Role of Amphiphilicity in Low-Dosage Biodegradable Inhibitors for Clathrate Hydrate: A Kinetic Study of Inhibition by Highly Branched Poly(l-lysine)s

Abstract

The rapid growth of natural gas clathrate hydrate has been a significant safety hazard in off-shore natural gas drilling and gas transport. Alleviation of hydrate formation can be achieved by applying low-dosage hydrate inhibitors of polymers during operation. In order to combine the advantages of high adsorption potential and environmental friendliness, we study the kinetics of hydrate inhibition of amphiphilic highly branched poly(l-lysine) (PLL) by NMR relaxometry. The introduction of n-butyl and cyclohexyl groups in PLLs was found to be significantly effective in prolonging the induction times of sII clathrate hydrates. Key physical–chemical factors involved in the mechanism previously proposed by molecular dynamics simulations can be clarified by NMR relaxometry. Induction times for the formation of hydrate are associated with the interfacial water amount but are not related to the hydrogen bonding strength or dynamics of the free water component. The amphiphilic PLLs in the interface interact with the hydrate surfaces through their hydration shell. Analysis of the molecular motion and freezing kinetics of water in the fluid before and during hydrate formation reveals that the polymers can reduce the rates of hydrate growth from the free water by a factor of 5 to 10. A further significant decrease in the growth rates from small hydrate particles by a factor of 10 to 2600 was observed in the presence of the amphiphilic PLLs with n-butyl and cyclohexyl groups. The distinct crystal growth kinetics in the heterogeneous interface are influenced by the terminal alkyl groups of the polymers, with changing growth geometry or crystal shapes and consistent with the interfacial Gibbs–Thomson effect. A high Avrami index value of the isothermal crystal growth and nonfreezable water amount favors a long induction time. The PLL inhibitors adsorb to the hydrate surface and act via an interfacial mechanism.