ACOUSTIC PROPERTIES OF WATER-ETHANOL MIXTURES AT LOW TEMPERATURES

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ACOUSTIC PROPERTIES OF WATER-ETHANOL MIXTURES AT LOW TEMPERATURES

G. d’Arrigo, O. Conde

To cite this version:

G. d’Arrigo, O. Conde. ACOUSTIC PROPERTIES OF WATER-ETHANOL MIXTURES AT LOW TEMPERATURES. Journal de Physique Colloques, 1984, 45 (C7), pp.C7-185-C7-193.

�10.1051/jphyscol:1984721�. �jpa-00224286�

JOURNAL DE

PHYSIQUE

Colloque C7, suppl6ment au

n 0 9 ,

Tome 45, septembre 1984 page C7-185

ACOUSTIC PROPERTIES OF WATER-ETHANOL M I X T U R E S A T LOW TEMPERATURES

G. DIArrigo and 0. ~ o n d e *

Dipartirnento d i F i s i c a , Universitc? d i Roma "La Sapienza", P.le

A.

Moro

2, 1-00185

Roma, I t a l y

and Gruppo NazionaZe d i S t r u t t u r a deZla Materia IGNSM) deZ CNR, Roma, I t a l y

* ~ a b o r a t o i r e de Physique Thermique, EcoZe de Physique e t Chimie, 10, r u e Vauque Zin,

75231

Paris Cedex, France

R6sum6 - On presente des r6sultats exp6rlmentaux sur la vitesse et l'absorption ultrasonore dans les solutions Eau-Ethanol au dessous de 0°C. Cesdonnges

sontanalys6es,aveclesd6terminations

pr6exlstantas de haute temp6rature, dans le cadre des pr6dictlons que des modBles rscents pour l'eau surfondue donnent quand on ajoute 5 l'eau des 1mpuret6s formant liaisons hydrog&e.

Abstract - We present experimental results on ultrasonic velocity and absorption in Water-Ethanol mixtures below O°C. These data, together with preexisting high temperature determinations, are analyzed and discussed in the framework of the predictions that recent models for supercooled water give when an H-B forming impurity is added to water.

I

- INTRODUCTION

The interaction of water with simple hydrocarbon derivatives such as alcohols, ketones, amines, etc. has been studied by almost every experimental technique.

Although still not explained completely at a molecular level, the thermodynamic and transport properties of such systems show well known systematic deviations from ideality /I/. Sound propagation exhibits similar trends. A considerable amount of experimental work has been carried out in the past at room temperature: it shows large positive deviations of both sound velocity and absorption coefficient from their ideal behavior when plotted as a function of solute concentration. Such ef- fects are very sharp around particular concentratiowand depend on the type of so- lute, temperature and frequency.

A renewed interest on the acoustic properties of such systems (in particular water-alcohols) grew up recently as a consequence of the great experimental and theoretical activity on liquid water at low temperatures. It is known / 2 / , in fact, that the anomalous behaviour of thermodynamic and transport properties of water above 0°C are enhanced when the liquid is cooled in the supercooled region. The addition of small impurities can reduce or increase such enhanced behavior.

Different models have been proposed to interpret such findings. For example, in the correlated-site percolation model by Stanley and Teixeira /3/ water consists of a random hydrogen-bond network where tiny bulky regions (patches) of four-bonded molecules are present. As temperature decreases, it is envisaged an amplification ,methanism of patches which produces density and entropy fluctuations. These, in turn, give rise to the anomalous behavior of isothermal compressibility, specific heat and thermal coefficient. According to this picture the addition of an H-B forming impdrity like alcohol should increase the local density of patches and then remove density fluctuations in water. The lower

K

/2/and the large increase of

T

sound velocity exhibited by dilute water-alcohol solutions are qualitatively Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984721

C7-186 30URNAL

DE PHYSIQUE

explained on these grounds.

Results from molecular dynamics studies

/4/

lead Stillinger

/5/

to suggest the formation of particular geometric structures (polyedra) as responsible for the enhanced properties of supercooled water. According to the Stillinger picture water consists of a three dimensional network of strained or broken hydrogen bonds .

As temperature decreases near and below the freezing point, unstrained H-B cages or polyedra emerge from the surrounding network. These cages are supposed similar or equal to the clathrate hydrates structures found around non polar solutes in aqueous solutions. Adding such solutes to water would promote the formation of water cages around the guest molecules so that one expects that addition of alcohol is equivalent to lowering the temperature of pure water.

