J. Electroanal. Chem., 103 (1979) 399--408 399 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

SURFACE PHASE AT THE IDEAL POLARIZED MERCURY ELECTRODE

PART III. THE INTERFACE OF MERCURY-KPF6 SOLUTION OF PROPYLENE CARBONATE

NGUYEN HUU CUONG, A. JENARD and H.D. HURWITZ

Universitd Libre de Bruxelles, Facultd des Sciences, C.P. 160, 50, avenue F.D. Roosevelt, 1050 Brussels (Belgium)

(Received 21st March 1979)

ABSTRACT

Electrocapillary measurements with a 0.1 mol kg -1 KPF 6 solution in propylene carbonate at the Hg electrode are reported at different temperatures. The charge densities vs. potential obtained by means of a coulostatic injection technique agree well with the values measured by the conventional a.c. bridge method. The inner layer differential capacity as a function of the charge density displays a water-like behavior. The inner layer adsorption entropy passes through a maximum at a negative charge of the electrode. This reflects a preferential orienta- tion of the solvent with the carbonate group towards the metal at the p.z.c. However con- trarily to the case of water, the molecular arrangement in the adsorbed layer of propylene carbonate seems to be more disordered than in the bulk.

INTRODUCTION

It has been suggested in some recent works [1,2] that the presence of clusters of molecules in the solvent monolayer at the mercury electrode surface can account, at least qualitatively, for some features of the electrocapillary behavior of water. Whether, however, the existence of adsorbed clusters might be specifi- cally linked to the intermolecular hydrogen bonding structure of the adjacent liquid phase is still open to question. In order to tackle this problem, it seems of highest interest to investigate the adsorption properties of some aprotic sol- vents, since most of them are unassociated in the bulk.

The present work focuses at tention particularly on the thermodynamic quan- tities related to the adsorption of propylene carbonate (PC) at the polarized mercury surface. It leads to an investigation of the structure of the electrode solution interphase similar to that published earlier on water [3,4], where the entropies of adsorption of the solvent have been evaluated as a function of the charge density. Hills and Reeves [5] have shown that no specific adsorption of KPF6 is detectable on mercury in PC. Therefore, this salt has been used as a ref- erence electrolyte for electrocapillary investigation, thus playing the same role as NaF in water.

EXPERIMENTAL

Reagents

The KPF6 was dissolved in distilled acetone, filtered, precipitated, twice from acetone with distillated ether, then recrystallized from acetone and dried for an

400

extended time under vacuum over P2Os. This operation was done in a PTFE con- tainer and in a dry box.

Other reagents and solution preparation have been described in ref. 6.

Cell

The cell and conditions of thermostatisation are the same as in ref. 3. The ref- erence electrode is the calomel electrode [6] Hg/Hg2C12/(C2Hs)4NC1, 0.5 mol kg -1 in PC (0.5 M CE). The differential capacity was measured using an a.c. bridge in conjunction with a dropping mercury electrode. The mercury flow rate was about 0.57 mg s- ' at 7.0 s drop age (the total life was about 16 s).

The potential of zero charge (p.z.c.) was found using a streaming mercury elec- trode [7].

Interfacial tension was determined by drop time method based on the value of 379.0 × 10 -3 N m- ' in a 0.1 M KPFa solution at the electrocapillary maximum at 25°C [51.

Measurements of the temperature coefficients of the reference electrode po- tential Eg ,Hg have been performed in ref. 6. The variation of the 0.5 molal calo- mel electrode potential with temperature fits the equation

V - - V2s = 0.5303 (t -- 25) + 0.0012 (t -- 25) 2 (1)

(Vis in mV and t in °C).

RESULTS AND DISCUSSION

(a) Differential capacity measurements

The frequency effect on the differential capacity measurement is very seldom mentioned. Dzhaparidze et al. [8] have found that in ethylene glycol solvent the dispersion of values with frequencies was inferior to 3--4% in the range 400--1200 Hz. Their results were presented for the lowest frequency (400 Hz) only.

