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The hydrophobic hydration in aqueous solutions of allyl-substituted ammonium salts
Journal of Molecular Liquids 131–132 (2007) 101 – 104 www.elsevier.com/locate/molliq
The hydrophobic hydration in aqueous solutions of allyl-substituted ammonium salts A. Lileev a,⁎, D. Loginova a , A. Lyashchenko a , L. Timofeeva b , N. Kleshcheva b a
Institute of General and Inorganic Chemistry RAS, 119991 Leninsky pr. 31 Moscow, Russia b Institute of Petrochemical Synthesis RAS, 119991 Leninsky pr. 31 Moscow, Russia Available online 9 October 2006
Abstract The complex dielectric permittivity for the series of aqueous allyl-substituted ammonium salt solutions is investigated in microwave range (13–25 GHz) in a wide range of salt concentrations. The measurements were made at 298 K for diallylammonium and diallylmethylammonium trifluoroacetates and in the temperature interval 288–308 K for diallyldimethylammonium chloride. The values of dielectric constant εs and parameters of the dielectric relaxation process were calculated. The increase of relaxation time τ in comparison with pure water is observed for all investigated solutions. It indicates the decrease of orientation mobility of water molecules in the H-bond network upon action of allyl-substituted ammonium ions connected with hydrophobic hydration of these ions. © 2006 Published by Elsevier B.V. Keywords: Hydrophobic hydration; Allyl-substituted ammonium salts; Dielectric permittivity
1. Introduction The phenomenon of hydrophobic hydration of ions and molecules has long been known [1–3]. It is demonstrated in electrolyte solutions for ions with a large number of nonpolar groups. Characteristic examples of such ions are tetraalkylammonium salts. The aqueous solutions of allyl-substituted and alkyl-substituted ammonium salts have much in common. There is no information on hydration of allyl-substituted ammonium salts in the literature. The presence of unsaturated double bonds CfC in hydrocarbon radicals causes differences in volume, form, amount of nonpolar groups, etc. in comparison with tetraalkylammonium salts. Therefore, it is interesting to find out how these differences are shown in hydration of allyl-substituted ammonium ions. The problem of hydration of allyl-substituted ammonium ions has not only theoretical, but also practical interest. The molecular-kinetic state of aqueous media renders the influence on speed and degree of polymerization at preparation of polyelectrolyte on the base of these salts [4]. Microwave dielectric spectroscopy (ε⁎) is an informative method for studying hydrophobic hydration in aqueous ⁎ Corresponding author. E-mail address: [email protected] (A. Lileev). 0167-7322/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.molliq.2006.08.028
solutions. It gives information about changes in molecularkinetic mobility of water molecules under the influence of dissolved ions. The hydrophobic hydration in solutions of tetraalkylammonium salts was determined using this method [5–9]. In the present work, the features of the hydration of allylsubstituted ammonium salts were investigated with an example of aqueous solutions of diallylammonium (DAA) and diallylmethylammonium (DAMA) trifluoroacetates (FA) and diallyldimethylammonium (DADMA) chloride by means of microwave dielectric spectroscopy. 2. Experimental The complex dielectric permittivity (ε′ and ε″) was measured by the method of a thin dielectric rod in the wave-guide at frequencies 13, 16, 18.9, 22 and 25 GHz. These frequencies were chosen because they correspond to the range of maximum of dielectric losses of water and aqueous electrolyte solutions. As it was shown in previous work [10–12], in this range we can find the changes of water molecule mobility under the action of the dissolved salts. The measurements of the dielectric properties of diallylammonium and diallylmethylammonium trifluoroacetate solutions were executed at 298 K. The complex dielectric permittivity of diallyldimethylammonium chloride