On looking at these models we can see that experiments on low temperature water-alcohol solutions should be very useful to investigate the nature of the me- chanism responsible for the enhanced properties of supercooled water. With this in mind, we performed ultrasonic (u.s.) velocity and absorption measurements in some Water-Ethanol mixtures from 20 to -30°C. Low frequency sound velocity (v) is

2

-1

related to the static adiabatic compressibility K =(Pv

)

so that it allows the test of thermodynamic implications in the previous models. On the other hand, the sound absorption coefficient (M) as a function of frequency can give useful infor- mations on the dynamics of structural processes occurring in these systems.

In view of the further discussion of our data, we need to briefly summarize the main features of the

U.S.

velocity and absorption behaviors previously found in monohydric alcohols mixtures at room temperature. Limitately to the absorption, extensive reviews have been given by Blandamer /6,7/.

I1

-

EARLY U.S. RESULTS IN WATER-ALCOHOL SOLUTIONS (ROOM TEMPERATURE)

A) Absorption

-

At given frequency (f) and temperature (T) the

U.S.

absorption, as expressed by a / f 2 , exhibits a maximum as a function of concentration in the rich water region. The magnitude of the absorption peak noticeably increases, as a rule, with the size of the hydrophobic group (MeOH-bEtOHd i-Pr0H-cPrOH-r t-BuOH, see Table I). The corresponding concentration x(PSAC) (whereas x is the mole fraction of alcohol) decreases with increasing the size of the alkyl group in the solute (see Table I). The absorption peak decreases with increasing T and markedly decreases as frequency incrases. Such dispersive effects sharply reduce in a small composition range around x(PSAC).

In the very low x, Blandamer /7/ found an d/f2 tlplateaull independent on the amount of the added alcohol. The "plateau" length x (Plateau) decreases with the size of alcohols (Table I) and increases when temperature decreases.

B) Sound velocity

-

The low frequency sound velocity at constant T incr&es suddenly with addition of alcohol and reaches a peak value (vp) at a concentration x(PSVC) which, again, depends on the size of the d k y l group (Table I). v is larger thanthveelocities in both pure components and it is always x(PSVC)(x(PSAC). P In contrast with the absorption peak, we observe that at 2 5 T v slightly depends

P

on the type of alcohol

(+

2% around 1600 m/s, Table I). Such findings are not peculiar of monohydric alcohol solutions. On looking at the existing data in the literature we found that similar features (i.e. x(PSVC)< x(PSAC) and v

P

independent on the solute at 250C) are exhibited by many aqueous solutions such

as 2-Chlorethanol

/ 8 / ,

Aceton

/ 9 / ,

Dioxan /lo/, Diethylamine /II/, Methyl-,

Ethyl- and Buthyl-Glycol Ethers /9/. For such systems v at 25OC ranges from 1580 m/s to 1640 m/s. P

MeOH

0.40

E ~ O H 0.089 0.24 i-PrOH 0.057 0.18 PrOH 0.046 0.14 t-BuOH

0.040

0.11

TABLE I - Characteristic u.s. quantities in Water-Alcohol solutions at 25OC.

plateau), x(PSAC) and x(PSVC) are respectively, the length of the

U.S.

absorption "plateau", the concentration of the low frequency ab- sorption peak (0(/f2) and the concentration oT the sound velocity

P peak

v

P'

Detailed experiments /12/ in Water-Ethanol mixtures in the range 5 to 45OC showed that x(PSVC) shifts to higher values as T decreases. They also gave evi- dence for the existence of a "cross" composition x(CR) at which the temperature coefficient of velocity dv/dTN

0.

For x ( x(CR) is dv/dT >

⁰

(like in water) while for x > x(CR) is dv/dT <

⁰

(like in ethanol)

_:

it means that different isotherms in the plane v-x cross together at a point. It is found that x(CR)

(

x(PSVC) and approximately x(CR)Z plateau). A final finding concerns the sound velocity dispersion: going from

U.S.

to hypersonic frequencies veloci- ty increases. However, dispersion effects reach a maximum not at x(PSVC) hut at x(PSAC) and they become negligible far from a small composition range around x(PSAC). Like for&/f 2 , the v-dispersion at x(PSAC) decreases as T increases.

111

- EXPERIMENTAL RESULTS

Ultrasonic velocity from 20 to -30°C and in the frequency range 5-55 MHz was measured by a phase comparison method employing an ArembegMod.PG-650C Power Oscil- lator operating as gated amplifier. Details of the technique are described elsewhere /13/. The accuracy of measurements is f

l%o.

