I have made a cocrystal which shows three melting endotherm .All these endotherms do not corresponds to the compound and coformer . The first two endo therms are showing a wt loss of about 2.5% and 52% and their peaks are not sharp. However the last endotherm appears at temperature 240 with loss of 14% . the given temperature is higher than that of the compound and coformer . Isformed compound a cocrystal or salt
Dear Zeeahan,
In your case, the lack of information on compound potential structure make too hard to advice you about your problem solution because it is too specific question.
What are you interest in? ... in the pure product or the co-crystal?. What did you expected from your synthesis or preparation?.....a new cocrystal made it from how many elements or compounds?....
TGA coupled to DSC analysis is a powerful technique to undertands materials behaviour. I think you will have to make a weight and/or molar balance of your mixture. Lets suppose your preparation is a mixture of fixed amount of different products. Then after the crystallization process (due to organic synthesis or any other technique) they will have to be incorporated in fixed proportions to the crystals. If you have the TG-DSC analysis for pure initial reagents then you could have a better understanding of the cocrystal behavious because the product that are not incorporated in cocrystal will show their weigh loss near the same temperature of the initial pure reagents. Some times a slightly shift in decomposition temperature appears. But if your starting product have an exothermal decomposition obviously they are not the same compounds.
Decomposiition temperature are also another parameter to have in mind in TGA analysis. Sometimes water losses from crystal structures occurs and also solvent impurities remaining from the synthesis process will be shown (both processes will occur at temperatures below 100°C). besides, there are different type of water associated to crystals structure and sometimes the water loss can appear at higher temperatures. I suggest you to try to do a TGA-FTIR or TGA-MS analysis of your sample. At the same time you can do a XRD anaysis to be sure you have what you want.
best regards,
Gustavo
Dear Zeeahan Fatima,
The difference between a crystalline salt and a cocrystal lies merely in the transfer of a proton. The transfer of protons from one component to another in a crystal is dependent on the environment. For this reason, crystalline salts and cocrystals may be thought of as two ends of a proton transfer spectrum, where the salt has completed the proton transfer at one end and an absence of proton transfer exists for cocrystals at the other end. Therefore, neither DSC nor TGA can tell if your compound is a salt or a cocrystal. Only x-ray structure will be the tool to confirm the structural features of your compound.
The following is reference discusses salts vs. cocrystals:
Stahly, G. P. (2007). "Diversity in Single- and Multiple-Component Crystals. The Search for and Prevalence of Polymorphs and Cocrystals". Crystal Growth & Design 7 (6): 1007.
Hoping this will be helpful,
Rafik
Hi zeeahan,.....in addition to previous answers, I think to know whether your sample is a cocrystal (a crystalline structure composed of at least two components, where the components may be atoms, ions or molecules). I suggest you to run a melting point phase diagram experiment which is constructed by slow heating of the slide under a microscope and observation of the melting points of the various portions of the slide. For example : if you get a simple binary phase diagram, and if one eutectic point is observed then the substances do not form a cocrystal. If two eutectic points are observed, then the composition between these two points corresponds to the cocrystal.
I hope this information will be useful
Rudy Situmeang
Dear Zeeahan,
I do agree with the above responses with the lack of information on structure. However, from the TGA data, it shows that second endotherm indicating 50% loss. Therefore, it is possible some transformation at this region. In addition , phase diagram will also help in finding the formation of any new phase. Hence, drawing the conclusion (whether cocrystal or salt ) primarily on DSC or TGA will lead to the erroneous result. Thanks, Jamshed
Hi Zeeahan,
In general cocrystal melting lies in between the coformer and the main compound, however there are numerous cases with high/low melting cocrystals. I understand your concern about the difference in pKa values as the methylparaben doesn't have proton donation possibility (pKa=-6.8 is for C=O) and phenolic hydrogen is weak donor here, I would imagine it's a high melting cocrystal. One more suggestion is never ever publish your compound names in the public sites, sometimes it ruin your years of hard work.
Good luck with your research.
I think IR spectroscopy should be exploited to get an idea whether the new binary system is a cocrystal or salt, in case PKa diff in between 2 and 3. Along with IR, one can get the melting point from DSC. Generally salts have more mp than cocrystal or individual mixture. Ofcourse final confirmation will be from single crystal X-ray structure. TGA shows ur compound desolvation/dehydration behavior along with decomposition at higher temperature. Last endotherm at 240C may be because of decomposition of ur material. Both thermal techniques DSC/TGA need to be coupled to get structural information.
- Badges
- Science method
- Similar topics
- Methodology
- Crystallography
- X-ray Diffraction
- XRD Analysis