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LOW FREQUENCY DYNAMICS IN WATER AND AQUEOUS SOLUTIONS OF ZnCl2 : A
COMPARATIVE INVESTIGATION BY RAMAN, RAYLEIGH WING AND INELASTIC NEUTRON
SCATTERING
M. Fontana, G. Maisano, P . Migliardo, M.-C. Bellissent-Funel, A. Dianoux
To cite this version:
M. Fontana, G. Maisano, P . Migliardo, M.-C. Bellissent-Funel, A. Dianoux. LOW FREQUENCY
DYNAMICS IN WATER AND AQUEOUS SOLUTIONS OF ZnCl2 : A COMPARATIVE INVESTI-
GATION BY RAMAN, RAYLEIGH WING AND INELASTIC NEUTRON SCATTERING. Journal
de Physique Colloques, 1984, 45 (C7), pp.C7-151-C7-159. �10.1051/jphyscol:1984716�. �jpa-00224281�
JOURNAL DE PHYSIQUE
Colloque C7, suppl6rnent au n09, Tome 45, septernbre 1984 page C7-151
LOW FREQUENCY DYNAMICS IN WATER A N D AQUEOUS SOLUTIONS OF ZnC12 : A COMPARATIVE INVESTIGATION BY RAMAN, RAYLEIGH WING AND INELASTIC NEUTRON SCATTERING
M . P . Fontana, G . ~ a i s a n o * , P . Migliardo*, M.-C. f ell is sent-Funel** and A. J . ~ianoux***
I s t i t u t o d i F i s i c a and G.N. S . M., Parma, I t a Z y
* ~ s t i t u t o d i F i s i c a and G. Ilr. S . M., Messina, I t a Z y
**
Laboratoire Le'on BriZZouin, CEIlr ~ a c Z a y + , 91191 Gif-sur-Yvette Cedex, France***
I .L. L., B. P. 256 X, 38042 GrenobZe Cedex, France
Resume
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Nous presentons l e s r e s u l t a t s d'une etude (syst6matique)des spectres d e r a t i o n s ti basse ( u s 4 0 0 cm-1) e t t r e s basse (0.2 cm-1 5 ws
40 cm-1) frequences e t de l a dynamique s t r u c t u r a l e de l ' e a u e t de solutions aqueuses de ZnC12 (pour des concentrations s1@tendant jusqu'ti l a s a t u r a t i o n ) . Des techniques de diffusion inelastique de neutrons par temps de vol e t de spec- troscopie Raman ont 6t6 u t i l i s e e s pour determiner l e s d i s t r i b u t i o n s de f r e - quence dues aux vibrations e t l a dependance en frequence de l a fonction de couplage Raman e n t r e l e s e t a t s Slectroniques e t l e s @ t a t s vibrationnels des systemes. Des temps de relaxation s t r u c t u r a l e ont 6t6 determines par d i f f u - sion de l a lumiPre Rayleigh di'polarisee. Les r e s u l t a t s confirment que czs systsmes peuvent Otre consideres comme des "solides localement amorphes.
Ladiscussion e s t f a i t e en termes d'amas localement ordonnes e t dynamiquement tort-616s dont l a s t r u c t u r e e t l a dynamique sont fortement influencees par l e s o l u t e e t en p a r t i c u l i e r par l ' a c t i o n de celui-ci sur l e s l i a i s o n s hydrogene de l ' e a u .