Results at high temperatures agree very well with previous determinations /12,14,15/. Measurements were performed in three solutions (x =0.049; x =0.12; x =0.236) and in pure alcohol. x nearly cor-

1 2 3 1

responds to a concentration where previous Brillouin light scattering experiments /16/ gave a minimum at -5OC in the curve v vs.T. x and

x

are, respectively, the

\ 2 3

concentrations where previous high temperature deterniinatiors gave x(PSVC) and x(PSAC). Sound velocity of pure water was taken from the literature /17/. The free- zing temperatures of the studied solutions are, respectively, t =-5.Z°C, t =-16.Z°C,

1 2

and t =-32.7OC while the minimum temperatures we were able to reach for x and

x

3 1

2

were, respectively, -10 and -18OC. The small supercooling we could attain probably depends on the required large amount of samples (-50 cc) and on the stainless steel walls of the employed u.s. cell.

Fig.1 shows sound velocity as a function of temperature for the studied mixtures.

It is evident that a change of the sign of the slope dv/dT must occur at a composi-

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PHYSIQUE

tion between x and x A better

1

2.

evidence for such zero-slope compo- sition is given by the v-isotherms plotted in Fig.2: the cross point corresponds to x(CR)h 0.065.

Although the number of solutions studied are not sufficient for a good definition, isotherms in Fig.2 confirm the trend anticipated in Sec.11, i.e. a larger x(PSVC) as

1400

- T decreases.

The peak velocity v as a function P

of temperature is displayed in Fig.3 In the range -20 to 20°C the slope is almost constant with a value

I I I I I I w

(-2.2 m/sGC) lower than in Ethanol

-2 0 0 20 T(OC) (-3.7

m/s°C). In the same figure

there also shown the relative de- viations of v from, respectively,

P

Fig.1 - Sound velocity vs.T for the the corresponding velocity in pure investigated mixtures. Barred symbols water 4v/v

=

(v -v )/v and in an

P W W denote other determinations (/14,15/).

ideal mixture &v/v

=

(v -v. )/v.

P ld ld' whereasv,

=

xv + (1-x) vW.

ld EtOH These curves give evidence for an enhanced Av/v increase as T decreases.

Curves in Figs.1-3 refer to data at 5

M H z .

Results in the experimental range 5-55 MHz show indeed small dispersion effects which increase on going from

x

to x We'll1 assume velocities at 5

M H z

as zero frequency values.

¹

3'

Fig.2 - Isotherms of sound velocity at low temperatures. Curve at 40°C is from /15/.

Fig.3

-

Sound velocity peak vs. T ; # other determinations. Relative deviations of v from the velocity in pure water _P

( V )

and in an ideal mixture ( A ) .

Sound absorption coefficients were measured byastandard pulse technique appa- ratus (MATEC) in the range 5-235 MHz. Fig.4 shows the enormous increase of

a ( / f 2

at 15 MHz when temperature decreases below O°C. The noticeable dependence of absorption on the frequency is evident from Fig.4. We can see that at x(PSVC) dispersive effects are much lower than at x(PSAC). Low T curves in Fig.5 give also evidence for a broad distribution of relaxation times.

IV

-

DISCUSSION

Let us discuss separately low fre- quency sound velocity and absorption results.

A) Low frequency sound velocity

Early attempts /18/ to explain the striking effect of alcohol to increase the sound velocity of water were based on a simple two-state model of water where a tetrabonded bulky species is in equilibrium with a closed, less com- pressible one. Alcohol molecules were supposed to be able to break up hydro-

gen bonds in the bulky species, shif- +

ting the equilibrium towards the closed

one. However this simple model of water

^X (alcohol mole fraction)

has been largely criticized so that we compare our findings in the frame-

Fig.4 - Isotherms of sound absorp- work of recent models suggested by

tion at low temperatures (f=15 MHz).

Stanley and Teixeira (S-T)/3/ and by

Curve at 25OC is taken from Ref./l4/

Stillinger /5/.

In the

(S-T)

model it is predicted

that when an H-B forming impurity, like

alcohol , is added to water density

JOURNAL DE PHYSIQUE

Fig.5 - Sound absorption dispersion for some temperatures at x(PSAC). Dashed curve refers to x(PSVC) at

- 20°C.

fluctuations are reduced, i.e. K decreases. In the temperature range down to - 2 0 " ~ T

it is reasonable to assume r( ^{=K /K} as constant /2/ so that the behavior of K

T S S

reflects the behavior of KT, i.e. the increased sound velocity in the solutions can be qualitatively explained on the grounds of density fluctuations reduction.

