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The glass transition and the distribution of voids in room-temperature ionic liquids: A molecular dynamics study
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10.1063/1.4723855
/content/aip/journal/jcp/136/20/10.1063/1.4723855
http://aip.metastore.ingenta.com/content/aip/journal/jcp/136/20/10.1063/1.4723855

Figures

Image of FIG. 1.
FIG. 1.

Schematic representation of the [C3mim]+ and [C4mim]+ cations and [Tf2N], [PF6] anions considered in the present simulation study.

Image of FIG. 2.
FIG. 2.

Comparison of the atomic and pseudo-electron charge density of carbon. The two radial functions match continuously and with continuous first derivative at the core radius r c = 2.2 a.u. The atomic charge density is computed within the local spin density approximation of density functional theory. Atomic and pseudo-atomic charge densities are assumed to be spherically symmetric.

Image of FIG. 3.
FIG. 3.

Comparison of the pseudo-electron charge density of carbon with its reconstruction from a Fourier representation with E cut = 69 Ry, corresponding to the lowest cut-off used in our analysis of voids.

Image of FIG. 4.
FIG. 4.

Temperature dependence of the average potential energy of [C4mim][PF6] (per ion pair) during the quench whose time schedule is described in the text. Dots: simulation results. Blue (full) line: linear interpolation of the 20 ⩽ T ⩽ 120 K range. Red (dash) line: linear interpolation of the 340 ⩽ T ⩽ 500 K range. The error bar on the simulation data is less than the size of the dot.

Image of FIG. 5.
FIG. 5.

(a) Temperature dependence of ΔU(T) = U(T) − a low b low T for [C4mim][Tf2N] during the quench whose time schedule is described in the text. U(T) is the average potential energy per atom pair, and a low + b low T is the linear interpolation to the 20 ⩽ T ⩽ 120 T data. Dots: simulation results. Dash line (blue): linear interpolation of the 340 ⩽ T ⩽ 500 K range. The gray area points to the temperature range over which U(T) falls below the high temperature linear interpolation. (b) Same plot of part (a) for [C4mim][PF6].

Image of FIG. 6.
FIG. 6.

Constant pressure (P = 1 atm) specific heat C p of [C3mim][Tf2N], [C4mim][Tf2N], and [C4mim][PF6], computed by differentiating the Padé interpolation of the system enthalpy (see text). N a is the total number of atoms and k B is the Boltzmann constant.

Image of FIG. 7.
FIG. 7.

Temperature dependence of the average volume of [C4mim][Tf2N] during the quench whose time schedule is described in the text. Dots: simulation results. Full line (blue): linear interpolation of the 20 ⩽ T ⩽ 120 K range. Dash line (red): linear interpolation of the 340 ⩽ T ⩽ 500 K range. P = 1 atm.

Image of FIG. 8.
FIG. 8.

Radial distribution functions for [C4mim][Tf2N] computed in the point-particle representation of cations and anions (see text). Upper panel: T = 320 K, in the equilibrium liquid phase; lower panel: T = 140 K, in the amorphous phase.

Image of FIG. 9.
FIG. 9.

Arrhenius plot of the diffusion coefficient of cations and anions in [C3mim][Tf2N], [C4mim][Tf2N], and [C4mim][PF6]. The full (straight) lines are a guide to the eye. In each panel, the vertical straight lines identify the glass transition temperature T g .

Image of FIG. 10.
FIG. 10.

Volume fraction of voids (in percent of the total volume) as a function of T for [C3mim][Tf2N], [C4mim][Tf2N], and [C4mim][PF6].

Image of FIG. 11.
FIG. 11.

Simulation snapshot of voids in [C3mim][Tf2N] at T = 300 K. Clusters have been painted in six different colours to identify them, and to highlight their size.

Image of FIG. 12.
FIG. 12.

Probability distribution p(r v ) of the voids' radius r v in [C3mim][Tf2N], computed at three different temperatures from the voids' volume upon assuming a spherical hole geometry (see text).

Image of FIG. 13.
FIG. 13.

Volume fraction of voids (in percent of the total volume) for [C3mim][Tf2N] as a function of T, computed at (r s = 8), and at (r s = 6), for [C3mim][Tf2N], [C4mim][Tf2N], and [C4mim][PF6].

Image of FIG. 14.
FIG. 14.

Probability distribution p(r v ) of the voids' radius r v in [C3mim][Tf2N] at T = 360 K computed at two different values of the cut-off density (see text).

Image of FIG. 15.
FIG. 15.

Average volume ⟨v h ⟩ of voids in [C3mim][Tf2N] as a function of temperature. The simulation data for ⟨v h ⟩ have been rescaled by the factor ⟨n⟩ accounting for the coalescence of primary holes (see text).

Image of FIG. 16.
FIG. 16.

Probability distribution p(r v ) of the voids' radius r v from the [C3mim][Tf2N] simulations shown on a semi-logarithmic scale.

Tables

Generic image for table
Table I.

Cut-off radii (atomic units) used to generate the pseudo-charge densities (see text).

Generic image for table
Table II.

Glass transition temperature estimated by simulation and measured in experiments.

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/content/aip/journal/jcp/136/20/10.1063/1.4723855
2012-05-31
2014-04-19
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: The glass transition and the distribution of voids in room-temperature ionic liquids: A molecular dynamics study
http://aip.metastore.ingenta.com/content/aip/journal/jcp/136/20/10.1063/1.4723855
10.1063/1.4723855
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