Molecular Dynamics Study on the Spontaneous Adsorption of Aromatic Carboxylic Acids to Methane Hydrate Surfaces: Implications for Hydrate Antiagglomeration

Abstract

Spontaneous adsorption of aromatic carboxylic acids (phenylacetic acid, 2-napthylacetic acid, and 1-pyreneacetic acid) to the CH4 hydrate surface in both liquid hydrocarbon and aqueous phases has been investigated using molecular dynamics simulations, aiming to provide implications for hydrate antiagglomeration. Simulation results indicate that the liquid-phase environment, that is, the liquid hydrocarbon phase or aqueous phase, especially its hydrophilic/hydrophobic property, could profoundly affect the interfacial structures of CH4 hydrate and the adsorption behavior of aromatic carboxylic acids. In the hydrophobic hydrocarbon phase, with many CH4 molecules dissolved, more interfacial hydrate structures decompose and form a thin quasiliquid water film on the hydrate surface; aromatic carboxylic acids act as surfactants, that is, strongly adsorb to the hydrate/hydrocarbon interface and significantly lower the interfacial tension. Moreover, they adsorb to the interfacial water film on the hydrate surface with their carboxylic groups, which may destabilize the capillary liquid bridges formed among hydrate particles and then prevent hydrate coalescence. By contrast, fewer interfacial hydrate structures decompose in the aqueous phase, as CH4 molecules rarely dissolve in water but stay at the hydrate/water interface and stabilize the hydrate solid; only a few aromatic carboxylic acids adsorb to the hydrate/water interface by inserting their aromatic rings into the semicages on the hydrate surface, which may kinetically disturb the hydrate growth. Such adsorption is not very strong and mainly depends on the size matching between aromatic rings and semicages. Consequently, many more aromatic carboxylic acid molecules strongly adsorb to the hydrate surface in the hydrocarbon phase than in the aqueous phase, which can explain why antiagglomerants generally show a higher performance in the hydrocarbon phase and easily lose efficacy at high watercuts. Additionally, the molecular structures could also affect the adsorption behavior of aromatic carboxylic acids: with more aromatic rings, acid molecules can form stable aggregates via the π–π stacking interactions of the aromatic rings, adversely affecting the adsorption in the aqueous phase.

Methane Hydrate Formation in the Salty Water Confined in Clay Nanopores: A Molecular Simulation Study

“…CH 4 hydrate formation in the porous clay minerals in the seafloor is affected by many factors, such as the pH and salinity of pore water, surface charge, and organic matters, which can impact the nucleation time and formation kinetics. − Experimental studies revealed that the formation conditions of CH 4 hydrates in clay minerals are milder than those in pure water, indicating that clay minerals can promote hydrate formation. − Cha and co-workers found that the promoting effect of clay minerals may be because the surfaces of solid particles can serve as nucleation sites . Other experimental studies also found that the confined environments and surface properties of clay minerals could significantly influence the formation conditions and stability of gas hydrates. − Molecular dynamics (MD) simulation has been frequently applied to investigate the microscopic behavior of gas hydrates in bulk and in nanopores. − The MD study of Yan et al reported that a higher gas–water ratio and larger nanopore size can promote the diffusion of CH 4 molecules and shorten the nucleation time of gas hydrates. , By using MD simulation, Li and co-workers revealed the significant effects of surface properties, solution cations, and surface charges of clay minerals on CH 4 hydrate formation. − Recent experimental and MD studies indicated that gas hydrate formation in nano-confined environments is much more complex than that in the bulk phase. − On the other hand, salt ions in seawater can adsorb to clay surfaces and affect their physical and chemical properties . Cations and anions are reported to selectively adsorb to different basal surfaces of clay minerals .…”

Outlook on the Role of Natural Surfactants on Emulsion Stability and Gas Hydrate Antiagglomeration in Crude Oil Systems

“…Acids and Hydrates. Several authors 31,43,196,197,207,208 report the importance of crude oil acids on hydrate antiagglomeration. Some of them state that acids change hydrate wettability from water-wet to oil-wet and consequently reduce hydrate agglomeration and plugging.…”

Competition between CH4 hydrate formation and phase separation in a wetted metal–organic framework MIL-101 at moderate subcooling: molecular insights into CH4 storage

“…24 Several MD simulation studies explored the nucleation and growth of CH 4 hydrate in the slit-nanopores of silica or clay to reveal the effects of geological factors on hydrate formation in marine sediments, including sediment solid surfaces, seawater ions, and organic matter. [25][26][27][28] Recent experimental studies inferred that the crystal structure and stability of CH 4 hydrate conned in sediments might be mainly affected by the hydrophilic/hydrophobic properties of sediment surfaces. 29 Despite many achievements, the microscopic mechanism of hydrate formation in porous materials is still not fully elucidated.…”