Abstract
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In t h i s paper we report the r e s u l t s of a comprehensive investiga- tion on the low (w ,$ 400 cm-1) and very low (0.2 cm-1 ,$ w 40 cm-l) f r e - quency vibrational and s t r u c t u r a l dynamics i n water and aqueous solutions of ZnC12 ( f o r concentrations up t o s a t u r a t i o n ) . Time-of-flight i n e l a s t i c neutron s c a t t e r i n g and Raman spectroscopy were used t o obtain the vibrational frequency d i s t r i b u t i o n s and the frequency dependence of the Raman electron vibration coup1 ing function. Structural relaxation times were determined by depolarized Rayleigh wing s:attering. The r e s u l t s , which confirm t h a t these systems a r e best viewed as l o c a l l y amorphous s o l i d s " , a r e discussed i n terms of locally ordered, dynamically correlated patches i n which s t r u c t u r e and dynamics are strongly influenced by the s o l u t e , especially by i t s action on the water hydroqen bond.I - INTRODUCTION
Several experimental techniques can y i e l d information on the relaxation times which characterize the short time microscopic motion in water and aqueous solutions ( o r , more generally, in liquids and amorphous or disordered s o l i d s ) . Most of these tech- niques ( f o r instance NMR, ultrasonic attenuation, d i e l e c t r i c relaxation, neutron, X-ray and l i g h t s c a t t e r i n g ) rely t o some extent on a dynamical model of the system t o connect the experimental r e s u l t s with the fundamental dynamical quantities of i n t e r e s t . We feel however t h a t in the short time domain ( T 5 10-lo sec.) scattering spectroscopy yields experimental q u a n t i t i e s which a r e c l o s e s t t o the i n t r i n s i c dy- namics of the system, i . e . , they require the smallest amount of intermediary mode- l i n g . Thus the connection between what i s actually measured and what one wishes t o , or thinks hd i s measuring i s c l e a r e r and more d i r e c t . A f u r t h e r advantage of t h i s + ~ a b o r a t o i r e commun C E A , CNRS
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984716
C7-152 JOURNAL DE PHYSIQUE
is t h e p o s s i b i l i t y of q u a n t i t a t i v e comparisons between t h e q u a n t i t i e s determined by d i f f e r e n t types o f s c a t t e r i n g spectroscopy a p p l i e d t o t h e same system, and o b t a i n t h u s a s e l f - c o n s i s t e n t and d e t a i l e d p i c t u r e of t h e time development of t h e micros- copic motions i n t h e system.
To our knowledge such approach has never been t e s t e d f u l l y i n any system. In t h i s paper we p r e s e n t t h e r e s u l t s of such a t y p e of study on pure H20, D20, and aqueous s o l u t i o n of ZnC12. O u r main i n t e r e s t was t h e determination of t h e low frequency (w 5 50 meV) and very low frequency (0.01 meV
x
w s 5 meV) v i b r a t i o n a l and d i f f u - sional motions i n such systems and t h e i r i n t e r p r e t a t i o n i n terms of c o l l e c t i v e motion i n l o c a l l y ordered s t r u c t u r e s , i n which t h e c o r r e l a t i o n range and t h e s t r u c - t u r e could be v a r i e d by changing t h e s o l u t e concentration a t c o n s t a n t temperature.The experimental techniques used were t i m e - o f - f l i g h t i n e l a s t i c and q u a s i - e l a s t i c neutron s c a t t e r i n g , Raman and depolarized Rayleigh wing spectroscopy. O u r r e s u l t s concerning q u a s i - e l a s t i c s c a t t e r i n g a r e reported elsewhere / I / , although we s h a l l use them i n t h i s paper a l s o . An obvious extension of our measurements i s t h e study of t h e e f f e c t s of temperature v a r i a t i o n , e s o e c i a l l y i n t h e supercooled region.
Actually B r i l l o u i n and Rayleigh wing s p e c t r a have already been taken i n supercooled water down t o 247K, and t h e r e s u l t s w i l l be published elsewhere / 2 / .
Thus i n t h i s paper we s h a l l l i m i t our account t o room temperature ( e . g . 298K) vibra- t i o n a l and d i f f u s i o n a l dynamics, with p a r t i c u l a r emphasis on what can be learned by t h e simultaneous a p p l i c a t i o n of t h e t h r e e experimental techniques j u s t mentioned.
