Wetting Properties of Clathrate Hydrates in the Presence of Polycyclic Aromatic Compounds: Evidence of Ion-Specific Effects

Polycyclic aromatic hydrocarbons (PAHs) have attracted remarkable multidisciplinary attention due to their intriguing π–π stacking configurations, showing enormous opportunity for their use in a variety of advanced applications. To secure progress, detailed knowledge on PAHs’ interfacial properties is required. Employing molecular dynamics, we probe the wetting properties of brine droplets (KCl, NaCl, and CaCl2) on sII methane–ethane hydrate surfaces immersed in various oil solvents. Our simulations show synergistic effects due to the presence of PAHs compounded by ion-specific effects. Our analysis reveals phenomenological correlations between the wetting properties and a combination of the binding free-energy difference and entropy changes upon oil solvation for PAHs at oil/brine and oil/hydrate interfaces. The detailed thermodynamic analysis conducted upon the interactions between PAHs and various interfaces identifies molecular-level mechanisms responsible for wettability alterations, which could be applicable for advancing applications in optics, microfluidics, biotechnology, medicine, as well as hydrate management

Microscopic insights on clathrate hydrate growth from non-equilibrium molecular dynamics simulations

Clathrate hydrates form and grow at interfaces. Understanding the relevant molecular processes is crucial for developing hydrate-based technologies. Many computational studies focus on hydrate growth within the aqueous phase using the 'direct coexistence method', which is limited in its ability to investigate hydrate film growth at hydrocarbon-water interfaces. To overcome this shortcoming, a new simulation setup is presented here, which allows us to study the growth of a methane hydrate nucleus in a system where oil-water, hydrate-water, and hydrate-oil interfaces are all simultaneously present, thereby mimicking experimental setups. Using this setup, hydrate growth is studied here under the influence of two additives, a polyvinylcaprolactam oligomer and sodium dodecyl sulfate, at varying concentrations. Our results confirm that hydrate films grow along the oil-water interface, in general agreement with visual experimental observations; growth, albeit slower, also occurs at the hydrate-water interface, the interface most often interrogated via simulations. The results obtained demonstrate that the additives present within curved interfaces control the solubility of methane in the aqueous phase, which correlates with hydrate growth rate. Building on our simulation insights, we suggest that by combining data for the potential of mean force profile for methane transport across the oil-water interface and for the average free energy required to perturb a flat interface, it is possible to predict the performance of additives used to control hydrate growth. These insights could be helpful to achieve optimal methane storage in hydrates, one of many applications which are attracting significant fundamental and applied interests

Cyclopentane hydrate slurry viscosity measurements coupled with visualization

One of the advanced strategies in hydrate plug prevention is to obtain an in-depth knowledge of the rheological properties of hydrate slurries. A major challenge in hydrate rheological measurements is that the viscosity profile can be difficult to attribute to physical phenomena, such as particle agglomerate breakup, particle bedding/settling and wall growth. In this work, a novel visual rheometer has been developed to help overcome these previous limitations by enabling the visualisation of the evolution of cyclopentane hydrate slurries during viscosity measurements. Two different model systems were used in this investigation: (1) non-emulsified and (2) emulsified systems. The physical phenomena, including initial hydrate formation, hydrate wall growth, bedding and sloughing were visually observed and directly correlated to the corresponding viscosity profile. For the non-emulsified system, there are four different stages of hydrate slurry development including initial hydrate formation, wall adhesion and growth and sloughing that caused changes in the viscosity profiles. Large fluctuations in the viscosity profile for a non-emulsified system were found to be the result of a sloughing phenomenon. On the other hand, the emulsified system showed a well-dispersed hydrate slurry with minimal wall and impeller growth, corresponding to a smooth viscosity profile