A classical density functional theory for the asymmetric restricted primitive model of ionic liquids
(2018) In Journal of Chemical Physics 148(19). Abstract
A new threeparameter (valency, ion size, and charge asymmetry) model, the asymmetric restricted primitive model (ARPM) of ionic liquids, has recently been proposed. Given that ionic liquids generally are composed of monovalent species, the ARPM effectively reduces to a twoparameter model. Monte Carlo (MC) simulations have demonstrated that the ARPM is able to reproduce key properties of room temperature ionic liquids (RTILs) in bulk and at charged surfaces. The relatively modest complexity of the model raises the possibility, which is explored here, that a classical density functional theory (DFT) could resolve its properties. This is relevant because it might generate great improvements in terms of both numerical efficiency and... (More)
A new threeparameter (valency, ion size, and charge asymmetry) model, the asymmetric restricted primitive model (ARPM) of ionic liquids, has recently been proposed. Given that ionic liquids generally are composed of monovalent species, the ARPM effectively reduces to a twoparameter model. Monte Carlo (MC) simulations have demonstrated that the ARPM is able to reproduce key properties of room temperature ionic liquids (RTILs) in bulk and at charged surfaces. The relatively modest complexity of the model raises the possibility, which is explored here, that a classical density functional theory (DFT) could resolve its properties. This is relevant because it might generate great improvements in terms of both numerical efficiency and understanding in the continued research of RTILs and their applications. In this report, a DFT for rodlike molecules is proposed as an approximate theoretical tool for an ARPM fluid. Borrowing data on the ion pair fraction from a single bulk simulation, the ARPM is modelled as a mixture of dissociated ions and connected ion pairs. We have specifically studied an ARPM where the hardsphere diameter is 5 Å, with the charge located 1 Å from the hardsphere centre. We focus on fluid structure and electrochemical behaviour of this ARPM fluid, into which a model electrode is immersed. The latter is modelled as a perfect conductor, and surface polarization is handled by the method of image charges. Approximate methods, which were developed in an earlier study, to take image interactions into account, are also incorporated in the DFT. We make direct numerical comparisons between DFT predictions and corresponding simulation data. The DFT theory is implemented both in the normal mean field form with respect to the electrostatic interactions and in a correlated form based on hole formation by both steric repulsions and ionion Coulomb interactions. The results clearly show that ionion correlations play a very important role in the screening of the charged surfaces by our ARPM ionic liquid. We have studied electrostatic potentials and ion density profiles as well the differential capacitance. The meanfield DFT fails to reproduce these properties, but the inclusion of ionion correlation by a simple approximate treatment yields quite reasonable agreement with the corresponding simulation results. An interesting finding is that there appears to be a surface phase transition at relatively low surface charge which is readily explored by DFT, but seen also in the MC simulations at somewhat higher asymmetry.
(Less)
 author
 Lu, Hongduo ^{LU} ; Nordholm, Sture; Woodward, Clifford E. and Forsman, Jan ^{LU}
 organization
 publishing date
 20180521
 type
 Contribution to journal
 publication status
 published
 subject
 in
 Journal of Chemical Physics
 volume
 148
 issue
 19
 publisher
 American Institute of Physics
 external identifiers

 scopus:85041478442
 ISSN
 00219606
 DOI
 10.1063/1.5013134
 language
 English
 LU publication?
 yes
 id
 4796b5ce22fa4ef8bd8b9536c509be3e
 date added to LUP
 20180220 08:37:29
 date last changed
 20180220 08:37:29
@article{4796b5ce22fa4ef8bd8b9536c509be3e, abstract = {<p>A new threeparameter (valency, ion size, and charge asymmetry) model, the asymmetric restricted primitive model (ARPM) of ionic liquids, has recently been proposed. Given that ionic liquids generally are composed of monovalent species, the ARPM effectively reduces to a twoparameter model. Monte Carlo (MC) simulations have demonstrated that the ARPM is able to reproduce key properties of room temperature ionic liquids (RTILs) in bulk and at charged surfaces. The relatively modest complexity of the model raises the possibility, which is explored here, that a classical density functional theory (DFT) could resolve its properties. This is relevant because it might generate great improvements in terms of both numerical efficiency and understanding in the continued research of RTILs and their applications. In this report, a DFT for rodlike molecules is proposed as an approximate theoretical tool for an ARPM fluid. Borrowing data on the ion pair fraction from a single bulk simulation, the ARPM is modelled as a mixture of dissociated ions and connected ion pairs. We have specifically studied an ARPM where the hardsphere diameter is 5 Å, with the charge located 1 Å from the hardsphere centre. We focus on fluid structure and electrochemical behaviour of this ARPM fluid, into which a model electrode is immersed. The latter is modelled as a perfect conductor, and surface polarization is handled by the method of image charges. Approximate methods, which were developed in an earlier study, to take image interactions into account, are also incorporated in the DFT. We make direct numerical comparisons between DFT predictions and corresponding simulation data. The DFT theory is implemented both in the normal mean field form with respect to the electrostatic interactions and in a correlated form based on hole formation by both steric repulsions and ionion Coulomb interactions. The results clearly show that ionion correlations play a very important role in the screening of the charged surfaces by our ARPM ionic liquid. We have studied electrostatic potentials and ion density profiles as well the differential capacitance. The meanfield DFT fails to reproduce these properties, but the inclusion of ionion correlation by a simple approximate treatment yields quite reasonable agreement with the corresponding simulation results. An interesting finding is that there appears to be a surface phase transition at relatively low surface charge which is readily explored by DFT, but seen also in the MC simulations at somewhat higher asymmetry.</p>}, articleno = {193814}, author = {Lu, Hongduo and Nordholm, Sture and Woodward, Clifford E. and Forsman, Jan}, issn = {00219606}, language = {eng}, month = {05}, number = {19}, publisher = {American Institute of Physics}, series = {Journal of Chemical Physics}, title = {A classical density functional theory for the asymmetric restricted primitive model of ionic liquids}, url = {http://dx.doi.org/10.1063/1.5013134}, volume = {148}, year = {2018}, }