In situ Raman spectroscopic investigation of flux-controlled crystal growth under high pressure: A case study of carbon dioxide hydrate growth in aqueous solution

Abstract We developed a high-pressure in-situ non-invasive technique to measure the solute concentration distribution in the solution around a growing crystal, in order to study the effect of increasing flux to the crystal surface on the growth process. Growth rates of carbon dioxide hydrate in pure water in a capillary high-pressure optical cell were determined at 20 and 40 MPa and 275.15, 278.15, 280.15, and 283.15 K. During the growth of the hydrate, CO 2 was supplied through diffusion in the aqueous phase from the liquid CO 2 –aqueous solution interface, and the dissolved CO 2 concentrations along the diffusion path were monitored by Raman spectroscopy. We demonstrated that the hydrate growth rates could change a lot under constant driving force, such as under the same degrees of super-saturation, super-cooling or over-pressurization of the system, and that the growth rates depend on the mass transfer flux to the surface of the hydrate crystal; with the increasing flux to the crystal surface, the rate-dominant process changes from “diffusion controlled” to “interfacial reaction controlled” after the diffusion flux reaches a critical (maximum) value. The interfacial reaction coefficient k r is estimated to be 1.31 × 10 −6  m/s at 20 MPa and 280.15 K, and the molar volume and hydration number of CO 2 hydrate vary from 21.7 to 22.0 cm 3 /mol and 7.09 to 7.67, respectively. These observations and measurements could be useful for the design of crystal growth.