Hyungmook Kang,1,2 David E. Suich,3 Robert Kostecki,3 Jeffrey J. Urban 1,*

1 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA2 Department of Mechanical Engineering, University of California, Berkeley, CA

3 Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, CA

Molecular Insight into the Lower Critical Solution Temperature Transition of Ionic Liquids

2019 LAMMPS Workshop and Symposium August 15, Albuquerque NM

Introduction

Te

mp

era

ture

composition

1 phase

2 phases

LCST

2 phases

UCST

Visualization

50 wt.% [P4444][DMBS]

LCST behaviorA thermo-responsive spontaneous phase-separation behavior

At temperatures below LCST, the system is completely miscible in all proportions, whereas above LCST partial liquid miscibility occurs.

with the critical temperature, called the Lower Critical Solution Temperature

Forward Osmosis desalination process based on Ionic Liquids with LCST behavior

Feed DrawH2O

NFFO

solar or waste heat

draw agent regeneration

water with small molecule IL

sea water of brackish waterpurified water

draw agentrecycling

H2O

H2O

H2O

Nile Red dye dissolved by IL

39 oC35 oC

T ↑

Particle size by Dynamic Light Scattering (DLS)50 wt.% [P4444][DMBS]

Scientific QuestionsHow does the molecular environment depend on temperatureand LCST phase transition ?

Why can a minor chemical change induce markedly different phenomena ?

[P4444][DMBS] [P4444][BnzSO3]

with LCST behavior without LCST behavior

1.

2.

At above Tc range, a stronger attractive interaction

was observed between the cations and anions.

Radial Distribution Functions between P cation – S anion

The mostly charged atom represents the position of each ion. i.e. P atoms for the cations, S atoms for the anions

The more thermal motion at higher temperature

0 5 10 150

1

2

3

4

0 5 10 150

0.5

1

1.5

g(r)

r (A)o

anion-anion

0 5 10 150

0.5

1

1.5

2

g(r)

r (A)o

cation-cation

0 5 10 150

1

2

3

4

cation-anion

g(r)

r (A)o

2 4 6 80

1

2

3

water-anion

g(r)

r (A)o

0 5 10 150

0.2

0.4

0.6

0.8

1

1.2

water-cation

g(r)

r (A)o

2 4 6 801234567

water-water

g(r)

r (A)o

a)

b)

c)

d)

e)

f)

T = 20 Co

50 Co

P+

Bu

Bu Bu

Bu

SO3-

0 5 10 150

1

2

3

4

g(r

)

r (Å)

Typical trend

P S

0 5 10 150

1

2

3

0 5 10 150

1

2

3

0 5 10 150

1

2

3

0 5 10 150

1

2

3

0 5 10 150

1

2

3

4

5

cation-anion

g(r)

r (A)o

0 5 10 150

0.5

1

1.5

2

g(r)

r (A)o

0 5 10 150

1

2

3

g(r)

r (A)o

cation-cation

anion-anion

0 5 10 150

0.20.40.60.81

1.2

water-cation

g(r)

2 4 6 80

2

4

6

water-water

g(r)

r (A)o

2 4 6 80

0.5

1

1.5

2

2.5

water-anion

g(r)

r (A)o

r (A)o

a)

b)

c)

d)

e)

f)

T = 10 Co

20 Co

60 Co50 Co

r (Å)

g(r

)

P S

Counter-intuitive

0 5 10 150

0.5

1

1.5

2

the peaks between 5 and 12 Å can be interpreted as all located in the first coordination shell.

between P cation – P cation

g(r

)

r (Å)

0 5 10 150

1

2

3

4

5

cation-anion

g(r)

r (A)o

0 5 10 150

0.5

1

1.5

2

g(r)

r (A)o

0 5 10 150

1

2

3

g(r)

r (A)o

cation-cation

anion-anion

0 5 10 150

0.20.40.60.81

1.2

water-cation

g(r)

2 4 6 80

2

4

6

water-water

g(r)

r (A)o

2 4 6 80

0.5

1

1.5

2

2.5

water-anion

g(r)

r (A)o

r (A)o

a)

b)

c)

d)

e)

f)

T = 10 Co

20 Co

60 Co50 Co

[P4444][DMBS] with LCST behavior

~ 6 Å

[P4444]+

Since the RDF is averaged data, the multiple peaks come from different orientations so thus mean the ions are loosely tied.

The peaks are merged at elevated temperatures and a major peak is observed at above Tc range.

- The ions are compactly tied to each other by the strong electrostatic forces

- The ion-pair aggregation is in the dense form of multiple layers of cation-anion shells.

Evidence of clustering at above Tc range

but, what drives this phenomena?

P+

P+

Considering the size of the single cation with even C4H9 group,

P P

Counting Hydrogen bonds in MD simulation

2

( )( )4n

dn rg rr drr p

= !"#$%&'(&) = +,

-./0423-4 3 5"63

g(r

)

RDF between O and H in water

H-bonds< r = 2.45 A

r[A] r[A]

g(r

)

RDF between O in anions and H in water

H-bonds< r = 2.45 A

(The first coordination shell)

Target Application

Motivation

LAMMPS set up

Validation of MD results

39 oC35 oC

Draw Agents: Ionic Liquids with LCST behaviorcation

tetrabutylphosphonium

[P4444]+

anion

4-toluenesulfonate

[ToS]-

2,4-dimethylbenzenesulfonate4-vinylbenzenesulfonate

[VBS]- [DMBS]-

0

1

2

3

4

5

6

Dra

wn

wa

ter

in 1

h (

g)

[VBS] [ToS] [DMBS]

Common

[P4444]+ cation

FO performance using 10 wt.% ILs as draw solutions

Best

candidate

More water molecules participate in perfecting the hydration shell of ILs

(there is essentially no free water for solutions > 50 wt.%)

