Experimental and theoretical studies of the structure of binary nanodroplets Gerald Wilemski Physics...
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Transcript of Experimental and theoretical studies of the structure of binary nanodroplets Gerald Wilemski Physics...
Experimental and theoretical Experimental and theoretical studies of the structure of binary studies of the structure of binary
nanodroplets nanodroplets
Gerald Wilemski Physics Dept. Missouri S&T
Physics 1
Missouri S&T
25 October 2011
Acknowledgments
• Part I – Supersonic nozzle and small angle neutron scattering (SANS) studies of nucleation and nanodroplet structure
• Barbara Wyslouzil (OSU) • Reinhard Strey (Köln U), • Christopher Heath and Uta Dieregsweiler (WPI)
• Part II – Structure in binary nanodroplets from density functional theory (DFT), lattice Monte Carlo (LMC), and molecular dynamics (MD) simulations
• Fawaz Hrahsheh, Jin-Song Li, and Hongxia Ning (Missouri S&T)
OUTLINEOUTLINE
Importance of structure for nanodropletsExperimental overview Experimental and theoretical results for
binary nanodropletsSANSDensity Functional TheoryLattice Monte CarloMolecular Dynamics
Conclusions
simulation
reality
Nucleation occurs all around us…Nucleation occurs all around us…
Organic matter is a common component of atmospheric particles
Aqueous core + organic layer with polar heads (●)
Inverted micelle model for aqueous organic aerosols was recently revived. (Ellison, Tuck, Vaida, JGR 1999)
Why is this important ?
Aerosols affect the Earth’s climateAerosols change the properties of clouds Sites for chemical reactions:
heterogeneous chemistry, ozone destruction
Fine particles (<100 nm) affect human health
Particle structure influences particle activity – nucleation and growth rates
Clouds effect the global energy balance. They modify earth’s albedo and LW radiation.
Radiative forcing by aerosols:
Direct (scattering and absorption) Indirect (affecting cloud formation and cloud properties)
How are small clusters involved?
… …
Critical cluster properties
growth
Nucleation rates
VV LL
Supersonic nozzleSupersonic nozzle
neutron or X-rayBeam (λ = 0.1 – 2 nm)
N2(g)
H2O(g)
120
100
80
60
40
20
0
120100806040200
-3.0
-2.5
-2.0
-1.5
N2(g)
H2O(l)
Dp = 2-20 nm 10-6
10-5
10-4
10-3
10-2
10-1
I (c
m-1
)
8 90.01
2 3 4 5 6 7 8 90.1
2 3
q (Å-1
)
3.75 m SDD 2.00 m SDD
Log Normal Distributionrg = 10.25 ± 0.05 nm
ln = 0.184 ± 0.004
N = ( 4.91 ± 0.05 ) × 1011
cm-3
Nozzle APo = 59.7 kPaTo = 308.1 KPD2O,o = 1.37 kPa
Experimental Setup at NIST
Is there evidence for structure Is there evidence for structure in larger nanodroplets?in larger nanodroplets?
Well-mixed Core-shell Partly nested or Russian doll
Use small angle neutron scattering (SANS) to find out.
CoreCore vs.vs. Shell Shell scattering scattering using contrast variationusing contrast variation
In high q region
sphere
I q–4
shell structure
I q–2
[q = (4π/λ)sin(θ/2)]
Evidence for shell scatteringEvidence for shell scattering
H2O – d-butanol/D2O – (h)butanolWyslouzil, Wilemski, Strey, Heath, Dieregsweiler, PCCP Wyslouzil, Wilemski, Strey, Heath, Dieregsweiler, PCCP 8, 8, 54 (2006)54 (2006)
Summary
• SANS: first direct experimental evidence for Core-Shell structure in aqueous-organic nanodroplets
Density Functional Theory applied to nanodroplets
Treat nanodroplets as large critical nuclei in supersaturated binary vapors. The species densities ρi (r) vary with position r.As a typical aqueous-organic system use nonideal water-pentanol mixtures modeled as hard sphere - Yukawa fluids (van der Waals mixtures). Use classical statistical mechanics to find the unstable equilibrium density profiles: Solve Euler-Lagrange Eqs.
D. E. Sullivan, J. Chem. Phys. 77, 2632 (1982).X. C. Zeng and D. W. Oxtoby, J. Chem. Phys. 95, 5940 (1991).
J.-S. Li and G. Wilemski, PCCP 8, 1266 (2006)
A droplet is a region with higher density than the surrounding fluid
The red line shows how the density (ρ) varies with radial position (r) within the droplet.
This example is fora pure droplet.
