  The relation between collective properties and performance of
antiagglomerants (AAs) used in hydrate management is handled using molecular
dynamics simulations and enhanced sampling techniques. A thin film of AAs
adsorbed at the interface between one flat sII methane hydrate substrate and a
fluid hydrocarbon mixture containing methane and n-dodecane is studied. The AA
considered is a surface-active compound with a complex hydrophilic head that
contains both amide and tertiary ammonium cation groups and hydrophobic tails.
At sufficiently high AA density, the interplay between the surfactant layer and
the liquid hydrocarbon excludes methane from the interfacial region. In this
scenario, we combine metadynamics and umbrella sampling frameworks to study
accurately the free-energy landscape and the equilibrium rates associated with
the transport of one methane molecule across the AA film. We observe that local
configurational changes of the liquid hydrocarbon packed within the AA film are
associated with high free-energy barriers for methane transport. The time
scales estimated for the transport of methane across the AA film can be, in
some cases, comparable to those reported in the literature for the growth of
the hydrates, suggesting that one possible mechanism by which AAs delay the
formation of hydrate plugs could be providing a barrier to methane transport.
Considering the interplay between structural design and collective properties
of AAs might be of relevance to improve their performance in flow assurance.
