Computer simulation of liquid tetramethylurea and its aqueous solution

Thermodynamic properties and correlation functions for the pure liquid 1,1,3,3-tetramethylurea (TMU) and its aqueous solution were obtained by Monte Carlo simulation in the isothermic and isobaric (NPT) ensemble at 25 degrees C and 1.0 atm. An eight site potential model combining Lennard-Jones plus coulombic functions was developed to calculate intermolecular interaction between TMU molecules. In this model the methyl groups are represented by a united atom approach. The partial charges needed for Coulomb interactions and the geometry of the TMU molecule were calculated at the HF level using a 6-31g* basis set with the CHELPG formalism. The parameters needed for the Lennard-Jones potential functions were optimised to reproduce experimental values for the density and enthalpy of vaporisation of the pure liquid at 298 K and 1.0 atm. The results obtained for density, enthalpy of vaporisation and other thermodynamic properties for the pure liquid TMU are in good agreement with experimental data. Radial distribution functions (rdf) obtained for liquid TMU are broad indicating a low degree of molecular organisation. Dipole-dipole correlation shows a preference for anti-parallel molecular orientation at short distances. Therefore, the present results are consistent with experimental data indicating the formation of dimers due to dipole-dipole interaction. A further test for the potential model was also provided by studying the hydration of TMU on TIP4P water. Potential functions for water-TMU intermolecular interaction were obtained by standard combining rules. The value obtained for the free energy of hydration using statistical perturbation theory was Delta G=-16.82 kJ mol(-1), to be compared with the value Delta G=-56.48 kJ mol(-1) obtained for urea. Radial distribution functions for water-TMU interaction show features indicating hydrogen bonding between the TMU oxygen site and hydrogen of water. Compared to pure water, our results shows that the water-TMU hydrogen bonding is more stable. The results also show that in dilute TMU-water solution the influence of TMU on the energetics of water-water hydrogen bonding is negligible. Contrasting with gas phase results for the TMU-water dimer the present results do not indicate the formation of hydrogen bonding interaction with the nitrogen site of TMU in aqueous solution. This finding is in agreement with previous hydration studies of dimethylformamide and also with the hydrophobic behaviour of TMU observed in experiments. Dipole-dipole correlation results obtained for TMU and water molecules in the water-TMU solution exhibit significant differences when compared to the ones for the pure liquids.