Spatial Heterogeneity in Ionic Liquids - ACS Symposium Series (ACS

Chapter 20 Spatial Heterogeneity in Ionic Liquids Yanting Wang, Wei Jiang, and Gregory A. Voth Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0975.ch020

Center for Biophysical Modeling and Simulation and Department of Chemistry, University of Utah, 315 South 1400 East Room 2020, Salt Lake City, UT 84112-0850

Electronically polarizable atomistic molecular dynamics models have been developed for the 1-alkyl-3methylimidazolium nitrate ionic liquids with the alkyl-chain length ranging from 2 to 12. The molecular dynamics simulations with these models confirm the spatial heterogeneity previously discovered by the multiscale coarse -grainedmodels (Wang & Voth, J. Am. Chem. Soc. 2005, 127, 12192). The global structures are monitored by a heterogeneity order parameter. The tail groups of cations are found to aggregate and distribute more heterogeneously than the headgroups of cations and the anions, forming discrete tail domains. The anions always stay close to the headgroups. The charged groups retain their local structures relatively unchanged, forming a continuous polar network, and leading to different global structures when varying the side-chain length. The ionic diffusion is slower with increasing alkyl -chain length. Based on these simulation results, a refined mechanism can be proposed, considering the competition among the Coulombic interactions, the collective short-range interactions, and the geometrical constraint from the intramolecular bonds. This mechanism can explain many experimental and simulation results, and is expected to be general for most kinds of ionic liquids. The finite size effects due to the limited simulation size are also discussed.

272

© 2007 American Chemical Society

In Ionic Liquids IV; Brennecke, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

273

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0975.ch020

Introduction Understanding the structural properties of ionic liquids is essential for the systematic design and application of ionic liquids. Ionic liquids are room temperature molten salts which may be used as a potential substitute for the traditional environment-unfriendly organic solvents. They have a much lower melting point compared to the inorganic molten salts, which is achieved by involving a bulky organic cation (7). Generally most of the partial charges on the cation are distributed around its headgroup. When the side chain is short, the tail groups also have non-zero effective partial charges. With increasing side-chain length, the partial charges on the tail groups become smaller, and the amphiphilic feature for cations increases accordingly. Because there are numerous ionic liquid systems (2) to be chosen from to meet specific applications, a theoretical understanding of the "tunability" of ionic liquids, such as the changes of their physical properties with various side-chain lengths, is important for guiding their efficient design and applications. Experiments have been done to investigate the behavior of ionic liquids with various cationic side-chain lengths. A liquid crystal phase has been observed for pure ionic liquids with long side chains (5-^ © -0.0826 C W / 0.2085 H4

_ y CW -0.1754 \ H4 0.2439 0

Q

9

2

2

5

C

0.0143 0.0063 0.0063 HI 0.0143 H1 0.0064 H1 0.0064 HI HI .HI


302

A2.5. CIO 0.0094 0.0094 0.0251 0.2221 0.1165 H1.

0.1165 ni ?

-0.1001

CT

H

H5 ι I

0.0596V

Λ


•0.0956

H1

Λ

0.0353 C T ^0 0182

©

J

C W

C W

/ 0.2101 H 4

HI

\

M

HI

0.0900

Ε

C T

/

X

C

^ H1

0

"00131 X /

T

C

/ H1

0.0144

J

Q

Q

0

H1

0

008

0

Q

H1

T

CT

Q

-0.0032 CT /

H1

0

0

5

9

0

0

H1

CT /

0.0174

H1

0.0031 0.0031

0

0

2

5

O.0055

.0069 H1

HC

\ / CT /

χ

\

0.0250

0

HC

\ /

χ

\

Q

5

H1

\ /

-nnnn

H1 0.0144

Q

H1

\ /

\ ^ « . CT 0 . 0 5 4 ^

HI

0 . 0 5 8 7 ^ ^ 0.1165

V ,

0.0900

^v.

V

0.0251

°T

0

/

-0.1006 X

.0363

\

H1

H

C

0.0250

H1

-0.0015 -0.0015 0.0019 0.0019

-0.1592

\ H 4 0.2394

A2.6. C12

0 2 2 0 1

n i 1

,

0

1

1

5

9

H

5

0

0

9

0

9

0

H1 Τ -0.0977V 01159

0

0

5

9

/

\

I «

H

»

0.0037 0.0037 0.0013 -0.0013 -0.0067 -0.0067 0.0013 00013 0.0211 0.0211 0.0238 0 0238 H1 H1 H1 HI H1 HI H1 H1 HC HC H1 HI \ / \ / \ / \ / \ / \ / CT 0.0053 CT 0.0091 CT 0.0147 CT 0.0080 CT -0.0899

i

J * ™ " , 5 ' ^ 0 2 2 9 7^.0005

/ 0 0 0 8 1

^

s *

0 0 1 6 4

/

X / * ™™

^

/ j*

HI H1 H1 H1 HI H1 H1 H1 Hi HI 0.0127 0.0127 0.0011 0.0011 -0 0032-0.0032 -0.0052-0.0052 0.0000 0.0000

5

-0.1023 CW / 0.2129 H4

X

CW -0 1549 \ H4 0.2374


\

303 A3.

Force Field Parameters 2

Bond parameters [k in kcal/(mol À ) and r


r

eq

in A ] .

