Interface water-methane

A number of other researchers have also confirmed that nucleation and subsequent growth typically occurs at the water-hydrocarbon interface for methane hydrate (Huo et al., 2001 0stergaard et al., 2001 Taylor, 2006) and carbon dioxide hydrate (Kimuro et al., 1993 Fujioka et al., 1994 Hirai et al., 1995 Mori, 1998). 

Molecular dynamics (MD) simulation studies also indicate that the initial formation of methane hydrate occurs preferentially near the water-methane interface where there is a significant concentration gradient (Moon et al., 2003). 

Figure 3.27 Methane hydrate film development at the water-methane interface from dissolved methane in the aqueous phase, as indicated from Raman spectroscopy (a) and methane solubility predictions (b). (a) A series of Raman spectra of dissolved methane collected at different temperatures during the continuous cooling process. Spectra marked A through E correspond to temperatures of 24°C, 20°C, 15.6°C, 10.2°C, and 2.8°C, respectively. (b) A schematic illustration of temperature dependencies of the equilibrium methane concentration in liquid water (C = without hydrate, Qjh = with hydrate). The scale of the vertical axis is arbitrary, but the Raman peak area is proportional to methane dissolved in water. Points A through F correspond to different temperatures during the continuous cooling process. (From Subramanian, S., Measurements ofClathrate Hydrates Containing Methane and Ethane Using Raman Spectroscopy, Ph.D. Thesis, Colorado School of Mines, Golden, CO (2000). With permission.)...

The mechanisms of nucleation followed by subsequent growth of the ice crystal discussed below are similar to that observed in the simulation of homogeneous nucleation of methane hydrate. More importantly, results of MD simulations show hydrate formation always initiate near the water/methane interface " where there is a large concentration gradient difference exists between the methane gas and the solution. This was observed in slab calculations that have a distinct water/methane interface and around the methane bubble in the spontaneous nucleation study. The nucleation prediction is... 

Figure 29 (a) Molecular formula for poly(vinylpyrrolidone) (PVP). (b) Location of PVP relative to the water/methane interface (top) initially and (bottom) after 1 ns. PVP is represented by thick lines, water with thin lines, and methane as spheres N atoms are black, C dark grey, O light grey, and H white, (c) Snapshots of the hydrate network and PVP following insertion of PVP. Adapted from Refs. 121 and 127. (For a color version of this figure, please see plate 11 in color plate section.)... 

R. W. Hawtin, D. Quigley, and P. M. Rodger, Phys. Chem. Chem. Phys., 10, 4853 (2008). Gas Hydrate Nucleation and Cage Formation at a Water/Methane Interface. 

Thus, the characteristic topographic function increases continuously from the surface (z = z0) to the lake bottom (z = 0) where it becomes infinitely large. In fact, at the lake bottom a tiny lake volume stays in contact with a finite sediment area. This explains the great spatial and temporal gradients often found close to the bottom of lakes for compounds which are exchanged at the sediment-water interface (oxygen, phosphorus, methane, etc.). 

Figure 3.26 Final methane hydrate film thickness vs. subcooling. (From Taylor, C.J., Adhesion Force between Hydrate Particles and Macroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface, Masters Thesis, Colorado School of Mines, Golden, CO (2006). With permission.)...

The trends shown from the predicted curves, Csh and Cs, are in qualitative agreement with corresponding dissolved methane Raman peak intensities. Therefore, the Raman spectra (Figure 3.27a) support the proposed mechanism that hydrate growth occurs in part as a result of methane diffusing from the bulk aqueous phase to the hydrate film formed at the vapor-liquid interface. This decreases the methane concentration in the bulk water phase. Hydrate growth from an aqueous... 

Englezos et al. (1987a,b) generated a kinetic model for methane, ethane, and their mixtures to match hydrate growth data at times less than 200 min in a high pressure stirred reactor. Englezos assumed that hydrate formation is composed of three steps (1) transport of gas from the vapor phase to the liquid bulk, (2) diffusion of gas from the liquid bulk through the boundary layer (laminar diffusion layer) around hydrate particles, and (3) an adsorption reaction whereby gas molecules are incorporated into the structured water framework at the hydrate interface. 

Figures 7.11a,b are arbitrary examples of the depths of hydrate phase stability in permafrost and in oceans, respectively. In each figure the dashed lines represent the geothermal gradients as a function of depth. The slopes of the dashed lines are discontinuous both at the base of the permafrost and the water-sediment interface, where changes in thermal conductivity cause new thermal gradients. The solid lines were drawn from the methane hydrate P-T phase equilibrium data, with the pressure converted to depth assuming hydrostatic conditions in both the water and sediment. In each diagram the intersections of the solid (phase boundary) and dashed (geothermal gradient) lines provide the lower depth boundary of the hydrate stability fields.

Figure 4.15. Block diagram for formation and transport of methane in waterlogged country. Notation FlCHi is the methane flux across the atmosphere/water body interface F2CHi is the oxidation of methane in aerobic zones FCH is the intensity of the methane source M is methane concentration.

Liikanen A. and Martikainen P.J. (2003). Effect of ammonium and oxygen on methane and nitrous oxide fluxes across sediment-water interface in a eutrophic lake. Chemo sphere, 52(8), 1287-1293. 

Conventional high pressure NICI spectra were obtained using a Hewlett-Packard 5985B quadrupole GC/MS, as described previously (1). Methane was used as the Cl reagent gas and was maintained in the source at 0.2-0.4 torr as measured through the direct inlet with a thermocouple gauge. A 200 eV electron beam was used to ionize the Cl gas, and the entire source was maintained at a temperature of 200° C. Samples were introduced into the spectrometer via the gas chromatograph which was equipped with a 25 meter fused silica capillary column directly interfaced with the ion source. For all experiments, a column coated with bonded 5% methyl phenyl silicon stationary phase, (Quadrex, Inc.) was used and helium was employed as the carrier gas at a head pressure of 20 lbs. Molecular sieve/silica gel traps were used to remove water and impurities from the carrier gas. 

We conclude that the proximal radial distribution function (Fig. 1.11) provides an effective deblurring of this interfacial profile (Fig. 1.9), and the deblurred structure is similar to that structure known from small molecule hydration results. The subtle differences of the ( ) for carbon-(water)hydrogen exhibited in Fig. 1.11 suggest how the thermodynamic properties of this interface, fully addressed, can differ from those obtained by simple analogy from a small molecular solute like methane such distinctions should be kept in mind together to form a correct physical understanding of these systems. 

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