Thread: Crystallization

hi guys,

maybe you can help me, i have recently taken an interest in salt and other crystalline forms,

If we were to melt salt with a heating element, then let it cool it would still crystallize correct?

I get that crystals form due to supersaturation but could we please elaborate on how this works?

I would like to use a vacuum chamber to decrease the pressure enough so that the salt will melt,
but then i want it to crystallize when i bring it back up closer to STP, is this going to work?
since i would consider a supersaturated state to have a positive pressure, where as i am using negative pressure.

would my method work to get my salt crystals, or is there something else i could do to achieve my goal using my negative pressure system?

Originally Posted by doggy

If we were to melt salt with a heating element, then let it cool it would still crystallize correct?

Yes. But note that the melting point of salt (NaCl) is about 800C so quite hard to achieve. You would need to cool it slowly to get good sized crystals to grow.

I get that crystals form due to supersaturation but could we please elaborate on how this works?

Supersaturation happens when crystals fail to form as a solution is concentrated (by cooling and/or evaporation) below the saturation point. Normally, at this point the solute will start coming out of solution. But if the solution is very pure then you may be able to get a supersaturated solution. Adding a seed crystal (or almost any impurity, or even a physical shock) will cause the liquid to rapidly crystallise.

Note that supersaturation applies to a solution of salt. The equivalent for the molten state is supercooling; reducing the emperature below the melting point without it solidifying. Again, a seed of some sort will cause it to rapidly solidify.

I would like to use a vacuum chamber to decrease the pressure enough so that the salt will melt,

I don't believe that pressure will have a significant effect on the melting point.

Originally Posted by PhDemon

Jinx, Strange...

He does that to me all the time.

800C so quite hard to achieve.
--NOTED!

(boiling points on the other hand vary greatly with pressure, maybe this is what you were thinking of?)
--it was actually, i do recall that now that you mention it! overall if i test things, i will be doing it in a near perfect vacuum and thought it may help things along, but i also want to make sure it doesn't have a negative impact on my seeding process.

i am referring to positive pressure as PSI > STP, ie inside a tire
and negative pressure as PSI < STP, ie inside a vacuum tube
perhaps improper terms there..


You would need to cool it slowly to get good sized crystals to grow.
Second question, it's a trade-off between kinetic and thermodynamic factors as described in the link.

-- these two points i think i am the most interested in. i have read that link and of what i could grasp!, it was very informative


from it, i can tell you guys that my system will have high temperature and very low pressures.
and that i am attempting a PRIMARY NUCLEATION which means i need to follow this design:

This can occur in two conditions. I will be applying the second condition
Homogeneous nucleation rarely occurs in practice due to the high energy necessary to begin nucleation without a solid surface to catalyse the nucleation.
Primary nucleation (both homogeneous and heterogeneous) has been modelled with the following:[1]
B=\dfrac{dN}{dt} = k_n(c-c^*)^n


and B) building crystals:

Points i have highlighted:

--a)The supersaturated solute mass the original nucleus may capture in a time unit is called the growth rate expressed in kg/(m2*h),
--b)Growth rate is influenced by several physical factors, such as surface tension of solution, pressure, temperature, relative crystal velocity in the solution,
--c)Supersaturation value, as an index of the quantity of solute available for the growth of the crystal;
--d)Total crystal surface in unit fluid mass, as an index of the capability of the solute to fix onto the crystal;
--e)Retention time, as an index of the probability of a molecule of solute to come into contact with an existing crystal;
--f)Flow pattern, again as an index of the probability of a molecule of solute to come into contact with an existing crystal (higher in laminar flow, lower in turbulent flow, but the reverse applies to the probability of contact).

i can consider my attempt to be a pure solution gas, in a high energy medium where solute bombardmaent will be constant,
there for there will be an adequate rentention time and flows.
so i am prety sure im am meeting all these requirements.


essentally i will be bombarding my seed crystal with the solute

wow, i just found this:

HRD Antwerp Research

which is basically what im trying to do, but what is the substrate used there? and how does it fit to the process? exactly what point does the element release the rest of the hydrogen and bond with the crystal?