Article pubs.acs.org/JPCB
Universality of Viscosity Dependence of Translational Diffusion Coefficients of Carbon Monoxide, Diphenylacetylene, and Diphenylcyclopropenone in Ionic Liquids under Various Conditions Y. Kimura,*,†,‡ Y. Kida,‡ Y. Matsushita,§ Y. Yasaka,† M. Ueno,† and K. Takahashi§ †
Department of Molecular Chemistry and Biochemistry, Faculty of Science and Engineering and ‡Department of Applied Chemistry, Graduate School of Science and Engineering, Doshisha University, Kyotanabe 610-0321, Japan § Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan S Supporting Information *
ABSTRACT: Translational diffusion coefficients of diphenylcyclopropenone (DPCP), diphenylacetylene (DPA), and carbon monoxide (CO) in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIm][NTf2]) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIm][NTf2]) were determined by the transient grating (TG) spectroscopy under pressure from 0.1 to 200 MPa at 298 K and from 298 to 373 K under 0.1 MPa. Diffusion coefficients of these molecules at high temperatures in tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide ([P4441][NTf2]), and tetraoctylphosphonium bis(trifluoromethanesulfonyl)imide ([P8888][NTf2]), and also in the mixtures of [BMIm][NTf2], N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([Pp13][NTf2]), and trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide ([P66614][NTf2]) with ethanol or chloroform have been determined. Diffusion coefficients except in ILs of phosphonium cations were well scaled by the power law of T/η, i.e., (T/η)P, where T and η are the absolute temperature and the viscosity, irrespective of the solvent species, pressure and temperature, and the compositions of mixtures. The values of the exponent P were smaller for the smaller size of the molecules. On the other hand, the diffusion coefficients in ILs of phosphonium cations with longer alkyl chains were larger than the values expected from the correlation obtained by other ILs and conventional liquids. The deviation becomes larger with increasing the number of carbon atoms of alkyl-chain of cation, and with decreasing the molecular size of diffusing molecules. The molecular size dependence of the diffusion coefficient was correlated by the ratio of the volume of the solute to that of the solvent as demonstrated by the preceding work (Kaintz et al., J. Phys. Chem. B 2013, 117, 11697). Diffusion coefficients have been well correlated with the power laws of both T/η and the relative volume of the solute to the solvent.
1. INTRODUCTION Translational mobility of a molecule dissolved in ionic liquids (ILs) has attracted much attention of a lot of chemists due to its importance for application, and numerous researches have been performed on the mobility of ions and neutral molecules dissolved in ILs by using various kinds of methods such as conductivity, pulse−field gradient (PFG) NMR, transient grating spectroscopy, and so on.1,2 Watanabe’s group has compared the mobility of ion estimated from the conductivity with the translational diffusion coefficient derived from the PFG-NMR method, and discussed the iconicity of ionic liquids by taking the ratio of them.3−7 They considered that the ratio represents the correlated motion between cations and anions and that the ionic liquid has high iconicity if the ratio is close to 1. Through these works it has been found that the diffusion coefficients of the solvent ion are almost proportional to the reciprocal viscosity (η−1, η the viscosity of the solvent) as is expected by the Stokes−Einstein (SE) relationship, although the proportional constant is not exactly the same as the molecular radius estimated by the van der Waals volume. For © XXXX American Chemical Society
neutral solute molecules in ionic liquids, several interesting properties have been revealed until now. Diffusion coefficients of small neutral molecules such as gases (oxygen, carbon monoxide, carbon dioxide, and so on) are much larger than expected from the SE predictions by an order of magnitude.8−13 The diffusion coefficients of nonpolar solute molecules are generally not proportional to η−1, but rather expressed by η‑P, where P is the constant dependent on the molecular size of the solute. Recently Kaintz et al. reported a detailed collection of the translational diffusion coefficients in various kinds of ILs together with new data measured by PFGNMR.14,15 They evaluated the ratio of the diffusion coefficient determined experimentally to that estimated from the SE relation, and showed a good correlation of the ratio to the relative volume of the solute molecule to that of the solvent ions. Received: March 26, 2015 Revised: May 10, 2015
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DOI: 10.1021/acs.jpcb.5b02898 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
pressure should be taken into account separately, although we found that the molecular size dependence is well explained by the correlation found in the previous work.14
