Recent insights into the anomalous dual nature (both promotion and Inhibition) of chemical additives on gas hydrate formation
Ngoc N. Nguyen, Anh V. Nguyen
Abstract
Gas hydrates are gas-carrying water structures which present both opportunities and threats to the energy industry and environment. Chemical additives are used either to promote gas hydrate formation (GHF) to realize the opportunities or inhibit GHF to mitigate the threats. Understanding the functionality of hydrate additives followed by tailoring their structures for a better performance is a crucial requirement. However, recent studies showed that best-known hydrate promoters can act as hydrate inhibitors, and vice versa, when their concentrations are changed. These puzzling observations demonstrate the anomalous dual nature of hydrate additives, which has not been discussed thoroughly in the literature. Here, we critically discuss the dual nature of popular hydrate additives such as salts, surfactants, alcohols and polymers, with a particular focus on their anomalous behaviors at low concentrations. We discuss the underpinning physics of the dual nature using the concepts of ion-specificity, hydrophobic nature of soft ions, hydrophobic effect and host–guest interactions to provide new insights into the dual nature of hydrate additives relevant to energy and environmental applications.