electrolyte contacts

electrolyte contacts

Surface Technology, 8 (1979) 171 - 183 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands 171 ELECTROCHEMICAL RELAXATION AT Hg/ELECTROL...

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Surface Technology, 8 (1979) 171 - 183 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands

171

ELECTROCHEMICAL RELAXATION AT Hg/ELECTROLYTE CONTACTS*

G. ACHATZ, F. BRAUNSPERGER, O. FRUHWIRTH and G. W. HERZOG Institute of Inorganic Chemical Technology and Analytical Chemistry, Technical University of Graz, Graz (Austria) (Received July 11, 1978)

Summary E l e c t r o c h e m i c a l r e l a x a t i o n p h e n o m e n a consisting o f loss m a x i m a and c a p a c i t a n c e dispersion w e r e i n v e s t i g a t e d at H g / b o r a x and H g / K F c o n t a c t s in t h e f r e q u e n c y range 0.1 - 1 0 0 0 Hz. A t all the c o n t a c t s a distinct loss m a x i m u m n e a r 10 Hz is f o u n d , w h i c h o c c u r s at positive p o t e n t i a l s versus H g / H g O / 0 . 1 M b o r a x r e f e r e n c e a n d is a t t r i b u t e d t o a relaxing m e r c u r y o x i d a t i o n . A s e c o n d kind o f loss m a x i m u m , w h i c h decreases w i t h t i m e , is l o c a t e d n e a r the d o u b l e - l a y e r loss p e a k a n d is ascribed to a m o d i f i e d d o u b l e l a y e r r e l a x a t i o n . T h e l o w f r e q u e n c y (0.1 - 3 Hz) i m p e d a n c e and p h a s e angle m e a s u r e m e n t s were carried o u t b y m e a n s o f an X Y r e c o r d e r a n d t h e m e a s u r e m e n t s a b o v e 40 Hz w i t h a bridge t h a t has b e e n described p r e v i o u s l y . T h e o b s e r v e d a b s o r p t i o n p h e n o m e n a are i n t e r p r e t e d in t e r m s o f a simple r e l a x a t i o n m o d e l derived f r o m c h e m i c a l r e l a x a t i o n principles. It leads to f e a t u r e s o f a b s o r p t i o n similar t o t h o s e arising f r o m p u r e dielectrics.

1. I n t r o d u c t i o n As a l r e a d y r e p o r t e d [1, 2] a d o u b l e - l a y e r r e l a x a t i o n at H g / e l e c t r o l y t e c o n t a c t s is f o u n d n e a r 10 k H z , d e p e n d i n g o n e l e c t r o l y t e c o n c e n t r a t i o n , a n d is believed t o be c a u s e d b y an a d s o r p t i o n process w h i c h changes t h e ionic charge d e n s i t y in t h e o u t e r o r i n n e r H e l m h o l t z plane. T h e d o u b l e l a y e r can be c o m p a r e d w i t h a c a p a c i t o r and a resistance in parallel, leading to a descript i o n b y D e b y e e q u a t i o n s f o r t h e c o m p l e x dielectric c o n s t a n t . Similar behavi o u r m i g h t be e x p e c t e d f o r e l e c t r o c h e m i c a l r e a c t i o n s o c c u r r i n g at certain p o t e n t i a l s , e.g. f o r t h e o x i d a t i o n o f m e r c u r y t o Hg 2÷ (aq.) or t o a HgO layer, t h e l a t t e r b e i n g a m o d e l r e a c t i o n f o r o x i d a t i v e c o r r o s i o n . It is a w e l l - k n o w n e l e c t r o c h e m i c a l p r i n c i p l e [3, 4] t h a t a l t e r n a t i n g c u r r e n t causes a p h a s e shift b e t w e e n c u r r e n t a n d o v e r v o l t a g e b u t , p r o b a b l y b e c a u s e o f difficulties arising *Dedicated to Professor Dr. W. Limontschew on the occasion of his 60th birthday.

