The adsorption of sucrose at the mercury-water interface

J. Electroanal. Chem., 122 ( 1 9 8 1 ) 2 9 9 - - 3 1 2

299

Elsevier Sequoia S.A., L a u s a n n e -- Printed in T h e N e t h e r l a n d s

THE ADSORPTION OF SUCROSE AT THE MERCURY--WATER INTERFACE

R O G E R P A R S O N S * and R O B E R T P E A T **

Department of Physical Chemistry, The University, Bristol, BS8 1TS (England) (Received 23rd S e p t e m b e r ; in revised f o r m 27th N o v e m b e r 1980)

ABSTRACT The a d s o r p t i o n of sucrose on m e r c u r y f r o m a q u e o u s 0 . 7 9 5 3 M N a F has been d e t e r m i n e d f r o m differential capacity and electrocapillary m e a s u r e m e n t s . T h e c o m p o u n d shows the typical characteristic features of an organic a d s o r b a t e and o b e y s a F r u m k i n i s o t h e r m with a s a t u r a t i o n m o l e c u l a r area of 0.68 n m 2 per molecule. T h e interparticle i n t e r a c t i o n is repulsive and the s t a n d a r d free energy of a d s o r p t i o n is a quadratic f u n c t i o n of the electrical variable with m a x i m u m a d s o r p t i o n occurring at the p o i n t of zero charge. T h e contrasting behaviour of sucrose at the u n c h a r g e d m e r c u r y and air interfaces is discussed.

INTRODUCTION

Despite the biological significance of sugars and related polyhydroxy compounds their adsorption characteristics have been little studied. D-ribose and 2'-deoxy-D-ribose were investigated [ 1 ] in an attempt to explain the adsorption properties of the constituents of nucleic acids. Mucic acid, Myo-inositol and sodium phytate [2] have been studied in this laboratory. However, the most general study is due to Gouy [ 3 ] who records 12 electrocapillary curves for the compounds xylose, glucose, galactose, sucrose, lactose, maltose, raffinose and dextrin. Frumkin [ 4] reproduced and discussed Gouy's results and noted that sucrose lowered the interfacial tension at the mercury interface while it raised the surface tension at water. He concluded from the small shift of the point of zero charge (pzc) and the surface potential of water that the molecules were adsorbed flat on the electrode surface. Damaskin et al. [ 5] argued that the flat orientation enabled the polar groups to interact favourably with the mercury surface and therefore to increase the absorbability with mercury compared with air. Frumkin [4 ] pointed out that a large number of organic molecules were similarly adsorbed at the air interface and the uncharged mercury interface, and concluded that this was determined by the water structure in the bulk and at the surface. Therefore, an alternative explanation for the behaviour of * Present address: L a b o r a t o i r e d ' E l e c t r o c h i m i e Interfaciale, C.N.R.S., 9 2 1 9 0 M e u d o n , France. ** Present address: D e p a r t m e n t of C h e m i s t r y , T h e University, S o u t h a m p t o n , England.

0 0 2 2 - 0 7 2 8 / 8 1 / 0 0 0 0 - - 0 0 0 0 / $ 02.50, © 1 9 8 1 , Elsevier Sequoia S.A.

