Colloids 3 (electrical properties of colloids)

Question 1

What is going to be discussed in this lecture?

Answer 1

electrical properties of colloids

• acquisition of charge
• electrical double layer
• zeta potential

Measurement of zeta potential

Question 2

For electrical properties of colloids;

A) What is dependent on/affected by?

B) Most particles in contact with a polar medium have a surface charge. How can the surface electric charge be acquired?

Answer 2

A)

• Dependent on or affected by the surface charge of a particle

B)

Surface electric charge can normally be acquired by:

• Ionisation
• Ion adsorption
• Ion dissolution

Question 3

What is ionisation? Discuss its properties for proteins. (acquisition of charge)

Answer 3

Ionisation: eg ionisation of surface functional groups

For protein:

• pH > pI (isoelectric point): -ve charge (COO^-)
• pH < pI: +ve charge (NH₃⁺)

Question 4

What is ion adsorption (acquisition of charge)

Answer 4

Ion adsorption: acquired by unequal adsorption of oppositely charged ions. Can be negative or positive

• Smaller, less hydrated and more polarizing anions are more prone to be specifically adsorbed to particles in aqueous medium –> particles often being negatively charged.
• Ionic surfactants are strongly adsorbed and have a major influence on surface charge, resulting in either + or – charge depending upon the ionic character.
• Particles which are already charged tend to adsorb counter-ions (esp multivalent counter-ions). Adsorption of counter-ions may lead to charge reversal on the particle.

Question 5

What is ion dissolution? Provide an example (acquisition of charge)

Answer 5

Ion dissolution: acquired by the unequal dissolution of the charged ions of which they are composed

Eg AgI –> AgI[I^-] or AgI[Ag⁺]

AgNO₃ + NaI –> Ag I (↓) + NaNO₃

• If AGNO3 is in excess, AgI [Ag+] will be formed –> surface +ve charge
• If an excess or equimolar amount of NaI is used, AgI[I-] is produced –> surface - ve charge.

Question 6

What does the charge on the surface of the particle cause the formation of? How does this formation come about?

Answer 6

• Charge on the surface of particles causes the formation of Electrical double layer: (distribution of ions at the charged interfaces)
• Charged particles attract ions of opposite charge and repel co-ions in the dispersion medium –> formation of the electric double layer.

Question 7

What is the importance of the electrical double layer?

Answer 7

The electrical double layer plays an important role in colloid stability and is the only stabilizing force for lyophobic colloids.

Question 8

Discuss the structure of the electrical double layer. What is it made up of?

Answer 8

Made up of:

• inner region which includes adsorbed ions & counter ions, also known as stern layer
• a diffuse region in which ions are distributed according to the influence of electrical forces and random thermal motion. This is also known as diffuse layer or mobile layer
• The two parts of the double layer are separated by the Stern plane, which is located at about a hydrated ion radius from the surface.
• A solvating layer is held to the Stern plane and the edge of the layer is termed the plane of shear and represents the boundary of movement between solid particle with its attached ions and liquid.

Question 9

Discuss the properties of the electrical double layer

Answer 9

• Charged particle has an electrical potential ψ₀ or E
• Counter-ions are strongly attracted to surface – approach inhibited by hydration sphere.
• Counter-ions form Stern layer. Stern plane cuts through the centres of the tightly bound ions. There will be an electrical potential at the Stern plane ψ_δ that is often (but not always) lower than the surface potential.
• Stern layer is the first part of the double layer
• Beyond the Stern plane, more counter-ions are attracted – aiming for electrical neutrality. These ions are subject to thermal motion and move around – diffuse/mobile/Gouy-Chapman layer – second part of double layer.​
• The thickness of the double layer (δ) is arbitrarily set at the distance over which the potential drops to 0.37 (1/e) of Stern potential – equivalent to 1/K.
• Stern layer ions are tightly bound and will move with the particle. When particle moves it will also take some solvent and ions from beyond the Stern plane with it.
• This boundary of movement is the Plane of Shear. Its exact location is unknown but as it is further out from the Stern plane, the potential at the Shear Plane will be slightly lower than the Stern Potential.
• The potential at the Shear Plane is the zeta potential (ζ)
• ζ is of significance, because it is this potential that governs interactions between particles.​

Question 10

What does a schematic representation of an electrical double layer look like?

Answer 10

Question 11

Define the following terms;

A) Electrothermodynamic (Nernst) potential E:

B) Electrokinetic potential (Zeta potential) ζ:

C) Thickness of double layer

D) Effect of electrolytes

E) Effect of valency of counter ions

F) Effect of specific adsorption

Answer 11

A)

• Electrothermodynamic (Nernst) potential E: potential difference between the actual surface and electroneutral region of the solution (also known as ψ₀).

B)

• Electrokinetic potential (Zeta potential) ζ: difference in potential between the surface of shear (ie shear plane) and electroneutral region of the solution

C)

• Thickness of double layer: 1/K (Debye-Huckel length Parameter). Distance for Stern potential to drop to 0.37 (1/e) of its value. Usually 1-100nm.

D)

• Effect of electrolytes: Conc. of Electrolytes ↑, K increases with consequent decrease in 1/K – shrinking or compressing the double layer, zeta potential ↓ quickly.

E)

• Effect of valency of counter ions: valency ↑, total conc. of electrolyte is held constant –> shrinks double-layer ↓ zeta potential eg: NaCl vs CaCl2 – Schulze-Hardy rule.

F)

• Effect of specific adsorption: specific adsorption of surface active ions via hydrophobic interaction. Zeta potential may increase or decrease

Question 12

What is the Importance of ζ (zeta potential)? What are some applications?

Answer 12

• ζ rather than ψ₀ (surface/nernst potential) governs the degree of repulsion between adjacent dispersed particles. ζ therefore is related to the dispersion stability.

Applications: flocculation, colloid stability.

Question 13

A colloidal particle with a negative surface charge. What might happen to zeta potential if a cationic surfactant was adsorbed onto the particle?

Answer 13

Zeta potential will decrease

Question 14

Compare what a graph would look like for adsorption of counterions and adsorption of co-ions.

Answer 14

Watch lecture for clarification

Question 15

What electrokinetic phenomena can be used to measure zeta potential?

Answer 15

• Electrophoresis
• Electro-osmosis
• Sedimentation potential
• Streaming potential

Question 16

What is electrophoresis? How does it work

Answer 16

Electrophoresis is a general term that describes the migration and separation of charged particles (ions) under the influence of an electric field

>Positive pole called anode (anions migrate here)

> Negative pole called the cathode (cations migrate here)

• Microelectrophoresis: movement is observed using an ultramicroscope.

Particle with a greater charge and smaller size move faster

• Rate of particle migration is a function of the charge on the particle
• The rate-determining potential is zeta potential
• Mobility μ=v/E. V: velocity of particle movement (cm/sec). E: potential gradient in volts/cm.
• Zeta potential ξ is calculated from the mobility ζ = (4 πημ/ε) x (9x104)

n: viscosity of medium

ε: dielectric constant of medium

Question 17

What does zeta potential depend on?

Answer 17

• surface characteristics
• pH
• ionic strength of solution
• presence of ionic surface-active agents

Question 18

What are some applications of zeta potential measurements?

Answer 18

• Monitor and predict the stability of emulsion and suspension
• Study the effect of electrolytes on colloidal stability
• Obtain info about delivery systems eg DNA–liposome interaction
• As a tool for investigation of the cause of change of particle size in some drug delivery system eg liposomes
• Isoelectric points of ampholytic drugs

see attached image