No abstract

The specific surface free energy (γ sv ) of a solid is the work required to create a unit area of a solid in a vacuum. γ sv is a fundamental physical quantity that plays a central role in cohesion, adhesion, and premelting of a solid and other phenomena like the nucleation work of a solid phase. However, due to experimental difficulties, no specific surface free energy values have been reported for clathrate hydrates. Here we applied the "Zisman plot" to the determination of the critical surface tension, γ c , of ice and tetrahydrofuran (THF) hydrate. The γ c value of ice was found to be very similar to the value reported by Adamson et al. (J. Colloid Interface Sci. 1970, 34, 461−468) for the same homologous series. Then we used the measured γ c value of THF hydrate and the γ sv values of ice reported in the literature to deduce the γ sv value of THF hydrate. The deduced γ sv value of THF hydrate was somewhat lower than that of ice and in the range from 60.4 to 124.2 mJ/m 2 , with an average of 92.3 mJ/m 2 .

Nucleation kinetics of clathrate hydrate is poorly understood because of the difficulties in determining the essential parameter, nucleation rate. Nucleation rate enables quantitative comparisons of the impacts of various additives and solid walls on nucleation. We made a setup that determines the nucleation curves of structure Iforming CO2 hydrate in quiescent quasi-free water droplets supported by a bulk of liquid perfluoromethyldecalin under isobaric conditions. The results were compared to the nucleation rates of CO2 hydrate in quiescent water that was in direct contact with stainless-steel walls. We assessed the convergence of the nucleation curves with the increasing numbers of nucleation data and compared our results to the nucleation rates of methane/propane mixed gas hydrate in quiescent quasi-free water droplets and the nucleation rates of gas hydrates of various guest types in the presence of solid walls reported in the literature. We found that (1) 400 nucleation events were sufficient to construct a reliable nucleation curve; (2) the addition of stainless-steel walls promoted the nucleation kinetics of CO2 hydrate, as it did to methane/propane mixed gas hydrate; (3) the kinetic parameter was significantly lower than the theoretically expected value, whereas the thermodynamic parameter was comparable to both the theoretically expected value and the experimentally determined value reported in the literature; and (4) CO2 hydrate nucleated over a substantially shallower supercooling range and had higher nucleation rates than those of methane/propane mixed gas hydrate, both in the presence and in the absence of a solid wall.

The adhesive force of gas hydrates to a solid surface is an important factor that influences the likelihood of a blockage of oil and natural gas pipelines by a hydrate plug. In this work, we investigated the adhesive shear strength of Structure II (sII)-forming tetrahydrofuran (THF) hydrate on substrates with different hydrophobicity. Five solid substrates with the contact angle of water in air ranging from 0° to 117 ± 3° were used. The critical shear stress (tangential adhesive strength) of THF hydrate was compared to that of tetrabutylammonium bromide (TBAB) semiclathrate hydrate on the same five substrates. We also investigated the impact of a common kinetic hydrate inhibitor (KHI), polyvinylpyrrolidone (PVP), on the critical shear stress of these hydrates. The results showed that the critical shear stress diminished with heating and became unmeasurably small just below the dissociation temperature of the hydrates. This lowering of the critical shear stress with heating is in contrast to the normal adhesive strength between a gas hydrate particle and a solid surface that has been known to increase with heating due to capillary action. Addition of PVP increased the critical shear stress of both THF clathrate hydrate and TBAB semiclathrate hydrate on all substrates. Interestingly, the critical shear stress of the hydrates was higher on the hydrophobic substrates than on the hydrophilic ones. Our results imply that the quasi-liquid layer may have acted as an effective lubricant layer that reduced the static friction. Our findings may be valuable to flow assurance related to hydrate deposition on pipeline walls during hydrodynamic transport.