The gain or loss of the directional bonding

has been used to explain the behavior of small molecules

[G.D. Smith et. al., Phys Rev Lett, 2000]

HydrationHydrogen bonds

or

Water molecules in these systemsessentially contribute to Main H-bonds in these systems

H

0 5 10 150

0.5

1

1.5

g(r)

r (A)o

anion-anion

0 5 10 150

0.5

1

1.5

2

g(r)

r (A)o

cation-cation

0 5 10 150

1

2

3

4

cation-anion

g(r)

r (A)o

2 4 6 80

1

2

3

water-anion

g(r)

r (A)o

0 5 10 150

0.2

0.4

0.6

0.8

1

1.2

water-cation

g(r)

r (A)o

2 4 6 801234567

water-water

g(r)

r (A)o

a)

b)

c)

d)

e)

f)

T = 20 Co

50 Co

P+

Bu

Bu Bu

Bu

SO3-

Common cation

[P4444]+

The hydration shell of ILs is the necessary requirement for the LCST

25 30 35 40100

101

102

103

104

25 30 35 40 45 50 550

1002003004005006007008009001000

a

b Temperature ( C)o

Temperature ( C)o

Par

ticle

siz

e (n

m)

Par

ticle

siz

e (n

m)

Miscible phaseAqueous phaseIonic liquid phase

SO3- P+

Bu

BuBu

Bu

SO3- P

+Bu

BuBu

Bu

Tc

25 30 35 40100

101

102

103

104

25 30 35 40 45 50 550

1002003004005006007008009001000

a

b Temperature ( C)o

Temperature ( C)o

Par

ticle

siz

e (n

m)

Par

ticle

siz

e (n

m)

Miscible phaseAqueous phaseIonic liquid phase

SO3- P+

Bu

BuBu

Bu

SO3- P

+Bu

BuBu

Bu

Tc

50 wt.% [P4444][BnzSO3]vs.

Discussion

Work at the Molecular Foundry was supported by the Office ofScience, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Acknowledgement

0 10 20 30 40 50 60 701

1.2

1.4

1.6

1.8

2

2.2

2.4

0 10 20 30 40 50 60 701

1.2

1.4

1.6

1.8

2

2.2

2.4

Temperature ( C)o

Num

ber o

f H-b

onds

per

O a

tom O -H A W

O -H W W*SO3

-

[BnzSO3]

SO3-

[DMBS]

without LCST behavior

0 10 20 30 40 50 60 701

1.2

1.4

1.6

1.8

2

2.2

2.4

T ( C)o

Num

ber o

f H-b

onds

O -H A W

O -H W W*

0 10 20 30 40 50 60 701

1.2

1.4

1.6

1.8

2

2.2

2.4

Temperature ( C)o

Num

ber o

f H-b

onds

per

O a

tom O -H A W

O -H W W*SO3

-

[BnzSO3]

SO3-

[DMBS]

0 10 20 30 40 50 60 701

1.2

1.4

1.6

1.8

2

2.2

2.4

Temperature ( C)o

Num

ber o

f H-b

onds

per

O a

tom O -H A W

O -H W W*SO3

-

[BnzSO3]

SO3-

[DMBS]

0 10 20 30 40 50 60 701

1.2

1.4

1.6

1.8

2

2.2

2.4

Temperature ( C)o

Num

ber o

f H-b

onds

per

O a

tom O -H A W

O -H W W*SO3

-

[BnzSO3]

SO3-

[DMBS] with LCST behavior

Number of Hydrogen bonds per O atom

Hydrogen bonds

- Below Tc: 10 oC, 20 oC

- Above Tc: 50 oC, 60 oCRef. critical T = 36 oC at 50 wt.%

MD Running

Timestep : 1.0 fs

Total timesteps : 10 M timesteps = 10 ns for every T cases (after pre-running for equilibration)

Isothermal-Isobaric ensemble (NPT)

• [P4444][DMBS] : 80 pairs of ILs & 1920 water molecules (= 50.8 wt.%)

with LCST behavior

• [P4444][BnzSO3] : 80 pairs of ILs & 1440 water molecules (= 49.6 wt.%)without LCST behavior

Initial placement

Potentials • pair style : lj/cut/tip4p/long, cutoff distance 15 A (LJ),13 A (charge) + pppm algorithm

• bond, angle style : harmonic

• dihedral style : charmm

• special style : amber[G. Zhou et. al., J Phys Chem B, 2007]

[X. He et. al., J Phys Chem B, 2010]

Parameters from

Ideal draw agents should possess

• high osmotic pressure

• being easily regenerated with only low grade heat

• low reverse diffusion, viscosity

• good chemical and thermal stability

• low toxicity

[DMBS]-

[P4444]+

P+

S

[P4444][DMBS] with LCST behavior50 wt.%

[P4444][BnzSO3] without LCST behavior50 wt.%

[BnzSO3]-

[P4444]+

P+

S

0 5 10 150

1

2

3

4

5

cation-anion

g(r)

r (A)o

0 5 10 150

0.5

1

1.5

2

g(r)

r (A)o

0 5 10 150

1

2

3

g(r)

r (A)o

cation-cation

anion-anion

0 5 10 150

0.20.40.60.81

1.2

water-cationg(

r)

2 4 6 80

2

4

6

water-water

g(r)

r (A)o

2 4 6 80

0.5

1

1.5

2

2.5

water-anion

g(r)

r (A)o

r (A)o

a)

b)

c)

d)

e)

f)

T = 10 Co

20 Co

60 Co50 Co

O

HO

H

email: hmkang@berkeley.edu (H.Kang) jjurban@lbl.gov (J.J.Urban)

Published as [H. Kang et. al., Comms Chem, 2 (51) 2019]