Branching in molecular structure enhancement of solubility in CO₂

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.