I don't see why not. In a gas cell IR can and has picked up CO (2143.27 cm-1) and CO(1+) ion (2183.90 cm-1). You should see all the functional groups and the polyelectrolyte's characteristic IR spectra. You may see some changes in vibrational frequencies when adding acid/base/DI wafer. Now I am not sure how well you will get the charged groups out of this as care is needed in interpretation of shifts.

I used CPAM in my thesis work and to quantify the amount of charge I used a Mutek Particle Charge Detector. This was used to determine changes in the remaining charge when I was binding enzyme/CPAM complexes to cellulose. So any remaining charge was determined measuring the change in overall charge. Details for that are in the following link. I do like using IR and never needed it for my thesis. What a shame but again, I did not force the use of it. Anyhow I am using IR now for a completely different project I am working on now with gaseous species. Hope this does help you. Thesis ENHANCED ENZYMATIC HYDROLYSIS OF CELLULOSIC FIBERS BY CATION...

It is one of the easiest things to see. For example, a carboxylic acid in a neutral form will have extremely strong band near 1700 cm-1 (depending on the state, for example if it is hydrogen bonded or not, and how strong it is). However, when this is in an base solution, it will form -COO(minus). This shows a significant frequency shift as well as a doublet around 1579 and 1406 cm-1, being the higher frequency band stronger. This is caused by the significant change in the force constant surrounding the carbonyl group by changing the electronic structure. Same for other acids that are ionized. The number of bands observed depends on the symmetry and selection rule of the molecule.

If your molecule is ionized in distilled water, the acidified solution may bring the ionized structure to an acid form, depending on the strength of acidic solution and the polyelectrolyte. Then, the opposite thing to the example shown above may happen.

Ishida's answer is correct. However the experimental issue is distinguishing the vibrations of polyelectrolyte functional groups from the strong bands from water and acid, in a liquid sample. It would depend on the concentration of the polyelectrolyte and how close their bands are to the broad water bands.

An alternative method would be Raman spectroscopy, because the Raman spectrum of water is relatively weaker, so it may be easier to distinguish the polyelectrolyte bands.

Thank You all for the answer.

Mr. Paulo Barbeitas Miranda , I have already tried observing samples for Raman, but i couldn't get any decent peak, rather my spectrum had a very steep absorbance about 400 cm-1 followed by a straight line of almost the same absorbance intensity.

Dear John Reye ,

Thanks for your response.

Regarding particle charge detector; I guess from the name that your compounds should be in a some sort of distinct particle shape?? right?? how about if its a loose structure of polymer chains, as mostly the case for polyelectrolytes. Would I still be able to get the reasonable results.

Forgive my bad understanding of your work, if it is.

You say you have tried Raman. Charged groups, as correctly described by Hatsuo Ishida, can be quite strong in the IR spectrum being anti symmetrical. However Raman is strong with symmetrical groups so Raman bands due to charged groups are quite weak and less likely to be detecte.

Hatsuo Ishida , Sir, Being the zwitterionic in nature, my polymer for sure have several carboxylic groups. However, for powdered sample, I am unable to observe any carboxylic group peak about 1700 cm-1. Due to limited resources, Instead of measuring liquid IR, I have measured ATR-FTIR of liquid sample on germanium crystal. Signals were extremely weak at first, but after quickly processing the spectrum, I could see some peaks, but still carboxylic group was absent. is it due to the hydrogen bonding or the inherent limitation of ATR, which I really doubt.

Thanks for your previous answer, It really helped a lot.

If you have zwitterion then it is unlikey you will see bands for the acidic carbonyl at around 1700 cm-1. Instead as mentioned above it will be adoublet due to the salt. The position of the carbonyl doublet depends on the strength of the bond and the size of the counter ion. In a zwitter ion bands, which may be multiple, can be anywhere i the 1650-1530 cm-1 region with the other abut 1400 cm-1

Geoffrey Dent and Hatsuo Ishida ,

I have tried measuring aqueous sample using CaF2 cell, but water has shown strong absorption in the region of 1300-1700 cm-1, and therefore I cant observe any amide I or II band in my IR analysis.

How can I overcome this issue?

Depending on the path length of the cell and the concentration of the solution you may be able to subtract the water band. However the water reference needs to be recorded at the same temperature and pH as your solution. If not the bands move and you end up with a mess

Geoffrey Dent , Thank You sir for your quick response.

I cant increase my concentration above 1.0 mg/mL (0.1 wt%).

However, for the path length I have tried space of thickness as low as 25 micron. What I find contradictory to my understanding is the relation of concentration and path length in the attached picture. Because for higher path lengths, IR will absorb more intensely in water.

Please have a look, and give some advice.

IR is generally referred to as a % technique. This means that providing that there is no band overlap one component in a two component, non polar, mixture may be detected at about 1% concentration. To examine lower concentrations then the path length needs to be increase. Hence the table you show. However a usual capillary film between flats is about about 12 micron path length. At this path length water, being a very strong absorber, can completely absorb at the wavelengths you are interested.