Physical Properties at the Base for the Development of an All-Atom Force Field for Ethylene Glycol (original) (raw)

2011, The Journal of Physical Chemistry B

Ethylene glycol (1,2-ethylenediol), the simplest of the diols, has a vast number of applications in technology, chemistry, and chemical engineering. It is commonly used as a solvent, as well as an antifreeze agent and coolant; and it is a precursor in production of poly(ethylene glycol), PEG, a polymer commonly used for biocompatibility purposes and nanoparticle stabilization. 1 Recently, it has been also applied as a component of ionic liquids (ILs). 2,3 In molecular modeling, the glycol molecule is used as a template to develop force fields for more complex systems with vicinal hydroxyl groups, like sugars for example. 4 As a result, several force fields for glycol have been developed and published. 5-7 In this work, we focus our attention on those compatible with the all-atom OPLS force field, 8 commonly used to simulate molecular liquids, including ionic liquids. 9 It has to be mentioned that both the all-atom and the united-atom force fields exist; 5,7 however, here we restrict our attention to the all-atom force fields only. Unfortunately, most of the OPLS-AA-based force fields for glycol were developed to reproduce the microscopic structure of the liquid (e.g., equilibrium between trans and gauche conformers) 6,10 or to design force fields for biological molecules, like sugar. 4,11 This came at the sacrifice of the ability to reproduce the physical properties of the liquid glycol itself. In the OPLS-AA force field, nonbonding interactions between atoms separated by three bonds (1-4 interactions) are scaled. The standard scaling factor is 0.5 for both Coulomb and Lennard-Jones interactions. For the glycol molecule, it has been proposed to use different scaling factors 5,6 or even apply an extra scaling factor for the interaction between atoms separated by four bonds (1-5 interactions). 4 Although this approach has been found to improve the quality of the force field, it is somewhat troublesome, when the force field is used for mixtures. Namely, molecular dynamics packages like GROMACS 12 usually permit us to define only a single scaling factor for the whole system in a convenient way. This can be problematic if the goal of the study is to simulate complex systems, like ILs (e.g., composed of glycol and choline chloride) 2 or mixtures, e.g., with other alcohols. 13 Therefore, in our work we attempt to optimize the force field for liquid glycol, with the following two requirements in mind: (i) the physical properties of liquid glycol, like density, thermal expansion, compressibility, heat of vaporization, and surface tension, should be reproduced as good as possible and (ii) the force field should be kept as simple as possible, without introducing "exotic" parameters and maintaining full compatibility with the OPLS-AA force field. Such a force field should be robust and easily combined with other compounds. We also pay attention to the interaction of ethylene glycol with water in aqueous solutions; these kinds of