Structural Properties and Aggregation Behavior of 1-Hexyl-3-methylimidazolium Iodide in Aqueous Solutions

The structural properties of 1-hexyl-3-methylimidazolium iodide ([C6mim]I)/water mixtures with molar ratios ranging from 1:1 to 1:200 have been investigated using molecular dynamics (MD) simulations with extended X-ray absorption fine structure (EXAFS) experimental data. The presence of a complex network of interactions among cations, anions, and water molecules has been highlighted from the MD simulations, even if water molecules have been found to interact preferentially with the I(-) anion. The EXAFS results show that, also for the 1:1 [C6mim]I/water mixture, the water molecules are placed next to the I(-) anion, and the I(-) hydration shell becomes more and more crowded with increasing water content. Tight ion pairs have been detected in the [C6mim]I/water mixtures with molar ratios from 1:1 to 1:12, while no ionic pairs were found in the most diluted solutions. The aggregation behavior has been determined from MD simulations with the aid of S(q) functions. For the most concentrated IL/water mixtures with molar ratios between 1:1 and 1:12 the existence of long-range structural correlations has been evidenced, even if the apolar chains are not completely segregated as expected for micelle-like structures. Conversely, for the 1:200 mixture, that is above the experimental critical aggregation concentration value, the alkyl chains are completely separated from each other.