Anisotropy of the Proton Kinetic Energy as a Tool for Capturing Structural Transition in Water Confined in a Graphene Nanoslit Pore.

The proton dynamics of a 2D water monolayer confined inside a graphene slit pore is studied in Cartesian and molecular frames of reference using molecular dynamics simulations. The vibrational density of states of the proton was calculated versus temperature and was further used to deduce the mean kinetic energy of the hydrogen atoms, Ke(H), in both frames of reference. The directional components of Ke(H) are in good agreement with experimental observations for bulk as well as nanoconfined water. Nonetheless, while in the molecular frame of reference the effect of temperature on the anisotropy ratios of Ke(H) (the ratio between its directional components) are practically invariant between the 2D and 3D cases, those in the Cartesian frame of reference reveal a rather notable reduction across 200 K, indicating the occurrence of an order-disorder transition. This result is further supported by the calculated entropy and enthalpy of the confined water molecules. Overall, it is shown that Ke(H) anisotropy ratios may serve as a valuable order parameter for detecting structural transformations in hydrogen bonds containing molecular systems.

Microscopic Study of Proton Kinetic Energy Anomaly for Nanoconfined Water.

The reported anomalies of the proton mean kinetic energy, Ke(H), in nanoconfined water, as measured by deep inelastic neutron scattering (DINS), constitute a longstanding problem related to proton dynamics in hydrogen-bonded systems. A considerable number of theoretical attempts to explain these anomalies have failed. The mean vibrational density of states (VDOS) of protons in water nanoconfined inside single wall carbon nanotubes (SWCNTs) is calculated as a function of temperature and SWCNT diameter, DCNT, by classical molecular dynamics (MD) simulation using the TIP4P-2005f water model. The calculated VDOS are utilized for deducing the mean kinetic energy of the water protons, Ke(H), by treating each phonon state as a harmonic oscillator. The calculation depicts a strong confinement effect as reflected in the drop of the value of Ke(H) at 5 K for DCNT < ∼12 Å, while absent for larger diameters. The results also reveal very significant blue and red shifts of the stretching and bending modes, respectively, compared to those in bulk ice, in agreement with experiment.

Measuring the water content in freshly-deposited fingermarks.

The literature view regarding the composition of deposited fingermarks has long been that the average water content is in the range of 98-99wt.%. This value has recently been challenged by Kent, claiming that it should be 20wt.% at most. Herein we have measured the weight percentage of water content in freshly-deposited fingermarks, with and without hand pre-washing. Two complementary techniques were utilized for the measurements, namely quartz crystal microbalance (QCM) for determining the relative mass-loss and its rate at ca. 37°C, and temperature-programmed desorption-mass spectrometry (TPD-MS) for establishing that the mass loss arises solely from the complete evaporation of all the water content in the fingermarks (done with hand pre-washing only). Unlike the traditional narrow-range values of 98-99% and the limiting value of 20wt.% suggested by Kent, our measurements indicate the occurrence of a broad 20-70% water content. Higher contents of water in fingermarks were found post hand pre-washing, most probably due to removal of the sebum from the fingertips, but none of the results exceeded 90%.

Sensing the physicochemical nature of he and ne in micropores by adsorption measurements.

He and Ne in contact with molecular sieves in the form of crystalline A zeolites and amorphous carbon molecular sieves fibers (CMSF) were studied by adsorption measurements. Classification of the effective enclosure of zeolitic apertures and of graphitic constrictions, as determined by recent temperature-programmed desorption mass spectrometry (TPD-MS) studies of adsorption of He and Ne onto these materials, was utilized in making a prudent choice of samples and experimental conditions. In view of the former TPD information, the behaviors of adsorption and volumetric measurements reported herein are straightforwardly interpreted. The combined TPD, adsorption isotherms, and dead volume data deepen the understanding of the physicochemical nature of adsorbed gas, where gas adsorption in the vicinity of pore constrictions and/or apertures as well as on the inner surface areas of pores and/or cages could be resolved. Previous conclusions that the huge activation energies measured for Ne/CMSF at high temperatures are unlikely to characterize chemical desorption but reflect those required for overcoming the barrier of effectively constricted apertures were confirmed by the volumetric data presented here. At 77 K, considerable He adsorption was observed in the porous solids and found to be responsible for abnormal deduced values of dead volumes. The occurrence of significant adsorption of He onto A zeolites and CMSF at 77 K warrants the realization that in cases concerning porous materials, volumetrically deduced quantities should not be taken for granted, but should be carefully considered and uniquely interpreted in relation to the specific experimental conditions under which they are taken.

A new route of oxygen isotope exchange in the solid phase: demonstration in CuSO4.5H2O.

Temperature-programmed desorption mass spectrometry (TPD-MS) measurements on [(18)O]water-enriched copper sulfate pentahydrate (CuSO(4).5H(2)(18)O) reveal an unambiguous occurrence of efficient oxygen isotope exchange between the water of crystallization and the sulfate in its CuSO(4) solid phase. To the best of our knowledge, the occurrence of such an exchange was never observed in a solid phase. The exchange process was observed during the stepwise dehydration (50-300 degrees C) of the compound. Specifically, the exchange promptly occurs somewhere between 160 and 250 degrees C; however, the exact temperature could not be resolved conclusively. It is shown that only the fifth, sulfate-associated, anionic H(2)O molecule participates in the exchange process and that the exchange seems to occur in a preferable fashion with, at the most, one oxygen atom in SO(4). Such an exchange, occurring below 250 degrees C, questions the common conviction of unfeasible oxygen exchange under geothermic conditions. This new oxygen exchange phenomenon is not exclusive to copper sulfate but is unambiguously observed also in other sulfate- and nitrate-containing minerals.