Colloids 1

Question 1

What is a colloid?

Answer 1

dispersions in which the size of the dispersed particles is within the range of 1nm to about 1µm

Question 2

What are the types of colloids?

Answer 2

lyophilic = solvent loving
- dispersed particles have an affinity for the dispersion medium
= leads to spontaneous formation of colloidal dispersions

lyophobic = solvent hating
- particles do not have an affinity for the dispersion medium

Question 3

What are examples of lyophilic and lyophobic dispersions?

Answer 3

lyophilic

• acacia
• tragacanth
• methylcellulose

lyophobic
- oil droplets in water/water droplets in oil

Question 4

How can lyophobic dispersions be prepared?

Answer 4

dispersion methods

• colloid mills
• ultrasonic treatment

condensation methods
- chemical reactions that result in small, fine particles
= colloidal silver iodide

Question 5

What are the properties of colloids?

Answer 5

size
- 1-1000nm
shape
- spherical, ellipsoid

have kinetic properties

• brownian motion = move in zigzags/randomly
• diffusion/fick’s law = can move from one side to another across a membrane
• sedimentation/stoke’s law = can sediment over time

optical properties
- tyndall effect = cannot be seen by naked eye/refraction of light proves the presence of particles

electrical properties
- electrical double layer

Question 6

What are the components of a micelle?

Answer 6

shell/corona
- outer part of the micelle with the polar heads

core
- inner part of the micelle with the lipophilic chain

Question 7

What are the timescales for fast and slow relaxation during micellization? What is the different between them?

Answer 7

fast relaxation
- microseconds
= is the loss of one surfactant which can rejoin to form the micelle again

slow relaxation
- milliseconds to minutes
= is the loss of a surfactant followed by another until there is complete disassociation

Question 8

When do micelles form?

Answer 8

micelles form when the concentration of surfactant monomers exceeded critical micelle concentration

Question 9

What are the factors affecting critical micelle concentration?

Answer 9

CMC

• decreases with the addition of electrolytes
• decreases with an increase in hydrophobic chain length of a surfactant
• decreases with increasing temperature

decreasing CMC makes it easier for micelle to form

Question 10

How do micelles improve solubility?

Answer 10

surfactants are added to poorly soluble substances and after CMC is reached, solubility increases

drugs with polar groups attach to the micelle as it has polar heads facing outward

drugs which are non-polar are contained within the micelle to protect it against the solvent

Question 11

What are the benefits of using micelles?

Answer 11

solubilisation

reduces hydrolytic degradation - micelle acts as a shield against water/increases stability

release rates - can be controlled as the drug must partition across the surfactant layers to escape the micelle

taste masking - most drugs are basic and carry a negative charge which results in a bitter/metallic taste
= micelles improve adherence as the drug being contained inside masks the taste

immunology - drugs can start immunological reactions inside the body but micelles hide them therefore reducing reactions

medical diagnostic imaging - micelles deliver contrast agents to specific sections of the body

study of cell membranes and micellar catalysis

Question 12

What is the electrical double layer? What causes its formation?

Answer 12

electrical double layer forms due to electrical charge arising on colloidal particles
- is used to determine the stability of a micellar system

electrical charge arises due to

• ionisation of the surface groups
• adsorption of the ions

Question 13

What are the different layers of the electrical double layer?

Answer 13

solid surface
- charged ions are adsorbed onto the particle surface via electrostatic force
= are usually negatively charged ions
= make up the surface potential

stern layer
- carries the counter ions to the absorbed surface ions
= make up the stern potential

diffused layer
- carry both positive and negative charge which neutralises the overall charge on the micelle
= boundary layer is known as the shear plane
= make up the zeta potential

Question 14

Why is zeta potential important? How can it be measured?

Answer 14

surface and stern potential cannot be measured therefore the only way to know what they are is to measure zeta potential
- zeta is lower than the surface potential as shown in the drop in the potential curve

can be measured via electrophoresis

Question 15

What affects the magnitude of the surface potential?

Answer 15

surface charge

thickness of the double layer
- as distance from the surface increases, the surface potential drops
= reaches zero at the boundary of the electrical double layer