SMALL AMPLITUDE COLLECTIVE PROTON MOTIONS IN WATER NETWORKS - APPLICATION TO ICES, CLATHRATES AND AQUEOUS SOLUTIONS

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Submitted on 1 Jan 1987

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SMALL AMPLITUDE COLLECTIVE PROTON MOTIONS IN WATER NETWORKS - APPLICATION

TO ICES, CLATHRATES AND AQUEOUS SOLUTIONS

J. Green, A. Lacey, M. Sceats

To cite this version:

J. Green, A. Lacey, M. Sceats. SMALL AMPLITUDE COLLECTIVE PROTON MOTIONS IN WATER NETWORKS - APPLICATION TO ICES, CLATHRATES AND AQUEOUS SOLUTIONS.

Journal de Physique Colloques, 1987, 48 (C1), pp.C1-53-C1-58. �10.1051/jphyscol:1987108�. �jpa-

00226238�

JOURNAL D E PKYSIQUE

C o l l o q u e C1, suppl6ment au n o 3, T o m e 48, mars 1987

SMALL AMPLITUDE COLLECTIVE PROTON MOTIONS I N WATER NETWORKS -

APPLICATION TO ICES, CLATHRATES AND AQUEOUS SOLUTIONS

J . L . GREEN, A . R . LACEY and M.G. SCEATS

Department of Physical Chemistry, University of Sydney, Sydney, N. S. W. 2006, Australia

Resume. Une c a r a c t e r i s t i q u e de la s p e c t r o s c o p i e Rarnan de I'eau en r e l a t i o n avec l e mouvement c o l l e c t i f de p e t i t e amplitude des p r o t o n s , e s t employ6e pour Btudier la s t r u c t u r e de r6seau de I1eau l i q u i d e , des g l a c e s , s e m i c l a t h r a t e s , a l c o o l s , e l e c t r o l y t e s e t des melanges H O/H 0

2 2 2'

Abstract. A f e a t u r e i n t h e Raman spectrum o f water networks, assigned t o small amplitude c o l l e c t i v e proton motions, is used t o probe t h e network s t r u c t u r e

in ices, liquid water, H O/H 0 mixtures, s e m i c l a t h r a t e s , a l c o h o l s and e l e c -

t r o l y t e s . 2 2 2

1 . I n t r o d u c t i o n

The OH s t r e t c h i n g spectrum of water is s t r o n g l y perturbed by t h e s t r e n g t h of hydrogen bonding [I]. However t h e s t r e t c h i n g motions of a d j a c e n t molecules a r e s t r o n g l y coupled

[ 2 ]

a s expected f o r a t r a n s i t i o n which e x h i b i t s a l a r g e t r a n s i t i o n d i p o l e moment. This complexity was circumvented by study of t h e OH s t r e t c h i n g motions i n heavily deuterated systems

[ 2 ] ,

where t h e OH o s c i l l a t o r s a r e mechanically decoupled from t h e surrounding OD o s c i l l a t o r g . Their spectrum e x h i b i t s t h e range of OH s t r e t c h i n g f r e q u e n c i e s induced by t h e v a r i a t i o n of t h e hydrogen bonding, i.e.

inhomogeneous broadening. Data t h u s obtained provide a probe of t h e c h a r a c t e r of t h e hydrogen bonding and have been used t o develop s t r u c t u r a l models of l i q u i d water C31-

I n t h i s paper r e c e n t experiments w i l l be reviewed i n which t h e spectrum of coupled OH o s c i l l a t o r s is s t u d i e d . Whereas t h e decoupled spectrum provides information on t h e s i n g l e t bond d i s t r i b u t i o n , t h e coupled spectrum is influenced by higher order d i s t r i b u t i o n f u n c t i o n s which more d i r e c t l y r e f l e c t t h e s t r u c t u r e of t h e networks and hydrogen bond c o r r e l a t i o n s .

2. C o l l e c t i v e Proton Motions i n Defect-Free T e t r a h e d r a l Networks

The i c e s a r e continuous t e t r a h e d r a l networks t41. The s t r o n g coupling of OH o s c i l l a t o r s i n i c e I was f i r s t revealed by Haas and Hornig [51 who observed t h e coupling between a d j a c e n t OD o s c i l l a t o r s 0-D....O-D i n d i l u t e D,O/H,O i c e I. The consequences of t h i s coupling on t h e spectrum of H,O i c e I was pointed out by Whalley

[2].

C a l c u l a t i o n s by Rice and coworkers [6-81 have shown t h a t t h e observed i n f r a r e d and Raman s p e c t r a can be modelled t o a high p r e c i s i o n , and have confirmed t h a t t h e spectrum is dominated by intermolecular coupling.

