Surface Premelting and Interfacial Interactions of Semi-Clathrate Hydrate

Abstract

Clathrate hydrates are a demanding field of research because of their fundamental impact on human life and the environment. Interfacial forces involving hydrates are vital factors in numerous natural and industrial processes, but the topic has been relatively neglected in the scientific literature. By using atomic force microscopy, we are now able to measure for the first time the forces between a semi-clathrate hydrate formed by tetrabutyl ammonium bromide and a silica microsphere in air and in undecane. Significantly, the analyses of jump-in (attractive) and pull-off (adhesive) forces in force–distance curves indicate the presence of a quasi-liquid layer (QLL) on the hydrate surface resulting from interfacial premelting. We have shown that interfacial forces between the sphere and the hydrate surface are dominated by the capillary formation between the QLL and the sphere. van der Waals forces are still active in the region several nm above the QLL, prior to the sphere touching the QLL. Our study introduces a new quantitative parameter to the hydrate literature, that is, the thickness of the QLL on the hydrate surface. This parameter, being around 11 nm at −4 °C, is crucial to the modeling of hydrate interactions and agglomerations.

Clathrate Adhesion Induced by Quasi-Liquid Layer

“…Our recent experiments showed δ ≈ 10 nm for the QLL on the surface of semiclathrate formed by tetrabutyl ammonium bromide at −4 °C. 7 This value (10 nm) agrees with typical thickness of QLL on pristine ice surfaces at temperatures of several minus degrees centigrade. 41 , 42 Therefore, we used δ = 10 nm as a representative and constant value for the clathrate in our model, without considering a likely temperature dependence of the QLL thickness.…”

“…In this region, the QLL thickness is on the order of 10 nm. 7 , 41 , 42 We recorded optical images close to the maximum force values in both premelting and dissociated regimes (insets in Figure 3 a). For the dissociated clathrate at 4 °C, the liquid deforms visually over a large vertical range and creates a macroscopic capillary bridge.…”

Premelting-Induced Agglomeration of Hydrates: Theoretical Analysis and Modeling

“…From then on, capillary forces dominate. 19 Figure 4 depicts the concepts of capillary interactions. When the hydrate sphere attaches to the substrate, the quasi-liquid in the volume V 1 is squeezed out and forms a meniscus which is indicated by green color.…”

“…17,18 However, the recent literature indicates that hydrate surfaces are likely not solid but rather quasi-liquid because of premelting. 19,20 Therefore, classical hard-sphere models can be insufficient for modeling the interactions of hydrates. In particular, a quasi-liquid layer (QLL) was found which alters the interfacial forces of hydrate surfaces.…”

“…In particular, a quasi-liquid layer (QLL) was found which alters the interfacial forces of hydrate surfaces. 19 In this case, forces between a silica microsphere and a tetra-butyl ammonium bromide semi-clathrate hydrate surface are essentially dominated by capillary attraction and are much larger than van der Waals (vdW) forces. 19 Hence, the theory of hydrate interactions has to be revisited as capillary attraction cannot be neglected.…”