Branching in molecular structure enhancement of solubility in CO2

Abstract

Most compounds of some 1,000 amu molecular weight (MW) and higher are poorly soluble in carbon dioxide (CO2). Only at very high pressure, there may be mild solubility. This limits the use of CO2 as a solvent and modifications of CO2 properties through additives. We have developed a coarse-grained molecular model to investigate the dependency of the solubility of hydrocarbon oligomers (MW of ∼1,000 amu) in CO2 and on the molecular structure. The coarse-grained model is optimized by the particle swarm optimization algorithm to reproduce density, surface tension, and enthalpy of vaporization of a highly branched hydrocarbon oligomer (poly-1-decene with six repeating units). We demonstrate that branching in molecular structure of oligomers significantly increases solubility in CO2. The branching in molecular structure results in up to 270-time enhancement of solubility in CO2 than an n-alkane with the same MW. The number of structural edges (methyl group) is a key in improved CO2-philicity. The solubility of poly-1-decene with nine repeating units (MW of 1,264.4 amu) is higher in CO2 than poly-1-dodecene with six repeating units (MW of 1,011.93 amu) because it has more structural edges (10 vs. 7). These results shed light on the enhancement of CO2-philicity by altering molecular structure rather than modifying chemical composition in compounds.