Appl. Phys. A 74 [Suppl.], S516–S518 (2002) / Digital Object Identifier (DOI) 10.1007/s003390201783
Applied Physics A
Materials
Science & Processing
QENS and NMR studies of 3-picoline–water solutions
L. Alm´asy1,2,∗, P. B ´anki1, M.C. Bellissent-Funel2, M. Bokor1, L. Cser1, G. Jancs ´o3, K. Tompa1, J.M. Zanotti2
1Research Institute for Solid State Physics and Optics, Budapest-1525, POB 49, Hungary
2Laboratoire L´eon Brillouin, CEA-CNRS, CE Saclay, 91191 Gif-sur-Yvette, France
3Atomic Energy Research Institute, Budapest-1525, POB 49, Hungary Received: 19 July 2001/Accepted: 11 December 2001 –Springer-Verlag 2002
Abstract. Quasi-elastic neutron scattering measurements were performed on aqueous solutions of 3-picoline (3-methyl- pyridine) at room temperature. H–D substitution on both the solute and the water was used to separate the dynamics of the two species. The analysis of the translational diffusive mo- tion at different concentrations shows that at high picoline content the diffusion coefficient of water decreases strongly and becomes similar to that of the solute, indicating strong coupling between the motions of the solute and the solvent.
Activation energies characteristic of the dynamic behavior of the methyl group were determined from1H spin–lattice re- laxation rate measurements for H2O and D2O solutions of 3-picoline above 310 K.
PACS:64.75.+g; 61.12.Ex; 61.18.Fs
3-picoline (3-methylpyridine, 3MP) dissolved in heavy wa- ter has a closed-loop immiscibility region between 38.5 and 117◦C at normal pressure, while it is fully miscible with light water at any concentration and temperature [1]. The partial miscibility is usually explained by strong orientational bond- ing between molecules of the two species. In this mixture, the water molecules can form strong hydrogen bonds with the N atom of the pyridine ring, thus connecting the 3MP molecule to a cluster of water molecules. This picture has been used in a simulation work to model the reentrant phase separation in aqueous solution of 3MP [2]. A recent quan- tum chemical study of the interaction between the molecules of methyl-substituted pyridine and water revealed the corre- lation between the miscibility behavior of the different MP – water mixtures and the strength of the N – H-O hydrogen bond [3]. Small-angle neutron scattering (SANS) measure- ments indicated strong clustering of the solute even far away from the immiscibility region [4]. Molecular dynamics (MD) simulations have not been yet carried out on these systems due to the complexity of the water – picoline intermolecular potential field.
∗Corresponding author. (E-mail: almasy@sunserv.kfki.hu)
Experimental and simulation investigations of aqueous solutions, e.g. QENS [5, 6], nuclear magnetic resonance (NMR) [7] and MD [6, 8] studies of aqueous solutions of dimethylsulfoxide [5, 7, 8] and 1,2-dimetoxyethane [6] usu- ally indicate strong hindering of the translational motion of water in solutions of organic molecules. In the present work the dynamic behaviour in the 3MP – water system was studied to obtain more information which can be useful to understand the strong non-ideal behaviour of this mixture.
1 Experimental
Incoherent quasi-elastic neutron scattering measurements have been performed on the MIB ´EMOL time-of-flight spec- trometer at the reactor Orph´ee, Laboratoire L´eon Brillouin, Saclay. 3MP – water mixtures were prepared using both ordi- nary and deuterated 3MP and light and heavy water, in order to separate the signals arising from the hydrogen atoms of the different species. The measurements were done at 25◦C. Inci- dent neutron wavelengths of 6 Å, and 9 Å were used to cover a wide energy and momentum transfer range, giving energy resolutions of 96 and 28µeV FWHM, respectively.
