Rotational diffusion of nonpolar and charged solutes in propylammonium nitrate-propylene glycol mixtures: does the organized structure of the ionic liquid influence solute rotation?

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.