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

Granite weathers into components

Quartz,clays

& other minerals

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?

The Colorado

The Nile

TheGanges

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.

+

+

+ +

+

+

+

++

+

++

+

+

+

The Brownian danceTwo 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

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

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