A

f e a t u r e of t h e spectrum is t h e appearance i n t h e low frequency edge of t h e I,, Raman Spectrum of an i n t e n s e band due t o t h e in-phase c o l l e c t i v e motion of t h e OH o s c i l l a t o r s E91. I t is t h i s f e a t u r e t h a t we w i l l use a s a probe of t h e c o l l e c t i v e motions. A s shown i n Figure 1 , t h i s band appears, a l b e i t broader, i n t h e spectrum of amorphous s o l i d water H,O(as) and, with reduced i n t e n s i t y , i n l i q u i d water.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987108

JOURNAL DE PHYSIQUE

C o n s i d e r t h e l i n e w i d t h , A,of t h e in-phase c o l l e c t i v e band i n i c e I . T h i s h a s been measured by Sivakumar e t a 1 1 9 1 , and is f i t t e d t o t h e form

A,

⁺

~ / ( e x p ( h c v / k ~ ) - ¹

⁾

The f r e q u e n c y

v

is 200 em-', and c o r r e s p o n d s t o t h e 0-0 s t r e t c h i n g r e g i o n o f t h e t r a n s l a t i o n a l s p e c t r u m [ l o ] . The c o l l e c t i v e mode s c a t t e r s o f f t h e t r a n s l a t i o n a l o p t i c a l phonons. The w i d t h

AYORPHOUS SOLID

H,O o f t h e uncoupled OH o s c i l l a t o r s p e c t r u m

DEPOSITION AT 10 K

i n D,O is o n l y weakly t e m p e r a t u r e

dependent [9] and is p r i m a r i l y a t t r i b u t e d t o inhomogeneous broadening a r i s i n g from p r o t o n d i s o r d e r i n i c e I . T h i s w i d t h e x t r a p o l a t e d t o 0 K i s 25.8 em-'. The e x p e r i m e n t s show t h a t AIH > A o , w i t h A,

=

1 9 cm-l and t h i s we a t t r i b u t e t o m o t i o n a l n a r r o w i n g of t h e c o l l e c t i v e mode. The OH s t r e t c h i n g e x c i t a t i o n , a v i b r o n , moves

POLYCRISTALLINC

r a p i d l y t h r o u g h t h e network a t low

DCPOSITION AT IS

t e m p e r a t u r e s and t h e s p e c t r u m is

u n p e r t u r b e d by t h e inhomogeneous d i s t r i b u t i o n . I n p r i n c i p l e , t h e l i n e w i d t h n e a r 0 K s h o u l d approach a 6 - f u n c t i o n . The s c a t t e r i n g due t o o p t i c a l and a c o u s t i c m o t i o n s is f r o z e n o u t a s T

+

0 K because t h e in-phase mode l i e s a t t h e low e n e r g y edge of t h e v i b r o n

UM)

d e n s i t y of s t a t e s . The r e s i d u a l w i d t h A.

V

(em-')

o f t h e c o l l e c t i v e band c o u l d be due t o

f i g . 1: The Raman spectrum of water, H20(as) and

v i b r a t i o n a l r e l a x a t i o n . A l i k e l y

i c e .

mechanism is c o u p l i n g t o t h e bending

o v e r t o n e by Fermi r e s o n a n c e [ I l l . The e l e g a n t c o d e p o s i t i o n e x p e r i m e n t s of D e v l i n and c o w o r k e r s [ 1 2 , , 1 3 ] have a l l o w e d measurement of t h e s p e c t r u m o f i n t a c t H,O m o l e c u l e s i n D,O i c e I. The r o l e of Ferrni r e s o n a n c e h a s been deduced from t h e s e s p e c t r a [14] and t h e c o u p l i n g e l e m e n t W i s 60 cm-I. I n t h e c o u p l e d s p e c t r u m t h e impact of Fermi r e s o n a n c e i s l e s s pronounced, b u t we s u g g e s t t h a t i t c o u l d be t h e s o u r c e o f t h e r e s i d u a l w i d t h a t 0 K of t h e in-phase c o l l e c t i v e mode, by t h e i r r e v e r s i b l e p r o c e s s of e x c i t a t i o n of t h e bending o v e r t o n e on one m o l e c u l e f o l l o w e d by f i s s i o n i n t o bending e x c i t a t i o n s on a d j a c e n t m o l e c u l e s o r e x c i t a t i o n of t h e l i b r a t r o n a l modes. The l i n e w i d t h is g i v e n by F e r m i ' s Golden r u l e , Z T W ~ ~ ( V , ) M , where

p

is t h e o v e r t o n e d e n s i t y o f s t a t e s . The r e s i d u a l w i d t h i s c o n s i s t e n t w i t h a d e n s i t y of s t a t e s a t vc o f 4 . 2 x l 0 - ~ ( c m - l 1-l. T h i s is i n r e a s o n a b l e agreement w i t h t h e s i m u l a t i o n s of t h e l i q u i d w a t e r s p e c t r u m 1151 i n which t h e bending o v e r t o n e d e n s i t y of s t a t e s a t 3086 cm-l is c a l c u l a t e d t o be 1 . 3 ~ 1 0 - 3 ( c m - ~ ) - I . The r e s u l t s s u g g e s t t h a t t h e G a u s s i a n form f o r p u s e d i n s i m u l a t i o n s

