Determination of temperature dependence of limiting activity coefficients for a group of moderately hydrophobic organic solutes in water

Limiting activity coefficients γ1∞ in water were measured for 17 moderately hydrophobic mono- and bifunctional compounds containing hydroxy, keto, ether, ester and nitrile groups at several temperatures from 298 to 353 K. Three experimental techniques, viz. Rayleigh distillation, inert gas stripping (IGS), and headspace analysis (HSA), were alternatively employed. With the objective to establish the reliable γ1∞(T) dependences, the γ1∞ values were combined with calorimetric data on limiting partial molar excess enthalpies H̄1E,∞ and heat capacities C̄P,1E,∞ determined previously in our laboratories. This database was further supplemented with critically evaluated literature data. The equation, ln γ1∞=A+B/τ+C ln τ+Dτ, where τ=T/(298.15 K), was employed for the simultaneous thermodynamic treatment of γ1∞, H̄1E,∞ and C̄P,1E,∞ data. With the exception of 1-butanol, for all solutes studied the three-parameter form of this equation (D=0) assuming temperature independent C̄P,1E,∞ was found to represent available information adequately. For all the solutes, γ1∞(T) exhibits a maximum between 273 and 373 K. As shown, by processing simultaneously the equilibrium and thermal data, thermodynamically consistent, accurate γ1∞(T) dependences can be obtained for the range 273–373 K even in cases of minimum underlying information (room temperature values of H̄1E,∞ and C̄P,1E,∞, a single temperature value of γ1∞). In general, the described methodology should be the route of choice when determining γ1∞(T) of aqueous semi-hydrophobic or non-hydrophobic solutes. Furthermore, prediction of γ1∞ for the systems studied was attempted by two group contribution methods: the modified UNIFAC (Dortmund) and the method of Cabani et al. Both were found to perform unsatisfactorily for the bifunctional derivatives.