Context of this research Economical and ecological sustainability

Synthesis • molecular design: anion / kation

renewable raw materials

•hydrophobic - hydrophylic ILs

• renewable resources

• screening melting point

• large scale synthesis

Glass formation of ionic liquids

• viscosity; electrical conductivity

• recycling in catalysis and electrodeposition

feedback

Elektrodeposition recycling ILs

• nucleation, growth, morphology of coatings • optimized deposition

standard and exotic coatings

Christ Glorieux

Physical and chemical behavior • thermophysical behavior • solvability

Catalysis recycling ILs

new ionic liquids

• elektrochemical window

Applications

Laboratory for Acoustics and Thermal Physics Department of Physics and Astronomy Katholieke Universiteit Leuven Celestijnenlaan 200D, B-3001 Heverlee, Belgium

feedback

relaxation / viscosity

• (T-dep.) mixing/demixing reaction agents/products

• T-dependent phase behavior

• catalytic selectivity and activity

Leuven Summer School on Ionic Liquids, Leuven (Belgium), 23-27 August 2010

Technologically relevant physical properties of ionic liquids Technological relevance property/application

solvent in chemical reactions

electrolyte for electrodeposition

high electrical conductivity

0

++

low viscosity

++ ++

++ +

fast thermal transport

temperature driven demixing

++

+

Technologically relevant physical properties of ionic liquids

Scientific relevance

Material Physics of glass formation

Physics of critical phenomena

Þ study of ionic mobility, visco-elasticity and thermal transport properties Þ study of temperature driven mixing/demixing Þ relation between ionic structure and macroscopic physical behavior Þ tuning of physical properties – molecular design

Silver Copper Annealed Copper Gold Aluminium Sea water Drinking water Deionized water (10-7M ions)

Electrical conductivity (S·m-1) 63.0 × 106 59.6 × 106 58.0 × 106 45.2 × 106 37.8 × 106 4.8 x 100 5 x 10-2 – 5 x 10-4

Electrical conductivity • Typical electrolytes: (100 - 101) S.m-1 • Ionic liquids (100M ions): (10-2 – 102) S.m-1

5.5 × 10-6

Thermal conductivity of • water: 0.6 W.m-1.K-1 • other liquids: (0.1-0.3) W.m-1.K-1

Viscosity (at room temperature) of • water: 10-2 Poise = 10-3 Pa.sec • ionic liquids: 10-1 - 101 Poise = 10-2 - 100 Pa.sec

http://en.wikipedia.org/wiki/Electrical_conductivity, last viewed on 13-8-2010

Outline

Outline

Mobility and glass formation

Mobility and glass formation

Temperature dependence of electric transport properties by impedance spectrometry

Temperature dependence of electric transport properties by impedance spectrometry

Thermal characterization by photopyroelectric spectroscopy

Thermal characterization by photopyroelectric spectroscopy

Thermoelastic characterization by impulsive stimulated scattering

Thermoelastic characterization by impulsive stimulated scattering

Mobility and glass formation

Mobility and glass formation

Mechanical relaxation in supercooled liquids

Mechanical relaxation in supercooled liquids

0.2 0.15

Intermolecular potential

0.1

high temperature: soft low temperature: stiff

0.05

V

V

0 -0.05 -0.1 -0.15 -0.2 -0.25 1

1.2

1.4

1.6

1.8

rr àCurvature of intermolecular potential V(r) ~ stiffness

2

à softening with increasing temperature

Impulsive stimulated light scattered from glass-forming liquids. II. Salol relaxation dynamics, nonergodicity parameter, and testing of mode coupling theory, Yongwu Yang and Keith A. Nelson, J. Chem. Phys., Vol. 103, No. 18, 8 November 1995

à1D plane longitudinal acoustic wave àdisordered molecular networks act stiff on a short time scale (high frequencies), and soften towards longer times (low frequencies), due to the increased possibilities for cooperative network rearrangements (Û more statistical coverage of pathways towards new minima in soft potential energy landscape) Þ time scale and frequency dependent mechanical response

Mobility and glass formation

Mobility and glass formation

Mechanical relaxation in supercooled liquids

Mechanical relaxation in supercooled liquids àTime (scale) dependent (effective) potential energy landscape 2

