VISCOSITY B COEFFICIENTS FOR THE AQUEOUS SOLUTIONS OF TETRAALKYLAMMONIUM BROMIDE UNDER HIGH PRESSURE

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1 Materials Science Research International, Vol.1, No.2, pp (1995) General paper VISCOSITY B COEFFICIENTS FOR THE AQUEOUS SOLUTIONS OF TETRAALKYLAMMONIUM BROMIDE UNDER HIGH PRESSURE Yukihiro YOSHIMURA*, Seiji SAWAMURA** and Yoshihiro TANIGUCHI** *Department of Chemistry, National Defense Academy, Yokosuka, Kanagawa 239, Japan. **Department of Chemistry, Faculty of Science and Engineering Ritsumeikan University, Kusatsu, Shiga , Japan. Abstract: The viscosities of aqueous solutions of tetraalkylammonium bromide (R4NBr; R=Me, Et, n-pr, and n-bu) were measured in the range of molkg-1 and Ž respectively up to a pressure of 375MPa, using a high pressure rolling-ball viscometer. The activation energy for viscous flow (Ev) and Jones-Dole B coefficient were estimated The viscosity vs. pressure minimum at 5.0 Ž observed for pure water becomes shallow or disappears with increasing concentration of R4NBr salt and increasing temperature. Ev for aqueous solution of Me4NBr is lower than that of pure water in the whole concentration range under high pressure. Two types of the pressure dependence of the Jones-Dole B coefficients are given. These phenomena were interpreted based on the stand point of pressure, temperature, concentration effects for the structure of water. Key words: Viscosity, Tetraalkylammonium bromide, High pressure, B coefficient, Activation energy for viscous flow 1 INTRODUCTION Viscosities of aqueous electrolyte solutions have given us much information about the dynamic structure of the solutions concerning the nature of ion-solvent interactions [1-5]. In general, the concentration dependence of the relative viscosities can be represented by the Jones-Dole equation [6] where ƒå and ƒå0 are the viscosities of solution and water, respectively and c is the concentration (mol1-1). The positive A coefficient related to the long-range coulombic forces among solute ions can be calculated from the theory [7]. The B coefficients are due to both the size of the ions and the ion-solvent interactions. They take both positive and negative signs depending of ions. The ions with the positive B coefficient, i.e., the structure maker, enhance the structure of water and the ions with the negative B coefficient, i.e., the structure breaker, break the structure of water. Temperature dependence of the viscosities of ionic aqueous solutions is given by the activation energy for viscous flow (Ev) as proposed by Eyring [8] such as Ev is one of the important parameters for understanding the structure of ionic hydration [9] because Ev is due to the maximum energy required to push the neighbor ions and move to the next position. Received Dec. 12, 1994 Equation (3) is derived from Eqs. (1) and (2), assuming that the term A ãc in Eq. (1) is neglected for dilute solutions, because the A term is much smaller than the B term. Thus Ev is related only to the B coefficient, Ev(sol)-Ev(H2O)=R[d in (1+Bc)/d(1/T)]p (3) where Ev(sol) and Ev(H2O) are the activation energies of solutions and water, respectively. If the B coefficient increases with increasing temperature, Ev(H2O) will be larger than Ev(sol), and suggests that the structure of water is broken by adding electrolytes to water. The viscosity of water in the range of 0-35 Ž under high pressure shows a minimum at MPa [10-13]. This anomaly of viscosity by compression has been explained due to breaking up of the structure of water. The effects of temperature, concentration and pressure on the viscosities for aqueous solutions of tetraalkylammonium bromides, R4NBr (R=Me, Et, n-pr, and n-bu) were measured in the temperature range of Ž, with the concentration range of molkg-1, and pressures up to 375MPa [14, 15]. R4NBr salts are known to form ice like (iceberg) structure around the hydrocarbon groups in water, i.e., hydrophobic hydration [16]. The properties of R4NBr salts in water change depending on the alkyl chain length. In the present work, we focused on the ion-water interaction in aqueous solutions of R4NBr under high pressure, especially on the relationship between the structure of water and the hydrophobic hydration of R4N+ ions under high pressure, so that the pressure dependences of the activation energies for viscous flow and

