An optical microscopy method for determining stable and metastable phase equilibria involving gas hydrate and ice: Application to CO2+water systems

Abstract

The temperatures and pressures of the three-phase equilibria between liquid water (L${}_w$), gas hydrate (H) and a guest-rich phase (a vapor, V, or a condensed liquid, L${}_c$) and between L${}_w$, ice (I), and V, are determined experimentally by monitoring the disappearance of the hydrate or ice phases near the meniscus between the water-rich and guest-rich phases when temperature or pressure are varied slowly. This monitoring is carried out by optical microscopy used in the transmission mode with or without crossed polarizers, the latter serving to identify the presence or absence of hexagonal (birefringent) ice. The guest molecule (or hydrate-former) taken as an example is CO${}_2$. The triple lines corresponding to L${}_w$-H-V, L${}_w$-I-V and L${}_w$-H-L${}_c$ equilibria are determined together with their intersections, i.e., the lower quadruple point Q${}_1$ (L${}_w$-I-H-V coexistence) and upper quadruple point Q${}_2$ (L${}_w$-H-V-L${}_c$ coexistence). The metastable extension of the three-phase line L${}_w$-H-V for temperatures and pressures below those of Q${}_1$ is also determined. A Clausius–Clapeyron treatment of this line and its metastable extension shows that a similar dissociation process exists, whether the dissociation is to supercooled liquid water or not.