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The high-frequency dielectric spectroscopy of the aqueous solutions of tetrabutylammonium carboxylates
Journal of Non-Crystalline Solids 351 (2005) 2882–2887 www.elsevier.com/locate/jnoncrysol
The high-frequency dielectric spectroscopy of the aqueous solutions of tetrabutylammonium carboxylates D. Loginova a
a,*
, A. Lileev a, A. Lyashchenko a, L. Aladko
b
Institute of General and Inorganic Chemistry RAS, 119019 Leninskii pr. 31. Moscow, Russia b Institute of Inorganic Chemistry SBRAS, Novosibirsk, Russia
Abstract The studies of complex permittivity and low-frequency conductivity of aqueous tetrabutylammonium formate and propionate solutions were carried out in the frequency region of maximum dispersion of dielectric permittivity of water. The measurements were made over a wide range of concentrations, in the temperature interval 288–308 K. The dielectric relaxation time (s) and activation enthalpy ðDH þþ e Þ were calculated. There is an increase in these parameters in aqueous tetrabutylammonium formate and propionate solutions in comparison with pure water. The concentration dependences of s and DH þþ relate to the structure-forming effect of e tetrabutylammonium and propionate ions on water through hydrogen bonding. The stabilization of the water structure is determined by the hydrophobic hydration of these ions. The hydrophobic hydration increases in going from tetrabutylammonium formate to tetrabutylammonium propionate aqueous solutions. Ó 2005 Published by Elsevier B.V. PACS: 77.22.d; 77.22.Ch; 77.22.Gm
1. Introduction The phenomenon of the hydrophobic hydration is determined by the stabilization of H-bonds of water near non-polar groups of molecules or ions and with the ordering of the tetrahedral water structure. This effect has been investigated for a long time [1–5]. Microwave dielectric spectroscopy is an informative method for study of hydration in aqueous solutions. Parameters of dielectric spectra for hydrophobic and hydrophilic hydration are different from those of electrolyte solutions [6,7]. Characteristics of dielectric relaxation give information about total changes in molecular-kinetic mobility of water molecules under the influence of dissolved molecules and ions. They have been received for the large amount of non-elec*
Corresponding author. Tel.: +7 095 2364610. E-mail address: [email protected] (D. Loginova).
0022-3093/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2005.06.019
trolyte systems [8]. In the case of hydrophobic hydraþþ tion ssol sH2 O > 0 and DH þþ e ðsolÞ DH e ðH2 OÞ > 0. The aqueous solutions of tetraalkylammonium (TAA) salts are the typical examples of the hydrophobic hydration of electrolyte solutions. The dielectric relaxation in these solutions has been investigated by several workers [9–14]. The data [12–14] are obtained in a wide frequency range but only at one temperature. Only in work [14], 1.85 m Pr4NBr and 1.23 m Bu4NBr solutions are investigations over a temperature interval. The dielectric characteristics in works [9,10] have been studied only in the area of maximal dispersion of water molecules. However, in this case the measurements were made over a temperature interval. So, the activation parameters of the dielectric relaxation process have been calculated. In this case ssol sH2 O > 0 þþ and DH þþ e ðsolÞ DH e ðH2 OÞ. The changes of s and þþ DH e in TAA solutions in D2O are also considered [11].
D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887
The hydrophobic hydration in TAA solutions can depend not only on the sizes and geometrical forms of cations but also on anions. However, systematic studies of this problem have not been carried out yet. The aqueous solutions of tetrabutylammonium (TBA) carboxylates are a suitable model system for this purpose. It is known that the tetrabutylammonium ion has hydrophobic hydration. The presence of the hydrophobic hydration of propionate ion in aqueous propionate solutions of alkaline metals was revealed in some works [15–17]. The transition from the TBA formate to TBA propionate solutions allows looking at the changes of the molecular-kinetic characteristics of water at the transition from hydrophilic hydration of anion to hydrophobic hydration of anion. The present paper is devoted to research of this question.
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The low-frequency conductivity (j) of potassium propionate solutions and tetrabutylammonium formate and propionate solutions was measured at frequency 1 kHz using the U-figurative glass cell with smooth platinum electrodes at the same temperature interval. The cell was thermostated with the help of the thermostat U-8 with accuracy ±0.05°. The accuracy of measurement of j was not more than 0.5%. Aqueous tetrabutylammonium formate and propionate solutions were prepared in equimolecular parts of aqueous solution of tetrabutylammonium hydroxide and formic or propionic acids. Initial reagents from ÔSouzreakhimÕ have qualification Ôchemical pureÕ. Ordinary distilled water was used.