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Table 1 Parameters of dielectric relaxation for aqueous solutions of allylammonium trifluoracetates at 298K M (mol/l)
0 0.25 0.50 0.75 1.0 1.5 2.0 2.5
Diallylmethylammonium trifluoracetate
Diallylammonium trifluoracetate
εs
τ (ps)
α
εs
τ (ps)
α
78.4 75.5 70.2 67.4 64.1 56.8 50.1 43.3
8.25 9.2 10.1 11.3 12.6 15.2 18.7 23
0.00 0.05 0.04 0.07 0.1 0.12 0.15 0.17
78.4 74.8 70.6 66.7 63.1 57.1 52 46.3
8.25 9.0 9.7 10.6 11.5 13.3 15.7 18.8
0.00 0.03 0.04 0.05 0.07 0.11 0.16 0.20
solutions was studied in the temperature range 288–308 K. The technique of the measurements was described previously [10,13,14]. The thin-wall capillary filled up by the investigated solution was placed into the wave-guide. The diameter of the capillaries was varied from 0.6 to 1.1 mm depending on frequency. The ratio of the capillary diameter to the width of the wall of the wave-guide was less than 0.01 for all frequencies; thus, the disturbance of the electromagnetic field by the capillary with the investigated solution was small. Therefore, the conditions for high-precision determination of the values of ε′ and ε″ were optimal [15]. The wave-guide part with capillary was supplied with a water thermostat-controlled jacket. The temperature was maintained with the accuracy ± 0.1 K using a thermostat U-8 type. It was controlled by means of a copperconstantan thermocouple. The errors of measurements of ε′ and ε″ were ± 1.0–1.5 and ± 2.0–2.5%, respectively (depending on frequency, temperature and salt concentration). The solutions were prepared by weight. DAA and DAMA trifluoroacetates were obtained from trifluoroacetic acid (Germany, “Riedel-de Haen AG” the content of basic substance is 99%). N,N-Diallylamine (Germany, “Fluka”) or N,N-diallylN-methylamine [16], by techniques described in [4,17]. The composition of the synthesized salts was confirmed by chemical elemental analysis and spectra of nuclear magnetic resonance 1H. Specific conductivity (σ) of above-mentioned solutions was investigated to account for ionic losses. It was measured in an U-shaped glass cell with smoothly platinum electrodes using a digital bridge E7-8 at frequency 1 kHz. The cell was calibrated with 1 M KCl solution. It was thermostatted in an U8 thermostat with an accuracy of ± 0.05°. The error of σ measurements did not exceed ± 0.5%. The ionic losses were estimated by the relation εi″ = σ/ε0ω [18]. The dispersion of the complex dielectric permittivity in the investigated systems was described by the Cole–Cole equation pa 1 þ ðxsÞ1−a sin 2 e V¼ el þ ðes −el Þ pa 1−a 1 þ 2ðxsÞ sin þ ðxsÞ2ð1−aÞ 2 pa ðxsÞ1−a cos 2 eW ¼ ðes −el Þ pa 1−a 1 þ 2ðxsÞ sin þ ðxsÞ2ð1−aÞ 2
where εs is the low-frequency limit of the dispersion region (static dielectric constant), τ is he dielectric relaxation time, α is the parameter of relaxation time distribution, ε∞ is the highfrequency limit of considered dispersion region. The same value of ε∞ was accepted for aqueous solutions as for pure water (ε∞ ≈ 5) as well as in our other work [10,12]. The temperature and concentration dependencies of ε∞ are not taken into account. The effect of assuming ε∞ ≈ 5 at all temperatures and concentrations on the values of relaxation time τ is negligible within the experimental errors for τ. The experimental data ε′ and εd″ are described by the Cole–Cole relations for all solutions. It means that the process of water molecule relaxation in these solutions corresponds to one most probable relaxation time with small values of α (Table 1). The parameter α increases in concentrated solutions. The examples of Cole–Cole diagrams are given in Fig. 1. 