Nanoparticles at fluid-fluid interfaces: self-assembly, stability...
Transcript of Nanoparticles at fluid-fluid interfaces: self-assembly, stability...
Nanoparticles at fluid-fluid interfaces: self-assembly, stability and disassembly
Valeria Garbin Department of Chemical Engineering Imperial College London garbinlab.ce.ic.ac.uk
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Nanoparticles at fluid-fluid interfaces
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1. Two-dimensional nanomaterials with tunable properties
2. Promoting self-assembly onto fluid-fluid interfaces
3. Tuning the interfacial microstructure (stability)
4. Disassembly
Kathleen Stebe @ University of Pennsylvania John Crocker Talid Sinno, Ian Jenkins
Self-assembly of colloidal particles at fluid-fluid interfaces
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Reduction in Helmoltz free energy F
Strong trapping:
a = 2 nm
γ0 = 30 mN/m
∆F ∼ 100 kBT
∆F = −πa2γ0 (1− | cos θ|)2
Particle monolayers at fluid-fluid interfaces
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Self-assembly of colloidal particles at fluid-fluid interfaces
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Colloidosomes Dinsmore et al., Science (2002) Particle-stabilized foams
Martinez et al., Soft Matter (2008)
Phase-selective catalysis with interfacial nanoparticles
(Crossley et al., Science 2010)
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Anisotropic particles Lewandowski et al., Langmuir (2010)
encapsulation catalysis interface mobility
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Nanoparticle monolayers at fluid interfaces
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Liquid-like mirrors Luo et al., Soft Matter (2012)
Nanoparticles Functional, tunable monolayers Interparticle distance-dependent collective properties
Tunable plasmonic films Tao et al., Nat. Nanotech. (2007)
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Outline
• Promote nanoparticle self-assembly
from suspension
• Tune interfacial microstructure
(equilibrium, reversible)
• Control dynamic behavior
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Nanoparticles are passivated by capping ligands
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surfactants polymers ions
• shape of nanoparticle (synthesis)
• colloidal stability in bulk suspension
Ye et al., ACS Nano (2012)
100 nm
Ye et al., PNAS (2010)
Personick et al., JACS (2011)
200 nm
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Promote nanoparticle self-assembly from suspension
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a = 2 nm
γ0 = 30 mN/m
∆F ∼ 100 kBT
1 2 γ0
P 1 γP1
P 2 γP2
cos θ =γP2 − γP1
γ0
|γP2 − γP1| < γ0
use ligands to tune surface energy to promote adsorption
adsorption to fluid-fluid interface if θ ≠ 0
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Controlling stability and dynamics
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Mechanics of particle-laden interface? Link with interparticle interactions?
Datta, Schum, Weitz, Langmuir (2010)
Mulligan & Rothstein, Langmuir (2011)
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• 2D osmotic pressure Π • Π due to entropy and interparticle interactions
Ligand-mediated interactions?
Mechanics of nanoparticle-laden fluid interface
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Particle change the effective surface tension
Π
γeff = γ0 −Π
γ0
• Measurements of Laplace pressure γeff • Mechanical equilibrium between gravity and surface tension γeff
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γ =
� x2
x1
[PN(x)− PT(x)] dx Rowlinson & Widom, Molecular Theory of Capillarity
Pendant drop method
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Measuring surface pressure of nanoparticle monolayer
• 5 nm Au nanoparticles • amphiphilic capping ligand MUTEG
• Aqueous suspension NPs (φ ~ 10-5)
• Form drop of oil at t = 0
• Adsorption: t ~ 103 s
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CH2(CH2)9CH2 S [ ]4 O OH Au
undec-11-yl tetra(ethylene glycol) mercapto
1 mm
Du, Glogowski, Emrick, Russel, Dinsmore, Langmuir (2010)
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Measuring surface pressure of nanoparticle monolayer
• surface tension from drop shape
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γeff = γ0 −Π
reversible
Garbin, Crocker, Stebe, Langmuir (2012)
1 mm
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speed 20x
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Mechanically forced desorption of nanoparticles
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Π = Π(Adrop)
need to measure area density Γ
Π = Π(Γ), Γ = N/Adrop• Optical absorbance A • Number of particles in plume • Extract area density Γ
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Critical area density for nanoparticle desorption?
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a∗eff = 3.3 nm
= acore + 1 nm
2aeff
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Energetics of nanoparticle desorption
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Upper bound for desorption energy: work done upon compression ΠcdA ≥ ∆F
Πc ≈ 13 mN/m
dA ≈ πa∗eff∆F ≤ 111 kBT
Π
∆F = −πa2γ0 (1− | cos θ|)2
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• 2D osmotic pressure Π • Π due to entropy and interparticle interactions:
Π = −�∂F
∂A
�= −
�∂U
∂A
�+ T
�∂S
∂A
�
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Ligand-capped nanoparticles are not hard disks
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experiment a = 2.3 nm a*eff = 3.3 nm
fluid: Henderson, Mol. Phys. (1975) crystal: Sturgeon & Stillinger, J. Chem. Phys. (1991)
Π = −�∂U
∂A
�
N,T
+ T
�∂S
∂A
�
N,T
r 2a
U
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Ligand-mediated soft repulsion
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Steric repulsion dense grafted polymer in bulk
P (s) = kBTσ32
��2L
s
� 94
−� s
2L
� 34
�Pressure between flat plates:
Derjaguin approximation Force between spheres Potential
De Gennes, Adv. Colloid Interface Sci. (1987)
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• particle at oil-water interface • amphiphilic ligand
Ranatunga et al. J. Phys. Chem C (2010)
Ligand configuration depends on grafting density σ
Hard disks with soft repulsion
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L = 2 nm
σ = 2 nm−2
NVT ensemble 2D Brownian Dynamics simulations (LAMMPS)
experiment a*eff = 3.3 nm
BD simulations
r 2a
U
2(a+L)
Garbin, Jenkins, Sinno, Crocker, Stebe, in preparation
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Nanoparticle interactions determine fate of monolayer
• Πc < γ0 • AND equilibrium • AND stability upon compression
Desorption
Garbin, Crocker, Stebe, Langmuir (2012)
Datta, Schum, Weitz, Langmuir (2010)
Buckling
• Πc = γ0 • OR non-equilibrium (aggregates, gel) • cohesive upon compression
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Garbin, Crocker, Stebe, J. Coll. Interf. Sci. (2012)
Reynaert et al., Langmuir (2006)
Reynaert et al., Langmuir (2006)
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Desorption: desirable for recycling/green chemistry
Other strategies for desorption Charge-mediated adsorption/desorption Reincke et al., Phys Chem Chem Phys (2006) Luo et al., Soft Matter (2012)
Interfacial displacement by surfactants Vashisth et al., J. Coll. Interf. Sci. (2010)
THIS WORK: Mechanically forced desorption Garbin, Crocker, Stebe, Langmuir (2012)
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Phase-selective catalysis with interfacial nanoparticles Crossley et al., Science (2010)
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water water
oil
bulk interface ligand rearrangements
stable interfacial suspension
2D gel network
adsorption repulsive
interactions
attractive interactions
Garbin, Crocker, Stebe, J. Colloid Interf. Sci. (2012)
SUMMARY: Ligands control adsorption, stability and dynamics
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NEXT: non-equilibrium?
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