Our measured capacities are considerably influenced by the frequency in the more positive potential region (above 200 mV v.s. 0.5 M CE). We observe that the effect decreases with increasing water concentration. This dependence suggests a case of diffusion control, where the adsorption process of water leads to an additional capacity or adsorption pseudo-capacity. Frumkin and Melik- Gaikazyan [9,10] have shown that, if the rate determining step in the adsorption of the neutral solute is diffusion, the additional capacity will become equal to

( Ow 1 Cadd(CO, Cw) ---- \-~W]E \~-]c w ---- Cadd(O9 ---- O, Cw)

where

co = 2~f

and

M= x/-~w (arw/aCw)E

2(V-~/M + 1)

(2x/-~IM + 1) 2 + 1 (2)

(3)

(4)

401

Ac/pC m~

0.5

450/mV vs, 0.5 i~l CE - - ~ 10-3m

3 0 0 ~ I I I I

15 20 25 30 V T "

Fig. 1. Var ia t ion of the d i f fe rence AC = Cmes(f ) - - Cmes ( 1 0 0 0 Hz) as a f u n c t i o n of square r o o t of f r equency f a t d i f f e ren t wa te r c o n c e n t r a t i o n s c w and e lec t rode po ten t i a l s a t 25°C.

In eqns. (2), (3) and (4), Dw, 0w, P~ and c~ are respectively the diffusion coeffi- cient, surface coverage, surface concentrat ion and bulk concentrat ion of water; f, qa and E are respectively the frequency, the density of charge and potential of the electrode.

We have used a plot of our measured capacities Crees versus q~ at various values of c~ and v ~ i n order to evaluate Cadd(W, c~) as a function of potential, fre- quency and bulk concentration. It appears from these data that M ~ 12.5 s - in and is only weakly frequency dependent (Fig. 1).

For any Crees(co, c) -- Cadd(~, C) a value of C = (aq~/aE)¢w independent of frequency and water content in the domain 400--2000 Hz can be computed. At small water concentrat ion (cw < 10 -2 M) this value corresponds to the capaci- tance of an adsorbed monolayer of solvent where (aO~/aE)~, has been kept equal to zero. That is t an tamount to saying that C yields the differential capacity of a PC solvent monolayer at the Hg electrode surface at constant and very low coverage of water molecules, frozen in their state of adsorption.

(b ) Consideration on electrocapillary data

Electrocapillary curves for 0.1 M KPF6 solution in PC were determined at 15, 25 and 35 ° C. Some of these data are given in Tables 1 and 2.

Comparison of potential-charge curves obtained from integration of C and from direct measurements by means of coulostatic injection [3] are shown in Fig. 2. With the first method, measurements are accomplished at constant potential after 7 s drop age. In the second method the potential is taken at 5.8 ms after the injection. Since both methods produce nearly identical curves, although the adsorption equilibrium of water should not be established in the second case, it might be inferred that negligibly small traces of water are adsorbed in our sys- tems. The same conclusion can be reached from comparison of the value deter- mined for the p.z.c, by back integration of the C curves starting at a very nega-

402

TABLE 1

Electrocapillary data at the mercury electrode in 0.1 mol kg -1 KPF5 Error on C +- 0.1 pF cm -2, on qa within + 1%, on 9" within ± 0.2 mN to +-1.0 mN m -1 at extreme negative charges

PC solution at 25°C m -1 at p.z.c, and increase

EImV vs. CIpF cm -2 qC~lpC cm -2 9'/raN m -1 0.5 M TEAC CE

+700 22.7 +19.4 296.0 +600 22.4 17.1 314.3 +500 23.2 14.9 330.3 +400 24.0 12.5 343.9 +300 24.2 10.1 355.2 +200 22.8 7.71 364.1 +100 20.7 5.53 370.9

