Colloids Flashcards
What is a colloid?
A colloid contains an interface between two phases
These interfaces are high energy; they bring things together that do not readily form productive interactions
What is the Tyndall effect?
The scattering of light as it passes through a colloid
The individual particles are large enough to scatter light, thus making the beam visible e.g. headlights through fog
What is a key property of a surfactant?
They must be partially soluble in both phases e.g. must have a hydrophobic tail and a polar head group
How does a surfactant lower the interface energy?
They organise at the interface, which lowers the interface energy and the surface tension
What are the three steps when surfactants are put into water?
(1) At low concentrations, the surfactant is at the air-water interface. Hydrophobic chains are in the air and the polar head group is in the water
(2) Once the air-water interface is full (when the [surfactant] increases), they have to enter the bulk water. The hydrophobic chain stays rigid.
(3) At a critical concentration (CMC), the energy cost is too high and the surfactant starts to form micelles
What are the thermodynamic effects that drive micelle formation?
H-bonds are re-established in the surrounding water - enthalpically good
Ordered water is released from the hydrophobic chains - entropically good
The hydrophobic chains become more mobile - entropically good
The monomer cannot move freely - entropically bad
Charged head groups are close together - enthalpically bad
What is the Critical Micelle Concentration?
The point at which delta G becomes negative with regard to micelle formation
What three methods can we use to determine the CMC?
Surface tension
Conductivity
Fluorescence
How does analysing the surface tension enable us to calculate the CMC?
As the [surfactant] increases, the surface tension decreases as there is more surfactant at the air-water interface
Once the surface is fully covered (at the CMC), any extra surfactant will not affect the surface tension
How do we determine the surface tension?
We place a wetted object e.g. wire, plate etc… and measure the force required to remove the ring
What is the limitation of determining the CMC by surface tension?
It can be difficult instrumentally to get robust results - sophisticated instrumentation is needed
How can we determine the CMC by conductivity?
Conductivity increases as surfactant concentration increases
However, once the CMC is reached, additional surfactant becomes less mobile, decreasing the rate of increase of conductivity
What is the limitation of determining the CMC by conductivity?
It only works for ionic surfactants
How can we determine the CMC by fluorescence?
A fluorescent chromophore is chosen that fluoresces in an organic solvent (like in the interior of a micelle) but not in water
The increase in fluorescence occurs at the CMC
What is the limitation of determining the CMC by fluorescence?
The presence of the fluorophore may change the CMC by encouraging or discouraging assembly
How does increasing the hydrophobic chain length impact the CMC?
As the hydrophobic chain length increases, the CMC decreases
This is because the hydrophobic effect becomes more significant
How does changing to a non-ionic surfactant impact the CMC when compared to an ionic surfactant?
It greatly reduces the CMC as there are no ion-ion repulsions between head groups, hence making them easier to assemble
How does increasing the charge of the cation on a surfactant impact the CMC?
The more highly charged ion forms better interactions with the anionic micelle surface, assisting surfactant packing and lowering the CMC
Why do polymer micelles have significantly lower CMCs?
They have high thermodynamic stability due to them being large, lowering the cost of translational entropy of organising themselves into a micelle
The large apolar chains have a very large hydrophobic effect - they are stable
How do we determine the structure of a micelle?
We calculate (v / lmax a)
When do we form a spherical micelle?
When (v / lmax a) is less than a third
When do we form a cylindrical micelle?
When (v / lmax a) is in between a third and a half
When do we form a vesicle bilayer structure?
When (v / lmax a) is in between a half and 1.
What are gels?
They are solid-in-liquid colloids
Traditionally, many gels are polymeric
What are Low Molecular Weight Gelators (LMWGs)?
They are small molecules that self-assemble into a non-covalent polymer (e.g. H-bonds)
These can form extended networks and trap solvents
How do gels form?
(1) The molecules are freely dissolved
(2) They self-assemble through non-covalent interactions into fibrils
(3) Fibrils bundle (non-covalently) to form fibres
(4) Nanofibres form an interactive sample spanning network
(5) The network makes a bulk gel
What is the Minimum Gelation Concentration?