On such a picture one is able to account for the various features of sound velocity reported in Sects.11 and 111. For example x(PSVC) should correspond to the amount of impurities necessary to produce a maximum density fluctuations reduction. The peak velocity v for a given T should then approximately correspond to the velocity

P

of a "normal" liquid water where anomalous fluctuations are removed. This conjuctu- qeak re is corroborated by two observations: a) the temperature coefficient of veloci y dvp/dT is negative (as usual in liquids) and its value is of the same order of ma- gnitude as found in normal liquids

/18/;

b) we showed in Sec.11 that the v values

P at 25OC are approximately the same in aqueous solutions with different alcohols (Table I) or with other hydrocarbon derivatives. It means that, whatever the solute, v reflects a property related to pure water.

P

Along this scheme of interpretation one also would expect that the composition x(PSVC) at which the reduction of density fluctuations is maximum should depend on the size of the alkyl group. The shift of x(PSVC) towards lower x on going from lower to higher alcohols (Table I) confirms such expectation. The enhancement of the relative deviation &v/v (Fig.3) and the shift of v towards higher concentra-

P

tion on lowering temperature are coherent with this scheme: since, in fact, the patches are predicted to increase faster below O°C one must remove larger and lar- ger density fluctuations by means of more and more amount of impurities.

Although the overall features of sound velocity in alcohol solutions can be

qualitatively accounted for by the S-T model, the problem related to the quantita-

t i v e a s p e c t s s t i l l r e m a i n s . More d e t a i l e d f u t u r e e x p e r i m e n t s a s a f u n c t i o n o f T, p r e s s u r e and t y p e o f s o l u t e o r s u b s t i t u t i n g D 0 t o w a t e r s h o u l d b e v e r y u s e f u l f o r a d e e p e r t e s t o f S-T model p r e d i c t i o n s . 2

L e t u s d i s c u s s sound v e l o c i t y i n t h e framework o f S t i l l i n g e r s u g g e s t i o n . Halfpap and S o e r e n s e n /19/ have r e c e n t l y e x p l a i n e d t h e enhanced v i s c o s i t y o f low t e m p e r a t u r e Water-Ethanol s o l u t i o n s on t h e b a s i s o f c a g e s f o r m a t i o n . I n o t h e r words t h e y suppose t h a t t h e i n c r e a s e d v i s c o s i t y o f s o l u t i o n s i s due t o t h e f o r m a t i o n o f w a t e r c a g e s around t h e a l c o h o l m o l e c u l e s s o t h a t t h e y c o n c l u d e t h a t a d d i t i o n o f a l c o h o l i s e q u i v a l e n t t o l o w e r i n g t e m p e r a t u r e i n p u r e w a t e r . However t h i s e q u i v a - l e n c e does n o t work f o r low T sound v e l o c i t y s i n c e v increases when a l c o h o l i s added w h i l e I n p u r e w a t e r ~t d e c r e a s e s on l o w e r i n g t e m p e r a t u r e . I n o r d e r t o r e t a i n t h e c a g e s h y p o t h e s i s , we s u g g e s t two p o s s i b i l i t i e s : a ) t h e d e n s l t y f l u c t u a - t i o n s p l a y t h e same r o l e a s i n t h e S-T model. I n s u c h a c a s e s i n c e t h e f i l l e d c a g e s s h o u l d b e d e n s e r t h a n t h e hollow o n e s e n v i s a g e d I n s u p e r c o o l e d w a t e r , a d e n s i t y f l u c t u a t i o n s r e d u c t i o n o c c u r s ; b ) t h e f i l l e d c a g e s d i f f e r from t h e empty o n e s i n t h a t t h e y have a s m a l l e r a d i a b a t i c c o m p r e s s i b i l i t y ( i . e . t h e y a r e more r i g i d ) and a n e g a t i v e t e m p e r a t u r e c o e f f i c i e n t o f v e l o c i t y dv/dT. The g r e a t e r r i g i d i t y o f t h e f i l l e d c a g e s c o u l d b e d u e , f o r example, t o t h e i n c r e a s e d number o f H-B f o r t h e w a t e r m o l e c u l e s engaged i n t h e p o l y e d r a s t r u c t u r e s . To assume h y p o t h e s i s b ) a l l o w u s t o q u a l i t a t i v e l y r a t i o n a l i z e t h e sound v e l o c i t y b e h a v i o r i n s o l u t i o n s a s f o l l o w s . i ) A d d i t i o n o f v e r y s m a l l q u a n t i t y ( u p t o w x ( C R ) = 0 . 0 6 5 ) o f EtOH promotes t h e forma- t i o n o f few w a t e r c a g e s around t h e s o l u t e m o l e c u l e s . These l e s s c o m p r e s s i b l e c a g e s , p r o b a b l y n o t i n t e r a c t i n g , i n c r e a s e t h e sound v e l o c i t y o f t h e m i x t u r e s b u t t h e o v e r a l l b e h a v i o r 1s s t i l l due t o t h e predominant s u r r o u n d i n g w a t e r (random H-B n e t - work) where dv/dTZO. Then, f o r t h i s r a n g e o f c o n c e n t r a t i o n , dv/dT i s s t i l l p o s i t i - v e (like i n p u r e w a t e r ) b u t s m a l l e r ( s e e F i g . 1 ) . On f u r t h e r a d d i t i o n o f EtOH, more c a g e s a r e formed and t h e two o p p o s i t e e f f e c t s b a l a n c e a t x(CR) where dv/dTI110.