3. Results 2. Experimental The measurements of the high-frequency dielectric permittivity (e 0 ) and losses (e00 ) of aqueous formate and propionate TBA solutions were fulfilled in the frequency region of the maximum dispersion of water (in the microwave range) by the method of thin dielectric rod in the waveguide [18]. In this method the standing electro-magnetic wave is established in the empty waveguide. The thin glass capillary is filled by the investigated solution. The capillary with a solution is located in the waveguide. The intensity of an electrical field in units of standing wave becomes distinct from zero at entering of sample in the waveguide. It is caused by the absorption of the electro-magnetic wave by the investigated solution. The amplitude of the reflected wave becomes less than the amplitude of the falling wave. The minima of the standing wave in the waveguide are displaced in the direction of the sample. The changes of parameters of a standing wave are directly connected with the dielectric permittivity and losses of sample. The capillary was calibrated on the data of the high-frequency permittivity e 0 and losses e00 of water at corresponding frequency. These values were calculated by the Debye relations. The values of the dielectric relaxation time and static dielectric permittivity of water were chosen from the literature data: s = 11.0, es = 82.0; (288 K); s = 8.25, es = 78.4 (298 K); s = 6.45, es = 74.9 (308 K) [19,20]. The waveguide section with sample was supplied with water thermostat jacket. The temperature in the cell was maintained using the thermostat U8 (VEB MLW PRUGERATE – WEBK MEDINGEN, DDR) and controlled by the copper–constantan thermocouple with accuracy ±0.05°. The measurements were carried out at seven frequencies in the interval 7– 25 GHz, at temperatures 288, 298 and 308 K. The accuracy of measurement of e 0 and e00 is ±1.0–1.5% and ±2.0–2.5%, respectively.
The aqueous electrolyte solutions are conductive liquids. Therefore, the measured values of e00 consist of two parts. The ionic losses were calculated using equation e00i ¼ j=e0 x [21], where x is the circular frequency, e0 is the dielectric permittivity of vacuum. Dipole losses of solutions were calculated by the equation e00d ¼ e00 e00i . The concentration dependences of dipole losses e00d of aqueous TBA formate and propionate solutions have specific features for different frequencies, which change with temperature. The example of such dependences is represented in Figs. 1(a) and 2(a). The bends of the curves were observed at initial concentration for decencies e00d vs m at 7.71 and 9.45 GHz. The values of e00d smoothly decrease with salt concentration at other frequencies (288 K). At 298 K the bend is transformed in maximum at frequency 7.71 GHz and there is a bend of the curve at 13 GHz. The bend of the curve at 308 K already took place at four frequencies (at 7.71, 9.45, 13 and 16 GHz), and the bend is transformed in maximum at 7.71 and 9.45 GHz. In the dilute solutions the values of e00d are larger than that of e00i by an order of magnitude. Value of e00d decreases with the growth of salt concentration and at the same time the values of e00i increase. So, in the concentrated solutions the values of e00d and e00i have the values of the same order. But the values of the e00d remain higher than the values of the e00i in all the cases. The values of the high-frequency permittivity e 0 decrease with growth of salt concentration for all systems. The obtained experimental data of e 0 , e00d , e00i , j were compared to the earlier investigated microwave properties of potassium chloride, formate, propionate [15–17,22–24] and TBA chloride [10] aqueous solutions. The values of e 0 of these solutions at the frequency 16 GHz decrease in the following order: potassium formate ! potassium propionate ! tetrabutylammonium formate tetrabutylammonium propionate (Fig. 3).
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D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887
40
50 ε'
ε"d, ε"i
1234-
40
30
1234567-
a 20
30
20 10
b 10 0 0
1
2 3 mol/1000 g H2O
4
5
Fig. 1. The concentration dependences of dipole (a) and ionic (b) losses of tetrabutylammonium formate aqueous solutions at 308 K. 1 – 9.45 GHz, 2 – 7.71 GHz, 3 – 13 GHz, 4 – 16 GHz, 5 – 18.9 GHz, 6 – 22 GHz, 7 – 25 GHz.
0 0
2
4
6
8
mol/1000 g H2O
Fig. 3. The concentration dependences of dielectric permittivity of aqueous solutions at the frequency 16 GHz and 298 K. 1 – potassium propionate [16], 2 – potassium formate [15,17], 3 – tetrabutylammonium formate, 4 – tetrabutylammonium propionate. 40
ε"d, ε"i
30 1234567-
20
a 10
b 0 0
1
2
3
4
mol/1000 g H2O
Fig. 2. The concentration dependences of dipole (a) and ionic (b) losses of tetrabutylammonium propionate aqueous solutions at 298 K. 1 – 9.45 GHz, 2 – 7.71 GHz, 3 – 13 GHz, 4 – 16 GHz, 5 – 18.9 GHz, 6 – 22 GHz, 7 – 25 GHz.
