Phase diagram and glass transition of confined benzene.

We used differential scanning calorimetry, neutron scattering, and proton NMR to investigate the phase behavior, the structure, and the dynamics of benzene confined in a series of cylindrical mesoporous materials MCM-41 and SBA-15 with pore diameters, d, between 2.4 and 14 nm. With this multitechnique approach, it was possible to determine the structure and, for the first time to our knowledge, the density of confined benzene as a function of temperature and pore size. Under standard cooling rates, benzene partially crystallizes in SBA-15 matrixes (4.7 <or= d <or= 14 nm) but not in MCM-41 (2.4 <or= d <or= 3.5 nm). Structure factors of the confined phases were recorded at different temperatures and compare to those of the bulk. The confined liquid has the same structure as the bulk above the bulk melting point. In SBA-15, the confined crystals are defective and have the same structure as the bulk. In MCM-41, the liquid undergoes a glass transition at low temperature regardless of the cooling rate or the thermal history of the sample. The density as a function of temperature was measured by neutron scattering contrast matching, and the glass transition temperatures were determined from the density versus temperature curves. The pore size dependence of T(g) does not show any evidence of finite size effects. A temperature versus pore diameter phase diagram of confined benzene is proposed combining liquid, supercooled liquid, crystal states, and glassy states. NMR relaxation time measurements showed that the dynamics of the confined liquids are slower than those of the bulk above its melting point. In the partially crystallized samples, the liquid and the crystal have the same relaxation times. The activation energies of reorientation motions in the confined phases, determined from spin lattice relaxation times, are smaller than the bulk ones.