SIi

Structure II, abbreviated sII, is a crystalline form of gas hydrates (clathrate hydrates) in which water molecules form a lattice that traps guest gas or vapor molecules inside defined cages. It is one of the principal hydrate structure types, alongside structure I (sI) and structure H (sH). The sII lattice is cubic and contains two types of cages: small cages with the formula 5^12 6^4 and larger cages with the formula 5^12 6^8. The larger cages can accommodate relatively bulky guest molecules, while the smaller cages may host smaller ones, allowing a range of gas compositions.

Guest molecules that form sII hydrates are generally larger than those typical of sI, including propane, isobutane, and other C3+ hydrocarbons, though methane can occupy the smaller cages in mixed hydrates. The stability of sII hydrates depends on temperature, pressure, and composition; under the appropriate low-temperature, high-pressure conditions they crystallize from water and guest gas mixtures. Structure II hydrates can coexist with liquid water and free gas phases in natural settings and industrial systems.

Occurrence and significance vary by context. In nature, sII hydrates are found in deep-sea sediments and permafrost where sufficient pressure and low temperatures prevail, contributing to natural gas hydrate deposits. In industry, hydrate formation, including sII, poses a flow assurance challenge in pipelines and storage, potentially causing blockages, but hydrates are also explored for energy storage, gas separation, and CO2 sequestration research. In laboratory studies, sII hydrates are characterized by techniques such as X-ray diffraction, Raman spectroscopy, and solid-state NMR to elucidate cage occupancy and guest–host interactions.