Phase Equilibrium

What is the phase rule?

Expand and explain

F = C - P + 2

Where
F = degrees of freedom
- Least number of intensive variables (eg. temperature, pressure, concentration etc)
- components that must be fixed in order to describe the system

C = no. components
- smallest number of constituents by which composition of each phase in system at equilibrium can be expressed in a formula

P = no. phases present
- homogeneously distinct portions of a system that is separated from other portions

Using the phase rule, if F =1, what does this mean?

If F = 1, the system is UNIVARIANT
- Must fix 1 variant in the system (eg. if temperature is known, pressure is known)
- In order to maintain equilibrium, there must be corresponding change between variables
- exist along line between 2 phase regions

Using the phase rule, if F = 2, what does this mean?

System is BIVARIANT
- Must describe 2 intensive properties such as pressure and temperature to describe sample
- Able to change independently without affecting the state of the system
- exists within the areas marked

Using the phase rule, if F = 0, what does this mean?

System is INVARIANT
- 0 degrees of freedom
exists at the intersection of lines bounding the 3 phase regions

What is a "consulate temperature"?

The temperature where 2 phases disappear, forming only 1 phase

How to work out mole fraction?

no. moles of A / moles A + B
- Should range between 0-1
- Used to normalise axis

What is the Eutectic composition?

Lowest melting point of both materials
- Both components are solid (crystalline material- microfine dispersion of crystals)

When will eutectics not occur?

When melting point is very different

What happens if a mixture, containing more A than the eutectic mixture, is cooled?

Crystals of pure A will appear
- when it reaches eutectic temperature = both crystallise out

What is a pharmaceutical advantage of this phenomenon of eutectic mixtures?

- eutectic mixture of the drug and substance readily soluble in water
- soluble "carrier" dissolves
- leaves behind the drug in vivo

Phase diagram of water?

Triple point of water: all 3 phases are present

What is sublimation and how does it occur?

Direct conversion from solid to vapour
eg. dry ice, freeze-drying for preservation

Lower pressure to form ice, and heat slightly

3 phase diagram (Ternary)?