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Experimental and theoretical studies of the structure of binary nanodroplets

Experimental and theoretical studies of the structure of binary nanodroplets. Gerald Wilemski Physics Dept. Missouri S&T. Physics 1 Missouri S&T 25 October 2011. Acknowledgments.

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Experimental and theoretical studies of the structure of binary nanodroplets

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  1. Experimental and theoretical studies of the structure of binary nanodroplets Gerald Wilemski Physics Dept. Missouri S&T Physics 1 Missouri S&T 25 October 2011

  2. 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)

  3. OUTLINE • Importance of structure for nanodroplets • Experimental overview • Experimental and theoretical results for binary nanodroplets • SANS • Density Functional Theory • Lattice Monte Carlo • Molecular Dynamics Conclusions

  4. Nucleation occurs all around us… simulation reality

  5. Organic matter is a common component of atmospheric particles Inverted micelle model for aqueous organic aerosols was recently revived. (Ellison, Tuck, Vaida, JGR 1999) Aqueous core + organic layer with polar heads (●)

  6. Why is this important ? • Aerosols affect the Earth’s climate • Aerosols 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

  7. Radiative forcing by aerosols: Direct (scattering and absorption) Indirect (affecting cloud formation and cloud properties) Clouds effect the global energy balance. They modify earth’s albedo and LW radiation.

  8. … … How are small clusters involved? V L growth Nucleation rates Critical cluster properties

  9. Supersonic nozzle N2(g) H2O(g) N2(g) H2O(l) neutron or X-ray Beam (λ = 0.1 – 2 nm) Dp = 2-20 nm

  10. Experimental Setup at NIST

  11. Is there evidence for structure in larger nanodroplets? Use small angle neutron scattering (SANS) to find out. Well-mixed Core-shell Partly nested or Russian doll

  12. In high q region sphere I q–4 shell structure I q–2 Corevs. Shell scattering using contrast variation [q = (4π/λ)sin(θ/2)]

  13. Evidence for shell scattering Wyslouzil, Wilemski, Strey, Heath, Dieregsweiler, PCCP 8, 54 (2006) H2O – d-butanol/D2O – (h)butanol

  14. Summary • SANS: first direct experimental evidence for Core-Shell structure in aqueous-organic nanodroplets

  15. 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)

  16. 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 for a pure droplet.

  17. Two types of droplet structureswell-mixed core-shell

  18. Structural Phase Diagram from DFT at 250 K

  19. DFT predicts nonspherical oil( )/water( ) droplets

  20. 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

  21. 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

  22. 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

  23. Ideal binary droplet at kT=2.5 1400 ● +3264 ● (Erb=0)

  24. Nonideal binary droplet at kT=2.5 1400 ● +3264 ● (Erb=0.5) Density profile indicates surface enrichment of red beads.

  25. Core-Shell droplet at kT=2.5 1400 ● +3400 ● (Erb=0.8) Interior depletion and surface enrichment of red beads.

  26. Russian doll droplet at kT=2 1400 ● +3400 ● (Erb=0.8)

  27. Russian doll axial density profile at kT=2 1400 ● +3400 ● (Erb=0.8) 0<r<1

  28. Core-Shell droplet at kT=2.5formed by heating Russian Doll 1400 ● +3400 ● (Erb=0.8)

  29. Antonow’s Rule: Interfacial Tensions and Wetting Transitions γ(bv) = γ(rv) + γ(rb) γ(bv) < γ(rv) + γ(rb) Partial wetting Perfect wetting

  30. By Analogy with Antonow’s Rule and Wetting Transitions Partial wetting Perfect wetting heat cool Russian doll Core-shell γ(bv) < γ(rv) + γ(rb) γ(bv) = γ(rv) + γ(rb)

  31. Cool the Core-Shell droplet to observe the dewetting transition 1400 ● +3400 ● (Erb=0.8) kT=2.4 kT=2.5 The backside is more evenly covered. There is a large dewetted patch; the backside is evenly covered.

  32. Cool the Core-Shell droplet to observe the dewetting transition 1400 ● +3400 ● (Erb=0.8) kT=2.3 kT=2.2 As the temperature is reduced further, the droplet elongates.

  33. Cool the Core-Shell droplet to observe the dewetting transition 1400 ● +3400 ● (Erb=0.8) T=2.0 kT=2.0 kT=2.1 At the lowest temperatures dewetting and elongation are pronounced.

  34. 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

  35. 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

  36. MD of nonane/water droplet The water droplet partly emerges from the oil droplet. Nonane molecules (blue-green) surround a droplet of water (red-white). initial final

  37. Double click on the slide to see the simulation.

  38. 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

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