3. Results and discussion Static dielectric constants of solutions were determined from the Cole–Cole diagrams by means of the circular extrapolation on the zero frequency. The values of static dielectric constant εs decrease with increasing salt concentration for all investigated solutions. The typical examples of concentration dependencies of dielectric constant for allyl-substituted ammonium salt solutions are presented in Fig. 2. The decrease of εs values for solutions in comparison with pure water is connected with partial decrease of rotary degrees of freedom of water molecules in the hydration spheres and with replacement of water molecules by ions with lower dielectric permittivity. The decrease of εs is observed in the series of salts: DAAFA ≈ DAMAFA→DADMACl. The values of dielectric relaxation time were found by means of the graphical decision of Cole–Cole relations. The frequency dependence of function [(εs − ε′)2 + (εd″)2] / [(ε′ − ε∞)2 + (εd″)2] in logarithmic coordinates becomes a straight line, which crosses the x-axis at a point corresponding to the maximum of dipole losses ω0 = 1/τ. The values of τ characterize the changes of water molecule mobility in the hydration shells of the ions. The values of dielectric relaxation time for solutions increase in comparison with pure water for all investigated salts (Tables 1 and 2). The examples of concentration dependences of τ at 298 K are
Fig. 1. The Cole–Cole diagrams for water (a) and aqueous solutions of DAMAFA (b: 0.25, c: 0.50, d: 0.75, e: 1.0, f: 1.5, g: 2.0 and h: 2.5 M) at 298 K. Frequencies: (1) 13.0, (2) 14.0, (3) 16.0, (4) 18.9, (5) 22.0 and (6) 25.0 GHz.
A. Lileev et al. / Journal of Molecular Liquids 131–132 (2007) 101–104
Fig. 2. The concentration dependencies of static dielectric permittivity of aqueous allyl-substituted ammonium salt solutions at 298 K. (1) DAA trifluoracetate, (2) DAMA trifluoracetate, (3) DADMA chloride.
presented in Fig. 3. The changes of τ for other solutions with hydrophilic and hydrophobic hydrations also are given on this figure. In solutions of KCl, the decrease of the values of τ is observed with increase of concentration [19], as well as for other solutions that consist of ions with hydrophilic hydration [20]. On the other hand, the relaxation time increases for (C4H9)4NCl solutions [5,8] where hydrophobic hydration of tetrabutylammonium is established. At 298 K, the values of τ increase in the series: KCl→H2O→(CH3)4NCl→DADMACl→(C4H9)4NCl and H2O→DAAFA→DAMAFA. All substituted ammonium salts cause decrease of orientational mobility of water molecules in solution. In the case of chloride solutions, the largest effect is caused by (C4H9)4N+-ion as it has the largest hydrocarbon radicals. In the series of trifluoracetate solutions, the DAMA ion has a stronger effect than DAA as it has one CH3-group more. The growth of τ for the considered allyl-substituted ammonium salt solutions indicates the similar to alkyl-substituted ammonium salt influence on the mobility of water molecules in tetrahedral H-bond net. The DADMACl causes the least changes of τ, though DADMA-ion contains more nonpolar groups in comparison with other allyl-substituted ammonium ions.
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Fig. 3. The concentration dependencies of dielectric relaxation time for aqueous electrolyte solutions: (1) tetrabutylammonium chloride [5], (2) diallylammonium trifluoracetate, (3) diallylmethylammonium trifluoracetate, (4) diallyldimethylammonium chloride, (5) tetramethylammonium chloride [5], (6) KCl [19].
