In Silico Analysis of the Effect of Alkyl Tail Length on Antiagglomerant Adsorption to Natural Gas Hydrates in Brine

Antiagglomerant (AA) gas hydrate inhibitors can prevent blockages in oil and gas flowlines and could help enable safe and profitable production from deepwater environments. AAs, which are generally surfactants, often have an ionic head and one or more alkyl tails. Here, we use molecular dynamics simulations to investigate the mechanistic and energetic effects of alkyl tail length on adsorption to an sII methane/propane hydrate for well-known AA molecules (n-alkyl-tri(n-butyl)ammonium salt surfactants) of varying tail length. We consider alkyl tails from 8 to 16 carbon atoms and show that the dodecyl (12-carbon) tail has the highest percentage of the strongest binding configuration (i.e., the simultaneous head and tail binding configuration) and the lowest free energy of binding among the five molecules investigated. This maximum in strongest-configuration binding statistics and minimum in binding free energy at a tail length of 12 carbons may result from the appropriate size of the AA molecule with the dodecyl tail for spanning the most abundant binding sites on the hydrate surface. Furthermore, competition between the enthalpic gain of removing thehydrophobic tail from the aqueous solution and the entropic penalty of binding gives an optimum binding free energy for the dodecyl tail. Notably, experimental work on the same quaternary ammonium cations of varying tail length shows a maximum in performance for the dodecyl tail. This suggests that free energy of binding and more computationally efficient measures such as head-plus-tail binding probability are possible metrics for using molecular simulation to predict experimental and field performance of surfactant-type AAs, a capability which could accelerate the chemical innovation process.