Colloids Flashcards
What is the difference between a colloidal and coarse suspension? (4 marks)
Colloidal. - dispersed phase dimension 1-1000nm
Coarse -larger dispersed phase >1000nm
Coarse has a larger size meaning they are affected by gravity and thus sedimentation
Define the term block copolymer and provide an example (4 marks)
Blocks of polymers linked together Takes on structure of - A-A-A-A-B-B-B-B diblock AAA-BBBB-AAAAA - triblock Example - non ionic surfactant pluronic - PPO -PEO - PPO (also draw structures of PEO and PPO)
Explain how the addition of electrolytes affects the CMC of non-ionic and anionic surfactants and define all terms
Non ionic surfactant -hydrophobic tail, hydrophilic head with no charge
Anionic surfactant - hydrophobic tail, positively charged hydrophilic head.
CMC = concentration at which aggregates of 20 or more molecules called micelles form.
Addition of electrolytes: The non Ionic CMC is NOT affected by the electrolyte.
The ANIONIC CMC is lowered by addition. This Is because the counter ion balances the charge of the head group reducing impact of electrostatic repulsion of head groups in micelle (shulz hardy)
Define the schulz-hardy rule (2 marks)
The critical coagulation concentration for an added electrolyte to a suspension will be affected by efficiency of flocculation, which increases with charge number of the counter ion of the added electrolyte
Of the electrolytes - CaCl2, KCl - which would you choose as a flocculating agent for a suspension containing negatively charged particles? explain
- Remember - always define terms - CCC, flocculation, charge number, counter ion
CaCl2 flocculating agent
This is because the Schulz-hardy rule states that the efficiency of flocculation increases with charge number of the counter ion of the added electrolyte - CaCl2 has a charge of 2 whereas KCl has a charge of 1.
Define - CCC(critical coagulation concentration)
the electrolyte concentration at which coagulation will occur (flocs form)
Define - flocculation
this is the secondary minimum, particles sediment entrapping continuous phase and remain discrete
Define counter ion and charge number
Counter ion - ion that has opposite charge to the suspension
Charge number - charge number of the counter ion e.g if suspension has a negative charge, we use a high positive charge number
Explain why the formation of micelles is entropy driven (4 marks)
Despite the apparent ordering of surfactant molecules in micelles - micelle formation is entropically driven by hydrophobic interactions.
This is governed by the limited movement and caging of water molecules around hydrophobic domains that works to minimise contact of water with the hydrophobic region of molecules.
- Within a micelle, the hydrophobic tails have increased mobility
True or false - the higher the potential at the surface of the particle, and thus throughout the double layer - the larger the repulsion between particles?
true - thicker layer
What internal factors affect critical micelle concentration? (3)
Principle factor affecting CMC is the length of the hydrocarbon chain - increasing hydrophobicity = decreases the CMC. (affect is more pronounced for non ionic)
Non ionic surfactants - length of hydrophilic head group - head group sizes increases, increases the CMC
Ionic - second factor in determining CMC is the nature of the head group. - charge and charge of counter ions (micellar size is affected by counter ion)
what external factors affect CMC? (3)
temperature - more pronounced for non ionics
Addiction of simple electrolytes - (ionic) - an increase in electrolytes reduces repulsion between charged groups, so micelle formation is favoured and CMC reduced. (micellar size increase)
Addition of organic molecules - can increase or decrease
(a) addition of mid length alcohols - reduce repulsions and steric hindrance, lower CMC
(b) addition of sugars - structure makers, decrease in CMC as structure water
(c) urea ‘formaldehyde’ water structure breakers - lead to increase in CMC
what are colloidal dispersions?
2 phase systems
dispersed 1-100nm
thermodynamically unstable - don’t form spontaneously, there are ways to form them (e.g dispersion methods, ultrasonic treatment or condensation methods)
apparent kinetic stability due to kinetics of collapse
How do we measure size of lyophobic colloids?
most common is light scattering - provide a weight average Mw
How do we measure optical properties of lyophobic colloids?
light scattering - colloids scatter light (Tyndall effect) - can calculate size and shape from light scattering measurements as a function of scattering angle and concentration
