Hydrogen bonding. Part 34. The factors that influence the solubility of gases and vapours in water at 298 K, and a new method for its determination
The solubility of 408 gaseous compounds in water at 298 K has been correlated through eqn. (i), where the solubility is expressed as the Ostwald solubility coefficient, Lw, and the solute explanatory variables are R2 an excess molar refraction, π2H the dipolarity/polarizability, Σα2H and Σβ2H the effective hydrogen-bond acidity and basicity, and Vx the McGowan characteristic volume. A similar equation using the log L16 parameter instead of Vx can also be used; L16 is the Ostwald solubility coefficient on hexadecane at 298 K. log Lw=–0.994 + 0.577R2+ 2.549 π2H+ 3.813Σα2H+ 4.841Σβ2H– 0.869 Vx(i), n= 408 ρ= 0.9976 sd = 0.151 F= 16810 The main factors leading to increased solubility are solute π2H, Σα2H and Σβ2H values; conversely, the corresponding properties of water are dipolarity/polarizability, hydrogen-bond basicity and hydrogen-bond acidity. Solute size plays a minor role, and slightly decreases solubility, contrary to observations on all non-aqueous solvents. It is shown that this peculiar behaviour of water is due to (a) a greater increase in the unfavourable cavity effect with increase in solute size, for solvent water, and (b) a smaller increase in the favourable general dispersion interaction with size, for solvent water.
A new method for the determination of log Lw values is put forward, using the relationship Lw=L16/P where L16 is as above, and P is either the water–hexadecane partition coefficient or the water–alkane partition coefficient. For 14 solutes using the former P-value, agreement with values calculated through eqn. (i) is 0.08 log units on average and for 45 solutes using the latter P-value, the corresponding agreement is 0.15 log units, with log Lw values ranging up to 8 log units.