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Colloids

Colloids. Mesoscopic particles (10nm-10 m m) suspended in solvent Exhibit Brownian motion Phase behavior analogous to atoms and molecules But, easier to study Longer time and larger length scales Real-space and real time observations using confocal microscopy Perfect model system. Colloids.

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Colloids

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  1. Colloids • Mesoscopic particles (10nm-10mm) suspended in solvent • Exhibit Brownian motion • Phase behavior analogous to atoms and molecules • But, easier to study • Longer time and larger length scales • Real-space and real time observations using confocal microscopy • Perfect model system

  2. Colloids • Shape • Spherical (silica, PMMA) • Colloidal molecules (dumbbells, beadchains) • Rodlike and flexible (TMV and fd virus, silica rods) • Platelike (clays) • Tunable effective interactions • Hard-sphere like • Long-range repulsive (Coulombic) • Short-range attractive (depletion) • Dipolar • Long-range attractive (Oppositely charged colloids) • Patchy interactions

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  4. Round table discussion New Jersey (1957): Uhlenbeck: Recently, there has been some indication, numerically, that a gas of hard spheres has a transition point. Kirkwood: We are now speaking of the classical case for hard spheres, and the question comes up as to whether, for sufficiently high densities, the fluid of hard spheres will crystallize. Uhlenbeck: Last fall, there was something similar and there I finished the discussion on hard spheres with a vote. The vote was simply on the matter of your belief in what you have heard, whether a gas of hard spheres has a transition point or not; at Seattle, the vote was even. May I ask first of the panel to put up their hands if they believe there is a transition in the classical case ….. Now those who believe there is not a transition …. Even again! Kirkwood: I think it is quite unnecessary to have an attractive force to achieve a crystalline phase, and one can produce simple intuitive arguments for that. …… Uhlenbeck: I would like to close this discussion, for I am quite sure that the transition goes a little bit against intuition; that is why so many people have difficulty with it, and surely I am one of those. But this transition – it still might be true, you know – and I don’t think one can decide by general arguments. Kirkwood: Well, I think we are all aware that we have to take numerical calculations relating to transitions with a grain of salt.

  5. Simulation of freezing of hard spheres Fluid solid Glass F + S 0.494 0.545 0.58

  6. Glass Solid F+S Fluid

  7. Computer simulations: Alder and Wainwright, JCP 27, 1208 (1957) Wood and Jacobson, JCP 27, 1207 (1957). F=E-TS E=0 Fase overgang kan alleen plaatsvinden als dit overeenkomt met een toename in de entropie! Entropie maat voor wanorde: vloeistof meer wanordelijk!, vaste fase meer geordend----------spontane kristallisatie/ordening????

  8. Definities van Entropie: Clausius (1850) De entropie is altijd maximaal voor een gesloten systeem Boltzmann De entropie is gerelateerd aan het aantal microtoestanden of configuraties dat een systeem kan hebben. S = kB log  Aantal configuraties groot : entropie hoog Hoe groter dit aantal, hoe minder we van het systeem weten: Wanorde is groot

  9. 1 4 1 3 2 5

  10. 4 21 31 23 13 25 27 2 17 30 7 11 19 1 15 28 5 32 9 22 3 18 26 14 10 8 29 33 12 24 16 20 6 7 16 19 22 25 28 31 1 4 10 13 8 20 2 5 23 26 29 14 17 32 11 3 6 18 21 27 30 33 9 12 15 24

  11. (Vrij)-Asakura-Oosawa model for colloid-polymer mixtures Colloid-colloid r Colloid-polymer r Polymer-polymer

  12. c A A P = zp 0 0 p Rij

  13. Phase separation in colloid-polymer mixtures Observed in experiments (Illet, Poon et al.) and computer simulations (Meijer, Frenkel) E. de Hoog et al., J. Phys. Chem. B 103, 10657 (1999) Depletion

  14. Colloid-Polymer mixture PMMA (=60 nm) + Polystyrene (Rg~30 nm) in Decalin Dirk Aarts and Henk Lekkerkerker

  15. Direct visual observation of thermal capillary waves • D. G. A. L. Aarts, M. Schmidt, H. N. W. Lekkerkerker, Science304, 847-850 (2004).

  16. Depletion Mechanism Entropic picture The volume accesible for the polymer increases: more entropy Kinetic picture Unbalanced osmotic pressure

  17. Colloid-polymer mixture • (1954) Asakura-Oosawa: attractive interaction between colloids due • to presence of polymer. • However, phenomenon already know in other areas: • Stack formation/ clustering of red bloodcells due to plasma proteins (1777). • proteins: albumin 15 nm • fibrinogen 50 nm • concentration increases when patient is ill • rouleaux/stacks aggregation stronger with fibrinogen • sedimentation rate increases 1mm/hr ---- 100 mm/hr • blood sedimentation test 7.8 m

  18. cream solution 2) Creaming of rubber latex (1923) Concentration of rubber latex from an aqueous solution Addition of plant extracts 0.2 % cheap: lots of research in this area (best molecule, minimum concentration?) 3) Purification/isolation of plant viruses 1942 Agricultural purposes: addition of polymer to TMV solution --- TMV precipitates 4) Aggregation of oil droplets add surfactant to O/W emulsion --- stabilisation add too much surfactant ---- attraction between oil droplets 50-60 % rubber

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