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The dynamics of solvent relaxation in ionic liquid (IL)-water, IL-methanol, and IL-acetonitrile mixtures have been investigated using steady state and picosecond time-resolved fluorescence spectroscopy. We have used Coumarin 153 (C-153) and 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF(6)]) as fluorescence probe and IL, respectively. The steady-state emission spectra showed that the gradual addition of cosolvents increases the polarity of the mixtures. In neat [hmim][PF(6)] and all IL-cosolvent mixtures, solvation occurs in two well-separated time regimes within the time resolution of our instrument. A substantial portion of the solvation has been missed due to the limited time resolution of our instrument. The gradual addition of cosolvents decreases the viscosity of the medium and consequently solvation time also decreases. The decrease in solvation time is more pronounced on addition of acetonitrile compared to water and methanol. The rotational relaxation time of the probe is also decreasing with gradual addition of the cosolvents. The decrease in viscosity of the solution is responsible for the decrease in the rotational relaxation time of the probe molecule.

The effects of confinement of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate on solvation dynamics and rotational relaxation of Coumarin 153 (C-153) in Triton X-100/cyclohexane microemulsions have been explored using steady-state and picosecond time-resolved emission spectroscopy. The steady-state and rotational relaxation data indicate that C-153 molecules are incorporated in the core of the microemulsions. The average rotational relaxation time increases with increase in w ([bmim][BF(4)]/[TX-100]) values. The solvent relaxation in the core of the microemulsion occurs on two different time scales and is almost insensitive to the increase in w values. The solvent relaxation is retarded in the pool of the microemulsions compared to the neat solvent. Though, the retardation is very small compared to several-fold retardation of the solvation time of the conventional solvent inside the pool of the microemulsions.

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The interaction of ionic liquid with water in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6])/Triton X-100 (TX-100)/H2O ternary microemulsions, i.e., "[bmim][PF6]-in-water" microregions of the microemulsions, has been studied by the dynamics of solvent and rotational relaxation of coumarin 153 (C-153) and coumarin 151 (C-151). The variation of the time constants of solvent relaxation of C-153 is very small with an increase in the [bmim][PF6]/TX-100 ratio (R). The rotational relaxation time of C-153 also remains unchanged in all micremulsions of different R values. The invariance of solvation and rotational relaxation times of C-153 indicates that the position of C-153 remains unaltered with an increase in R and probably the probe is located at the interfacial region of [bmim][PF6] and TX-100 in the microemulsions. On the other hand, in the case of C-151, with an increase in R the fast component of the solvation time gradually increases and the slow component gradually decreases, although the change in solvation time is small in comparison to that of microemulsions containing common polar solvents such as water, methanol, acetonitrile, etc. The rotational relaxation time of C-151 increases with an increase in R. This indicates that with an increase in the [bmim][PF6] content the number of C-151 molecules in the core of the microemulsions gradually increases. In general, the solvent relaxation time is retarded in this room temperature ionic liquid/water-containing microemulsion compared to that of a neat solvent, although retardation is very small compared to that of the solvent relaxation time of the conventional solvent in the core of the microemulsions.