The similar order of changes is observed for the concentration dependences of specific conductivity (Fig. 4) and ionic losses. The concentration dependences of ionic losses e00i , as well as j pass through the maximum for all above-mentioned systems. The maximum of e00i is moved in the area of lower concentration with increase of the non-polar part of anion as in aqueous formate and propionate solutions of TBA, so in aqueous formate and propionate solutions of potassium. The largest e00i takes place in aqueous formate solutions of potassium and the smallest in aqueous propionate solutions of TBA. Different relaxation models can be used for description of dielectric spectra [6–8,21]. As it was revealed in the previous investigations one Debye-like relaxation process takes place in aqueous electrolyte solutions at the frequency range that corresponds to maximal dispersion of water (excluding high concentrated solutions and aqueous melts [20]). We used the Cole–Cole relaxation model for calculation of dielectric relaxation parameters for systems investigated in this work at all temperatures and salt concentrations: es e1 e ðxÞ ¼ e1 þ 1a ð1 þ isxÞ The equation is divided on the real and imaginary part:
D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887
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κ , Om-1m-1
30
20 1234-
10
0 0
4
8
12
16
mol/1000 g H2O Fig. 4. The concentration dependences of specific conductivity of aqueous solutions. 1 – tetrabutylammonium propionate, 2 – tetrabutylammonium formate, 3 – potassium propionate [16], 4 – potassium formate [21].
e0 ¼ e1 þ e00 ¼
1 þ ðsxÞ1a sin pa 2 1 þ ðsxÞ
2ð1aÞ
þ 2ðsxÞ
ð1aÞ
ðsxÞ1a cos pa 2 2ð1aÞ
1 þ ðsxÞ
ð1aÞ
þ 2ðsxÞ
sin pa 2
sin pa 2
Fig. 5. The Cole–Cole diagrams of aqueous solutions (308 K). (a) Tetrabutylammonium formate; I – H2O, II – 0.4 m (mol/1000 g H2O), III – 1.23 m, IV – 4.56 m; 1 – 7.71 GHz, 2 – 9.45 GHz, 3 – 13 GHz, 4 – 16 GHz, 5 – 18.9 GHz, 6 – 22 GHz, 7 – 25 GHz. (b) Tetrabutylammonium propionate; I – H2O, V – 0.38 m, VI – 1.08 m, VII – 3.74 m; 1 – 7.71 GHz, 2 – 9.45 GHz, 3 – 13 GHz, 4 – 16 GHz, 5 – 18.9 GHz, 6 – 22 GHz, 7 – 25 GHz.
ðes e1 Þ
ðes e1 Þ
in which es is the low-frequency limit of the dispersion region (static dielectric constant), s is the dielectric relaxation time, a is the parameter of relaxation time distribution, e1 is the high-frequency limit for considered dispersion region. The value of e1 was accepted equal to 5 for water and aqueous solutions as well as in other works [15–17,22]. The temperature and concentration dependences of e1 are not taken into account, because the variations of es and s with small variations of e1 are also small and lie within the limits of experimental error for es and s. All experimental data are described by the Cole–Cole circular diagrams (Fig. 5). The value of a varies from 0.06 in diluted to 0.3 in concentrated solutions. The parameter of relaxation time distribution (a) increases with the growth of salts concentration. Static dielectric constant es is determined from the Cole– Cole diagrams by means of circular extrapolation on the zero frequency. As well as in other cases dielectric constant decreases at transition from water to carboxylate solutions. The least values of es are observed for TBA propionate solutions, the greatest – for potassium formate propionate solutions. Such dependence saves at all temperatures.
The dielectric relaxation time s is calculated by using the graphical decision of the Cole–Cole equation. The 2 frequency dependence of the function f ¼ ½ðes e0 Þ þ 2 00 2 0 00 2 ðed Þ =½ðe e1 Þ þ ðed Þ represents a straight line in logarithmic scale. The function f across frequency axis in the point that corresponds to the frequency of the maximum dipole losses – x0, correspondingly s = 1/x0. The activation enthalpy of dielectric relaxation ðDH þþ e Þ was determined from the temperature dependence of s (for interval 288–308 K) with the use of the Eyring relation: DH þþ ¼ e
R ðln s1 ln s2 Þ R ðT 1 þ T 2 Þ=2 ð1=T 2 1=T 1 Þ
The growth of the dielectric relaxation time s and the is observed in aqueous potasactivation enthalpy DH þþ e sium propionate and TBA formate and propionate solutions in studied temperature interval (Figs. 6 and 7).
4. Discussion As it is visible from Figs. 6 and 7 the following order of changes of s and DH þþ for investigated aquee ous solutions is observed: potassium chloride < potassium formate < water < potassium propionate < TBA
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D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887 τ, ps
40
30
∆ Hε ++, kJ/mol
30
25 20
10
20 a 0 0 30
0.5
1
1.5
τ, ps
2
15
123456-
20
1234-
2.5
10
10
0 b
0 0
0.5
1
1.5
2
2.5
τ, ps
20
10
c 0 0
0.5
1 1.5 mol/1000 g H2O
2
2.5
Fig. 6. The concentration dependences of dielectric relaxation time of aqueous solutions at 288 K (a), 298 K (b), 308 K (c). 1 – Tetrabutylammonium propionate, 2 – tetrabutylammonium formate, 3 – potassium propionate [16], 4 – potassium chloride [22], 5 – potassium formate [16], 6 – tetrabutylammonium chloride [10].