In n a r t i c u l a r , we s h a l l compare t h e v i b r a t i o n a l information obtained by i n e l a s t i c neutron and by Raman s c a t t e r i n g , and we s h a l l show t h e complementary n a t u r e of t h e information obtained by q u a s i - e l a s t i c neutron and by depolarized Rayleigh winq s c a t - t e r i n g r e s o e c t i v e l y . F i n a l l y , we s h a l l p r e s e n t a reasonably d e t a i l e d narametrization of t h e time e v o l u t i o n of o r i e n t a t i o n a l , s t r u c t u r a l and v i b r a t i o n a l degrees of f r e e - dom i n water and how a l l t h i s i s a f f e c t e d by t h e a d d i t i o n of ZnC12.
I1
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LOCAL STRUCTURE AND DIFFUSIONAL MOTIONIn pure water each molecule i s surrounded on t h e average by 4-5 molecules i n preva- l e n t t e t r a h e d r a l coordination, o r i g i n a t e d by t h e s a t u r a t i o n of t h e f o u r H-bonds of which t h e s i n g l e molecule i s caoable /3/. Although each molecule w i l l r e t a i n i t s
p o s i t i o n i n t h e s t r u c t u r e f o r an estimated time of t h e o r d e r of picoseconds, a t any given time t h e r e i s a high p r o b a b i l i t y of f i n d i n g f o u r molecules i n t h e t e t r a h e d r a l p o s i t i o n s around a given one. From t h e point of view of d i f f u s i o n a l dynamics we should exoect t h e r e f o r e t h a t t h e d i f f u s i o n be somewhat s o l i d - l i k e . Independently of t h e s ~ e c i f i c model chosen /4/ t h e most r e l e v a n t parameter w i l l be t h e "residence time" T! o f a molecule i n i t s p o s i t i o n , which can be defined a s "quasi-equilibrium"
a s long a s r! i s l a r g e r t h a t t y p i c a l v i b r a t i o n a l periods. I t i s t h e r e f o r e t h i s quan- t i t y which i s determined i n a q u a s i - e l a s t i c neutron s c a t t e r i n g exoeriment, which probesotime c o r r e l a t i o n s on t h e nicosecond t o 100 nsec s c a l e over d i s t a n c e s of up t o 10 A . I s T! synonimous with t h e s o - c a l l e d H-bond l i f e t i m e , o r with t h e H-bond breaking time T~ ? The answer may be found by an indenendent, d i r e c t measurement of r S . This can be obtained by depolarized Rayleigh wing s c a t t e r i n g . Such e f f e c t i s connected t o t h e a n i s o t r o o i c p o l a r i z a b i l i t y f l u c t u a t i o n s induced by molecular motions i n t h e long time l i m i t ( a t s h o r t e r times t h e v i b r a t i o n a l component would dominate). Therefore t h e non-vibrational p a r t of t h e d e ~ o l a r i z e d spectrum w i l l be a c t i v a t e d , i . e . w i l l be v i s i b l e a s i n a f l a s h , only when t h e molecules a r e i n t h e process of breaking a p a r t ( o r coming t o g e t h e r ) . When i n t h e i r " s t a b l e " t e t r a h e d r a l o o s i t i o n s , t h e molecules may c o n t r i b u t e only t o t h e v i b r a t i o n a l d e n s i t y of s t a t e s g(w). Assuming an exponentially decaying time c o r r e l a t i o n f u n c t i o n of t h e a n i s o t r o - pic p o l a r i z a b i l i t y f l u c t u a t i o n s , t h e r e s u l t i n g spectrum w i l l be a Lorentzian cen- t e r e d a t z e r o frequency s h i f t with a half-width a t h a l f maximum (HWHEI) T-rl4/-cs.
Therefore i n p r i n c i p l e t h e combined q u a s i - e l a s t i c neutron and l i g h t s c a t t e r i n g measurements should r e s o l v e t h e question of whether t h e H-bond breaking time coin-
c i d e s with t h e molecular residence time.