000 18.5 3.57 375.2 --100 16.5 1.83 377.9 --200 15.1 +0.257 378.9 --217.2 14.9 0.00 379.0 --800 14.3 --1.21 378.5 --400 13.9 --2.61 376.6 --500 13.4 --3.97 373.3 --600 12.8 --5.28 368.6 --700 12.0 --6.52 362.7 --800 11.2 --7.68 355.6 --900 10.5 --8.77 347.4

--1000 9.87 --9.78 31~8.1 --1100 9.31 --10.7 327.8 --1200 8.88 --11.7 316.6 --1300 8.56 --12.5 304.5 --1400 8.33 --13.4 291.6 --1500 8.23 --14.2 277.8 --1600 8.52 --15.0 263.2

t i ve c h a r g e a n d b y u s ing a s t r e a m i n g m e r c u r y e l e c t r o d e . T h e d i f f e r e n c e o f p .z .c .

b e t w e e n t h e t w o p r o c e d u r e s is less t h a n 1 m V .

T h e i n n e r l a y e r c a p a c i t y C i was c a l c u l a t e d b y m e a n s o f t h e r e l a t i o n :

I/C = 1/Ci + I/C d

and assuming the absence of specific ionic adsorption. Figure 3 represents the variations of C i as a function of charge density q~ for

different temperatures.

TABLE 2

Coordinates of electrocapillary measurements for solution of 0.1 mol kg -1 KPF5 in PC

Temperature/°C Potent i~ /mV vs. 0 . 5 M Surfacetension TEAC CE a t tempera ture T 9'/mN m -1

15 --216.6 381.7 25 --217.2 379.0 35 --217.7 376.1

q~IpC cm -~

I

403

0 c o . l ~ c~pAc+

-5

-10

-15

0 -500 -I000 -1500 EAS/mv vs. 0.5 m EC

Fig. 2. Charge dens i ty as a func t ion of potent ia l measured by coulos ta t ic and bridge me thods .

One sees that the curves resemble those obtained in the case of NaF aqueous solution with a " h u m p " in the positive region and two common points at which the differential capacity is independent of temperature. The temperature coeffi- cient of the differential capacity happens to be much smaller than in the aqueous c a s e .

(c) Determination of the molar adsorption entropy of PC

The calculation procedure is identical to that used in the previous paper [3]. We have represented the adsorption entropy (or the inner layer formation entropy) by

S a = PC F P C ( S ~ C - - 8~C)T,p,q°l m

where Fpc is the surface density of PC in the monolayer, ~a PC and S~c are respec- tively the molar surface entropy and partial molar ent ropy of PC in the bulk. The variation of the adsorption entropy of PC as well as the theoretical contri- but ion of the diffuse layer Fs,d(K ÷, PF~) to the formation ent ropy of the sur- face phase are depicted in Fig. 4. The adsorption entropy culminates at about --5.3 pC cm -2 and presents two inflexion points at +14 and --14 pC cm -~.

It is easy to show that the charge density at these two inflexion points is not dependent on the accuracy of the entropy measurements since their coordinates must correspond to the value of q~ at which the differential capacities are tern-

404

$l - CI/pF ¢m-2

21

I!

II

35°c ~ 15°c

0 +20 +10 0 -I q'/pc m -2

Fig. 3. Inner layer differential capacity as a function of charge densities at 15 and 35°C.

perature independent. This condit ion can be readily proved as follows using for this purpose the following relations derived in ref. 11.

( 8 ~/a T)p, m,qa -- qC~E~r'Hg = l~pc(L~l~C - - S ~ C ) T , P , q O l m - - Fs,d(K ÷, PF~)

(a~/aq~)p,m,T = E

where

~ = 7 + q ~ E

and

(5)

(6)

(7)

q ~

Fs,d(K +, PF~) = - - y (~(~(~)/(}T)p,j,a do (8) 0

In these equations 7 is the surface tension, ¢(5), the potential at the outer Helmholtz plane, m the molality and J the ionic strength of KPF6.

