An asymmetric restricted primitive model (ARPM) of electrolytes is proposed as a simple three parameter (charge q, diameter d, and charge displacement b) model of ionic liquids and solutions. Charge displacement allows electrostatic and steric interactions to operate between different centres, so that orientational correlations arise in ion-ion interactions. In this way the ionic system may have partly the character of a simple ionic fluid/solid and of a polar fluid formed from ion pairs. The present exploration of the system focuses on the ion pair formation mechanism, the relative concentration of paired and free ions and the consequences for the cohesive energy, and the tendency to form fluid or solid phase. In contrast to studies of similar (though not identical) models in the past, we focus on behaviours at room temperature. By MC and MD simulations of such systems composed of monovalent ions of hard-sphere (or essentially hard-sphere) diameter equal to 5 Å and a charge displacement ranging from 0 to 2 Å from the hard-sphere origin, we find that ion pairing dominates for b larger than 1 Å. When b exceeds about 1.5 Å, the system is essentially a liquid of dipolar ion pairs with a small presence of free ions. We also investigate dielectric behaviours of corresponding liquids, composed of purely dipolar species. Many basic features of ionic liquids appear to be remarkably consistent with those of our ARPM at ambient conditions, when b is around 1 Å. However, the rate of self-diffusion and, to a lesser extent, conductivity is overestimated, presumably due to the simple spherical shape of our ions in the ARPM. The relative simplicity of our ARPM in relation to the rich variety of new mechanisms and properties it introduces, and to the numerical simplicity of its exploration by theory or simulation, makes it an essential step on the way towards representation of the full complexity of ionic liquids.

