ABSTRACT
The self-diffusivity of cyclohexane and n-octane adsorbed in hierarchical zeolite monoliths has been investigated by using PFG-NMR. In these samples, the intrinsic FAU-X zeolite microporosity combines with a complex macroporous network composed of aggregated zeolite nanocrystals. As temperature is increased, cyclohexane self-diffusivity apparently decreases, reaches a minimum, and then starts increasing upon further increasing the temperature. Such striking, i.e., non-Arrhenius, temperature dependence is not observed for n-octane in the same samples and for cyclohexane adsorbed in purely microporous FAU-X. Through thermodynamic modeling, we show that this anomalous behavior can be rationalized by considering the evolution in the adsorbate populations when changing the temperature. In more detail, we show that the slow and fast diffusing species present in the microporosity and secondary porosity arising from the packing of zeolite nanocrystals vary significantly with a strong impact on the effective diffusivity. Applying the temperature evolution of their relative fractions to a simple two-phase diffusion model helps obtain insights into the physicochemical factors responsible for the complex behavior of effective self-diffusivity in hierarchical zeolites.
ABSTRACT
Binary liquid mixtures can exhibit nanosegregation, albeit being fully miscible and homogeneous at the macroscopic scale. This tendency can be amplified by geometrical nanoconfinement, leading to remarkable properties. This work investigates the molecular dynamics of tert-butanol (TBA)-toluene (TOL) mixtures confined in silica nanochannels by quasielastic neutron scattering and molecular dynamics simulation. It reveals a decoupling of the molecular motion of each constituent of the binary liquid, which can be followed independently by selective isotopic H/D labeling. We argue that this behavior is the signature of spatially segregated dynamic heterogeneities, which are due to the recently established core-shell nanophase separation induced by mesoporous confinement.
ABSTRACT
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 RII and RI phases, whereas C28H58 exists in the RIII and RIV phases. Over a certain range of composition, this binary mixture was found to exist in RII, RI and an intervening mesophase was reported to be the hexatic phase, wherein the long-range two-dimensional in-plane hexagonal lattice order of the RII 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.
ABSTRACT
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 RIV phase and undergoes complete disappearance of its RIII 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 RI 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.
ABSTRACT
The confinement of liquid mixtures in porous channels provides new insight into fluid ordering at the nanoscale. In this study, we address a phenomenon of microphase separation, which appears as a novel fascinating confinement effect for fully miscible binary liquids. We investigate the structure of tert-butanol-toluene mixtures confined in the straight and mono-dispersed cylindrical nanochannels of SBA-15 mesoporous silicates (D = 8.3 nm). Small angle neutron scattering experiments on samples with carefully designed isotopic compositions are performed to systematically vary the scattering length density of the different compounds and assess the radial concentration profile of the confined phases. The resulting modulation of the Bragg reflections of SBA-15 is compared with the predictions from different core-shell models, highlighting a molecular-scale phase-separated tubular structure with the tert-butanol forming a layer at the pore surface, surrounding a toluene-rich core. The present structural study suggests that the microphase separation phenomenon in confinement, which so far had only been reported for a smaller pore size (D = 3.65 nm) and a unique mixture composition, must be considered as a general phenomenon. It also highlights the strength of neutron scattering method with isotopic substitution, which is a unique experimental approach to reveal this phenomenon.
ABSTRACT
MCM-41 nanoporous silicas show a very high selectivity for monoalcohols over aprotic molecules during adsorption of a binary mixture in the gas phase. We present here an original use of gravimetric vapour sorption isotherms to characterize the role played by the alcohol hydrogen-bonding network in the adsorption process. Beyond simple selectivity, vapour sorption isotherms measured for various compositions help to completely unravel at the molecular level the step by step adsorption mechanism of the binary system in the nanoporous solid, from the first monolayers to the complete liquid condensation.
