Fedorov, Maxim V

Lynden-Bell, Ruth M

Varela, Luis M

English

Apollo (Cambridge)

Simulations of ionic liquids nearcharged wallsRuth Lynden-BellDepartment of Chemistry, University of Cambridge11. Room temperature ionic liquidsHow are these liquids different from ‘ordinary ’solvents?• They have singly charged cations and anions (cfmolten NaCl).• Electrostatic interactions are very important.• The liquid is easily polarised• But solvent cations and anions are larger than Na+or Cl−, so dispersion interactions are alsoimportantThey are interesting solvents as well as being important.examples - dimethylimidazolium salts [dmim][Cl] and [dmim][PF6]butylmethylimidazolium salts [bmim][Cl], [bmim][N(SO2CF3)2], [bmim][PF6],tetraalkyl ammonium salts, tetraalkylphosphonium salts.[dmim]+22. Computer simulation of liquidsAim: to make a computer liquid with model molecules moving as in a real liquid.We need finite temperatures and moving molecules.Why: To understand molecular scale properties and processes.In the method of Molecular Dynamics we follow the motion of molecules usingNewton’s Laws of motion with interatomic forces. The temperature is determinedfrom the average kinetic energy.Many configurations are needed to explore liquid phase space and to determineproperties.In classical simulations the forces are determined from a model intermolecularpotential (force field).In ab initio simulations the forces on the nuclei are calculated from quantummechanics at each step.I use classical simulations as long simulations of fairly large systems are necessary33. Ionic Liquids near charged walls-setup→ zIn order to study the behaviour of ionic liquids near charged walls we set up asimulation with 576 [dmim]+ cations and 576 Cl− anions confined between twoplanar walls separated by 10.5nm.These walls had equal and opposite charges on them.Periodic boundaries in x and y directions give infinite slabs. In the z direction theseinfinite slabs were separated by 7.5nm of vacuum and periodically repeated.44. Ionic Liquids near charged walls -simulationsLong simulations are needed as the motion of the liquid is sluggish.We also studied systems with additional probe particles fixed at various distancesfrom the walls.55. CamgridThese seemed to be good candidates for ‘farming’ as each position of the probe is anindependent calculation. These calculations need to be long (1-2 weeks) as themotion is sluggish and the system size is large.I started withuniverse = Vanillarequirements = Arch == "x86_64" && OpSys == "LINUX" && Memory > 500executable = /home/rmlb/bin/dlpolyilinitialdir = w1$(Process)log = test.log output = test.out error = test.errshould_transfer_files = YESwhen_to_transfer_output = ON_EXIT_OR_EVICTtransfer_input_files = /home/rmlb/bin/dlpolyil, CONTROL, FIELD, CONFIGQueue 8However the chemistry grid was then rather unstable and tended to crash after a dayor two.66. CamgridI then collaborated with Maxim Fedorov and Andrej Frolov who did the probecalculations on a supercomputer.However I continued to use CamGrid to get properties of the neat ionic liquid nearthe wall. These calculations are shorter but independent runs are still useful. Toovercome the problem with crashing, I divided the runs into 48hour portions andused DAG to run them in succession, averaging the results afterwards with the scriptcat.csh.DAGJOB A runjobJOB B runjobJOB C runjobSCRIPT POST A cat.csh 1 mba95SCRIPT POST B cat.csh 2 mba95SCRIPT POST C cat.csh 3 mba95PARENT A CHILD BPARENT B CHILD Ccat.csh#! /bin/csh#cp $2d.his $2$1.hiscp $2d.zpr $2$1.zprcat $2d.out » $2all.outcat $2d.sen » $2all.senwhich deals with the output files mba952d.his,mba952d.out, ... from each run77. Layering in the ionic liquidDensity changes as a function of wallcharge density. The wall is at z = 0.Cation orientation - 〈P2(cosθ)〉.The cations are flat rings and areoriented by the wall.88. Simple Continuum theories for charged walls & mediumq -q' q' -qIn a polar solvent the solvent is polarisedso that the field F due to the walls insidethe medium is reduced by a factor of εs(the relative permittivity or dielectricconstant).Fz =qε0εsThis can be described by surface charges±q′ on