Induced Charge Density and Thin Liquid Film at Hydrate/Methane Gas Interfaces

The hydrate/methane gas interface is studied by molecular dynamics simulations. Below the hydrate melting temperature a thin liquid film forms with an associated surface charge density and electrostatic potential. The thickness of the thin liquid film, the charge density, and electrostatic potential at the hydrate/gas interface are established at different subcooling temperatures for the first time. The hydrate interface has mixed polarity, being predominantly positive. A comparison is made with the ice/methane interface, which reveals similarities and differences in the induced charge density. The thin liquid film and the induced charge density have important implications for the interfacial properties of methane hydrates

Hydrophobic Hydration and the Effect of NaCl Salt in the Adsorption of Hydrocarbons and Surfactants on Clathrate Hydrates

Adsorption of functional molecules on the surface of hydrates is key in the understanding of hydrate inhibitors. We investigate the adsorption of a hydrocarbon chain, nonionic and ionic surfactants, and ions at the hydrate–aqueous interface. Our results suggest a strong connection between the water ordering around solutes in bulk and the affinity for the hydrates surface. We distinguish two types of water ordering around solutes: (i) hydrophobic hydration where water molecules form a hydrogen bond network similar to clathrate hydrates, and (ii) ionic hydration where water molecules align according to the polarity of an ionic group. The nonionic surfactant and the hydrocarbon chain induce hydrophobic hydration and are favorably adsorbed on the hydrate surface. Adsorption of ions and the ionic headgroups on the hydrate surface is not favorable because ionic hydration and the hydrogen bond structure of hydrates are incompatible. The nonionic surfactant is adsorbed by the headgroup and tail while adsorption of the ionic surfactants is not favorable through the head. Water ordering is analyzed using the hydrogen bond and tetrahedral density profiles as a function of the distance to the chemical groups. The adsorption of solutes is studied through the free energy profiles as a function of the distance to the hydrate surface. Salt lowers the melting temperature of hydrates, disrupts hydrophobic hydration, reduces the solubility of solutes in the aqueous solution, and increases the propensity of solutes to be adsorbed on hydrate surfaces. Our studies are performed by the unbiased and steered molecular dynamics simulations. The results are in line with experiments on the effect of salt and alkanes in hydrate antiagglomeration

Contact Angle, Liquid Film, and Liquid–Liquid and Liquid–Solid Interfaces in Model Oil–Brine–Substrate Systems

Oil–water–substrate wettability is of prime importance in most branches of science and technology, from biology and nanomaterials to geology and petroleum science. Wetting is a three-phase interaction phenomenon as expressed in Young’s equation. Microscopically wetting is from the fluid–substrate interactions and surfaces are designated as lyophilic and lyophobic (fluid-wet and nonwet). Here we investigate the microscopic mechanisms of wettability changes by salt concentration in oil–water–mineral substrate systems. A model oil droplet (<i>n</i>-decane) placed in an aqueous electrolytic solution next to a solid substrate surface (muscovite mica) is simulated. A thin water layer between oil and the substrate regulates the oil–substrate interaction. We find that at zero and low salt concentrations, the oil adsorption on the hydrophilic mineral substrate is stabilized by a thin layer of water giving rise to a nonzero contact angle (partial oil wetting). As the salt concentration increases ionic adsorption and the water layer thickness increase reducing the oil–substrate wettability. Ions adsorb unsymmetrically on the substrate and promote water adsorption into the water layer. Ionic adsorption is higher away from the droplet than under the droplet. Our contact angles by molecular dynamics simulations are in agreement with experimental measurements

Nucleation of Methane Hydrates at Moderate Subcooling by Molecular Dynamics Simulations

Methane hydrates are crystalline structures composed of cages of hydrogen-bonded water molecules in which methane molecules are trapped. The nucleation mechanisms of crystallization are not fully resolved, as they cannot be accessed experimentally. For methane hydrates most of the reported simulations on the phenomena capture some of the basic elements of the full structure. In few reports, formation of crystalline structures is reached by imposing very high pressure, or dynamic changes of temperature, or a pre-existing hydrate structure. In a series of nanoscale molecular dynamics simulations of supersaturated water–methane mixtures, we find that the order of the crystalline structure increases by decreasing subcooling. Crystalline structures I and II form and coexist at moderate temperatures. Crystallization initiates from the spontaneous formation of an amorphous cluster wherein structures I, II, and other ordered defects emerge. We observe the transient coexistence of sI and sII in agreement with experiments. Our simulations are carried out at high methane supersaturation. This condition dramatically reduces the nucleation time and allows simulating nucleation at moderate subcooling. Moderate temperatures drive hydrates to more ordered structures

Tunable Substrate Wettability by Thin Water Layer

In oil–water–mineral substrate systems, we show that the contact angle can be tuned by ionic structures in the water layer confined between an oil droplet and the substrate. We perform molecular dynamics simulations of a complex oil droplet in a NaCl aqueous solution on a mica surface; the oil is a mixture of <i>n</i>-decane and surfactant molecules. The surfactant head contains an OH group and an aromatic ring. A thin water layer between the oil droplet and the substrate and ionic stratification regulate the wetting behavior. The concentration of salt ions in the thin film is nonmonotonic; it first increases, then decreases, and starts increasing again as the salt concentration in the bulk increases. On the other hand, the surfactant head adsorption in the thin film first increases as the bulk salt concentration increases. Then, it decreases with further increase in the bulk salt concentration. The change of contact angle with salt concentration also shows a nonmonotonic behavior; the contact angle is first nearly constant to a low salt concentration of 0.1 wt % NaCl. Then, it decreases sharply as the salt concentration increases from 0.1 to 1.1 wt % NaCl. A reverse trend in contact angle follows with further salt concentration increase. The nonmonotonic trend unlike the monotonic trend of interfacial tension with salt concentration is in line with recent measurements of contact angle of oil–brine–substrate systems. A sharp increase of surfactant head adsorption in the thin film, the decrease of ion adsorption, and the minimum of contact angle are all related. This is the first report of such correlations with change of wetting in the brine–complex oil–mineral substrate predicted from molecular simulations

