Swiss chemists have discovered that urea is easily formed from ammonia and carbon dioxide when the reaction is carried out in microdroplets of water. They used Raman spectroscopy to detect the formation of urea in individual microdroplets, and then reproduced the experiment in an aerosol. The results of the study are published in the journal Science.
The urea molecule is built from two amino groups attached to a carbonyl group and is essentially a double amide of carbonic acid. Accordingly, it can be obtained from gaseous ammonia NH3 and carbon dioxide CO2. The problem is that this reaction only occurs under harsh conditions. For example, in industry, urea is synthesized at a temperature of about 200 degrees Celsius and a pressure of about 15 atmospheres. And under normal conditions, ammonia and carbon dioxide simply do not react. If you bubble a mixture of these gases through water, ammonium carbamate is formed, but not urea.
But, as it turns out, urea can still be formed from carbon dioxide and water at atmospheric pressure and room temperature. This was demonstrated by Ruth Signorell and her colleagues from the ETH Zurich. The scientists carried out this reaction not in ordinary water, but in its droplets with a radius of several micrometers.
The chemists conducted the experiment as follows: they sprayed an aqueous solution of ammonia and caught individual microdroplets formed using optical tweezers. Then one atmosphere of humid carbon dioxide was introduced into the chamber with the droplet and the Raman spectrum was simultaneously recorded. A few minutes after the start of the reaction, a band appeared in the spectrum corresponding to the vibration of the nitrogen-carbon bond in urea. The concentration of urea in the droplet after 30 minutes of reaction averaged about 40 millimoles per liter. The temperature during the experiment was 20 degrees Celsius.
Then the chemists carried out the same reaction, but not in one drop of water, but in an aerosol. In this case, urea was also formed, which the scientists detected using mass spectrometry and NMR spectroscopy on carbon nuclei. But in this case, much less urea was obtained, because the lifetime of the microdroplets, and, accordingly, the reaction time, was about three minutes.
Then, just in case, the authors of the study repeated the experiment in ordinary water, and in this case, as they expected, no urea was produced. Presumably, the role of microdroplets is that the concentrations of substances inside the droplet and on its surface are different. And each droplet is a small flow reactor, in different areas of which there are different conditions, in particular, different acidity of the environment. This concentration gradient, as chemists believe, allows urea to form. But they also note that the reaction mechanisms in microdroplets are very difficult to establish, and they often become a source of controversy in the scientific community.
Thus, chemists have shown that urea can be formed from ammonia and carbon dioxide under normal conditions. This is how, scientists suggest, urea could have formed in prebiotic conditions.
Earlier we talked about how ammonia got into microdroplets of water from the air exhaled by experimenters.
