Interfacial water studies and their relevance for the energy sector

Interfacial water is ubiquitous, and its investigation has attracted the interest of many for both fundamental and applied purposes. This perspective provides a few highlights concerning how molecular and sometimes multi-scale investigations on the properties of interfacial water could be of practical relevance for the energy sector. The discussion focuses on the transport of electrolytes through narrow pores, and on the solubility and transport properties of confined hydrocarbons, with particular attention in attempting to describe how interfacial water can affect such properties, especially within narrow pores. Recent results on the self-assembly of surfactants on heterogeneous surfaces are also briefly discussed. Finally, a few possible directions for future research are suggested. It should be stressed that this is not a comprehensive review on the possible impact of interfacial water studies on the energy sector, but rather a brief commentary on some personal interests, with the intent of stimulating debate

Chemical Promoter Performance for CO2 Hydrate Growth: A Molecular Perspective

Carbon dioxide (CO2) hydrates, which contain a relatively large amount of captured CO2 (almost 30 wt % of CO2 with the balance being water), represent a promising CO2 sequestration option for climate change mitigation. To facilitate CO2 storage via hydrates, using chemical additives during hydrate formation might help to expedite formation/growth rates, provided the additives do not reduce the storage capacity. Implementing atomistic molecular dynamics, we study the impact of aziridine, pyrrolidine, and tetrahydrofuran (THF) on the kinetics of CO2 hydrate growth/dissociation. Our simulations are validated via reproducing experimental data for CO2 and CO2 + THF hydrates at selected operating conditions. The simulated results show that both aziridine and pyrrolidine could perform as competent thermodynamic and kinetic promoters. Furthermore, aziridine seems to exceed pyrrolidine and THF in expediting the CO2 hydrate growth rates under the same conditions. Our analysis unveils direct correlations between the kinetics of CO2 hydrate growth and a combination of the free energy barrier for desorption of CO2 from the hydrate surface and the binding free energy of chemical additives adsorbed at the growing hydrate substrate. The detailed thermodynamic analysis conducted in both hydrate and aqueous phases reveals molecular-level mechanisms by which CO2 hydrate promoters are active, which could help to enable the implementation of CO2 sequestration in hydrate-bearing reservoirs

Self-assembly of mono- and poly-dispersed nanoparticles on emulsion droplets: antagonistic vs. synergistic effects as a function of particle size

In this work, using Dissipative Particle Dynamics simulations, we provide fundamental insights into the self-assembly of nanoparticles (NPs) on droplet surfaces in an oil-in-water emulsion. We highlight the effect of particle size on the arrangement of NPs for different interparticle interactions. NPs of two different sizes were considered. In general, when the NP–NP interaction is changed from repulsive to attractive, a transition in the NP arrangement occurs from weekly-connected networks to clusters of NPs separated by particle-free domains. When NP–NP interactions are strongly attractive, NPs yield small 3D aggregates on the droplet surface. These arrangements seem to agree with experimental observations reported in the literature. In addition, our simulations suggest that small NPs are able to diffuse more easily on the droplet surface, which leads to prompt self-organisation, while large NPs are more likely to form metastable structures, perhaps because of slow mobility and strong adsorption to the interface. Our analysis suggests that thermal fluctuations could provide the activation energy for the small NPs to escape local minima in the free energy landscape. The results obtained for systems containing NPs of two sizes provide evidence of size segregation on the droplet surface, which could be useful when NP self-assemblies are used, for example, to template supra-molecular materials. However, analysis of the simulated trajectories suggests that the results depend strongly on the initial configuration, as the larger NPs seem to impose barriers for the small NPs to adsorb and diffuse on the droplet surface

