Dr Tim Senden
Dept Applied Mathematics,
Research School of Physics
and Engineering
12 lectures - 4 tutes
3021
Course Outline
– 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
Foundation Demonstrations
Part 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
Granite weathers into
components
Quartz,
clays
& other minerals
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.
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”
bacterium
Looking at clay first…
1 micron
Red blood cell
(6 micrometres)
Scanning electron micrograph of kaolin
Why doesn’t muddy water clear?
Salts also weather from rocks
ClNa+
What happens in water?
Why does salt dissolve?
What happens to the muddy water?
The Colorado
The Nile
The
Ganges
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?
Ganges River Delta
The nanoscale characterises a strong cross over
between physics and chemistry (both matter and
energy levels are discrete.)
Getting a sense of scale
metres
pico10-12
nano10-11
10-10
10-9
micro10-8
10-7
colloids
ions
molecules
10-6
milli10-5
10-4
fog / mist
oil / smoke
pollen
macromolecules
viruses
micelles
bacteria
Surface tension beats gravity
Thermal fluctuations
Electronic effects
10-3
Nanoscale measurements
Nanoscale leads to
pico-, femto-, attoeffects
Scale of forces
1 N ≈ force required to hold an apple against gravity
1 mN ≈ force required to hold a postage stamp against gravit
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 ener
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-
Energy (exothermic)
Jmol-1
Processes involving changes;
- in the nuclei of atoms
235U + n  Ba + Kr + 3n
1012
- in molecular structure
H2 + 1/2O2  H2O
105.5
- in valence electrons
e + H+  H
105
- changes of state
H2O(g) H2O(l)
104.5
- molecular translational, rotational & vibrational energy
H2O(g, 1000K) H2O(l, 300K)
This compares with RT (2500 Jmol-1)
103
- mechanical potential energy
H2O(l, 555 metres) H2O(l, sea level)
102
- mechanical kinetic energy
H2O(l, 10 ms-1) H2O(l, rest)
(adapted from Rossini)
101
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.
+
+ +
+
+
+
+
+
+
+
+
+
+
The Brownian dance
Two forces in balance
• One repels
• The other attracts
+ +
+ +
+
+ +
+
+
The Darkened Hall analogy
Bulk properties
• Some bulk properties scale with size –
but the explanation might not
Elasticity
Consider a rubber band
Viscosity
stretch
Cooling molecule down
Thermal fluctuations
Ordered layer
etc…..
Now consider boiling/melting point, reflectivity, solubility……
For solids
•The surface atoms “squeeze” the
internal atoms. In nanoscopic systems
this could be 1000s of atmospheres.
• Physical properties such as optoelectronic, phase state, solubility,
reactivity and conductivity may change
Each atom on the surface has
different properties (colour indicated)
thus the surface is defective.
energy
Mg
MgO
2Mg + O2
2MgO
Population of atoms
with a given energy
Reactivity
“tipping point”
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
Empty
Conduction band
400 kT
Conduction band
40 kT
Filled
valence band
Insulator
valence band
Semiconductor
Partially filled
Conduction band
valence band
Metal
Density of States in semiconductors
Bulk
(3D)
r(E)
Quantum Well
(2D)
r(E)
r(E)
Energy
Quantum Wire
(1D)
Energy
Quantum Dot
(0D)
r(E)
Energy
Energy
Reduced Dimensionality leads to higher efficiency, lower threshold
current, reduced power consumption and higher operating speed
Photoluminescence
1.6 nm
4 GaAs QW with AlGaAs barriers
1
2.2 nm
S
25000
2
2
3.4 nm
20000
PL Intensity (a.u.)
3
6.8 nm
3
15000
1
4
4
10000
S
Transmission Electron Micrograph
5000
S
0
600
650
700
750
800
850
Wavelength (nm)
Colloidal CdSe
quantum dots
Courtesy of Prof. Jagadish, ANU
For gases
It’s curvature that matters
q
Contact angle is due to
balance of surface energies
• 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
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.
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.
Summary of forces
Force
approx. range
for colloidal
sized objects
Attractive (negative force)
van der Waals
<15 nm
Hydrophobic
<500 nm
Ion correlation
<100 nm
Depletion
<10 nm
Polymer entanglement
<5000 nm
Capillary condensation
<2000 nm
Repulsive (positive force)
Double layer repulsion
Hydration
Steric
<100 nm
<5 nm
<20 nm
min/max force
< -1 nN
< -10 nN
< -5 nN
< -1 nN
< -5 nN
< -50 nN
< +5 nN
< +10 nN
< +10 nN
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
H+
• Surface SiOH are acidic
–O
Si
O
O
O
Si
O
O
Si
Si
O
• Some metal oxides are amphoteric;
eg alumina, goethite (a-FeO(OH))
-M+–OH2
H+
-M–OH
OH–
-M–O– + H2O
The origin of surface charge
• 4 classes of clays (kaolinite, montmorillonitesmectite, 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
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