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1. Understanding the effect of moderate concentration SDS on CO2 hydrates growth in the presence of THF

   https://doi.org/10.1016/j.jcis.2023.11.136  

   Cai, Xinrui; Worley, Joshua; Phan, Anh; Salvalaglio, Matteo; Koh, Carolyn; Striolo, Alberto (March 2024, Journal of Colloid and Interface Science)

   Hypothesis: Additives like Tetrahydrofuran (THF) and Sodium Dodecylsulfate (SDS) improve Carbon Dioxide (CO2) hydrates thermal stability and growth rate when used separately. It has been hypothesised that combining them could improve the kinetics of growth and the thermodynamic stability of CO2 hydrates. Simulations and Experiments: We exploit atomistic molecular dynamics simulations to investigate the combined impact of THF and SDS under different temperatures and concentrations. The simulation insights are verified experimentally using pendant drop tensiometry conducted at ambient pressures and high-pressure differential scanning calorimetry. Findings: Our simulations revealed that the combination of both additives is synergistic at low temperatures but antagonistic at temperatures above 274.1 K due to the aggregation of SDS molecules induced by THF molecules. These aggregates effectively remove THF and CO2 from the hydrate-liquid interface, thereby reducing the driving force for hydrates growth. Experiments revealed that the critical micelle concentration of SDS in water decreases by 20% upon the addition of THF. Further experiments in the presence of THF showed that only small amounts of SDS are sufficient to increase the CO2 storage efficiency by over 40% compared to results obtained without promoters. Overall, our results provide microscopic insights into the mechanisms of THF and SDS promoters on CO2 hydrates, useful for determining the optimal conditions for hydrate growth. 

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2. Investigating water/oil interfaces with opto-thermophoresis

   https://doi.org/10.1038/s41467-022-31546-3  

   Kim, Youngsun; Ding, Hongru; Zheng, Yuebing (June 2022, Nature Communications)

   Abstract Charging of interfaces between water and hydrophobic media is a mysterious feature whose nature and origin have been under debate. Here, we investigate the fundamentals of the interfacial behaviors of water by employing opto-thermophoretic tweezers to study temperature-gradient-induced perturbation of dipole arrangement at water/oil interfaces. With surfactant-free perfluoropentane-in-water emulsions as a model interface, additional polar organic solvents are introduced to systematically modify the structural aspects of the interface. Through our experimental measurements on the thermophoretic behaviors of oil droplets under a light-generated temperature gradient, in combination with theoretical analysis, we propose that water molecules and mobile negative charges are present at the water/oil interfaces with specific dipole arrangement to hydrate oil droplets, and that this arrangement is highly susceptible to the thermal perturbation due to the mobility of the negative charges. These findings suggest a potential of opto-thermophoresis in probing aqueous interfaces and could enrich understanding of the interfacial behaviors of water. 

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3. A data-driven and topological mapping approach for the a priori prediction of stable molecular crystalline hydrates

   https://doi.org/10.1073/pnas.2204414119  

   Hong, Richard S.; Mattei, Alessandra; Sheikh, Ahmad Y.; Tuckerman, Mark E. (October 2022, Proceedings of the National Academy of Sciences)

   Predictions of the structures of stoichiometric, fractional, or nonstoichiometric hydrates of organic molecular crystals are immensely challenging due to the extensive search space of different water contents, host molecular placements throughout the crystal, and internal molecular conformations. However, the dry frameworks of these hydrates, especially for nonstoichiometric or isostructural dehydrates, can often be predicted from a standard anhydrous crystal structure prediction (CSP) protocol. Inspired by developments in the field of drug binding, we introduce an efficient data-driven and topologically aware approach for predicting organic molecular crystal hydrate structures through a mapping of water positions within the crystal structure. The method does not require a priori specification of water content and can, therefore, predict stoichiometric, fractional, and nonstoichiometric hydrate structures. This approach, which we term a mapping approach for crystal hydrates (MACH), establishes a set of rules for systematic determination of favorable positions for water insertion within predicted or experimental crystal structures based on considerations of the chemical features of local environments and void regions. The proposed approach is tested on hydrates of three pharmaceutically relevant compounds that exhibit diverse crystal packing motifs and void environments characteristic of hydrate structures. Overall, we show that our mapping approach introduces an advance in the efficient performance of hydrate CSP through generation of stable hydrate stoichiometries at low cost and should be considered an integral component for CSP workflows. 

