CO2 and C2H2 in cold nanodroplets of oxygenated organic molecules and water

Abstract

Recent demonstrations of subsecond and microsecond timescales for formation of clathrate hydrate nanocrystals hint at future methods of control of environmental and industrial gases such as CO2 and methane. Combined results from cold-chamber and supersonic-nozzle [A. S. Bhabhe, "Experimental study of condensation and freezing in a supersonic nozzle," Ph.D. thesis (Ohio State University, 2012), Chap. 7] experiments indicate extremely rapid encagement of components of all-vapor pre-mixtures. The extreme rates are derived from (a) the all-vapor premixing of the gas-hydrate components and (b) catalytic activity of certain oxygenated organic large-cage guests. Premixing presents no obvious barrier to large-scale conditions of formation. Further, from sequential efforts of the groups of Trout and Buch, a credible defect-based model of the catalysis mechanism exists for guidance. Since the catalyst-generated defects are both mobile and abundant, it is often unnecessary for a high percentage of the cages to be occupied by a molecular catalyst. Droplets represent the liquid phase that bridges the premixed vapor and clathrate hydrate phases but few data exist for the droplets themselves. Here we describe a focused computational and FTIR spectroscopic effort to characterize the aerosol droplets of the all-vapor cold-chamber methodology. Computational data for CO2 and C2H2, hetero-dimerized with each of the organic catalysts and water, closely match spectroscopic redshift patterns in both magnitude and direction. Though vibrational frequency shifts are an order of magnitude greater for the acetylene stretch mode, both CO2 and C2H2 experience redshift values that increase from that for an 80% water-methanol solvent through the solvent series to approximately doubled values for tetrahydrofuran and trimethylene oxide (TMO) droplets. The TMO solvent properties extend to a 50 mol.% solution of CO2, more than an order of magnitude greater than for the water-methanol solvent mixture. The impressive agreement between heterodimer and experimental shift values throughout the two series encourages speculation concerning local droplet structures while the stable shift patterns appear to be useful indicators of the gas solubilities.

Transient Translational and Rotational Water Defects in Gas Hydrates

“…Within the five independent simulations at each temperature, we observed a total of four hydrogen-interchange events at 250 K and eight hydrogen-interchange events at 260 K, which correspond to a hydrogen-interchange rate of ∼10 4 s –1 per water molecule, whereas hundreds of such events, which corresponds to a hydrogen-interchange rate of ∼10 6 s –1 per water molecule, can be observed at the high temperature of 300 K. With the measured the hydrogen-interchange rate, we can estimate the associated free energy of activation using eq . Figure shows the linear regression results of the measured hydrogen-interchange rates (ln­( k / T )) versus temperatures (1/ T ), from which we can calculate that the 95% confidence interval for the rate of the hydrogen interchange is 1.3 × 10 5 –2.4 × 10 5 s –1 per water molecule, and the 95% confidence interval for the associated free energy of activation is 38.2–39.4 kJ/mol at 270 K. These values are comparable to previous results from all-vapor experiments, − supersonic nozzle experiments, and thermodynamics calculations , of clathrate hydrates with hydrogen-bonding guests but under much lower temperatures, indicating the effects of the hydrogen-bonding guests on inducing the defect formation and accelerating the mass diffusion in gas hydrates. Note while previous experimental studies using NMR techniques have studied the water reorientation processes within gas hydrate systems, − for example, the activation energy of water reorientations in THF hydrates is determined to about 31 kJ/mol, ,, the particular type of water reorientations discussed in this paper may not be identified directly in these NMR experiments where all the possible water orientations are sampled over time.…”

“…The transient defects generated during these processes may facilitate guest rotational and translational motions in the gas hydrate cages and hence have implications for the unusual behavior of the thermal conductivity and mechanical properties of gas hydrates. − , Note similar rotation processes have been observed in ice Ih when the central water molecules are surrounded by a Bjerrum L-type defect . This rotation process is also essential to the proton transport and the isotopic exchange processes as demonstrated before. ,,, It has been shown that the hydrogen-bonding guests, such as small ether molecules, H 2 S, etc., can enhance the defect concentration in gas hydrates and catalyze the formation of gas hydrates, , which are consistent with the all-vapor gas hydrate formation experiments with various guest molecules. − In future studies it would be interesting to investigate the possible role of the guest molecules in inducing and stabilizing the transient defects in gas hydrates.…”

Molecular Modes and Dynamics of HCl and DCl Guests of Gas Clathrate Hydrates

“…The acetylene classical s-c frequency is decreased slightly from the gas-phase value of 3289 to ∼3280 cm –1 , but is downshifted another ∼30 cm –1 upon bonding to the cage wall . Nevertheless, that downshift is small compared to the ∼100 cm –1 amount for C 2 H 2 dissolved in oxygenated organics, or within the corresponding heterogeneous dimers . The large shift is a testimony to an acidity of the acetylene hydrogen as well as the strength of its electron-acceptor charge-transfer interactions .…”

“…The premixing was completed with addition of ∼100 Torr of He(g). During the present study, the vapor premixtures at different times contained (a) THF alone, (b) THF with HCN, (c) THF with HCl/DCl, (d) THF with HCN followed by HCl/DCl added in a second pulse (as previously described in a study of the droplet compositions), 23 (e) THF with HCl with HCN added in a second pulse, (f) all three guests in a single pulse, (g) THF alone followed by HCN in the second pulse, and (h) THF alone followed by HCl. It is noteworthy that guest molecules of any second pulse are not premixed with vapors of the other guests but occupy the CH particles only through rapid transport into pre-existing CH aerosol particles available from the first pulse.…”

Raman Spectroscopic Studies of Clathrate Hydrate Formation in the Presence of Hydrophobized Particles

“…Experimental studies of clathrate nucleation also reflect a wide variety of pathways by which it can occur. For example, Devlin et al 14 have shown that hydrates can form from all-vapour mixtures at sufficiently large driving forces (which are comparable or greater than those used in MD simulations) via the formation of nanodroplets. Hawtin et al 15 reviewed the use of technologies such as neutron scattering, [16][17][18] nuclear magnetic resonance, and X-ray tomography 19 .…”