Characterization of Concentration-Dependent Infrared Spectral Variations of Urea Aqueous Solutions by Principal Component Analysis and Two-Dimensional Correlation Spectroscopy

Abstract

The association behavior of urea as a function of urea concentration was elucidated by applying principal component analysis (PCA) and two-dimensional (2D) correlation spectroscopy to concentration-dependent FTIR spectra of aqueous urea solutions. The first principal component revealed changes that occurred during the initial increase in urea concentration, which led to the formation of dimers. The second principal component gave information on changes involving the formation of higher aggregates at higher urea concentration. By applying 2D correlation analysis to two sets of FTIR spectra collected above and below 2 M of urea, which were divided on the basis of the PCA, further details on the association process were extracted. Synchronous and asynchronous 2D correlation spectra constructed from the first group of spectra (urea concentration up to 2 M) identified intensity changes due to the formation of ribbon and chain dimers, while the 2D correlation analysis indicated that for urea concentrations above 2 M, urea undergoes further chain polymerization.