172

f r o m low f r e q u e n c y m e a s u r e m e n t s , a b s o r p t i o n p h e n o m e n a due t o electrochemical reactions have n o t been studied intensively. F o l l o w i n g the c o n c e p t o f chemical r e l a x a t i o n it is possible to describe a m o d e l f o r e l e c t r o c h e m i c a l relaxation. Chemical r e l a x a t i o n is based on the linear response e q u a t i o n [5, 6] r~ + x = 0

(1)

w h e r e x is the c o n c e n t r a t i o n change, 2 is the rate and ~ is the r e l a x a t i o n time given b y 1

=

(2) g(CA + eB) + CA and CB are the c o n c e n t r a t i o n s o f A and B due to the r e a c t i o n A + B AB with f o r w a r d and b a c k w a r d rate c o n s t a n t s k and ~-. T h e solution o f eqn. (1) is simply (3)

X = XO e - t p -

and c o r r e s p o n d s t o the d e c a y f u n c t i o n a f t e r a released p e r t u r b a t i o n . However, if the p e r t u r b a t i o n is a periodic field (with f r e q u e n c y co), which p r o d u c e s the new oscillating equilibrium c o n c e n t r a t i o n change (i = x / - ~ i )

2 = ae i~t

(4)

the solution o f (5)

TFC + x = a e i¢'t

is given by x =

a exp{i/(¢o t -- ~)}

[1 + (~T) 2] 1/2

tg ~v = ¢o7

(6)

or by x

1 -

(7)

1 +i~ T h e physical result is a phase shift ~ b e t w e e n x and ~. Carrying over this c o n c e p t to an e l e c t r o c h e m i c a l r e a c t i o n involving charged species, the phase shift m u s t be related t o the observable phase angle b e t w e e n the c u r r e n t and t h e overvoltage. This can be d o n e b y t r a n s f o r m i n g ~ into an alternating current

~[

i¢o e iaJt =

aF

-

-

1 +icor

(8)

T o simplify the e q u a t i o n t h e e l e c t r o n transfer n u m b e r was t a k e n as 1, F being the Faradaic c o n s t a n t . If we i d e n t i f y a F ~ with ( 1 / R ° ) A ~ , R ° corres p o n d i n g t o t h e resistance o f an a r b i t r a r y c u r r e n t p o t e n t i a l curve at a fixed p o t e n t i a l , eqn. (8) b e c o m e s

173

(9)

R ° 1 + icoT E q u a t i o n (9) c o n t a i n s t h e c o m p l e x a d m i t t a n c e o r c o n d u c t i v i t y 1

1

-

o

i

i

-

Z

-

o °

R ° 1 +icot

-

-

(10)

1 +icoT

a n d d e s c r i b e s t h e r e l a x i n g p a r t o f t h e a.c. c o n d u c t i v i t y o w i t h o u t a n y l i m i t i n g c o n d i t i o n s . I n t r o d u c t i o n o f a high f r e q u e n c y c o n d u c t i v i t y o ~ = co C ~ ( w h i c h c o r r e s p o n d s t o t h e l o w l i m i t o f t h e d o u b l e l a y e r c a p a c i t a n c e CD) a n d o f a d.c. c o n d u c t i v i t y a ° ( c o r r e s p o n d i n g t o 1 / R °) y i e l d s i o(co ) = io ~ + o ° + o ° - = o r + io i 1 +icoT

(11)

Thereby o ~ was taken as a complex part of the conductivity because of the imaginary character of coCD. After splitting eqn. (11) into a real and an imaginary part we obtain or = o ° 1 +

(12)

1 + ((~gT) 2

and ao o i =

o ~

+

-

(13)

-

1 + (coT) 2 C o m p a r i s o n o f t h e real a n d i m a g i n a r y p a r t s w i t h t h e a d m i t t a n c e o f a c a p a c i t o r a n d r e s i s t a n c e in parallel 1

O r + i o i --= . . . . + ico Cp

(14)

Rp gives R

RO 1 + 1 + (coT) 2

(15)

and 1 coCp

¢°CD+R°

1 1 + (COT) 2

(16)

B e c a u s e tg 0 = - - ( c o R p C p ) -1 = or/o i t h e loss is f o u n d t o be

1 + cot tg6

+

(097) 2

c o R ° C D { l + (coT) 2} + 1

(17)

A c o m p a r i s o n w i t h t h e a d m i t t a n c e o f a c a p a c i t o r a n d a r e s i s t a n c e in series w o u l d lead t o m u c h t h e s a m e f r e q u e n c y d e p e n d e n c e o f tg O b u t t o m o d i f i e d expressions for the capacitance and the resistance: (Or) 2 + (o'i) 2

Cs -

.--

0901

Or

- tg - - + Cp CO

(18)

174

%0"

I

I

I

]

