Dr Tim Senden

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

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 doesnt 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 isnt size alone that makes a material nano
its 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 dont 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
Getting a sense of scale
metres
fog / mist
ions molecules
oil / smoke
pollen
macromolecules
bacteria
viruses
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
10-18 atto- 10-15 femto- 10-12 pico- 10-9
nano- 10-6 micro-
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
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
Density of States in semiconductors
Reduced Dimensionality leads to higher
efficiency, lower threshold current, reduced
power consumption and higher operating speed
20
Photoluminescence
1.6 nm
4 GaAs QW with AlGaAs 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 qgt90
• roughness makes a huge difference
• If the vapour doesnt adsorb then surface is not
  wet

Its curvature that matters

q
Contact angle is due to balance of surface
energies
22
Summary

• Its not so much the size that matters, its 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 lt15 nm lt -1 nN Hydrophobic lt500
nm lt -10 nN Ion correlation lt100 nm lt -5
nN Depletion lt10 nm lt -1 nN Polymer
entanglement lt5000 nm lt -5 nN Capillary
condensation lt2000 nm lt -50 nN Repulsive
(positive force) Double layer repulsion lt100
nm lt 5 nN Hydration lt5 nm lt 10
nN Steric lt20 nm lt 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
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-sme
  ctite, illite, and chlorite)
• silicate tetrahedra, aluminate octohedra, and
  maybe an interlayer cation (21 types only)
• 11 clay if one tetrahedral and one octahedral
  group in each layer
• 21 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

• 11 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
21 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