# Packing ionic liquids of desirable Molarity in A given simulation box size

I want to format a supercapacitor system where there are two porous electrodes on both sides with void volume. Should I consider the void volume in the electrodes?

Hi Kaushik,

It’s not easy to help you when you leave all the relevant information behind in an older thread.

Here are summary calculations for setting up this kind of simulation:

At room conditions, the molar concentrations and molar volumes of relevant pure substances are:

Acetonitrile: 19.2 mol/L; 52.2 mL/mol (note 1)
BMIm-PF6: 4.86 mol/L; 206 mL/mol (note 1)

So as a first guess, 1.5 mol of BMIm-PF6 in 1 L would take up ~300 mL of volume, leaving ~700 mL of volume to the acetonitrile corresponding to 13.4 mol of acetonitrile.

As a nice hack, 1 mol/L corresponds, in particle concentrations, to 0.6 particles/nm^3 (note 2). Thus the earlier molar ratio (1.5 : 13.4) corresponds to 0.9 ion pairs and 8 acetonitrile molecules per nm^3, or a ratio of 0.113 ion pairs per acetonitrile molecule. That’s only 6% off from the reported value of 0.107 (230/2146), and one can quickly surmise that their molar ratio is lower because BMIm-PF6, dissolved in a polar solvent, has lower ionic strength and thus increased partial molar volume relative to the neat IL.

Hope that makes sense.

Note 1

The molar concentration of a pure substance is just the number of moles contained in 1 L of volume - so, for example, water, with molar mass 18 g/mol and room condition density 1000 g/L, has (room condition) molar concentration 1000/18 = 55 mol/L. (Thus we can consider aqueous solutions up to 2-3 M as being “dilute” – solute:solvent molar ratios under 5% – while more concentrated aqueous solutions are considerably less colligative.)

Thus, pure acetonitrile is 19.1 M (786 g/L / 41 g/mol) and pure BMIm-PF6 is 4.86 M (1380 g/L / 284 g/mol).

Note 2

1 mol / L = 6e23 particles / 1e24 nm^3 = 0.6 particle / nm^3.

Very thanks for your generous response.

Can I perform npt simulations for equilibrating the ionic liquid for the system where I have filled the ionic liquid betweem the space of two electrodes such that the volume of space in between can fluctuate to equilibrate the system, the electrodes can move in z direction, the equilibrating solution can enter the system but the final structure of the system such that the volume between the electrodes shrinks or expands for achieving the equlibrium density in bulk?
Attaching my system below so that you can get the idea
embf15M.dat (1.7 MB)