Water Confinement in Nanoporous Silica Materials

International audienceThe influence of the surface polarity of cylindrical silica nanopores and the presence of Na + ions as compensating charges on the structure and dynamics of confined water has been investigated by molecular dynamics simulations. A comparison between three different matrixes has been included: a protonated nanopore (PP, with SiOH groups), a deprotonated material (DP, with negatively charged surface groups), and a compensated-charge framework (CC, with sodium cations compensating the negative surface charge). The structure of water inside the different pores shows significant differences in terms of layer organization and hydrogen bonding network. Inside the CC pore the innermost layer is lost to be replaced by a quasi bulk phase. The electrostatic field generated by the DP pore is felt from the surface to the centre of pore leading to a strong orientation of water molecules even in the central part of the pore. Water dynamics inside both the PP and DP pores shows significant differences with respect to the CC pore in which the sub-diffusive regime of water is lost for a superdiffusive regime

Microphase separation of a miscible binary liquid mixture under confinement at the nanoscale

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Unravelling the Anomalous Dielectric Permittivity of Nanoconfined Electrolyte Solutions

International audienceThe dielectric properties of sodium chloride solutions confined in a hydrophilic nanocavity were investigated by means of molecular dynamics simulations. Unlike what is observed in the bulk phase, three dielectric regimes were evidenced, namely an anomalous increase in the dielectric permittivity at low concentrations (with respect to confined pure water), a dielectric plateau at intermediate concentrations and finally a bulk-like behavior for salt concentrations higher than a critical value. It was shown that this peculiar behavior results from the competition between dielectric saturation due to the electric field generated by ions (which tends to lower the dielectric permittivity) and the ion-induced perturbation of pre-oriented water molecules inside the nanocavity which gain some rotational degrees of freedom (entropic contribution) leading to an increase in dipolar fluctuations responsible for the increase in the dielectric permittivity

Computation of the hindrance factor for the diffusion for nanoconfined ions: molecular dynamics simulations versus continuum-based models

Special Issue: Thermodynamics 2011 ConferenceInternational audienceWe report the self-diffusion coefficients and hindrance factor for the diffusion of ions into cylindrical hydrophilic silica nanopores (hydrated silica) determined from molecular dynamics (MD) simulations. We make a comparison with the hindered diffusion coefficients used in continuum-based models of nanofiltration (NF). Hindrance factors for diffusion estimated from the macroscopic hydrodynamic theory were found to be in fair quantitative agreement with MD simulations for a protonated pore, but they strongly overestimate diffusion inside a deprotonated pore

Zhu et al. Reply

International audienceA Reply to the Comment by S. Gekle and A. Arnold. Original Article: Stephan Gekle and Axel Arnold, Comment on "Anomalous Dielectric Behavior of Nanoconfined Electrolytic Solutions", Phys. Rev. Lett. 111, 089801 (2013)

How does the electronic continuum model perform in the prediction of the surface tension of salt solutions?

International audienceThe electronic continuum (EC) model uses a scaling of the charges of the ions in order to model implicitly the polarization into nonpolarizable models. This scaling procedure is applied here to two standard nonpolarizable force fields to investigate the salt concentration dependence of the surface tension and density of NaCl aqueous solutions. The composition of the interface and the orientation of the water molecules at the water surface are reported for different combinations of force fields

Is V̇O2peak a Valid Estimation of V̇O2max in Swimmers with Physical Impairments?

