Identifying Structural Domains of Super-cooled Water Confined in Mesoporous Silica Gel via FTIR Spectroscopy

The structure and properties of deeply super-cooled water have intrigued researchers for many years, and confinement within mesoscopic pores has emerged as a common strategy to study the properties of liquid water in this temperature range. Here, we monitor the FTIR spectrum of HOD as a probe of the structure of super-cooled water confined in mesoporous silica gel. The range of pore sizes investigated spans a reported transition in pore-filling mechanism from axial pore filling, condensation in the full pore followed by extension of the filled section, to radial pore filling where surface layers thicken until full condensation occurs. In the super-cooled range the influence of pore size on the local structure of water was apparent. Evidence of an LDL-like phase with a broad liquid-like OD peak near 2435 cm^-1 was observed in all pore sizes except the smallest pores, 2.2 nm (in contrast to MCM-41 where LDL phase was reported in narrower pores). We explain our findings based on disorder along the pores which limits cooperative interactions of the hydrogen bond network that are required for conversion to an LDL-like phase. This work demonstrates that the occurrence of a liquid-liquid phase transition for water confined in mesoporous materials strongly depends on the pore morphology.