Mireille Privat

Coadsorption at the air/water interface likely explains some pollutants transfer to the atmosphere: benzene and lead case

This study attempts to evidence a physical interfacial mechanism for the passing of some non-volatile harmful molecules from water, where they are dissolved, to the atmosphere. The idea developed here is that an organic substance, at its solubility limit, forms a surface layer whose properties induce the coadsorption of another dissolved substance; both are then able to pass to the atmosphere by a bubbling mechanism. Experiments were made with benzene close of its solubility limit in an aqueous solution of lead nitrate, which is non-volatile and normally does not adsorb at water surface. Coadsorption really occurred. The impact of such a mechanism on the environment is discussed.

Polyethylene glycol aggregates in water formed through hydrophobic helical structures

The present study was aimed at elucidating the mechanism of aggregation in water of hydroxyl-terminated polyethylene glycol (PEG) of low molecular weight (600 g/mol). The results from fluorescence spectroscopy at different temperatures were consistent with surface tension measurements, suggesting aggregate formation. Indeed, the process of aggregation is accompanied by an increase in the fluorescence emission of a hydrophobic probe. So, PEG aggregates in the form of internal hydrated helices covered with CH(2) groups are shown to yield hydrophobic regions. These regions created upon PEG aggregation in water and at a temperature close to 35°C result from a balance between H bonding and entropic effects. By providing the first experimental evidence for hydrophobic mediation of aggregation with OH-terminated oxy-ethylene chains of low molecular weight, this study highlights their surfactant-like behaviour.

Surface transitions in binary liquid mixtures

Abstract Using a reasoning based on the classical thermodynamics of adsorption, the occurrence of a surface transition is described in terms of a phase diagram. The surfaces involved are the liquid-vapor and liquid-solid interfaces, when only physisorption takes place. Cahns conclusion concerning the existence of a prewetting line in the monophasic zone of the phase diagram of the binary system has been verified. Two experimental examples have been considered, viz., the methanol-cyclohexane and water-2-butoxyethanol systems.

Conditions of phase separation, both at the interface (occasionally considered as a monolayer) and in solution for a binary liquid vapor system: The adsorption isotherm and the consequences of critical phenomena on the behavior of the system

Abstract The effects of modifications in the bulk and superficial phases of a binary mixture in equilibrium with its vapor, on the form of the surface adsorption isotherm, have been studied, and in particular the influence of deviations from ideality and of temperature. One observes, according to the value of the latter as compared to the critical temperatures of the surface and in the solution that a phase separation is possible, which may lead to a system evolving in a different manner from one simply separating into two phases. The monolayer model was used in a part of the work to describe the surface phase.

Wetting transitions and other wetting properties of water–2,5 lutidine system

The wetting behavior of the water–2,5 lutidine system has been studied around the lower consolute point (Tc=13.1 °C). We have measured contact angles and surface tensions by varying the concentration and temperature. In the diphasic region, a wetting transition has been observed at 46–47 °C on a silica wall by direct observation of the solid–liquid–liquid contact angle. The perfect wetting occurs close to Tc, the wetting phase is water rich. At the liquid–vapor interface, the analysis of the values of the surface tensions shows that, close to Tc, they obey critical laws and that a lutidine rich phase perfectly wets the vapor and the water rich phase. These behaviors have been analyzed on the basis of the contact angles of the monophasic on a silica wall on both sides of the coexistence curve, and the general variation of the surface tensions. A second wetting transition has been shown on a differently washed glass surface. Reference is made to the theoretical and experimental works following the early wor...

Ab initio calculations on the electronic structure of the divalent lead-water complex.

We studied the electronic structure of the Pb (2+)-4H 2O system. Analysis of the complex orbital evidenced no mixing between the 6s lone pair orbital of the lead and the 6p orbital components. Moreover, we found that the HOMO is widely described by the mixture of the 6p components with the 7s valence orbital of the lead. This orbital shows an important elliptical electron charge density around the lead ion and opposite the direction of the short lead-water bonds. From these results, we demonstrated that the hemidirected conformation of the Pb (2+)-4H 2O system could be easily explained by the shape of the electron charge density distribution of the HOMO rather than by the stereochemically active character of the 6s (2) lone pair of lead electrons.

Linear rheological properties of low molecular weight polyethylene glycol solutions

This experimental study of the linear viscoelastic properties of PEG600 aqueous solutions at various concentrations and temperatures mainly aims at getting a better understanding, in concentrated regimes, of the role played on the structural and rheological properties by PEG assembly properties previously evidenced in the dilute regime. The results indicate a peculiar and unique viscoelastic behavior: the elastic modulus versus frequency curves are nearly the same for all concentrations and temperatures investigated. The key role played by hydrogen bonds in the rheological properties of PEG solutions is highlighted. The relaxation on long time scales is indicative of large scale complex associative polymeric structures. A schematic complete phase diagram of PEG600 in water is finally proposed.

Prewetting transition influenced by layeringlike transitions at a binary liquid/solid interface

The prewetting transition in the silica-water-2,5-dimethylpyridine system has been studied through the adsorption isotherms. Steep wave shapes for these isotherms lead to the conclusion of alternate solid and liquid surface demixing very close to bulk coexistence and wetting temperature. Liquid demixing has the generic aspects of prewetting. An empirical model for demixings, surface phase diagrams in several representations, and critical end points are discussed, as well as the phenomena universality.

Universal amplitude ratios of surface tensions near a critical point in a liquid binary system: Water‐2,5 lutidine

Experimental data of surface tensions along the critical isochore (1) and the two branches of the coexistence curve (2,3) for the water‐2,5 lutidine system have been used in order to check signs and values of the universal amplitude ratios of the critical laws in the forms σα,j=σc+A(T−Tc)+Bj(T−Tc)μ, j=1,2,3, with imposed μ=1.26. A and the Bj’s being highly correlated, several statistical tests using the values of the ratios have been used. Experimental values are compatible with P+=B/Bi=−0.59 (B: isochore; Bi: interfacial tension in the two phases system), maybe with Q=B/B‘=−0.83 (B‘ one branch of the coexistence curve) and certainly with its sign. The other ratios cannot be verified at all by the experimental data.

Layering in a two-component liquid system undergoing a phase separation

The phase change possibilities in the surface phase formed in a binary liquid system (water 2,5dimethylpyridine) in contact with a solid (silica) have been examined on the basis of adsorption isotherms determined at ten different temperatures. The liquid mixture undergoes a liquid–liquid demixing but also, some 20° below the critical point, a solid phase separation. On the l.h.s. of a liquid–liquid coexistence curve 2,5dimethylpyridine, diluted in water, adsorbs step by step, and on the r.h.s. the relative adsorption is wave shaped. Both behaviors can be explained by a layering process, i.e., a solid–liquid surface demixing, which unexpectedly does not exclude a separate liquid–liquid surface demixing, which could constitute the prewetting process. This analysis is compared with known theories and its consistency is carefully checked on a thermodynamical basis.