300 sucrose is that there is a different structure of water at the mercury--water interface to that at the air interface, which plays a role for some compounds but is u n i m p o r t a n t for others. In view of the recent interest in the structure of water at interfaces [ 6 ], it appeared useful to study the adsorption properties of sucrose for comparison with thiourea [7 ], urea [ 8 ] and glycine [ 9], which also show contrasting behaviour at the mercury--solution and air--solution interfaces. EXP ERIM ENT AL Both electrocapillary and differential capacity measurements were carried out for 11 concentrations of sucrose in aqueous solutions containing 0.7953 M NaF. The reference electrode used t h r o u g h o u t was an aqueous 0.7953 M NaC1 calomel electrode. The temperature was 25.00 + 0.05 ° C. Double-layer capacities were measured using the bridge described by Hills and Payne [ 10 ] and the small t w o - c o m p a r t m e n t cell described previously [ 8 ]. The working electrode was a siliconized dropping mercury electrode of internal radius 0.04 mm. The mercury column height was typically 70 cm which produced an average mercury flow rate of 0.3 mg s -1. The bridge was normally set to balance 6.8 s after the birth of a mercury drop whose total life was about 10 s. The measuremenis were recorded at a frequency of 800 Hz and a peak-topeak amplitude of 10 mV. The capacity was independent of frequency over the range 0 . 4 - 6 kHz for all the concentrations studied. Electrode potentials were measured with a digital voltmeter accurate to +0.5 mV. The electrolyte was freshly prepared and deoxygenated with presaturated oxygen-free nitrogen. The pzc was then determined using the streaming electrode [ 11 ]. The capacity could not be measured at extreme negative potentials due to erratic behaviour of the dropping mercury electrode. This was characterised by the formation of a spray of tiny mercury droplets instead of a single spherical drop. Electrocapillary curves were measured using a classical electrometer. The capillary was unsiliconized. The instrument was calibrated with aqueous 0.1 M KC1 solution, assuming a value for the interfacial tension at the electrocapillary m a x i m u m of 426.2 mN m -~ [ 12 ]. Very recent absolute determination by Vos and Los [ 13 ] shows that this value is 0.6 mN m -~ too high. However, this leads to a proportional error (0.15%) in the derived quantities which is quite negligible. The capillary constant was found to change systematically with use due to capillary wear and it was necessary to recalibrate the instrument after alternate runs. At extreme positive potentials the measurements were impossible to obtain owing to adhesion of the mercury to the walls of the capillary. These peculiarities of fluoride solutions are well d o c u m e n t e d [ 14 ], but require further detailed experimental investigation. The densities of the working solutions were measured using a p y c n o m e t e r that had been previously calibrated with triply distilled water. Analar NaC1 and KC1 were twice recrystallised from water and dried at red heat in a platinum crucible. The NaF was BDH Analar high-purity grade and was used without further purification. The sucrose was British Sugar Corporation commercial grade. Solutions were prepared in triply distilled water. Mercury was twice distilled after wet purification.

301

RESULTS AND ANALYSIS

Differential capacity curves for sucrose concentrations in the range 0.001-1 mol l-' are shown in Fig. 1. The shape is typical for an organic species with adsorption occurring in the region of the pzc and desorption taking place at the extremes of electrode charge. However, the shape is very different to that obtained for the aliphatic alcohols [ 15 ] and is characteristic of a repulsive interaction between the molecules in the adsorbed layer. It is also consistent with the rounded electrocapillary curves obtained for this system [ 3 ]. Some typical electrocapillary curves are compared with twice-integrated capacity curves in Fig. 2. The agreement is satisfactory at potentials negative of the ecm, but on the positive side the curves are somewhat depressed with respect to the twiceintegrated capacity curve. This is typically a depression of 2 mN m - ' a t - - 0 . 1 V

35

30 'E U

tL

BASE /ImM

k)

/

25

2ram 3ram

~

6raM

20 21 mM "'47raM j

100mM / 215mM j 464rnM /

15

1000 mM

'

6

. . . .

0:4

'

'

-Ej/v

,:2

Fig. 1. D i f f e r e n t i a l c a p a c i t y o f a m e r c u r y e l e c t r o d e in c o n t a c t w i t h a q u e o u s 0 . 7 9 5 3 M N a F c o n t a i n i n g sucrose. T h e c o n c e n t r a t i o n o f sucrose is i n d i c a t e d b y each curve in m m o l 1-1 .

302 430 BASE

420 lOOmM

z E

410

400

! r

390 fr r

0

0-5

-E/V

~

I-0

Fig. 2. Comparison between twice-integrated differential capacity data (o,e) and electrocapillary curves (A,×) for a mercury electrode in contact with aqueous 0.7953 M NaF containing sucrose. Concentration of sucrose is indicated by each curve in mmol 1-1.