I

I

tg -

-

T=QO48s

Log C s Log Cp

% -I

0

1

2

3

log

v

Fig. 1. F r e q u e n c y d e p e n d e n c e o f o r, a i, tg 0, log Cp, log C s c a l c u l a t e d n u m e r i c a l l y w i t h R 0 = 3 2 0 ~ c m 2, C D = 16 p F c m - 2 , ~ = 0 . 0 0 1 6 , 0 . 0 3 2 , 0 . 0 4 8 s (data f r o m H g / 0 . 1 M K F c o n t a c t at 2 3 0 m V ) . O r

Rs

(or)2 +(oi) 2

(19)

Figure 1 shows the numerically calculated frequency dependence of o ~, o i, tg 0 and log Cs, log Cp for a special contact. At low frequencies a linear relationship between log Cp (log Cs) and log co with slope - I as well as tg 0 values near 1 are predicted. In contrast to the condition cot equal to 1 at the position of the o r m a x i m u m , cot equal to 1 - 3 at the position of the tg 0

175 m a x i m u m is expected. Because of the invariance of tg 0 on equivalent circuits the best representation of data in the f r e q u e n c y domain is found to be that of the loss factor. The kinetics of an electrode reaction enter the derived equations only through the term R °, which is identical to the impedance Z(w -+ 0). Because of this fact, we expect loss maxima and capacity dispersion due to any electrode reaction, e x c e p t for the case R ° -~ ~ . The aim of this work was to prove experimentally this simple model of electrochemical relaxation with the help of tg 0 measurements at low frequencies.

2. Experimental F o r low f r eq u e nc y measurements phase angle methods as well as bridge m e t h od s can be used. For our purpose both m et hods proved to be necessary. The bridge m e t h o d has already been described in a previous paper [1]. The details o f the electrical circuit applied for impedance and phase angle measurements are given in Fig. 2. The input a.c. signal produced by a generator (Philips PM 5167) passes the electrolytic cell and is fed together with the o u t p u t signal into the X Y recorder. Thus ellipses are produced, the shape o f th em depending on impedance and phase angle. Before feeding the recorder the signals are made highly ohmic by two transformers. Polarisation of the Hg/electrolyte c o n t a c t is made by a bat t ery and the electrode poten-

battery

I

generator

X

X/Y'-

I ~hermost

I Fig. 2. Electrical circuit for low frequency phase angle measurements.

recorder

176

2 HZ

I HZ

ux 0,5 HZ"

0,2 H z

Uz mcl x ql Hz

Fig. 3. Ellipses of a Hg/0.01 M borax contact at +30 m V and at different frequencies; = 5 k ~ , recorder sensitivity 2 m V cm 1.

R x

f

/

%[.,FI

0,3-

T 300



Q

0,2-

200-

0,1

i

100-

I

5

r

10

--'115

,~

I

20

~;

:0

,; f'~ 10 3 [crn 2]

(a)

(b)

Fig. 4. Dependence of (a) electrolyte resistance Re1 and (b) double layer capacity C D o n drop area (data from Hg/0.1 M borax contact at different potentials).

tial is m e a s u r e d b y a v o l t m e t e r v e r s u s H g / H g O / 0 . 1 M b o r a x r e f e r e n c e electrode. T h e ellipse m e t h o d has b e e n c h e c k e d with d i f f e r e n t e q u i v a l e n t circuits o f k n o w n i m p e d a n c e s , yielding an a c c u r a c y of a b o u t 10 - 15% w i t h r e s p e c t to d a t a for Rp, Cp, Z a n d tg 0. A certain e x t e n s i o n o f a p p l i c a b i l i t y is given

177

to tOSS Of d o u b l e - l a y e r c o u p l e d to r e a c t / o n

17 h

2-

tOSS of dou hie - lay er

I 2

I

3

1

~

log V

Fig. 5. Tg 0 m a x i m a o f a H g / 0 . 0 0 5 M b o r a x c o n t a c t at 0 m V s h o w i n g the c o u p l i n g o f t h e d o u b l e layer to t h e r e a c t i o n a f t e r various r e a c t i o n times.

by the variation of the resistance R x. Most of the results refer to Hg/0.1 M borax and Hg/0.1 M KF contacts with electrolyte resistances (Rel) between 200 and 1000 S2, depending on the size of the hanging Hg drop electrode. In the case where the impedances became of the order of the electrolyte resistance, a correction was made by the vector subtraction Zexp --Re1 through IZ¢orr{ = (RZel + Z 2 x , - - 2 R e l Z ~ x p sin 0exp) 1/2

