Tetrabutylammonium lactate semiclathrate hydrate phase equilibria and polymorphs

Abstract

Clathrate hydrates can be used as a way of storing and separating gases at elevated pressures. Ionic semiclathrate formers, miscible in water, can promote the formation of hydrates, allowing for milder conditions in potential industrial applications such as cold energy storage and carbon sequestration. However, understanding semiclathrate formation has been difficult due to the complexity associated with the numerous stable and metastable phase changes possible. Using tetrabutylammonium lactate (TBAL) and differential scanning calorimetry, here we show that the formation of polymorphs with different dissociation temperatures is highly sensitive to the TBAL concentration - with gradual transitions from one phase to the next over small concentration increments. Furthermore, we show that the cooling rate may influence the formation/dissociation of a polymorph, leading to complexities such as recrystallization on warming. Given the discrepancies between reported semiclathrate thermodynamic properties, such as dissociation temperature and latent heat, this work highlights the need for caution when using calorimetry (and other methods) to determine semiclathrate properties. This caution is needed to ensure that the same polymorph is referred to when reporting thermodynamic properties. Precise reference to polymorph conditions is also important but also when projecting such data to potential future applications where a different polymorph/polymorph mixture may form with different properties than those anticipated.