2 VISCOSITY B COEFFICIENTS FOR R4NBr UNDER HIGH PRESSURE Jones-Dole B coefficients could be estimated. 2 EXPERIMENTAL R4NBr (R=Me, Et, n-pr, and n-bu) salts provided from Nakarai Tesque, Inc. (extra pure grade) were recrystallized twice from water, and dried in a vaccum desiccator with P2O5. The purity of the salts was checked by titration with 0.1 N AgNO3 solution. All salts had a purity qualified to be 99.8%. Solutions with concentration of 0.100, 0.500, 0.700, ( }0.0002) molkg-1 were prepared using an analytical balance. Sample solutions were filtrated through the membrane filters (d=25mm, pore size 0.1ƒÊm) in order to remove dust. The viscosity of each sample was measured up to 375MPa with every 25MPa at the temperature of 5.0, 10.0, 25.0, 40.0, 50.0 Ž. The reproducibility was within } 1%. The apparatus and experimental procedures under high pressure were previously described in detail [17]. 3 RESULTS AND DISCUSSION 3.1 Viscosity at high pressure The observed viscosity data under high pressure are represented by the interpolating equation in Eq. (4), The maximum deviation of the viscosity in Eq. (4) from the experimental data is within }1%. The coefficients f2, f1, f0 are presented in Tables 1 and 2. Figures 1 and 2 show the typical pressure dependence of the viscosities for aqueous solutions of (a) Me4NBr, (b) n-bu4nbr, and pure water at 5.0 and 50.0 Ž. Solid curves for aqueous solutions of R4NBr in Figs. 1 and 2 are calculated by Eq. (4), and dotted lines are for pure water [10]. At 5.0 Ž (Fig. 1) the viscosities for aqueous solutions of Me4NBr at each concentration under high pressure show minima similar to that of pure water, but these of n-bu4nbr at higher concentrations above 0.5molkg-1 do not show any minima At low temperatures we can not measure the viscosities for aqueous solutions of n-bu4nbr under high pressure, because the aqueous solutions of n-bu4nbr are frozen by pressure. For example at 5.0 Ž, aqueous solutions of n-bu4nbr froze at about 750MPa for 0.5molkg-1, 250MPa for 0.7molkg-1, and 5MPa for 1.0molkg-1, respectively. At 25.0 Ž, the viscosities for all systems under high pressure do not show any minima at higher concentrations. The viscosity minima become shallow and then disappear with higher concentration for the same sample. At 50.0 Ž (Fig. 2), the viscosities of all the solutions increase by compression similar to that of pure water. The viscosity minimum disappears not only with increasing temperature but also with the amount of R4NBr. The effect of R4NBr on the viscosity is larger Fig. 1. Pressure dependences of the viscosity for aqueous solutions of R4NBr and water at 5.0 Ž. with longer alkyl chain of R4NBr, i.e., Me, Et, n-pr, and n-bu. 3.2 Activation energy for viscous flow Viscous flow can be treated as the rate process and the activation energy for viscous flow (Ev) is calculated from Eq. (2). The relationship between In ƒå and 1/T at 0.5molkg-1 aqueous solution of Me4NBr is shown in Fig. 3. The curve at 0.1MPa crosses the curve of 200MPa at 15.0 Ž. This corresponds to the viscosity minimum in Fig. 1 for (a) Me4NBr. These plots are fitted

3 Yukihiro YOSHIMURA, Seiji SAWAMURA and Yoshihiro TANIGUCHI Table 1. Coefficients of f2, f1, and foin Eq.(4) for aqueous solutions of Me4NBr and Et4NBr. Table 2. Coefficients of f2, f1, and foin Eq.(4) for aqueous solutions of n-pr4nbr and n-bu4nbr.

4 VISCOSITY B COEFFICIENTS FOR R4NBr UNDER HIGH PRESSURE Fig. 3. The plots of In ƒå vs. 1/T for 0.5molkg-1 aqueous solution of Me4NBr at several pressures. Fig. 2. Pressure dependences of the viscosity for aqueous solutions of R4NBr and water at 50.0 Ž. to a polynomial of the second degree. Ev at 25 Ž was estimated from the slope of this curve at 25.0 Ž as shown in Fig. 3, and the pressure dependences of Ev of the viscous flow for (a) Me4NBr and (b) n-bu4nbr are shown in Fig. 4. The dotted line is for pure water [10]. The activation energy of pure water has a minimum at about 250MPa. The decrease of Ev for pure water under high pressure corresponds to the disruption of the water structure and the increase of Ev under high pressure is due to the restriction on the movement of water molecules by compression [20]. We reported previously that at 0.1molkg-1 Ev for aqueous solution of Me4NBr is lower than that of pure water [15]. In the present study Fig. 4 shows that each Ev for aqueous solution of Fig. 4. Pressure dependences of the activation energy for mmolkg-1 aqueous solutions of R4NBr and water at 25.0 Ž.

