Short/Medium-to-Long Range Order correlations in Room Temperature Ionic Liquids.

Our group is presently working on a new, attractive class of materials, namely room temperature ionic liquids (RTILs): they are composed solely of ionic species that, because of their chemical architecture, have melting point below ambient temperature. These systems are attracting great attention because of their low vapour pressure and other properties that make them green replacements for the noxious organic volatile solvents. Recently we investigated a peculiar set of RTILs that is characterized by an alkaline metal as the cation (Na, Li, K) and a complex anion (Figure 1). Their interest lies in the fact that so far alkali metals were not known to be able to lead to low-melting salts. The Kunz group [1,2], which we are collaborating with on this topic, proposed two models for their morphology: the alkali metal can be either coordinated by a crown of ether-oxygen atoms (belonging to the chain) or by the chelating carboxy (CO2-) group. Synchrotron high energy X-ray diffraction (HEXRD) measurements were collected on the Na-RTIL at ambient temperature. Similarly to other RTILs, this system shows a distinct low-Q peak that fingerprints the existence of an enhanced long range order over the spatial scale of several nm. [3] MD calculations were developed for this system at 500 K. The calculated diffraction pattern is in good agreement with the experimental data, thus validating our potential. MD-derived pair distribution functions (pdf´s) indicate a strong coordination between the carboxy-group oxygen and the Na atoms. A much weaker correlation exists between the ether-oxygen and the Na. These observations support the structural scenario where the Na is coordinated by chelating carboxy rather than the ether groups, thus forming well-defined carboxy-coordinated sodium clusters. A more detailed analysis allows detecting that the low Q peak is the fingerprint of the structural correlation between the mentioned clusters that are kept at fairly well defined distance by the ether chains that are separating them. Accordingly on the basis of a comparison between HEXRD and simulation data, we propose a structural scenario where short-to-medium range interactions-driven clusters (carboxy-coordinated sodium ones) are able to induce the occurrence of long range order, showing up as a low-Q peak in the diffraction pattern. [1] Kunz et al., Chem. Eur. J., 15, 1341 (2009); PCCP 12, 14341 (2010) [2] Russina et al., JPCB 111, 4641 (2007)