Three samples containing 2, 8.4 and 25 mol% 3MP have been selected as three characteristic concentrations – the di- lute and the concentrated samples remain fully miscible at any temperature, while 8.4 mol% corresponds to the criti- cal concentration of the lower phase separation point of the 3MP – D2O mixture [9]. At the temperature of the measure- ments all samples were macroscopically homogeneous.
In this paper we describe only the results of the high- resolution (λ=9 Å, FWHM=28µeV) measurements of the translational diffusion for both water and 3MP solute in the aqueous solutions of 3MP. A more detailed analysis of the spectra obtained at 6 and 9 Å, will be presented in a subse- quent publication.
Data analysis was performed using the standard proced- ures developed in the LLB. The intensities measured with the large set of detectors were grouped to points of fixed mo- mentum transfer. At each point the time-of-flight spectrum was fitted by a single Lorentzian component convoluted with
S517
HWHM,meV
2
Fig. 1.Experimental linewidths obtained from high energy resolution spec- tra (λ=9 Å) in H2O–C6D7N mixtures. Thelinesare fits using the jump diffusion model
the experimental resolution function, and a linear background term was added to correct for the small contribution of inelas- tic processes.
The results of the analysis of the measurements on the deuterated 3MP – H2O solutions are shown in Fig. 1. The be- haviour of the Lorentzian linewidth as a function ofQ2is well accounted for by a jump diffusion model [10] from which the diffusion coefficient and the residence time of the water pro- tons are obtained. The diffusion coefficient of the protiated 3MP in heavy water was calculated applying the model of translational self-diffusion [11].
Proton spin-lattice relaxation rates 1H(R1) were meas- ured in H2O and D2O solutions of protiated 3-picoline at ν0=86.68 MHz on Bruker SXP 4-100 spectrometer by using inversion recovery method. The stability of the frequency and magnetic field was better than±1×10−6. The tempera- ture was controlled by an open-circle Oxford cryostat and an Oxford ITC4 temperature controller using N2 gas flow.
The uncertainty of the temperature control was less than 1 K.
The recovery of magnetization was exponential. Activation energies were obtained from the measured R1(T)data by im- plying Arrhenius type dependence.
2 Results and discussion
The obtained diffusion coefficients of 3MP and water in the aqueous solutions of 3MP are compared in Fig. 2. With in- creasing concentration both components show strong slowing down, the diffusion coefficient of water tends to approach that of the solute. In the more dilute solutions the observed diffusion coefficients of the water represent only the aver- age characteristics of water molecules having different envi- ronment: surrounded by water molecules, or attached to the solute molecules with or without hydrogen bonding. In the mixtures that exhibit strong composition fluctuations, such as picoline – water mixtures, this effect is more pronounced as separate regions with different solute concentrations are present [4].
The decrease of the diffusion coefficient of the water with increasing solute concentration is a common feature of many aqueous solutions [5, 6, 12]. The dynamics of the solute has been studied much more scarcely by neutron scattering. We
Diffusioncoefficient,*10-6 cm2 /s
Fig. 2.Self-diffusion coefficients of H2O and 3MP in the mixtures at room temperature. The value for pure light water is taken from [6]
are aware of two neutron studies of aqueous solutions of small organic molecules [5, 12]. In [12] the mobility of the tetra- methylurea was found to be independent of the concentration, while a slowing down of the water motion similar to the present results was observed. In [5] at a single concentration of about 33 mol% of dimethyl-sulfoxide (DMSO) the diffu- sion coefficient of DMSO was found to be clearly different
1 HR1(s-1 )1 HR1(s-1 )
Fig. 3.1H spin-lattice relaxation rates in 3MP–D2O and H2O solutions at ν0=86.68 MHz. Size of symbols represents error of measurement. Solid linesare fits used for activation energy determination fromT>310 K data (see text);dashed lines: extrapolation
S518
from that of the water, indicating the absence of stable solute- solvent complexes. In the concentrated 3MP – water mixtures the observed behavior of the two species can be interpreted as an indication of the strong coupling between the motions of the water and 3-picoline.