Probably, it is connected with the strong structure-breaking action of the Cl− ion on the H-bond network of water. Analysis of temperature dependence of dielectric relaxation time for DADMACl solutions shows that the values of ΔHε++ increase with increasing of salt concentration (Fig. 4). So, the concentration changes of ΔHε++ are similar for Bu4NCl and DADMACl solutions and are contrary for KCl solutions [19]. Since this parameter characterizes the strength of the H-bond net of water, it is possible to report on structure-making effect of diallyldimethylammonium ions as well as for (C4H9)4N+-ion. In
Table 2 Parameters of dielectric relaxation for aqueous solutions of diallyldimethylammonium chloride M 288 K (mol/l) εs τ (ps) α
298 K εs
τ (ps) α
εs
0 0.25 0.50 1.0 1.49 2.0 2.51
78.4 72.3 65.6 56.9 50.8 42.8 34
8.25 8.5 8.9 10 10.7 13.6 15.2
74.9 6.45 68.9 6.5 64.0 7.0 56.0 7.9 50.5 8.6 41.6 10.4 33.9 11.7
82.1 78.3 73.1 64.6 61.8 54.6 62.0
11.0 11.8 12.7 15.0 18.4 26.5 53.8
0.00 0.04 0.05 0.09 0.14 0.22 0.35
308 K
0.00 0.00 0.00 0.01 0.01 0.08 0.10
τ (ps) α 0.00 0.00 0.00 0.01 0.01 0.04 0.05
ΔH++ ε (kJ/mol) 17.2 19.5 19.6 21.3 25.7 31.7 30.0
Fig. 4. The concentration dependencies of activation enthalpy of relaxation processes for aqueous electrolyte solutions: (1) tetrabutylammonium chloride [5], (2) diallyldimethylammonium chloride, (3) KCl [19].
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the cases of DAAFA and DAMAFA solutions, we see stronger increase of τ values in comparison with DADMACl solutions. It can be assumed that hydrophobic hydration manifests for diallylammonium and diallylmethylammonium ions. 4. Conclusion The molecular-kinetic properties of water in aqueous allylsubstituted ammonium salt solutions were investigated. Allylsubstituted ammonium ions decrease the orientation mobility of water molecules in H-bond net. The structure-making effect is established for diallyldimethylammonium chloride solutions on the basis of concentration changes of τ and ΔHε++. They have opposite sign in comparison with electrolyte solutions where typical hydrophilic hydration is present (KCl, etc.). It is connected with hydrophobic hydration of DADMA ions. It is assumed that diallylammonium and diallylmethylammonium ions cause the same effect on water structure. Acknowledgement This work was supported by the RFBR, Project No. 05-0332100 and Program of Department of RAS. References [1] F. Franks (Ed.), Water, A Comprehensive Treatise, vol. 4, Plenum Press, New York, 1975, p. 1. [2] Yu.M. Kessler, A.L. Zaytsev, Solvofobniye Effecty. Teoriya, Experiment, Praktika, Leningrad, Khimiya, 1989, 312 pp.
[3] W.-Y. Wen, in: R.A. Horne (Ed.), Water and Aqueous Solutions— Structure, Thermodynamics, and Transport Processes, Wiley, New York, 1972. [4] L.M. Timofeeva, N.A. Kleshcheva, Yu.A. Vasil'eva, G.L. Gromova, G.I. Timofeeva, M.P. Filatova, Vysokomol. Soedin. A. (Polym. Sci. Russ. A.) 47 (2005) 551. [5] G.V. Kokovina, A.K. Lyashchenko, P.S. Yastremsky, J. Struct. Chem. 24 (1983) 143 (Engl. Transl. from Zh. Strukt. Khim. 24 (1983) 152). [6] P.S. Yastremsky, G.V. Kokovina, A.K. Lyashchenko, O.Ya. Samoylov, Yu.A. Mirgorod, Zh. Fiz. Khim. 49 (1975) 1442. [7] G.V. Kokovina, A.K. Lyashchenko, P.S. Yastremsky, Zh. Struct. Khim. 24 (1983) 152. [8] R. Buchner, C. Hölz, J. Stauber, J. Barthel, Phys. Chem. Chem. Phys. 4 (2002) 2169. [9] D.V. Loginova, A.S. Lileev, A.K. Lyashchenko, L.S. Aladko, J. NonCryst. Solids 351 (2005) 2882. [10] Yu. Zasetsky, A.S. Lileev, A.K. Lyashchenko, Russ. J. Inorg. Chem. 39 (1994) 990. [11] A.K. Lyashchenko, A.Yu. Zasetsky, J. Mol. Liq. 77 (1998) 61. [12] A.S. Lileev, Z.A. Filimonova, A.K. Lyashchenko, J. Mol. Liq. 103–104 (2003) 299. [13] J. Le Bot, S. Le Montagner, C. R. Acad. Sci. 236 (1953) 469. [14] A.K. Lyashchenko, V.S. Kharkin, A.S. Lileev, P.V. Efremov, Russ. J. Phys. Chem. 75 (2001) 195. [15] Yu. Radin, Izv. Vuzov. Radiofiz. 