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.
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.
The dynamics of fluids confined in hierarchical porous materials is gaining increasing attention. Here, using pulsed field gradient nuclear magnetic resonance, we report an experimental study of the selfdiffusivity of cyclohexane confined at different temperatures in the bi-porous structure of hierarchical faujasite zeolites (i.e., combining the intrinsic zeolite microporosity of < 2 nm diameter and an ordered MCM-41-like mesoporosity of 4 nm diameter). For different mesoporous volumes, we consider cyclohexane self-diffusivity at chemical potentials where the porosity is either totally or only partially filled (in the latter case, the microporosity is completely filled, while the mesoporous surface is only covered by a molecular thin film). On one hand, in completely filled materials, as expected, the effective cyclohexane self-diffusivity is found to increase as the mesoporous volume increases. Moreover, in this regime, the self-diffusivity follows an Arrhenius behavior with an activation energy that is close to�although slightly smaller than�that for bulk cyclohexane. On the other hand, for partially filled materials, a striking behavior is observed as the measured self-diffusivity decreases upon increasing the loading and remains nearly constant upon increasing the temperature. We propose here that such a behavior can be rationalized by considering (1) the population redistribution between the microporosity and mesoporosity (including the gas phase in the mesopores) and (2) the effective number of molecules that contribute to spin echo attenuation upon increasing the temperature. In particular, while molecules diffuse faster overall with the increasing temperature, we show here that our measurements rely at each temperature on a slower and slower population (therefore leading to unexpected temperature variations).
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 HD labelling. We argue that this behavior is the signature of spatially
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