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 (in the nanometre range)

Molecular dispersions

• <1nm

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

Difference between gels and sols

Sols:

Low viscosity

Gels:

High Viscosity

Dispersed and continuous phase particles intertwined

Name the 2 Classifications of dispersions by viscosity

Sols - LOW viscosity

Gels - HIGH viscosity and intertwined particles

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

What would a system where the two phases have low affinity and a continuous phase of water be called

A dispersed system with a water continuous phase and a low affinity between phases would be a hydrophobic dispersion

Lyophobic colloids characteristics

Possess low affinity for the continuous phase

They form thermodynamically unstable dispersions

Energy equation for lyophobic colloids

Gamma = surface tension

A = area

need to remember

Lyophobic colloid solvent order

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

How does forming a lyophobic colloid affect the entropy in the system

Forming a lyophobic colloid causes a highly ordered structure of solvent around the colloid, the increased order decreases entropy and therefore also decreases spontaneity

Name 2 methods of preparing a lyophobic colloid

• Breakdown of large particles via mechanical force

• Controlled aggregation

  • Chemical reactions

    • Oxidation, reduction, hydrolysis etc.

  • Solvent changes

    • Colloids that are in a good solvent being transferred to a bad solvent

Using the Gibbs Free Energy equation, explain why lyophilic colloids form spontaneously

ΔG = γ ΔA - T ΔS

If there is high affinity between the continuous and dispersed phase, then the (γ ΔA) section of the equation is nearly negligible, as the change in area is minor. This means the (T ΔS) section is larger, and so gives a negative Gibbs free energy, and means the reaction needs no additional energy to occur (spontaneous)

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 surfactants behave like and why

lyophilic, as there will always be a part of the molecule that likes the continuous phase, it will just flip depending on medium (polar head inside and nonpolar tails outside in a oil, and vice-versa in water)

They also have spontaneous formation (when above a minimal concentration (CMC))

And thermodynamic stability (up to a point)

How do surfactants reduce surface tension?

Their amphiphilic structure allow them to act as a bridge between the molecules and disrupt the stronger cohesive forces between the bulk molecules. This brings the two phases closer together and decreases surface tension

How can you tell when a dispersion is saturated with surfactants (reached its critical micellar concentration)?

When surface tension stops dropping, as that means there is no more surface for the surfactant to attach to, and so stops dropping surface tension

What do the surfactants do when the dispersion is oversaturated with surfactants

They form micelles of just surfactant to protect the phobic part of the surfactant molecules

What effect does micelle formation have on entropy, and is it energetically favourable

Increases entropy, as lone surfactant molecules require a large amount of water molecules around them to form the ordered structure around it, but when surfactants combine to form a micelle, less water molecules are needed

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 to think about

• 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 up to 5 micrometers

Why is brownian motion relevant in regards to sedimentation

It is what stops colloidal dispersions from spontaneously sedimenting, as it provides an upwards force.

Sedimentation

Downward motion under gravitational forces. Due to Brownian motion, will not spontaneously occur under normal gravity, may need centrifugation.

Fick’s first law + what is it used for

Diffusion rate

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

Why does a colloidal dispersion look blue and cloudy under a light

Faraday-tindall effect causes the dispersion to scatter light, causing a cloudy look, and blue wavelengths are shortest, so they scatter more

Electrical properties of colloids

A partticle may become charged through:

• Being naturally charged

• Ion adsorption

  • Adsorption of oppositely-charged ions from dispersed phase

• Ion dissolution

  • Excess ions in solution

• Ionisation of surface groups

  • Ionisation of surface groups

    • Permanent/pH-dependent charge (e.g. acids & bases)

• Affects distribution of other ions

describe particle surface, stern layer and slipping plane

Particle surface - Just the surface of the dispersed phase in the suspension, no additives

Stern layer - If the particle surface is negatively charged (for example), then it will attract positively charged cations, which forms a layer around the surface (the ions are not the dispersing phase, just other random ions in the solution)

Slipping plane - Other ions may be pushed in/out by the stern layer, causing one final layer to form around it. The potential around the slipping plane is the Zeta potential, and is essentially the overall surface charge.

Stern and slipping plane both move with the particle

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 - highly charged particles will repel eachother

Attractive

Steric

• Linked to solvation

  • shell of solvent around dispersed phase means it is more difficult for particles to aggregate

• Stabilisation of suspensions

  • Means large, solid, poorly solvated particles can be covered in lyophilic colloids to form a stable suspension

What does a weak attractive energy signify on the DVLO chart? (secondary minimum)

Flocculation - can easily redisperse particles by shaking

What does a strong attractive energy signify on the DVLO chart? (primary minimum)

Coagulation - shaking will not be strong enough to redisperse particles

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