Freezing the Moment: Monitoring Freezing Processes Under MAS Conditions

NMR spectroscopy under magic angle spinning (MAS) conditions can be employed to study the freezing of aqueous solutions and gel-like systems without the need for specialized equipment. This is attractive as one sample can be studied in its liquid and solid form without changing the experimental setup. There are multiple areas that benefit from such an analysis. Here, we will thoroughly study the freezing process of various aqueous systems containing multiple NMR-active nuclei (¹H, ¹¹B, ¹³C, ²³Na, and ⁷⁹Br) to gain detailed insights into what the prerequisites are, what problems one must be aware of and what limitations arise, how reproducible the measurements are, and what additional information can be gained from them for different sample types. As a further aspect it will be shown how the described setup can be used for the study of aqueous systems in solution at subfreezing conditions.