Flashcards in Disperse systems part 2 Deck (41):
What are the 5 things involved in the kinetics of a disperse system?
1. Brownian motion
3. Osmotic pressure
What is Brownian motion?
The path that particles take is irregular due to collisions with molecules of the continuous phase. The Brownian moton leads us to diffusion and this tells us how particles move
What size molecules are affected by the Brownian motion?
2-5μm micron size
What happens to the veloity of particles as their size increases and the vehicle viscosity also increases?
The velocity (speed in a given direction) will decrease as the size of particles increases or if the viscocity of the vehicle increases
What is Diffusion?
Particles move from an area of high conc to low conc in order to give a uniform concentration throughout the system
What is Fick's Law?
The rate of Diffusion which is described as The amount of substance that diffuses over time relating to the change in its concentration.
What affects diffusion? (5)
1) Mass of substance diffusing (dm)
2) Time (dt)
3) Area (A)
4) Concentration gradient (dC/dx)
5) Diffusion coefficient (D)
What is the diffusion coefficient?
An incorporation of all the factors that are important in influencing the movement of particles
What affects the diffusion coefficient?
Gas constant (R)
Osmotic pressure (π)
Viscosity of medium (η)
Radius of particle (a)
Avogadro's number (NA)
What will happen if the top of the equation increases?
The diffusion coefficient will increase
What will happen if the bottom of the equation increases?
The diffusion coefficient will Decrease.
In what context do we look at Osmotic pressure?
Think about how the liquid is moving rather than particles
What will happen if a semi-permeable membrane prevents particles from moving to achieve equal concentration throughout the system?
The dispersant (the liquid) will move instead.
What is osmosis?
The flow of the dispersant through a semi-permeable membrane from a region will low particle concntration to a region of high particle concentration
what is osmotic pressure?
The pressure created by the liquid pushing against the membrane
What is osmotic pressure useful in calculating?
Molecular weight of colloidal particles and therefore if the particles or dispersant will move (a high MW will mean that the particles will not be able to pass the semi-permeable membrane so the dispersant will)
Sedimentation: What does the Brownian motion do?
It keeps small particles moving in a colloidal dispersion, but as the particles get bigger, the influence of gravity is greater.
What size particles won't be affected by gravity?
Small particles (1μm and below) won't be affected and will stay suspended
What will happen to larger molecules?
PArticles over 1μm will be affected by gravity and will eventually sediment and settle at the bottom of the formulation
Sedimentation: Stoke's Law:
Describes the velocity of sedimentation for a particle falling through liquid.
What affects the velocity of sedimentation? (5)
Liquid viscosity (η)
Particle radius (a)
Particle density (σ)
Liquid density (ρ)
What represents the interaction between liquid and solid?
Particle density (σ) - Liquid density (ρ) = Difference in Density
What is the only thing that can decrease sedimentation velocity?
An increase in liquid viscosity.
What happens if there is no difference in density? i.e. σ-ρ = 0?
There is no sedimentation at any time
What is viscosity?
The visocsity of a substance is its resistance to flow.
It is to do with the internal friction of a fluid produced by the movement of the molecule against each other.
Factors influencing viscosity: (6)
Shape of molecules
Size of molecules
Chemistry of molecules
Concentration of molecules
Nature of solvent
What happens to the viscosity with large molecules?
Lots of large molecules are more likely to bond and get tangled, causing more friction, and an increase in the viscosity.
What happens to the viscosity with smaller molecules?
Smaller molecules (e.g. shorter chain length) do not bond as much and cause less friction. The viscosity will be decreased.
What will a small difference in chain length do to the visocity?
A small difference in the chain length will greatly affect the friction of the molecules and therefore the viscosity
How does a long slim shape of a molecule affect the viscosity?
They tend to lie along side each other, with more opportunites for bonds to form between them. This makes it harder for them to flow past each other with increased friction therefore icresed viscosity
How does a big, lumpy shape of a molecule affect the viscosity?
The molecules will have a tendancy to repel each other, so will be less viscous.
Chemistry of the molecule: How will pentanol's viscosity differ from hexanol?
Pentanol will form hydrogen bonds between its molecules, making it harder for them to flow easily (more friction) therefore pentanol will be more viscous than hexanol. Hexanol has a slighlty longer chain lengthe, but the effects of the hydrogen bonds on the pentanol will be greater.
Concentration of the molecules: How will a high molecule concentration affect the viscosity?
The higher the conentration of particles, the greater the chance of particles meeting and forming temporary bonds, increasing the friction and therefore the viscosity.
What is the equation used to calculate the effect of concnetration of molecules?
volume fraction of dispersed particles (ϕ) = Dispersed phase vol /Dispersion vol (total) = 5/25 = 0.2 = volume fraction
What is the Einstein equation and what is it used for?
It is used to make ASSUMPTIONS about the viscosity of a dispersion.
η = η0 (1+2.5ϕ)
(ϕ = volume fraction of dispersed particles)
η0 = viscosity of continuous phase
What are the neccessary assumptions for the Einstein equation to work? (5)
1) We have a dilute suspension (up to 5% particle conc)
2) The particles are spherical and rigid = consistant behavious
3) There are no interparticulate interactions (no hydrogen bonding etc)
4) The liquid behaves in a simple, predictable manner
5) Brownian motion effects are unimportant (however we know that this is not true)
Nature of the solvent: What happens if a solvent does not interact with the particles?
No interactions between sovelnt and particles means that the there will be lots of small particles that will flow past each other easily with little friction therefore low viscosity
What happens if the solvent and the particles have similar properties and will interact with each other?
If both are hydrophilic/lipophilic, then the particles will surround the continuous phase molecules creating bigger, and bulkier molecules, increasing the friction and thereofre the viscosity.
What can we do to change the viscosity of the dispersion?
change or add another solvent like ethanol which will decrease the viscosity.
Temperature: How does increasing the temperature of a liquid affect the viscosity?
The viscosity DECREASES because more energy is put into the system so they particles can move past each other easier.