Thermodynamics of binary gas adsorption in nanopores

Nanometer Confinement-Driven Promotion and Stabilization of a Hexatic Phase Intervening between Ordered Rotator Phases

Bulk phase binary mixture of two rotator phase forming alkanes, n-tricosane (C₂₃H₄₈) and n-octacosane (C₂₈H₅₈), has been previously studied. C₂₃H₄₈ exists in the R_II and R_I phases, whereas C₂₈H₅₈ exists in the R_III and R_IV phases. Over a certain range of composition, this binary mixture was found to exist in R_II, R_I and an intervening mesophase was reported to be the hexatic phase, wherein the long-range two-dimensional in-plane hexagonal lattice order of the R_II is lost and what remains is molecules present in hexagonal geometry without long-range positional correlation between individual hexagons. Upon confinement in cylindrical anodized alumina pores 200 nm wide, on the one hand, the temperature range of the hexatic phase was found to extend, and on the other hand, it underwent increased molecular ordering compared to the hexatic phase in bulk, exhibiting two counter-reacting behaviors in confinement. We provide here a temperature-dependent X-ray diffraction study and a theoretical approach combining the Landau and Flory-Huggins theories to, first, understand the underlying mechanism leading to emergence of the hexatic phase and then to explain the effect of confinement on it in the light of finite size and interfacial interaction between the alkanes and alumina pores.

Confinement-driven radical change in a sequence of rotator phases: a study on n-octacosane

Rotator-phase forming n-alkanes have been studied extensively in both their bulk state and in nanoconfinement. While some alkanes maintain their bulk-state rotator phases in nanoconfinement albeit with increased disorder, there are others exhibiting new rotator phases upon confinement. We present here a temperature dependent X-ray diffraction (XRD) and differential scanning calorimetric (DSC) study on n-octacosane (C₂₈H₅₈), which almost completely loses its bulk state R_IV phase and undergoes complete disappearance of its R_III phase. In their place, when confined in cylindrical anodized alumina nanopores, a phase highly resembling the hexatic mesophase is formed at a higher temperature and the R_I rotator phase at a lower temperature. The effects of finite size, interfacial interactions with the host matrix and alkyl chain flexibility are used to provide an explanation for such unexpected behaviour.