- Email: [email protected]
The investigation of hydrophobic hydration effects
Physica B 276}278 (2000) 413}414
The investigation of hydrophobic hydration e!ects A.G. Novikov , M.N. Rodnikova, O.V. Sobolev * Institute for Physics and Power Engineering, Bondarenko Sq.1., Obninsk, Kaluga Region, Russia Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 117907, Moscow, Russia Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, 141980, Dubna, Moscow Region, Russia
Abstract The results of neutron scattering experiments on [CH ] NCl (2 M), [C H ] NCl (1 M) and [C H ] PCl (0.3 M) aqueous solutions are presented. The generalised frequency distributions (GFD) for water molecules hydrating [C H ] N> and [C H ] P> ions were obtained from the neutron scattering spectra. These data were compared with the results of ionic hydration previously obtained for LiCl (Novikov et al., Chem. Phys. Lett. 259 (1996) 391) and CsCl (Novikov et al., Physica B 234}236 (1997) 340) solutions. The comparison of GFD of hydration water molecules and GFD of pure water gives information concerning the ion in#uence on the rotation}vibration motions of surrounding water molecules and provides the data about changes in hydrogen-bounds network structure. The di!erence between the e!ects of ionic and hydrophobic hydration is discussed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen bonds; Proton dynamics; Water
Neutron scattering experiments on [CH ] NCl (2 M) [C H ] NCl (1 M) and [C H ] PCl (0.3 M) aqueous solutions were carried out by our group with the use of DIN-2PI double time-of-#ight spectrometer operating on a neutron beam of the IBR-2 pulsed reactor (Frank Laboratory of Neutron Physics, JINR, Dubna) [1]. Experiments and data handling methods were described in our previous papers [2}4]. It is well known that inelastic neutron scattering experiment provides information about di!usion characteristics of liquid molecules studied as well as their vibration}rotation motions. As for ionic and hydrophobic hydration e!ects on di!usion of water molecules, our results are generally in good agreement with the results of other methods (NMR [5,6], molecular-dynamics simulations [7,8]), and we touch upon them only brie#y. The main conclusion is that the e!ect of hydrophobic hydration under room temperature in the in#uence on hydration molecules di!usion is formally similar to the ionic hydration, hampering the di!usion mobility of the hydration shells
* Corresponding author. Fax: #7-95-883-3112. E-mail address: [email protected] (O.V. Sobolev)
molecules. As for tetraalkylammonium ions, this e!ect is stronger for the ions with longer alkyl chains [4]. We shall consider in more detail the vibration}rotation motion of hydration water molecules, because inelastic neutron scattering is practically the only method to obtain direct information on the spectrum of elementary excitations in the intermolecular interactions force "eld. It is useful to compare the generalised frequency distributions (GFD) for water molecules in [C H ] NCl and [C H ] PCl solutions with the results of ionic hy dration previously obtained for LiCl [2] and CsCl [3] solutions. The GFD of pure water molecules and GFDs of water molecules comprising the hydration shells of Cs> and Li> ions were presented in our previous papers [2,3]. Although Li> and Cs> ions belong to di!erent types of ionic hydration (positive and negative), as it follows from presented data, both of these ions lead to hydrogen-bounds (H-B) network disruption in the hydration shell. This is evidenced by decreasing of the weight of the "rst translation mode (e+7 meV) corresponding to deformation of tetrahedral angle O}O}O, which is usually regarded as the evidence for the existence of the H-B network [9]. The Bu N> and Ph P> ions a!ect molecules GFD of their hydration shell di!erently from the ions mentioned
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 6 1 1 - 7
414
A.G. Novikov et al. / Physica B 276}278 (2000) 413}414
H-B network in the hydration water surrounding the hydrophobic particles [14]. It follows from the analysis of hydrophobic particles e!ects on vibration}rotation motions of water molecules, that hydrophobic hydration does not a!ect the characteristics of these motions and hence does not bring about signi"cant deformations of H-B network in the surrounding water.
Acknowledgements The Russian Federation Program `Actual Investigation of Condensed Mattera supported this research.
