Small Molecule Cyclic Amide and Urea Based Thickeners for Organic and sc-CO₂/Organic Solutions

A series of cyclic amide and urea materials were prepared and screened as small molecule thickeners for organic solvents, dense CO₂, and mixtures thereof. In addition to a cyclohexane or benzene core, both of which are mildly CO₂-phobic, these molecules contained either ester, amide, or urea groups responsible for establishing intermolecular interactions necessary for increasing solution viscosity. These groups also function to connect siloxane or heavily acetylated CO₂-philic segments to the cyclic core of the thickener molecule. Many of these compounds were shown to thicken conventional organic liquids (e.g., toluene, hexane), usually after heating and cooling the mixture. A propyltris­(trimethylsiloxy)­silane-functionalized benzene trisurea material was also shown to thicken compressed liquid propane and butane. Attaining solubility and self-assembly in CO₂ proved more challenging, however. Several ester, amide, and urea containing compounds were discovered that are soluble in dense CO₂ at low loadings (0.5–2 wt %). For linear siloxane segments, increasing the number of silicon atoms provides greater solubility in dense CO₂. Branched siloxane segments were shown to have superior solubility characteristics in dense CO₂ to linear siloxanes of similar silicon content. However, only the propyltris­(trimethylsiloxy)­silane-functionalized benzene trisurea and trisureas functionalized with varying proportions of propyltris­(trimethylsiloxy)­silane and propyl-poly­(dimethylsiloxane)-butyl groups exhibited remarkable viscosity increases (e.g., 3–300-fold at 0.5–2.0 wt %) in CO₂, although high concentrations of an organic cosolvent (18–48 wt %) such as hexane were required to attain dissolution in CO<sub>2