1.
H.
Spohr
and
G.
Patey
, “
Structural and dynamical properties of ionic liquids: The influence of ion size disparity
,”
J. Chem. Phys.
129
,
064517
(
2008
).
2.
H. V.
Spohr
and
G.
Patey
, “
Structural and dynamical properties of ionic liquids: Competing influences of molecular properties
,”
J. Chem. Phys.
132
,
154504
(
2010
).
3.
H. V.
Spohr
and
G.
Patey
, “
The influence of water on the structural and transport properties of model ionic liquids
,”
J. Chem. Phys.
132
,
234510
(
2010
).
4.
E. K.
Lindenberg
and
G. N.
Patey
, “
How distributed charge reduces the melting points of model ionic salts
,”
J. Chem. Phys.
140
,
104504
(
2014
).
5.
E. K.
Lindenberg
and
G. N.
Patey
, “
Melting point trends and solid phase behaviors of model salts with ion size asymmetry and distributed cation charge
,”
J. Chem. Phys.
143
,
024508
(
2015
).
6.
M.
Malvaldi
and
C.
Chiappe
, “
From molten salts to ionic liquids: Effect of ion asymmetry and charge distribution
,”
J. Phys.: Condens. Matter
20
,
035108
(
2008
).
7.
W.
Silvestre-Alcantara
,
L.
Bhuiyan
,
S.
Lamperski
,
M.
Kaja
, and
D.
Henderson
, “
Double layer for hard spheres with an off-center charge
,”
Condens. Matter Phys.
19
,
13603
(
2016
).(2016).
8.
W.
Silvestre-Alcantara
,
D.
Henderson
,
J.
Wu
,
M.
Kaja
,
S.
Lamperski
, and
L. B.
Bhuiyan
, “
Structure of an electric double layer containing a 2:2 valency dimer electrolyte
,”
J. Colloid Interface Sci.
449
,
175
179
(
2015
).
9.
M. V.
Fedorov
and
A. A.
Kornyshev
, “
Ionic liquid near a charged wall. Structure and capacitance of electrical double layer
,”
J. Phys. Chem. B
112
,
11868
11872
(
2008
).
10.
M.
Fedorov
,
N.
Georgi
, and
A.
Kornyshev
, “
Double layer in ionic liquids: The nature of the camel shape of capacitance
,”
Electrochem. Commun.
12
,
296
299
(
2010
).
11.
M. A.
Gebbie
,
M.
Valtiner
,
X.
Banquy
,
E. T.
Fox
,
W. A.
Henderson
, and
J. N.
Israelachvili
, “
Ionic liquids behave as dilute electrolyte solutions
,”
Proc. Natl. Acad. Sci. U. S. A.
110
,
9674
9679
(
2013
).
12.
M. A.
Gebbie
,
H. A.
Dobbs
,
M.
Valtiner
, and
J. N.
Israelachvili
, “
Long-range electrostatic screening in ionic liquids
,”
Proc. Natl. Acad. Sci. U. S. A.
112
,
7432
7437
(
2015
).
13.
A. J.
Fry
, “
Strong ion-pairing effects in a room temperature ionic liquid
,”
J. Electroanal. Chem.
546
,
35
39
(
2003
).
14.
K. J.
Fraser
,
E. I.
Izgorodina
,
M.
Forsyth
,
J. L.
Scott
, and
D. R.
MacFarlane
, “
Liquids intermediate between ‘molecular’ and ‘ionic’ liquids: Liquid ion pairs?
,”
Chem. Commun.
2007
,
3817
3819
.
15.
B.
Kirchner
,
F.
Malberg
,
D. S.
Firaha
, and
O.
Holloczki
, “
Ion pairing in ionic liquids
,”
J. Phys.: Condens. Matter
27
,
463002
(
2015
).
16.
A. M.
Smith
,
A. A.
Lee
, and
S.
Perkin
, “
The electrostatic screening length in concentrated electrolytes increases with concentration
,”
J. Phys. Chem. Lett.
7
,
2157
2163
(
2016
).
17.
Y.
Zhang
and
E. J.
Maginn
, “
Direct correlation between ionic liquid transport properties and ion pair lifetimes: A molecular dynamics study
,”
J. Phys. Chem. Lett.
6
,
700
705
(
2015
).
18.
M.
Sha
,
H.
Dong
,
F.
Luo
,
Z.
Tang
,
G.
Zhu
, and
G.
Wu
, “
Dilute or concentrated electrolyte solutions? Insight from ionic liquid/water electrolytes
,”
J. Phys. Chem. Lett.
6
,
3713
3720
(
2015
).
19.
O.
Holloczki
,
F.
Malberg
,
T.
Welton
, and
B.
Kirchner
, “
On the origin of ionicity in ionic liquids. Ion pairing versus charge transfer
,”
Phys. Chem. Chem. Phys.
16
,
16880
16890
(
2014
).
20.
M. A. B. H.
Susan
,
A.
Noda
, and
M.
Watanabe
, in
Electrochemical Aspects of Ionic Liquids
, 2nd ed., edited by
H.
Ohno
(
Wiley
,
New York
,
2011
), pp.
65
85
.
21.
D.
Wu
,
D.
Chandler
, and
B.
Smit
, “
Electrostatic analogy for surfactant assemblies
,”
J. Phys. Chem.
96
,
4077
4083
(
1992
).
22.
G. S.
Fanourgakis
, “
An extension of Wolfs method for the treatment of electrostatic interactions: Application to liquid water and aqueous solutions
,”
J. Phys. Chem. B
119
,
1974
1985
(
2015
).
23.
D.
Wolf
,
P.
Keblinski
,
S. R.
Phillpot
, and
J.
Eggebrecht
, “
Exact method for the simulation of Coulombic systems by spherically truncated, pairwise r−1 summation
,”
J. Chem. Phys.
110
,
8254
8282
(
1999
).
24.
S.
Plimpton
, “
Fast parallel algorithms for short-range molecular dynamics
,”
J. Comput. Phys.
117
,
1
19
(
1995
).
25.
J.
Jover
,
A. J.
Haslam
,
A.
Galindo
,
G.
Jackson
, and
E. A.
Müller
, “
Pseudo hard-sphere potential for use in continuous molecular-dynamics simulation of spherical and chain molecules
,”
J. Chem. Phys.
137
,
144505
(
2012
).
26.
S.
Plimpton
,
R.
Pollock
, and
M.
Stevens
, in
Proceedings of the Eighth SIAM Conference on Parallel Processing for Scientific Computing, 1997
.
27.
K.
Ma
,
J.
Forsman
, and
C. E.
Woodward
, “
Influence of ion pairing in ionic liquids on electrical double layer structures and surface force using classical density functional approach
,”
J. Chem. Phys.
142
,
174704
(
2015
).
28.
M.
Neumann
, “
Dipole moment fluctuation formulas in computer simulations of polar systems
,”
Mol. Phys.
50
,
841
858
(
1983
).
29.
J.
Kolafa
and
L.
Viererblova
, “
Static dielectric constant from simulations revisited. Fluctuations or external field?
,”
J. Chem. Theory Comput.
10
,
1468
1476
(
2014
).
30.
M.-M.
Huang
,
Y.
Jiang
,
P.
Sasisanker
,
G. W.
Driver
, and
H.
Weingärtner
, “
Static relative dielectric permittivities of ionic liquids at 25 °C
,”
J. Chem. Eng. Data
56
,
1494
1499
(
2011
).
31.

It will be a fluid, since the volume fraction is chosen to be slightly below the freezing point of a hard-sphere fluid, and the corresponding dumbbell fluid will freeze at even higher volume fractions.

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