1. Colloidal stabilization by nano-particle halos Ard Louis Dept. of Chemistry

2. Utrecht-LMC-July05 2 Stabilizing colloids against van der Waals 1. Charge Stabilization DLVO potential (1940’s) Derjaguin Landau Verwij Overbeek picture1968

3. Utrecht-LMC-July05 3 Stabilizing colloids against van der Waals 1. Charge Stabilization 2. Steric Stabilization 3. Nano-halo Stabilization?

4. Utrecht-LMC-July05 4 V. Tohver, J. Lewis et al. (2001) Proc. Natl. Acad. Sci. USA 98, 8950-8954 Nano-particle halo stabilization? depletion v.d. Waals 3 nm nanoparticles 285 nm colloids

5. Utrecht-LMC-July05 5 V. Tohver, J. Lewis et al. (2001) Proc. Natl. Acad. Sci. USA 98, 8950-8954 Nano-particle halo stabilization? 1.Uncharged colloids 2.Charged nano-particles 3.Extreme size-ratios 4.Re-entrant attraction 5.Low surface coverage Features depletion v.d. Waals 3 nm nanoparticles 285 nm colloids

6. Utrecht-LMC-July05 6 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) Nano-particle halo stabilization? 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

7. Utrecht-LMC-July05 7 V eff (r) depends on V ss (r), V bs (r), Total potential: V(r) = V bb (r) + V eff (r) V ss (r) =0, V bs (r) = V HS (r) gives Asakura Oosawa model depletion (A. Vrij 1976 – also in Utrecht) Calculating effective potentials beyond simple depletion

8. Utrecht-LMC-July05 8 V eff (r) depends on V ss (r), V bs (r), Total potential: V(r) = V bb (r) + V eff (r) HS + Yukawa is flexible Calculating effective potentials beyond simple depletion: HS + Yukawa model

9. Utrecht-LMC-July05 9 Two strategies for repulsive potentials Simple depletion: Simulations for

10. Utrecht-LMC-July05 10 Two strategies for repulsive potentials 1. Small-small repulsion Simulations for

11. Utrecht-LMC-July05 11 Two strategies for repulsive potentials 2. big-small attraction Simulations for

12. Utrecht-LMC-July05 12 Flexible ways to calculate V eff (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

13. Utrecht-LMC-July05 13 Accuracy of HNC integral equations HNC works well for soft repulsions small-small repulsion

14. Utrecht-LMC-July05 14 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

15. Utrecht-LMC-July05 15 using strategy 1 (small-small repulsion) S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004) small particle packing fraction is varied

16. Utrecht-LMC-July05 16 using strategy 1 (small-small repulsion) small particle packing fraction is varied S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004)

17. Utrecht-LMC-July05 17 using strategy 1 (small-small repulsion) S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004) small particle packing fraction is varied Stabilization?

18. Utrecht-LMC-July05 18 using strategy 1 (small-small repulsion) S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004) Stabilization?

19. Utrecht-LMC-July05 19 using strategy 1 (small-small repulsion) S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004)

20. Utrecht-LMC-July05 20 using strategy 1 (small-small repulsion) S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004)

21. Utrecht-LMC-July05 21 using strategy 1 (small-small repulsion) S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004)

22. Utrecht-LMC-July05 22 using strategy 1 (small-small repulsion) S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004) Correlation attraction: (not depletion!)

23. Utrecht-LMC-July05 23 Bridging We can engineer almost any potential shape we want! using strategy 2 (big-small attraction) S. Karanikas and AAL, Phys. Rev. Lett. 93, 248303 (2004)

24. Utrecht-LMC-July05 24 using strategy 2 (big-small attraction) J. Liu and E. Luijten, Phys. Rev. Lett. 93, 247802 (2004) q=0.01

25. Utrecht-LMC-July05 25 using strategy 2 (big-small attraction) J. Liu and E. Luijten, Phys. Rev. Lett. 93, 247802 (2004) It’s amazing HNC works so well! metastability reminiscent of DLVO?

26. Utrecht-LMC-July05 26 Second stabilization window: re-re entrant?

27. Utrecht-LMC-July05 27 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!

28. Utrecht-LMC-July05 28 Derjaguin scaling of potentials

29. Utrecht-LMC-July05 29 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 Padding

30. Utrecht-LMC-July05 30 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