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3021 Course Outline

3021 Course Outline. Dr Tim Senden Dept Applied Mathematics, Research School of Physics and Engineering 12 lectures - 4 tutes Introduction Foundation demonstrations What are colloids? Where are they found in nature? How do surfaces become charged? How to colloids interact?

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3021 Course Outline

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  1. 3021Course Outline Dr Tim Senden Dept Applied Mathematics, Research School of Physics and Engineering 12 lectures - 4 tutes • Introduction • Foundation demonstrations • What are colloids? • Where are they found in nature? • How do surfaces become charged? • How to colloids interact? • The Electrical Double Layer • van der Waals Forces • DLVO theory • Other forces (adhesion, hydrophobic) • Molecules at interfaces • Capillarity and wetting • Surfactant behaviour and adsorption • Self assembly • Tools of the trade

  2. Foundation DemonstrationsPart I Gold colloid (colloids scatter light) sulfur colloids (why nano- is special) Salt induced flocculation colloids van der Waals attraction (in air, in hexane, in water) cold welding of gold leaf

  3. Granite weathers into components Quartz, clays & other minerals

  4. Mary Kathleen uranium mine, near Cloncurry, Qld. Tyndall effect Named after the Irish scientist John Tyndall. Light with shorter wavelengths scatters better, thus the color of scattered light has a bluish tint. This is the reason why the sky looks blue; the blue component of sun light is more highly scattered.

  5. Scattering • Finely divided insulators become whiter • Finely divided metals become black and then coloured Aussie sky blue European sky blue Colour in metals comes from plasmon resonance, just ask Paul “Blue” Karason

  6. bacterium Looking at clay first…. 1 micron Red blood cell (6 micrometres) Scanning electron micrograph of kaolin Why doesn’t muddy water clear?

  7. Salts also weather from rocks Cl- Na+ What happens in water? Why does salt dissolve? What happens to the muddy water?

  8. The Colorado The Nile The Ganges

  9. It isn’t size alone that makes a material “nano” it’s how nanoscopic phenomena play on that material that does matter. • Summary (some questions to be explored) • How does matter interact with light? • How does matter interact with matter? • Which bulk properties don’t scale with size? • Why does surface chemistry matter? • What keeps nano-materials dispersed? The nanoscale characterises a strong cross over between physics and chemistry (both matter and energy levels are discrete.) Ganges River Delta

  10. pico- nano- micro- milli- 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 colloids metres Getting a sense of scale fog / mist ions molecules oil / smoke pollen macromolecules viruses bacteria micelles Surface tension beats gravity Thermal fluctuations Electronic effects

  11. Nanoscale measurements Nanoscale leads to pico-, femto-, atto- effects Scale of forces 1 N ≈ force required to hold an apple against gravity 1 mN ≈ force required to hold a postage stamp against gravity 1 µN ≈ force required to hold an eye lash against gravity 1 nN ≈ covalent bonds; force between clay particles in water 10 pN ≈ a single H-bond Scale of energy 100 J ≈ the energy released by a sleeping person per second 1 J ≈ work required to pick an apple of the ground (1 metre) 1 fJ ≈ energy required to bend lipid membrane 1 aJ ≈ energy required to do cis - trans rotation (thermal energy) thermal energy (kT) = is maxm work available to a molecule 10-18atto- 10-15femto- 10-12pico- 10-9nano- 10-6micro-

  12. Energy (exothermic) Jmol-1 Processes involving changes; - in the nuclei of atoms 1012 235U + n Ba + Kr + 3n - in molecular structure 105.5 H2 + 1/2O2 H2O - in valence electrons 105 e + H+ H - changes of state 104.5 H2O(g) H2O(l) - molecular translational, rotational & vibrational energy 103 H2O(g, 1000K) H2O(l, 300K) This compares with RT (2500 Jmol-1) - mechanical potential energy 102 H2O(l, 555 metres) H2O(l, sea level) - mechanical kinetic energy 101 H2O(l, 10 ms-1) H2O(l, rest) (adapted from Rossini) The amount of energy required to raise the temperature of one kilogram of water by one degree Celsius. It equals roughly the energy required to raise a spoonful of food to your mouth.