5 Yukihiro YOSHIMURA, Seiji SAWAMURA and Yoshihiro TANIGUCHI Me4NBr at each concentration under pressure is smaller than that of pure water. Furthermore, Ev for aqueous solution of Me4NBr decreases with the concentration at 0.1MPa, but at higher pressures above 100MPa it increases with the concentration. Me4N+ ion is known to show structure-breaking properties [7, 16, 19]. Hence, Ev for aqueous solution of Me4NBr at 0.1MPa decreases with increasing concentration. The structure of water would be also broken by compression and the pressure above 100MPa restricts the movement of ions. For aqueous solution of n-bu4nbr, Ev is larger than that of pure water in the whole concentration range and Ev value increases with the concentration. As the Ev vs. pressure plots show minima for every concentration of aqueous solution of n-bu4nbr, the structure of water is not completely destroyed at the high concentration of 1.0molkg Jones-Dole B coefficient under high pressure The extended Jones-Dole equation [20] is given by where A, B, and D are constants, and ƒå0 and ƒå are the viscosity of water and solutions. c is the concentration Fig. 5. The plots of(ƒå/ƒå0-1-a ãc)/c vs. c for aqueous solution of Me4NBr at 25.0 Ž and several pressures. (mol1-1) of the electrolytes. This equation can be changed to Eq. (6) as a linear function of c, The viscosity data for aqueous solutions of R4NBr at various pressures are plotted in this way as shown in Fig. 5, where the A ãc term was calculated from the theory [2, 21]. The plots for aqueous solution of Me4NBr at 25.0 Ž are given in Fig. 5 along with the data of Out et al. [19] and Desnoyers et al. [20]. In general, our data at 0.1MPa is in good agreement with their data. Figure 5 shows that the value of (ƒå/ƒå0-1-a ãc) is a linear function of the concentration at all pressures. The B coefficients can be determined from the intercept of the linear line at zero concentration. The linear line is derived by a least square analysis of the present data. The pressure dependences of the B coefficients are shown as Fig. 6. The B coefficient for aqueous solution of Me4NBr shows a maximum at about 150MPa, and those for another aqueous solutions of R4NBr decrease with increasing pressure. The pressure dependence of the B coefficient for aqueous solution of Me4NBr is similar to that for aqueous solution of NaCl [22], showing its structure-breaking property [3]. That is, the increase of B coefficients at pressure up to 150MPa may be related to the fact that the structure of water at 0.1MPa is broken by Me4NBr. This breaking effect becomes less clear at high pressures since the structure of water is already broken by pressure. Larger hydrocarbon groups of R4NBr enforce the ice-like structure of water around the hydrocarbon groups, i.e., hydrophobic hydration, and it increases the degree of hydrogen bonding in their vicinity of R4N+ ions. So that the steep decrease of the B coefficients for these solutions with increasing pressure Fig. 6. Pressure dependences of the Jones-Dole B coefficients for aqueous solutions of R4NBr at 25.0 Ž. may be explained not only bulk structure of water broken by pressure but also the ice-like structure of water around the hydrocarbon groups broken or weaken by pressure. For aqueous solution of Et4NBr, the decrease of the B coefficients at high pressure is less and is almost constant. his is interpreted by assuming that