chloride < TBA formate < TBA propionate. The decrease of s and DH þþ of potassium chloride and formate e solutions testifies to disordering of H-bond net of water. The growth of s and DH þþ values is observed in aqueous e potassium propionate solutions. The increase of values of s and DH þþ for solutions of formate and chloride e of TBA indicate on the structure-making influence of cation on the tetrahedral H-bond water net. The ions of propionates and TBA stabilize water structure because of the hydrophobic hydration. The hydrophobic hydration is stronger at low temperatures. In the first approximation structure-breaking action of ions K+ and Cl on the H-bond net of water is equal. In that case, the higher values of s in TBA chloride solutions than in potassium propionate solutions testify to stronger hydrophobic hydration of TBA than that of
2
4 mol/1000 g H2O
6
8
Fig. 7. The concentration dependences of dielectric relaxation enthalpy of aqueous solutions. 1 – Tetrabutylammonium propionate, 2 – tetrabutylammonium formate, 3 – potassium propionate, 4 – potassium formate.
propionate ion. The additional increase of the relaxation characteristics in the TBA propionate solutions in comparison with TBA formate and chloride solutions is caused by the hydrophobic hydration of anion. So, the joint hydrophobic hydration of cation and anion are displayed in aqueous TBA propionate solutions.
5. Conclusions The influence of anions on hydrophobic hydration of TBA cation was studied. The molecular-kinetic properties of water in aqueous solutions were investigated at the transition from hydrophilic hydration of cation and anion (the potassium formate and chloride solutions) to hydrophobic hydration of anion and hydrophilic hydration of cation (the potassium propionate solutions), further to hydrophobic hydration of cation and hydrophilic hydration of anion (the TBA formate solutions) and to hydrophobic hydration of anion and cation (the TBA propionate solutions). In row of investigated systems the TBA propionate caused stronger stabilization of H-bond net of water. In this case, the values of s for the concentrated solutions are three times more than that of water. Thus, the present research shows that the dielectric spectroscopy is the effective method for investigation of hydrophobic hydration.
D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887
Acknowledgment This work was supported by the Scientific Programs of the Russian Academy of Science (2004). References [1] F. Franks (Ed.), Water – A Comprehensive Treatise, vol. 4, Plenum Press, New York, 1975, pp. 1–95. [2] W.-Y. Wen, in: R.A. Horne (Ed.), Water and Aqueous Solutions – Structure, Thermodynamics, and Transport Processes, Wiley, New York, 1972. [3] Yu.M. Kessler, A.L. Zaytsev, Solvofobniye Effecty, Teoriya, Experiment, Praktika, Leningrad, Himiya, 1989. [4] A.K. Lyashchenko, Adv. Chem. Phys. 87 (1994) 379. [5] D.L. Bergman, R.M. Lynden-Bell, J. Mol. Liquids 99 (2001) 1011. [6] A.K. Lyashchenko, T.A. Novskova, V.I. Gaiduk, J. Mol. Liquids 94 (2001) 1. [7] A.K. Lyashchenko, A.S. Lileev, A.U. Zasetsky, T.A. Novskova, V.I. Gaiduk, J. Chem. Soc. Faraday Trans. 89 (1993) 1985. [8] A.K. Lyashchenko, A.S. Lileev, T.A. Novskova, V.S. Kharkin, J. Mol. Liquids 93 (2001) 29. [9] P.S. Yastremsky, G.V. Kokovina, A.K. Lyashchenko, Yu.A. Mirgorod, Z. Struct. Khim. 16 (1975) 1002 (in Russian). [10] P.S. Yastremsky, G.V. Kokovina, A.K. Lyashchenko, O.Ya. Samoylov, Yu.A. Mirgorod, Russ. J. Phys. Chem. 49 (1975) 850.