Quantification of Ostwald Ripening in Emulsions via Coarse-Grained Simulations

Ostwald ripening is a diffusional mass transfer process that occurs in polydisperse emulsions, often with the result of threatening the emulsion stability. In this work, we design a simulation protocol that is capable of quantifying the process of Ostwald ripening at the molecular level. To achieve experimentally relevant time scales, the Dissipative Particle Dynamics (DPD) simulation protocol is implemented. The simulation parameters are tuned to represent two benzene droplets dispersed in water. The coalescence between the two droplets is prevented via the introduction of membranes, which allow diffusion of benzene from one droplet to the other. The simulation results are quantified in terms of the changes of the droplets volume as a function of time. The results are in qualitative agreement with experiments. The agreement with the Lifshitz-Slyozov-Wagner theory becomes quantitative when the simulated solubility and diffusion coefficient of benzene in water are considered. The effect of two different surfactants was also investigated. In agreement with both experimental observations and theory, the addition of surfactants at moderate concentrations decreased the Ostwald ripening rate because of the reduction in the interfacial tension between benzene and water; as the surfactant film becomes dense, other phenomena are likely to further delay Ostwald ripening. In fact, the results suggest that the surfactant that yields higher density at the benzene-water interface delayed more effectively Ostwald ripening. The formation of micelles can also affect the ripening rate, in qualitative agreement with experiments, although our simulations are not conclusive on such effects. Our simulations show that the coarse-grained DPD formalism is able to capture the molecular phenomena related to Ostwald ripening, and reveal molecular-level features that could help to understand experimental observations. The results could be useful for predicting, and eventually controlling the long-term stability of emulsions

Numerical analysis of Pickering emulsion stability: insights from ABMD simulations

The issue of the stability of Pickering emulsions is tackled at a mesoscopic level using dissipative particle dynamics simulations within the Adiabatic Biased Molecular Dynamics framework. We consider the early stage of the coalescence process between two spherical water droplets in a decane solvent. The droplets are stabilized by Janus nanoparticles of different shapes (spherical and ellipsoidal) with different three-phase contact angles. Given a sufficiently dense layer of particles on the droplets, we show that the stabilization mechanism strongly depends on the collision speed. This is consistent with a coalescence mechanism governed by the rheology of the interfacial region. When the system is forced to coalesce sufficiently slowly, we investigate at a mesoscopic level how the ability of the nanoparticles to stabilize Pickering emulsions is discriminated by nanoparticle mobility and the associated caging effect. These properties are both related to the interparticle interaction and the hydrodynamic resistance in the liquid film between the approaching interfaces

Buckling in armored droplets

The buckling mechanism in droplets stabilized by solid particles (armored droplets) is tackled at a mesoscopic level using dissipative particle dynamics simulations. We consider one spherical water droplet in a decane solvent coated with nanoparticle monolayers of two different types: Janus (particles whose surface shows two regions with different wetting properties) and homogeneous. The chosen particles yield comparable initial three-phase contact angles, selected to maximize the adsorption energy at the interface. We study the interplay between the evolution of droplet shape, layering of the particles, and their distribution at the interface when the volume of the droplets is reduced. We show that Janus particles affect strongly the shape of the droplet with the formation of a crater-like depression. This evolution is actively controlled by a close-packed particle monolayer at the curved interface. In contrast, homogeneous particles follow passively the volume reduction of the droplet, whose shape does not deviate too much from spherical, even when a nanoparticle monolayer/bilayer transition is detected at the interface. We discuss how these buckled armored droplets might be of relevance in various applications including potential drug delivery systems and biomimetic design of functional surfaces

Evidence of Facilitated Transport in Crowded Nano-Pores

Fluid transport in nature often occurs through crowded nanopores, where a number of phenomena can affect it, because of fluid−fluid and fluid−solid interactions, as well as the presence of organic compounds filling the pores and their structural fluctuations. Employing molecular dynamics, we probe here the transport of fluid mixtures (CO2−CH4 and H2S−CH4) through silica nanopores filled with benzene. Both CO2 and H2S are strongly adsorbed within the organic-filled pore, partially displacing benzene. Unexpectedly, CO2/H2S adsorption facilitates CH4 transport. Analysis of the trajectories suggests that both CO2 and H2S act as vehicle-like carriers and might swell benzene, generating preferential transport pathways within the crowded pore. The results are useful for identifying unexpected transport mechanisms and for developing engineering approaches that could lead to storage of CO2 in caprocks

Equimolar mixtures of aqueous linear and branched SDBS surfactant simulated on single walled carbon nanotubes

