Vibrational properties of confined and interfacial water : thermodynamic consequences

Les études en laboratoires montrent que les propriétés de l'eau liquide sont perturbées lorsqu'elle est proche d'une surface (≈ 1nm) ou occluse dans des pores de taille inférieure à la dizaine de nanomètres : au-delà de ces seuils, ses propriétés sont supposées volumiques. Paradoxalement, des observations de terrains suggèrent que le comportement de l'eau peut s'écarter de son comportement volumique pour des tailles de pores de l’ordre de quelques microns. Nous présentons une étude expérimentale des effets, et de leur portée, d'une surface solide, paroi interne d’une cavité fermée, sur les propriétés vibrationnelles et thermodynamiques de l'eau occluse. D’abord, nous avons développé et calibré une fonction de partition prenant en compte les modes inter- et intramoléculaires de l'eau, afin de convertir ses propriétés vibrationnelles en propriétés thermodynamiques. Cette fonction nous permet de calculer la variation d’enthalpie libre que représente une déviation du spectre IR par rapport au spectre de référence. Des mesures IR dans des canaux ont été réalisées en fonction de leur hauteur, de 100 à 5 nm, qui ont mis en évidence que les propriétés de l'eau sont modifiées entre 5 et 20 nm. Si ces distances restent dans les valeurs admises pour l’influence interfaciale, l’intensité thermodynamique des variations (jusqu’à 1.5 kJ/mol) est surprenante, de même que leur sens de variation : l’activité de l’eau augmente. Le moteur de cette évolution pourrait être la restriction géométrique, et des effets de pression de disjonction. Au contraire, des mesures de micro-spectroscopie IR haute résolution et Raman ont été menée dans des micro-cavités fermées (inclusions fluides synthétiques), en fonction de la distance aux interfaces solide/liquide et liquide vapeur. Elles ont montré que les propriétés de l'eau sont progressivement perturbées sur une distance de 1 à 3 μm. Proche de la surface, la variation thermodynamique est au-delà du kJ/mol d’eau, et traduit une augmentation de l’activité de l’eau. Le moteur de ce qu’on appelle une « interphase », vu son épaisseur, associe la tension de surface solide-solution aqueuse et des effets osmotiques, liés à une stratification chimique de la solution depuis la surface jusqu’au centre de la cavité. D'un point de vue thermodynamique, l’eau confinée et l’eau impliquée dans l’interphase apparaissent plus réactives que l’eau volumique : ces mesures ouvrent donc des perspectives d’interprétation des interactions solide-solution dans les milieux naturels. Quelques applications à des observations sont évoquées.Laboratory studies shows that water properties are disturbed when it is located close to a surface (≈1nm) or occluded in a pore with size less than ten nanometers: beyond these thresholds, its properties are supposed to be bulk. Paradoxically, field observations suggest that the water behavior can deviate with respect to its bulk one for pore sizes of few microns. We present an experimental study of the effects and the scope of a solid surface, internal wall of closed cavity, on the occluded water vibrational and thermodynamic properties. First, we have developed and calibrated a partition function, that takes into account the water inter- and intramolecular modes, to convert its vibrational properties into its thermodynamic counterparts. This function allows us to calculate the free enthalpy corresponding to an IR spectrum deviation with respect to a reference spectrum. IR measurements were performed in channel in function of their height, from 100 to 5 nm. They highlighted that water properties are modified between 5 and 20 nm. Although these distances are within the accepted values for the interfacial influence, the intensity of thermodynamic variations are surprising, as its variation direction: the water activity increases. This evolution could be drive by the geometrical restrictions and the disjoining pressure effects. On the contrary, high resolution IR and Raman micro-spectroscopies measurements were carried out in closed micro-cavities (synthetic fluid inclusions), in function of the distance to the solid/liquid and liquid/vapor interfaces. They demonstrated that water properties are progressively disturbed on a distance from 1 to 3 μm. Close to the surface, the thermodynamic variation is beyond kJ/mol of water, and reflects an increasing of water activity. The driver of the so-called interphase, because of its thickness, involves the solid-aqueous solution surface tension and osmotic effects, related to a chemical stratification of the solution from the surface to the cavity center. From a thermodynamics point of view, confined water and water in the interphase appear more reactive than bulk water: these measurements offer perspectives for the solid-solution interaction interpretation in the natural media. Some applications of the observations are discussed