Long time limit: more V(r)(t) probed à softer average environment

Note: V(r) involves many molecules/ions with many coordinates

1.5

2 1 0 -1

snapshot of soft V(r) V

1

V

0.5

1

0

1.5

2

r

1D plane longitudinal acoustic wave -0.5 -1

1

1.2

-5

200

220 240 260 temperature (K)

viscosity (Pa.sec)

10

10

10

10

280

10

10

300

5

0

-5

-10

180

200

220 240 260 temperature (K)

280

20

300

10

10

10

5

0

-5

3.5

4

4.5 1000/T (K-1)

10

15

viscosity (Pa.sec)

180

10

10

5

10

10 10

2

structural relaxation time (sec) glycerol

0

10

1.8

2 1 0 -1

1

1.5

2

r

Mechanical relaxation in supercooled liquids

10

structural relaxation frequency (Hz) glycerol

10

structural relaxation time (sec) glycerol

structural relaxation frequency (Hz) glycerol

10

5

1.6

Mobility and glass formation

Mechanical relaxation in supercooled liquids 10

1.4 r

àdisordered molecular networks act stiff on a short time scale (high frequencies), and soften towards longer times (low frequencies), due to the increased possibilities for cooperative network rearrangements (Û more statistical coverage of pathways towards new minima in soft potential energy landscape)

Mobility and glass formation 10

Short time limit: less V(r)(t) probed à stiffer average environment

snapshot of stiff V(r)

V

à crystal: only strong and coherent net forces in direction of propagation, simple potential energy landscape àamorphous network: net spring forces to be overcome are weaker in direction of propagation, and main wave motion couples to motions in other directions. Slower motions see a spring (potential energy) landscape with increased variability and delocalized features Þ more dissipation of energy to incoherent, cooperative motions. Fast motions encounter a more rigid landscape. Þ time scale and frequency dependent mechanical response

5

5.5

10

10

10

5

0

-5

3.5

4

5

5.5

4.5 1000/T (K-1)

5

5.5

15

10

5

0

150

200 250 temperature (K)

300

Glass formers at a given temperature are characterized by a strongly temperature dependent - characteristic relaxation time trelax below which they act stiff - characteristic relaxation frequency frelax = 1/( 2 p trelax ) above which they act stiff - viscosity h

3.5

4

4.5 1000/T (K-1)

The strong temperature dependence trelax(T), frelax(T) of a physical process related to molecular dynamics, and h(T), is typically represented in an Arrhenius plotà10log(trelax) vs 1000/T, 10log(frelax) vs 1000/T, 10log(h) vs 1000/T

Mobility and glass formation

Mobility and glass formation

PG(a)

PG(a’)

10

log (fpeak)

4

Glycerol(a) PG in pores(a’)

2 0 -2 -4 3.3

3.6

3.9

4.2

4.5

4.8

5.1

5.4

5.7

Glass transition temperature Tg t(Tg)=102 sec

10

h(Tg)=1012 Pa.sec

2 0

10 -2 10 -4 10 -6 10 -8 10

12

10 10 10 8 10 6 10 4 10 2 10

Tg 200 220 240 260 280 Tg temperature temperature (K)

Tg200 220 240 260 280 temperature (K)

Calorimetric Tg: from kink in differential scanning calorimetry (DSC) enthalpy curve H(T) at 10 K/min scanning rate

Dynamic Tg: trelax(Tg)= 102 seconds

6.0

1000/T K-1

viscosity (Pa.sec)

8 6

Different relaxation processes in supercooled liquids … some definitions

This work 3w method VFT fit structural relaxation ultrasonic relaxation Dielectric relaxation

DSC heat flow

10

structural relaxation time (sec) glycerol

Different relaxation processes in supercooled liquids

and PG specific heat capacity(acoustic, relaxation time versus temperature compared to rotational,… à Glycerol different physical processes thermal, thermal expansion, response) exhibit a Arrhenius plot with, for a given glass former, - a time/frequency scale that is of the same order but typically not equal - a curvature (º ‘fragility’) which is very similar “Broadband photopyroelectric thermal spectroscopy of a supercooled liquid near the glass transition”, E.H.Bentefour, C.Glorieux, M.Chirtoc and J.Thoen, Journal of Applied Physics 93(12), 9610-9614(2003) “Thermal relaxation of glycerol and propylene glycol studied by photothermal spectroscopy”, E. H. Bentefour, C. Glorieux, M. Chirtoc and J. Thoen, Journal of Chemical Physics 120(8), 3726-3731 (2004)