We can differentiate eqns. (5) and (6) with respect to q% thus obtaining the condit ion of inflexion points:

I "O "

L ~ J r.P.., ~ J T.P.,,

405

7 ~T p q~ mjp T m

1

The existence of a maximum of adsorption en t ropy at a negative charge of the electrode expresses a preferential orientat ion of the PC molecule at the Hg sur- face such that at the p.z.c, the dipole turns its negative pole towards the elec- trode. It is thus the carbonate group as sketched in Fig. 5 which is bound to the metal. The adsorption en t ropy maximum is about 29 X 10 -s J m -2 K -~. I t has already been stressed [11 ] that the quant i ty 1-'pc(S~c- Sgc) contains implicitly the contr ibut ion o f FHg(SHg - - 8Hg), related to the part of the interphase on the mercury side (phase ~). We have assumed that this contr ibut ion is invariant with q~ and is equivalent to the temperature coefficient of the surface tension of mercury at equilibrium with its vapor phase [3]. Under these condit ions

) P,T, m,q =/J K-' m~ 2

Fsj

' ° i - - T ' ' ' ' '

q~/pc m -2

Fig. 4; Entropy of formation of the inner layer and diffuse layer as a function of charge den- sity at 25°C. Estimated errors are represented on the curve.

406

a b

Fig. 5. Or ienta t ion of the p ropy lene ca rbona te molecule on the mercury surface.

['Hg(S~g - - ~g) becomes equal to 21 × 10 -s J m -2 K -1 and we obtain for the adsorption entropy of PC at the maximum a value of about 8 × 10 -s J m -2 K -~.

(d) Evaluat ion o f the molar surface excess o f p ropy l ene carbonate

The most simple hypothesis is to consider the average density of PC to be constant from the bulk to the surface. This yields an average value of NAvFpc = N~c equal to

l~p c = (NAp~W) 213 ~- 3.68 × 1018 molecules m -2

where NAv is the Avogadro constant, p and W respectively the density and mole- cular weight of PC.

A more physical evaluation of Fpc can be based on Courtauld's molecular model (Fig, 5).

One observes that two positions of the molecule are plausible, both with their carbonate group oriented towards the metal. The area occupied by each type of position has been assimilated to an equivalent circular surface. If these surfaces are ordered in an hexagonal compact arrangement, the position " lying" (Fig. 5a) yields N~p c = 2.76 × l0 Is molecules m -2 and the position " u p " (Fig. 5b) yields N~c = 4.07 × 1018 molecules m -2. It should be noticed that these positions pre- sent practically the maximal and minimal values of the projected area for all possible positions of the molecule.

According as to whether we put the molecules on position " lying" or "up" , the molar adsorption entropy at the maximum (S~,c - - 8~C)max becomes respec- tively 18.5 J K -1 mol- ' or 12.5 J K -~ mol- ' .

These values for PC contrast with those derived from a similar evaluation made on water [3], where the maximum molar adsorption entropy was nega- tive. The positive sign of the maximum entropy of adsorption in the case of PC seems to indicate that the molecular arrangement of solvent is more disordered in the adsorbed monolayer than in the bulk, at least in the region of the p.z.c. This finding is quite convincing and again focuses at tent ion on the specificity of water and its ability to form ice like hydrogen bounded molecular clusters in the vicinity of the mercury surface which acts partially as a hydrophobic macroion.

407

0 -301

II o 3, "-o _,,,

51 / C+132 , C:, +50

-6 ~ H ~ H - 37~i~H -7 -12 H

Fig. 6. Electron density distribution (in cule [12].