a n d has been observed in a n u m b e r of systems e m p l o y i n g N a F as the base electrolyte. In view of the difficulties e x p e r i e n c e d with the electrocapillary meas u r e m e n t s at positive charge values, the integrated capacity curves were used in the s u b s e q u e n t analysis. The c o o r d i n a t e s of the pzc are r e c o r d e d in Table 1. G o u y [3] o b t a i n e d a value of 23.9 mN m - ' for the capillary depression at uncharged m e r c u r y for 1 M sucrose which agrees f a v o u r a b l y with these results. The relative surface excess (F) of sucrose was d e t e r m i n e d at c o n s t a n t charge by n u m e r i c a l d i f f e r e n t i a t i o n of t h e f u n c t i o n ~(= 7 + oE) with respect to the log of sucrose c o n c e n t r a t i o n . This p r o c e d u r e involves some error due to the nonideality of the s o l u t i o n s a n d the neglect of t h e change of t h e activity coefficient of N a F [ 16 ]. T h e r m o d y n a m i c d a t a are unavailable for the t e r n a r y s y s t e m N a F + w a t e r + sucrose, b u t t h e sucrose + w a t e r and NaC1 + sucrose + w a t e r systems have been studied in some detail [ 1 7 - - 1 9 ]. If it is assumed t h a t N a F behaves in a similar m a n n e r to NaC1 these results suggest t h a t errors due to this

303 TABLE 1 Coordinates of the pzc of mercury in contact with 0.7953 tool 1-t NaF containing sucrose at 25°C. Potentials are measured with respect to an aqueous 0.7953 mol 1-l NaC1 calomel electrode c/mmol 1-1

--Epzc/mV

7pzc/mN m -1

Cpzc/PF cm -2

0 1 2 3 6 10 21 47 100 215 464 1000

0.481 0.481 0.481 0.481 0.481 0.481 0.481 0.481 0.481 0.481 0.481 0.481

428.0 426.5 426.2 425.8 425.2 424.2 422.2 419.4 416.0 412.5 408.5 404.1

25.39 23.42 22.93 21.99 21.12 20.30 19.36 18.46 17.75 16.92 16.28 15.43

a p p r o x i m a t i o n b e c o m e significant for sucrose c o n c e n t r a t i o n s above 0.5 m o l 1-'. T h e m e d i u m effect o n t h e activity of t h e s u p p o r t e l e c t r o l y t e is negligible in this c o n c e n t r a t i o n range. T h e relative surface excess is s h o w n as a f u n c t i o n of elect r o d e charge in Fig. 3 a n d c o n f i r m s t h e c o n c l u s i o n s derived f r o m t h e c a p a c i t y curves. M a x i m u m a d s o r p t i o n occurs w h e n t h e charge on t h e e l e c t r o d e is zero. The a d s o r p t i o n i s o t h e r m was a n a l y s e d b y s u p e r i m p o s i n g t h e surface pressure ( ~ ) - - l o g c o n c e n t r a t i o n curves on t o p of t h e curve c o r r e s p o n d i n g t o t h e charge of m a x i m u m a d s o r p t i o n b y t r a n s l a t i o n along t h e abscissae [7]. T h e c o m p o s i t e curve is s h o w n in Fig. 4. T h e e x p e r i m e n t a l s c a t t e r is of t h e o r d e r of +0.5 m N m - ' w h i c h is well w i t h i n t h e a c c u r a c y for t h e m e t h o d [7]. A c o m parison of t h e log ( P - l o g c o n c e n t r a t i o n p l o t w i t h a f a m i l y of generalized isot h e r m s shows t h a t t h e results fit a F r u m k i n i s o t h e r m of t h e f o r m : log tic = l o g [ 0 / ( 1 -- 0)] +

a0/2.303

(1)

