The application of attenuated total reflectance Fourier transform infrared spectroscopy to monitor the concentration and state of wate in solutions of a thermally responsive cellulose ether during gelation

This paper reports the use of ATR-FTIR with PLS data analysis to probe the thermal gelation behaviour of aqueous solutions of the cellulose ether, hydroxypropyl methylcellulose (HPMC). Spectroscopic changes in the ν(CO) region of the infrared spectra (collected using ATR) were shown to mark the onset of gelation and information about the temperature of gelation and the effect of the gel structure on the water hydrogen bonding network was elucidated. The use of PLS data analysis to quantify the water concentration within the gel at the ATR interface is highlighted. The dominance of intermolecular H-bonding over intramolecular H-bonding within the cellulose ether in solution was also observed. The ATR-FTIR data was in good agreement with rheological and DSC measurements conducted on the same systems. A discussion regarding the changes in shape of the ν(OH) band of the water within the gel is provided and an interpretation of these changes in terms of modifications of the hydrogen bond strength of associated water during syneresis is given.