(20)

The corrected loss angle is then given by sin 0

-

1 IZcorr[(Z~xp sin 0exp --R~l)

(21)

A correction due to the impedance of the counter electrode was not necessary because of its large size. Some ellipses obtained with the contact Hg/

178

0

1

2

3 log v

-,

\

\

\

\ 2-

\

\

"\

\ \

2h

.,h

\

o

-i

1

2

3

Io 9 ~

P

Fig. 6. Tg 0 m a x i m a a n d log C s c u r v e s o f a H g / 0 . 1 M K F c o n t a c t at + 2 3 0 m V m e a s u r e d a f t e r 1, 2 a n d 4 h.

0.01 M b o r a x at a p o t e n t i a l o f +30 m V are s h o w n in Fig. 3. D a t a o f p h a s e angles, losses and i m p e d a n c e s were t a k e n f r o m the Ux a n d / o r Uy values o f such ellipses sin

Uy 0

U~

-

Uy, nlax

~Zl -

gy,

nlax

(22)

U~-, max

Rx

[]~v, m a x

By m e a n s o f parallel and series splitting o f Z data, Cp and Cs d a t a were obtained

(23)

179

'g~ q

T3i JJ

--I

j

j r

I

I

i

I

I

-I

0

1

2

3

4

-I

0

1o9 V

log C

3-

11•

= 75 5 s 4

7

2

3 log V

Fig. 7. Tg 0 maxima, log Cs and log Cp curves of a Hg/0.1 M borax contact at +5 mV measured after 0,5, 1, 2 and 4 h.

Cp =

1 co IZl(1 + tg20) 1/2

C s = Cp(1 + tg20)

(24) (25)

O n e a d d i t i o n a l e x p e r i m e n t a l a s p e c t c o n c e r n s eqn. (20), f o r w h i c h Re1 d a t a are n e c e s s a r y ; t h e y w e r e d e t e r m i n e d t h r o u g h h i g h - f r e q u e n c y e x t r a p o l a t i o n in a R~, 1/co p l o t ( d a t a o b t a i n e d w i t h bridge m e t h o d at d i f f e r e n t p o t e n t i a l s ) . Figure 4(a) s h o w s t h a t Re] d e p e n d e d linearly on 1/(A) 1/2 or 1/r, A being the area o f t h e Hg d r o p a n d r its radius. D r o p areas were d e t e r m i n e d experi m e n t a l l y b y weighing t h e d r o p s . T h e d e p e n d e n c e on 1 / ( A ) 1/2 or 1/r respectively was in g o o d a g r e e m e n t with t h e t h e o r e t i c a l p r e d i c t e d Re1 = f(A) f o r an e l e c t r o l y t i c cell m a d e o f c o n c e n t r i c e l e c t r o d e s , f o r which the f o l l o w i n g r e l a t i o n h o l d s [7, 8]

180

t

-

-

- 700

mY

/'x~\

0.Orl~ 0,m , / / / ~/ /

~ l /___

2

3

.~

~.

Io9 v --V~,-

Fig. 8. Tg 0 m a x i m u m o f a Hg/0.01 M b o r a x c o n t a c t at 0 m V ( r e a c t i o n ) a n d tg 0 m a x i m a of the same c o n t a c t at - - 7 0 0 m V ( n o r e a c t i o n ) at d i f f e r e n t b o r a x c o n c e n t r a t i o n s .

p

1

p is t h e specific e l e c t r o l y t e resistance, r is the radius o f t h e Hg d r o p and r ' the radius o f the c o u n t e r e l e c t r o d e . Because r ' >> r Rel -~

p 1 4~ r .

.

.

.

1 .

.

.

.

(27)

.

( A ) 1/2

T h e r e f o r e Rel has to be n o r m a l i s e d w i t h r e s p e c t t o (A) 1/2 (or r) a n d n o t to A (or r 2) as is the case f o r the resistance o f a d o u b l e l a y e r or i m p e d a n c e o f a r e a c t i o n . A p p l i c a t i o n o f eqn. (26) in this case yields PD { r ' - - r \

RD = 4 ~ [~ r'r )

- - P D const.

4n

r'r

1

A

(2S)

if we m a k e t h e plausible a p p r o x i m a t i o n s r ' - - r = c o n s t a n t and r, r ' >> r ' - - r . Because o f the r e l a t i o n s h i p

181

f

tg b

7-

/

~

/'\/

, ,"/ii '

i! I/xxx, '< /

~.