[ 7 ]

f a l l s o f f t o o r a p i d l y . I t is a l s o p o s s i b l e t h a t t h e combined d e n s i t y of s t a t e s of bending and l l b r a t i o n a l modes s h o u l d b e used. I f o u r h y p o t h e s i s is c o r r e c t , t h e OH s t r e t c h i n g v i b r a t i o n a l l i f e t i m e i n i c e I i s 0.6 ps.

The w i d t h of t h e v i b r o n band is e s t i m a t e d from 2(vOH-vc) where

VOH

i s t h e f r e q u e n c y of t h e decoupled o s c i l l a t o r and vc t h a t o f t h e in-phase c o l l e c t i v e mode.

For i c e s I , 11, I11 and V , H,O(as) and l i q u i d w a t e r t h e v i b r o n bandwidth s p a n s a

r a n g e of 300-400 cm-1. The s i m i l a r i t y o f t h e s e w i d t h s s u g g e s t s t h a t t h e magnitude

of t h e r e s o n a n c e c o u p l i n g between OH o s c i l l a t o r s is t h e same i n a l l t h e s e

t e t r a h e d r a l networks. T h i s is confirmed by a n a l y s i s of t h e s p e c t r a of p r o t o n

o r d e r e d i c e s I1 and I X , which y i e l d s a c o u p l i n g f o r c e c o n s t a n t [17] i n agreement

w i t h t h a t r e q u i r e d t o model t h e i c e

I

spectrum. While t h e v i b r o n bandwidth of t h e

n e t w o r k s w i t h s e v e r e l y d i s t o r t e d hydrogen bonds i s s l i g h t l y l e s s t h a n t h a t of i c e I ,

i t is c l e a r t h a t t h e c o l l e c t i v e behaviour i s not s e n s i t i v e t o t h e network topology f o r t e t r a h e d r a l networks, a conclusion borne o u t i n c a l c u l a t i o n s of t h e spectrum of H,O(as)[18,191.

The e x t e n t of t h e c o l l e c t i v e mode c h a r a c t e r is d e f i n e d by t h e r e l a t i v e c o n t r i b u t i o n of t h e in-phase c o l l e c t i v e mode t o t h e I g Raman spectrum. We have devised C201 a s p e c t r a l s t r i p p i n g procedure f o r e x t r a c t i o n of t h i s q u a n t i t y i n c a s e s where in-phase c o l l e c t i v e mode is not well r e s o l v e d , such a s i n l i q u i d water ( F i g u r e 1 ) . The p o l a r i z a t i o n r a t i o Ill /I& f o r t h e remainder of t h e spectrum does not vary s i g n i f i c a n t l y with frequency, and is very c l o s e t o t h a t of uncoupled o s c i l l a t o r s . Presumably t h i s a r i s e s from c a n c e l l a t i o n s of c o n t r i b u t i o n s due t o t h e complex phase r e l a t i o n s h i p s among t h e OH o s c i l l a t o r s i n t h e s e modes. The c o l l e c t i v e spectrum Ic(w) is t h u s d e f i n e d by I n (w) - a-lIL(w) where a is an a d j u s t a b l e parameter determined by f i t t i n g t h e high frequency shoulder t o minimize Ic(w) i n t h i s region.

The value of a is t y p i c a l l y O.2li0.02 C201. The a r e a , C , of Ic(w) r e l a t i v e t o 111 (a) is taken t o be a measure of t h e c o l l e c t i v e c h a r a c t e r . S p e c t r a of i c e I a t a wide range of temperatures were t h u s analysed [20] and t h e value of Cite was found t o be 0.54f0.03 with no temperature dependence. This i m p l i e s t h a t t h e thermal d i s o r d e r , which l e a d s t o an i n c r e a s e i n t h e l i n e w i d t h of t h e in-phase c o l l e c t i v e mode with i n c r e a s i n g temperature [9], does not reduce t h e e x t e n t of t h e c o l l e c t i v e motion, b u t merely c a u s e s a s c a t t e r i n g of t h e vibron modes by i n t e r a c t i o n with t h e l a t t i c e phonons. The value of C f o r H,O(as) is t h e same a s t h a t of i c e I C201 showing t h a t t h e c o l l e c t i v e c h a r a c t e r is not a f f e c t e d by t h e t o p o l o g i c a l d i s o r d e r of H,O(as), nor by a modest i n c r e a s e i n t h e s h o r t range d i s o r d e r [ 9 ] .

