Thermodynamic analysis of the mutual solubilities of normal alkanes and water

Abstract

Mutual solubility data for C 5–C 16 alkanes and water close to 298 K are investigated, along with calorimetric heats of solution, as a function of the carbon number. The solubility of alkanes in water at 298 K drops steeply with increasing CN (carbon number), but the rate of decrease becomes significantly smaller for CN>11, possibly suggesting a transition to a `collapsed' conformation that reduces contact of the alkane with water. Calorimetric data for the heat of solution of normal alkanes (and normal alkylbenzenes) in water strongly suggest that the heat of solution is a linear function of temperature, in excellent agreement with solubility data, and both calorimetric and solubility data indicate a minimum in solubility (heat of solution=0) at around 303 K (vs. 290 K for the alkylbenzenes). The solubility of water in alkanes presents a much different picture from that of alkanes in water. First, the solubility of water in alkanes at 298 K is fairly insensitive to CN, rising slightly with increasing CN, while calorimetric measurements give a heat of solution that is nearly independent of temperature and CN. Its magnitude, 35 kJ mol −1 (8.4 kcal mol −1), is that of a normal hydrogen-bond energy, suggesting that the dissolution of n water molecules leads to the breaking of n hydrogen bonds. The temperature dependence of the solubility data is well represented by an equation that assumes a temperature-independent heat of solution that decreases slightly with increasing CN.