Polymorphism of the glass former ethanol confined in mesoporous silicon

TL;DR

This study investigates how ethanol confined in mesoporous silicon exhibits different polymorphic behaviors, with vitrification occurring at much lower cooling rates than in bulk, due to confinement effects.

Contribution

It demonstrates that confinement in mesoporous silicon alters ethanol's polymorphism and reduces vitrification rates, highlighting the impact of geometrical constraints and disorder.

Findings

Capillary condensed ethanol reproduces bulk polymorphism.

Vitrification rate is at least 100 times lower than in bulk ethanol.

Confinement influences ethanol's phase transition behavior.

Abstract

X-ray diffraction patterns of ethanol confined in parallel-aligned channels of approx. 10 nm diameter and 50 micrometer length in mesoporous silicon have been recorded as a function of filling fraction, temperature and for varying cooling and heating rates. A sorption isotherm, recorded in the liquid state, indicates a three monolayer thick, strongly adsorbed wall layer and a capillary condensed fraction of molecules in the pore center. Though the strongly adsorbed film remains in an amorphous state for the entire temperature range investigated, the capillary condensed molecules reproduce the polymorphism of bulk solid ethanol, that is the formation of either crystalline or glass-like states as a function of cooling rate. The critical rate necessary to achieve a vitrification in the mesopores is, however, at least two orders of magnitude smaller than in the bulk state. This finding can be traced both to pure geometrical constraints and quenched disorder effects, characteristic of confinement in mesoporous silicon.