Mesoscopic Structure in Mixtures of Water and 1-Butyl-3-methyl imidazolium tetrafluoborate: A Multinuclear NMR Study

This work is devoted to highlighting some peculiar aspects of ionic liquid self-aggregation in water, which is still a matter of wide debate. Here, the whole water /1-butyl-3-methyl imidazolium (bmim+) tetrafluoborate ($$ {\text{BF}}_{4}^{ - } $$) phase diagram was thoroughly re-investigated, at a molecular level, by means of 1H and 19F PGSTE and 11B relaxation NMR experiments performed at room temperature. The analysis of H2O, bmim+ and $$ {\text{BF}}_{4}^{ - } $$ ions self-diffusion coefficients and the 11B relaxation times revealed that ion-pair dissociation is a progressive process starting at a H2O molar fraction equal to 0.2 and ending at high water content. More importantly, the collected results indicate that H2O and ions diffuse within different domains, strongly suggesting that the system under investigation is actually nanostructured. This fact agrees with some other recent works focusing on the possible mesoscopic order that ionic liquids possess when dissolved in water because of their hydrophobic tail segregation.