Melting/freezing behavior of a fluid confined in porous glasses and MCM-41: Dielectric spectroscopy and molecular simulation

We report both experimental measurements and molecular simulations of the m elting and freezing behavior of fluids in nanoporous media. The experimenta l studies are for nitrobenzene in the silica-based pores of controlled pore glass, Vycor, and MCM-41. Dielectric relaxation spectroscopy is used to de termine melting points and the orientational relaxation times of the nitrob enzene molecules in the bulk and the confined phase. Monte Carlo simulation s, together with a bond orientational order parameter method, are used to d etermine the melting point and fluid structure inside cylindrical pores mod eled on silica. Qualitative comparison between experiment and simulation ar e made for the shift in the freezing temperatures and the structure of conf ined phases. From both the experiments and the simulations, it is found tha t the confined fluid freezes into a single crystalline structure for averag e pore diameters greater than 20 sigma, where sigma is the diameter of the fluid molecule. For average pore sizes between 20 sigma and 15 sigma, part of the confined fluid freezes into a frustrated crystal structure with the rest forming an amorphous region. For pore sizes smaller than 15 sigma, eve n the partial crystallization did not occur. Our measurements and calculati ons show clear evidence of a novel intermediate "contact layer" phase lying between liquid and crystal; the contact layer is the confined molecular la yer adjacent to the pore wall and experiences a deeper fluid-wall potential energy compared to the inner layers. We also find evidence of a liquid to "hexatic" transition in the quasi-two-dimensional contact layer at high tem peratures. (C) 2001 American Institute of Physics.