Structure-directing effects in microheterogeneous ionic liquids

Abstract

The man-made climate change and contamination of the world's oceans with plastics demand for the development of alternative solvents, in particular to establish more efficient material synthesis routes and extraction processes. In that regard Ionic Liquids (ILs) represent a promising material class, whose special properties are based on their molecular composition. However, their molecular-level working principles are still far from being understood completely. Thus, this thesis focuses on the microheterogeneity (MH) in ILs and its potential to function as structure-directing or templating agent, using molecular dynamics simulations. <br /> In the first part, from carrying out simulations of a Te₄Br₂ molecule dissolved in one pure IL and one IL mixed with AlCl₃, the IL's degree of MH, the role of AlCl₃, and their effects on the Te₄Br₂ molecule are elucidated. While, within the pure IL a notable degree of MH is observed, the second system is dominated by the formation of larger anionic species. For the Te₄Br₂ molecule dissolved in both ILs, significant Te-Br bond elongations are detected, however, only in the AlCl₃-containing system the it converts from an open form into a positively charged square-like Te₄ species. <br /> In the second part, the MH in ILs and its tunability are investigated in more detail, particularly with respect to its potential to act as template for alcohol molecules. It is observed that an elongation of the cations alkyl side chain leads to an increasing segregation of the non-polar from the polar moieties. Thereby, the cations order in a parallel way, thus forming a micelle-like structure. While the alcohol molecules tend to self-aggregate in the solvents with shorter side chains, they show a rising degree of dispersion when moving towards longer alkyl tails. The increasing order within the ILs comprising longer side chains is imprinted into the alignment of the alcohol molecules: the alkyl chain stretches out parallel to the alkyl groups of the IL cations, while their polar part anchors to the polar network of the IL, indicating an unambiguous template effect of the IL on the solute molecules in it. <br /> The last part deals with the question whether ILs are suitable solvents for the extraction of nanoplastic particles from aqueous media. By simulating a polyethylene nanoparticle in a set of various ILs, it is found that the particle tends to disentangle into a loosely associated structure in order to increase its contact surface area with the solvent. Thereby, the degree of disentanglement strongly depends on the degree of MH present in the system, so the stability of the particles can be controlled. Modeling a phase transfer of a polyethylene nanoparticle from an aqueous to an IL phase reveals that this process is thermodynamically and kinetically favorable, so employing ILs can indeed lead to promising processes to handle nanoplastic contamination.