Molecular Dynamics Simulations of Dynamics and Diffusion Coefficients of Tetrabutylphosphonium Amino Acid Based Room Temperature Ionic Liquids

Molecular dynamics simulations are used to study the dynamics and transport properties of 6 [TBP][AA] roomtemperature ionic liquids composed of the tetrabutylphosphonium ([TBP]+, or [P(C4)4]+) cation with the amino acid ([AA]–) anions, such as glycine ([Gly]–), alanine ([Ala]–), leucine ([Leu]–), aspartic acid ([Asp]–), glutamic acid ([Glu]– ), and glutamine ([Gln]–). The explicit atom force field of Zhou et al. [J. Phys. Chem. B 111, 7078 (2007)] is used in the simulations. The dynamics of the ionic liquids are characterized by studying the mean-square displacement (MSD) and the velocity autocorrelation function (VACF) for the centers of mass of the ions at ~ 280 K. The diffusion coefficients evaluate from the linear slope of MSD(t) functions in the range of 2–4 ns and the cationic transference numbers estimated for study the contributions of the anions and cations to the transport of charge in these ionic liquids. We determined the role of the amino acid anion structures on the dynamical behavior of this family of ionic liquids. The MSD and self-diffusion of relatively heavier [TBP]+ non-planar cation are smaller than that of the lighter amino acids anions from the Einstein results, except of case of [TBP][Gln] and [TBP][Leu]. The trends in the cationic transference numbers with different length of alkyl side chain in the amino acid anions are [Gly]- < [Ala]- < [Leu]- and the trends in the cationic transference numbers with different functional side chain in the amino acid anions are [Asp]- < [Glu]- < [Gln]-. The trends in the diffusion coefficient for anions with identical cations are [Gly]- > [Ala]- > [Leu]- and [Asp]- > [Glu]- > [Gln]-. The results for the cases of [Leu]- and [Gln]- have a relatively high uncertainly due to the lower values of b exponent and relatively higher melting points for these two ILs. The major factor for determining the magnitude of the self-diffusion is the length of alkyl side chain and the type of functional group in the side chain of the anion of the amino acid ionic liquids.

کلیدواژه ها:

Ionic Liquids ، Amino Acid ، Molecular dynamics Simulation ، Diffusion ، Transport Properties