Pressure dependence of small-cage occupancy in the cyclopropane hydrate system

Abstract

Laser Raman microspectroscopy and stability boundary observation for the cyclopropane hydrate system were investigated in a pressure range up to 460 MPa and temperature range from 278 to 320 K . The cyclopropane molecule is one of the largest guest species which build up the structure I hydrate lattice. It has been argued that the cyclopropane hydrate lattice would become stable in the presence of the perfectly vacant small cage. In the pressure range lower than 200 MPa , we can confirm the perfect vacancy of small hydrate cages. We have ascertained that the cyclopropane molecule is able to occupy the small hydrate cage in the pressure range higher than 200 MPa along with the three-phase coexisting curve. It becomes clear that the degree of small-cage occupancy increases in proportion to pressure without any phase transition in a pressure range up to 460 MPa .