Q u a s i - e l a s t i c neutron s c a t t e r i n g y i e l d s , f o r pure water a t 298K, t h e value 4 - 1 . 7 psec f o r t h e molecular residence time / 4 / . In f i g . 1 we show t h e depolarized
Fig.1
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Depolarized Rayleigh wing spectra f o r pure Hz0 a t 298K ; spectral resolution was varied according t o the frequency s h i f t , from b e t t e r than 0.01 meV t o 0.06 meV.Rayleigh wing spectra we obtained f o r pure H20 a t 298K. All experimental procedures are described elsewhere-/5/. Here we wfsh only t o mention-tTat i n t%e lTwest f r e - quency s h i f t region, our spectral bandpass was b e t t e r than 0.01 meV. The spectrum could be f i t t e d excellently with a resolution limited Lorentzian plus a broader one, from which the value I - ~ = 0.7 psec was obtained. Since the difference in the experi-, mental value of
r l
and .rs i s well outside the experimental e r r o r , our r e s u l t s clearly indicate t h a t H-bond breaking time and molecular residence time a r e not synonimous.Actually any two molecules may break and re-form t h e i r H-bond several times before actually breaking away from t h e i r quasi-equilibrium positions. This important r e s u l t i n t u r n impl i e s a many-molecule c o l l e c t i v e interaction, i .e. the s t r u c t u r a l cage which keeps n molecules together holds even i f some s i n g l e bonds a r e broken here and there.
The addition of ZnC12 complicates the composition of the system considerably.
However some e f f e c t s on the residence and structural relaxation times a r e c l e a r l y evident. In p a r t i c u l a r , r! i s found t o increase up t o 8 psec a t s a t u r a t i o n . Since even a t t h i s concentration, where the ZnC12/H20 molecular r a t i o i s about 0.5, the dominant contribution t o the s c a t t e r i n g of neutrons originates from the water nrotons, t h i s r e s u l t indicates t h a t ZnC12 s t a b i l i z e s the water molecules i n a s t r u c t u r e which, as we shall see and as has been discussed i n previous papers / 6 / , i s strongly connected t o the local c r y s t a l l i n e s t r u c t u r e of t h e solute.
The structural relaxation time r s , which f o r nure water reduces t o the H-bond breaking time, features a more complex behavior. A t s a t u r a t i o n , there seems t o be only one relaxation time, r S = 1 . 4 psec, whereas a t intermediate concentrations there are a t l e a s t two s t r u c t u r a l relaxation times, of which the shorter goes i n t o the s i n g l e relaxation time observed both a t high and low (zero) concentrations (Fig.2). Such time i s therefore connected with the bond between water molecules among themselves and/or with the metal ion. I t must be emohasized t h a t i t reduces t o the H-bond breaking time only i n the l i m i t of low concentrations. Otherwise, the bond i s more complex, involving, and being modified by, the interaction between the water molecule and t h e metal ions. A t saturation (and nossibly also a t lower concentrations), the existence of a s i n g l e time impl i e s t h a t the water molecules, together with the Zn and C1 ions are packed i n a f a i r l y s t a b l e local s t r u c t u r e , and i n t e r a c t s u f f i c i e n t l y strongly t o minimize differences i n the resoective bond dynamics. In t h i s framework t h e existence of a slower component ( r s = 5 psec) a t intermediate concentrations may be ascribed t o the breaking of the metal-halogen bond. Such bond however, i n order t o y i e l d a seaarate relaxation time, must belong
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Fig.2
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Depolarized Rayleigh wing s p e c t r a f o r 6M s o l u t i o n o f ZnC12 i n H20. Same experimental c o n d i t i o n s as f i g . 1 .t o i s o l a t e d Zn-C1 complexes, o r a t l e a s t t o complexes n o t embedded i n t h e "polymeric"
s t r u c t u r e t o which most o f t h e s o l u t e and s o l v e n t belong i n the s a t u r a t e d s o l u t i o n / 7 / . Thus t h e i n t e n s i t y o f t h e slower r e l a x a t i o n should y i e l d the r e l a t i v e propor- t i o n o f " i s o l a t e d " and "oolymeric" Zn-C1 comol exes as c o n c e n t r a t i o n i s increased.