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[11] G.V. Kokovina, A.K. Lyashchenko, P.S. Yastremsky, J. Struct. Chem. 24 (1983) 143 (English Translation from Z. Struct. Khim. 24 (1983) 152). [12] U. Kaatze, R. Pottel, J. Mol. Liquids 52 (1992) 181. [13] U. Kaatze, Radiat. Phys. Chem. 45 (1995) 549. [14] R. Buchner, C. Holzl, J. Stauder, J. Barthel, Phys. Chem. Chem. Phys. 4 (2002) 2169. [15] D.V. Loginova, A.S. Lileev, A.K. Lyashchenko, V.S. Kharkin, Mendeleev Commun. 2 (2003) 66. [16] D.V. Loginova, A.S. Lileev, A.K. Lyashchenko, V.S. Kharkin, Russ. J. Neorg. Chem. 48 (2003) 278. [17] D.V. Loginova, A.S. Lileev, A.K. Lyashchenko, K.S. Ivanova, V.S. Kharkin, Russ. J. Neorg. Chem. 48 (2003) 1541. [18] J. Le Bot, S. Le Montagner, C.R. Acad. Sci. 236 (1953) 469. [19] U. Kaatze, V. Uhlendorf, The dielectric properties of water at microwave frequencies, Z. Phys. Chem. Bd. 126 S (1981) 151. [20] U. Kaatze, J. Chem. Eng. Data 34 (1989) 371. [21] J. Barthel, R. Buchner, M. Munsterer, Electrolyte Data Collection. Part 2: Dielectric Properties of Water and Aqueous Electrolyte Solutions, DECHEMA, Chemistry Data Series, vol. 12, 1995. [22] A.K. Lyashchenko, A.Yu. Zasetsky, J. Mol. Liquids 77 (1998) 61. [23] K.S. Ivanova, A.S. Lileev, A.K. Lyashchenko, S.M. Portnova, Russ. J. Phys. Chem. 34 (1989) 2148. [24] D.V. Loginova, A.S. Lileev, A.K. Lyashchenko, Russ. J. Neorg. Chem. 47 (2002) 1426.
The high-frequency dielectric spectroscopy of the aqueous solutions of tetrabutylammonium carboxylates D. Loginova a
a,*
, A. Lileev a, A. Lyashchenko a, L. Aladko
b
Institute of General and Inorganic Chemistry RAS, 119019 Leninskii pr. 31. Moscow, Russia b Institute of Inorganic Chemistry SBRAS, Novosibirsk, Russia
Abstract The studies of complex permittivity and low-frequency conductivity of aqueous tetrabutylammonium formate and propionate solutions were carried out in the frequency region of maximum dispersion of dielectric permittivity of water. The measurements were made over a wide range of concentrations, in the temperature interval 288–308 K. The dielectric relaxation time (s) and activation enthalpy ðDH þþ e Þ were calculated. There is an increase in these parameters in aqueous tetrabutylammonium formate and propionate solutions in comparison with pure water. The concentration dependences of s and DH þþ relate to the structure-forming effect of e tetrabutylammonium and propionate ions on water through hydrogen bonding. The stabilization of the water structure is determined by the hydrophobic hydration of these ions. The hydrophobic hydration increases in going from tetrabutylammonium formate to tetrabutylammonium propionate aqueous solutions. Ó 2005 Published by Elsevier B.V. PACS: 77.22.d; 77.22.Ch; 77.22.Gm
1. Introduction The phenomenon of the hydrophobic hydration is determined by the stabilization of H-bonds of water near non-polar groups of molecules or ions and with the ordering of the tetrahedral water structure. This effect has been investigated for a long time [1–5]. Microwave dielectric spectroscopy is an informative method for study of hydration in aqueous solutions. Parameters of dielectric spectra for hydrophobic and hydrophilic hydration are different from those of electrolyte solutions [6,7]. Characteristics of dielectric relaxation give information about total changes in molecular-kinetic mobility of water molecules under the influence of dissolved molecules and ions. They have been received for the large amount of non-elec*
Corresponding author. Tel.: +7 095 2364610. E-mail address: [email protected] (D. Loginova).
0022-3093/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2005.06.019
trolyte systems [8]. In the case of hydrophobic hydraþþ tion ssol sH2 O > 0 and DH þþ e ðsolÞ DH e ðH2 OÞ > 0. The aqueous solutions of tetraalkylammonium (TAA) salts are the typical examples of the hydrophobic hydration of electrolyte solutions. The dielectric relaxation in these solutions has been investigated by several workers [9–14]. The data [12–14] are obtained in a wide frequency range but only at one temperature. Only in work [14], 1.85 m Pr4NBr and 1.23 m Bu4NBr solutions are investigations over a temperature interval. The dielectric characteristics in works [9,10] have been studied only in the area of maximal dispersion of water molecules. However, in this case the measurements were made over a temperature interval. So, the activation parameters of the dielectric relaxation process have been calculated. In this case ssol sH2 O > 0 þþ and DH þþ e ðsolÞ DH e ðH2 OÞ. The changes of s and þþ DH e in TAA solutions in D2O are also considered [11].
D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887
The hydrophobic hydration in TAA solutions can depend not only on the sizes and geometrical forms of cations but also on anions. However, systematic studies of this problem have not been carried out yet. The aqueous solutions of tetrabutylammonium (TBA) carboxylates are a suitable model system for this purpose. It is known that the tetrabutylammonium ion has hydrophobic hydration. The presence of the hydrophobic hydration of propionate ion in aqueous propionate solutions of alkaline metals was revealed in some works [15–17]. The transition from the TBA formate to TBA propionate solutions allows looking at the changes of the molecular-kinetic characteristics of water at the transition from hydrophilic hydration of anion to hydrophobic hydration of anion. The present paper is devoted to research of this question.
