Freezing behavior in porous glasses and MCM-41

We report experimental measurements of the melting and freezing behavior of fluids in nano-porous media. The experimental studies are for nitrobenzene in the silica based pores of controlled pore glass (CPG), Vycor and MCM-41 . Dielectric relaxation spectroscopy was used to determine melting points a nd the orientational relaxation times of the nitrobenzene molecules in the bulk and the confined phase. It was found that the confined fluid freezes i nto a single crystalline structure for average pore diameters greater than 20 sigma, where sigma is the diameter of the fluid molecule. For average po re sizes between 20 and 15 sigma, part of the confined fluid freezes into a frustrated crystal structure with the rest forming an amorphous region. Fo r pore sizes smaller than 15 sigma, even the partial crystallization did no t occur. (C) 2001 Elsevier Science B.V. All rights reserved.