Peak and maximal oxygen uptake ([Formula: see text] and [Formula: see text], respectively) are used in assessing aerobic power. For swimmers with physical impairments, it is unclear whether the physiological variables obtained in 200-m and Nx200-m tests are similar. The objective of this study is to assess the validity of [Formula: see text] as an estimator of [Formula: see text] and complementary physiological variables, in particular, carbon dioxide production ([Formula: see text]), respiratory exchange ratio (RER), minute-ventilation ([Formula: see text] and absolute (HR) and relative (%HRmax) heart rates-which were obtained in a time trial test (200-m) and an incremental intermittent test (Nx200-m) performed by swimmers with physical impairments. Methods: Eleven well-trained swimmers with physical impairments performed 200-m all-out and Nx200-m from low to all-out (controlled by a visual pacer), both with a respiratory valve system and a portable gas analyzer. Results: A paired Student's t-test showed no statistical difference (p > .05) for all comparisons. The intraclass correlation coefficient (ICC) was 0.97 and 0.98 for [Formula: see text] in l/min and ml/kg/min, respectively; ICC = 0.75 to 0.9 for [Formula: see text] (l/min and ml/kg/min),[Formula: see text] (in l/min) and HR (beats/min); ICC = 0.5 and 0.75 for %HRmax; and ICC < 0.5 for RER. Passing-Bablok regression showed that the dispersions were acceptable, considering the proportionality, except for HR and %HRmax. Bland-Altman method showed a high level of agreement for all variables. Conclusions: The [Formula: see text] and [Formula: see text], as well as the physiological variables [Formula: see text] and HR obtained, respectively, by 200-m and Nx200-m tests in swimmers with physical impairment were not different.The study was approved by the local ethics committee (number 2.274.037) and was performed in accordance with the Declaration of Helsinki.info:eu-repo/semantics/publishedVersio

Experimental and theoretical line parameters for self-and H2-broadened transitions in the first overtone band of CO

International audienceIn this study we have re-analyzed high-resolution spectra of pure CO and CO broadened by hydrogen recorded in the spectral range of the first overtone band [1]. Self-and H2-Lorentzian pressure-broadened half-width, pressure-induced shift parameters, line mixing coefficients as well as line centers and intensities were obtained for 48 (P(24) to R(23)) ro-vibrational transitions belonging to the first overtone (2←0) band of 12 C16O at the ambient temperature (~298 K)

Molecular origin of the prepeak in the structure factor of alcohols

International audiencePrepeak in the structure factor of alcohols is known for a half century and was attributed to one of two mechanisms (i) self-assembly in aggregates and (ii) existence of spatial heterogeneity. Although both explnations are often argued the molecular origin is yet unclear. In this work, molecular dynamics simulation of neat alcohols and their mixtures in the presence of an apolar liquid in bulk and in confined phases is performed to unveil and to clarify the origin of the prepeak at the molecular scale. Unambiguously, we show that the existence of the prepeak is the result of the self-assembly in clusters leading to long-range correlations rather than the spatial heterogeneity. We also establish that the confinement of neat liquids at the nanoscale does not erase the clustering and the prepeak but strongly reduce the spatial heterogeneity. Regarding the binary alcohol/toluene mixtures, we highlight the possibility to erase the clustering and the spatial heterogeneity from nanoconfinement inducing the formation of a core−shell structure. By tuning the interfacial chemistry and pore size, we shed light on the possibility to control the spatial heterogeneity, the self-assembly, and the microphase separation. © 2020 American Chemical Societ

Metal-organic-framework transparency to water interactions for enhanced CO2 adsorption

International audienceToday, the capture of post-combustion CO2 has become a global priority, particularly in order to curb rising temperatures in the coming decades. CO2 physisorption in porous materials such as metal-organic frameworks is likely one of the most effective approaches to addressing this problem, thanks to its low energy requirement and improved regeneration process. In this study, I investigated CO2 capture in humid conditions using the Al-MIL-53-TDC metal-organic framework through molecular simulations. I observed that water filling occurs gradually through pore capillary condensation. Remarkably, the presence of water (at a hydration rate of 4.4 wt%) resulted in a significant increase of 283% in the adsorbed amount of CO2 (1.8 mmol/g at 0.2 bar) due to a unique transparency property. The thin molecular walls between the uniaxial channels enable van der Waals and electrostatic interactions between water and adsorbed gas, which allow media on opposite sides of the walls to influence each other. The increase in isosteric heat of adsorption (46 kJ/mol) is attributed to the contribution of water/water energy resulting from the truncation of the hydration shell of water. This truncation increases the number of hydrogen bonds, which can be linked to a kosmotropic effect. Additionally, the selectivity of hydrated Al-MIL-53-TDC for CO2/N2 increased by 185% at 0.2 bar (composition 20:80), compared to the unhydrated MOF material with QsT &lt; 50 kJ/mol