I t s disappearance a t s a t u r a t i o n i s i n good agreement w i t h t h e o b s e r v a t i o n t h a t t h e polymeric s t r u c t u r e dominates f o r c
2
l O M and w i t h EXAFS data, which i n d i c a t e t h a t near s a t u r a t i o n a t l e a s t 85 % o f t h e Zn ions belong t o such s t r u c t u r e /8/.I 1 1
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VIBRATIONAL EXCITATIONS, COUPLING AND CORRELATIONSA l l dynamical i n f o r m a t i o n on a system o f many i n t e r a c t i n g p a r t i c l e s i s contained i n t h e v e l o c i t y t i m e a u t o - c o r r e l a t i o n f u n c t i o n , whose s p e c t r a l densi t~ i s t h e g e n e r a l i z e d frequency d i s t r i b u t i o n gN(w). For r e l a t i v e l y l o n g times g (w) describes the d i f f u s i v e m o t i o n o f t h e p a r t i c l e s , whereas f o r s h o r t times i t i s connected t o t h e v i b r a t i o n a l motion. I n f a c t , f a r a s o l i d , and a l s o f o r a s t r o n g l y associated 1 iq u i d , gN(w) should reduce t o t h e v i b r a t i o n a l d e n s i t y o f s t a t e s g(w).
I t i s w e l l known t h a t i n a system w i t h a s i z e a b l e c o n t r i b u t i o n from incoherent s c a t t e r i n g gN(w) may be obtained by t h e s o - c a l l e d E g e l s t a f f - S c h o f i e l d e x t r a p o l a t i o n , according t o which
where Ss(q,w) i s t h e i n c o h e r e n t s c a t t e r i n g law.
I f t h e e x t r a p o l a t i o n i s p r o p e r l y c a r r i e d out, and coherent s c a t t e r i n g n o t t o o interne, one may use i n p l a c e of Ss(q,w) t h e experimental s c a t t e r e d i n t e n s i t y . However, care must b e taken t o check f o r eventual modulation o f gN(w) by r e s i d u a l coherence e f f e c t s . Using t h e IN6 t . 0 . f . snectrometer a t I.L.L.(Grenoble) we have obtained p r e c i s e i n e l a s t i c s c a t t e r i n g s p e c t r a f o r H20, D20, and r e s p e c t i v e s o l u t i o n s o f ZnC12, SrC12, CuBr-2. Here we s h a l l discuss t h e r e s u l t s obtained f o r pure water and ZnC12 s o l u t i o n s i n D20. Since we are s t u d y i n g low frequency v i b r a t i o n s , we chose an
i n c i d e n t neutron wavelength of 5.9 A ; thus our a n a l y s i s has an upper frequency l i m i t of approximately 50 meV, due t o t h e poor s i g n a l - t o - n o i s e r a t i o a t higher energy t r a n s f e r s . Other experimental and d a t a hand1 ing d e t a i l s a r e discussed elsewhere / 5 / . In f i g . 3 and 4 we show t h e gN(w) d i s t r i b u t i o n obtained f o r ZnC12
Fig.3
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Generalized frequency d i s t r i b u t i o n gN(w) f o r H20 and 3M, 6M, 12.6M, ZnC12 s o l u t i o n s r e s p e c t i v e l y .Fig.4 - Generalized frequency d i s t r i b u t i o n gN(w) f o r D20 and 3M, 6M, 12.6M, ZnC12 s o l u t i o n s r e s p e c t i v e l y .
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s o l u t i o n s i n Hz0 and D20 r e s p e c t i v e l y . The two sets o f d i s t r i b u t i o n s a r e very s i m i - l a r , c o n f i r m i n g e m p i r i c a l l y t h e procedure o f u s i n g t h e t o t a l s c a t t e r e d i n t e n s i t y i n t h e E g e l s t a f f - S c h o f i e l d e x t r a p o l a t i o n .