Enhanced Hydrate Nucleation near the Limit of Stability

Clathrate hydrates are crystalline structures composed of small guest molecules trapped into cages formed by hydrogen-bonded water molecules. In hydrate nucleation, water and the guest molecules may stay in a metastable fluid mixture for a long period. Metastability is broken if the concentration of the guest is above a certain limit. Here we study propane hydrates by means of molecular dynamics simulations. First we simulate three-phase equilibrium of water, propane, and propane hydrates; the simulated melting temperature and solubility of propane in water are agreement with experimental measurements. In the main part we simulate hydrate nucleation in water–propane supersaturated solutions. At moderate temperatures we show that hydrate nucleation can be very fast in a very narrow range of composition, namely, close to the limit of stability. Propane density fluctuations near the fluid–fluid demixing are coupled with crystallization, producing enhanced nucleation rates. This is the first report of propane-hydrate nucleation by molecular dynamics simulations. We observe motifs of the crystalline structure II in line with experiments and new hydrate cages not reported in the literature. Our study relates nucleation to the fluid–fluid spinodal decomposition and demonstration that the enhanced nucleation phenomenon is more general than short-range attractive interactions as suggested in nucleation of proteins

Hydrophobic Hydration and the Effect of NaCl Salt in the Adsorption of Hydrocarbons and Surfactants on Clathrate Hydrates

Adsorption of functional molecules on the surface of hydrates is key in the understanding of hydrate inhibitors. We investigate the adsorption of a hydrocarbon chain, nonionic and ionic surfactants, and ions at the hydrate–aqueous interface. Our results suggest a strong connection between the water ordering around solutes in bulk and the affinity for the hydrates surface. We distinguish two types of water ordering around solutes: (i) hydrophobic hydration where water molecules form a hydrogen bond network similar to clathrate hydrates, and (ii) ionic hydration where water molecules align according to the polarity of an ionic group. The nonionic surfactant and the hydrocarbon chain induce hydrophobic hydration and are favorably adsorbed on the hydrate surface. Adsorption of ions and the ionic headgroups on the hydrate surface is not favorable because ionic hydration and the hydrogen bond structure of hydrates are incompatible. The nonionic surfactant is adsorbed by the headgroup and tail while adsorption of the ionic surfactants is not favorable through the head. Water ordering is analyzed using the hydrogen bond and tetrahedral density profiles as a function of the distance to the chemical groups. The adsorption of solutes is studied through the free energy profiles as a function of the distance to the hydrate surface. Salt lowers the melting temperature of hydrates, disrupts hydrophobic hydration, reduces the solubility of solutes in the aqueous solution, and increases the propensity of solutes to be adsorbed on hydrate surfaces. Our studies are performed by the unbiased and steered molecular dynamics simulations. The results are in line with experiments on the effect of salt and alkanes in hydrate antiagglomeration

Molecular Dynamics Simulation of the Adsorption and Aggregation of Ionic Surfactants at Liquid–Solid Interfaces

Structure of surfactants adsorbed on solid surfaces is a key knowledge in various technologies and applications. It is widely accepted in the literature that the surface–surfactant headgroup electrostatic interaction is a major driving force of adsorption of ionic surfactants on charged substrates. Our result shows that the adsorption of surfactants as monomers is driven by both electrostatic and nonelectrostatic interactions. Further adsorption of surfactants in aggregates is essentially driven by the tail–tail interaction. To a great extent, the substrate–tail interaction determines the structures of the adsorbed surfactant aggregates. Water and counterions influence the headgroup–substrate and tail–substrate interactions. We investigate two vastly different surfactants and substrates by molecular dynamics simulations: (1) SDS on alumina (SDS–Al₂O₃), and (2) CTAB on silica (CTAB–SiO₂). We study the adsorption of a single surfactant at the solid surface by the density profiles and free energy of adsorption. In the SDS–Al₂O₃ system, we analyze the free energy of adsorption on the substrate covered by aggregates of different sizes. We examine the configurations of surfactants and the distribution of water and ions at the liquid–solid interface as the number of adsorbed molecules on the substrate increases. In the SDS–Al₂O₃ system, the headgroup adsorption is mediated by the Na⁺ counterions; the adsorbed water molecules may be displaced by the surfactant headgroup but unlikely by the hydrocarbon tails. As a function of the surfactant adsorption, we observe single surfactants, aggregates of different morphologies, and bilayers. The CTAB–SiO₂ system combines both electrostatic attraction of the surfactant headgroup and affinity for the surfactant’s hydrocarbon tail. At low surfactant adsorption, aggregates and single surfactant molecules lie on the substrate; hemimicelles form at intermediate adsorption; and micelles form at high surfactant adsorption. Our results agree with experimental observations and indicate two different surfactant adsorption mechanisms where the tail–tail and tail–substrate interactions play a fundamental role