Beste Henk,

I think that equilibrium hydrations at given vapour pressures are not affected by the presence of oxygen/nitrogen. The diffusion of the vapour towards the hydrate may, however be hampered by the atmospheric pressure.

Hi Henk

Practically speaking, I see hardly differences between air and vauum for this process.

From the principle point of view, the thermodynamic potential of a salt at 1 bar differs a bit from that at vacuum.

Depending on the set-up, Jacques is right about diffusion differences in air and vauum, although I guess that the diffusion limitation may primarily be in the transport in deeper layers within the salt,

In vacuum it may be harder to control the humidity. If done by the presence  of a water reservoir (required for saturation) make sure that the temperature wished is realised, independent on cooling/heating by evaporation/condensation.

regards

jos

Thx. Jos and Jacques. Indeed, in terms of driving forces I do not expect differences as long as my gases behave ideally. Main source of difference should be kinetic in origin.

The point is that some people state that there are differences between these to methods (even when the water vapor pressures are kept similar). However, I have not found a good report in the literature on this issue.

Hello,

Vapor-involving thermogravimmetry is very pleasant and straightforward research when the adequate area of science is used, otherwise there is only confusion and frustration. Thermal decomposition of hydrates involves both physics and chemistry, but physics dominates:

A decomposition / recombination equilibrium is chemical in respect to the result (hydrate or thy anhydrous) but when the decomposition is already enabled by exceeding the thermodynamic (not "procedural") decomposition temperature, all the controlling processes and otherwise involved ones, and the parameters are of purely physical nature:

Transport processes (diffusion within the sample and outside of it, forced flow from the inside of the sample to the outside, concentration gradients, heat flow), the value of the equilibrium vapor pressure, total pressure, partial pressure (related to concentration or activity "a").

Chemical kinetics is certainly involved too, but as always, in multi-stage processes only the slowest one counts in the kinetics, and in the discussed kind of thermogravimetry, the physical processes evidently dominate and control. No matter how small the sample, thermogravimmetrically we are recording and describing the MACRO-kinetics: the SAMPLE of the substance, not the substance abstracted from its external conditions.

Let's take copper hydrate pentahydrate for the example. The first water molecule is bonded extremely weakly: during heating that salt "melts", but that is not melting: the water leaves the crystals but stays liquid, and so it dissolves the produced tetra-hydrate in itself. Then the water boils out from the solution, often disrupting the sample. Since this decomposition step (5->4) occurs below the boiling point, it is not recorded by TG. What is recorded is the sloping loss due to evaporation, followed by boiling; that is not chemistry, is it? The loss of the last water molecule (monohydrate to anhydrous) is a high-temperature, true chemical decomposition. The intermediate decomposition steps are chemical at the origin, but controlled physically.

We could continue this subject making a book of it, but the "bottom line" is: For TG of hydrates, forget the micro-kinetics, approach these problems from the viewpoint of physics, and "the fog will clear".

On the particular question about the role of presence or absence of the air, or another neutral gas:

Whether in the "vacuum" the vapor (water or any other) is alone or it is diluted, from the chemical viewpoint it is the same, because the activity (a) is the same. So, from the chemical viewpoint 0.05 bar of water vapor alone is the same as 0.10 bar of 1:1 mixture of the vapor and a gas.

From the point of view of PHYSICAL kinetics, (this is what counts with hydrates TG) it is not the same: diffusion of the active component in vapor alone is much faster than in the mixture.

Sorry for pushing my own write-ups, but this is the only source that I know:

1) More about vapors in TG: Website  goo.gl/TduRcK    This is the website of the company I used to have.

Three articles about the physical kinetics in TG:

2) In the book “Thermal Physics and Thermal Analysis” to be published very soon by Springer, there will be a pertaining article “The physical kinetics of reversible thermal decomposition”.

3) J Therm Anal Calorim (2015) 120:157-166 DOI 10.1007/s10973-014-4385-z

4) J Therm Anal Calorim (2015) 120:139-147 DOI 10.1007/s10973-014-4384-0

Jerry Czarnecki, [email protected]

goo.gl/TduRcK

Thanks for your extensive answer Dr. Czanecki. Please inform me when your book is available.

Considering my specific question, I agree with your statement that kinetics matter and that it that sense vacuum or the presence of an inert gas make a difference in kinetics.

However, do the references you put forward compare these two cases? Or are you aware of literature comparing these two cases on one specific substance?

Hello Dr. Huinink,

This answer consists of several, only loosely related thoughts, addressing your question at the end.

Thermogravimetry in vapors is an under-explored area.

It is not clear for me whether your interest is of practical nature, or if it is pure research.

In both cases the apparatus of the interpretation (equations etc.) should be of the kind which matches the reality:

- kinetic apparatus when the process is controlled by one of several possible micro-kinetic steps, on the theoretical molecule or a micro-crystal level , OR-

- physical apparatus, when describing the sample, not a theoretical molecule or a single micro-crystal.

It is merely my personal opinion that in the gas-evolving reversible thermal decomposition (like hydrates), thermogravimetry always is controlled by, and records, only the physical processes.

It is not an opinion but a fact though, that the question: chemical kinetics or physical kinetics? - has to be DETERMINED, not ASSUMED.

How to determine it? By changing the physical parameters and recording how the process responds.

If we just assume the chemical nature but the process happens to be controlled by physical factors, we will be calculating activation energy etc. for diffusion and heat transfer, which makes as much sense as fishing using an accordion.

Several decades ago I loaded 1 gram of a blue complex compound into a long glass test tube and started heating it gently with a small flame. The powder began changing its color from blue (tetra-complex)  to green (bi-complex), and the heavy vapors of the ligand were visibly traveling upwards in that tube. I stopped heating, the heavy vapors returned down, and the green powder became blue again.  Repeated the process several times, and the blue-green-blue-green transformation was repeating back and forth.

Being a complete ignorant of chemical kinetics, I did not see any problem or sophistication in it. The whole thing was simple and obvious: I was seeing a reversibly decomposing substance: You remove the gaseous product, and the equilibrium shifts to the right side. You re-supply the gaseous product, and the reaction goes back. Qualitatively the process did not differ much from evaporation and boiling.

You can repeat the same with any hydrate, copper sulfate for instance, or with calcium carbonate. The difference will be that carbon dioxide, being not a vapor, would not condense but would escape faster, thus irreversibly, even though the process is reversible.

If an equation is needed to describe that, any chemistry student, innocently unaware of the kinetic sophistication, would use Clausius-Clapeyron's, maybe Van't Hoff's. Anyone proposing Arrhenius's, would be advised to go back to the basic textbooks.

This nudges us not to forget experimenting - no computer simulation can reveal anything new beyond the model used.

The above discussion was addressing the need to establish the reality-reflecting basis for the planning of the experiments and their data interpretation.

I know too little to answer your question how vapors diluted with a gas, as opposed by vapor-alone (controlled vacuum), affect the kinetics. I only know that will be the difference in the "physical kinetics" (diffusion, heat transfer), not the chemical kinetics. The best person to ask such questions is Prof. Erich Robens, ( [email protected] ), the greatest guru and the doyen in this area, and many others. My speciality is only practical and instrumental: how to provide the desired kind of environment of vapors and gases in thermogravimetry.

Concerning the pertaining literature, many articles and books touch these problems only fragmentarily and indirectly, none of them covering these issues in the way we would like to read. I see this deficiency as natural for any new discipline which is only emerging. The industry is awaiting thermogravimetry in vapors to become a usable and well-understood tool.

Jerry Czarnecki, [email protected]