The c o l l e c t i v e c h a r a c t e r of t h e motion can be s t r o n g l y p e r t u r b e d by t h e i n t r o d u c t i o n of d e f e c t s which cause t h e OH o s c i l l a t o r f r e q u e n c i e s t o l i e o u t s i d e t h e v i b r o n bandwidth. The upper l i m i t t o t h e vibron band is about 3500 cm-l f o r a l l t h e t e t r a h e d r a l networks, while t h e lower l i m i t is about 3100 cm-l. Broken hydrogen bonds have s t r e t c h i n g f r e q u e n c i e s above 3500 cm-l and because they l i e o u t s i d e t h e v i b r o n band they a c t a s d e f e c t s with r e s p e c t t o t h e c o l l e c t i v e s t r e t c h i n g motions.

I n i c e I t h e c o n c e n t r a t i o n of broken bonds is t o o small t o i n f l u e n c e t h e c o l l e c t i v e motions. However t h e e f f e c t of d e u t e r a t i o n is t o introduce mass d e f e c t s which v i b r a t e near 2400 em-'. The i n t e n s i t y of t h e c o l l e c t i v e band t h u s d e c r e a s e s with

i n c r e a s i n g OD mole f r a c t i o n POD. and t h e r e s u l t s ~ 2 1 1 can be f i t t e d by ~ ~ ~ ~ ( 1 - p ~ ~ ) ~ . This i s expected because t h e c o l l e c t i v e motions a r i s e from a coupling of n e a r e s t

neighbour OH o s c i l l a t o r s . I f C f o r a l l d e f e c t f r e e t e t r a h e d r a l networks is 0.54 then we propose t h a t f o r networks with i n t r i n s i c d e f e c t s of mole f r a c t i o n P I , t h a t C

=

~ ~ ~ ~ ( 1 - p ~ ) ~ . Thus a measurenent of C can be used t o e s t i m a t e PI. I n t h e absence of mass d e f e c t s , we can assume t h a t i n most c a s e s t h e d e f e c t s w i l l a r i s e from broken hydrogen bonds with OH s t r e t c h i n g f r e q u e n c i e s above 3500 cm-l a s r e q u i r e d by t h e magnitude of t h e vibron band. Thus

p~

can be broadly i n t e r p r e t e d a s t h e mole f r a c t i o n of broken hydrogen bonds. This i s t h e concept used t o analyse t h e d e f e c t i v e network of l i q u i d water i n t h e next s e c t i o n . A l t e r n a t i v e l y , when o t h e r molecules a r e added, i t is p o s s i b l e t h a t a new network topology w i l l a r i s e , c h a r a c t e r i z e d by water molecules with a lower c o o r d i n a t i o n number, hence a lower v a l u e of C.

3. Liquid Water - ^A Random Network with Defects

I t has been proposed t h a t l i q u i d water can be regarded a s a continuous random network [31. However, t h e l i f e t i m e of a c o n f i g u r a t i o n of water molecules is small and t h e random network must be d e f e c t i v e . A s water i s cooled, t h e d e f e c t d e n s i t y d e c r e a s e s and t h e behaviour of t h e thermodynamic and t r a n s p o r t p r o p e r t i e s of water e x h i b i t i n c r e a s i n g l y anomalous behaviour [22,23] s u g g e s t i n g approach towards a thermodynamic s i n g u l a r i t y , near TS - - 4 5 ' ~ . A s water is cooled a t 1 atm towards Ts t h e r e a r e i n c r e a s i n g f l u c t u a t i o n s between c o n f i g u r a t i o n s of water molecules with s i m i l a r e n e r g i e s but s i g n i f i c a n t l y d i f f e r e n t d e n s i t i e s . The w e l l known anomalous p r o p e r t i e s of water i n t h e normal l i q u i d regime stem from a r e s i d u e of t h e behaviour e x h i b i t e d i n t h e supercooled region. The s t r u c t u r a l o r i g i n of t h e behaviour Of l i q u i d water remains t h e c e n t r e of much debate.