1 (1958) 177 (in Russian). [16] S. Harada, K. Arai, Makromol. Chem. 107 (1967) 7. [17] L.M. Timofeeva, N.A. Kleshcheva, Yu.A. Vasileva, G.L. Gromova, G.I. Timofeeva, M.P. Filatova, Macromol. Chem. Phys. 203 (2002) 2296. [18] J. Barthel, R. Buchner, M. Munsterer, Electrolyte data collection: dielectric properties of water and aqueous electrolyte solutions. Chemistry Data Series. V.12 Part 2. DECHEMA Frankfurt, 1995, Chemistry Data Series. V.12 Part 2. [19] D.V. Loginova, A.S. Lileev, A.K. Lyashchenko, Russ. J. Neorg. Chem. 47 (2002) 1426. [20] W. Wachter, W. Kunz, R. Buchner, G. Hefter, J. Phys. Chem., A 109 (2005) 8675.
The hydrophobic hydration in aqueous solutions of allyl-substituted ammonium salts A. Lileev a,⁎, D. Loginova a , A. Lyashchenko a , L. Timofeeva b , N. Kleshcheva b a
Institute of General and Inorganic Chemistry RAS, 119991 Leninsky pr. 31 Moscow, Russia b Institute of Petrochemical Synthesis RAS, 119991 Leninsky pr. 31 Moscow, Russia Available online 9 October 2006
Abstract The complex dielectric permittivity for the series of aqueous allyl-substituted ammonium salt solutions is investigated in microwave range (13–25 GHz) in a wide range of salt concentrations. The measurements were made at 298 K for diallylammonium and diallylmethylammonium trifluoroacetates and in the temperature interval 288–308 K for diallyldimethylammonium chloride. The values of dielectric constant εs and parameters of the dielectric relaxation process were calculated. The increase of relaxation time τ in comparison with pure water is observed for all investigated solutions. It indicates the decrease of orientation mobility of water molecules in the H-bond network upon action of allyl-substituted ammonium ions connected with hydrophobic hydration of these ions. © 2006 Published by Elsevier B.V. Keywords: Hydrophobic hydration; Allyl-substituted ammonium salts; Dielectric permittivity
1. Introduction The phenomenon of hydrophobic hydration of ions and molecules has long been known [1–3]. It is demonstrated in electrolyte solutions for ions with a large number of nonpolar groups. Characteristic examples of such ions are tetraalkylammonium salts. The aqueous solutions of allyl-substituted and alkyl-substituted ammonium salts have much in common. There is no information on hydration of allyl-substituted ammonium salts in the literature. The presence of unsaturated double bonds CfC in hydrocarbon radicals causes differences in volume, form, amount of nonpolar groups, etc. in comparison with tetraalkylammonium salts. Therefore, it is interesting to find out how these differences are shown in hydration of allyl-substituted ammonium ions. The problem of hydration of allyl-substituted ammonium ions has not only theoretical, but also practical interest. The molecular-kinetic state of aqueous media renders the influence on speed and degree of polymerization at preparation of polyelectrolyte on the base of these salts [4]. Microwave dielectric spectroscopy (ε⁎) is an informative method for studying hydrophobic hydration in aqueous ⁎ Corresponding author. E-mail address: [email protected] (A. Lileev). 0167-7322/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.molliq.2006.08.028
