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Keywords:

  • phase transition;
  • acetonitrile;
  • FT Raman;
  • mesoporous silica;
  • confinement

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 (equation image and equation image) phases exist in bulk acetonitrile, only the liquid [RIGHTWARDS ARROW]β phase transition was detected for acetonitrile confined in the nanopores of mesoporous silica. The equation image solid-to-solid phase transition was not observed. For the mesoporous silica with the smallest pore size of 4.6 nm, neither the liquid equation image nor the equation image 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. Copyright © 2011 John Wiley & Sons, Ltd.