Global phase behavior for CO2-philic solids: the CO2 + β-d-maltose octaacetate system

High-pressure phase behavior for a binary mixture of the CO2-philic solid β-d-maltose octaacetate (MOA, Tm = 432 K) with carbon dioxide was determined. Both critical end points (LCEP and UCEP) and pressure versus composition isotherms at 283, 298, and 323 K were measured in order to identify the global phase behavior for this system. Limited pressure-composition measurements were also made down to 253 K to determine whether liquid-liquid equilibria re-emerged at lower temperatures. The pressure-temperature projection of this system has the fluid-phase features of a Scott and Van Konynenburg Type V system. The solid-fluid behavior consists of a continuous, three-phase VL2S2 curve that goes through a pressure maximum before running just below and parallel to the three-phase VL1L2 line to lower pressures and temperatures. Another distinguishing feature of the CO2 + MOA system is a long VL1L2 line, which extends over a range of more than 20 K. The solubility of MOA in CO2 can be explained by the interaction between a Lewis base (the carbonyl groups of MOA) and a Lewis acid (CO2). The observed phase behavior, which is relatively rare, is likely to be observed for other high-melting CO2-philic solids, including poly(fluoroalkyl acrylate), poly(vinyl acetate), and other sugar acetates.