Why does methyl cellulose dissolve in cold water but not hot water?

[CrimpJiggler]

I was reading about cellulose derivatives there and came across the wiki page for methyl cellulose:
https://secure.wikimedia.org/wikipedia/en/wiki/Methyl_cellulose
which states that it is a hydrophilic compound that dissolves in cold water but not hot water. Up until now, every compound I've come across has a solubility curve that increases with temperature. Why does methyl cellulose have this unusual property?

 

[Phrak]

That is really weird. Do you think it was a typo?

 

[Borek]

Read the solubility and temperature section of the wiki entry.

 

[Phrak]

OK, I did. How does a decreasing dielectric constant of water with increased temperature reduce solubility?

 

[Borek]

I am not sure it depends on the dielectric constant. That's not the only thing that changes.

See also https://secure.wikimedia.org/wikipedia/en/wiki/Lower_critical_solution_temperature and https://secure.wikimedia.org/wikipedia/en/wiki/Upper_critical_solution_temperature

 

[Phrak]

Why so cryptic?

 

[sankalpmittal]

OK, I did. How does a decreasing dielectric constant of water with increased temperature reduce solubility?

Hi Phrak !

Reduce solubility? Well it depends on which substance is being added to water. Presumably I think you are talking about methyl cellulose.

Dielectric constant is the property due to which water has the ability of electrostatic charge separation and can dissolve various (nearly all) substances especially electrovalent compounds and hence is known as dielectric constant. Due to this property only water acquire a very strong polar covalent bond.

H₂O ----------> H⁺ + OH^-
Now
H⁺ + H₂O ---------------> H₃O⁺

Overall equation :
2H₂O --------------> H₃O⁺ + OH^-

[C₆H₇O₂(OH)x(OCH₃)_y]_n is the formula of methyl cellulose (stolen from http://www.fao.org/ag/agn/jecfa-additives/specs/Monograph1/Additive-277.pdf)

H₃O^δ+​

OH^δ-
​

[C₆H₇O₂(OH)x(OCH₃)_y]_n​

where δ is delta here represents net charge.


These have high electric dipoles and hence can easily dissolve various substances.

Solubility curves : Temperature vs solubility


So now you see that in many cases some substances behave differently.
For instance is case of solids solubility of Caesium Sulphate [ Ce₂(SO₄)₃ ] and Calcium Sulphate [ CaSO₄ ] is decreasing with rise in temperature. There are reasons for it but I can't explain them here.

Solubility curves : Pressure vs solubility

Answer to your question : If you reduce dielectric constant of water then solubility of methyl cellulose will decrease. But if you increase the temperature then its solubility will decrease at the greater rate. This is because methyl cellulose is more covalent in nature than electrovalent or ionic. On increasing the temperature it uses that heat energy to increase its bond statistics like increasing the vander waals forces. Hence the solubility will decrease.

Hope this helps :) .

 

[DrDu]

I would guess that the "dissolution" of methylcellulose is better described as a swelling process, i.e. water being absorbed into the molecular network of the cellulose. This process reduces the entropy of the water absorbed in the network. Hence the process becomes unfavorable at higher temperatures. Generally entropic effects are much more important in polymer solutions than in simple solutions.
The relevant theory is called Flory Huggins theory:
http://en.wikipedia.org/wiki/Flory–Huggins_solution_theory