the boundaries of the medium.Fz =q−q′ε0with q′ = (1− ε−1s ).In a conducting medium the field inside the medium is zero and q′ = q.In the continuum model this charge is localised on the surface. Real liquids, however,do not have sharp edges to cavities and the compensating charge q′ is spread overmolecular distances.99. Solvent polarisation by charged wallSolvent charge density ρ(r) as a functionof distance from positive and negativecharged walls (±1e/nm2) in [dmim][Cl].The total charge per nm2 withindistance z from the walls. Successivelayers overcompensate the chargeincluded within them.M.V.Fedorov and RMLB (in preparation)1010. Screening - electrostatic potential in liquidThe electrostatic potential in the liquid between charged walls is the sum of thepotential due to the walls and the potential due to the solvent charge density.ψ(r) = ψwall(z)+ ψsolv(z)whereψwall(r) =−qzε0and ψsolv(r) can be calculated from the charge density in th liquid using Poisson’sequation to giveψ(z) =−(ε0)−1Z r0(z− z′)ρ(z′)dz′.We can calculate these potentials from the simulation results for ρ(z).1111. Potentials - near wallThese curves are plotted relative tozero potential at the charged walls.Again the solvent potential tends tocancel the bare ion potential so thatthe field due to the wall is screenedby the solvent and the potential isconstant in the bulk liquid.The potential reaches the bulkvalue by 2nm (20Å) and from 6 Å(0.6nm) to 20Å it oscillates aboutthe bulk value due to alternatinglayers of anions and cations.Wall charges are measured in e/nm2.1212. Free energy profilesPoisson potentials are probed by an infinitesimal charge in the liquid. In realexperiments we need to consider ions with finite size and charge.We constructed the free energy profiles(potentials of mean force) of ionsapproaching charged walls from the bulk.We averaged the force on probe ions at 30fixed distances from charged walls duringlong simulations of the confined ionicliquid. The free energy profile is then theintegral of the averaged force as a functionof distance.Large probe spheres 5Å in diameter and small probes 3.77Å were placed in theliquid. The large spheres are approximately the size of a BF−4 ion or the [dmim] ring,and the small spheres the size of a Cl− ion.1313. Free energy profilesThe neutral probe shows considerable oscillations as it approaches a wall, especiallyif it is positively charged. This is due to the layering in the ionic liquid.Charged probes show larger oscillations.There are large barriers for negative probes to reach contact with positive walls (orpositive probes to contact negative walls). This must slow electrochemical processesat the electrode.1414. Correlation with IL layering - negative probeThe minima in the probe free energiescorrelate with the maxima in the chlorideion density.The probe ‘likes’ to replace an ionic liquidanion.The barriers increase in positively chargedwalls.1515. Correlation with IL layering - positive probeSimilarly positive probes prefer thepositions which the [dmim] rings prefer.Consider the redox process where apositive ion is reduced to a neutral specieson contact with the wall.Consider a redox process where a neutralspecies is reduced to a negative ion oncontact with the wall.1616. Charged walls• The ionic liquid is layered.• The layering is affected by the wall charge.• The free energy profile of probes relative to charged walls is determinedmainly by the ionic liquid layering.• It is difficult for a cation to approach a negative wall due to a high barrier.After reduction the neutral species could diffuse away.• It would be easier for a neutral species to be reduced to an anion.• Similarly near a positive wall it would be easier to oxidise a neutral than ananion.1717. Conclusions• Ionic Liquids provide excellent screening of electrostatic interactions.• Simulations give insight at the molecular level.• CamGrid was useful, but for me there was a steep learning curve.1818. ThanksI would like to thank my collaborators:• Maxim Fedorov and Andrej Frolov (MPI Leipzig)

Simulations of ionic liquids near charged walls

Abstract

Similar works

Full text

Available Versions

Apollo (Cambridge)