supercritical fluids: investigation of local density inhomogeneities … · 2017. 5. 10. ·...
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Supercritical fluids: Investigation of Supercritical fluids: Investigation of local density inhomogeneities and local density inhomogeneities and related properties using computer related properties using computer
simulations.simulations.Ioannis SkarmoutsosIoannis Skarmoutsos
In collaboration with: In collaboration with: Elvira Guardia (DFEN, UPC, Barcelona, Spain)Elvira Guardia (DFEN, UPC, Barcelona, Spain)
Dimitris Dellis (Chemistry Dept., Univ. Athens, Greece)Dimitris Dellis (Chemistry Dept., Univ. Athens, Greece)Jannis Samios (Chemistry Dept., Univ. Athens, Greece)Jannis Samios (Chemistry Dept., Univ. Athens, Greece)
What is a supercritical (SC) fluid?What is a supercritical (SC) fluid?
P – TPhase Diagrams
T – VPhase Diagram
P – VPhase Diagram
CriticalPoint
0
0
0
2
2
c
c
c
TT
PP
TT
VP
VT
VP
TVTVT P
VVcc
1lim
,,
PVTVT TV
Vcc
1lim,,
Isothermal Compressibility
Thermal Expansion
SF6 : Going from vapor liquid equilibrium to supercritical state and back!!
Characteristic Properties of SC fluidsClose to the critical point:
●VERY SMALL pressure changes cause VERY LARGE changes in density and dielectric constant
consequences
● Tuning of dissolving capability of SC solvents by adjusting the pressure
● Significant increase of the chemical reaction rates in SC solvents
Transport properties of SC fluids in comparison with liquids:
● Self Diffusion: 1 order of magnitude higher than in the liquid state
● Viscosity: 1 order of magnitude lower than in the liquid state
Applications of SC fluidsIndustrial Toxic Waste Treatment
Separation Processes: Supercritical ChromatographyFragmentation of macromolecules
Synthesis of fine metal particlesMaterial Science: Synthesis of nanoparticles
Synthesis of porous materials
Green Chemistry
See:Eckert, C.A., Knutson, B.L., Debenedetti, P.G., Nature, 383, 313 (1996)Tucker, S.C., Chem. Rev., 99, 391 (1999)Kajimoto, O., Chem. Rev., 99, 355 (1999)Akiya, N., Savage, P., Chem. Rev., 102, 2725 (2002) … etc
NO hazardous waste streams, NO harsh organic chemicalsRecyclable solvents
Characteristic Local Microstructure in SC FluidsMicrostructure in SC Fluids
Local Density Inhomogeneities(LDI)
Why are we interested in these Local Density Why are we interested in these Local Density Inhomogeneities?Inhomogeneities?
They affect significantly the properties of SC fluids
And the answer is:
The isothermal compressibility factor is directly related with these density fluctuations and their spatial extent (correlation length ξ)
WHY?
Close to the Critical Point: COMPRESSIBLE REGIME
Open problems:Open problems:
● Influence of thermodynamic parameters (P,V,T) on the formation of LDI
● Interrelation of these inhomogeneities with the bulkthermodynamic, structural and dynamic propertiesof molecular SC fluids
● Effect of intermolecular interactions on the behavior of LDI
● Dynamic behavior of LDI, Mechanisms of local environment reorganization
Theoretical Investigation of LDITheoretical Investigation of LDI
cR2
c c0
N (R ) 4 r g(r)dr
c ceff ,l c ref
c c ref
N (R , )(R , )N (R , )
Statistical Mechanics – Molecular Simulation
How can we calculate local densities? From the pair radial distributionfunction g(r)
Effective local density
cR2
c c0
N (R ) 4 r g(r)dr
ref cρ 2ρ
eff ,l c eff ,l c(R , ) (R , )
Liquid-like structure
Coordination number
Local density augmentation (LDA)
Local density enhancement (LDE)
eff ,l cenh c
(R , )F (R , )
SC Ethanol
Skarmoutsos, I., Samios, J., J. Phys. Chem. B., 110, 21931 (2006)
SC Water
Skarmoutsos, I., Guardia, E.,J. Phys. Chem. B, 113, 8887
(2009)
Influence of Intermolecular Interactions Influence of Intermolecular Interactions on LDA and LDEon LDA and LDE
•• LDE decreases with density.LDE decreases with density.
