Local organization of water and its effect on the structural heterogeneities in room-temperature ionic liquid/H2O mixtures

Fourier-transform infrared and Raman spectroscopies have been used to investigate the role played by water on the structural organization of 1-butyl-3-methyl-imidazolium tetrafluoroborate and H2Omixtures, over a wide composition range at room temperature. Our measurements provide clear experimental evidence of the prompt association between 'free'watermolecules and anions at very smallwater contents. Moreover, in the case of higher water contents, we obtain indications of the local aggregation of water molecules in the network. Such an aggregation is found to occur even before the saturation of all the anions that are available for H-bonding. We propose that the water clusters favour the organization of ionic liquid in the polar network, where they are embedded, and the aggregation of hydrophobic alkyl tails in amicelle-like structure.Whenmixtureswith watermolar fraction exceeding 0.7 are considered, this local organization starts to weaken owing to the gradual break up of the ion-pair interactions.