Near-Infrared Spectral Evidence for Water Structures in Icing by Chemometrics and Simulation

Icing of water is a natural phenomenon when the temperature decreases. However, the ice can be much different in shape or even in crystal structure depending on the freezing conditions. Much effort has been made to study the dynamic change of water structures in the phase transition, but current investigations remain inadequate for describing the structural change due to the difficulty in capturing the information in icing. In this work, near-infrared (NIR) spectroscopy, which is very sensitive to the vibration of OH, is adopted to monitor the water-freezing process under different conditions. Through spectral analyses using the wavelet transform (WT), principal component analysis (PCA), and moving window evolving factor analysis (MWEFA), the information on the intermediate states involved in ice nucleation and crystal growth is obtained. Before the formation of ice nuclei, short-lived distorted tetrahedral structures emerge. During the growth of ice, the ice-water interface is formed by water molecules with three or four hydrogen bonds, which undergo mutual conversion between hexagonal and cubic structures. These structures bridge the ice and liquid phases, facilitating the growth of ice crystals. Moreover, molecular simulation is used to observe the structures obtained by spectral analysis. This study provides a new way to analyze the spectral data of the icing process and new evidence and insights into the transformation of water structures during icing.