solutions. It gives information about changes in molecularkinetic mobility of water molecules under the influence of dissolved ions. The hydrophobic hydration in solutions of tetraalkylammonium salts was determined using this method [5–9]. In the present work, the features of the hydration of allylsubstituted ammonium salts were investigated with an example of aqueous solutions of diallylammonium (DAA) and diallylmethylammonium (DAMA) trifluoroacetates (FA) and diallyldimethylammonium (DADMA) chloride by means of microwave dielectric spectroscopy. 2. Experimental The complex dielectric permittivity (ε′ and ε″) was measured by the method of a thin dielectric rod in the wave-guide at frequencies 13, 16, 18.9, 22 and 25 GHz. These frequencies were chosen because they correspond to the range of maximum of dielectric losses of water and aqueous electrolyte solutions. As it was shown in previous work [10–12], in this range we can find the changes of water molecule mobility under the action of the dissolved salts. The measurements of the dielectric properties of diallylammonium and diallylmethylammonium trifluoroacetate solutions were executed at 298 K. The complex dielectric permittivity of diallyldimethylammonium chloride
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Table 1 Parameters of dielectric relaxation for aqueous solutions of allylammonium trifluoracetates at 298K M (mol/l)
0 0.25 0.50 0.75 1.0 1.5 2.0 2.5
Diallylmethylammonium trifluoracetate
Diallylammonium trifluoracetate
εs
τ (ps)
α
εs
τ (ps)
α
78.4 75.5 70.2 67.4 64.1 56.8 50.1 43.3
8.25 9.2 10.1 11.3 12.6 15.2 18.7 23
0.00 0.05 0.04 0.07 0.1 0.12 0.15 0.17
78.4 74.8 70.6 66.7 63.1 57.1 52 46.3
8.25 9.0 9.7 10.6 11.5 13.3 15.7 18.8
0.00 0.03 0.04 0.05 0.07 0.11 0.16 0.20
solutions was studied in the temperature range 288–308 K. The technique of the measurements was described previously [10,13,14]. The thin-wall capillary filled up by the investigated solution was placed into the wave-guide. The diameter of the capillaries was varied from 0.6 to 1.1 mm depending on frequency. The ratio of the capillary diameter to the width of the wall of the wave-guide was less than 0.01 for all frequencies; thus, the disturbance of the electromagnetic field by the capillary with the investigated solution was small. Therefore, the conditions for high-precision determination of the values of ε′ and ε″ were optimal [15]. The wave-guide part with capillary was supplied with a water thermostat-controlled jacket. The temperature was maintained with the accuracy ± 0.1 K using a thermostat U-8 type. It was controlled by means of a copperconstantan thermocouple. The errors of measurements of ε′ and ε″ were ± 1.0–1.5 and ± 2.0–2.5%, respectively (depending on frequency, temperature and salt concentration). The solutions were prepared by weight. DAA and DAMA trifluoroacetates were obtained from trifluoroacetic acid (Germany, “Riedel-de Haen AG” the content of basic substance is 99%). N,N-Diallylamine (Germany, “Fluka”) or N,N-diallylN-methylamine [16], by techniques described in [4,17]. The composition of the synthesized salts was confirmed by chemical elemental analysis and spectra of nuclear magnetic resonance 1H. Specific conductivity (σ) of above-mentioned solutions was investigated to account for ionic losses. It was measured in an U-shaped glass cell with smoothly platinum electrodes using a digital bridge E7-8 at frequency 1 kHz. The cell was calibrated with 1 M KCl solution. It was thermostatted in an U8 thermostat with an accuracy of ± 0.05°. The error of σ measurements did not exceed ± 0.5%. The ionic losses were estimated by the relation εi″ = σ/ε0ω [18]. The dispersion of the complex dielectric permittivity in the investigated systems was described by the Cole–Cole equation pa 1 þ ðxsÞ1−a sin 2 e V¼ el þ ðes −el Þ pa 1−a 1 þ 2ðxsÞ sin þ ðxsÞ2ð1−aÞ 2 pa ðxsÞ1−a cos 2 eW ¼ ðes −el Þ pa 1−a 1 þ 2ðxsÞ sin þ ðxsÞ2ð1−aÞ 2