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4. Analysis of Electrochemical Capture of CO2 From Oceanwater Coupled With Hydrates-Based Seabed Sequestration

   https://doi.org/10.1115/ES2024-132209  

   Hamalian, Mark; Bhati, Awan; Bahadur, Vaibhav (July 2024, American Society of Mechanical Engineers)

   Abstract Novel energy efficient and scalable carbon capture and sequestration technologies are critical to meeting the goals of the Paris Agreement. In this study, we present a first-order system-level assessment of an integrated carbon capture and carbon sequestration plant that couples electrochemical CO2 capture from oceanwater with co-located long-term carbon sequestration as CO2 hydrates (ice-like solids) on the seabed. Separate recent experimental results associated with electrochemical capture and hydrate formation form the basis for this energetics-focused analysis, which evaluates power consumption of all the key components associated with capture and sequestration. Hydrates can be formed from both pure water as well as seawater, and the implications of including a desalination plant to provide pure water for hydrate formation are studied. All analysis is conducted for a 1 plant which captures and sequesters 1 megaton CO2 annually. Our results indicate the carbon capture will consume significantly more energy than carbon sequestration despite the use of a low-energy consuming electrochemical technique. From a sequestration standpoint, there are clear benefits to forming hydrates at high pressures, since the elevated formation rates reduce the number of hydrate formation reactors significantly. It is also seen that the addition of a desalination plant to provide pure water for hydrate formation (which speeds up hydrate formation) will not affect the energetics of the overall process significantly; however the CAPEX and operational aspects of including a desalination plant need to be analyzed in greater detail. Overall, this study seeds a novel CCS concept which can be deployed via decommissioned oil-gas platforms to capture CO2 from surface oceanwater and store CO2 right below on the seabed after appropriate sealing (artificial or natural). 

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5. Cyclopentane hydrate wettability measurements used to evaluate the efficacy of oil natural surfactants using ultra-low volume samples

   https://doi.org/10.1016/j.fuel.2023.129422  

   Stoner, Hannah M.; Moak, Jefferson; Delgado-Linares, Jose G.; Koh, Carolyn A. (January 2024, Fuel)

   Hydrate surface wettability is a fundamental aspect to better understand agglomeration present in oil bearing petroleum pipelines. Coupling these measurements with hydrate film growth gives further information on kinetic effects that may also be present from natural surfactants in different oils. In situ measurements of wettability (quantified by the contact angle) and film growth rates were performed on cyclopentane hydrate surfaces at atmospheric pressure and subcooling of 4 ◦C. Contact angle and film growth results were obtained for the baseline system (pure cyclopentane), one model oil, and seventeen natural oils (diluted to 0.02 vol% in cyclopentane). Results showed a wide variety of contact angles and film growth values where higher asphaltene contents in the oils corresponded to higher contact angles and lower film growth rates, thought to be from better alignment of natural surfactant molecules at the hydrate/hydrocarbon interface. It was also shown for select oils that increasing the oil concentration in the cyclopentane increases the contact angle and decreases the film growth rate compared to the baseline system. For select oils that had higher contact angles, increasing the water content of the system decreases their contact angle and film growth compared to the baseline system. Isolating different oil fractions for select oils also shows which fractions tend to play a larger role in wettability behavior. Typically, the fractions with more surface active components (asphaltene and resins) are shown to contribute to the higher contact angle and slower film growth rates for select oils. Evidence of the competition between film growth and capillary suction of water into the hydrate has been shown, and a mechanistic breakdown of three different transient scenarios has been proposed. Each of these observed interfacial behaviors gives information on what can be expected from larger scale phenomena, including hydrate agglomeration, with very small oil samples. 

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