References Fig. 1. Generalized frequency distributions of water molecules. Solid circles } pure water, open circles } water molecules in aqueous solutions: (a) [C H ] NCl (1 M); (b) [C H ] PCl (0.3 M).
above. It follows from Fig. 1 that there are no signi"cant changes in the low-frequency part of spectra. This fact is indicative of the persistence of H-B network existing in pure water in the vicinity of large hydrophobic particles. In our opinion, this result directly correlates with the results of neutron di!raction investigations on water structure in hydration spheres of tetraalkylammonium ions [10}13]. The absence of signi"cant in#uence of these ions (at least, up to and including Bu N>) on hydration water molecular structure, which appears as practically indistinguishable from pure water, was shown clearly. It should be noted that the study of hydrophobic e!ects by Raman scattering and infrared adsorption also did not show the remarkable growth of the intermolecular interactions and the space coordination of
[1] Yu.V. Taran (Ed.), User Guide, Neutron Experimental Facilities at JINR, JINR Press, Dubna, 1992. [2] A.G. Novikov, M.N. Rodnikova, V.V. Savostin, O.V. Sobolev, Chem. Phys. Lett. 259 (1996) 391. [3] A.G. Novikov, M.N. Rodnikova, V.V. Savostin, O.V. Sobolev, Physica B 234}236 (1997) 340. [4] A. Novikov, M. Rodnikova, J. Barthel, O. Sobolev, J. Mol. Phys. 79 (1999) 203. [5] H. Hertz, B. Lindman, V. Siepe, Ber. Bunsenges. Phys. Chem. 73 (1969) 542. [6] S. Bradl, E. Lang, J. Phys. Chem. 97 (1993) 10 463. [7] A. Geiger, A. Rahman, F. Stillinger, J. Chem. Phys. 70 (1979) 263. [8] A. Geiger, M.N. Rodnikova, C.A. Zasipkin, Russ. J. Phys. Chem. 69 (1995) 1173. [9] M. Seats, S. Rice, J. Chem. Phys. 72 (1980) 3236. [10] J. Turner, A. Soper, J. Finney, Mol. Phys. 70 (1990) 679. [11] J. Turner, A. Soper, J. Finney, Mol. Phys. 77 (1992) 411. [12] J. Turner, A. Soper, J. Chem. Phys. 101 (1994) 6116. [13] J. Turner, A. Soper, J. Finney, J. Chem. Phys. 102 (1995) 5438. [14] Z. Kecki, P. Dryjnski, J. Mol. Struct. 275 (1992) 135.
The investigation of hydrophobic hydration e!ects A.G. Novikov , M.N. Rodnikova, O.V. Sobolev * Institute for Physics and Power Engineering, Bondarenko Sq.1., Obninsk, Kaluga Region, Russia Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 117907, Moscow, Russia Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, 141980, Dubna, Moscow Region, Russia
Abstract The results of neutron scattering experiments on [CH ] NCl (2 M), [C H ] NCl (1 M) and [C H ] PCl (0.3 M) aqueous solutions are presented. The generalised frequency distributions (GFD) for water molecules hydrating [C H ] N> and [C H ] P> ions were obtained from the neutron scattering spectra. These data were compared with the results of ionic hydration previously obtained for LiCl (Novikov et al., Chem. Phys. Lett. 259 (1996) 391) and CsCl (Novikov et al., Physica B 234}236 (1997) 340) solutions. The comparison of GFD of hydration water molecules and GFD of pure water gives information concerning the ion in#uence on the rotation}vibration motions of surrounding water molecules and provides the data about changes in hydrogen-bounds network structure. The di!erence between the e!ects of ionic and hydrophobic hydration is discussed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen bonds; Proton dynamics; Water
Neutron scattering experiments on [CH ] NCl (2 M) [C H ] NCl (1 M) and [C H ] PCl (0.3 M) aqueous solutions were carried out by our group with the use of DIN-2PI double time-of-#ight spectrometer operating on a neutron beam of the IBR-2 pulsed reactor (Frank Laboratory of Neutron Physics, JINR, Dubna) [1]. Experiments and data handling methods were described in our previous papers [2}4]. It is well known that inelastic neutron scattering experiment provides information about di!usion characteristics of liquid molecules studied as well as their vibration}rotation motions. As for ionic and hydrophobic hydration e!ects on di!usion of water molecules, our results are generally in good agreement with the results of other methods (NMR [5,6], molecular-dynamics simulations [7,8]), and we touch upon them only brie#y. The main conclusion is that the e!ect of hydrophobic hydration under room temperature in the in#uence on hydration molecules di!usion is formally similar to the ionic hydration, hampering the di!usion mobility of the hydration shells
