78–85%) is in the HB forms, dominantly as (H2O)5. Without hydrogen bonding there would be no ocean, and nowhere in the bulk ocean does the simple H2O molecule exceed 20% of the total. The fraction of water molecules in the solvation shell around ions in seawater represents a third type of water, bound to an ion but not hydrogen bonded. Approximately 6% of the water molecules present at 35 salinity are in this form, and their concentration is not affected by temperature or pressure. The HB forms are a continuum of species in rapid exchange. The fraction of nHB is driven solely by temperature. We find no evidence that increased pressure changes the population of the free singlet nHB molecule; pressure drives the dominantly (H2O)5 population toward a lower volume, nested, molecular state.