w h e r e a is t h e i n t e r a c t i o n p a r a m e t e r , / ~ t h e a d s o r p t i o n c o e f f i c i e n t a n d 0(= F/F~) t h e surface coverage. T h e V o l m e r , virial a n d L a n g m u i r i s o t h e r m s were also t e s t e d b u t in every case t h e fit was p o o r . T h e s a t u r a t i o n coverage, F~, o b t a i n e d f r o m t h e F r u m k i n fit was 2.42 × 10 -~° m o l c m -2 w h i c h c o r r e s p o n d s t o a m o l c u l a r area on the surface of 0.68 n m 2. This area is feasible for h e x a g o n a l close p a c k i n g of sucrose m o l e c u l e s if o n e ring is a d s o r b e d flat on the m e r c u r y surface while t h e plane of the s e c o n d ring is inclined at a small angle t o it (Fig. 5). T h e m o d e l was c o n s t r u c t e d f r o m the disp o s i t i o n of the m o l e c u l e in the solid crystal [ 20]. However, the m o l e c u l e is c o m p l e x a n d p r o b a b l y a d o p t s a c o n f o r m a t i o n t h a t is d e p e n d e n t on the struct u r e of the solvent [ 21]. C o n s e q u e n t l y , it is only possible to c o n c l u d e t h a t t h e value o b t a i n e d for F s is of the right o r d e r of m a g n i t u d e , a n d any c o n c l u s i o n relating to the o r i e n t a t i o n of the m o l e c u l e can o n l y be t e n t a t i v e . The i n t e r a c t i o n p a r a m e t e r , a, has a value of +2.0 w h i c h c o r r e s p o n d s t o a repulsive i n t e r p a r t i c l e i n t e r a c t i o n . T h e a d s o r p t i o n coefficient/3 has a value of

304 2"5

lO00mM

•

GO o o 0

•

2"0

@

?EU

o\

1"5

o

@

o[-"-0 I'©

Q

Q

0"5 o

•

, Q

0

12

01~

2mM

.

~tI~

6

Q

O : 011 0

Qtt

0 a

-12 -6 M/pC crl' l -2

Fig. 3. The relative surface excess o f sucrose as a f u n c t i o n o f charge d e n s i t y on t h e m e r c u r y surface at 25°C. T h e sucrose c o n c e n t r a t i o n in m m o l 1-1 is i n d i c a t e d b y each curve. T h e lines were c a l c u l a t e d using t h e F r u m k i n i s o t h e r m (eqn. 1) w i t h a = 2.0, Ps = 2.42 × 10 -1° tool cm -2, ~ = 135 1 mo1-1 and t h e e x p e r i m e n t a l values for f(a).

25

20

~ 15 z E e IO

* % q qPO

0

-4"0

-3"0

-2.0

- I . 0 LOG C + f ( 2 }

Fig. 4. C o m p o s i t e surface pressure curve f o r sucrose a d s o r b e d on m e r c u r y in c o n t a c t with 0 . 7 9 5 3 M N a F at 25°C. T h e line was c a l c u l a t e d f r o m t h e F r u m k i n i s o t h e r m with a = 2.0, Ps--2.42×10 l°molcm -2and~=1351mol -].

305 C6H 2 06 H

C 3 1\ aH

L_.L&~

H C " /L.

1

c

H

,

C~

6'

OH

SI DE VIEW

d'

6 5.

H

side

vIew

0 l

.

.

.

.

0.5 ,

.

.

.

.

l;O

n m

Fig. 5. S c h e m a t i c s t r u c t u r e a n d scale d r a w i n g o f a m o l e c u l e o f s u c r o s e a c c o r d i n g t o l i t e r a t u r e d a t a [ 20 ].

135 1 mol -'. If the adsorption process is treated as a solvent replacement reaction in which an adsorbate molecule from the bulk replaces one solvent molecule at the surface, then choosing standard states of unit mole fraction for the adsorbate and solvent in the bulk and surface phases respectively, the standard free energy of adsorption can be calculated from: = (1/Cs.b)

exp(--~G°/RT)

(2)

where C~.b is the bulk solvent concentration. The reference states correspond to the pure solvent and solute in the bulk phase and to the ideal monolayer in the surface phase. Substituting the value for/3 into eqn. (2), the standard free energy of adsorption at the point of m a x i m u m adsorption (~G~a~) is --22.1 kJ m o l - ' . This is indicative of physical adsorption rather than chemisorption forces. A meaningful comparison of standard free energies for a variety of neutral molecules is severely limited by the assumption t h a t the adsorbate replaces a single solvent species at the interface, as for such a large molecule as sucrose this seems highly unlikely. The success of the Frumkin isotherm in describing the adsorption of molecules of different sizes has been explained by a model in which the adsorbate replaces a water cluster [ 22 ]; indeed cluster models are able to predict fairly successfully the properties of the inner layer [23,24]. However, the success may be due to the intensitivity of the isotherm approach and at present the experimental data are insufficiently accurate to allow the more general isotherms to be tested with confidence. The variation of the standard free energy of adsorption with the electrical variable was obtained at constant potential and constant charge from the hori-

