[HaganeM]

Hello,

 I am a graduate of physics and just got the chance to work with some professors on a project about growth of a sugar monocrystal. Since I have also just started a new job from 8:00 to 18:00 I am unable to go to my university and consult with professors or request a book from the library on the subject, that is why I would like to ask you, if anyone can help me:

 - at what temperature does sugar begin to form crystals?
 - how saturated should the solution be?
 - are there any specifics one should bear in mind in this case?

thank you 

[Enthalpy]

The amount of sugar soluble in water depends on the temperature.
Sugar should begin to crystallize when the temperature doesn't allow any more to dissolve as much as you've put in solution, so it's a temperature versus concentration relation, rather than one concentration and one temperature.

I expect the solution must be de-stabilized enough to get a significant crystallization speed.

[HaganeM]

Could you advise me on a proper procedure to do that?

[Babcock_Hall]

When you say sugar, do you mean sucrose or something else?  I would not attempt to recrystallize sucrose from hot water by slowly cooling it (a common method of recrystallizing something).  One gram of sucrose dissolves in 0.2 mL of hot water and 0.5 mL of room temperature water, if I am interpreting the Merck Index correctly.  There should be a solvent with a larger solubility difference at high vs. low temperature.  I don't know of a protocol offhand, but there probably is one.

[Arkcon]

If you mean sucrose, then children have been conducting an analogous experiment since the 1940's at least.  A supersaturated solution of sucrose in water will grow rock candy crystals on a string or a stick.  I did once try to grow a very large perfect sucrose crystal by suspending it on a supersaturated sucrose solution suspended in gelatin.  But it cracked after getting less than 3/4" in size. 

Making a supersaturated solution of sucrose in water isn't easy.  By volume, it ends up requiring 3 times the mass of sucrose to water  -- i.e. for the kids, 3 or 4 cups of sugar to one cup of water.  My gelatin experiment was even worse, I needed way more of both sugar to make a supersaturated solution, and more gelatin than normal to gel the solution.

What, exactly, is your application?  As I understand it, these days, as long as the crystal is quite pure, it can be pretty small and still be useful for x-ray crystallography, and the like.

[HaganeM]

Yes, I mean store-bought sugar. We will dissolve a small portion in water and let it evaporate to collect seed crystals for the actual project.

My professors grow all sorts of crystals for x-ray analysis and etc. but I want to grow a sugar mono-crystal for educational purposes. I do hope it will be able to grow to about an inch or two.

As far as rock candy is concerned, I am aware of that but since we have a more specialised machine my professors tasked me with obtaining some actual numbers data.

Am I to understand that if I reach a supersaturated solution at some point, decreasing its temperature will immediately force it to form a crystal? And as far as I know if the temperature decrease is real slow, a very good mono-crystal will begin to form.

[Babcock_Hall]

In general slower cooling would be expected to produce better crystals than rapid cooling.  You might try using powdered sugar (confectioner's sugar) in seeding experiments, but it might be that there would be too many seed crystals if you did.

[Enthalpy]

To make excellent and big single-crystals of molten mineral compounds, the best process is Czochralski
http://en.wikipedia.org/wiki/Czochralski_process
which provides one seed, and avoids multiple nucleation by having solidifying conditions only near the already existing crystal, and melting conditions elsewhere in the bath. Fantastic for semiconductor, lasing or detector crystals.

It is my hope that Czochralski's method works also for dissolved organic compounds
http://www.chemicalforums.com/index.php?topic=65386.0
to make excellent single-crystals. This process could serve for instance to separate enantiomers.