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

Polymer/surfactant interactions at the air/water interface

The development of neutron reflectometry has transformed the study and understanding of polymer/surfactant mixtures at the air/water interface. A critical assessment of the results from this technique is made by comparing them with the information available from other techniques used to investigate adsorption at this interface. In the last few years, detailed information about the structure and composition of adsorbed layers has been obtained for a wide range of polymer/surfactant mixtures, including neutral polymers and synthetic and naturally occurring polyelectrolytes, with single surfactants or mixtures of surfactants. The use of neutron reflectometry together with surface tensiometry, has allowed the surface behaviour of these mixtures to be related directly to the bulk phase behaviour. We review the broad range of systems that have been studied, from neutral polymers with ionic surfactants to oppositely charged polyelectrolyte/ionic surfactant mixtures. A particular emphasis is placed upon the rich pattern of adsorption behaviour that is seen in oppositely charged polyelectrolyte/surfactant mixtures, much of which had not been reported previously. The strong surface interactions resulting from the electrostatic attractions in these systems have a very pronounced effect on both the surface tension behaviour and on adsorbed layers consisting of polymer/surfactant complexes, often giving rise to significant surface ordering

The adsorption of oppositely charged polyelectrolyte/surfactant mixtures: Neutron reflection from dodecyl trimethylammonium bromide and sodium poly(styrene sulfonate) at the air/water interface

The interactions between the cationic surfactant dodecyl trimethylammonium bromide (C 12TAB) and the anionic polymer sodium poly(styrene sulfonate) (NaPSS) have been studied using surface tension and tron reflectivity techniques. Neutron reflection shows that the surface consists of a mixture of polymer and surfactant over surfactant concentrations ranging from twice the critical micelle concentration (CMC) to 1/100 of the CMC for polymer concentrations between 20 and 140 ppm. In the lower surfactant concentration range the amount of surfactant adsorbed approximately corresponds to a surfactant monolayer (area per molecule ∼ 50 Å 2), but at a higher concentration this increases to an amount approximately corresponding to three adsorbed layers (area per molecule = 17 Å 2). A similar increase is observed in the amount of adsorbed polyelectrolyte. The jump in the adsorbed amounts is accompanied by an increase in the overall thickness of the adsorbed layer from 20 Å to about 60 Å. Based on fits to the neutron reflectivity data from different isotopic compositions the structure of the higher concentration layer is a sandwich structure with an outer surfactant layer and a submerged micellar layer or defective bilayer. This composite layer may be associated with adsorption of the polymer/micelle complex at the interface

Polyelectrolyte/surfactant mixtures at the air-solution interface

This review presents some of the recent developments in our understanding of the behaviour of polyelectrolyte/surfactant mixtures at the air-solution interface. The existence of a strong surface polyelectrolyte/surfactant interaction results in a complex pattern of surface adsorption. Recent studies, using a range of surface sensitive techniques, which include ellipsometry, neutron and X-ray reflectivity, surface tension and interfacial rheology, have considerably enhanced the understanding of their surface behaviour, which can be rationalized in terms of the competition between the formation of surface active polymer/surfactant complexes and solution polymer/surfactant micelle complexes. © 2006 Elsevier B.V. All rights reserved

The adsorption of oppositely charged poly-electrolytes/surfactant mixtures, neutron reflection from alkyl trimethyl ammonium bromide and Na PSS at the air-water interface: the effect of surfactant chain length

The interactions between a series of cationic surfactants (alkyltrimethylammonium bromides)(C nTAB) and the anionic polymer sodium poly(styrenesulfonate)(NaPSS) were studied using surface tension and neutron reflectivity. The effect of surfactant chain length on surfactant/polymer adsorption was also studied using chain lengths from 10 to 16 carbon atoms. The surface tension and neutron reflection results were explained in terms of a model in which there were three kinds of polymer/surfactant complex