bond

Κ

eq

bond

Κ

feq

CR-NA

954.0

1.343

CW-NA

854.0

1.381

CT-NA

674.0

1.475

CT-CT

620.0

1.526

CW-H4

734.0

1.080

CT-H1

680.0

1.090

CWCW

1040.0

1.370

CR-H5

734.0

1.080

CT-HC

680.0

1.090

NA-OS

600.0

1.260

r

2

Angle parameters [k in kcal/(mol radian ) and 0 in degree]. e

angle

eq

0

angle CW-NA-CT

k 140.0

125.8

120.0

NA-CR-H5

70.0

120.0

70.0

120.0

CW-CW-H4

70.0

128.2

NA-CT-H1

70.0

109.5

CT-CT-H1

100.0

109.5

H1-CT-H1

70.0

109.5

HC-CT-HC

70.0

109.5

CR-NA-CT

140.0

125.8

NA-CW-CW

140.0

120.0

NA-CT-CT

160.0

111.2

CT-CT-HC

100.0

109.5

CT-CT-CT

80.0

109.5

OS-NA-OS

300.0

120.0

CR-NA-CW

ke 140.0

120.0

NA-CR-NA

140.0

NA-CW-H4

eq

e


304


Dihedral angle parameters [V„ in kcal/mol and χ in degree]. dihedral

v„

r

η

dihedral

v„

r

Ν

HC-CT-CTNA

0.156

0.0

3

HC-CT-CTHl

0.156

0.0

3

CT-NA-CRH5

2.325

180.0

2

CT-NACR-NA

2.325

180.0

2

CT-NACW-CW

1.500

180.0

2

CT-NACW-H4

1.500

180.0

2

NA-CRNA-CW

2.325

180.0

2

CR-NACW-H4

1.500

180.0

2

CW-NACR-H5

2.325

180.0

2

CR-NACW-CW

1.500

180.0

2

NA-CWCW-NA

5.375

180.0

2

NA-CWCW-H4

5.375

180.0

2

CT-CT-CTHl

0.156

0.0

3

CT-CT-CTCT

0.156

0.0

3

Hl-CT-CTHl

0.156

0.0

3

NA-CT-CTCT

0.156

0.0

3

NA-CT-CTHl

0.156

0.0

3

HC-CT-CTCT

0.156

0.0

3

Hl-CT-NACW

0.0

0.0

2

Hl-CT-NACR

0.0

0.0

2

CW-NACT-CT

0.0

0.0

2

CR-NACT-CT

0.0

0.0

2

H4-CWCW-H4

5.375

180.0

2

H5-CR-NANA°

1.100

180.0

2

CR-CWNA-cr

1.100

180.0

2

H4-CWCW-NA"

1.100

180.0

2

OS-OS-NAOS°

50.000

180.0

2

"Improper torsions.



Van der Waals parameters [ε in kcal/mol and σ ί η Â].

6

pair

ε

σ

pair

ε

σ

CW— CW*

0.0860

3.400

CW— NA

0.121

3.325

CW—H5

0.0359

2.911

CW— HC

0.0371

3.025

CT—CT

0.109

3.400

CT—HI

0.0418

2.936

CT—HC

0.0418

3.025

CT—H4

0.0404

2.955

CT—CW

0.0968

3.400

HI—HI

0.0160

2.471

HI—HC

0.0160

2.560

HI—H4

0.0155

2.491

2.422

H5—HC

0.0155

2.536

H5—H5

0.0150

H5—NA

0.0505

2.836

HC—HC

0.0160

2.650

HC—NA

0.0522

2.950

H4—H4

0.0150

2.511

CW—HI

0.0371

2.936*

CW--H4

0.0359

2.955

CT--H5

0.0404

2.911

CT---NA

0.136

3.325

HI—H5

0.0155

2.446

HI—NA

0.0522

2.861

H5—H4

0.0150

2.466

HC—H4

0.0155

2.580

H4—NA

0.0505

2.880

OS—OS

0.170

3.001

NA—NA

0.170

3.250

NA—OS

0.170

3.125

HI—OS

0.0522

2.736

H4—OS

0.0505

2.756

CW—OS

0.121

3.200

CT—OS

0.136

3.200

H5—OS

0.0505

2.711

HC—OS

0.0522

2.825

C R type atom has the same van der Waals parameters as C W atom.


306

References 1. 2.


3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

24. 25.

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Spatial Heterogeneity in Ionic Liquids - ACS Symposium Series (ACS

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