In this paper, we have investigated the pressure and temperature dependence of the translational diffusion of nonpolar solute molecules (tracer diffusion) in imidazoliumbased ionic liquids; 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIm][NTf2]) and 1ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIm][NTf2]) by using the transient grating spectroscopy. One of the authors previously investigated the translational diffusion coefficients of the molecules associated with the photodissociation of diphenylcyclopropenone (DPCP) in various kinds of ILs.13 DPCP is photodissociated into carbon monoxide (CO) and diphenylacetylene (DPA) by UV light. By using the transient grating spectroscopy, the diffusion coefficients of these three chemical species were determined simultaneously. Since the molecular size of CO is quite different from those of DPA and DPCP, the reaction system is suitable for the investigation of the molecular size effect on the translational diffusion. It has been reported that the diffusion coefficients of CO were much larger than those predicted by SE relation, and the deviation from the SE prediction became smaller for DPA and DPCP.13 In this work, we use a highpressure optical cell designed for the TG measurement, and report the translational diffusion coefficients of CO, DPA, and DPCP under pressure up to 200 MPa. We have also measured the temperature effect on the diffusion coefficients up to 373 K for the same system under ambient pressure. By the combination of pressure and temperature variations, we can change the value of T/η (T the absolute temperature), which is a key factor affecting the diffusion, by 2 orders of magnitude. There are several reports on the temperature dependence of the diffusion coefficients on neutral molecules in ILs, and it has been reported that the activation energy of the diffusion coefficient is not exactly the same as that of 1/η.14 However, the pressure effect on the translational diffusion has scarcely been reported,16 although there are several reports on thermodynamic data such as density and viscosity under pressure.17−21 It is an interesting issue how the effects of pressure and temperature will appear in the different sizes of solute molecules. In parallel with the study on the pressure and the temperature effects in the imidazolium cation-based ionic liquids, we have performed a study on the solvent molecular size effect on the solute diffusion coefficient using phosphonium cation-based ILs with longer alkyl chains and the mixtures of the ILs with conventional liquid solvents. As mentioned, it has been known that molecules in ILs with longer alkyl chains such as trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide ([P66614][NTf2 ]) diffuse much faster than expected from the SE relationship.11,13 The report by Kaintz et al. indicated that the relative volume of the solvent ions to that of the solute is the determining factor.14 In order to pursue this issue and compare the results with the imidazolium-cation cases, we have measured the DPCP system in two different phosphonium cation-based ILs (tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide ([P4441][NTf2]) and tetraoctylphosphonium bis(trifluoromethanesulfonyl)imide ([P8888][NTf2])), and in the mixture of several ILs with conventional liquid solvents. We chose three ILs of different cations as [BMIm]+, N-methyl-Npropylpiperidinium ([Pp13]+), and [P66614]+. By using the mixture, the relative size of the solvent molecules can be continuously varied. For the better understanding of the size effect of the diffusion coefficient, the effects of temperature and
2. EXPERIMENTAL SECTION 2.1. Materials. DPCP was purchased from Nacalai Tesque or Aldrich and used without further purification. Bromocresolpurple (BCP) and methanol were purchased from Nacalai Tesque and used without further purification. [P4441][NTf2] (>99.98%, product of Nippon Chemical Industrial Co.), [Pp13][NTf2] (>98%, product of Merck), [P66614][NTf2] (>98%, product of Cytec), ethanol (>99.5%), and chloroform (>99.5%) were purchased from Kanto Kagaku. For the measurements of the high pressure or the high temperature solutions, specially prepared high purity grade of [EMIm][NTf2] and [BMIm][NTf2] (>99.99%), supplied from Kanto Kagaku were used for the measurements. For the measurements of the mixture solution, products of Merck (>98%) were used. [P8888][NTf2] were synthesized as is described in the Supporting Information. The purity of [P8888][NTf2] (>99%) was confirmed by NMR. All ILs were transparent at 355 nm (the excitation wavelength of the laser). For the measurements under high pressure or high temperature, ILs were evacuated under 0.4 Pa overnight at about 50 °C. The concentrations of DPCP were typically 30 mM, and the solutions were evacuated for more than 2 h before use. The solution was kept in a drybox under argon atmosphere. The ILs used for the measurements of the mixtures contained somewhat large amount of water (
