Ice-induced structural reconfigurations in nanoconfined water–glycerol mixtures

Abstract

We elucidated the mesoscopic organization emerging in water–glycerol mixtures confined within the nanoporous cylindrical pores of SBA-15 and MCM-41 silicas, with pore diameters of 8.1 and 3.5 nm, respectively. Neutron diffraction was used to track variation in the intensity of Bragg reflections originating from the crystalline pore arrangements after filling and as a function of composition and temperature. In addition, isotopic substitution was employed to systematically adjust the scattering length density contrast between the different components of the mixtures. The radial concentration profile within the pore was determined by fitting various form factor models to the experimental intensity data. At room temperature, our findings support a uniform compositional distribution within the pore. Upon cooling, we observe partial ice crystallization at Tf ≈ 200–230 K in solutions where the water content exceeds the maximally freeze-concentrated solution threshold (30% w/w), while vitrification of the entire solution is observed otherwise. The partial freezing triggers phase separation into distinct ice and liquid domains. Remarkably, the morphology adopted by these domains is shaped by the geometry of the confining cylindrical nanopore, resulting in a core–shell structure: a pure ice core surrounded by a glycerol-rich layer adjacent to the pore surface. In contrast to the unfreezable layer commonly found in pure confined water, typically around 0.6 nm thick, our findings demonstrate that the size of the interfacial glassy solution is governed by both the pore size and the overall mixture composition. In the systems studied, this interfacial thickness varies between 0.3 and 1.4 nm, with its composition aligning with that of the MFCS.