The Raman s p e c t r a of l i q u i d water a t 1 atm were o b t a i n e d i n t h e range - 2 8 ' ~ t o

9 0 ° c , and v a l u e s of

C

were o b t a i n e d 1201 using t h e procedures d e s c r i b e d i n S e c t i o n

C1-56 JOURNAL DE PHYSIQUE

2 , and t h e r e s u l t s a r e p l o t t e d i n Figure 2. The most important f e a t u r e of t h e r e s u l t s is t h a t they l i n e a r l y e x t r a p o l a t e t o t h e value of i c e I and H,O(as) near t h e temperature Ts of t h e purported thermodynamic s i n g u l a r i t y . From t h e a n a l y s i s of S e c t i o n 2 we would conclude t h a t

p~

+O a s T+Ts. I t should be noted t h a t homogeneous n u c l e a t i o n t o i c e I p r e v e n t s experiments i n t h e v i c i n i t y of T s and t h e range of e x t r a p o l a t i o n is l a r g e . Furthermore, i t should not be construed t h a t t h e c o n f i g u r a t i o n of water is tending t o i c e I l i k e s t r u c t u r e s ( 6 membered r i n g s ) a s T+T, but r a t h e r t o a low d e n s i t y continuous t e t r a h e d r a l network.

The d a t a obtained a t 1 atm do not r e v e a l t h e f u l l complexity of l i q u i d water.

S t u d i e s a t constant d e n s i t y a r e more revealing. Raman s p e c t r a of water under t e n s i o n i n s e a l e d c a p i l l a r i e s were obtained and analysed. I n t h i s c a s e t h e d e n s i t y of water is approximately c o n s t a n t , 18.2 cm3mol-l below 3 0 ' ~ . The values of C obtained i n t h i s study ( a c o l l a b o r a t i v e p r o j e c t with D r . R. Speedy) a r e a l s o p l o t t e d i n Figure 2 f o r a sample with an OD mole f r a c t i o n of 0.18. A t temperatures below 20°C t h e value of C, hence

p ~ ,

does not change s i g n i f i c a n t l y . This i n d i c a t e s t h a t t h e d e f e c t s a r e s t r o n g l y coupled t o t h e d e n s i t y , i n s o f a r a s t h e c o n f i g u r a t i o n a l composition of t h e l i q u i d is temperature independent under c o n d i t i o n s of constant volume. The d a t a of Figure 2 emphasize t h e importance of performing s t u d i e s a t approximately c o n s t a n t volume conditions. Water under tension (e.g. -210 bar a t 9 . 1 2 ' ~ f o r t h e sample i n Figure

2 )

and i n t h e supercooled region e x h i b i t s an a m p l i f i c a t i o n of t h e anomalous p r o p e r t i e s of water t o such an e x t e n t t h a t they a r e r e f l e c t e d

by

observable changes i n spectroscopic p r o p e r t i e s i n t h e

OH

s t r e t c h i n g region. Further s t u d i e s a r e r e q u i r e d t o v e r i f y t h e s e r e s u l t s .

1

¹

I

^0.0

^I I

-40 -20 0

20

40

60

80

100 ^{1 .O 2 0 3.0}

TEMPERATURE°C

td

(molarity of clathrate)

2: Collective band intensity f o r H 0 a t 1 atm

(a),

and Fig. 3: Collective band i n t e n s i t y o f (C4H9) 4NOH

b

^D20/H20

^from

^{capillary ( 0 )} corfected for OD content

(el.

solutions at 30°C.

4.

Chemical d e f e c t s i n Water Networks - H,O,/H,O S o l u t i o n s

The p r o p e r t i e s of aqueous s o l u t i o n s a r e extremely important. I f pure water is t o be viewed a s a d e t e c t i v e random network [3], then one of t h e primary q u e s t i o n s concerning aqueous s o l u t i o n s r e l a t e s t o t h e e f f e c t of s o l u t e s on t h e water s t r u c t u r e . There can be no s i n g l e answer t o t h i s question because of t h e wide v a r i e t y of behaviour e x h i b i t e d and o f t e n c l a s s i f i e d a s s t r u c t u r e I1making" o r

"breaking". The s i m p l e s t c l a s s of s o l u t e s a r e those which can themselves be incorporated i n t o t h e network. Such an example is H,O,. This molecule adds an excess of two l o n e p a i r s f o r each peroxide molecule, such t h a t each OH 0 S ~ i l l a t 0 r should, i n p r i n c i p l e , be unperturbed i n H20-H202 networks because strong 0-H...O bonds can always be formed. The uncoupled o s c i l l a t o r spectrum of mole f r a c t i o n x=

0.56 of H,O, i n water shows a d i s t r i b u t i o n of OH s t r e t c h i n g f r e q u e n c i e s which is not

d i s s i m i l a r t o t h a t of pure water and, although t h e d e t a i l e d i n t e r p r e t a t i o n is not

c l e a r , t h e s p e c t r a i n d i c a t e t h a t t h e hydrogen bonds i n t h e s e mixtures a r e s i m i l a r ' i n

s t r e n g t h t o t h o s e i n l i q u i d water. The Raman s p e c t r a of H,O,/H,O mixtures over a

wide range of compositions and temperatures have been obtained and t h e small

amplitude c o l l e c t i v e proton motions a r e analysed t o y i e l d values of C,(T). In a l l

c a s e s Cx(T) was reduced from t h a t of H20 i n d i c a t i n g a breakdown of t h e c o l l e c t i v e c h a r a c t e r a t t r i b u t e d t o s t r u c t u r e breaking.