•• LDA maximum in range 0.6LDA maximum in range 0.6--0.8 0.8 ρρcc..
•• LDE and LDA higher in polar, Hydrogen bonded LDE and LDA higher in polar, Hydrogen bonded fluids.fluids.
•• Amplitudes of LDE and LDA :Amplitudes of LDE and LDA :•• Xe < CO2, N2, CH4 < NH3 < MeOH < H2O Xe < CO2, N2, CH4 < NH3 < MeOH < H2O
•• No significant differences in LDE and LDA No significant differences in LDE and LDA between nonbetween non--dipolar systems.dipolar systems.
•• For Hydrogen Bonding fluids, Water LDE and For Hydrogen Bonding fluids, Water LDE and LDA sufficiently higher than those of MeOH and LDA sufficiently higher than those of MeOH and NH3.NH3.
benh
eF
0
1
11)(
Skarmoutsos, I., Dellis, D., Samios, J., J. Phys. Chem. B.,113,2783 (2009)
T/Tc = 1.03
Representative SnapshotsMethanol Ammonia
Xenon Water
First Shell of sc ethanol going from low to high densities
Nc = 2
Nc = 7
Nc = 4
Nc = 10
Local Density ReorganizationLocal Density Reorganization
l l l(t) (t)
l
l lρ 2
l
ρ (0) ρ (t)C (t)
ρ (0)
l l0
C (t) dt
Local Density Reorganization time
Length Scale Effects on theLength Scale Effects on theMechanisms of Local Density ReorganizationMechanisms of Local Density Reorganization
3 length scale regions with different density dependence oflocal reorganization times (and different time decay)
00
1
R3
R1
Second ShellFirst Shell
g (r
)
r
First Peak
R2
Region 1 : r --> 0 – R1
Region 2 : r --> R1 – R2
First Shell : r --> 0 – R2
Second Shell : r --> 0 – R3
At very short intermolecular distances:
‘Direct’ Potential induced mechanism
Goodyear, G., Maddox, M. W., Tucker, S.C., J. Phys. Chem. B, 104, 6240-6266 (2000)
At higher length scales: Collective effects start to affect thelocal density reorganization
As the length scale increase
The Local Density Reorganization timeis maximized in the density range close
to the critical density
Effect of critical slowing down on local density dynamics
Density Dependence of local reorganization Density Dependence of local reorganization timestimesSC Ethanol
Skarmoutsos, I., Guardia, E., J. Phys. Chem. B. 113, 8887 (2009)
Local density reorganization as a function of densityand length scale
The Local Density Reorganization timeis indeed maximized
at higher lengthscales in the density range close
to the critical density
WHAT DO WE PREDICT?
Benzene
D. Dellis, I. Skarmoutsos, J. Samios, J. Mol. Liq. , 153, 25 (2010)
Local density reorganization processes Local density reorganization processes inside the first solvation shellinside the first solvation shell
‘Direct-Pair Potential’ and ‘Collective’ contributions:
212
0
22
1
1
44 NNdrrgrdrrgrRNR
R
R
cc
Skarmoutsos, I., Guardia, E., J. Phys. Chem. B. 113, 8887 (2009)
00
1
R1
First Shell
g (r
)
r
First Peak
R2
Length scale contributions on the reorganization mechanism
SC Ethanol
21 )1()()( 21tt
tt
leccetCtCtC
2121 )1( tctcl
First Solvation Shell
Second Solvation Shell
The time decay is differentespecially at higher densities
Another indication ofthe change in the relaxation
mechanism at larger length scalesSkarmoutsos, I., Samios, J., J. Chem. Phys., 126, 044503 (2007)
Time scale contributions
Residence DynamicsResidence Dynamics
ji ij
tijijres
h
thhtC
,20
0 *
dttCresres
0
● hij (t) = 1, if molecule j is inside the solvation shell of molecule i at times 0 and t and the molecule jhas not left in the meantime the shellfor a period longer than t*.