A gelator must be soluble enough to dissolve with a stimulus (e.g. heating or ultrasound), but upon cooling/standing, it should be insoluble enough to form into a gel
These fibres need to be insoluble enough to interact to form bundles and networks, but not so insoluble that they precipitate out of the solution
What happens if our gelator is too soluble?
It just dissolves
What happens if our gelator is too insoluble?
It just precipitates
What three methods can we use to view the macroscopic behaviour of gels?
Vial Inversion
Dropping ball method
Rheology
What is the vial inversion method?
It involves inverting a vial of the gel and watching to see if any flow occurs
What is the dropping ball method?
A small metal ball is placed onto the gel, and is watched
In an ideal scenario, the ball is immobile in the gel, but drops rapidly in the sol
What is rheology?
Involves a sample of gel being placed between two parallel plates which are rotated with respect to each other
The response of the material to the operating shear is recorded
The amplitude of oscillation, or the frequency, can be varied, allowing us to monitor the G’ and G’’ values
What is G’?
The storage modulus; the measure of the elasticity associated with the energy stored in elastic deformation
What is G’’?
The loss modulus; represents the ability of the system to flow under stress
What are two methods of viewing the nanoscale behaviour of gels?
EM and Cryo-EM
Why is Cryo-EM better than EM?
In EM, the sample is left to dry ambiently, but this can cause the collapse of the structure
In Cryo-EM, the sample is frozen and the solvent evaporated in vacuo. This reduces the collapsing of the structure
What are the five methods main methods of monitoring molecular assembly?
IR
NMR
Circular Dichroism Spectroscopy
Calorimetry
X-Ray Scattering
How can IR spectroscopy tell us about the molecular assembly?
We can detect the interactions between the molecules, and compare them with the self-assembled gel
How can NMR tell us about the molecular assembly?
The solid-like network is ‘invisible’ because of its low mobility, whereas anything in the liquid-like phase has sharp peaks
It can be used as an effective quantifying process if we use a known amount of mobile internal standard
What is the purpose of the builder?
It acts as a chelating agent to bind Ca2+ and Mg2+, softening the water
This is because Ca2+ and Mg2+ can react with the surfactant to precipitate out of solution, hence making it worthless
What is the main composition of laundry detergents?
Builders (50%)
Surfactants (15%)
Bleach (7%)
Enzymes (2%)
How can Circular Dichroism Spectroscopy tell us about molecular assembly?
It is UV spectroscopy involving circularly polarised light
The CD signal of self-assembled nano-fibres is large, but as the fibres disassemble into isolated molecules (e.g. by heating), the signal becomes smaller
What is the purpose of bleach in a laundry detergent?
It targets oxidisable stains by releasing hydrogen peroxide
What is the purpose of enzymes in a laundry detergent?
They remove biological stains e.g. egg (protein/fat)
Why are vesicle structures good as drug-delivery agents?
The bilayer structure is more stable than normal micelles due to a larger hydrophobic interaction
This reduces the side effects
What are three key principles that improve the action of drugs by using vesicle structures?
(1) Using PEG lipids - ‘stealth’ polymers to enhance circulation times
(2) High drug loading that is released at the tumour
(3) Optimised vesicle for performance
What is the enhanced permeation and retention (EPR) effect?
Nanosystems cannot pass membranes into the bloodstream; they circulate around our body
Tumours however are ‘leaky’, allowing our nanosystem to enter and accumulate in the tumour
What is the limitation of calcium grease, and what grease did this lead to next?
Poor thermal stability, which led to sodium grease
What is the limitation of sodium grease, and what grease did this lead to next?
It has poor water stability, leading to lithium grease
What are the benefits of lithium grease?
They are stable to a high temperature and water
What greases did the Japanese invent?
They invented synthetic greases based on urea, with tunable properties
Why are gels used in tissue engineering?
They are excellent media for tissue growth because they are like the ECM
We can tune the properties to create different stem cells (the stiffer the gel the harder the material it makes)
What are the benefits of stem cells?
Low cost
Avoid rejection
Tissue on-demand