The most important f e a t u r e i n t h e spectra i s t h e peak a t 7 meV, which decreases s t r o n g l y w i t h i n c r e a s i n g s o l u t e c o n c e n t r a t i o n . Such a peak i s due t o t h e l i b r a t i o n a l deformation o f t h e H-bond cage about a g i v e n water molecule. I t s decrease due t o t h e Zn and C1 i o n s i n h i g h c o n c e n t r a t i o n s i n d i c a t e s then a breaking-up o f t h e H- bond s t r u c t u r e . T h i s r e s u l t helps t o understand t h e behaviour o f t h e s t r u c t u r a l r e l a x a t i o n times described i n t h e preceding paragraoh. I n f a c t , whereas a t low concentrations t h e s t r u c t u r a l r e l a x a t i o n i s dominated by t h e H-bond b r e a k i n g time, a t h i g h concentrations, where e s s e n t i a l l y no o r d i n a r y H-bonds e x i s t as shown by t h e behaviour o f t h e 7 meV peak, we see a d i f f e r e n t r e l a x a t i o n time.
We want t o discuss now t h e very low frequency r e g i o n o f t h e v i b r a t i o n a l spectrum (w 5 6 meV). I n t h i s r e g i o n i n f a c t t h e v i b r a t i o n a l dynamics m i g h t be s e n s i t i v e t o t h e existence o f l o c a l l y ordered patches. The e f f e c t o f l o c a l order, and i t s change w i t h i n c r e a s i n g s o l u t e concentration, may be o f two types : i t may a1 t e r t h e v i b r a t i o n a l d e n s i t y o f s t a t e s g(w), and i t may a l t e r t h e response o f the v i b r a - t i o n a l ensemble t o some p e r t u r b a t i o n , such as an u l t r a s o n i c wave (and i n t h i s case s p e c i f i c r e l a x a t i o n e f f e c t s have been observed / 9 / ) o r an i n c i d e n t e l e c romagnetic
a
wave (such as i n B r i l l o u i n o r Raman s c a t t e r i n g ) . I n f i g . 5 we show t h e g (w) d i s t r i - b u t i o n s f o r H20, D20 and t h e s a t u r a t e d s o l u t i o n s o f ZnC12 i n Hz0 and D20 respectively.
Fig.5
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Very low frequency gN(w) f o r H20, D20 and s a t u r a t e d s o l u t i o n s o f ZnC12 i n Hz0 and D20 r e s p e c t i v e l y .We n o t e t h a t i n t h i s frequency r e g i o n t h e r e seems t o be a small d i f f e r e n c e i n t h e s p e c t r a o f pure Hz0 and D20, centered a t about 1 meV. Since a t low frequencies eventual coherence e f f e c t s a r e expected t o be more important, such d i f f e r e n c e i s probably due t o r e s i d u a l i n t e r f e r e n c e e f f e c t s i n t h e D20 spectra. Thus t h e t r u e g(w) should be g i v e n by t h e Hz0 data. I n b o t h cases t h e frequency d i s t r i b u t i o n deviates from t h e Debye w2-dependence expected i n t h i s low frequency r e g i o n i f o n l y a c o u s t i c v i b r a t i o n s were present. Such d e v i a t i o n i m p l i e s t h e existence o f
o t h e r motional degrees o f freedom, very probably connected w i t h s t r u c t u r a l r e l a x a - t i o n : i n t h i s , as i n o t h e r ways, water and aqueous s o l u t i o n s o f t h e type s t u d i e d here behave as amorphous m a t e r i a l s .
