Molecular Simulation of Aqueous Electrolyte Solutions · Laboratory of Engineering Thermodynamics...

Laboratory of Engineering Thermodynamics Prof. Dr.-Ing. H. Hasse

Molecular Simulation of Aqueous

Electrolyte Solutions

Stephan Deublein1, Steffen Reiser1, Jadran Vrabec2, Hans Hasse1

InMoTher Lyon, March, 19th, 2012

1 Laboratory of Engineering Thermodynamics, University of Kaiserslautern, Germany 2 Thermodynamics and Energy Technology, University of Paderborn, Germany


02.04.2012 2 InMoTher 2012

Electrolytes in Aqueous Solution –

Applications

Buffer solutions in pharmaceutical

and biochemical industry /

purification of proteins

CO2 -storage in saline aquifers


Ions

1 CLJ

1 point charge

-

Simulation of aqueous electrolyte solution

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Molecular models

Water

1 CLJ

3 point charges + +

-

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Reference property:

• Density ρ

Minimize the influence of uncertainties in water model:

• Reduced density:

Electrolyte solution

Solvent


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Literature models NaCl

• Scattering of model parameters also for other salts

• Large deviations from experiments InMoTher 2012

Parameters Cl-:

3.5 < sCl-/ Å < 4.8

38.0 < eCl-/ K < 118.0

Parameters Na+ :

1.9 < sNa+/ Å < 4.1

0.06 < eNa+/ K <1068.8

Reduced density of NaCl solutions (T = 293 K, p = 1 bar)


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Adjustable parameters

• Ions: 1 CLJ with 1 point charge (±1e) – 2 parameters

Parameter optimization for alkali halides

Simulation conditions

• MC simulations at T = 293 K, p = 1 bar

• SPC/E water model

• Simulation code: extended version of ms21

1: Deublein et al.; CPC; 2011 ; http://www.ms-2.de

-

sIon, eIon

Target

• Reduced density for varying salinity

El

S

( , , , , )x

s s e e

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Sensitivity study

Sensitivity of simulation

results of :

• sIon dominant

• eIon negligible

Sim Sim

+ -(m) (m)

d d( , )

d dm

x x

s s

Parameter adjustment:

Sim Sim

Ions

Sim

+ -

( , )

( , , )

x

x

s

s s

m

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Reduced density of NaCl solutions (T = 293 K, p = 1 bar)

Sim


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Ion parameterization

Electrolyte systems:

5 cations: Li+, Na+, K+, Rb+, Cs+

4 anions: F-, Cl-, Br-, I-

20 different salts

9 parameters

Sim Sim

+ -(m) (m)

d d( , )

d dx x

s s

Exp Sim

+ -(m) (m)

!d d( , )

d dx x

s s

with s+ [1.5;4.5]

s- [2.0;4.5]

Global fitting approach:


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sLi = 1.88 Å sNa = 1.89 Å sK = 2.77 Å

sRb = 3.26 Å sCs = 3.58 Å

Anions

sF = 3.66 Å

sCl = 4.41 Å

sBr = 4.54 Å

sI = 4.78 Å

Aqueous electrolyte solutions - Results

Reduced density (T = 293 K, p = 1 bar)


Osmotic coefficient of water

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SS (n)

S

ln( )

ln( )

a

x

• Large statistical

uncertainties

• Reasonable match:

eNa = eCl = 200 K

Adjustment of the LJ energy parameters eIon to the osmotic

coefficient S of water in NaCl solutions (T = 293 K, p = 1 bar)

• Reasonable choice:

e+ = e- = 200 K

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r1. Max / Å r1. Min / Å

Sim. Exp. Sim. Exp.

Na – O 2.2 2.3 3.0 3.0

Cl – O 3.4 3.3 3.9 4.0

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Radial distribution functions of water

Na+

Cl-

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Dilute aqueous NaCl solutions (T = 298 K, p = 1 bar)


Self-diffusion coefficient of alkali cations

and halide anions in aqueous solution

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Predictions (T = 298K, p = 1 bar)

Water model: SPC/E

Concentration: xEL(n)= 0.018 mol/mol

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Electric conductivity of NaCl and CsCl in

aqueous solution at various salinities

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Predictions (T = 298K and p = 1 bar)

Water model: SPC/E

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Electric conductivity:

• Correlated motion of ions

in solution


-

Earth-alkali cations: Modeling

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1 CLJ

1 point charge

s, e

Adjustment of the cation model parameters:

• Reduced density of the aqueous earth-alkali halide

solution (T = 293 K, p = 1 bar):

El

s

Molecular model Counterion and solvent model

+ + +

- 2+

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Aqueous electrolyte solutions - Results

sBe = 1.69 Å

sMg = 1.77 Å

sCa = 2.58 Å

sSr = 2.69 Å

sBa = 3.12 Å

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Reduced density (T = 293 K, p = 1 bar)


Self-diffusion coefficient of earth-alkali

cations in aqueous solution

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Water model: SPC/E

Concentration:

xEL(n)= 0.018 mol/mol

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Electric conductivity of MgCl2 and BaCl2 in

aqueous solution at various salinities

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Water model: SPC/E



New atomistic force field for ions

Alkali cations: Li+, Na+, K+, Rb+, Cs+

Earth-alkali cations: Be2+, Mg2+, Ca2+, Sr2+, Ba2+

Halide anions: F-, Cl-, Br-, I-

Model adjustment

Reduced density

Osmotic coefficient

Good agreement with experimental data

Structural properties – radial distribution function

Self-diffusion coefficients

Electric conductivity

Summary

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BACKUP

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Alkali halides

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Self-diffusion coefficients of Na+ / Cl- in

aqueous solutions

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T = 298,15 K, p = 1 bar

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Activity of water in NaCl solution

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T = 298,15 K, p = 1 bar

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Hydration number of NaCl in solution

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Na+

Cl-

n (xNaCl=0.09 g /g ) n (xNaCl=0.15 g /g )

Sim. Exp. Sim. Exp.

Na – O 5.5 5 – 6 5.5 5 – 6

Cl – O 7.2 7 7.6 7


Electric conductivity

02.04.2012 23 InMolTher 2012

Time correlation function:

B

1( ) (0)

3 ot

Vk Ts

j j

Electric current flux:

Ion

1

( ) ( )N

k k

k

t q t

j v

Ion Ion Ion

2

1 1 1,

( ) (0) ( ) (0) ( ) (0)N N N

k k k k n k n

k k n n k

t q t q q t

j j v v v v

Split electric current flux:


Electric conductivity - split

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Separation in diffusion (Z) and correlated motion of ions (D)

MgCl2

BaCl2

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Dynamic properties – Residence time

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• Definition:

• First hydration shell ends at the first

minimum in the RDF

Pairs

0( t ) ( )( t )

N

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Temperature dependence

02.04.2012 26 Stephan Deublein

Lowering of vapor pressure:

Pure solvent Electrolyte

EL Sp p pD

T / K DpSim / bar DpExp / bar

373 0.03 0.04


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Earth-alkali halides

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Enthalpy of solvation

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D hsolvSim

/ kJ mol-1

D hsolvExp

/ kJ mol-1

MgCl2 2462 2683

MgBr2 2430 2615

MgI2 2378 2531

BaCl2 1972 2067

BaBr2 1940 1999

BaI2 1891 1915

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Complex salts

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Multicenter Lennard-Jones models –

Na2SOH4

02.04.2012 30

Sulfate - model

reproduces the reduced

density in aqueous

solution

InMolTher 2012

Molecular Simulation of Aqueous Electrolyte Solutions · Laboratory of Engineering Thermodynamics...

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