2883
The low-frequency conductivity (j) of potassium propionate solutions and tetrabutylammonium formate and propionate solutions was measured at frequency 1 kHz using the U-figurative glass cell with smooth platinum electrodes at the same temperature interval. The cell was thermostated with the help of the thermostat U-8 with accuracy ±0.05°. The accuracy of measurement of j was not more than 0.5%. Aqueous tetrabutylammonium formate and propionate solutions were prepared in equimolecular parts of aqueous solution of tetrabutylammonium hydroxide and formic or propionic acids. Initial reagents from ÔSouzreakhimÕ have qualification Ôchemical pureÕ. Ordinary distilled water was used.
3. Results 2. Experimental The measurements of the high-frequency dielectric permittivity (e 0 ) and losses (e00 ) of aqueous formate and propionate TBA solutions were fulfilled in the frequency region of the maximum dispersion of water (in the microwave range) by the method of thin dielectric rod in the waveguide [18]. In this method the standing electro-magnetic wave is established in the empty waveguide. The thin glass capillary is filled by the investigated solution. The capillary with a solution is located in the waveguide. The intensity of an electrical field in units of standing wave becomes distinct from zero at entering of sample in the waveguide. It is caused by the absorption of the electro-magnetic wave by the investigated solution. The amplitude of the reflected wave becomes less than the amplitude of the falling wave. The minima of the standing wave in the waveguide are displaced in the direction of the sample. The changes of parameters of a standing wave are directly connected with the dielectric permittivity and losses of sample. The capillary was calibrated on the data of the high-frequency permittivity e 0 and losses e00 of water at corresponding frequency. These values were calculated by the Debye relations. The values of the dielectric relaxation time and static dielectric permittivity of water were chosen from the literature data: s = 11.0, es = 82.0; (288 K); s = 8.25, es = 78.4 (298 K); s = 6.45, es = 74.9 (308 K) [19,20]. The waveguide section with sample was supplied with water thermostat jacket. The temperature in the cell was maintained using the thermostat U8 (VEB MLW PRUGERATE – WEBK MEDINGEN, DDR) and controlled by the copper–constantan thermocouple with accuracy ±0.05°. The measurements were carried out at seven frequencies in the interval 7– 25 GHz, at temperatures 288, 298 and 308 K. The accuracy of measurement of e 0 and e00 is ±1.0–1.5% and ±2.0–2.5%, respectively.
The aqueous electrolyte solutions are conductive liquids. Therefore, the measured values of e00 consist of two parts. The ionic losses were calculated using equation e00i ¼ j=e0 x [21], where x is the circular frequency, e0 is the dielectric permittivity of vacuum. Dipole losses of solutions were calculated by the equation e00d ¼ e00 e00i . The concentration dependences of dipole losses e00d of aqueous TBA formate and propionate solutions have specific features for different frequencies, which change with temperature. The example of such dependences is represented in Figs. 1(a) and 2(a). The bends of the curves were observed at initial concentration for decencies e00d vs m at 7.71 and 9.45 GHz. The values of e00d smoothly decrease with salt concentration at other frequencies (288 K). At 298 K the bend is transformed in maximum at frequency 7.71 GHz and there is a bend of the curve at 13 GHz. The bend of the curve at 308 K already took place at four frequencies (at 7.71, 9.45, 13 and 16 GHz), and the bend is transformed in maximum at 7.71 and 9.45 GHz. In the dilute solutions the values of e00d are larger than that of e00i by an order of magnitude. Value of e00d decreases with the growth of salt concentration and at the same time the values of e00i increase. So, in the concentrated solutions the values of e00d and e00i have the values of the same order. But the values of the e00d remain higher than the values of the e00i in all the cases. The values of the high-frequency permittivity e 0 decrease with growth of salt concentration for all systems. The obtained experimental data of e 0 , e00d , e00i , j were compared to the earlier investigated microwave properties of potassium chloride, formate, propionate [15–17,22–24] and TBA chloride [10] aqueous solutions. The values of e 0 of these solutions at the frequency 16 GHz decrease in the following order: potassium formate ! potassium propionate ! tetrabutylammonium formate tetrabutylammonium propionate (Fig. 3).
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D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887
40
50 ε'
ε"d, ε"i
1234-
40
30
1234567-
a 20
30
20 10
b 10 0 0
1
2 3 mol/1000 g H2O
4
5
Fig. 1. The concentration dependences of dipole (a) and ionic (b) losses of tetrabutylammonium formate aqueous solutions at 308 K. 1 – 9.45 GHz, 2 – 7.71 GHz, 3 – 13 GHz, 4 – 16 GHz, 5 – 18.9 GHz, 6 – 22 GHz, 7 – 25 GHz.