We can t e s t t h e o t h e r e f f e c t o f l o c a l o r d e r i n g on dynamics by determining t h e f r e - quency dependence o f t h e c o u p l i n g o f t h e v i b r a t i o n a l ensemble t o t h e e.m. r a d i a t i o n v i a t h e e l e c t r o n i c p o l a r i z a b i l i t y . I n o r d e r t o do t h i s we must separate t h e depola- r i z e d Rayleigh wing s c a t t e r i n g from t h e v i b r a t i o n a l Raman s c a t t e r i n g i n t h e t o t a l i n t e n s i t y . This may be e a s i l y done once t h e q u a s i - e l a s t i c d e p o l a r i z e d l i g h t scat- t e r i n g has been p r o p e r l y parametrized, as we have discussed i n t h e preceding para- graph. Thus we have s u b t r a c t e d from t h e experimental s p e c t r a t h e q u a s i - e l a s t i c c o n t r i b u t i o n ( i n t h e form o f two o r t h r e e l o r e n t z i a n s depending on s o l u t e concen- t r a t i o n ) ; t h e remaining i n t e n s i t y was then normalized by t h e (n(w,T)+l)/w s t a t i s - t i c a l f a c t o r , t o y i e l d t h e Raman reduced spectrum :
R R
where C ( w ) i s t h e e l e c t r o n v i b r a t i o n c o u p l i n g f u n c t i o n . I n f i g . 6 we show t h e g (w)
Fiq.6
-
Reduced Raman i n t e n s i t y a f t e r s u b t r a c t i o n o f q u a s i - e l a s t i c deoolarized~ a y l e i ~ h component f o r Hz0 ; D ~ O and s a t u r a t e d s o l u t i o n s o f ZnC12 i n ~ 2 0 and D20 r e s p e c t i v e l y .
s p e c t r a f o r H20, D20 and t h e s a t u r a t e d s o l u t i o n o f ZnC12 i n H20. I f t h e reasonable assumption i s made t h a t f o r these h i g h l y s t r u c t u r e d and associated f l u i d s gN(w)*g(w), then the c o u p l i n g f u n c t i o n CR(w) may be determined d i r e c t l y by experiment, by t a k i n g t h e r a t i o between gR(w) and gN(w) :
cR(w) = gR(w)/gN(w) (2)
The r e s u l t s are p l o t t e d i n f i g . 7 f o r t h e case o f s o l u t i o n s i n H20.
It i s important t o note t h a t unless combined measurements o f t h e type r e p o r t e d here are performed, i n order t o o b t a i n cR(w) one must r e s o r t t o more o r l e s s complex t h e o r e t i c a l models
/ l o /
; as f a r as we know, our's i s t h e f i r s t t o t a l l y experimentalJOURNAL DE PHYSIQUE
AE,meV
Fig.7
-
Raman c o u p l i n g f u n c t i o n C (w) f o r ZnC12 s o l u t i o n s i n H20. R d e t e r m i n a t i o n o f C (w). RFor c o u p l i n g t o a c o u s t i c v i b r a t i o n s , i t i s g e n e r a l l y assumed t h a t
C l e a r l y such behaviour c o u l d f i t o u r data o n l y i n t h e w+O l i m i t . A t h i g h e r frequen- cy t h e r e i s a damping i n t h e coupling. S i m i l a r behaviour has been observed i n B r i l l o u i n s c a t t e r i n g i n amorphous s o l i d s , and has been i n t e r p r e t e d i n t h e framework o f a v i s c o - e l a s t i c model i n which a c o u s t i c modes would be s c a t t e r e d by t h e r e l a x a - t i o n o f s t r u c t u r e s w i t h a g i v e n c o r r e l a t i o n range 20 /11/. On t h e b a s i s o f such model, cR(w) t u r n s o u t t o be :
where VL i s t h e h i g h frequency l o n g i t u d i n a l sound v e l o c i t y . Using values o f VL from t h e 1 it e r a t u r e / 7 / , we have f i t t e d eq. (4) t o t h e data o f f i g . 7 : t h e r e s u l t i s t h e s o l i d l i n e , whose obvious d e v i a t i o n a t h i g h frequencies i s o f course due t o t h e onset o f o p t i c a l modes.