6 VISCOSITY B COEFFICIENTS FOR R4NBr UNDER HIGH PRESSURE Et4N+ ion is an intermediate size for which the structuremaking and the structure-breaking effects are canceled each other [7]. The temperature dependences of the B coefficients at various pressures are given in Fig. 7. Kaminsky [3] and Frank and Wen [23] have introduced the quantity db/dt to distinguish solute ions between the structure-breaker (db/dt>0) and the structure-maker (db/dt<0). The temperature dependence of the B coefficients has been considered to be an important parameter related to the structure of water than the sign of the B values, because the sign of the B coefficients of electrolytes depends on temperature. The structure of water may depend on temperature, and so the temperature dependence of the B coefficients is useful to clarify the effect of R4NBr on the structure of water. The B coefficients for aqueous solution of Me4NBr at 0.1MPa increase with temperature, db/dt>0, but the slope of db/dt becomes smaller with increasing pressure, and then at 300MPa it changes negative. Therefore, Me4NBr of the structure-breaking property at 0.1MPa change into the structure-making property at 300MPa. On the other hand, aqueous solutions of n-pr4nbr and n-bu4nbr show the negative slope of db/dt at 0.1MPa, and at high pressure it becomes positive. The B coefficients for aqueous solution of n-bu4nbr at lower temperatures below 20 Ž under high pressure can not be estimated because of its freezing under high pressure. As n-pr4n+ and n-bu4n+ ions having the large hydrophobic groups are the strong structure-maker at 0.1MPa, the slope of db/dt is steep. With increasing pressure, hydrogen bonds in the bulk structure of water and the ice-like structure of water Fig. 7. Temperature dependences of the Jones-Dole B coefficients for aqueous solutions of R4NBr at several pressures. around the hydrocarbon chains of the salts would be broken or weaken, and the structure-making property becomes less clear. The water structure around Et4N+ ion almost seems not to depend on temperature and pressure. 4 CONCLUSION The pressure dependences of the B coefficients for aqueous solutions of R4NBr are classified into two types. One is the increasing of the B coefficients, and the other is the decreasing of the B coefficients with increasing pressure. This may be related to the breaking of the structure of water by pressure and the property of solutes, i.e., the structure-breaker or the structure-maker. The slope of db/dt also changes under high pressure, and the property of each solute becomes less clear as the structure of water is broken by compression. REFERENCES 1. M. Kaminsky, Discuss. Faraday Soc., 24 (1957) M. Kaminsky, Z. Phys. Chem. N.F., 5 (1955) M. Kaminsky, Z. Phys. Chem. N.F., 8 (1956) M. Kaminsky, Z. Phys. Chem. N.F., 12 (1957) G. Jones, M. Dole, J. Am. Chem. Soc., 51 (1929) H. Falkenhagen, E.L. Nernon, Philos. Mag., 14 (1932) R.L. Kay, T. Vituccio, C. Zawoyski, D.F. Evans, J. Phys. Chem., 70 (1966) S. Glasstone, K.J. Laidler, H. Eyring, The Theory of Rate Processes, McGrow-Hill, New York (1941). 9. D. Eisenberg, W. Kauzmann, The Structure and Properties of Water, Oxford Univ. Press, Oxford (1969). 10. International Association for the Properties of Steam, The I APS formulation 1985 for the Viscosity of Ordinary Water Substance (1984). 11. K.E. Bett, J.B. Cappi, Nature, 240 (1965) E.M. Stanley, R.C. Batten, J. Phys. Chem., 73 (1969) R.A. Home, D.S. Johnson, J. Phys. Chem., 70 (1966) Y. Yoshimura, S. Sawamura, J. Mastuda, K. Kitamura, Y. Taniguchi, J. Soc. Mat. Sci., Japan, 41 (1992) Y. Yoshimura, S. Sawamura, Y. Taniguchi, J. Soc. Mat. Sci., Japan, 42 (1993) W.Y. Wen, Water and Aqueous Solutions, (ed. by R.A. Home), John Wiley, New York (1972). 17. S. Sawamura, N. Takeuchi, K. Kitamura, Y. Taniguchi, Rev. Sci. Instrum., 61 (1990) T. Defries, J. Jonas, J. Chem. Phys., 66 (1977) D.J.P. Out, J.M. Los, J. Sol. Chem., 9 (1980) J.E. Desnoyers, G. Perron, J. Sol. Chem., 1 (1972) R.L. Kay, Water, Vol.3 (ed. by F. Franks), Plenum Press, New York (1973). 22. S. Sawamura, Y. Yoshimura, Y. Taniguchi, J. Phys. Chem., 96 (1992) H.S. Frank, W.Y. Wen, Discuss. Faraday Soc., 24 (1957) 507.