Infrared-Thermodynamics Conversion as a Function of Temperature: Towards Confined Water

International audienceAn experimental method has been developed to calculate the thermodynamic properties of water from its vibrational properties, relevant to study (in near future) the properties of adsorbed or confined water. The infrared absorption of the intra-molecular OH stretching mode of liquid water has been measured over a wide range of temperature (from -10°C to 90°C). The corresponding large band has been decomposed into three Gaussian components standing for three different water connectivities (percolation model) that feature the liquid state as a function of temperature: network, intermediate, multimer, water. Measurements evidenced that the components are differently shifted with temperature, giving a quantitative insights into the internal energy change of liquid. A vibrational partition function has been used to calculate the corresponding thermodynamic properties, neglecting all energy components except the present intra-molecular vibrational mode. Interestingly, the vibrational free enthalpy thus computed differs of the total free enthalpy only by a multiplicative constant all along the thermal range

Thermodynamic properties of interfacial water from its infrared signatures

International audienceWater liquid trapped in quartz micro-cavities is probed by infrared micro-spectroscopy to explore the influence of the solid/liquid and liquid/air interfaces on water thermodynamics. The sample is infraredly mapped with a 3x3 µm micro-beam allowed by the brilliance of synchrotron radiation source (SOLEIL, SMIS beamline) at 2.5 µm xy interval. It appears that the intramolecular OH-stretching band is changing as a function of its proximity to the interfaces. To refine these variations, the band is decomposed into Gaussian components, evidencing that water is interface-printed even at micrometric distance to the wall and/or to the bubble with the closest layer to the interface marked by a dangling-OH effect. Then, these vibrational changes are converted into Gibbs free energy, which range from 35 J.mol-1 4.5 µm away from the interface, to 1 kJ.mol-1 at about 1 µm

Chemo-mechanical coupling at the one-pore scale: fracturing quartz hostby increasing tension in water inclusion

International audienceWater-bearing porous media allow the infilling liquid to become superheated much easily than anyother system. Superheating liquids make them prone to develop a tensile state, that is to say an internalnegative liquid pressure or tension. In this sense, superheated liquid is a close analogue to capillarywater retained in many not-saturated porous media (unsaturated zone of soils, gas/oil-depletedaquifers, CO2-storing aquifers). In granular physics, the role of capillary water tension to rigidify thegranular assemblage is studied for long (sand–castles physics) but little attention has been turned tothe same effect in compacted/continuous systems (rock fissures, fluid inclusions, intra-mineralcavities, etc.).Using synthetic fluid inclusions trapped in quartz, we were able to put the occluded liquid (aqueoussolution, CsCl 12m) at very high tension by isochoric cooling of the samples. Starting with aliquid-vapour assemblage, we heat the sample up to a special temperature at which the vapour bubbledisappears (temperature of homogeneization, Th), and then turn to a cooling procedure that decreasesthe internal pressure of the occluded liquid at constant volume, as long as the bubble does notre-appear again (relaxing the tensile state of the liquid). At a given tension state, we mapped theRaman spectra at the two quartz bands frequencies, in the quartz matrix all around the inclusion undertension. Using frequency-pressure calibration of the literature, it turned out that the quartz host wassubmitted to a small compressive stress in response to the perpendicular traction from the liquid.In a second step, one sample was submitted during one month to repetitive cycles ofsuperheating-relaxation processes, after which the volume of the inclusion changed brutally. This wasrecorded by a change of the liquid density measured through a significant Th shift. In the meantime,another sample was submitted to a constant tension which, after a while, provoked the visiblefracturation of the quartz matrix.These observations and measurements demonstrate that the tension of water occluded in pores, orchannels, or any type of cavities in solids, organics or living cells, is able to exert a stress onto the hostsolid, quantitatively weak. However, it appears that the recurrence of such effect and/or itspreservation through time, create a fatigue in the host that is ultimately able to break out its cohesion,certainly owing to pre-existing matrix defaults. Consequences in terms of chemo-mechanical couplingin hydrosystems or materials submitted to wetting-drying cycles will be eventually highlighted

