************ The 14th Lab Seminar **************************************

Title : Molecular dynamics simulation of imidazolium-based ionic liquids.
        I. Dynamics and diffusion coefficient

M. H. Kowsari, Saman Alavi, Mahmud Ashrafizaadeh and Bijan Najafi, 
THE JOURNAL OF CHEMICAL PHYSICS 129, 224508 (2008)

Speaker : Takafumi Noguchi

Date : Fri. Feb. 14th, 10:30 am

Place : Seminar room of 4F, Graduate School of Information Science

Abstract :
Molecular dynamics simulations are used to study the dynamics and trans-
port properties of 12 room-temperature ionic liquids of the 1-alkyl-3-
methylimidazolium [amim+] (alkyl=methyl, ethyl, propyl, and butyl) 
family with PF6-, NO3-, and Cl- counterions. The explicit atom transfer-
able force field of Canongia Lopes et al. [J. Phys. Chem. B 108, 2038 
(2004)] is used in the simulations. In this first part, the dynamics of 
the ionic liquids are characterized by studying the mean-square displace-
ment (MSD) and the velocity autocorrelation function (VACF) for the 
centers of mass of the ions at 400 K. Trajectory averaging was employed 
to evaluate the diffusion coefficients at two temperatures from the 
linear slope of MSD(t) functions in the range of 150-300 ps and from the 
integration of the VACF(t) functions at 400 K. Detailed comparisons are 
made between the diffusion results from the MSD and VACF methods. The 
diffusion coefficients from the integration of the VACFs are closer to 
experimental values than the diffusion coefficients calculated from the 
slope of MSDs. Both methods can show good agreement with experiment in 
predicting relative trends in the diffusion coefficients and determining 
the role of the cation and anion structures on the dynamical behavior of 
this family of ionic liquids.
The MSD and self-diffusion of relatively heavier imidazolium cations are 
larger than those of the lighter anions from the Einstein results, except 
for the case of [bmim][Cl]. The cationic transference number generally 
decreases with temperature, in good agreement with experiments. For the 
same anion, the cationic transference numbers decrease with increasing 
length of the alkyl chain, and for the same cation, the trends in the 
cationic transference numbers are [NO3]-<[Cl]-<[PF6]-. The trends in 
the diffusion coefficient in the series of cations with identical anions 
are [emim]+>[pmim]+>[bmim]+ and those for anions with identical cations 
are [NO3]->[PF6]->[Cl]-. The [dmim]+ has a relatively low diffusion 
coefficient due to its symmetric structure and good packing in the 
liquid phase. The major factor for determining the magnitude of the self-
diffusion is the geometric shape of the anion of the ionic liquid. Other 
important factors are the ion size and the charge delocalization in the 
anion.


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