  13. The Brownian dance • Two forces in balance • One repels • The other attracts + + + + + + + + + + + + + + + + + + + + + + The Darkened Hall analogy

  14. Bulk properties • Some bulk properties scale with size – but the explanation might not Elasticity stretch Cooling molecule down Consider a rubber band Viscosity Thermal fluctuations Ordered layer etc….. Now consider boiling/melting point, reflectivity, solubility……

  15. For solids • The surface atoms “squeeze” the internal atoms. In nanoscopic systems this could be 1000s of atmospheres. • Physical properties such as opto-electronic, phase state, solubility, reactivity and conductivity may change Each atom on the surface has different properties (colour indicated) thus the surface is defective.

  16. Heating or finely dividing 2Mg + O2 2MgO Reactivity “tipping point” Population of atoms with a given energy energy Mg MgO Thermal energy

  17. Why are nanomaterials stable? • Chemical stability - surface passivation • Physical stability - against aggregation - A balance of forces Sulfur is hydrophobic, gold has huge attraction • Dissociation - (Oxides, acidic or amphoteric) • Crystal lattice effects (Clays) • Ion adsorption (specific)

  18. Energy Band Representation of Insulators, Semiconductors and Metals Empty Conduction band Conduction band 400 kT 40 kT Partially filled Conduction band Filled valence band valence band valence band Insulator Semiconductor Metal

  19. Bulk (3D) Quantum Well (2D) Quantum Wire (1D) Quantum Dot (0D) r(E) r(E) r(E) r(E) Energy Energy Energy Energy Density of States in semiconductors Reduced Dimensionality leads to higher efficiency, lower threshold current, reduced power consumption and higher operating speed

  20. Photoluminescence 1 S 2 3 4 S Transmission Electron Micrograph 1.6 nm 4 GaAs QW withAlGaAs barriers 2.2 nm 2 3.4 nm 6.8 nm 3 1 4 S Colloidal CdSe quantum dots Courtesy of Prof. Jagadish, ANU

  21. For gases • depends on vapour pressure and a balance of surface energies • hydrophobic is q>90° • roughness makes a huge difference • If the vapour doesn’t adsorb then surface is not wet It’s curvature that matters q Contact angle is due to balance of surface energies

  22. Summary It’s not so much the size that matters, it’s the dominance of microscopic phenomena at that length scale. Bulk, macroscopic properties give way to the fact matter is corpuscular, electronic and fluctuating with thermal energy.

  23. Colloid Stability • All atoms experience a short range attraction that arises from dipole/dipole interactions of electron clouds-van der Waals attraction • Therefore a repulsive force is required to obtain stable colloids • In practice, this repulsion can arise in many ways.

  24. Summary of forces Force approx. range min/max force for colloidal sized objects Attractive (negative force) van der Waals <15 nm < -1 nN Hydrophobic <500 nm < -10 nN Ion correlation <100 nm < -5 nN Depletion <10 nm < -1 nN Polymer entanglement <5000 nm < -5 nN Capillary condensation <2000 nm < -50 nN Repulsive (positive force) Double layer repulsion <100 nm < +5 nN Hydration <5 nm < +10 nN Steric <20 nm < +10 nN

  25. The origin of surface charge • Dissociation - (Oxides, acidic or amphoteric) • Crystal lattice effects (Clays) • Ion adsorption (specific) • Point of zero charge - titration of surface charge • Surface charge vs. surface potential (first mention)

  26. H+ – O Si O O O Si O Si Si O O H+ OH– -M+–OH2 -M–OH -M–O– + H2O The origin of surface charge • Surface SiOH are acidic • Some metal oxides are amphoteric; • eg alumina, goethite (a-FeO(OH))

  27. The origin of surface charge • 4 classes of clays (kaolinite, montmorillonite-smectite, illite, and chlorite) • silicate tetrahedra, aluminate octohedra, and maybe an interlayer cation (2:1 types only) • 1:1 clay if one tetrahedral and one octahedral group in each layer • 2:1 clay if two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet.

  28. The origin of surface charge • 1:1 no free hydroxyl groups between layers - only van der waals attraction so easy to cleave. From: Hunter, R.J. Foundations of Colloid Science, Vol. 1,1989

  29. 2:1 are highly charged as silicate layer has some aluminum substitution. Ions can exchange and clay layers can swell with great pressure. From: Hunter, R.J. Foundations of Colloid Science, Vol. 1,1989

  30. Ion adsorption • Specific ions can absorb to surfaces leaving an excess of charge at the interface. • Eg. Ag+ or I- on AgI Ca2+ on silica

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