0 -3 electron unit) of the propylene carbonate mole-

As regards PC, on the contrary, the molecules adsorbed in the monolayer, which are oriented predominantly near the p.z.c, as indicated in Fig. 5, are linked through weak Van der Waals forces with the adjacent molecules of the bulk and might behave quite independently in the electric field. If one refers to the elec- tronic density distribution of PC molecules (Fig. 6) [ 12 ], a considerable electro- nic density is found on the oxygen atom in the ring and in the carbonyl oxygen atom. Consequently, the overlapping of the electronic orbitals with the metal should increase the probability to encounter the "lying" position compared to the "up" position at the p.z.c, and on the anodic side of the entropy maximum. This lying position exhibits the largest projected area. The position "down", which is the opposite of position "up", in Fig. 5b, has been taken into account recently with position "up" and "lying" in a three state model of adsorption of aprotic solvents [13]. It would arise predominantly on the cathodic side of the adsorption entropy maximum. It presents the same projected molecular area as position "up". The occurrence of the position "up" is not evident from the herein reported changes of entropy of adsorption with q% It would be expected at very positive charge densities. The weakly marked inflexion point on the anodic side seems however to denote a lesser extent of orientation polarization than in water.

The projection of molecular dipole moments on the perpendicular to the elec- trode surface corresponding to position "lying" (where the molecule ring "lies" on the electrode surface) and "down" (where the C= O bond is perpendicular to the surface) are respectively 0.94 D and 4.91 D.

4 0 8

C O N C L U S I O N

The results here described show that the general morphologies of the adsorp- tion entropy curves plot ted against the density of charge at the mercury elec- t rode are quite similar for PC and water adsorbed in the inner Helmholtz layer.

The most essential differences consist in the positive value of the maximum adsorption entropy of PC and the more weakly marked inflexion points.

The recorded inner layer differential capacity curve as a function of qa pre- sents also a similar shape for PC and water. Parsons [14] recently has extended to other solvents his model which initially referred to water [15]. Thereupon, he classified the solvents into three groups with regard to the experimental differ- ential capacity curves.

(1) the curves displaying a hump and two minima on each side (water-like sys- tems).

(2) the curves showing only one hump. In this category, Parsons included aprotic solvents like PC.

(3) The curves presenting an inverted parabolic form (water-unlike systems). A recent publication of Borkowska [16] and our own data seem to indicate

that the classification of Parsons might be inappropriate for some solvents. In the case of PC it seems that the classification has been based on measure-

ments which are performed in a too narrow voltage range and which might have been complicated by the specific adsorption of the electrolyte.

R E F E R E N C E S

1 B.B. Damaskin and A.N. Frumkin, Electrochim. Acta, 19 (1974) 173. 2 R. Parsons, J. Electroanal. Chem., 59 (1975) 229. 3 Nguyen Huu Cuong, C.V. D'Alkaine, A. Jenard and H.D. Hurwitz, J. ElectroanaL Chem., 51 (1974) 377. 4 J.A. Harrison, J.E,B. Randles and D.J. Schiffrin, J, Electroanal. Chem., 48 (1973) 359. 5 G.J. Hills and R.M. Reeves, J. Electroanal. Chem., 38 (1972) 1. 6 Nguyen Huu Cuong, A. Maiornikoff and H.D. Httrwitz, J. Electroanal. Chem., 72 (1976) 107. 7 D.C. Grahame, R.P. Larsen and M.A. Poth, J. Amer. Chem. Soe., 74 (1952) 1207. 8 D.I. Dzhaparidze, G.A. Tedradze and Sh.S. Dzhaparidze, Elekt rokhimiya , 8 (1968) 955. 9 A.N. Frumkin and V.I. Melik-Gaikazyan, Dokl. Akad. Nauk SSSR, 77 (1951) 885.

10 V.I. Melik-Gaikazyan, Zh. Fiz. Khim., 26 (1952) 560. 11 H.D. Hurwitz and C.V. D'Alkaine, J. Electroanal. Chem., 42 (1973) 77. 12 H.L. Yeagex, J.D. Fedyk and R.J. Parker, J. Phys. Chem., 77 (1973) 2407. 13 W.R. Fawcett , J. Phys. Chem., 82 (1979) 1385. 14 R. Parsons, Electrochim. Acta, 21 (1976) 681. 15 R. Parsons, J. Electroanal. Chem., 59 (1975) 229. 16 Z. Borowska, J. Eleetroanal. Chem., 79 (1977) 206.