Two regimes of behaviour were observed a s a f u n c t i o n of mole f r a c t i o n x of H,O,.

For x<0.08, t h e d a t a g e n e r a l l y follows a (1-mx) behaviour with m= 2-3 i n d i c a t i n g t h a t each H,O, c r e a t e s approximately two t o t h r e e d e f e c t s i n t h e network of 0-H o s c i l l a t o r s . For higher values of x, t h e dependence tends t o a weaker (I-x) dependence and we i n t e r p r e t t h i s i n terms of network r e c o n s t r u c t i o n . S o l i d s o l u t i o n s of H,O/H,O, e x h i b i t a complex phase diagram and t h e c r y s t a l s t r u c t u r e s of H2O,:2H,O and H,O, a r e c h a r a c t e r i z e d by networks which have a lower coordination than the t e t r a h e d r a l networks of pure H,O. The number of n e a r e s t neighbour o s c i l l a t o r s a r e reduced, and i t follows t h a t t h e vibron bandwidth w i l l decrease.

Furthermore, f o r an approximately i n v a r i a n t d i s t r i b u t i o n of o s c i l l a t o r f r e q u e n c i e s a p r o p o r t i o n a l l y l a r g e r f r a c t i o n w i l l f a l l o u t s i d e t h e vibron bandwidth thereby reducing C. We suggest t h a t t h e decrease of C a t high mole f r a c t i o n s of H,O, is a s s o c i a t e d with r e c o n s t r u c t i o n of t h e network topology t o t h a t of a network of lower c o n n e c t i v i t y . I t is i n t e r e s t i n g t o note t h a t H,O, a l s o gradually suppresses t h e d e n s i t y f l u c t u a t i o n s of water

[25]

over t h e range i n which we observe d e f e c t behaviour (x<0.1) and is completely quenched i n t h e r e c o n s t r u c t i o n regime.

5. S e m i c l a t h r a t e s - A model f o r Hydrophobic Hydration Networks

C l a t h r a t e s t r u c t u r e s a r e c h a r a c t e r i z e d by a continuous t e t r a h e d r a l water network 1261 a s i n t h e i c e s , and e x h i b i t c o l l e c t i v e small amplitude proton motions s i m i l a r t o t h a t i n i c e I . The s e m i c l a t h r a t e of t e t r a n-butyl ammonium hydroxide [27] has been s t u d i e d using t h e techniques described above. We have s t u d i e d t h e c r y s t a l , t h e l i q u i d , t h e supercooled l i q u i d and i t s aqueous s o l u t i o n s . This type of c l a t h r a t e was chosen f o r study because i t is a convenient model f o r understanding hydrophobic hydration, which is of considerable importance f o r understanding b i o l o g i c a l s t r u c t u r e s . The s e m i c l a t h r a t e c r y s t a l has a broad d i s t r i b u t i o n of hydrogen bond l e n g t h s , and t h u s e x h i b i t s a broad d i s t r i b u t i o n of decoupled OH s t r e t c h i n g f r e q u e n c i e s . However t h e in-phase c o l l e c t i v e band is well e s t a b l i s h e d and has a value of C=0.48. This is c l o s e t o t h e value expected f o r a d e f e c t f r e e t e t r a h e d r a l network, perhaps with a d e f e c t d e n s i t y of 0.06. Upon melting t h e r e is a l a r g e i n c r e a s e i n t h e decoupled o s c i l l a t o r spectrum above 3500 cm-l corresponding t o broken bonds, and C i n t h e coupled spectrum is reduced t o 0.27, suggesting a d e f e c t p r o b a b i l i t y of 0.30. In t h i s c a s e t h e f l u i d p r o p e r t i e s a r i s e from extensive breaking of t h e hydrogen bonds; indeed, more s o than i n t h e melting of i c e . However, upon d i l u t i o n with H,O, C r i s e s again t o a value l a r g e r than t h a t of bulk water a t t h e same temperature, a s shown i n Figure 3. We a t t r i b u t e t h i s behaviour t o network r e s t a b i l i z a t i o n i n which each s o l u t e molecule e s t a b l i s h e s i t s own ordered hydration s h e l l , perhaps with a s e p a r a t e d l a y e r of water molecules between t h e hydration s h e l l s of neighbouring s o l u t e s , i n comparison t o t h e shared hydrated water molecules of t h e parent c l a t h r a t e . The d i l u t i o n d a t a provide information on network r e c o n s t r u c t i o n during hydrophobic aggregation.

