Demonstration of Critical Opalesence

The photographs on this page show what happens when hexane and methanol are mixed, heated above roughly 42 celsius and allowed to cool. The tables below chart what happens on heating and cooling. Below the tables are large versions of each picture, together with a more detailed commentary. Use the links in the left hand column to get either more technical information, or my attempt at an explanation!

Heating

Picture 1

Picture 2

Picture 3

T = 18 °C

T = 30 °C

T = 45 °C

Image 1

Cooling

Picture 4

Picture 5

Picture 6

T = 46 °C / t = 0 minutes

T = 42 °C / t = 4 minutes

T = 41 °C / t = 5 minutes

Picture 7

Picture 8

Picture 9

T = 39 °C / t = 6 minutes

T = 37.5 °C / t = 7 minutes

T = 37 °C / t = 8 minutes

Picture 10

Picture 11

Picture 12

T = 36.5 °C / t = 9 minutes

T = 35.5 °C / t = 11 minutes

T = 33 °C / t = 14 minutes

Picture 13

Picture 14

Picture 15

T = 28 °C / t = 21 minutes

T = 26 °C / t = 26 minutes

T = 18 °C / t = 81 minutes

 

 

1. Picture taken after mixing hexane and methanol at = 18 °C. The liquid vapour boundary is at the top the picture and the phase boundary between the lower methanol-rich phase is and the upper hexane-rich phase is visible about 1/4 of the way up the picture. The silver "blob" in the centre is a thermometer.

 

2. Now the liquid has been put onto a hot-plate and heated to around 30 °C. Notice the presence of so-called capillary waves at the interface between the two fluids. The waves are very persistent and move slowly around the container giving a kind of "slow motion" effect.

 

3. Still on the hot-plate the liquid has now reached 45 °C. The liquid appears to "boiling" but it is not: 45°C is well below the boiling temperature of both liquids (64.7 °C and 68.7 °C for methanol and hexane respectively). What is happening is that one phase is "boiling" into the other. Indeed, just before this picture was taken, small jets of the lower (hotter) methanol-rich phase could be seen exploding into the other!

 

4. Now the flask has been removed from the hot-plate and is being held in a clamp stand and allowed to cool. It is at about 46 °C. Notice that the liquid is clear (compared with Picture 3) and that the phase boundary visible in Picture 1 has disappeared. This is the high temperature mixed phase.

 

5. Now the flask is continuing to cool and the nominal temperature is about 46 °C. Notice that the liquid is becoming cloudy at the bottom of the container. Due to convection, the bottom of the container is probably the colder than the top of the container by a few tenths of a degree celsius.

 

6. The flask is continuing to cool and the nominal temperature is about 41 °C. The cloudiness is now much more noticeable and reaches further up the container. With "the eye of faith" one can just begin to see the methanol-rich phase collecting at the bottom of the container.

 

7. The flask is continuing to cool and the nominal temperature is about 39°C. The cloudiness is now much more noticeable and reaches further up the container. The methanol-rich phase may now be more clearly seen at the bottom of the container.

 

8. The flask is continuing to cool and the nominal temperature is about 37.5°C. The cloudiness now reaches further up the container. The methanol-rich phase may now be clearly seen at the bottom of the container and a reasonably clear phase boundary is visible.

 

9. The flask is continuing to cool and the nominal temperature is about 37°C. The cloudiness now reaches its maximum extent. The methanol-rich phase may now be clearly seen at the bottom of the container and that too appears to be cloudy, but it is difficult to be sure.

 

10. The flask is continuing to cool and the nominal temperature is about 36.5°C. The cloudiness is now beginning to clear.

 

11. The flask is continuing to cool and the nominal temperature is about 35.5°C. The cloudiness is now definitely clearing.

 

12. The flask is continuing to cool and the nominal temperature is about 33°C. The cloudiness continues to clear and one can now (just) see droplets forming on the glass. These droplets are on the inside if the glassware and form in both phases. Those in the upper hexane-rich phase drip down to interface, but the drops in the lower methanol-rich phase drip upwards to the interface!

 

13. The flask is continuing to cool and the nominal temperature is about 28°C. The cloudiness is nearly gone and one can now clearly see the droplets described in Picture 12!

 

14. The flask is continuing to cool and the nominal temperature is about 26°C. The cloudiness is nearly gone and the droplets described in Picture 12 are clearly visible.

 

15. The flask is has now been left for one hour since picture 14 was taken and its nominal temperature is about 18°C. The cloudiness is nearly gone and the droplets described in Picture 12 are still clearly visible. Phase separation is essentially complete and the system has returned to its equilibrium state similar to that visible in Picture 1.