In our previous simulation study [J. Phys. Chem. C, 2011, 115, 17286], branched sodium dodecyl benzene-sulfonate (SDBS) surfactants showed self-assembled structures on single-walled carbon nanotubes (SWNTs) that were strongly dependent on tube diameter. Those results suggested that branched SDBS, as opposed to their linear counterparts, could specifically stabilize SWNTs of narrow diameter. Experimental data, however, show that SDBS stabilizes aqueous SWNTs of many diameters. This discrepancy between simulated and experimental results could be explained by the fact that experimental SDBS samples are isomeric mixtures. To test this possibility we report here molecular dynamics (MD) simulation results for equimolar mixtures of aqueous linear and branched SDBS on (6,6) and (20,20) SWNTs at ambient conditions. Our results suggest that there is no strong effect due to nanotube diameter on the morphology of mixed SDBS surfactant aggregates, although the adsorbed aggregate structure strongly depends on surfactant coverage. In-plane radial distribution functions suggest that linear and branched molecules distribute evenly onto the surfaces of (6,6) SWNTs, while some evidence of segregation, in which branched SDBS predominantly pack near other branched molecules, was obtained on (20,20) SWNTs at high surface coverage. These results suggest that the lack of specificity in stabilizing aqueous dispersions of carbon nanotubes using SDBS surfactants is probably due to the presence of multiple isomeric molecules in commercial surfactant samples. Perhaps more importantly, these simulations suggest that using mixtures of surfactants could affect the structure of the adsorbed aggregates, and the stability of aqueous dispersion of carbon nanotubes

Nanoparticles shape-specific emergent behaviour on liquid crystal droplets

Self-assembly attracts enormous research attention because it is at the core of important applications ranging from medical treatments to renewable energy production. Among several classes of self-assembling materials, liquid crystals (LCs) and nanoparticles yield ordered structures under well-defined thermodynamic conditions and could yield supra-molecular aggregates, respectively. In this work, nanoparticle self-assembly on LC nano-droplets is investigated. The LC nano-droplets act as templating agents on which homogeneous and Janus nanoparticles of various geometrical features are adsorbed. LC mesogens and water have low mutual solubility, and under the conditions chosen the LCs yield bipolar nano-droplets. Particle self-assembly on oil nano-droplets is also considered for comparison. Our results reveal that the mesogens can direct the assembly of the nanoparticles. This effect is mainly governed by the nanoparticle size and shape. In some cases, strong evidence of emergent behaviour is observed depending on entropic forces that arise because of the shape and patchiness of the nanoparticles. For example, while one small spherical homogeneous particle does not show preferential adsorption on specific LC nano-droplet locations, 100 spherical nanoparticles preferentially agglomerate at the nano-droplet boojums, providing evidence of emergent behaviour. On the contrary, Janus spherical nanoparticles do not show such a strong emergent behaviour. Cylindrical NPs manifest the opposite trend: while homogeneous nano-cylinders do not exhibit orientational order on the LC nano-droplet, Janus ones either locate at the LC nano-droplet boojums or orient towards the direction vector of bipolar droplets. Quantification of the orientational order within the LC nano-droplets suggests that the self-assembly of the LC mesogens does not significantly change upon nanoparticle adsorption. These simulations clearly suggest an interplay between nanoparticle size, shape and chemical composition upon their self-assembly on LC nano-droplets. The results could be helpful for the design of new sensors and for the directed self-assembly of advanced materials

Aqueous films on pore surfaces mediate adsorption and transport of gases through crowded nanopores

Interactions of trapped reservoir gases within organic-rich and brine-bearing sedimentary rocks have direct relevance to many geoenergy applications. Extracting generalizable information from experimental campaigns is hindered by the fact that geological systems are extremely complex. However, modern computational tools offer the opportunity of studying systems with controlled complexity, in an effort to better understand the mechanisms at play. Employing molecular dynamics, we examine here adsorption and transport of gases containing CH4 and either CO2 or H2S within amorphous silica nanopores filled with benzene. We explicitly quantify the effect of small amounts of water/brines at geological temperature and pressure conditions. Because of wetting, the presence of brines lessens the adsorption capacity of the aromatic-filled pore. The simulation results show salt-specific effects on the transport properties of the gases when either KCl or CaCl2 brines are considered, although adsorption was not affected. The acid gases considered either facilitate or hinder CH4 transport depending on whether they are more or less preferentially adsorbed within the pore as compared to benzene, and this effect is mediated by the presence of water/brines. Our simulation results could be used to extract thermodynamic quantities that in the future will help to optimize transport of various gases through organic-rich and brine-bearing sedimentary rocks, which is likely to have a positive impact on both hydrocarbon production and carbon sequestration applications. As a first step, a phenomenological model is presented here, which allows one to predict permeability based on interatomic energies