dr tim senden dept applied mathematics, research school of physics and engineering 12 lectures - 4...
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Dr Tim SendenDept Applied Mathematics,Research School of Physicsand 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
3021Course Outline
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
Tyndall effect
Mary Kathleen uranium mine, near Cloncurry, Qld.
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.
• Finely divided insulators become whiter
• Finely divided metals become black and then coloured
Scattering
Colour in metals comes from plasmon resonance, just ask Paul “Blue” Karason
Aussie sky blue European sky blue
bacterium
1 micron
Looking at clay first….
Why doesn’t muddy water clear?
Red blood cell(6 micrometres)
Scanning electron micrograph of kaolin
Na+Cl-
Salts also weather from rocks
What happens in water?Why does salt dissolve?What happens to the muddy water?
Ganges River Delta
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?
It isn’t size alone that makes a material “nano” it’s how nanoscopic phenomena play on that material that does matter.
The nanoscale characterises a strong cross over between physics and chemistry (both matter and energy levels are discrete.)
Getting a sense of scalemetres
colloidsfog / mistions
molecules
macromoleculespollen
bacteriamicelles
oil / smoke
viruses
10-10 10-9 10-8 10-7 10-6 10-5 10-4
micro-pico- milli-nano-10-310-12 10-11
Electronic effects
Thermal fluctuations
Surface tension beats gravity
Nanoscale measurements
Scale of forces1 N ≈ force required to hold an apple against gravity1 mN ≈ force required to hold a postage stamp against gravity1 µN ≈ force required to hold an eye lash against gravity1 nN ≈ covalent bonds; force between clay particles in water10 pN ≈ a single H-bond
Scale of energy100 J ≈ the energy released by a sleeping person per second1 J ≈ work required to pick an apple of the ground (1 metre)1 fJ ≈ energy required to bend lipid membrane1 aJ ≈ energy required to do cis - trans rotation (thermal energy)
10-18 atto- 10-15 femto- 10-12 pico- 10-9 nano- 10-6 micro-
Nanoscale leads to pico-, femto-, atto- effects
thermal energy (kT) = is maxm work available to a molecule
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.
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The Brownian danceTwo forces in balance• One repels• The other attracts
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The Darkened Hall analogy
Bulk properties
• Some bulk properties scale with size – but the explanation might not
Consider a rubber band
stretch
Now consider boiling/melting point, reflectivity, solubility……
Elasticity
Viscosity
etc…..
Thermal fluctuations
Ordered layer
Cooling molecule down
•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
For solids
Each atom on the surface has different properties (colour indicated) thus the surface is defective.
Reactivity“tipping point” 2Mg + O2 2MgO
Mg
MgO
ener
gy
Po
pula
tion
of a
tom
s w
ith a
giv
en
en
erg
y
Thermal energy
Heating or finely dividing
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)
Energy Band Representation of Insulators, Semiconductors and Metals
Insulator Semiconductor Metal
EmptyConduction band
Filledvalence band
Conduction band
Partially filledConduction band
valence band valence band
400 kT
40 kT
Bulk (3D)
Quantum Well (2D)
Quantum Dot (0D)
Quantum Wire (1D)
Energy
Energy
(E)
Energy
Energy
Density of States in semiconductors
Reduced Dimensionality leads to higher efficiency, lower threshold current, reduced power consumption and higher operating speed
4 GaAs QW with AlGaAs barriers
600 650 700 750 800 8500
5000
10000
15000
20000
25000
PL Intensity (a.u.)
Wavelength (nm)
1
2
3
4
S
Photoluminescence
1
2
3
4 S
S
Transmission Electron Micrograph
Courtesy of Prof. Jagadish, ANU
1.6 nm
2.2 nm
3.4 nm
6.8 nm
Colloidal CdSe quantum dots
• depends on vapour pressure and a balance of surface energies• hydrophobic is >90°• roughness makes a huge difference•If the vapour doesn’t adsorb then surface is not wet
For gases
It’s curvature that matters
Contact angle is due tobalance of surface energies
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.
Summary
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.
Force approx. range min/max forcefor colloidalsized objects
Attractive (negative force)van der Waals <15 nm < -1 nNHydrophobic <500 nm < -10 nNIon correlation <100 nm < -5 nNDepletion <10 nm < -1 nNPolymer entanglement <5000 nm < -5 nNCapillary condensation <2000 nm < -50 nN
Repulsive (positive force)Double layer repulsion <100 nm < +5 nNHydration <5 nm < +10 nNSteric <20 nm < +10 nN
Summary of forces
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)
The origin of surface charge
• Surface SiOH are acidic
Si
O
– O
Si
SiSiO
OO
O O
H+
• Some metal oxides are amphoteric; eg alumina, goethite (-FeO(OH))
-M+–OH2 -M–OH -M–O– + H2OH+ OH–
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.
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
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