5. Colloidal interaction and surface forces

Stability of colloidal dispersions

  • Almost always thermodynamically unstable systems
  • Kinetics and interaction determine stability
  • Time is important – stability from minutes to 100s of years (for example Italian salad dressing is short)

Colloidal interactions- Interactions between colloidal particles → forces between particles

  • Attraction (if it comes close enough) and repulsion
  • Range of interaction (how close can two particles get before anything happens) is important!

Flocculation- a process where a solute comes out of solution in the form of floccules or flakes

Aggregation- direct mutual attraction between particles (atoms or molecules) via van der Waals forces or chemical bonding

van der Waals interactions

• Caused by molecular dipole interactions (impermanent, permanent and induced)

• Comparably long-ranged, but weak at long distances.

• Strong at short distances (smooth surfaces tend to get closer to each other)

Attractive force between surfaces, is always present

Van der Waals force between surfaces- It decays were fast with larger distances

A=Hamaker constant (J)

L=separation between surfaces

P=attractive pressure (N/m2)

Positive direction is repulsive and negative direction is attractive

Hamaker constant, A, is proportional to the difference in density (Δρ) and the refractive index (polarizability of material)

Electrostatic repulsion

• Caused by charge at surfaces

• In low dielectric media → coulomb interaction (continuous phase typically air and oil/apolar solvents)

– Charge level could be very high and operating distance very long

– Important in clouds (thunder)

– Sparks

• In dielectric media (polar solvent, typically water) a diffuse counter-ion cloud is formed

– Neutralising the long range character of the coulomb forces.

Electrical field and charge densities

• A charged surface gives rise to an electric potential in a dielectric medium (such as water)

• The electric potential decays from the surface – depends on the charge density in the solution (i.e. ionic strength)

• Ions in solution ”screen” the charge of the surface

• Creation of the double layer (one charge on the surface and another a bit further away from surface (diffused ion cloud of counter ion, not every ion on surface has it own counter ion))

Concentration profile of the double layer

• The concentration profile of the double layer can be described

by the Boltzmann distribution

• Influenced by interaction (attraction) and entropy

where Z is valence, q is the unit charge, c∞ is the bulk ion concentration (far away from surface), ψ(x) the potential at distance x from the surface, the concentration at the distance x from the surface.

Higher charge → c(x) will decay more rapidly → thinner layer of counter ions

The double layer

Counter-ions are attracted to the charged surface and a “double layer” is formed. The counter ion concentration decreases with distance from the surface

Debye-Hückel length

• By assuming symmetrical electrolytes we obtain the Debye-Hückel screening length

Higher charge → shorter Debye-Huckel length

Increasing concentration → shorter Debye-Huckel length

LD=Debye-Huckel length

Potential- The potential as a function of distance from the surface

• Can be interpreted as the thickness of the electric double layer → the range of electrostatic repulsion

• Depends strongly on valence and also on concentration

Then two layers of different surfaces overlap → higher osmotic pressure → repulsion between the surfaces → solvent goes in

  • The screening length is long if the counter ion concentration is low.
  • The screening length is short if the counter ion concentration is high.
  • The screening length is short if the counter ions have a high valence

Na+<Ca2+<La3+, Al3+, Fe3+

Note that Al3+ and Fe3+ from hydroxide complexes at intermediate pH

Electrostatic repulsion

• Osmotic repulsion between overlapping clouds of counter ions (double layers)

• Resulting pressure:

• Range of the force depends on the thickness of the double layer

When two curves overlap → repulsion of surface

DLVO theory (Derjaugin, Landau, Verwey and Overbeek)

• van der Waals interaction dominates at very short distances → primary minimum

• van der Waals interaction dominates at long distances → secondary minimum

• The electrostatic repulsion may dominate at intermediate distances. This causes the electrostatic repulsive barrier.

Repulsion barrier: If it is too low it will pass

Examples of loss of electrostatic repulsion

• Flocculation of particles in rivers

– The clear water in the Mediterranean (salt water, more ions, increased ionic strength, reduced thickness of the double layer (repulsive barrier becomes thinner), aggregate, sink to the bottom of ocean, Vsed ～r2)

• pH induced protein aggregation

- protein typically more anionic than cationic

-Lower pH → more H+

→ shielding effect

→ acids get protonated at surface → less negative charge (think about what happens to charge at surface)

Steric repulsion

• Osmotic repulsion between protruding chains of adsorbed macromolecules.

• Caused by adsorbed (stuck to the surface) macromolecules

Steric repulsion requires:

• Adsorption of the macromolecule

• Sufficient coverage of the interface

• High solubility of the macromolecule (better than θ solvent)

• Larger polymer → more efficient

Low solubility → try to get away from water

High solubility → protein will stretch out

Examples

• Irish cream (i.e. Bailey’s-type drinks) - long (steric) stability (proteins (typically charged), phospholipids, H2O/EtOH, Na-caseinate)

• Block co-polymers (i.e. pluronic) in topical emulsions (PPO will be anchored to oil and PEO is the solution → steric stabilisation)

• Cheese (particle gel, casein micelle (nothing to do with micelle), at the surface from casein micelle, so casein should not stick to each other) curd -remove steric stabilisation by adding enzyme (chymosin or pepsin used cleave of surface protein) and decrease pH (not main)

• Stabilization of apolar organic solvent borne paints (for example faluröd, Fe2O3)

- add anionic surfactant, the tail is in contact with the organic solvent

Bridging (attraction)

• Partial coverage of surfaces can lead to the formation of macromolecular bridges between the surfaces.

• Attractive (destabilizing) force

Examples of bridging

• Flocculation additives.

• Optimal concentration with a low coverage

– Papermaking

– Waste water treatment

– Irish moss (carrageenan, anionically charged) used in beer brewing

Beneficial to do before filtration

Depletion (attraction)

• Caused by non-adsorbing polymers → no or low affinity for surface

• Polymers avoid contact with surfaces

• As polymers are depleted → osmotic gradient is created

• Occurs at intermediate polymer concentrations and large polymer size (high molar mass)

• Examples in aqueous dispersions are, dextran, PEG and other non-ionic polymers

Hydration forces (repulsion)

• Hydrated surfactants at the interface

• Caused by the interaction between the emulsifier layer and the aqueous phase

• Empirically described force

• Repulsive

Hydration forces- Examples

H2O as solvent compared to methylformamide as solvent for phosphatidyl choline.

Hydrophobic interaction (attraction)

• Between hydrophobic macroscopic surfaces

• Experimentally observed but, fundamentally, poorly understood force