Effect of confinement on the fluid properties of ammonia in mesopores of MCM-41 and SBA-15.

The effect of pore size on capillary condensation and solid-liquid phase changes of ammonia in MCM-41 and SBA-15 was studied by adsorption and FTIR measurements of condensed phases at low temperatures. Adsorption isotherms are all typical type IV on the fully hydroxylated surfaces, without hysteresis loops in the smaller pores (d < 2.4 nm). In the larger pores, hysteresis loops appear at lower temperatures and disappear with increasing temperature, i.e., the capillary critical phenomenon was detected (hysteresis critical point). The criticality of the adsorption hysteresis loop is very similar to that for nonpolar nitrogen in mesopores of various shapes, suggesting that this is a universal phenomenon among fluids in mesopores. Freezing and melting of capillary-condensed ammonia were observed by FTIR spectroscopy. The melting temperature of capillary-condensed ammonia decreased with decreasing pore size, which is similar in the behavior of freezing. In the smaller pores (d < 2.4 nm); however, ammonia was not frozen. It is suggested that the capillary-condensed inner part, i.e., inside the ammonia monolayer, is affected too much by the pore wall and/or is too small in volume to crystallize. In the larger pores of SBA-15, crystallization is remarkably segregated from ammonia molecules strongly coordinated to surface hydroxyls.