The electrosorption valency of organic electrosorbates

The electrosorption valency of organic electrosorbates

141 J. Electroanal. Chem., 229 0987) 141-164 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands THE ELECTROSORPTION VALENCV OF ORGANIC EL...

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141

J. Electroanal. Chem., 229 0987) 141-164 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

THE ELECTROSORPTION

VALENCV OF ORGANIC ELECTROSORRATES

PART II. AROMATIC AND HETEROCVCLIC

COMPOUNDS

*

D. ROLLE and J.W. SCHULTZE Institut fiir Physikalische

Chemre II der Universittit Diisseldorl,

Universitiitsstr.

I, 4ooO Diisseldorf (F. R.G.)

(Received 25th February 1987)

ABSTRACI Thermodynamic data (electrosorption valency y and its potential dependence au/&S, the Gibbs energy of adsorption AGad and others) are collected for the electrosorption of aromatic molecules at mercury. Two sets of data are obtained for the flat ( II) and perpendicular ( _I.) orientation, respectively. For most substances the flat orientation dominates at low surface concentrations FA < 3~10-‘~ mol/& and positive potentials. A charge transfer from the adsorbate to the metal is obtained which contributes 50 kJ/mol per transferred electron to the Gibbs energy of adsorption. Increasing A” diminishes the potential dependence 8y “/&S and increases the potential of maximum adsorption EA. The increasing charge of the adsorbate causes lateral repulsion between the molecules. In the perpendicular orientation, yk increases with the dipole term K,. Water desorption gives a small contribution but the charge transfer is negligible. Reorientation processes are determined by the potential and surface concentrations. General rules are derived and a-picoline and phenylthiourea are discussed as examples. Ionic adsorbates have yN values determined mainly by the charge z. Charge transfer depends on the electron density in the ring: compared with the neutral molecule, it is smaller for cations and larger for anions. Heterocyclic compounds behave similarly, but the charge transfer is smaller than for aromatics.

(I) INTRODUCTION

The electrosorption of organic compounds on electrodes is of interest not only for fundamental research [l-4] but also for electroanalysis [5], electrocatalysis and inhibition [6], polymerization [7,8] and photo processes [9]. In general, there is a strong difference between aliphatic and aromatic compounds. The adsorption of aliphatic compounds depends mainly on the chain length

In honour of Professor H. Gerischer on the occasion of his retirement as Director of the Fritz-Haber Institute.

l

0022-0728/87/$03.50

0 1987 Elsevier Sequoia S.A.

142

water

A

B

Q.0’

Q= 180”

C lp=90”

0 + i Fig. 1. Schematic potentials.

t

t

+

metal representation

of various orientations

of aromatic

molecules

on mercury at positive

and the functional group, which determine the double-layer thickness and orientation. Thus, the charge flow, the electrosorption equilibrium and the electrosorption valency can be analysed in terms of the minimum capacity Cti, the maximum surface concentration r, and the charge and dipole of adsorbed substances [lo]. Only a few systems show complex behaviour caused by reorientation and charge transfer. For aromatic substances, on the other hand, reorientation processes become dominant, as can be concluded from the shape of the adsorption isotherm or the capacity [ll]. This has been shown recently with phenol on mercury as an example [12]. Figure 1 gives a schematic model of the double layer with adsorbed molecules. Water and ions form the outer Helmholtz plane. Three borderline cases for the orientation of an aromatic compound have to be distinguished: the functional group X towards the metal (A), the solution (B), or flat alignment (C). The angles of the marked dipole vector are added. Therefore, two sets of data for the perpendicular ( _L, usually model B, but with varying dipole orientations) and the flat (II , model C) orientation have to be analysed. Special interests arises from the nature of “r-bonding” at positive potentials mentioned sometimes in the literature but never specified [13]. Chemical bond formation has been postulated also for adsorption from the gas phase, and correlations between adsorbability and work function changes have been discussed [14]. The concept of partial charge transfer was first introduced by Lorenz and Kriiger [15] but neglecting the dipole contribution the charge transfer has been overemphasized [16]. Taking into account both terms, the first reliable estimation was carried out with phenol and some other substances on mercury [12]. On the basis of the theoretical concepts for electron-transfer reactions derived by Gerischer [17], a correlation between the charge-transfer coefficient X and molecular data such as the oxidation potential E,,, the ionization potential I and the electron affinity A was derived [12].

