Effect of confinement on melting in slit-shaped pores: Experimental and simulation study of aniline in activated carbon fibers

We report both experimental and molecular simulation studies of the melting behavior of aniline confined within an activated carbon fiber having slit- shaped pores. Dielectric relaxation spectroscopy is used to determine the t ransition temperatures and also the dielectric relaxation times over the te mperature range 240 to 340 K. For the confined system two transitions were observed, one at 298 K and a second transition at 324 K. The measured relax ation times indicate that the low temperature phase (below 298 K) is a crys talline or partially crystalline solid phase, while that above 324 K is a l iquid-like phase; for the intermediate phase, in the range 298 - 324 K, the relaxation times are of the order 10 (- 5) s, which is typical of a hexati c phase. The melting temperature of the confined system is well above that of bulk aniline, which is 267 K. The simulations are carried out using the Grand Canonical Monte Carlo method together with Landau free energy calcula tions, and phase transitions are located as state points where the grand fr ee energies of two confined phases are equal. The nature of these phases is determined by analysis of in-plane pair positional and orientational corre lation functions. The simulations also show two transitions. The first is a transition from a two-dimensional hexagonal crystal phase to a hexatic pha se at 296 K; the second transition is from the hexatic to a liquid-like pha se at 336 K. Confinement within the slit-shaped pores appears to stabilize the hexatic phase, which is the stable phase over a wider temperature range than for quasi-two-dimensional thin films.