where εs is the low-frequency limit of the dispersion region (static dielectric constant), τ is he dielectric relaxation time, α is the parameter of relaxation time distribution, ε∞ is the highfrequency limit of considered dispersion region. The same value of ε∞ was accepted for aqueous solutions as for pure water (ε∞ ≈ 5) as well as in our other work [10,12]. The temperature and concentration dependencies of ε∞ are not taken into account. The effect of assuming ε∞ ≈ 5 at all temperatures and concentrations on the values of relaxation time τ is negligible within the experimental errors for τ. The experimental data ε′ and εd″ are described by the Cole–Cole relations for all solutions. It means that the process of water molecule relaxation in these solutions corresponds to one most probable relaxation time with small values of α (Table 1). The parameter α increases in concentrated solutions. The examples of Cole–Cole diagrams are given in Fig. 1. 3. Results and discussion Static dielectric constants of solutions were determined from the Cole–Cole diagrams by means of the circular extrapolation on the zero frequency. The values of static dielectric constant εs decrease with increasing salt concentration for all investigated solutions. The typical examples of concentration dependencies of dielectric constant for allyl-substituted ammonium salt solutions are presented in Fig. 2. The decrease of εs values for solutions in comparison with pure water is connected with partial decrease of rotary degrees of freedom of water molecules in the hydration spheres and with replacement of water molecules by ions with lower dielectric permittivity. The decrease of εs is observed in the series of salts: DAAFA ≈ DAMAFA→DADMACl. The values of dielectric relaxation time were found by means of the graphical decision of Cole–Cole relations. The frequency dependence of function [(εs − ε′)2 + (εd″)2] / [(ε′ − ε∞)2 + (εd″)2] in logarithmic coordinates becomes a straight line, which crosses the x-axis at a point corresponding to the maximum of dipole losses ω0 = 1/τ. The values of τ characterize the changes of water molecule mobility in the hydration shells of the ions. The values of dielectric relaxation time for solutions increase in comparison with pure water for all investigated salts (Tables 1 and 2). The examples of concentration dependences of τ at 298 K are
Fig. 1. The Cole–Cole diagrams for water (a) and aqueous solutions of DAMAFA (b: 0.25, c: 0.50, d: 0.75, e: 1.0, f: 1.5, g: 2.0 and h: 2.5 M) at 298 K. Frequencies: (1) 13.0, (2) 14.0, (3) 16.0, (4) 18.9, (5) 22.0 and (6) 25.0 GHz.
A. Lileev et al. / Journal of Molecular Liquids 131–132 (2007) 101–104
Fig. 2. The concentration dependencies of static dielectric permittivity of aqueous allyl-substituted ammonium salt solutions at 298 K. (1) DAA trifluoracetate, (2) DAMA trifluoracetate, (3) DADMA chloride.
presented in Fig. 3. The changes of τ for other solutions with hydrophilic and hydrophobic hydrations also are given on this figure. In solutions of KCl, the decrease of the values of τ is observed with increase of concentration [19], as well as for other solutions that consist of ions with hydrophilic hydration [20]. On the other hand, the relaxation time increases for (C4H9)4NCl solutions [5,8] where hydrophobic hydration of tetrabutylammonium is established. At 298 K, the values of τ increase in the series: KCl→H2O→(CH3)4NCl→DADMACl→(C4H9)4NCl and H2O→DAAFA→DAMAFA. All substituted ammonium salts cause decrease of orientational mobility of water molecules in solution. In the case of chloride solutions, the largest effect is caused by (C4H9)4N+-ion as it has the largest hydrocarbon radicals. In the series of trifluoracetate solutions, the DAMA ion has a stronger effect than DAA as it has one CH3-group more. The growth of τ for the considered allyl-substituted ammonium salt solutions indicates the similar to alkyl-substituted ammonium salt influence on the mobility of water molecules in tetrahedral H-bond net. The DADMACl causes the least changes of τ, though DADMA-ion contains more nonpolar groups in comparison with other allyl-substituted ammonium ions.