6. Alcohols and E l e c t r o l y t e s

Water-alcohol s o l u t i o n s e x h i b i t a wide v a r i e t y of thermodynamic [281 and t r a n s p o r t behaviour which is poorly understood i n terms of s t r u c t u r a l p r o p e r t i e s . We have s t u d i e d aqueous s o l u t i o n s of e t h a n o l , isopropanol and t-butanol with a view t o determining t h e e f f e c t of t h e s e molecules on t h e l i q u i d water random network s t r u c t u r e . The s e r i e s was chosen because i t r e p r e s e n t s a progressive incre.ase i n t h e s i z e of t h e hydrophobic alkane region. The n e a t a l c o h o l s e x h i b i t very small OH s t r e t c h i n g c o l l e c t i v e c h a r a c t e r because t h e number of n e a r e s t neighbour OH o s c i l l a t o r s is l i m i t e d and t h e vibron bandwidth is l e s s than t h e d i s o r d e r .

Measurements of C ( x , t ) show t h a t f o r x<0.16 ethanol and i-propanol a r e

accommodated i n t o t h e network with i n s i g n i f i c a n t change i n t h e c o l l e c t i v e c h a r a c t e r .

This implies t h a t t h e s e a l c o h o l s n e i t h e r introduce broken hydrogen bond d e f e c t s , nor

d e s t r o y t h e t e t r a h e d r a l c h a r a c t e r of t h e network. On t h e o t h e r hand, t h e t-butanol

i n c r e a s e s t h e value of C r e l a t i v e t o t h a t of pure water, and t h e l a r g e s t value of C

i s s i m i l a r t o t h a t of t h e comparable s e m i r c l a t h r a t e discussed i n t h e previous

s e c t i o n . This suggests t h a t t h e s o l u t i o n p r o p e r t i e s of t-butyl alcohol 1291 a r i s e

Cl-58 JOURNAL DE PHYSIQUE

f r o m f o r m a t i o n o f c l a t h r a t e - l i k e c a g e s .

F i n a l l y , s t u d i e s h a v e b e e n c o m p l e t e d o n L i C l e l e c t r o l y t e s o l u t i o n s . T h e s e s o l u t i o n s q u e n c h t h e a n o m a l o u s p r o p e r t i e s o f w a t e r [ 3 0 , 3 1 ] a n d ,

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c o n n e c t i v i t y t h a n t h e t e t r a h e d r a l n e t w o r k s o f p u r e w a t e r s y s t e m s . T h i s w o r k w a s s u p p o r t e d b y t h e A u s t r a l i a n R e s e a r c h G r a n t s S c h e m e . JLG a c k n o w l e d g e s t h e a w a r d of

a

Commonwealth P o s t g r a d u a t e S c h o l a r s h i p .

R e f e r e n c e s

[ 11

FALiK,

M. ,

KNOP, 0.

, Water, A Comprekensive T r e a t i s e ,

V o l . 2 , F . F r a n k s e d . ( P l e n u m P r e s s , N.Y. 1 9 7 3 ) C h a p t e r 2

[

2 1 wHALLEY, E . ,

Dev. Appl. Spee. 2

( 1 9 6 8 ) 2 7 7 - 2 8 1 .

[

3 1 SCEATS, M.G., RICE, S.A.,

Water, A Comprekensive T r e a t i s e ,

V o l . 7 , F . F r a n k s e d ( P l e n u m P r e s s , N.Y. 1 9 8 3 ) C h a p t e r 2

[

4 1 FLETCHER, N.H.,

The Chemicaz Physics of Ice

( C a m b r i d g e U n i v . P r e s s , C a m b r i d g e ) 1 9 7 0 .

[ 51

HAAS, C . , HORNIG, D.F.,

J . Ckem. Pkys. 32

( 1 9 6 0 ) 1 7 6 3 1 1 7 6 9 .

[

6 1 McGRAW, R., MADDEN, W.G., BERGREN, M.S., RICE, S.A., SCEATS, M.G.

,

J .

&em.

Phys. 69

( 1 9 7 8 ) 3 4 8 3 - 3 4 9 6 .

L

7 1 BERGREN, M . , RICE, S.A.,

J. Ckem. Pkys. 77

( 1 9 8 2 ) 583-602.

[ 81

RICE,

s

. A . , BERGREN,

M.s.,

BELCH, A.c., NIELSON, G . ,

J . Pkys. Conem.

