Ionic liquids (ILs) form a variety of nanostructures due to their amphiphilic nature. Recently, unusual structural phenomena have been found in glassy ILs near their glass transition temperatures; however, in all studied cases, IL cations and anions were in the form of separate moieties. In this work, we investigate for the first time such structural anomalies in zwitterionic IL glasses (ZILs), where the cation and anion are bound in a single molecule. Such binding reasonably restricts mutual diffusion of cations and anions, leading to modification of nano-ordering and character of structural anomalies in these glassy nanomaterials, as has been investigated using Electron Paramagnetic Resonance (EPR) spectroscopy. In particular, the occurrence of structural anomalies in ZIL glasses was revealed, and their characteristic temperatures were found to be higher compared to common ILs of a similar structure. Altogether, this work broadens the scope of structural anomalies in ionic liquid glasses and indicates new routes to tune their properties.
Active Pharmaceutical Ingredient-Ionic Liquids (API-ILs) draw increasing interest as a particular class of ILs that possess unusual physicochemical properties along with simultaneous potentials for pharmaceutical applications. Although nanostructuring phenomena were actively investigated in common ILs, their studies in API-ILs are scarce so far. In this work, using the complex methodology of Electron Paramagnetic Resonance (EPR) and dissolved spin probes, we investigate nanostructuring phenomena in a series of API-ILs: [Cnmim][Ibu], [Cnmim][Gly], and [Cnmim][Sal] with n = 2, 4, and 6, respectively. We reveal similar trends for API-ILs and common ILs, as well as peculiarities inherent to the studied API-ILs. Unusual behavior observed for [Cnmim][Ibu] has been assigned to the presence of a non-polar fragment in the [Ibu]- anion, which leads to the formation of more complex nanostructures around the radical compared to common ILs. Understanding general trends in the formation of such self-organized molecular structures is of fundamental interest and importance for applying API-ILs.
Ionic Liquids , Nanostructures , Electron Spin Resonance Spectroscopy , Ionic Liquids/chemistry , Molecular Structure , Nanostructures/chemistry , Pharmaceutical Preparations
Ionic liquid (IL) glasses have recently drawn much interest as unusual media with unique physicochemical properties. In particular, anomalous suppression of molecular mobility in imidazolium IL glasses vs. increasing temperature was evidenced by pulse Electron Paramagnetic Resonance (EPR) spectroscopy. Although such behavior has been proven to originate from dynamics of alkyl chains of IL cations, the role of electron spin relaxation induced by surrounding protons still remains unclear. In this work we synthesized two deuterated imidazolium-based ILs to reduce electron-nuclear couplings between radical probe and alkyl chains of IL, and investigated molecular mobility in these glasses. The obtained trends were found closely similar for deuterated and protonated analogs, thus excluding the relaxation-induced artifacts and reliably demonstrating structural grounds of the observed anomalies in heterogeneous IL glasses.
Intriguing heterogeneities and nanostructural reorganizations of glassy ionic liquids (ILs) have recently been found using electron paramagnetic resonance (EPR) spectroscopy. Alkyl chains of IL cations play the key role in such phenomena and govern the anomalous temperature dependence of local density and molecular mobility. In this paper we evidence and study similar manifestations in a variety of common non-IL glasses, which also contain molecules with alkyl chains. A series of phthalates clearly demonstrates very similar behavior to imidazolium-based ILs with the same length of alkyl chain. Glasses of alkyl alcohols and alkyl benzenes show only some similarities to the corresponding ILs, mainly due to a lower glass transition temperature hindering the development of the anomaly. Therefore, we demonstrate the general nature and broad scope of nanoscale structural anomalies in organic glasses based on alkyl-chain compounds. The 'roadmap' for their occurrence is provided, which aids in understanding and future applications of these anomalous nanoheterogeneities.
Intriguing nanostructuring anomalies have been recently observed in imidazolium ionic liquids (ILs) near their glass transition points, where local density around a nanocaged solute progressively grows up with temperature. Herewith, we for the first time demonstrate experimentally and theoretically, that these anomalies are governed by alkyl chains of cations and crucially depend on their length. Electron Paramagnetic Resonance (EPR) spectroscopy on a series of ILs [Cnmim]BF4 (n = 0-12) shows that only the chains with n = 3-10 favor anomaly. Moreover, remarkable even vs. odd n peculiarities were systematically observed. Finally, similar anomaly was for the first time observed for a non-IL glass of dibutyl phthalate, which structurally mimics cations of imidazolium ILs. Therefore, such anomalous density behavior in a glassy state nanocage goes far beyond ILs and proves to be a more general phenomenon, which can be structurally tuned and rationally adjusted for various potential applications in nanoscale materials.