306 zontal shift to superimpose the surface pressure--log concentration curves. In both cases the quadratic dependence characteristic of organic adsorption was found (Fig. 6a, c). According to the model of two capacitors in series [ 25] the variation of the standard free energy is given by

f(o) = log ~ - log ~max

:

--b( aM --

aMax):

(3)

where b = 2 × 2.303RTF~

d~

Co

(4)

and where Co is the capacity corresponding to zero coverage with adsorbate and C~ the capacity at monolayer coverage. This model is satisfactory at negative charge values (Fig. 6b) with b = 0.0085 cm 4 pC -2, but at positive charges the standard free energy is larger than predicted by the model. This trend has been observed in other systems [26 ]. One possibility is that the weak specific adsorption of fluoride ion has some effect of the standard free energy at positive charge values. The effective charge is given as the sum of the electrode charge and the specifically adsorbed charge a ~. The specific adsorption of fluoride ion from aqueous solutions containing sucrose has never been studied, but Schiffrin [ 27] has studied the adsorption of the pure electrolyte at two temperatures (0°C and 15 ° C) and he found that specific adsorption was weak and decreased with increasing temperature. At 15°C and for an electrode charge ( a M) of 10 pC cm -2 the value of o ~ is--4(+1) pC cm -~, whereas at o M = 6 pC cm -2 it is --1(+1 ) pC cm -2. This correction is sufficient to explain the deviations observed at positive charge values and in this respect specific adsorption effects cannot be ruled out. A more quantitative approach would require accurate data at 25°C which are not available at present. The alternative plot at constant potential corresponds to Frumkin's model of two capacitors in parallel for which [6]: f(E) = log ~ -- log ~max ------0~(E - - Emax) 2

(5)

where a = (Co -- C~)/2.303RT

F~

(6)

The model is more successful at predicting the overall variation of the standard free energy than the two-series capacitor model (Fig. 6d, a = 2.76 V-:); however, a simple quadratic result would not be expected from the assumptions of the model. Congruence of the adsorption isotherm implies that Fs is independent of the electrical variable and therefore, a and b will be constant only if the ratio (Co -- C~) or the iatio ( 1 / C ~ - - 1/C0) are constant respectively. This follows because it is an experimental fact that Co varies in a complex manner with the electrical variable. The experimental and calculated surface pressure curves are compared at constant charge in Fig. 7. Similar conclusions can be drawn from the shift of potential caused by the adsorption of sucrose which is plotted in Fig. 8. Here, A~M--2 is the change in rational potential drop across the inner layer and was calculated from: A ~0M-: = E ~ -- ~o=0vb a~e _ V~-~

(7)

307 23

-o.g

(a)

(b)

/

/

/(~)

/@

i

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(c)

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(d) f(E) i

io J

,FI

o21 E

-0.6

h

<) !

lg

- 0.3

p/~ //

Ovalues ÷ ve of E max .

J~

x

17

A

o

=

-

.Jk

o.s

.i

=

-E/v

,

~,

O-

,:o

_vo o, Emax

l i

"

0

" '~ '0-2 (E-Emax '2/'2J/v 0-4

Fig. 6. T h e v a r i a t i o n o f t h e s t a n d a r d free e n e r g y o f a d s o r p t i o n o f s u c r o s e w i t h c h a r g e (a) a n d p o t e n t i a l (c). T h e s a m e d a t a p l o t t e d as a f u n c t i o n o f (O - - oM) 2 (b) a n d ( E - E m a x ) 2 (d). T h e b r o k e n lines are c a l c u l a t e d f r o m eqns. (3") a n d (5) w i t h b = 0 . 0 0 8 5 c m 4 p C -2 a n d a = 2 . 7 6 V -2 r e s p e c t i v e l y .

where ~ - ~ is the diffuse layer potential drop, EoN the measured potential in the presence of adsorbate and EbaSeo=0is the pzc of the base electrolyte. The linear relationship implies congruence of the isotherm with respect to electrode charge and the zero gradient at the pzc corresponds to the charge of maximum adsorption. These plots may be extrapolated to F~ to obtain the potential shift