Rétroactions Porosité - Réactivité

International audienceLa structure des milieux poreux est de première importance lorsqu’on étudie les propriétés de transport, avec un intérêt dirigé versla taille des pores et la topologie 3d. Les propriétés réactives ont également des liens très forts avec la structure, et même la micro-structure, notamment la géométrie et la composition locales, les états de surface, des phases dissoutes et solides. Dans cette contribution, nous nous intéressons aux relations spécifiquement entre taille et réactivité, pour déterminer quelle est l’échelle dimensionnelle où les propriétésde l’eau porale changent. Trois situations contrastées sont étudiées : (1) l’eau est abritée dans un pore de grande taille (plusieurs dizaines de microns) ; (2) l’eau liquide est confinée entre deux surfaces de silice ; (3) une épaisseur d’eau entre 0.5 et quelques micromètres est déposée sur une surface. Deux types d’échantillons abritent ces situations : une inclusion fluide de quartz, c’est -à-dire une vacuole fermée au cœur même du solide, contenant soit seulement de l’eau liquide, soit un mélange biphasique eau-air; des canaux de nano-fluidique, à base de silicium monocristallin, dont la hauteur varie entre 5 et 100 nm. Des mesures infra-rouges ont été menées sur cette eau piégée, et les signaux enregistrés sont examinés en fonction de la hauteur de canaux ou de l’éloignement à l’interface.Ces mesures sont ensuite converties en propriétés thermodynamiques en champ moyen et les prédictions en termes de transitions de phase calculées par simulation thermodynamique

Water-solid interactions at the pore scale

International audienceWater and aqueous solution confined in restricted volumes (pore, channels, intra-solid cavities, etc) are largely encountered everywhere in nature, and their thermochemical properties define the possible driving forces of their interactions with any phase(s) of interest locally present. At given (T,Ptotal) pairs, liquids are considered to be bulk materials, except when the confinement reaches the nanometric scale. Despite this very usual choice, a growing number of evidences points to the existence of an intermediate-sized domain (1 μm - 50 nm) in which the interaction behaviors seems to change, most probably due to thermodynamic driving forces. This contribution reports on dedicated recent experiments, the measurements of which can be interpreted assuming modified reactive properties in the system

Gibbs free energy of liquid water derived from infrared measurements

International audienceInfrared spectra of pure liquid water were recorded from 20 cm À1 to 4000 cm À1 at temperatures ranging from 263 K to 363 K. The evolution of connectivity, libration, bending and OH stretching bands as a function of temperature follows the evolution of the inter-molecular dynamics, and so gives insight into the internal energy averaged over the measurement time and space. A partition function, which takes into account the inter-molecular and intra-molecular modes of vibration of water, all variable with the molecular networking, was developed to convert this vibrational absorption behavior of water into its macroscopic Gibbs free energy, assuming the vibrational energy to feature most of the water energy. Calculated Gibbs free energies along the thermal range are in close agreement with the literature values up to 318 K. Above this temperature, contributions specific to the non H-bonded molecules must be involved to closely fit the thermodynamics of water. We discussed this temperature threshold in relation to the well-known isosbestic point. Generally speaking, our approach is valuable to convert the IR molecular data into mean field properties, a quantitative basis to predict how water behaves in natural or industrial settings

