| Author | Message | ||
     Erik Skibsted (ersk) New member Username: ersk Post Number: 2 Registered: 1-2006 |
Howard and Tony Thanks for the valuable input. I might find time to repeat the experiment with EtOh:Water mixtures Here is a figure where the spectra are plotted after the spectra measured at 30C i substrated. Its very good for identifying what is increasing and what is decreasing as a function of temperature increase.  | ||
| Tony Davies (td) Moderator Username: td Post Number: 151 Registered: 1-2001 |
Hello Erik! Some time ago (1988) we published a study of deuterated methanol and ethanol to support our idea that there was evidence for a CH-COH combination. No lesser authority than Peter Griffiths said we were wrong (but didn�t give an alternative interpretation of our data). My first thought was that your experiment (which we failed to do in 1986) might give some additional evidence but I�m not sure if it does. Our band was at 1582 nm (6321 cm-1) I suggest that we need to be precise in discussing hydrogen bonding. My belief is that hydrogen bonds are not weakened with increasing temperature they become statistically less likely and what we observe is a shift in absorption that �suggests� a weakening of the bond. The absorption at 6850 cm-1 is un-bonded water OH; the water is too dilute to find another water molecule (well not many do) but they can find CH-OH and this could be the absorption at 6850 cm-1 (1460 nm). The structure of water is very complex and I think there are many theories and not much agreement. Liquid water contains a a large number of hydrogen bonded species. Some of these are short lived and some more stable. The most recent ideas are that the major change observed in NIR experiments is due to a switch between two relatively stable states and perhaps something similar exists in ethanol or you may be observing an interaction between water bonded to CH-OH and the CH-OH�CHOH hydrogen bonds. Do you have time to repeat the experiment with increasing amounts of water? Best wishes, Tony Reference: A.M.C. Davies and S.G. Rutland, Spectrochimica Acta. 44A(11), 1143 (1988). | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 93 Registered: 9-2001 |
Erik - the decrease in the -CH may be due to simple thermal expansion of the ethanol, making it less dense. \o/ /_\ | ||
| Erik Skibsted (ersk) New member Username: ersk Post Number: 1 Registered: 1-2006 |
Hi Just for the fun of it I measured 99.9% Ethanol with NIR from 30 to 70 degress celcius in 5C steps. As expected was the absorbance of the O-H (combinations and first overtone) increasing as a function of temperature. I can understand that the hydrogen bonds between the molecules are weakening as a function of increasing temperature. In the same time was the absorbance of C-H first overtones decreasing as a function of increasing temperature...I might have overlooked something in the literaure, but can anyone please help me to remember why it is like that? Also two charateristic bands at 4750 cm-1 and 5130 cm-1 were decreasing as a function of temperature. I saw an interesting thing when I did a PCA of the data. The two first components were significant 97.21% and 2.03% respectively. The first one was the temperature influence but the second one was much more tricky. By inspection of the loading it was spectral variation around 5190 cm-1 and 6850 cm-1 that were found in the 2. PCA score and it was interesting to observe that the absorbance at 5190 cm-1 was increasing from 30C to 45C and then from 45C to 70C the absorbance was decreasing ??? Could this have something to do with the tiny amount of water molecules present in the sample? I have read that liquid water can have zero, one or two hydrogen bonds, maybe does ethanol molecules also have different numbers of hydrogen bonds in liquid state and the change at 45C is somehow related to a equilibrium between number of hydrogen bonds?? Any comments on my observations are highly appreciated kind regards Erik Skibsted Score 1vs2  |