Rotational Dynamics of Coumarin-153 and 4-Aminophthalimide in 1-Ethyl-3-methylimidazolium Alkylsulfate Ionic Liquids: Effect of Alkyl Chain Length on the Rotational Dynamics

Abstract

Rotational dynamics of two neutral organic solutes, coumarin-153 (C-153) and 4-aminophthalimide (AP), with only the latter having hydrogen-bond-donating ability, has been investigated in a series of 1-ethyl-3-methylimidazolium alkyl sulfate ionic liquids as a function of temperature. The ionic liquids differ only in the length of the linear alkyl side chain (alkyl = ethyl, butyl, hexyl, and octyl) on the anionic moiety. The present study has been undertaken to examine the role of alkyl side chains on the rotational dynamics of the two solutes in these ionic liquids. Analysis of the results using Stokes-Einstein-Debye hydrodynamic theory indicates that the rotational dynamics of C-153 lies between the stick and slip boundary condition in the ethyl analogue and finally reaches subslip condition as in case of the octyl substituent. The observed rotational behavior of C-153 has been explained on the basis of an increase in the size of the solvent, which offers lower friction for solute rotation. On the other hand, AP shows superstick behavior in the ethyl system and exceeds the stick limit in the octyl derivative. Superstick behavior of AP has been attributed to the specific hydrogen-bonding interaction between AP and the sulfate moiety. Proton NMR investigation confirms the hydrogen-bonding interaction between the N-H hydrogen of AP and the ionic liquid. The decrease in rotational coupling constant values for AP with increasing length of alkyl side chains has been attributed to the decrease in the solute-solvent-specific interaction with an increase in the alkyl side chain length on the sulfate moiety.

Linking Diffusion–Viscosity Decoupling and Jump Dynamics in a Hydroxyl‐Functionalized Ionic Liquid: Realization of Microheterogeneous Nature of the Medium

“…[73] When the rotatings olute molecule is larger in size than the solvent molecule, C is unity (stick boundary condition).Inthe case of asolute molecule that is of asize comparable to or smaller than the solventm olecule, C is less than unity.T he shapes of the solute molecules are usually considered to be symmetrical or asymmetricale llipsoids in this theory. [32] The slip and stick boundary limits that were assigned with the help of SED hydrodynamic theory are shown in Figure 7w ith the experimentally measuredr eorientation times of C153 in the presentI Ls. For the calculation of the slip boundaryc ondition C slip , C153 is considered to be an asymmetricale llipsoid in determining the theoretical rotational time, and we took the probe properties that are already available in the literature.…”

“…The two extreme marginal conditions are stick and slip according to this hydrodynamic theory. [32] The van der Waals volume, shape factor,a nd calculated slip boundary condition parameter for C153 are 243 3 ,1 .5, and 0.18 respectively. For nonspherical molecules, f is generally greater than unity and the extento fd eviation from unity in the value of f describes the degree of nonspherical nature of the rotating species.…”

“…[42] Details of the calculation procedure have been described in our earlier publications. [32] The van der Waals volume, shape factor,a nd calculated slip boundary condition parameter for C153 are 243 3 ,1 .5, and 0.18 respectively. [32] The slip and stick boundary limits that were assigned with the help of SED hydrodynamic theory are shown in Figure 7w ith the experimentally measuredr eorientation times of C153 in the presentI Ls.…”

“…[32] The van der Waals volume, shape factor,a nd calculated slip boundary condition parameter for C153 are 243 3 ,1 .5, and 0.18 respectively. [32] The slip and stick boundary limits that were assigned with the help of SED hydrodynamic theory are shown in Figure 7w ith the experimentally measuredr eorientation times of C153 in the presentI Ls. With the help of Equation (4), the experimentally observed rotationalc oupling constant (C obs = t exp r /t stk r ), which is am easure of the extent of departures from normalh ydrodynamic behavior of as olute due to specific interactions,w as calculated and is listed in Ta ble3.…”

Influence of the Organized Structure of 1-Alkyl-3-methylimidazolium Tetrafluoroborates on the Rotational Diffusion of Structurally Similar Nondipolar Solutes

“…Nevertheless, viscosities of the ionic liquids have also been altered by increasing the length of the alkyl chain on one of the ions in quite a few studies. [17][18][19][20][21][22][23][24][25]39 In a majority of instances, it has been observed that the experimentally measured reorientation times did not comply with the trends predicted by the Stokes− Einstein−Debye (SED) hydrodynamic theory. 43,44 In other words, τ r versus η/T plots displayed a significant degree of nonlinearity, and also τ r values at a given η/T, especially for nonpolar and dipolar solutes, are found to be lower in ionic liquids with longer alkyl chains.…”