Abstract
The varying states of water confined in the nano-domain structures of typical room temperature ionic liquids (ILs) were investigated by 1H NMR and by measurements of self-diffusion coefficients while systematically varying the IL cations and anions. The NMR peaks for water in BF4-based ILs were clearly split, indicating the presence of two discrete states of confined water (H2O and HOD). Proton and/or deuterium exchange rate among the water molecules was very slowly in the water-pocket. Notably, no significant changes were observed in the chemical shifts of the ILs. Self-diffusion coefficient results showed that water molecules exhibit a similar degree of mobility, although their diffusion rate is one order of magnitude faster than that of the IL cations and anions. These findings provide information on a completely new type of confinement, that of liquid water in soft matter.
Highlights
Challenging to obtain sufficiently intense spectra for ionic liquids (ILs) in aqueous solutions, due to the large water peak that results from the dynamic range effect
The use of HOD may allow one to follow the H/D exchange reaction between H2O and D2O occurred inside the water pocket, which in a normal sense cannot be resolved on the NMR timescale, because the exchange is very fast in neat water
By the use of infrared and Raman spectroscopies as a complementary technique to 1H NMR, Yaghini et al.[20] pointed out that the effect of water on the local structure and phase behavior of imidazolium-based ILs is dependent on the particular cation-anion pair investigated
Summary
Koji Saihara[1], Yukihiro Yoshimura[2], Soichi Ohta1 & Akio Shimizu[1] received: 04 March 2015 accepted: 21 April 2015 Published: 29 May 2015. By the use of infrared and Raman spectroscopies as a complementary technique to 1H NMR, Yaghini et al.[20] pointed out that the effect of water on the local structure and phase behavior of imidazolium-based ILs is dependent on the particular cation-anion pair investigated. In this manner, we may expect that the equilibrium property of water molecules confined in ILs can be evaluated. The water was prepared by mixing H2O and D2O in different molar ratio
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