Formation of Methane Hydrate in the Presence of Natural and Synthetic Nanoparticles.

Natural gas hydrates occur widely on the ocean-bed and in permafrost regions, and have potential as an untapped energy resource. Their formation and growth, however, poses major problems for the energy sector due to their tendency to block oil and gas pipelines, whereas their melting is viewed as a potential contributor to climate change. Although recent advances have been made in understanding bulk methane hydrate formation, the effect of impurity particles, which are always present under conditions relevant to industry and the environment, remains an open question. Here we present results from neutron scattering experiments and molecular dynamics simulations that show that the formation of methane hydrate is insensitive to the addition of a wide range of impurity particles. Our analysis shows that this is due to the different chemical natures of methane and water, with methane generally excluded from the volume surrounding the nanoparticles. This has important consequences for our understanding of the mechanism of hydrate nucleation and the design of new inhibitor molecules.

Surfactin structures at interfaces and in solution: the effect of pH and cations.

We have investigated the interfacial and bulk phase structures of surfactin at different pH and in the presence of mono/divalent cations using neutron scattering techniques. Neutron reflectivity profiles were recorded at the air/water and sapphire/water interfaces as a function of pH and ionic strength. The air/water results show that surfactin has a hydrophobic ball-like structure and that changes in pH and cations lead to changes in the area per molecule and hydrophilicity of surfactin. However the adsorption of surfactin on the hydrophilic sapphire/water interface is highly pH dependent because of electrostatic interactions with the surface. The bulk phase structures were characterized by small angle neutron scattering (SANS) and are more sensitive to pH and cation than the interfacial structure. At high pH surfactin forms micellar structures with low aggregation numbers, but at low pH values the bulk phase structure becomes rod-like at pH 6.5 and lamellar at pH 5.5. The addition of cations in the subphase tends to neutralize the two acidic groups of the peptide ring and the neutralization seems to be more complete for divalent cations than for monovalent cations.