8 7 ( 1 9 8 3 ) 4 2 9 5 - 4 3 0 8 .

L

9 1 S I V A ~ ? & A R , T.C., RICE, S.A., SCEATS, M.G.,

J. Ckem. Pkys. 69

( 1 9 7 3 ) 3468-3476.

[lo]

HOBBS, P . V . ,

I c e Physics

( O x f o r d U n i v e r s i t y P r e s s , 1 9 7 4 )

[ll] SCEATS, M.G., STAVOLA, M., RICE, S.A., J .

&em. PkyS. 11

( 1 9 7 9 ) 983-990.

[ l 2 ] RITZHAUPT, G. a n d DEVLIN, J . P . ,

J. &em. Pkys.

( 1 9 7 7 ) 4 7 7 9 - 4 7 8 0 .

[13] RITZHAUPT, G., THORNTON, C . , DEVLIN, J . P . ,

Ckem. Pkys. L e t t . 59

( 1 9 7 9 ) 4 2 0 - 4 2 2 . [ 1 4 ] DEVLIN, J . P . , WOOLDRIDGE, P . J . , RITZHAUPT, G . ,

&em. Pkys. L e t t .

( i n p r e s s ) [15] MILLS, M.F., REIMERS, J . R . , WATTS, R.O.,

M O ~ . Pkys. 57

( 1 9 8 6 ) 777-791.

[16] TAYLOR, M., WHALLEY, E . J . ,

&em. Phys. 40

( 1 9 6 4 ) 1 6 6 0 - 1 6 6 4 . [17] BERTIE, J . E . , FRANCIS, B . F . ,

J. Ckem. Pkys. 72

( 1 9 8 0 ) 2 2 1 3 - 2 2 2 1 .

[18]

MADDEN, W.G., BERGREN, M.S., McGRAW, R . , RICE, S.A., SCEATS, M.G.,

J .

a e m . Phys. 69

( 1 9 7 8 ) 3 4 9 7 - 3 5 0 1 .

[19] NEILSON, G., RICE, S . A . , J .

&em. Pkys.78

( 1 9 8 3 ) 4 8 2 4 - 4 8 2 7 .

[20] GREEN, J . L . , LACEY, A.R., SCEATS, M.G., J .

Pkys. &ern. 90

( 1 9 8 6 ) 3 9 5 8 - 3 9 6 4 . 1 2 1 1 GREEN, J . L . , LACEY, A.R., SCEATS, M.G.,

&ern. Phys. L e t t . 130

( 1 9 8 6 ) 67-71.

[22] SPEEDY, R . J . , ANGELL, C.A.,

J . &em. Pkys. 65

( 1 9 7 8 ) 8 5 1 - 8 5 8 .

[23] ANGELL, C.A.,

Water, A Comprehensive T r e a t i s e ,

V o l . 7 , F . F r a n k s e d . ( P l e n u m P r e s s , N.Y., 1 9 8 3 ) C h a p t e r

1.

[24] BANSIL, R., WIAFE-AKENTEN, J . , TAAFFE, J . L . , J .

&ern. Pkys. 76

( 1 9 8 2 ) 2 2 2 1 - 2 2 2 6 . [ 2 5 ] OGUNI, M., ANGELL, C.A., J .

&em. Pkys. 73

( 1 9 8 0 ) 1 9 4 8 - 1 9 5 4 .

[ 2 6 ] DAVIDSON, D.W.

, Water, A Comprekensive T r e a t i s e ,

V o l . 2 , F . F r a n k s e d . ( P l e n u m P r e s s , N.Y., 1 9 7 3 ) C h a p t e r 3.

[27] McMULLAN, R.K., BONAMICO, M., JEFFREY, G.A., J .

Chem. Pkys. 39

( 1 9 6 3 ) 3 2 9 5 - 3 3 1 0 . [28] FRANKS, F . a n d REID, D.s.,

Water, A Comprehensive ~ r e a t i s e ,

V o l . 2 , F . F r a n k s ,

ed. ( P l e n u m P r e s s , N.Y., 1 9 7 3 ) C h a p t e r 5 .

[ 2 9 ] BENDER, T.M., PECORA, R., J .

Pkys. Chem.

9 0 ( 1 9 8 6 ) 1 7 0 0 - 1 7 0 6 .

[30] ANGELL,

c.A.,

SARE, J.E., DONNELLA, J., M ~ A R L A N E ,

D.R.,

J .

Pkys. hem.

85

( 1 9 8 1 ) 1 4 6 1 - 1 4 6 4 .

[31]

ANGEE,

C.A.,

Nature

( L o n d o n ) 2 9 6 ( 1 9 8 2 ) 1 3 8 - 1 4 1 .