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Phase Behavior of Carbon Dioxide and Polyhedral Oligomeric Silsesquioxanes with Two Different Functional Groups

Phase behavior of carbon dioxide and polyhedral oligomeric silsesquioxanes (POSS) modified with two different functional groups is studied in the temperature range of 308-323 K, up to 22 MPa. Different than the common types, one of the eight functional groups attached to the Si atoms of these POSS is a CO2-philic functional group, methacryl, while the rest are branched alkyl chains, either isooctyl (MIOPOSS) or isobutyl (MIBPOSS). Methacrylisobutyl POSS-CO2 binary system exhibits pressure-induced melting temperature depression. Both molecules have higher solubilities in scCO2 compared to their counterparts with single type of functionality, octaisobutyl POSS, isooctyl POSS and methacryl POSS. The measured highest solubility of MIBPOSS is 0.006 mole fraction at 323 K and 16.8 MPa, while it is 0.0017 mole fraction at 323 K and 22.1 MPa for MIOPOSS. Various density-based semi-empirical models used to model the solubility data gave good correlations with AARD of about 4%.