i i ) F u r t h e r i n c r e a s i n g o f EtOH promotes new and p e r h a p s clumped g r o u p s o f c a g e s h e l d t o g e t h e r by t h e "hydrophobic i n t e r a c t i o n " /5/ and dv/dT becomes n e g a t i v e . The c o n c e n t r a t i o n x(PSVC) o f t h e peak v e l o c i t y would t h e n c o r r e s p o n d t o a p o i n t where t h e promotion o f f i l l e d c a g e s r e a c h e s a maximum. T h i s c o n c e n t r a t i o n i s 0 . 0 9 a t 40°C a n d * 0 . 1 4 a t -20°C, w h l l e d e t a i l e d e x p e r i m e n t s by Giacomini /12/ g i v e 0.085 a t 45'C and 0 . 1 4 5 a t 5°C. E t h a n o l aqueous s o l u t i o n s a r e known t o form c l a t h r a t e s t r u c t u r e s . A s t a b l e t y p e I h y d r a t e o f s t e c h i o m e t r i c f o r m u l a C H OH-5 3 - H 0 h a s been

2 5 4 2

supposed t o e x i s t /19/ and i t c o r r e s p o n d s t o a 0 . 1 5 mole f r a c t i o n o f a l c o h o l . T h i s c o m p o s i t i o n d o e s n o t g r e a t l y d i f f e r from x(PSVC) a t low T s o t h a t one c a n s u p p o s e t h a t t h e assumed f i l l e d c a g e s i n EtOH s o l u t i o n s a r e s i m i l a r t o t h e c l a t h r a t e hydra- t e s p o l y e d r a a p p r o a c h i n g t h i s s t r u c t u r e a t t h e l o w e s t t e m p e r a t u r e s .

iii) F o r c o m p o s i t i o n s g r e a t e r t h a n x(P?#g) t h e amount o f w a t e r i s n o t enough t o s u p p o r t t h e f u l l s t r u c t u r i n g a b l l l t y o f s o l u t e . P o l y e d r a s t r u c t u r e s must b r e a k up and t h e s y s t e m s h l f t s t o a r e g u l a r s o l u t i o n where sound v e l o c i t y d e c r e a s e s t o w a r d s a l m o s t i d e a l v a l u e s .

We o b s e r v e t h a t t h e t h r e e composition r a n g e s h e r e c o n s i d e r e d a r e q u i t e s i m i l a r t o t h o s e proposed i n t h e model by Mik&lov-Ponomarova /1/ t o s u c c e s s f u l l y e x p l a i n t h e e x p e r i m e n t a l h e a t and volume o f m i x i n g i n a l c o h o l s o l u t i o n s . I n t h i s model i t i s assumed a s e t o f e q u i l i b r i a where w a t e r c a n b e i n v o i d s , i n a " i c e l i k e " network o r i n a r e g u l a r s o l u t i o n w h i l e t h e s o l u t e i s l o c a t e d i n t h e v o i d s o r i t i s found i n a r e g u l a r s o l u t i o n w i t h w a t e r . By i d e n t i f y i n g v o i d s w i t h c a g e s and " i c e l i k e "

network w l t h t h e s t r a l n e d H-B n e t w o r k , t h e Mikhailov model c a n e x p l a l n t h e sound v e l o c i t y b e h a v i o r a l o n g t h e l i n e s p r e v i o u s l y c o n s i d e r e d .

