The Structure of Water and Its Relationship to Hydrocarbon-Water Interactions

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Abstract
A comparison of entropy and enthalpy values for the dissolution of a hydrocarbon in water with those for the formation of ice show that in the dissolving process both the formation and melting of ice structures are involved. Instead of 20 or more water molecules per molecule of hydrocarbon as required for clathrate formation, about -1 to 6 water molecules per molecule of hydrocarbon at most are associated with the process. The solubility of hydrocarbons in concentrated salt solutions, partial molal volumes, and nuclear magnetic resonance data support the conclusion that ciathrate structures around such relatively inert molecules cannot exist. The solubility of a hydrocarbon depends on the amount of water that is unbonded (does not exist in clusters) since water clusters appear to react more unfavorably with hydrocarbons than unbonded water molecules. The physical properties of water suggest the presence of two types of water clusters: compressible and noncompressible. It is proposed that from about 60 or 65°C to 100°C the hydrogen bonding of water molecules produces exclusively noncompressible water clusters which have either the structure of ice II (rhombohedral crystallites) or tetrahedrally bonded water molecules. From 0 to about 18°C the main process in hydrogen -bond formation is the aggregation of these water clusters to form compressible clusters. Both processes coexist in the transition region of approximately 18–60°C. Hydrogen bonding between different-sized crystallites produces empty pockets that decrease the density of these cluster aggregates. The final aggregation of these aggregates to form ice produces unbonded water molecules as liquid microinclusions in the interstitial areas of the final aggregate. Ice I at 0°c therefore consists of interlocked water clusters, interdispersed H2O molecules, and about 20 vol.% of empty pockets. The proposed model for water explains such diverse phenomena as the decreases and increases in solubility of hydrocarbons with an increase in temperature; the formation of in- soluble water clathrates around methane under high pressure; the ability of inert molecules to raise the freezing-point temperature of pure water; viscosity-pressure isotherms; specific heat capacities; and the positive and negative values of ΔH and the decreases and increases in the values of ΔS for the solubilization of hydrocarbons and inert gases at different temperatures.

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