0 0
2
4
6
8
mol/1000 g H2O
Fig. 3. The concentration dependences of dielectric permittivity of aqueous solutions at the frequency 16 GHz and 298 K. 1 – potassium propionate [16], 2 – potassium formate [15,17], 3 – tetrabutylammonium formate, 4 – tetrabutylammonium propionate. 40
ε"d, ε"i
30 1234567-
20
a 10
b 0 0
1
2
3
4
mol/1000 g H2O
Fig. 2. The concentration dependences of dipole (a) and ionic (b) losses of tetrabutylammonium propionate aqueous solutions at 298 K. 1 – 9.45 GHz, 2 – 7.71 GHz, 3 – 13 GHz, 4 – 16 GHz, 5 – 18.9 GHz, 6 – 22 GHz, 7 – 25 GHz.
The similar order of changes is observed for the concentration dependences of specific conductivity (Fig. 4) and ionic losses. The concentration dependences of ionic losses e00i , as well as j pass through the maximum for all above-mentioned systems. The maximum of e00i is moved in the area of lower concentration with increase of the non-polar part of anion as in aqueous formate and propionate solutions of TBA, so in aqueous formate and propionate solutions of potassium. The largest e00i takes place in aqueous formate solutions of potassium and the smallest in aqueous propionate solutions of TBA. Different relaxation models can be used for description of dielectric spectra [6–8,21]. As it was revealed in the previous investigations one Debye-like relaxation process takes place in aqueous electrolyte solutions at the frequency range that corresponds to maximal dispersion of water (excluding high concentrated solutions and aqueous melts [20]). We used the Cole–Cole relaxation model for calculation of dielectric relaxation parameters for systems investigated in this work at all temperatures and salt concentrations: es e1 e ðxÞ ¼ e1 þ 1a ð1 þ isxÞ The equation is divided on the real and imaginary part:
D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887
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κ , Om-1m-1
30
20 1234-
10
0 0
4
8
12
16
mol/1000 g H2O Fig. 4. The concentration dependences of specific conductivity of aqueous solutions. 1 – tetrabutylammonium propionate, 2 – tetrabutylammonium formate, 3 – potassium propionate [16], 4 – potassium formate [21].
e0 ¼ e1 þ e00 ¼
1 þ ðsxÞ1a sin pa 2 1 þ ðsxÞ
2ð1aÞ
þ 2ðsxÞ
ð1aÞ
ðsxÞ1a cos pa 2 2ð1aÞ
1 þ ðsxÞ
ð1aÞ
þ 2ðsxÞ
sin pa 2
sin pa 2
Fig. 5. The Cole–Cole diagrams of aqueous solutions (308 K). (a) Tetrabutylammonium formate; I – H2O, II – 0.4 m (mol/1000 g H2O), III – 1.23 m, IV – 4.56 m; 1 – 7.71 GHz, 2 – 9.45 GHz, 3 – 13 GHz, 4 – 16 GHz, 5 – 18.9 GHz, 6 – 22 GHz, 7 – 25 GHz. (b) Tetrabutylammonium propionate; I – H2O, V – 0.38 m, VI – 1.08 m, VII – 3.74 m; 1 – 7.71 GHz, 2 – 9.45 GHz, 3 – 13 GHz, 4 – 16 GHz, 5 – 18.9 GHz, 6 – 22 GHz, 7 – 25 GHz.
ðes e1 Þ
ðes e1 Þ
in which es is the low-frequency limit of the dispersion region (static dielectric constant), s is the dielectric relaxation time, a is the parameter of relaxation time distribution, e1 is the high-frequency limit for considered dispersion region. The value of e1 was accepted equal to 5 for water and aqueous solutions as well as in other works [15–17,22]. The temperature and concentration dependences of e1 are not taken into account, because the variations of es and s with small variations of e1 are also small and lie within the limits of experimental error for es and s. All experimental data are described by the Cole–Cole circular diagrams (Fig. 5). The value of a varies from 0.06 in diluted to 0.3 in concentrated solutions. The parameter of relaxation time distribution (a) increases with the growth of salts concentration. Static dielectric constant es is determined from the Cole– Cole diagrams by means of circular extrapolation on the zero frequency. As well as in other cases dielectric constant decreases at transition from water to carboxylate solutions. The least values of es are observed for TBA propionate solutions, the greatest – for potassium formate propionate solutions. Such dependence saves at all temperatures.
The dielectric relaxation time s is calculated by using the graphical decision of the Cole–Cole equation. The 2 frequency dependence of the function f ¼ ½ðes e0 Þ þ 2 00 2 0 00 2 ðed Þ =½ðe e1 Þ þ ðed Þ represents a straight line in logarithmic scale. The function f across frequency axis in the point that corresponds to the frequency of the maximum dipole losses – x0, correspondingly s = 1/x0. The activation enthalpy of dielectric relaxation ðDH þþ e Þ was determined from the temperature dependence of s (for interval 288–308 K) with the use of the Eyring relation: DH þþ ¼ e
R ðln s1 ln s2 Þ R ðT 1 þ T 2 Þ=2 ð1=T 2 1=T 1 Þ
The growth of the dielectric relaxation time s and the is observed in aqueous potasactivation enthalpy DH þþ e sium propionate and TBA formate and propionate solutions in studied temperature interval (Figs. 6 and 7).
