# Homework Help: Colloidal Kinetics

1. Sep 4, 2007

### paperclip

Hi!

I want to make sure I have the right handle on this. Fick's law relates to dispersING solids, whereas Einstein's law of diffusion indicates disperSED particles, Stoke's law illustrates sedimentation and

D=kT/6πηNa=RT/6πηaN

is the overall equation relating all of the above to indicate an AVERAGE diffusion rate including when it's still mostly in the solid stage, while dispersing and while sedimentation is taking place?

Is this correct? Thank you for your help!

2. Sep 4, 2007

### Astronuc

Staff Emeritus
Fick's first law is also known as Fick's law of diffusion. http://en.wikipedia.org/wiki/Fick's_law_of_diffusion
It describes diffusion of particles, e.g. diffusion theory of thermal neutrons in a moderator.

Stoke's law pertains the viscous forces on a spherical particle.
http://en.wikipedia.org/wiki/Stokes_law
Einstein's work was on Brownian motion, after Fick's work.
http://en.wikipedia.org/wiki/Einstein_relation
http://en.wikipedia.org/wiki/Fluctuation_dissipation_theorem

I'm not sure what is meant by "still mostly solid".

3. Sep 4, 2007

### paperclip

I suppose I meant while the particles were still so close together that they were still more or less associated with one another, then they would still be 'mostly in the solid phase'.
Brownian motion refers to particulate matter widely dispersed in the solvent, so that WOULD mean 'dispersED', as in a currently homogenous phase, am I right?

About Fick's Law- that's the homogenous phase. The overall average diffusion rate that I'm talking about in the last sentence refers to a sort of integrated law which takes into account, for example, a tablet dropped into water.

Firstly, you have the tablet in the water. It's mostly compacted particles, the outside layer is dissolving into the liquid. That's where Fick's Law comes into play.
Next, you have the particles in the solvent moving under the influence of Brownian Motion. That's Einstein's dissolution theory.
Finally, there's the Stoke's theory, which tracks the particles as they precipitate out of solution and sink towards the bottom. There's the sedimentation.

The overall equation that I'm speaking of- I'm assuming that this integrates all three stages of the dissolution of the colloidal particles from the tablet?

4. Sep 5, 2007

### paperclip

Nobody? Ah, come on....sure it must be easy enough to just say 'yes, you got it right' or 'no way, you're way off the mark?'