When the main anions (negatively charged ions) present are chloride, sulphate or nitrate, the hardness is called permanent hardness, which cannot be removed by boiling. Rainwater may be even more acidic in areas where the highly soluble acidic gases sulphur dioxide and nitrogen dioxide (both produced by fossil fuel power generation, transportation and industrial processes) are present in the atmosphere. Natural rainwater is slightly acidic as a result of this reaction, with an average pH of 5.7, whereas the average pH of seawater is 8.2 (Box 1). A major difference in composition is the greater relative proportions of dissolved gases in rainwater, particularly carbon dioxide. Most of the dissolved salts in rainwater come from sea spray dispersed into the atmosphere. Rainwater and seawater (Figure 2a and b) have similar relative proportions of dissolved solids, although rainwater is much more dilute. TDS is a good indicator of water quality, and standards that have been set for drinking water and for water used in other ways include maximum values for TDS. TDS values for groundwater vary too much for an average to be meaningful. Average TDS values are: 7mg l -1 for rainwater, 118mg l -1 for river water and 34 400 mg l -1 for seawater. This gives a clear picture of the underlying mechanism of water replacement by urea.Rainwater, seawater and river water (Figure 2) and groundwaters (Figure 3) generally have very different chemical compositions and differ widely in their concentrations of total dissolved solids (TDS). Considering all the constituents as the hydrogen bond partners we calculate the possibility of a successful hydrogen bond formation with a central water molecule. A negligible contribution from the hydrogen bonds between water and bulky choline cations has also been found. Further insights are drawn from the characterization of the hydrogen-bonded network in water and we observe the gradual rupturing of water–water hydrogen bonds and their subsequent replacement by the water–urea hydrogen bonds. Increase in the q tet values are observed when highly electro-negative hetero-atoms like nitrogen, oxygen of urea and choline cations are counted as partners of the central water molecules. Our analyses show a monotonic increase in the structural disorder as the co-solvents are added. The extent of deviation of the water structure from tetrahedrality is quantified using the tetrahedral order parameter ( q tet). A disruption of the local hydrogen-bonded structure in water is observed upon inclusion of urea and choline chloride. We consider four aqueous solutions of reline, between 26.3 and 91.4 wt%. We carry out extensive all-atom molecular dynamics simulations to elucidate the effect of the gradual addition of co-solvents on the microscopic arrangements of water molecules. However, a quantitative understanding of the microscopic structural features of water in the presence of reline is still lacking. Owing to the presence of active hydrogen bond formation sites, urea and choline cations can disrupt the hydrogen-bonded network in water. Pure reline and its aqueous solution have large scale industrial use. Reline, a mixture of urea and choline chloride in a 2 : 1 molar ratio, is one of the most frequently used deep eutectic solvents.
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