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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