Thermodynamic analysis of the mutual solubilities of hydrocarbons and water

The analysis of mutual solubilities and calorimetric heats of solution carr ied out for normal alkanes and water close to 298 K [Fluid Phase Equilib. 1 56 (1999) 21] is extended to normal alkylcyclohexanes, linear 1-alkenes, an d normal alkylbenzenes. Extensive solubility data are available for alkylbe nzenes, but relatively little is known about alkylcyclohexanes and alkenes in water. All of these hydrocarbons are more soluble than the corresponding alkanes, but are less volatile than the alkanes (their Henry's constants d escending in the order alkanes > alkenes > alkylcyclohexanes > alkylbenzene s). The solubility minimum for hydrocarbons in water is confirmed by calori metric data for C-6-C-9 alkylbenzenes (similar to 291 K) and cyclohexane (2 98.5 K). It appears that the heat capacity of solution is independent of te mperature and increases linearly with carbon number, in the descending orde r alkanes > alkylcyclohexanes > alkylbenzenes. In the absence of calorimetr ic data, the relatively limited solubility data for alkenes in water do not allow a reliable determination of the solubility minimum or of the heat ca pacity of solution. As is the case for water in alkanes. the solubility of water in hydrocarbon s at 298 K is relatively insensitive to carbon number (CN), but, unlike tha t in alkanes and alkylcyclohexanes, the solubility of water in alkenes and alkylbenzenes decreases slightly with increasing CN. Less is known about th e heat of solution, but for each family it appears to be independent of tem perature and CN, covering the range 24 kJ mol(-1) (alkylbenzenes) to 35 kJ mol(-1) (alkanes and alkylcyclohexanes). The latter is a better measure of the hydrogen bond energy in hydrocarbon/water systems, since alkanes and al kylcyclohexanes have the least affinity for water. This affinity is measure d by the heat of hydrophobic interaction, which makes a negative contributi on to the heat of solution, and therefore, as observed, the heat of solutio n of water in alkenes (33 kJ mol(-1)) and alkylbenzenes is smaller than tha t in alkanes and alkylcyclohexanes. (C) 2001 Elsevier Science BN. All right s reserved.