* Corresponding author. Fax: #7-95-883-3112. E-mail address: [email protected] (O.V. Sobolev)
molecules. As for tetraalkylammonium ions, this e!ect is stronger for the ions with longer alkyl chains [4]. We shall consider in more detail the vibration}rotation motion of hydration water molecules, because inelastic neutron scattering is practically the only method to obtain direct information on the spectrum of elementary excitations in the intermolecular interactions force "eld. It is useful to compare the generalised frequency distributions (GFD) for water molecules in [C H ] NCl and [C H ] PCl solutions with the results of ionic hy dration previously obtained for LiCl [2] and CsCl [3] solutions. The GFD of pure water molecules and GFDs of water molecules comprising the hydration shells of Cs> and Li> ions were presented in our previous papers [2,3]. Although Li> and Cs> ions belong to di!erent types of ionic hydration (positive and negative), as it follows from presented data, both of these ions lead to hydrogen-bounds (H-B) network disruption in the hydration shell. This is evidenced by decreasing of the weight of the "rst translation mode (e+7 meV) corresponding to deformation of tetrahedral angle O}O}O, which is usually regarded as the evidence for the existence of the H-B network [9]. The Bu N> and Ph P> ions a!ect molecules GFD of their hydration shell di!erently from the ions mentioned
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 6 1 1 - 7
414
A.G. Novikov et al. / Physica B 276}278 (2000) 413}414
H-B network in the hydration water surrounding the hydrophobic particles [14]. It follows from the analysis of hydrophobic particles e!ects on vibration}rotation motions of water molecules, that hydrophobic hydration does not a!ect the characteristics of these motions and hence does not bring about signi"cant deformations of H-B network in the surrounding water.
Acknowledgements The Russian Federation Program `Actual Investigation of Condensed Mattera supported this research.
References Fig. 1. Generalized frequency distributions of water molecules. Solid circles } pure water, open circles } water molecules in aqueous solutions: (a) [C H ] NCl (1 M); (b) [C H ] PCl (0.3 M).
above. It follows from Fig. 1 that there are no signi"cant changes in the low-frequency part of spectra. This fact is indicative of the persistence of H-B network existing in pure water in the vicinity of large hydrophobic particles. In our opinion, this result directly correlates with the results of neutron di!raction investigations on water structure in hydration spheres of tetraalkylammonium ions [10}13]. The absence of signi"cant in#uence of these ions (at least, up to and including Bu N>) on hydration water molecular structure, which appears as practically indistinguishable from pure water, was shown clearly. It should be noted that the study of hydrophobic e!ects by Raman scattering and infrared adsorption also did not show the remarkable growth of the intermolecular interactions and the space coordination of
[1] Yu.V. Taran (Ed.), User Guide, Neutron Experimental Facilities at JINR, JINR Press, Dubna, 1992. [2] A.G. Novikov, M.N. Rodnikova, V.V. Savostin, O.V. Sobolev, Chem. Phys. Lett. 259 (1996) 391. [3] A.G. Novikov, M.N. Rodnikova, V.V. Savostin, O.V. Sobolev, Physica B 234}236 (1997) 340. [4] A. Novikov, M. Rodnikova, J. Barthel, O. Sobolev, J. Mol. Phys. 79 (1999) 203. [5] H. Hertz, B. Lindman, V. Siepe, Ber. Bunsenges. Phys. Chem. 73 (1969) 542. [6] S. Bradl, E. Lang, J. Phys. Chem. 97 (1993) 10 463. [7] A. Geiger, A. Rahman, F. Stillinger, J. Chem. Phys. 70 (1979) 263. [8] A. Geiger, M.N. Rodnikova, C.A. Zasipkin, Russ. J. Phys. Chem. 69 (1995) 1173. [9] M. Seats, S. Rice, J. Chem. Phys. 72 (1980) 3236. [10] J. Turner, A. Soper, J. Finney, Mol. Phys. 70 (1990) 679. [11] J. Turner, A. Soper, J. Finney, Mol. Phys. 77 (1992) 411. [12] J. Turner, A. Soper, J. Chem. Phys. 101 (1994) 6116. [13] J. Turner, A. Soper, J. Finney, J. Chem. Phys. 102 (1995) 5438. [14] Z. Kecki, P. Dryjnski, J. Mol. Struct. 275 (1992) 135.