103
Fig. 3. The concentration dependencies of dielectric relaxation time for aqueous electrolyte solutions: (1) tetrabutylammonium chloride [5], (2) diallylammonium trifluoracetate, (3) diallylmethylammonium trifluoracetate, (4) diallyldimethylammonium chloride, (5) tetramethylammonium chloride [5], (6) KCl [19].
Probably, it is connected with the strong structure-breaking action of the Cl− ion on the H-bond network of water. Analysis of temperature dependence of dielectric relaxation time for DADMACl solutions shows that the values of ΔHε++ increase with increasing of salt concentration (Fig. 4). So, the concentration changes of ΔHε++ are similar for Bu4NCl and DADMACl solutions and are contrary for KCl solutions [19]. Since this parameter characterizes the strength of the H-bond net of water, it is possible to report on structure-making effect of diallyldimethylammonium ions as well as for (C4H9)4N+-ion. In
Table 2 Parameters of dielectric relaxation for aqueous solutions of diallyldimethylammonium chloride M 288 K (mol/l) εs τ (ps) α
298 K εs
τ (ps) α
εs
0 0.25 0.50 1.0 1.49 2.0 2.51
78.4 72.3 65.6 56.9 50.8 42.8 34
8.25 8.5 8.9 10 10.7 13.6 15.2
74.9 6.45 68.9 6.5 64.0 7.0 56.0 7.9 50.5 8.6 41.6 10.4 33.9 11.7
82.1 78.3 73.1 64.6 61.8 54.6 62.0
11.0 11.8 12.7 15.0 18.4 26.5 53.8
0.00 0.04 0.05 0.09 0.14 0.22 0.35
308 K
0.00 0.00 0.00 0.01 0.01 0.08 0.10
τ (ps) α 0.00 0.00 0.00 0.01 0.01 0.04 0.05
ΔH++ ε (kJ/mol) 17.2 19.5 19.6 21.3 25.7 31.7 30.0
Fig. 4. The concentration dependencies of activation enthalpy of relaxation processes for aqueous electrolyte solutions: (1) tetrabutylammonium chloride [5], (2) diallyldimethylammonium chloride, (3) KCl [19].
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the cases of DAAFA and DAMAFA solutions, we see stronger increase of τ values in comparison with DADMACl solutions. It can be assumed that hydrophobic hydration manifests for diallylammonium and diallylmethylammonium ions. 4. Conclusion The molecular-kinetic properties of water in aqueous allylsubstituted ammonium salt solutions were investigated. Allylsubstituted ammonium ions decrease the orientation mobility of water molecules in H-bond net. The structure-making effect is established for diallyldimethylammonium chloride solutions on the basis of concentration changes of τ and ΔHε++. They have opposite sign in comparison with electrolyte solutions where typical hydrophilic hydration is present (KCl, etc.). It is connected with hydrophobic hydration of DADMA ions. It is assumed that diallylammonium and diallylmethylammonium ions cause the same effect on water structure. Acknowledgement This work was supported by the RFBR, Project No. 05-0332100 and Program of Department of RAS. References [1] F. Franks (Ed.), Water, A Comprehensive Treatise, vol. 4, Plenum Press, New York, 1975, p. 1. [2] Yu.M. Kessler, A.L. Zaytsev, Solvofobniye Effecty. Teoriya, Experiment, Praktika, Leningrad, Khimiya, 1989, 312 pp.
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