308

25

lO00mM

20

464mM

k

215mM

zI5 E I OOmM

I0

47mM

\0

lOmM

.~ O,

\

~m

%

•

. 6

12

•

®,

0

-6 oM/pccm2

-12

- Fig. 7. T h e surface pressure o f s u c r o s e as a f u n c t i o n o f charge d e n s i t y on the m e r c u r y surface at 25°C. The s u c r o s e c o n c e n t r a t i o n in m m o l 1-1 is i n d i c a t e d b y each curve. T h e lines were c a l c u l a t e d using t h e F r u m k i n i s o t h e r m with a = 2.0 l~s = 2 . 4 2 × 10 -1° tool c m -2, ~ = 135 1 tool -1 a n d t h e e x p e r i m e n t a l values for f(o).

d~Ccm "2 -I 0 "

-9

-0"5

'~-4 3 -2

e

|

5"

= ~=

-.0-0

~ ~


o

,----e

~-

~ ~ ~

~

-I

~ ~

0 ~

I

~-'----~

1

54 3 76

~J

,,,J

0.5

i.5 IO~°r"/mol

2-5 cn.i 2

Fig. 8. T h e change o f r a t i o n a l p o t e n t i a l d r o p across t h e i n n e r l a y e r u p o n a d s o r p t i o n o f s u c r o s e at c o n s t a n t charge. Charge values are i n d i c a t e d b y each curve.

309

due to a complete monolayer of sucrose. From these values C~ may be calculated. It is found that C1 is constant only at charges <--2 pC cm -~ where it has the value 13.4 pF cm -2. At higher charges it increases steadily to about 30 pF cm -2 at +10 pC cm -2. This variation means that the usual custom of taking Cl as a constant, equal to the slope of the charge--potential curve at m a x i m u m adsorption, is not valid in this system. Parsons [ 28] has derived an equation at constant charge for the relationship between A~M--2 and F based on a quadratic dependence for the standard free energy of adsorption:

([}A~M--2/~}r)oM

=

--RT(a In/3/8o M)

(8)

The gradients of Fig. 8 are related to the derivatives of the log ~ - - 0 "M plots and therefore, a plot of (~}A~M--2/dF)rM vs. o M gives a value for b in eqn. (4). This is shown in Fig. 9. A linear dependence is found in the charge range +1 to --6 pC cm -2 and corresponds to a value for b of 0.0092 pC cm -2. The deviations outside this charge range correspond to breakdown of the quadratic dependence of the standard free energy with charge. The change in potential drop across the double layer due to replacement of water molecules by sucrose molecules may be written [8]: AE = 0 {(P,/e)(p., -- npw) + oe-'(x, -- Xw)}

(9)

where e is the permittivity of the inner region at fixed molecular orientation,

/

,/ o K

2

u

>

[._

~-2

% -4

-6

-8

-I0

-5

0

5 I0 oM/pCcrfi ~

Fig. 9. Plot of the function (c)A~M--2/()r)oM against electrode charge for the adsorption of sucrose from aqueous 0.7953 M NaF at 25°C. The broken line was calculated using eqns. (3) and (8) with b = 0.0092 cm 4 pC -2.

310

p~ and Pw the perpendicular c o m p o n e n t s of the dipole m o m e n t for sucrose and water respectively, n the n u m b e r of water molecules replaced by each sucrose molecule and x~ and x~ the inner layer thickness in the presence of sucrose and water respectively. If the t e r m (F~/e)(p~ -- npw) is i n d e p e n d e n t of electrode charge which corresponds to a fixed orientation for b o t h sucrose and water in the inner layer, t h e n a plot of AE/0 v s . a M will be linear. This is confirmed in the charge range +1 to --6 pC cm -2 in Fig. 10. The gradient (+0.0240 cm 2 pF -~) corresponds to (xs -- x w ) e - ' , and if e is assumed to be 5.3 [23] then (xs -- xw) is 0.11 nm. The thickness of a water molecule in the inner layer is 0.33 nm, and x~ is therefore 0.44 nm. This small value is consistent with the molecular model for sucrose lying flat on the mercury surface; (x~ -- Xw)e-~ is equivalent to (1/C, -- l/C0) and substituting this value into eqn. (4) then b is 0.0087 cm 4 p F -2. If the parameters in eqn. (9) are i n d e p e n d e n t of 0, then from eqn. (8) the charge at m a x i m u m a d s o r p t i o n is given by 0 M =M,(p~ - - n p w ) / ( x ~ --Xw)

