Article pubs.acs.org/JPCB
Rotational Diffusion of Nonpolar and Charged Solutes in Propylammonium Nitrate−Propylene Glycol Mixtures: Does the Organized Structure of the Ionic Liquid Influence Solute Rotation? Sugosh R. Prabhu and G. B. Dutt* Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India S Supporting Information *
ABSTRACT: 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 (9PA) 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.
1. INTRODUCTION
evidence to suggest that the organized structure of the ionic liquids affects this process as well. In recent times, rotational diffusion studies have been performed with different kinds of solutes in ionic liquids by us23−26 and others,27−31 wherein reorientation times (τr) were measured as a function of viscosity (η) by varying the temperature (T) of the ionic liquid and also by increasing the length of the alkyl chain on one of the constituent ions. In a majority of these studies, significant deviations from the Stokes−Einstein−Debye (SED) hydrodynamic theory32,33 have been noticed. Essentially, for a given η/T, a faster rotation of the solute has been noticed with an increase in the length of the alkyl chain. Furthermore, nonlinear relationships have been obtained between τr and η/T with the degree of nonlinearity
The unusual physicochemical properties of ionic liquids and their utility in numerous fields have been extensively documented in the literature.1−6 Unlike conventional solvents, ionic liquids are known to form organized structures as a consequence of various types of interactions prevailing between the constituent ions.7−18 The spatial heterogeneity of ionic liquids in turn influences dynamical processes such as solvation dynamics, photoisomerization, intramolecular charge transfer, proton transfer reactions, and solute rotation.19−31 It has been noticed that the rates of most of these processes are governed by the choice of the excitation wavelength. In other words, depending on the excitation wavelength, a distribution of relaxation rates has been observed due to the preferential excitation of solute molecules residing in distinct environments. Even though excitation wavelength dependence has seldom been observed in the case of solute rotation,29,30 there is ample © 2014 American Chemical Society
Received: February 7, 2014 Revised: February 19, 2014 Published: February 21, 2014 2738
dx.doi.org/10.1021/jp501343k | J. Phys. Chem. B 2014, 118, 2738−2745
The Journal of Physical Chemistry B
Article
the temperature. For the sake of comparison, similar measurements have been carried out in an organic solvent, propylene glycol (PG), whose size and viscosity are comparable to PAN. Figure 1 gives the molecular structures of the solutes 9-PA and
becoming pronounced with an increase in the length of the alkyl chain. The observed deviations from the SED theory have been rationalized by taking into consideration the organized structure of the ionic liquids, since solute rotation in these organized domains depends on microviscosity rather than the bulk viscosity of the medium. It may be noted that one of the prerequisites for observing the apparent influence of organized structure of ionic liquids on solute rotation is the presence of either strongly associating ions or long alkyl chains on one of the ions and these facets can be illustrated by considering the following examples from the literature. Samanta and co-workers28,29 have measured the reorientation times of numerous organic solutes in N-alkyl-Nmethylmorpholinium bis(trifluoromethylsulfonyl)imides and found significant deviations from the SED theory. However, in the case of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides, the effect of organized structure on solute rotation has been noticed only from octyl derivative onward.23,26 The important point to be noted here is that bis(trifluoromethylsulfonyl)imide is a weakly associating anion because of the delocalization of charge over the bulky anion.34 Nevertheless, in the presence of N-alkyl-N-methylmorpholinium, which is a strongly associating cation, these ionic liquids become more structured. Similarly, when longer alkyl chains (octyl and above) are substituted on the imidazolium cation, they undergo aggregation due to the stronger van der Waals interactions between them, which results in organized domain structure of the ionic liquids even in the case of weakly associating anions.13,26 In the same vein, rotational diffusion studies carried out in 1-alkyl-3-methylimidazolium-based ionic liquids with strongly associating anions such as tetrafluoroborate and hexafluorophosphate suggest that the influence of organized structure on solute rotation can be noticed even when shorter alkyl chains are present on the imidazolium cation.24,25 Apart from morpholinium- and imidazolium-based ionic liquids, solute rotation has also been examined in N-alkylpyrrolidinium bis(trifluoromethylsulfonyl)imides in a recent study.31 Despite varying the length of the alkyl chain from propyl to decyl, no pronounced alkyl chain length dependence has been noticed for the three solutes employed. From the structural studies available in the literature and the results summarized in the preceding paragraph, it is evident that the presence of longer alkyl chains on one of the constituent ions is necessary for the formation of organized structure in ionic liquids. However, exceptions to this rule do exist, as small angle neutron scattering studies carried out by Atkin and Warr14 indicate that protic ionic liquids such as ethylammonium nitrate (EAN) and n-propylammonium nitrate (PAN) form nanostructures in spite of having short alkyl chains. On the basis of the volume ratio of ionic and alkyl components being close to unity, it has been suggested that a lamellar structure comprising alternating polar−nonpolar layers is more probable. Since both of these ionic liquids are not optically birefringent, it is most likely that the layering is quite disordered. The organized structure reported for EAN and PAN forms the motive for undertaking the present study. Essentially, we are interested in finding out whether the organized structure of PAN influences solute rotation as observed in the case of morpholinium- and imidazolium-based ionic liquids. For this purpose, the fluorescence anisotropies of two structurally similar solutes, 9-phenylanthracene (9-PA) and rhodamine 110 (R110), have been measured in PAN by varying
Figure 1. Molecular structures of the solutes and the solvents.
R110 along with the solvents PAN and PG. We believe that such a comparison will enable us to figure out if the organized structure of the ionic liquid affects rotational diffusion of 9-PA and R110. Furthermore, studies have also been performed in four different compositions of PAN−PG mixtures to get a better appreciation of how the organized structure of the ionic liquid is affected on the addition of an organic solvent and its subsequent influence on solute rotation.
2. EXPERIMENTAL SECTION The ionic liquid n-propylammonium nitrate was purchased from io-li-tec, Germany, and was purified as described in the literature.35 The water content of the purified PAN is
