SOM1 - Colloidal dispersions

Lecture 38+39

Disperse systems

What are the 3 size catergories of dispersions and their ranges

Coarse dispersions

• 10-50 mcm

Fine dispersions

• 1mcm-10mcm

Colloidal dispersions

• 1-1000nm or 1-500nm for some resources

Molecular dispersions

• <1nm

• Basically a solution, as the dispersed phase is so fine

Name the 2 Classifications of dispersions by viscosity

Solutions - LOW viscosity

Gels - HIGH viscosity and intertwined particles

can be described as a hydrogel/oleogel/alcogel depending on continuous phase

Classification by solvent affinity

Lyophobic colloids characteristics

Possess low affinity for the continuous phase

They form thermodynamically unstable dispersions

Energy equation for lophobic colloids

Gamma = surface tension

A = area

Lyophobic colloid solvent order

Very ordered solvent molecules, because they don’t want to touch the central molecule

Association colloids

Micelles behave like lyophilic colloids

• Spontaneous formation above a minimal concentration

• Thermodynamic stability

Micelles are typically spherical and made of a number of surfactant molecules (aggregation number

Not all amphiphiles form micelles

Different types of shape

What type of colloid do micelles behave like

lyophilic

What is the Krafft point?

Temperature above which the solubility of a surfactant increases sharply (critical micelle temperature)

At temperatures below Krafft point, surfactants just precipitate instead of forming micelles.

Basically the minimum temperature for any meaningful amount of micelles to form

what is the cloud point

Temperature above which the solubility of a surfactant decreases sharply (basically opposite of Krafft point)

Cloudy appearance is due to polar head being dehydrated, and precipitating

Reversible, can be fixed by cooling

What causes shape variety of dispersed phase components in colloids

Affinity with solvent

In a good solvent, what are the properties of a colloid in regards to:

• Lyophilic/lyophobic

• will it want more or less interaction with dispersing phase

• Will it form a scrunched or extended shape

• Contribution of surface tension to overall free energy

• Dispersion spontaneity

Lyophilic

More interactions

Extended - wants to maximise contact with dispersing phase

Surface tension will contribute almost nothing

Spontaneous dispersion, no additional energy required

What is dialysis

Separation of colloidal particles from small molecules/ions

How is dialysis performed

Semi permeable dialysis membrane allows ions and molecules to diffuse, but not colloids because… [they’re too big?]

Kinetic properties of colloids

• Brownian motion

• Diffusion

• Sedimentation

• Viscosity

• Osmotic pressure

• Colloids contribute to total osmotic pressure

• Donnan membrane effect

• Impact of charged colloids on diffusion of small ions across a membrane

Brownian motion

Random movement under thermal agitation

only affects particles u[ to 5 micrometers

Why is brownian motion relevant in regards to sedimentation

Sedimentation

Where downward motion

Viscosity

Resistance to flow under an applied force

Can be used to determine molecular weight of lyophilic colloids

Changes with:

• Solvation state (basically how much continuous and dispersed phase interacts, higher solvation state = more interactions)

  • Increases with degree of solvation

• Shape

  • Spherical vs elongated

Concentration

• Increases with concentration

Molecular weight

• Increases with the molecular weight of the colloid

Electrical properties of colloids

A partticle may become charged through:

• Being naturally charged

• Ion adsorption

  • Adsorption of oppositely-charged ions

• Ion dissolution

  • Excess ions in solution

• Ionisation of surface groups

  • Ionisation of surface groups

    • Permanent/pH-dependent charge

• Affects distribution of other ions

Zeta potential

Effective particle charge

• contribution of ions in solution

Impacts on stability, moreso on hydrophobic colloids

Why do we measure zeta potential

Indicates stability

3 Major forces on stability of colloid

Repulsive

• Electrostatic

Attractive

Steric

• Linked to solvation

• Stabilisation of suspensions

How do you figure out total potential energy of colloid interaction

DLVO theory, compare attractive Van der Waals forces to repulsive electrostatic energy,

Good indicator as to whether the dispersion will stay aggregate or split

Stability of lyophobic and lyophilic colloidal

dispersions