● hij (t) = 0 , otherwise
● t* = 0
● t* = 2 ps
Continuous
Intermittent- Like
Ethanol
The residence dynamics are also different atshort length scales, but not the same
with local density dynamics
Relation of HB network with LDIRelation of HB network with LDI
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,20,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
< n HB > Nc
c
< n H
B >
0
2
4
6
8
10
12
NC
Similar non-linear density dependence
with the coordination number
Ethanol
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,20,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
T / Tc = 1.03
sc H2O
< n H
B >
ρ / ρc
Water
Higher deviation from linearity
Possible reason for higher LDA valuesin SC water
Relation of spectral shifts with the local densityRelation of spectral shifts with the local density
Libration Bands in SCW
Now some experimentalists doubt previous assumptions that:
cmax
INDEEDWe observe a clearnon-linear behavior!
e.g. see Song, W., Maroncelli, M. , Chem. Phys. Lett., 378, 410 (2003)
Skarmoutsos, I., Guardia, E.J. Chem. Phys, 132, 074502 (2010)
HB Dynamics
ji ij
tijijHB
h
thhtC
,20
0 * dttCHBHB
0
● hij (t) = 1, if molecule j is hydrogen bonded with molecule i at times 0 and t and the bond has not been brokenin the meantime for a period longer than t*.
● hij (t) = 0 , otherwise
● t* = 0
● t* = ∞
Continuous
Intermittent
Effect of mutual diffusion and reorientation on HB Dynamics
ji ijd
ij
tij
dijd
HB hh
thhtC
, 00
0 *
● ifat times 0 and t and this bond length has not been largerin the meantime for a period longer than t*.
● , otherwise
Mutual Diffusion
dttC dHB
dHB
0
●
at times 0 and t and this angle has not been largerin the meantime for a period longer than t*.
● , otherwise
Intermittent Case
Mutual Reorientation
Skarmoutsos, I., Guardia, E., J. Phys. Chem. B. 113, 8898 (2009)
Effect of mutual diffusion - reorientation on HB Dynamics
Especially forContinuous dynamics
Mutual reorientation mechanism seemsto be more
important than mutual diffusionin the case of HB dynamics
Similar behavior with residence dynamicsat short length scales
Ethanol
Skarmoutsos, I., Guardia, E., J. Phys. Chem. B. 113, 8898 (2009)
What about single molecule dynamics?
Single Reorientational Dynamics
Legendre Reorientational tcfs
Skarmoutsos, I., Samios, J., J. Chem. Phys., 126, 044503 (2007)
Different density dependence for reorientational modes related withstronger interactions (e.g. O-H in methanol)
MeOH
Plateau
SC Ethanol
DifferentBehavior of1st, 2nd order
Legendre dynamics
3 groups of vectorsexhibiting similar
density dependence
Plateau for 2nd ordercorrelation times
related with weakerinteractions
Skarmoutsos, I., Guardia, E., J. Phys. Chem. B. 113,
8898 (2009)
Effect of the HB network on single reorientational dynamics
The HB network aroundeach molecule affectssignificantly its single
reorientational dynamics
Plateau for 2nd ordercorrelation timesare observed for
free (non-HB) moleculesAnd for modes relatedto weak interactions
● 1st order Legendre reorientational dynamics more affected by thelocal environment
● Modes related with weak interactions are not affected by LDI inthe case of 2nd order Legendre reorientational dynamics
Effect of the HB network on single translational dynamics
SCW (SPC/E)
Significant influence of the HB network on single translational dynamics!
The libration band peaks are shifted towards higher ω values for more strongly H Bonded water molecules
Skarmoutsos, I., Guardia, E.J. Chem. Phys, 132, 074502 (2010)
Bulk Density Dependence of librational modes as a function of H. Bonds
As the bulk density increases
the peaks shift towards higherfrequencies. Plateau behavior in the caseof more strongly H-Bondedmolecules (e.g. nHB = 3)
Dependence of the local HB network on the C.O.M. motion
Effect of the HB network on single O-H reorientational dynamics
Density Dependence HB Dependence
Further work in progress…..
FIN
Muchas gracias !Muchas gracias !