Kinematic Tg: viscosity h(Tg)=1013 Poise =1012 Pa.sec

Different relaxation processes in supercooled liquids … some definitions

Different relaxation processes in supercooled liquids … some definitions

Effective activation energy 10

10

10

æ E ö f relax = fT ®¥ exp ç - act ÷ è kT ø ß

5

Eact 10

10

10

10

0

Eact

Eact

log ( f relax ) =

10

log ( fT ®¥ ) - Eact

1 1000 1000 ln(10)k T

Arrhenius behavior (straight line): àtemperature independent activation energy Eact

-5

Eact -10

3

4

5

6

1000/T (K-1)

Non-Arrhenius behavior (curved line): àtemperature dependent activation energy Eact (T)

Temperature dependence of relaxation frequency: frelax = fvibrations x (probability of structural change per vibration period) ~ fvibrations x (Boltzmann factor) (àcfr Monte Carlo walk on the rhythm of vibrations) ~ fTॠx exp(-Eact/(kT)) with fvibrations ~ 1013 Hz ~ characteristic “structural information exchange frequency” º (shear velocity) /(characteristic length of regions of cooperativity) @ (103 m/s )/ (10-9 m) @ 1012 Hz

structural relaxation frequency (Hz) glycerol

Mobility and glass formation

structural relaxation frequency (Hz) glycerol

Mobility and glass formation

Temperature dependent effective activation energy à fragility 8

10 6 10 4 10 2 10 0 10 -2 10 -4 10 -6 10 -8 10

m= m’

m > m’ 0.7

0.8

Quasi-exponential behavior fTॠEact(T à¥)

0.9 Tg/T

d ( 10 log f relax ) æ Tg ö dç ÷ èT ø

m

1 T=Tg

Rather material independent

Curvature/fragility – glass transition temperature : material dependent

T =Tg

Mobility and glass formation

Outline

Different relaxation processes in supercooled liquids … some definitions Temperature dependent effective activation energy à fragility Angell plot of typical glass forming materials

Mobility and glass formation Low and high temperature limit of h(Tg/T) are rather material independent Þ mainly Tg and fragility are determining the molecular mobility (viscosity), which in turn should be maximized (minimized) for optimum ionic conductivity for electrolytic applications or chemical applications where rapid convective heat transfer or cooling is crucial.

Curvature – fragility : material dependent: strong/fragile glass formers Þ Physical understanding of microscopic situation resulting in curvature à chemical tuning of mobility of glass formers and thus the electrical transport properties of ILs at technologically relevant temperatures, by molecular design

Temperature dependence of electric transport properties by impedance spectrometry Thermal characterization by photopyroelectric spectroscopy

Thermoelastic characterization by impulsive stimulated scattering

C.A. Angell. Glass formation and glass transition in supercooled liquids, with insights from study of related phenomena in crystals. Journal of Non-Crystalline Solids, 354:4703–4712, 2008.

Temperature dependence of electric transport properties by dielectric spectroscopy

Temperature dependence of electric transport properties by dielectric spectroscopy

Extracting electric conductivity ionic liquids from dielectric spectroscopy

Extracting electric conductivity ionic liquids from dielectric spectroscopy

d

ìe'=e s ï º e '- ie '' Þ í s iw ïîe '' = w ì s '=s s eff (w ) = s + iwe º s '- is '' Þ í îs '' = -we

e eff (w ) = e +

S

Ionic liquid between electrodes of capacitor under electric voltage V: à rotational response: electric polarization (~ dielectric permittivity e) à translational response: electric current (~ electric conductivity s)

Note: e is real at low frequencies (w>ms) Light absorption at air-sensor interface

air Lp Ls

air Lp Ls

sensor sample backing

sensor sample backing

Thermal conductivity

Sample thermally thin (Ls