Thermodynamics of solubility, sublimation and solvation processes of parabens

Saturated vapor pressures for a number of parabens (methyl- (MePB); ethyl- (EtPB), n-propyl- (PrPB) and n-butyl- (BuPB)) were obtained and from their respective temperature dependences the sublimation enthalpy, ΔHsub°, and sublimation entropy, ΔSsub°, as well as their respective relative fractions in the process calculated. The sublimation enthalpies are: ΔHsub°(MePB) = 98.8 ± 0.8; ΔHsub°(EtPB) = 100.9 ± 0.7; ΔHsub°(PrPB) = 123.7 ± 0.6; ΔHsub°(BuPB) = 108.4 ± 0.8 kJ mol−1. The obtained values are discussed with regard to X-ray data from the literature. Theoretical calculations of the respective crystal lattice energies were carried out and compared to the experimental data. The following parameters were analyzed: (a) energetic contribution of van der Waals forces and hydrogen bonding to the total packing energy of the crystals; (b) contributions of the different fragments of the paraben molecules to the packing energy; (c) influence of bias of the supposed C-H distances on the result of the calculation procedure. Enthalpies of evaporation were estimated from the measured enthalpies of sublimation and enthalpies of fusion, and compared with literature data. Moreover, the thermodynamic functions of solvation of the molecules in water and in a number of n-alcohols were evaluated. The thermodynamic terms (Gibbs energy, enthalpy and entropy) of the solvation process were split up in their respective specific and nonspecific fraction, and these values compared for all combinations of parabens and solvents. The influence of mutual saturation of the phases in the water-octanol system on the partitioning process of the molecules is also discussed.