Miscibility and Thermodynamics of Mixing of Different Models of Formamide and Water in Computer Simulation

Abstract

The thermodynamic changes that occur upon mixing five models of formamide and three models of water, including the miscibility of these model combinations itself, is studied by performing Monte Carlo computer simulations using an appropriately chosen thermodynamic cycle and the method of thermodynamic integration. The results show that the mixing of these two components is close to the ideal mixing, as both the energy and entropy of mixing turn out to be rather close to the ideal term in the entire composition range. Concerning the energy of mixing, the OPLS/AA_mod model of formamide behaves in a qualitatively different way than the other models considered. Thus, this model results in negative, while the other ones in positive energy of mixing values in combination with all three water models considered. Experimental data supports this latter behavior. Although the Helmholtz free energy of mixing always turns out to be negative in the entire composition range, the majority of the model combinations tested either show limited miscibility, or, at least, approach the miscibility limit very closely in certain compositions. Concerning both the miscibility and the energy of mixing of these model combinations, we recommend the use of the combination of the CHARMM formamide and TIP4P water models in simulations of water-formamide mixtures.

Adsorption of Formamide at the Surface of Amorphous and Crystalline Ices under Interstellar and Tropospheric Conditions. A Grand Canonical Monte Carlo Simulation Study

“…This stable layer is, however, still an unsaturated monolayer, and the building up of the outer molecular layers starts somewhat before the first layer becomes saturated. This finding indicates, in accordance with earlier results, 95,107,108 that the adsorbate-adsorbent and lateral interactions are of roughly the same magnitude. Thus, adsorbed formamide molecules are not isolated from each other even at very low surface densities, and this effect is more marked at lower temperatures.…”

“…Thus, the obtained isotherms represent a transition between type II and type III isotherms (both describing multilayer adsorption) according to the IUPAC convention. Considering that type II isotherms correspond to systems where the adsorbate-adsorbent interaction is considerably stronger, while type III isotherms correspond to systems where it is considerably weaker than the lateral interaction between the adsorbed molecules, this result is in a clear accordance with earlier claims that the mixing of water and formamide is nearly ideal, 95,107,108 and hence there is no marked difference between the interactions of the like and unlike molecules. Furthermore, it is also seen that the steepness of the first rising part of the isotherm, corresponding to the building up of the first molecular layer, increases very strongly with decreasing temperature, which results in a very sharp rise of the low temperature isotherms even at extremely low prel values.…”

“…Water and formamide molecules have been described by the three-site SPC/E potential 93 and by the CHARMM27 force field, 94 respectively, since this model combination was previously found to best reproduce the mixing properties of water and formamide. 95 In both of these potential models, interactions are centered at the individual atoms. Thus, the interaction energy of a molecule pair is calculated as the sum of the Lennard-Jones and chargecharge Coulomb contributions of their individual atom pairs, while the total energy of the 7 system is calculated as the sum of the interaction energies of all molecule pairs.…”

Binary Liquid Mixture Contact-Angle Measurements for Precise Estimation of Surface Free Energy

“…No literature data of surface tension are available for the water–formamide system. The accuracy of the surface energy predictions for DMSO–water, ethylene glycol water, and glycerol–water mixtures builds confidence in the method and suggests the use of the water–formamide system, considering that formamide satisfies all other stated criteria …”

“…The accuracy of the surface energy predictions for DMSO−water, ethylene glycol water, and glycerol−water mixtures builds confidence in the method and suggests the use of the water−formamide system, considering that formamide satisfies all other stated criteria. 30 The next step was to decompose the overall mixture surface energy, γ L,mix , into its polar and dispersive components, using the approach outlined in the Theory section. An important question that must be addressed at this point is the ability of the simple estimation approach to account for potential differences in surface aggregation between polar and nonpolar liquid components, a concern raised previously by Kwok and Neumann.…”