,Y

/i

,I/I

/

\K, \

\

x x \

D

b

;

.~

$ log y

i

Fig. 9. Theoretical and experimental tg 0 maxima of 0.1 M borax contact at +5 mV (after 4 h). 1 - - = icoe p

(29)

(e, dielectric c o n s t a n t ) C s a n d Cp s h o u l d d e p e n d linearly on A (Fig. 4(b)). T h u s c o r r e c t i o n s such as Z --Rel have t o be p e r f o r m e d with a b s o l u t e and n o t w i t h n o r m a l i s e d data. 3. Results a n d discussion S o m e c h a r a c t e r i s t i c results are given in Figs. 5 - 7, all d i s p l a y i n g the f e a t u r e s o f loss m a x i m a a n d dispersive capacities. T o begin the discussion, t h r e e t y p e s o f m a x i m a can be o b s e r v e d . A t p o t e n t i a l s a r o u n d - - 7 0 0 m V (i.e. m i n i m u m c a p a c i t y w i t h r e s p e c t t o p o t e n t i a l ) o n l y o n e m a x i m u m at a b o u t l 0 - 50 k H z is o b s e r v e d , w h i c h is c a u s e d b y t h e charging o f t h e d o u b l e layer. With increase in t h e p o t e n t i a l t o zero a n d m o r e , w h e r e Hg o x i d a t i o n sets in, this m a x i m u m b e c o m e s larger and is s h i f t e d t o w a r d s l o w e r f r e q u e n c i e s (to

182

T t500

0,1n}

-

borax

7000

OJm K F R'~(~' : ÷ 2 J O cn V) = 3 0 0 (~ 2 0 ) ~ c m 2

- 500

;o

? [my]

Fig. 10. Current potential curves of the contacts Hg/0.1 M borax and Hg/0.1 M KF; potential scan 0.25 mV c m - 1 . a b o u t 1 k H z with 0 . 0 0 5 M b o r a x , Fig. 5). During o n g o i n g o x i d a t i o n this m a x i m u m is l o w e r e d a n d s h i f t e d to a f r e q u e n c y range a r o u n d 10 Hz. T h e 1 k H z m a x i m u m was p r e v i o u s l y [1] a t t r i b u t e d to the relaxat i o n o f the r e a c t i o n itself. H o w e v e r , b e c a u s e o f t h e e x i s t e n c e o f the 10 Hz m a x i m u m we m u s t c o r r e c t this i n t e r p r e t a t i o n in the following w a y . T h e 1 k H z m a x i m u m is still a d o u b l e - l a y e r e f f e c t , b u t c o u p l e d to the r e a c t i o n . Hg 2+ ions p r o d u c e d b y t h e r e a c t i o n or O H - ions c o n s u m e d f o r a kind o f HgO f o r m a t i o n l o w e r t h e negative excess charge d e n s i t y (at positive p o t e n tials) t o such an e x t e n t t h a t m a c r o s c o p i c a l l y t h e loss p e a k b e c o m e s equivalent to a " p u r e " d o u b l e l a y e r p e a k (---700 m V ) at m u c h l o w e r e l e c t r o l y t e c o n c e n t r a t i o n s . T o s h o w this e q u i v a l e n c e in Fig. 8 t h e loss p e a k of 0.01 M b o r a x c o n t a c t at zero p o t e n t i a l is c o m p a r e d with the p u r e d o u b l e l a y e r p e a k at .... 700 m V at various c o n c e n t r a t i o n s . T h e size a n d p o s i t i o n o f the f o r m e r is a p p r o x i m a t e l y t h a t o f a 0 . 0 0 1 M b o r a x c o n t a c t . With increasing r e a c t i o n t i m e the p e a k d i s a p p e a r s and a n e w o n e arises at a b o u t 10 Hz. This behavi o u r is s h o w n in Fig. 5. I n s p e c t i o n o f t h e tg 0 curves in Figs. 6 a n d 7 a n d c o m p a r i s o n with Fig. 1 lead t o the c o n c l u s i o n t h a t r e l a x a t i o n due to Hg o x i d a t i o n is a p h e n o m e n o n d e s c r i b a b l e in t e r m s o f the c o m p l e x c o n d u c t i v i t y , a l t h o u g h t h e r e are still s o m e d i s c r e p a n c i e s b e t w e e n e x p e r i m e n t a l and t h e o r e t i c a l tg 0

