9th Eastern Mediterranean Chemical Engineering Conference, Ankara, Turkey, 31 August - 02 September 2018, pp.1
Solubility and phase behavior investigations of novel materials in supercritical carbon dioxide (scCO2) permit design and advancement of new and environmentally friendly supercritical fluid processes. Supercritical carbon dioxide (scCO2) is called as a green solvent because it is non-toxic, non-flammable, inexpensive, and abundant. Its use in a process instead of an organic solvent can eliminate the emissions of volatile organic components. Also, due to its nonpolar structure, it can dissolve nonpolar molecules with small molecular weight. On the other hand, due to its quadrupole moment, it also exhibits specific interactions such as Lewis acid-base with some substances functionalized with certain polar groups, strong enough to dissolve them. Poly (methyl methacrylate) and sugar acetates are among those substances which can interact with scCO2 because of their carbonyl groups and be dissolved in scCO2. Certain materials such as poly (dimethyl siloxane) (PDMS) which have flexible siloxane groups can also dissolve in scCO2. In this study, our investigation on the phase behavior of polyhedral oligomeric silsesquioxanes (POSS)-CO2 binary systems are continued with different chemical structures of POSS. Earlier, the solubility of various POSS, which are trifluoropropyl, octamethyl, metacryl, isooctyl and octaisobutyl POSS, in scCO2 were studied [1,2]. All of these components were found to be soluble in scCO2, with different levels of solubility, except octamethyl POSS. The common characteristic property of these molecules was that all the functional groups attached to the Si atom of a certain POSS were identical. However in this study, we report the phase behavior of methacrylisobutyl POSS (MIBPOSS) and methacrylisooctyl POSS (MIOPOSS) with CO2, both of which have two different functional groups in their chemical structures. High pressure view cell was used to measure the cloud or dew points at the temperature range of 308-323 K, up to 22 MPa. The measured highest solubility was exhibited by MIBPOSS, which was 0.006 mol fr. at 323 K and 16.82 MPa. The solubility of POSS in scCO2 increases with increasing pressure for both POSS at the isothermal conditions. At the isobaric conditions, solubility decreases with increasing temperature due to the decreasing density of the solvent. On the other hand, at constant CO2 density, solubility of POSS in scCO2 increases with increasing temperature possibly due to the increasing vapor pressure of the solutes. An interesting solubility behavior was observed, which was that the dual functionality allows both POSS molecules attain a higher solubility in scCO2 compared to their counterparts with single functionality. This shows that the entropic contributions dominate the solubility of the double functional POSS molecules in scCO2. Due to the lack of the information on the critical properties or group contribution constants of these molecules, the solubility data was modeled using density-based semi-empirical models, which predicted the solubility of these molecules in scCO2 with a maximum absolute average relative deviation of 7%.
 Dilek, C. (2013). Supercritical carbon dioxide-soluble polyhedral oligomeric silsesquioxane (POSS) nanocages and polymer surface modification. The Journal of Supercritical Fluids, 73, 171-177. doi:10.1016/j.supflu.2012.10.012
 Kanya, B., & Dilek, C. (2015). Effects of functional groups on the solubilities of polyhedral oligomeric silsesquioxanes (POSS) in supercritical carbon dioxide. The Journal of Supercritical Fluids, 102, 17-23. doi:10.1016/j.supflu.2015.03.024