colloidal stabilization by nano-particle halos ard louis dept. of chemistry

Colloidal stabilization by nano-particle halos

Ard LouisDept. of Chemistry

Utrecht-LMC-July052

Stabilizing colloids against van der Waals

1. Charge Stabilization

DLVO potential (1940’s)

Derjaguin

Landau

Verwij

Overbeekpicture1968

Utrecht-LMC-July053

Stabilizing colloids against van der Waals

1. Charge Stabilization

2. Steric Stabilization

3. Nano-halo Stabilization?

Utrecht-LMC-July054

V. Tohver, J. Lewis et al. (2001) Proc. Natl. Acad. Sci. USA 98, 8950-8954

Nano-particle halo stabilization?

depletion

v.d. Waals

3 n

m n

an

opart

icle

s

285 nm colloids

Utrecht-LMC-July055

V. Tohver, J. Lewis et al. (2001) Proc. Natl. Acad. Sci. USA 98, 8950-8954


1. Uncharged colloids

2. Charged nano-particles

3. Extreme size-ratios

4. Re-entrant attraction

5. Low surface coverage

Featuresdepletion

v.d. Waals

3 n

m n

an

opart

icle

s

285 nm colloids

Utrecht-LMC-July056

Today’s Questions: Do these experiments show a new kind of stabilization? Are the features universal? Can this be used more broadly?

(this talk hopes to inspire experimentalists)


Outline of talk:

1. Intro to the experiments

2. Calculating effective interactions beyond simple depletion

3 Features of the nano-particle halo mechanism

4. Conclusion: answers to questions above: yes

Utrecht-LMC-July057

• Veff(r) depends on Vss(r), Vbs(r),

• Total potential: V(r) = Vbb(r) + Veff(r)

Vss(r) =0, Vbs(r) = VHS(r) gives Asakura Oosawa model depletion (A. Vrij 1976 – also in Utrecht)

Calculating effective potentials beyond simple depletion

€

ρs

Utrecht-LMC-July058

• Veff(r) depends on Vss(r), Vbs(r),

• Total potential: V(r) = Vbb(r) + Veff(r)

•HS + Yukawa is flexible

Calculating effective potentials beyond simple depletion: HS + Yukawa model

€

ρs€

Vbs(r) = VbsHS (r) +

ε bsσ bsr

exp −(r −σ bs)

λ bs

⎡

⎣ ⎢

⎤

⎦ ⎥

Vss(r) = VssHS (r) +

ε ssσ ssr

exp −(r −σ ss)

λ ss

⎡

⎣ ⎢

⎤

⎦ ⎥

Utrecht-LMC-July059

Two strategies for repulsive potentials

Simple depletion:

€

q =σ sσ b

= 0.2

Simulations for

Utrecht-LMC-July0510


1. Small-small repulsion

€

q =σ sσ b

= 0.2

€

q =σ sσ b

= 0.2

Simulations for



2. big-small attraction

€

q =σ sσ b

= 0.2

€

q =σ sσ b

= 0.2

Simulations for


Flexible ways to calculate Veff(r)?

Great many parameters

Needs a flexible method: simulations too slow

We finally settled on HNC integral equations

• Works well for soft repulsions

• Works well for low density

• Flexible and fast


Accuracy of HNC integral equations

€

q =σ sσ b

=1

5= 0.2

βε ss = 3

λ ss = 0.33σ ssλ bs = 0.8σ ss

φs =π

6ρsσ ss

3 = 0.1

HNC works well for soft repulsions

small-small repulsion


Accuracy of HNC integral equations

Conclusion: HNC works well for low densities and extreme size-ratios q trustworthy for qualitative effects

pure HS

q=0.01


using strategy 1 (small-small repulsion)

S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004)

€

q =σ sσ b

=1

100

βε ss = 6

λ ss = 5σ ss

φs =π

6ρsσ ss

3

small particle packing fraction is varied



€

q =σ sσ b

=1

100

βε ss = 6

λ ss = 5σ ss

φs =π

6ρsσ ss

3






€

q =σ sσ b

=1

100

βε ss = 6

λ ss = 5σ ss

φs =π

6ρsσ ss

3


Stabilization?




Stabilization?




Correlation attraction: (not depletion!)

€

∝φ2


Bridging

We can engineer almost any potential shape we want!

using strategy 2 (big-small attraction)


€

λbs = λ ss

€

λbs = 3λ ss



J. Liu and E. Luijten , Phys. Rev. Lett. 93, 247802 (2004)

q=0.01



J. Liu and E. Luijten , Phys. Rev. Lett. 93, 247802 (2004)

It’s amazing HNC works so well!

metastability reminiscent of DLVO?


Second stabilization window: re-re entrant?


Properties of the “nano-particle halo”

•halo = average accumulation at surface

•halos are very dilute

•Stabilisation does not correlate with detailed halo properties

•halos are fluctuating or “dynamic”

For same set of parameters 2d packing is almost same

but stabilization window is not!


Derjaguin scaling of potentials


Will stabilization persist in non-equilibrium?

Smaller particles (larger diffusion coefficients) will be better.

Large stabilization windows will be more robust

For more on hydrodynamics +Brownian forces see

poster P11.6 with Johan PaddingQuickTime™ and a

YUV420 codec decompressorare needed to see this picture.


Stabilization by nano-particle halos?: yes! Conclusions:

•Nano-particle halo mechanism is fundamentally different from previous steric and charge stabilisation.

•Adding nanoparticles helps engineer potentials:

•Depletion attraction

•Accumulation repulsion (can be re-entrant) (negative non-addivity)

•Correlation attraction

•Bridging

•Repulsive effects seen in large swathes of parameter space

•Works best for smaller added particles and large screening length, but q=0.2 is also possible

•Should be widely applicable in colloid science and biology

•GO TRY IT (Experiments needed)!

•Contact us at: www-louis.ch.cam.ac.uk

colloidal stabilization by nano-particle halos ard louis dept. of chemistry

Documents

r v ss r

v bb r v

v hs r

v bs r

varied slide

simple depletion slide

chemistry slide

fast slide