The dynamics of the methyl group of 3MP and of the solvent water can be characterized by analyzing the tempera- ture dependence of proton spin-lattice relaxation rate (Fig. 3).
Both the reorientational motions of the methyl group and the mobility of the hydrogen of water are responsible for the results obtained for 3MP – H2O solutions. The activation en- ergies (Ea) observed in light and heavy water solutions of 2 and 25 mol% picoline content are equal within the errors of measurement (Table 1). This result implies that the activation energies obtained for both light and heavy water solutions can be considered as being characteristic of the reorienta- tional behavior of the methyl group of 3MP molecules. As Ea values obtained for methyl group reorientation are very close to the activation energy for self-diffusion in pure wa- ter (18.3 kJ/mol at 25◦C [13]) we suggest that the methyl group of 3MP and the water molecules are correlated in their dynamic behavior.
Activation energies (Table 1) show an increasing trend with increasing concentration of the solute. This is in accor- dance with the slowing down of both components, although further experiments are needed to obtain more accurate acti- vation energy values. Thec=8.4 mol% solution of 3MP in heavy water undergoes a phase separation in the measured
Table 1.Activation energies of the methyl group rotation (Eain kJ/mol) in 3MP–H2O and D2O solutions obtained fromT>310 K1H spin-lattice relaxation rate data (Fig. 3)
c(mol%) 3MP–H20 3MP–D2O 2 17.8±0.2 18.3±0.4 8.4 18.3±0.6 20.0±1.2 25 19.1±0.5 18.7±0.2
temperature range, so the fitted Ea represents only an aver- age for the motions in the two phases. This can be seen on the high uncertainty of the calculatedEa, as well as on the larger deviations of the R1(T)data from linear behavior (see Fig. 3).
3 Conclusions
A slowing down of the diffusional processes of water molecules within the sphere of influence of the solute was observed by QENS. At the highest picoline concentration (25 mol%) the diffusion coefficients of the solvent and so- lute were found to be the same. The connection between the dynamics of the water and that of the methyl group of pico- line (observed by NMR) suggests that the anomalous mixing behavior of the aqueous solutions of picoline can be partly re- lated to the dynamics of the water hydrate shell surrounding the methyl group of the solute.
Acknowledgements.The Hungarian Research Fund under Grant No. OTKA T031829 is gratefully acknowledged. L.A. wishes to thank the LLB for their hospitality and the French Government for a Ph.D. scholarship.
References
1. J.D. Cox: J. Chem. Soc. 4606 (1952)
2. I.V. Brovchenko, A.V. Oleinikova: J. Chem. Phys.106, 7756 (1997) 3. I. P´apai, G. Jancs ´o: J. Phys. Chem. A104, 2132 (2000)
4. L. Alm´asy, L. Cser, G. Jancs ´o: Physica B276–278, 446 (2000) 5. J.T. Cabral, A. Luzar, J. Teixeira, M.C. Bellissent-Funel: J. Chem.
Phys.113, 8736 (2000)
6. D. Bedrov, O. Borodin, G.D. Smith, F. Trouw, C. Mayne: J. Phys.
Chem. B104, 5151 (2000)
7. K.J. Packer, D.J. Tomlinson: Trans. Faraday Soc.67, 1302 (1971) 8. I.A. Borin, M.S. Skaf: J. Chem. Phys.110, 6412 (1999) 9. J. Szydlowski: Nukleonika43, 423 (1998)
10. J. Teixeira, M.C. Bellissent-Funel, S.H. Chen, A.J. Dianoux: Phys.
Rev. A31, 1913 (1985)
11. K.S. Singwi, A. Sjolander: Phys. Rev.119, 863 (1960)
12. L. Cser, T. Gr´osz, G. Jancs´o, G. K´ali: Physica B234–236, 349 (1997) 13. H. Weingärtner: Z. Phys. Chem. NF132, 129 (1982)
