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CORRESPONDENCE Comment on “A Debye-Hu 1 ckel Model for Calculating the Viscosity of Binary Strong Electrolyte Solutions” Anil Kumar† Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, India
Sir: Esteves et al. have proposed a simple and useful model for calculating the dynamic viscosity of 21 binary solutions of electrolytes in water, methanol, ethanol, and 1-butanol at 0.1 MPa and 25 °C.1 Their model is based on Eyring’s absolute rate theory and the Debye-Hu¨ckel model, which are employed for calculating the excess free energy of activation of viscous flow. We have the following comments to offer on the application of their model: 1. The classic Jones-Dole equation that is applicable up to 0.01 M of electrolyte solution offers very significant information via the B coefficient extracted from the fitting of η/η1 versus c21/2 for a given electrolyte, where c2 is its molar concentration. This significant information from the B coefficient is related to the structuremaking and -breaking abilities of the ions, of which an electrolyte is made up.2 The structure-making and -breaking properties of ions define how the water structure is made or broken because of the presence of an ion. For example, Na+ orients water molecules around itself because of a strong electric field and is therefore called a structure maker. On the other hand, ClO4- breaks the water structure by disturbing hydrogen bonds and is known as a structure breaker. These structure-making and -breaking characteristics of ions in a solvent are shown by positive and negative B values. Accordingly, the B value for Na+ is +0.085 dm3 mol-1, while that for ClO4- is -0.058 dm3 mol-1 at 25 °C.2 The ionic B values are additive so it is possible to obtain information on a given electrolyte in a solvent. With the above guidelines, if we examine the B values for electrolytes given their Table 1, we fail to get a clear picture of the structure-altering properties of ions/ electrolytes in different solvents. For example, all of the electrolytes, except KBr in water, are seen to have very high B values. With this observation, it is clear that all of these electrolytes barring KBr are structure makers. If these values do not reflect the structure-making or -breaking properties, the negative B value for KBr in their Table 1 is quite puzzling. As reported by the authors in their Table 1, the B value of NaCl, a structure maker, is about 6 times lower than that of KC1, a structure breaker in water. In a well-accepted manner in the literature, B values for NaCl and KCl are +0.080 and -0.013 dm3 mol-1, respectively, clearly showing the manner in which these electrolytes influence the water structure.3 An examination of B values given in their Table 1 shows that all of the electrolytes, except KBr, cause electrostriction4,5 of water, a point that calls for interpretation of these values. Further, no systematic †
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trend is seen in the B values among the 1:1, 2:1, 1:2, and 2:2 types of electrolytes. If one moves from water to methanol, a given electrolyte which was originally a structurebreaker in water becomes a structure maker because of ion solvation.2,6 Thus, KCl with B ) -0.013 dm3 mol-1 in water increases to 0.764 to 1.35 dm3 mol-1 in methanol to 1-butanol.3 Such a trend in B values from water to methanol to 1-butanol is missing in their table. However, if the B value is merely an adjustable parameter obtained from the least-squares analysis of the viscosity data, it has significance in terms of the correlation power of the proposed model, as evidenced from the statistical analysis given by the authors. 2. Considering the whole work as a fitting exercise, the model works as a powerful predictive tool only for the structure makers. If their Figure 4 is examined, their models overestimate the viscosity by a greater degree through the concentration ranging up to 3 M. This is due to the fact that their equations must be refined for the structure breakers. This means that the proposed model is inadequate for accounting for the decreased viscosity upon addition of an electrolyte, like KC1. The results of Figure 6 do not show the success of the model but clearly bring out its limitations. LiI + l-butanol is a complex solvated system, which cannot be accounted for by the simple model of these authors. From Figure 6 it is clear that it is futile to rationalize the fitting in terms of the mean relative standard deviation as shown by the authors. In summary, though the proposed model is quite effective in correlating the viscosity-concentration data of several binary electrolytes, it requires refinement with applications to temperature- and pressure-dependent viscosity data to cover process engineering and other kinetic studies of organic reactions in electrolyte solutions.7 Acknowledgment The Department of Science and Technology, New Delhi University, New Delhi, India, is acknowledged for a grant-in-aid (SP/S1/G-19/99) to carry out work in this area. Literature Cited (1) Esteves, M. J. C.; Cardoso, M. J. E. de M.; Barcia, O. E. A Debye-Hu¨ckel Model for Calculating the Viscosity of Binary Strong Electrolyte Solutions. Ind. Eng. Chem. Res. 2001, 40, 50215028. (2) For example, see: Marcus, Y. Ion Solvation; Wiley: New York, 1985.
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(3) Jenkins, H. D. B.; Marcus, Y. Viscosity B-Coefficients of Ions in Solution. Chem. Rev. 1995, 95, 2695. (4) Millero, F. J. Molal Volumes of Electrolytes. Chem. Rev. 1971, 71, 147-176. (5) Kumar, A.; Phalgune, U.; Pawar, S. S. Salt Solutions in Different Solvents and their Effect on the Stereoselectivities of Diels-Alder Reaction. J. Phys. Org. Chem. 2001, 14, 577-582.
(6) Kumar, A. Alternate View on Thermal Stability of the DNA Duplex. Biochemistry 1995, 34, 12921-12925. (7) Kumar, A.; Pawar, S. S. Rate Acceleration and Subsequent Retardation of Diels-Alder Reactions in LiC1O4-Diethyl Ether: An Experimental Investigation. J. Org. Chem. 2001, 66, 7646-7652.
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