183 d a t a (Fig. 9). Because o f t h e special f o r m o f eqn. (17) t h e tg 0 m a x i m u m is s o m e w h a t shifted with r e s p e c t to t h e a ~ m a x i m u m , leading t o cot values b e t w e e n 1 a n d 3. This shift causes s o m e u n c e r t a i n t y in c a l c u l a t i n g r e l a x a t i o n t i m e s d i r e c t l y f r o m the p o s i t i o n o f t h e e x p e r i m e n t a l tg 0 m a x i m u m . A n o t h e r u n c e r t a i n t y arises f r o m a m o r e trivial e x p e r i m e n t a l fact: t h e polarising d.c. c u r r e n t , a n d t h e r e f o r e t h e resistance R °, is n o t s t a t i o n a r y , so t h a t tg 0 a n d C d a t a v a r y w i t h t i m e . S t a t i o n a r y c o n d i t i o n s are r e a c h e d a f t e r s o m e hours. In a d d i t i o n , Fig. 9 s h o w s t h a t t h e o r e t i c a l loss m a x i m a are n a r r o w e r t h a n the o b s e r v e d ones, a f a c t which f a v o u r s t h e idea o f a r e l a x a t i o n t i m e distribution. I n t r o d u c t i o n o f such a d i s t r i b u t i o n w o u l d b r o a d e n the t h e o r e t i c a l m a x i m u m a n d w o u l d t h e n give b e t t e r a g r e e m e n t . F r o m t h e e x p o n e n t i a l c u r r e n t p o t e n t i a l curves in Fig. 10 we a s s u m e f o r the o x i d a t i o n kinetics at t h e left side o f t h e c u r r e n t p e a k , a t r a n s f e r c o n t r o l . A t r a n s f e r o f Hg 2÷ ions f r o m t h e m e t a l to t h e e l e c t r o l y t e w o u l d i n d e e d at first result in an a c c u m u l a t i o n o f ions in the d o u b l e l a y e r a n d later o n in a d i f f u s i o n i n t o b u l k or l a y e r f o r m a t i o n . T h e s e processes d e f i n i t e l y d e p e n d on t h e d o u b l e l a y e r s t r u c t u r e , so t h a t the change o f ~ w i t h t i m e can be qualitatively u n d e r s t o o d . F u r t h e r m o r e , we are able to c h e c k R ° values o b t a i n e d f r o m c u r r e n t p o t e n t i a l curves a n d t h o s e f r o m C analysis, as o u t l i n e d in the i n t r o d u c t i o n . F o r e x a m p l e , we find f o r t h e 0.1 M b o r a x c o n t a c t at +5 m V : R ° ( c u r r e n t p o t e n t i a l ) = 110 + 10 ~2 c m 2, R ° (C analysis) = 110 ~ cm 2 a n d f o r t h e 0.1 M K F c o n t a c t at +230 m V : R ° ( c u r r e n t p o t e n t i a l ) = 300 + 20 g~ c m 2, R ° (C analysis) = 3 2 0 ~2 c m 2. As a c o n s e q u e n c e o f t h e f o r e g o i n g a g r e e m e n t we c o n c l u d e t h a t t h e e l e c t r o c h e m i c a l r e l a x a t i o n m o d e l b a s e d on t h e c o m p l e x c o n d u c t i v i t y describes p r i n c i p a l l y the l o w - f r e q u e n c y p h e n o m e n a o f losses a n d c a p a c i t a n c e s . In c o n t r a s t to this, d o u b l e l a y e r p h e n o m e n a at higher f r e q u e n c i e s are best e x p l a i n e d b y t h e c o m p l e x dielectric c o n s t a n t , w h i c h is n o t caused b y relaxing w a t e r dipoles b u t r a t h e r b y a d s o r b i n g h y d r a t e d ions. B o t h p r o p e r t i e s t o g e t h e r , in p u r e cases as well as c o u p l e d ones, c h a r a c t e r i s e t h e f r e q u e n c y s p e c t r u m b e t w e e n 0.1 Hz and 100 k H z .

Acknowledgment This research was s u p p o r t e d b y t h e F o n d s zur F S r d e r u n g der wissens c h a f t l i c h e n F o r s c h u n g in C)sterreich. References 1 2 3 4 5 6 7 8

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