Wanjun Lu

In situ study of mass transfer in aqueous solutions under high pressures via Raman spectroscopy: A new method for the determination of diffusion coefficients of methane in water near hydrate formation conditions

A new method was developed for in situ study of the diffusive transfer of methane in aqueous solution under high pressures near hydrate formation conditions within an optical capillary cell. Time-dependent Raman spectra of the solution at several different spots along the one-dimensional diffusion path were collected and thus the varying composition profile of the solution was monitored. Diffusion coefficients were estimated by the least squares method based on the variations in methane concentration data in space and time in the cell. The measured diffusion coefficients of methane in water at the liquid (L)–vapor (V) stable region and L–V metastable region are close to previously reported values determined at lower pressure and similar temperature. This in situ monitoring method was demonstrated to be suitable for the study of mass transfer in aqueous solution under high pressure and at various temperature conditions and will be applied to the study of nucleation and dissolution kinetics of methane hydrate in a hydrate–water system where the interaction of methane and water would be more complicated than that presented here for the L–V metastable condition.

Chapter 24 – A new optical capillary cell for spectroscopic studies of geologic fluids at pressures up to 100 MPa

A new optical capillary cell was constructed from square cross-sectioned fused silica-capillary tubing (300 µm × 300 µm with cavities of 100 µm × 100 µm or 50 µm × 50 µm) and a high-pressure valve that allows room-temperature studies of fluids at pressures up to 100 MPa. This capillary cell has the advantage of very precise study of fluid systems under 100 MPa pressures, not easily done in diamond anvil cells. Several key features of this cell include: (1) The ability to directly load sample fluids and monitor pressure during investigation, (2) The lack of optical distortion, (3) The small cell volume suitable for samples of limited supply (e.g. commercially available gas mixtures), (4) The high pressures that can be achieved, (5) The high-magnification, high-numerical aperture objective lens (e.g. 100×) with a short working distance that can be used due to the thin wall of the capillary tube, and (6) The heating–cooling stage or a circulating fluid bath that can be added, allowing for investigations at temperatures other than room temperature, particularly suitable for studies of gas hydrates. Raman spectra have been collected from the cell at room temperature for methane, ethane, propane, n-butane, and for two gas mixtures containing up to nine components as a function of pressure up to 70 MPa. The spectra document the shift in Raman bands with pressure as well as constrain the detection limits for various gas species in the mixtures. Preliminary experiments on the diffusion of methane in water were conducted by monitoring the concentration of dissolved methane in water as a function of time and distance from the vapor–water boundary, immediately after perturbation of an equilibrium state induced by a sudden incremental change in methane pressure.

Sensitive surface-enhanced Raman scattering (SERS) detection of nitroaromatic pollutants in water.

The increasing and urgent demand for clean water requires new approaches for identifying possible contaminants. In the present study, polymer substrates with embedded silver nanoparticles are employed to reveal the presence of traces of nitroaromatic compounds in water on the basis of the surface-enhanced Raman scattering (SERS) effect. These platforms provide an easy and sensitive method of detecting of low concentrations of these organic pollutants in contaminated water.

Pressure and Temperature Dependence of the Raman Peak Intensity Ratio of Asymmetric Stretching Vibration (ν3) and Asymmetric Bending Overtone (2ν2) of Methane

Raman peaks of the asymmetric stretching vibration (ν3) and the asymmetric bending overtone (2ν2) of methane were studied at elevated pressures and temperatures, from 3 to 51 MPa and from 298.15 to 473.15 K. The peak intensity ratios of ν3 and 2ν2 were calculated, and the relationship among peak intensity ratio, temperature, and pressure/density were derived using equations. Such relationships allow geologists to determine the pressure and density of methane fluid inclusions using Raman spectroscopic measurements of the peak intensity ratios of ν3 and 2ν2.