(10)

However, m a x i m u m a d s o r p t i o n occurs at the pzc which is equivalent to the condition t h a t the surface potential of a m o n o l a y e r of sucrose molecules is the same as t h a t for the m o n o l a y e r of water molecules replaced u p o n adsorption, i.e. the intercept of Fig. 10 is zero. It was p o i n t e d o u t by Trasatti [29] t h a t this behaviour can also occur if C, = 0. However, this condition does n o t seem to be

O'J

> ~O L~ <3

-04

-0-2

0 . . . .

5

. . . .

6

.....

-s

. . . . .

-,o

o M / p F c rr[2

Fig. 10. Test o f t h e t w o c a p a c i t o r s in series m o d e l f o r t h e a d s o r p t i o n o f s u c r o s e f r o m a q u e ous 0 . 7 9 5 3 M N a F at 2 5 ° C . T h e b r o k e n line was c a l c u l a t e d f r o m e q n . (9) w i t h (Fs/e)(ps - n p w ) = 0 and e - l ( X s - - Xw) = 0 . 0 2 4 0 c m 2 p F -1.

311

valid for this system. As a first approximation C~ can be calculated from the Helmholtz formula with e = 5.3 and xs = 0.44 and a value of 10.7 pF cm -2 is obtained. Increasing evidence indicates that water is oriented with the negative oxygen end of its dipole towards mercury [4,30], and therefore sucrose must be oriented likewise. The adsorption of ethylene glycol suggests that the surface potential of water (g~]~ (dip) at the uncharged surface is of the order of 80 mV [31]. The potential drop across a monolayer of oriented dipoles is ~M)(dip)

=p.~P~/e

(11)

from which it follows that the effective perpendicular c o m p o n e n t of the dipole m o m e n t of sucrose is 2.6 × 10 -30 C m. Franks et al. [ 32 ] have estimated a dipole m o m e n t of 1.5--2.0 × 1 0 -29 C m for several similar compounds in aqueous solution. However, it is not possible to be sure that these calculations will be valid at the surface. Pulidori et al. [ 33] have discussed the interaction coefficient of the Frumkin isotherm in terms of solvent--solute interactions compared with solvent--solvent and solute--solute interactions. The repulsive character of the net interaction observed in the present system may thus be interpreted by suggesting that the solvent--solute attraction in the interphase is weaker than the solvent-solvent and solute--solute interactions. This presumably must arise to some extent because of the orientation of both species in the interphase, because in bulk solution the behaviour is not too far from ideal and in fact may be represented as an ideal solution of sucrose pentahydrate [34]. Similar overall repulsive interaction coefficients were found by Brabec et al. [1 ] in the adsorption of D-ribose and two of its derivatives (although the reported signs are not entirely consistent) so that this type of behaviour seems to be fairly general. In summary the results agree with those obtained by Gouy [ 3] and confirm the conclusion made by Frumkin [4] that sucrose is adsorbed flat on the mercury surface. The low value for the standard free energy of adsorption suggests that the contrasting behaviour at the uncharged mercury and air interfaces is probably due to a different solvent structure at the two interfaces, rather than to a specific interaction of the molecule with mercury. A detailed comparison of the adsorption characteristics of sucrose, thiourea, urea and simple amino acids has been given elsewhere [ 35 ]. According to the old measurements of Traube [ 36,37] besides sucrose, mannitol, lactose and glucose are also negatively adsorbed at the air--water interface and these systems provide scope for further study. Mannitol, xylose and pentaerythritol have been investigated in this laboratory and these results will be published in a later paper. ACKNOWLEDGEMENT

We are grateful to Professor Trasatti for helpful comments and to the Science Research Council for a maintenance grant to R. Peat. REFERENCES

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