Oversolubility in the microvicinity of solid-solution interfaces

International audienceWater-solid interactions at macroscopic level (beyond ten of nanometers) are often viewed asthe coexistence of two bulk phases with a sharp interface in many areas spanning frombiology to (geo)chemistry and various technological fields (membranes, microfluidics,coatings, etc.). Here we present experimental evidence indicating that such a view may be asignificant oversimplification. High-resolution infrared and Raman experiments wereperformed in a 60x20 μm2 quartz cavity, synthetically created and initially filled withdemineralized water. The IR mapping (3x3 μm2 beam size) performed with the SOLEILsynchrotron radiation source displays two important features: (i) the presence of a danglingfree-OH component, a signature of hydrophobic inner walls; (ii) a shift of the OH-stretchingband which essentially makes the 3200 cm-1 sub-band to predominate over the usual maincomponent around 3400 cm-1. Raman maps confirmed these signatures (though less markedthan IR’s) and afford a refined spatial distribution of this interfacial signal. This spatialresolution, statistically treated, results in a puzzling image of a 1-3 μm thick marked-liquidlayer along the entire liquid-solid interface. The common view is then challenged by thesestrong evidences that a μm-thick layer analogous to an interphase forms at the solid-liquidinterface. The thermodynamic counterpart of the vibrational shifts amounts to around +1kJ/mol at the interface with a rapid decreasing signature towards the cavity centre, meaningthat vicinal water may form a reactive layer, micrometer thick, expected to have an elevatedmelting point, a depression of the boiling temperature, and enhanced solvent properties

Growing Negative Pressure in Dissolved Solutes: Raman Monitoring of Solvent-Pulling Effect

International audienceNegative pressure in liquids is both an experimental fact and a usually-neglected state of condensed matter. Using synthetic fluid inclusions, namely closed vacuoles fabricated inside one solid host by hydrothermal processes, a Raman study was performed to examine how a superheated solvent (under negative pressure) interacts with its dissolved solutes. As a result, this contribution not only illustrates this well-known tensile state, but also displays evidence that a stretched solvent is able to pull on its dissolved solutes and put them also under a stretched state. The dielectric continuum hypothesis may lead to expect a stretching effect in solutes similar to the solvent’s, but our measurements evidence a damping mechanical effect (growing with tension), most probably related to solvation shells. One practical consequence is that the (experimentally known) super-solvent properties of superheated solutions are certainly related to the change of the chemical potential of solutes which results from the damping effect. This change can determine as well a change in the thermodynamic driving force of the superheated solution towards bubble nucleation. A more complex than usual picture of the aqueous solution physical chemistry emerges from this study

Quartz Stressing and Fracturing by Pore Pressure Dropping Down to Negative Pressure

International audienceIn water-bearing porous rocks, pore pressure variations play a major role in deformation, through dissolution−precipitation and fracturing processes. An often-overlooked variation where pressure falls to negative pressure or tension can operate whenever aquifer formations dry out, for instance, in deep storage (nuclear or industrial wastes, long-term CO 2 mitigation, short-term energetic resources, etc.). This can generate capillary tension within the aquifers. This study investigates the mechanical effect of such in-pore tension in the surrounding crystal field, through laboratory experiments at the one-pore scale. Microthermometric procedures were carried out on synthetic fluid inclusions to generate large tensile stress and were combined with Raman microspectrometry to visualize the resulting stress fields in the host quartz. For comparison, we numerically modeled the stress field by linear elasticity theory. The experiments demonstrate that significant damage is produced in crystalline materials by the pore tension. Despite the induced stress measured by micro-Raman spectrometry to remain moderate, it is able to fracture the quartz. The volume of the cavity is a prominent controlling parameter for the stress amplitude. The crystalline heterogeneities of the solid are another major parameter for localizing the mean weak stress and accumulating overstress. Our results call for bringing pore-scale micromechanics into the safety assessment of the geological storage of various wastes inside depleted aquifers. They also show the magnifying effect of heterogeneities on propagating stress and localizing it along certain directions, promoting the final failure of water-bearing minerals, rocks, or pore networks