Theoretical Studies of Concentrated Aqueous Urea Solutions Using Computacional Monte Carlo Simulation

Abstract

Simulações de Monte Carlo foram realizadas para soluções aquosas concentradas de uréia nas concentrações de 4, 5, 6, 7 e 8 mol L -1 no ensemble NpT a 298 K e 1 atm. As superfícies de energia potencial foram representadas pela soma dos potenciais clássicos de Coulomb e de Lennard-Jones, nos quais a aditividade de pares foi considerada. Nas simulações a água foi descrita pelo modelo TIP4P e a uréia por dois modelos: o modelo OPLS planar e um modelo no qual a molécula de uréia não é totalmente planar. Os resultados das simulações para os dois modelos mostraram que a uréia não causa perturbações significativas na estrutura da água, na faixa de concentrações estudadas. No entanto, foi observada diminuição do número médio de ligações de hidrogênio entre as moléculas de água. Para as soluções mais concentradas os resultados obtidos com o modelo OPLS sugerem a formação de dímeros de uréia. Observou-se similaridade entre alguns dos resultados das simulações obtidos com os modelos: OPLS planar e não-planar da uréia , nas mesmas concentrações e condições.Monte Carlo simulations were carried out for concentrated aqueous urea solutions at 4, 5, 6, 7 and 8 mol L -1 in the NpT ensemble at 298 K and 1 atm. The potential energy surfaces were represented by the sum of Coulomb and Lennard-Jones potentials in which pair-wise additivity was considered. Water was described by the TIP4P model, while urea was described by the OPLS planar model and also by a proposed non-planar model. The simulation results for both urea models showed that urea does not induce any significant changes in the structure of water over all the concentrations studied. However, it was noted that the number of hydrogen bonds between the water molecules decrease as the concentration increases. The simulation results of the OPLS planar model suggest the formation of urea dimers in the more concentrated solutions. The simulations with the non-planar model showed several features similar to those of the OPLS planar model.

DIADORIM: a Monte Carlo Program for liquid simulations including quantum mechanics and molecular mechanics (QM/MM) facilities: applications to liquid ethanol

“…Earlier versions of this program, not including QM/MM facilities, were extensively used to study pure liquids [28][29][30] and solutions 31-33 using classical force fields. The present QM/MM version was used to study liquid ethanol and the results are presented in the next sections.…”

“…23 (viii) The trajectory generated along the Monte Carlo Metropolis sampling can be saved using the xtc file format; 24 (ix) interfaces with molecular visualization programs such as vmd, 25 molden 26 and the plotting tool xmgrace 27 are available. Earlier versions of this program, not including QM/MM facilities, were extensively used to study pure liquids [28][29][30] and solutions [31][32][33] using classical force fields. The present QM/MM version was used to study liquid ethanol and the results are presented in the next sections.…”

Intermolecular hydrogen bonds in urea–water complexes: DFT, NBO, and AIM analysis

“…Hydrogen bonded chains of urea can be formed in solution due to the large number of sites available on urea molecule for hydrogen bonding. The number of hydrogen bonds between water molecules was observed to decrease with the addition of urea [3], however, the whole structure of water was kept unchanged. Recent molecular simulation studies show that urea doesn't break the local tetrahedral structure of water even at high concentrations [4][5].…”

“…The geometries of all the considered models, urea-(H 2 O) n (n = [1][2][3][4][5], are optimized at the B3LYP/6-31+G(d, p) level and the more stable structures are given in Fig. 1 with atom numbering.…”

Effect of urea on the mullite crystallization

“…Thus, it appears that the reduction of homogeneity in samples TU-1 and TU-3 occurs after the addition of ANN and TEOS. This can be explained by the high affinity of silanol molecules, ANN and urea by water molecules [15,22,23]. For samples prepared by polymeric sol-gel, the unique water source was the crystallization water of the ANN.…”