Rotational
diffusion of two structurally similar nonpolar and charged solutes has
been examined in mixtures of an ionic liquid and an organic solvent of
comparable size and viscosity with an intent to find out whether the
organized structure of the former influences solute rotation. To this
effect, temperature-dependent fluorescence anisotropies of
9-phenylanthracene (9-PA) and rhodamine 110 (R110) have been measured in
n-propylammonium nitrate (PAN), propylene glycol (PG), and also four
different compositions of PAN−PG mixtures. Analysis of the data carried
out with the aid of Stokes−Einstein−Debye (SED) hydrodynamic theory
indicates that the reorientation times of 9-PA and R110 scale more or
less linearly with the ratio of viscosity to temperature and are found
to be independent of the mole fraction of PAN. In other words, apart
from the viscosity and temperature, rotational diffusion of both the
solutes is not affected by the composition of PAN−PG mixtures. It has
also been observed that the reorientation times of R110 are
significantly longer compared to those of 9-PA due to the specific
interactions prevailing between the cationic solute and PAN−PG mixtures.
However, the important finding of this work is that, even though PAN
forms an organized structure, rotational diffusion of the solute
molecules is similar in both the ionic liquid and the organic solvent.
The disordered lamellar structure present in PAN probably does not offer
compact organized domains unlike ionic liquids with long alkyl chains
wherein solute rotation is influenced significantly.