4. Discussion As it is visible from Figs. 6 and 7 the following order of changes of s and DH þþ for investigated aquee ous solutions is observed: potassium chloride < potassium formate < water < potassium propionate < TBA
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D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887 τ, ps
40
30
∆ Hε ++, kJ/mol
30
25 20
10
20 a 0 0 30
0.5
1
1.5
τ, ps
2
15
123456-
20
1234-
2.5
10
10
0 b
0 0
0.5
1
1.5
2
2.5
τ, ps
20
10
c 0 0
0.5
1 1.5 mol/1000 g H2O
2
2.5
Fig. 6. The concentration dependences of dielectric relaxation time of aqueous solutions at 288 K (a), 298 K (b), 308 K (c). 1 – Tetrabutylammonium propionate, 2 – tetrabutylammonium formate, 3 – potassium propionate [16], 4 – potassium chloride [22], 5 – potassium formate [16], 6 – tetrabutylammonium chloride [10].
chloride < TBA formate < TBA propionate. The decrease of s and DH þþ of potassium chloride and formate e solutions testifies to disordering of H-bond net of water. The growth of s and DH þþ values is observed in aqueous e potassium propionate solutions. The increase of values of s and DH þþ for solutions of formate and chloride e of TBA indicate on the structure-making influence of cation on the tetrahedral H-bond water net. The ions of propionates and TBA stabilize water structure because of the hydrophobic hydration. The hydrophobic hydration is stronger at low temperatures. In the first approximation structure-breaking action of ions K+ and Cl on the H-bond net of water is equal. In that case, the higher values of s in TBA chloride solutions than in potassium propionate solutions testify to stronger hydrophobic hydration of TBA than that of
2
4 mol/1000 g H2O
6
8
Fig. 7. The concentration dependences of dielectric relaxation enthalpy of aqueous solutions. 1 – Tetrabutylammonium propionate, 2 – tetrabutylammonium formate, 3 – potassium propionate, 4 – potassium formate.
propionate ion. The additional increase of the relaxation characteristics in the TBA propionate solutions in comparison with TBA formate and chloride solutions is caused by the hydrophobic hydration of anion. So, the joint hydrophobic hydration of cation and anion are displayed in aqueous TBA propionate solutions.
5. Conclusions The influence of anions on hydrophobic hydration of TBA cation was studied. The molecular-kinetic properties of water in aqueous solutions were investigated at the transition from hydrophilic hydration of cation and anion (the potassium formate and chloride solutions) to hydrophobic hydration of anion and hydrophilic hydration of cation (the potassium propionate solutions), further to hydrophobic hydration of cation and hydrophilic hydration of anion (the TBA formate solutions) and to hydrophobic hydration of anion and cation (the TBA propionate solutions). In row of investigated systems the TBA propionate caused stronger stabilization of H-bond net of water. In this case, the values of s for the concentrated solutions are three times more than that of water. Thus, the present research shows that the dielectric spectroscopy is the effective method for investigation of hydrophobic hydration.
D. Loginova et al. / Journal of Non-Crystalline Solids 351 (2005) 2882–2887
Acknowledgment This work was supported by the Scientific Programs of the Russian Academy of Science (2004). References [1] F. Franks (Ed.), Water – A Comprehensive Treatise, vol. 4, Plenum Press, New York, 1975, pp. 1–95. [2] W.-Y. Wen, in: R.A. Horne (Ed.), Water and Aqueous Solutions – Structure, Thermodynamics, and Transport Processes, Wiley, New York, 1972. [3] Yu.M. Kessler, A.L. Zaytsev, Solvofobniye Effecty, Teoriya, Experiment, Praktika, Leningrad, Himiya, 1989. [4] A.K. Lyashchenko, Adv. Chem. Phys. 87 (1994) 379. [5] D.L. Bergman, R.M. Lynden-Bell, J. Mol. Liquids 99 (2001) 1011. [6] A.K. Lyashchenko, T.A. Novskova, V.I. Gaiduk, J. Mol. Liquids 94 (2001) 1. [7] A.K. Lyashchenko, A.S. Lileev, A.U. Zasetsky, T.A. Novskova, V.I. Gaiduk, J. Chem. Soc. Faraday Trans. 89 (1993) 1985. [8] A.K. Lyashchenko, A.S. Lileev, T.A. Novskova, V.S. Kharkin, J. Mol. Liquids 93 (2001) 29. [9] P.S. Yastremsky, G.V. Kokovina, A.K. Lyashchenko, Yu.A. Mirgorod, Z. Struct. Khim. 16 (1975) 1002 (in Russian). [10] P.S. Yastremsky, G.V. Kokovina, A.K. Lyashchenko, O.Ya. Samoylov, Yu.A. Mirgorod, Russ. J. Phys. Chem. 49 (1975) 850.
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