To c o n c l u d e we o b s e r v e t h a t I n t h e framework o f t h e c a g e s model, t h e lndepen-

C7-192 JOURNAL

DE _PHYSIQUE

dence of v from the type of solute should implicate that the adiabatic compressi- P

bility of the filled cages depends only on the strength of the cages skeletons.

The reasons for such behavior should be explained. Moreover, the variation of x(PSVC) with different solutes would implicate a different stechiometric ratio of the solute-water molecules engaged in the polyedra.

B) u.s. absorption

The most important features of the u.s. absorption in the solutions are the large peak values (which depend on T, f and type of solute) and the corresponding compo- sitions x(PSAC) which are always lower than x(PSVC). The last finding suggests that the mechanism responsible for the velocity peak is different from that originating the absorption peak. This conclusion is supported by other two findings. Firstly, the absorption peak depends noticeably on the type of solute (see Table I) while v is almost independent on it. Secondly, the maximum dispersion effects for v and

P

D(/f 2 occur at the same concentration, i.e. x(PSAC). Early attempts to explain the d/f peaks were based on the hypothesis of 2

a

set of equilibria between specific water-alcohol intermolecular complexes perturbed by the sound waves

/ 2 0 / .

There are however many evidences against this model (large number of molecules reacting in a single kinetic process, experimentally observed distribution of relaxation times, etc.) and the most widely accepted point of view is now in favour of a dis- sipative mechanism due to concentration fluctuations.

The existence of large concentration fluctuations near x(PSAC) is also suppor- ted by the presence of large intensity peaks in light scattering experiments

/21/.

As a matter of fact, the large absorption peaks shown in Fig.4 strongly remind those observed in binary critical mixtures. In such mixtures anomalous sound absorption was successfully explained by Fixman /22/ on the grounds of diverging concentration fluctuations at the consolute point. On the other hand Franks /1/

E E

pointed out that thermodynamic conditions (AH , ^TAS

)

in alcohol aqueous solutions are near to those characterizing a system with LCST and suggested the possible exi- stence of a critical point just outside the range of thermodynamic stability of the solutions.

A theory for sound absorption in systems with large concentration fluctuations has been given by Romanov and Solov'ev (R-S)

/ 2 3 / .

In such theory, which is tailo- red on the Fixman approach for critical mixtures, the location and the amplitude of absorption peaks are related to the second derivatives of volume (V) and Gibbs energy (G) with respect to concentration. The R-S approach also predicts a distri- bution of relaxation times related to the mutual diffusion coefficient and to the wave numbers of the individual components of the Fourier expansion of spatial con- centration fluctuations. Although experiments qualitatively agree with R-S theory /24/ a quantitative comparison is very difficult due either to the scarce accuracy of the previous second derivatives (as obtained by the experimental V and G data) or to the lack of some microscopic parameters entering in the theory. As a matter of fact, our experiments at low temperatures show larger and larger absorption peaks with large dispersive effects (Fig.5) with a broad distribution of a relaxa- tion times. Such features seem to indicate that at x(PSAC) the :system. approaches a critical point which is located at a temperature below the investigated range.

From a structural point of 'view a possible mechanism for such behavior could be

envisaged in the dissolution of cages structures occurring for x 7 x(PSVC): once

a maximum structuring has been reached at about x(PSVC) the water content is not

enough to support more ordered structures and the system collapses at some "criti-

cal" composition breaking into a random arrangement of H-B fragments with few

polyedra and then into a regular solution at higher compositions.

V - CONCLUSIONS

We compared the peculiar behavior of sound velocity in Water-Ethanol solutions on the basis of density fluctuations reduction considered in the percolation model by Stanley-Teixeira. We found that every aspect of the observed behavior can be qualitatively explained on such grounds. We also found that a similar qualitative agreement can be obtained by

considering

the formation of water cages filled with solute molecules as suggested by Stillinger. In such case, however, one must asso- clate to the cages a smaller compressibility than in pure water and a "normal" tem- perature coefficient. In such scheme the observed large absorption peaks and broad the spectra of relaxation times are related to the large concentration fluctuations associated to the collapse of the filled cages structures.

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72 (1980) 3511

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