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

In summary, the lower dielectric constant of water at higher temperatures reduces the solubility of methyl cellulose.
  • #1
CrimpJiggler
149
1
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?
 
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  • #2
That is really weird. Do you think it was a typo?
 
  • #3
Read the solubility and temperature section of the wiki entry.
 
  • #4
OK, I did. How does a decreasing dielectric constant of water with increased temperature reduce solubility?
 
  • #6
Why so cryptic?
 
  • #7
Phrak said:
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)

H3Oδ+

OHδ-

[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 solubilitySo 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

solubility_pressure.png
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 :) .
 
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  • #8
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
 

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

The solubility of methyl cellulose in water is dependent on the temperature. At lower temperatures, the hydrogen bonds between the water molecules are stronger, making it easier for the methyl cellulose molecules to break apart and dissolve. However, at higher temperatures, the hydrogen bonds are weaker and the water molecules are more active, making it difficult for the methyl cellulose molecules to dissolve.

2. What is the molecular structure of methyl cellulose?

Methyl cellulose is a polymer made up of repeating units of glucose and methanol. It has a long, linear chain structure with many hydroxyl (-OH) groups, which allows it to interact with water molecules through hydrogen bonding.

3. Why is methyl cellulose often used as a thickening agent in food and cosmetics?

Due to its hydrophilic nature, methyl cellulose can absorb and hold onto large amounts of water. This property allows it to form a gel-like consistency, making it useful as a thickening agent in food and cosmetics.

4. Can methyl cellulose dissolve in other solvents besides water?

Yes, methyl cellulose can also dissolve in other polar solvents, such as methanol and ethanol. However, it is insoluble in non-polar solvents like oil.

5. How does the concentration of methyl cellulose affect its solubility in water?

The solubility of methyl cellulose in water increases with increasing concentration. This is because at higher concentrations, there are more methyl cellulose molecules present to interact with the water molecules, leading to easier dissolution.

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