Comptes Rendus de l'Académie des Sciences - Series IIC - Chemistry最新文献

Modelling consists of considering the response and recovery times of a sensor to the target chemical species, as the sum of an ‘intrinsic’ response/recovery time, independent of the experimental conditions, and of an ‘extrinsic’ one, linked to the volume of the measurement cell and to the fluid flow. The experimental response/recovery times being almost always dominated by the extrinsic part, a dissymmetry between the response and recovery signals is observed for sensors whose response is not linear with the concentration of the target chemical species.

We show that the magnetic field dependence of the proton spin–lattice relaxation rates 1/T₁=Aω^-b₀ in non-crystalline proteins may be quantitatively related to structural fluctuations localized along the backbone that modulate proton–proton dipolar couplings. The parameter A is related to the temperature, the dipolar coupling strength and the energy for the highest vibrational frequency in the polymer backbone. The parameter b is related to the fractal dimensionality of the spatial distribution of protons and to the spectral dimensionality that characterizes the anomalous diffusion. Extension of the theory is presented to treat the case of hydrated proteins. This theory is satisfactorily compared with field cycling experiments realized on lysozyme protein.

⁷Li nuclear magnetic resonance relaxation times, T₁ and T_1ρ, versus temperature are reported in the 150–900 K temperature range on the lithium lanthanum titanates, Li_3xLa_2/3–x□_1/3–2xTiO₃, which are fast ionic conductors. Two characteristic frequencies of Li⁺ motions are evidenced in these compounds: the first is in the range of the Larmor frequency when the second one is in the range of the radio-frequency field. These frequencies are respectively attributed to motion of the Li⁺ ion inside the cage formed by the oxygen ions and to jumps between the cages. The T₁ and T_1ρ studies on ⁶Li nuclei confirm the above results and show that the relaxation is not due to quadrupolar interaction at a variance, which is generally accepted.

Co-circulation of gas and liquid in a pipe can generate, depending on inlet conditions, various kinds of flow patterns. Few investigations have been performed on intermittent two-phase flows (slug flows) using classical techniques (optical probe, hot-wire anemonetry, etc.), because these techniques are difficult to apply in this flow regime. Here we show that nuclear magnetic resonance is a powerful technique to study such flows. The presented results deal with controlled isolated Taylor bubbles. In addition to a classical Pulsed Field Gradient Spin Echo (PFGSE), a magnetic field gradient was applied during the π/2)_X radio frequency pulse, which produces a selective irradiation. Thus, cutting up of the flow into slices provides the longitudinal evolution of the liquid fraction and of the velocity probability distribution in the entire region perturbed by the Taylor bubble. The existence of a recirculatory flow under the Taylor bubble is clearly demonstrated.

We propose proton NMR and longitudinal relaxation experiments to study the structure and dynamics of water within calibrated hydrophobic mesoporous silica samples. We show the relevance of these methods to qualify the hydrophobic treatment for a water–substrate combination corresponding to a non-wetting situation.

The ⁴³Ca nuclear magnetic resonance is measured in the normal state of the high temperature superconductor (Ca_xLa_1–x)(Ba_1.75–xLa_0.25+x)Cu₃O_y as a function of temperature. The samples are chosen in order to compare the effect of changing the calcium and the oxygen contents in the underdoped regime. We determine the quadrupolar parameters and the Knight shift (KS). The macroscopic magnetic susceptibility is measured and used to estimate the ⁴³Ca hyperfine field. The variation of KS when increasing the calcium content does not show the signature of an increase of the doping level, contrary to what is suggested by the variation of macroscopic properties.

The consequences in terms of microstructure and texture of a prolonged contact between concrete and a continuous flow of mineral water have been investigated here by Nuclear Magnetic Resonance (NMR) because of its non-invasiveness and sensitivity to local environment. In particular, we evidence the dissolution of residual anhydrous cement, which leads to the further precipitation of hydrates occurring over 12 months of leaching tests in High Performance Concrete (HPC) and Ultra High Performance Concrete (UHPC). The study of the longitudinal relaxation of proton magnetization shows that the difference of pore size distribution between these two types of concrete remains mostly in the number of capillary pores. Its evolution with the time of water leaching up to the end of our experiment is not significant.

When, in the method MAS, part of the sample is at the edge of the coil so as to experience a partly radial RF field, this part gives rise during the sample spinning to spectra shifted by –1 and +1 times the spinning frequency, which are superimposed on the normal spectra with possible sidebands due to the modulated interactions, anisotropic chemical shifts and dipolar or quadrupolar interactions. The amplitude of these shifted spectra depends on the RF field distribution over the sample and on the amplitude of the RF pulse preceding the observation of the FID. On the other hand, it is independent of the spinning frequency. The quantitative description of this effect necessitates the use of the Reciprocity Theorem. Following a simplified account of the relevant theory, experimental illustrations of this effect are presented. The analysis of these contributions to the amplitudes of the sidebands makes it possible to determine the conditions under which the perturbation of the spectra is negligible.

We present a time evolution of ¹H spin-lattice relaxation rates in the laboratory (1/T₁) and in the rotating (1/T_1ρ) frame of a synthetic cement paste. The typical results found for both rates allow us to follow the main hydration stages of the cement paste and the refinement of its microporosity. In particular the texturation of the porosity and the structuration of the surface of the material are evidenced on two model cement pastes. An interpretation in terms of fractal size distribution is considered as well as the effect of the curing temperature.

Cement slurry is always used to support the casing of oilwells. This slurry is pumped down the steel casing of the well and then placed in the annular space between the casing and the surrounding rock. Nowadays wells are becoming deeper and deeper so the setting conditions of the cement paste are crucial. The pressure and temperature limit conditions can reach 1 000 bar and 250 °C at the bottom of the well. Hydration of synthetic tricalcium silicate Ca₃SiO₅ – the main component of oil well cements – was performed at high temperature under high pressure to simulate the oilwell conditions. The objective of the study is the establishment of the phase diagram of the ternary system – SiO₂, CaO, H₂O – according to the parameters: pressure, temperature and Ca/Si ratio. In this study, NMR is largely used to identify and to quantify the synthesized silicate hydrates, with two setting conditions (6 days at 120 °C under 400 bar and 6 days at 200 °C under 600 bar). Mechanical tests were performed to evaluate the compression strength of those silicate hydrates synthesized under such drastic conditions and to verify that they were still efficient binders.