Freezing and melting behavior of CO2 in nanometer geometry

We present a comprehensive adsorption and desorption phase diagram for CO2 confined in 4 nm diameter pores of mesoporous VYCOR studied using positron/positronium annihilation spectroscopy. We concentrate on special features that occur at the freezing and melting part of the phase diagram. Both the freezing and melting of the confined CO2 occur at temperatures well below that of the bulk freezing phase transition, with pronounced hysteresis between the confined transitions. Freezing creates open voids within the pores. At the early stages of melting, we observe features that are similar to those seen during liquefaction, suggesting that gas-liquid interfaces are formed at this point. At pressures close to the adsorption "triple point" of the confined material, the freezing temperature increases with decreasing pressure, leading to a curvature of the freezing line. We observe that this is related to incomplete pore filling with liquid prior to the freezing transition. Such curvature is not observed during the melting transition. Hysteresis scanning curves show that partially frozen material is stable if the temperature is increased. On completion of the pore melting transition, the annihilation parameters for the pore liquid state do not revert to values prior to the commencement of freezing. The original state is obtained only after heating the sample above the bulk melting temperature.