- Created by: E.H13
- Created on: 29-05-20 10:16
Are colloids/emulsions stable? - They are meta stable. This means it is stabilised, but not permanently.
Ignoring chemical nature, we have;
- Gravitational force, which aims to pull larger particles downwards (which is opposed by)
- Random Thermal/ Brownian Motion, which has origins in entropy (and)
- Viscous drag (which is opposed by)
Including chemical naturel, we also have;
- Attractions /repulsions between ions and dipoles
- Dispersion interactions between species, both between the particles and the medium it's dispersed in
The balance between attractions and repulsions treated with DVLO theory. Basically means interactions are independent of each other and therefore you can add/subtract them.
Interactions between particles are opposed by thermal motion.
Forces acting on a particle
Gravitation force; Fgrav = g(4*pi*a3/3)(pp - pw), where a = radius of particle
- Gravity acts to pull particles downwards, opposed by viscus drag. Dependent on the density of the particles relative to the solvent
Viscous drag (Stokes Law for macroscopic spheres); Fdrag = 6*pi*a*v*n
- Dependent on velocity and size of the particle.
Constant/Settling velocity; v = 2ga2(pp - pw)/9n, units = m-2s-1,
- Dependent on the size of particle and the difference in viscosity.
Larger particles will sediment fasrwe, but have lower random thermal motion, which opposed sedimentation.
On average, particles will have thermal energy (a few)~ kBT, where kB = 1.38 x 10-23 JK-1. We know there will be a distribution of speeds.
Strategies for stabilisation;
- Keep particles 'light'
- Make particles charged, so they repel each other
- For metallic nanoparticles; coat particles in a ligand
- Stabilise the interface between the dispersed phase and the dispersing medium
Thought Experiment on ions
Solutions are stabilised by the presense of charge. Check powerpoint for maths etc.
Imagine there are equal numbers of anions and cations. Qe expect the same amount f repulsions and attrations. Repulsions destabilise solution and attractions stabilise solution, meaning owe wouldn't see an overall effect.
If motion of ions was copletely random cations would meed anions as much as other cations, however like charges repel, so cations are more likely to meet anions, due to electrostatic forces. This means motion isn't random at all, and there is an overall stabilisation.
Electrostatic interactions of ions causes them to distribute around oppositely charged ions is opposed by random thermal motion. This opposes but doesn't overcoe the electrostatic attractions.
Basic equation; log10y = -A | z+z- | sqrt(I)
- Non-ideality is caused only by ion-ion interactions, not ion-solvent interactions
- Ionic interactions are described quantitatively by Coulomb's Law for Point charges i.e. idea that unlike point charges attract
- Electrolyte is fully dissociated
Only works for ditule solutions (below 10-2M).
Debye length; the distance between an ion and the average location of the harge in its ionic atmosphere
Proportional to 1/sqrt(1)
As ionic strength increases, the distance between the ion and the charge in the atmosphere decreases. The stabilisation by the ionic atomosphere increases with ionic strength and the solution becomes more non-ideal.
EDHL/ Robinson and Stokes
Actual Debye-Huckel only works for really dilute solutions.
Extended Debye-Huckel Law takes into account the fact ions have size, so it is the limiting law, divided by a correction for the size of the ion. A and B are constante that depend on solvent and temperature.
Deviations from the D-H laws at higher concentrations are due to ion-solvent interactions.
This is taken into account in the Robinson and Stokes Equation, with the term +cI. This term has an opposing sign to the original, as this is an opposing effect.
Mean Ionic acitivity coefficient; In reality it is not possible to separate each yj, so we use an average.
For salt MaBb, y = (y+ay-b)1/(a+b)
How do charges form on a particle?
- Surface ionisation; can be controlled by pH e.g. protonation/deprotontion of groups on surfaces and in proteins
- Ion adsorption; Ionic surfactants can be added to non-ionisable materials to generate charge-stabilised suspensions
- Non-symmetric ion dissolution; If one ion dissolves more readily than the other the remaining particle is charged, e.g. in the presence of excess iodide ions, particles will be negatively charged, and in excess silver ions, particles will be positively charged.
- Isomorphous ion substitution; replacing one ion with another of a similar size, but with a different charge.
Different types of ions
Potential-determining ions; can affect the charge density and potential distribution, e.g. by affecting equilibria
Inert ions; don't directly affect the equlibrium, but can influence the potential by their local distribution (see DH theory).
Charge Density, distribution and potential
Charge on the surfae of the particle is matched by an equal and opposite charge in the solution.
The electrostatic potential of the surface and the surface charge density are related, with this potential often being used as a predictor of likely stability.
The rate of potential decay depends on 1/Debye length, where the Debye length (aka double-layer thickness) is the distance from the surface of the electrode/charged particle where electrical potential has fallen to 1/e of original value.
The dependence of potential on distance from a surface drops over a shorter distance at large electrolyte concentration.
When particles approach one another, ionic atmospheres overlap and repulsion occurs. The difference in concentration between the mid-point of the ions and the bulk pushes the particles apart.
Factors affecting Stability
- High salt concentrations can lead to coaguation - not good for stability
- Higher valency ions in solution are more effective in inducing coagulation - again not good for stability
- Colloid particle size; increasing radii increases electrostatic stabilisation
DVLO theory accounts of attractive/repulsive interactions, and predicts and energy barrier that should be overcome for coagulation to occur. The more repulsive the interactions, the higher the barrier, and the exact shape of the curve depends on the strength of inter-particle interactions.
Effects of electrolyte;
- Lower surface charge density > weaker repulsive interction > more rapid coagulation
- Increased ionis strength > decreased debye length > particles can approach more closely > increases attraction relative to repulsio > more rapid coagulation.