Probing the phase transition behavior of acetonitrile confined in mesoporous silica by FT Raman spectroscopy

Abstract

Phase transitions of acetonitrile confined in mesoporous silica SBA-15 and mesocellular silica foam (MCF) having different pore diameters of 39.0, 39.9, 28.4, 8.7, and 4.6 nm with corresponding pore openings of 20.9, 12.1, 10.0, 8.7, and 4.6 nm were investigated by FT Raman spectroscopy. Melting and freezing temperature depressions were found for acetonitrile confined in mesoporous silica with pore opening sizes of 20.9, 12.1, 10.0 and 8.7 nm. A thermal hysteresis between the cooling and heating cycles was also observed. It appears that the smaller the pore opening, the larger the depression of melting or freezing temperature. Although two solid (α and β) phases exist in bulk acetonitrile, only the liquid → β phase transition was detected for acetonitrile confined in the nanopores of mesoporous silica. The β → α solid-to-solid phase transition was not observed. For the mesoporous silica with the smallest pore size of 4.6 nm, neither the liquid → β nor the β → α transition was observed for the confined acetonitrile. The results demonstrate that FT Raman spectroscopy is a useful technique for studying the phase transition behavior of organic compounds confined in silica-based hosts.

Cryoporometry in Femtoliter Volumes by Confocal Raman Spectroscopy

“…In addition, this should also permit to map with high resolution the spatial variation of pore structure in heterogeneous porous materials. Indeed, spatial resolution conveyed by the confocal mode is another feature that distinguishes our results from previous related Raman studies. − …”

“…This deconvolution approach can provide a more quantitative insight of the freezing and melting processes inside the pores, which could be used to relate to various models. 14,15,37−40 We note that the sensitivity of the Raman spectrum and related observables to the phase transitions in pores, 42 including the freezing and melting temperature hysteresis, 43,44 have been previously demonstrated, albeit with insufficient resolution and with no clear intent for extracting information about the pore structure. In relation to those studies where the mere fact of transition is only revealed by shifts in the peak positions of given spectral features, 43,44 our experimental procedure is novel as it relies on the full spectral analysis and quantitative decomposition into frozen and molten fractions.…”

“…14,15,37−40 We note that the sensitivity of the Raman spectrum and related observables to the phase transitions in pores, 42 including the freezing and melting temperature hysteresis, 43,44 have been previously demonstrated, albeit with insufficient resolution and with no clear intent for extracting information about the pore structure. In relation to those studies where the mere fact of transition is only revealed by shifts in the peak positions of given spectral features, 43,44 our experimental procedure is novel as it relies on the full spectral analysis and quantitative decomposition into frozen and molten fractions. This latter feature permits the detection of a distribution of pore properties as reflected by the distribution of phase transition temperatures.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part VI

“…Li et al . probed the phase transition behavior of acetonitrile confined in mesoporous silica by FT‐Raman spectroscopy . Lima et al .…”

Evolution of Ultrafast Vibrational Dynamics During Sol–Gel Aging

“…Since their inception, , silica sol–gels have enabled a multitude of fundamental and applied studies. , Their relative ease of synthesis and pore size tunability have allowed scientists to answer important questions about the behavior of solvents − and solutes − in nanoscopic volumes, and to monitor the biological activity of proteins and enzymes as their solvation shells become vanishingly thin. − Historically, the silica structure has been used as scaffolding upon (or within) which to confine solvents and/or solutes. In a departure from this passive role, our group recently showed that a silicon hydride vibration on the silica surface could be used to probe the structural dynamics of molecular guests within the pore volume. , Rather than relying upon the spectroscopy of the infiltrating solvent or solute, or a chemically bound sensitizer, this approach transforms the silica matrix itself into a general sensing platform to monitor any infiltrating species.…”