Two types of droplet structureswell-mixed core-shell
1.0
0.8
0.6
0.4
0.2
0.0
W W
3
6543210
Distance (nm)
aP=1.001602
aW=1.178168
xP=2.64%
Water Pentanol BDS
1.0
0.8
0.6
0.4
0.2
0.0
W W
3
6543210
Distance (nm)
aP=1.001602
aW=1.178168
xP=2.64%
Water Pentanol BDS
Structural Phase Diagram from DFT at 250 K
DFT predicts nonspherical oil( )/water( ) droplets
Why interested in oil/water droplets?
• Offshore natural gas wells produce high pressure mixtures of methane, water, and higher hydrocarbons (i.e., oils)• Gas must be cleaned before pumping to shore and clean-up may involve droplet formation
DFT Summary
• DFT: provides a vapor activity “phase diagram” for the nanodroplet structures– bistructural region implies hysteresis for transitions
between well-mixed and core-shell structures
• Also predicts nonspherical shapes for droplets with immiscible liquids
Lattice Monte Carlo Simulations of Large Binary Nanodroplets
• Generalize the lattice MC approach of Cordeiro and Pakula, J. Phys. Chem. B (2005) for pure droplets
• Each site of an fcc lattice is occupied by a different particle type (red or blue beads) or by a vacancy.
• Beads and vacancies interact repulsively– Ebv = 1, Erv = 2/3, Erb = 0, 0.5, 0.8– Red beads ↔ lower surface tension, higher volatility (~alcohol)
Blue beads ↔ higher surface tension, lower volatility (~water)
• T range: 2.8 ≥ kT ≥ 2.0; Blue triple point is at kT= 2.8
Ideal binary droplet at kT=2.5
1400 ● + 3264 ● (Erb=0)
Density profile indicates surface enrichment of red beads.
1400 ● + 3264 ● (Erb=0.5)
Nonideal binary droplet at kT=2.5
Core-Shell droplet at kT=2.5
Interior depletion and surface enrichment of red beads.
1400 ● + 3400 ● (Erb=0.8)
Russian doll droplet at kT=2
1400 ● + 3400 ● (Erb=0.8)
Russian doll axial density profile at kT=2
1400 ● + 3400 ● (Erb=0.8) 0<r<1
-20 -10 0 10 200.0
0.2
0.4
0.6
0.8
1.0
1.2kT=2.0N1=1400
N2=3400
E3=0.8
0< r<1 component 1 component 2
D
imen
sio
nle
ss N
um
ber
Den
sity
Axial (z) position
Core-Shell droplet at kT=2.5formed by heating Russian Doll
1400 ● + 3400 ● (Erb=0.8)
Antonow’s Rule: Interfacial Tensions and Wetting Transitions
γ(bv) < γ(rv) + γ(rb) γ(bv) = γ(rv) + γ(rb)
Partial wetting Perfect wetting
By Analogy with Antonow’s Rule and Wetting Transitions
Russian doll Core-shell
Partial wetting Perfect wetting
heat
cool
γ(bv) < γ(rv) + γ(rb) γ(bv) = γ(rv) + γ(rb)
kT=2.5
The backside is more evenly covered.
kT=2.4
There is a large dewetted patch; the backside is evenly covered.
Cool the Core-Shell droplet to observe the dewetting transition
1400 ● + 3400 ● (Erb=0.8)
As the temperature is reduced further, the droplet elongates.
kT=2.2kT=2.3
Cool the Core-Shell droplet to observe the dewetting transition
1400 ● + 3400 ● (Erb=0.8)
Cool the Core-Shell droplet to observe the dewetting transition
At the lowest temperatures dewetting and elongation are pronounced.
T=2.0kT=2.1 kT=2.0
1400 ● + 3400 ● (Erb=0.8)
LMC Summary
• LMC: the core-shell - Russian doll structural change is a reversible wetting-dewetting transition that modulates the shape of the nanodroplet – May ultimately be a cause of droplet fission ?
• The RD droplet resembles the nonspherical structure found with DFT for oil/water droplets
Molecular Dynamics (MD)
• Solve Newton’s equations of motion for large numbers of interacting molecules
• Time step = 1 or 2 fs (10-6 ns)• Average over 2 ns long trajectories to
calculate properties of interest
MD of nonane/water droplet
initial final
Nonane molecules (blue-green) surround a droplet of water (red-white).
The water droplet partly emergesfrom the oil droplet.
Double click on the slide to see the simulation.
Grand Summary• SANS: experimental evidence for Core-Shell structure of
aqueous-organic nanodroplets• DFT: vapor activity “phase diagram” for CS and well-
mixed nanodroplet structures• DFT: nonspherical droplet shapes• LMC: core-shell - Russian doll structural transition
changes the shape of the nanodroplet• MD: realistic simulations of droplets with large numbers
of molecules