Computer simulation of liquid tetramethylurea and its aqueous solution

Abstract

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 ¡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. DipoleÈdipole 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 dipoleÈdipole interaction. A further test for the potential model was also provided by studying the hydration of TMU on TIP4P water. Potential functions for waterÈTMU intermolecular interaction were obtained by standard combining rules. The value obtained for the free energy of hydration using statistical perturbation theory was *G \ [16.82 kJ mol~1, to be compared with the value *G \ [56.48 kJ mol~1 obtained for urea. Radial distribution functions for waterÈTMU interaction show features indicating hydrogen bonding between the TMU oxygen site and hydrogen of water. Compared to pure water, our results shows that the waterÈTMU hydrogen bonding is more stable. The results also show that in dilute TMUÈwater solution the inÑuence of TMU on the energetics of waterÈwater hydrogen bonding is negligible. Contrasting with gas phase results for the TMUÈwater dimer the present results do not indicate the formation of hydrogen bonding interaction with the nitrogen site of TMU in aqueous solution. This Ðnding is in agreement with previous hydration studies of dimethylformamide and also with the hydrophobic behaviour of TMU observed in experiments. DipoleÈdipole correlation results obtained for TMU and water molecules in the waterÈTMU solution exhibit signiÐcant di †erences when compared to the ones for the pure liquids.

Structure and weak hydrogen bonds in liquid acetaldehyde

“…33 Consequently, the long-range electrostatic contribution to the electrostatic energy is lower than could be expected for a strictly Coulombic system. This is in accord with recent results 34 using the generalised Born model. 35,36 Also in good agreement with this, it is observed that site-site pair correlation functions, thermodynamic properties and single particle dynamics of polar liquids, appears to be quite insensitive to the long-range forces in the system.…”

Enthalpies of solution of 1,1,3,3-tetramethylurea in normal and branched alkanols (C2–C4) at 298.15K

“…However, there is a possibility that a TMU molecule interacts with another TMU molecule through dipole-dipole interactions [12,13,15]. In the case of dissolution of TMU in an alkanol, the breaking of hydrogen bonding among alcohol molecules and that of intermolecular dipole-dipole interactions in the pure solute take place [18].…”

“…MeOHÞ is negative only in the case of unsubstituted U, whereas TMU has a substantially more positive value of D tr H m than the other methyl-substituted ureas. The difference between standard molar enthalpies of solution of TMU in water ðD sol H m ffi À22: 19 Interest in solutions of TMU is due not only to peculiarities of its hydration (solvation) but also, like U, to biochemical importance of such systems [7][8][9][10][11][12][13][14][15]. Besides TMU has some unique properties among the ureas.…”

Solvent-induced lysozyme gels: Rheology, fractal analysis, and sol–gel kinetics

“…Microheterogeneities of the binary solvent media, leading to an inversion in the configuration of the liquid mesodomains, were found to be decisive for the onset of protein gelation [14][15][16]. From an academic point of view, such systems are very interesting models for investigating the driving forces involved in protein folding/unfolding and selfassembly events.…”