From t h e f i t we o b t a i n alues f o r t h e c o r r e l a t i o n range 20, which i s found t o increase from 20 = 3.4
1
f o r "ure H20 t o 20 = 7 . 51
f o r t h e s a t u r a t e d s o l u t i o n o f ZnC12 i n H20.We wish t o emphasize how s t r i k i n g t h i s r e s u l t i s : w i t h o u t any t h e o r e t i c a l "fudging", we o b t a i n f o r CR(w) a f u n c t i o n a l form which i s i d e n t i c a l w i t h t h a t obtained inde- pendently and by o t h e r methods f o r amorphous s o l i d s . Furthermore, t h e c o r r e l a t i o n range o f l o c a l o r d e r thus determined i s found t o increase w i t h s o l u t e concentration, i n good agreement w i t h t h e r e s u l t s obtained on t h e d i f f u s i o n a l dynamics by quasi- e l a s t i c neutron s c a t t e r i n g and by depolarized Rayleigh wing s c a t t e r i n g .
I V
-
CONCLUSIONSThe combined measurements o f i n e l a s t i c neutron s c a t t e r i n g and l i g h t s c a t t e r i n g r e p o r t e d i n t h i s paper, t o g e t h e r w i t h o t h e r r e s u l t s we obtained, p a r t i c u l a r l y by
quasi-el a s t i c neutron s c a t t e r i n g , y i e l d a f a i r l y complete ~ i c t u r e of the microscopic dynamics of room temperature water and of the perturbative e f f e c t s caused by a strong e l e c t r o l y t e solute such as ZnC12. For the f i r s t time, a c l e a r experimental separation of the various relaxation r a t e s affecting t h e diffusional and low f r e - quency vibrational dynamics of the water and s o l u t e molecules has been achieved.
Conclusive evidence f o r s o l u t e connected "optical c o l l e c t i v e excitations" has been obtained and, more generally, the opnortunity of describing the vibrational motion i n these liquids in terms of c o l l e c t i v e excitations in an amorphous s o l i d - l i k e matrix has been demonstrated.
An obvious extension of these experiments i s t h e study of temperature e f f e c t s especially in the supercooled region, and also the study of d i f f e r e n t types of strong e l e c t r o l y t e s , i n order t o find out nrecisely what a r e the interactions which a r e mainly responsible f o r the c o l l e c t i v e vibrational behaviour and f o r the strong local structuring e f f e c t s we have found i n the ZnC12 and similar solutions.
REFERENCES
Bellissent-Funel M.-C., Kahn R , Dianoux A-J., Fontana M.P., Maisano G . , Migliardo P. and Wanderlingh F., 1984, Mol.Physics ( t o be published) Maisano G . , Migliardo P . , Mallamace F . , MaqazCi S., Aliotta F . , Vasi C . ,
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For general information on water, aqueous solutions and t h e relevant expe- rimental methods see the volumes of "Water : a comprehensive t r e a t i s e " . F.Franks, ed. (Plenum Press : N . Y . )
See f . i . t h e monography by T. Springer, "Quasi-elastic neutron s c a t t e r i n g f o r the investigation of d i f f u s i v e motions i n s o l i d s and liquids" G.Hohler ed.
(Springer Verla N . Y . 1972)
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Fontana M.P., Maisano G . , Migliardo P . , and Wanderlingh F . , 1978, J,Chem.Phys.
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Darbari G.S., Richelson M . R . and Petrucci S . , 1979, J.Chem.Phys. - 53, 859 Lagarde P . , Fontaine A . , Raoux D., Sadoc A . , and Migliardo P . , 1980, J.Chem.
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See Weaire D . L . , 1981 Amorphous Solids, P h i l l i p s W.A., ed. (Ber1in:Soringer) Jackle J . , 1981 Amorphous Solids, P h i l l i p s W.A., ed. (Ber1in:Springer) Martin A.J. and Brenig W . , 1974, Phys.Stat.So1 . ( b ) - 64, 163.