143

Since our recent paper was confined to the analysis of charge transfer, we want to discuss now the general behaviour of aromatic substances adsorbed on mercury. Owing to the smaller accuracy of measurements at solid electrodes, other metals are not discussed here. General aspects will be discussed preferentially, and special or unreliable data will be mentioned in the Appendix. (II) THERMODYNAMIC

DESCRIF’TION AND MICROSCOPIC

MODELLING

In the following summary and analysis of experimental data of aromatic compounds we will discuss macroscopic, thermodynamic values, e.g. the electrosorption valency yN, the Gibbs energy of adsorption AGad, the maximum surface concentration I,,,, the minimum capacity C,,, electrode potentials of maximum adsorption E,,, and the shift of the potential of zero charge EL. We will then try to correlate these with molecular properties such as dipole terms K, the charge-transfer coefficient X and the desorption of Y water molecules. Before that, we will show the theoretical connections between various macroscopic and microscopic approaches. For an extended discussion of yN [18], thermodynamic derivatives and microscopic data, see refs. 10, 12 and 19. Frumkin was the first to present an equation in 1926 which correlates the parameters charge a,,,; E, referred to E, = 0; capacities C, ((3 = 0), C, (8 = 1); degree of coverage 0 = I/I’,,,; and shift of the potential of zero charge EL [l]: elII = C,,E - C&‘E + C#E - C,E;B

0)

The dependence of a,,, on I can be described by

-(auJar).=g=

((co- c,)E+ c,E#r,

For the potential of zero charge E,, YN

=

(2)

we obtain yN: (3)

GE;/Frm

Another important limiting case of eqn. (2) is y = 0, which yields the potential of maximum adsorption E,:

Em= -c&/(co

- cl> = -yNFrm/(cO -

Cl)

(4)

In the following decades, other formulations of the surface layer were proposed: Parsons’ [20] and Hansen’s [21] model and a generalized equation from Damaskin [22]. In terms of yN, it can be derived as follows: Parsons’ equation :

YN = E;C:/Fr&,

(5)

Hansen’s equation:

yN = EA(2Cr - C,)/Er,

(6)

E&C, (1 + n(1 - k)) yN --- F r

(7)

Generalized surface layer equation (GSL)

m

n

Damaskin [22] discussed extensively the physical meaning of the different formulae. It is easy to see that Frumkin’s (n = 1, k = l), Parsons’ (k = 1, n = Co/C,) and Hansen’s equation (n = 1, k = Co/C,) are special cases of eqn. (7).

144

A great problem in electrosorption is to link the experimental, macroscopic parameters (of eqn. (2) for example) with the molecular, microscopic properties of the adsorbed substance. The first approaches consider the dipole contribution only. The potential drop at em = 0 is given by the dipole drop Edit,, Edlp = W,m,/W&

(8)

which can be combined with the condenser formula

Equating the shift of EL from eqn. (3) with the dipole drop Edip, one finds that yN equals the dipole term K: yN

m,/el, =

=

K,

(10)

Neglecting the contribution of desorbed water molecules (i = adsorbed molecule S), yN is given only by the dipole term K, of the molecule. On the basis of the GSL eqn. (7), we obtain YN=

KS/n

+

K,(l

-

k)

(11)

Recently, Schuhmann has discussed eqns. (8)-(10) in great detail and very profoundly [23]. Although eqn. (10) fits the electrosorption data of some aliphatic alcohols, the limitations become obvious if one looks at the experimental yN values for the flat adsorbed molecules (Table l), with the dipole moment perpendicular to the field (m, cos ‘p = 0), e.g. pyridine yj!, = 0.05. The disadvantage of eqns. (Q-(10) is that both the influence of desorbed water molecules and the possibility of charge transfer are ignored. As an extension of the models given by Butler [24], Lorenz and Kruger [15] and Parsons [25], Vetter and Schultze derived eqn. (12) for any charged or neutral molecule [ 18] : yN=gZ-h(l-g)+K,-VK,

(12)

where g is a geometric factor, z is the charge of the adsorbate, X is the chargetransfer coefficient, v is the number of desorbed water molecules and K~, K, are the dipole terms of molecule S and water given by eqn. (10). From thermodynamic principles the potential dependence of y was derived: Y=

YN

-

1/F’~E(i3C/81-),

dE

(13)

EN

where C is the high frequency capacity at constant coverage. The Frumkin model (eqn. 1) and the following eqn. (2) give the same result for the potential dependence of y only if (Co - C,)/I’,,, = -(Z/i3I’),. Further, it was shown that y is defined by two additional derivatives [18]: ye=

(it~~,/ii~),=

where ps

is

-(abm/ar>,=

(~~AG,,~IE),

the chemical potential of the adsorbed substance in solution.

(14)

145

By integration of eqn. (14), we obtain the Gibbs energy of adsorption: AG,, = AG$ + yN FE -

E(X,/W), JJ EN

which can be determined experimentally AG,, = -RT

ln(O/(l

dE dE

(15)

since

- 0)) - RT ln(55.5/c,)

(16) where c, is the concentration of S in the solution. It can be correlated with the Gibbs energy of dissolution, as will be discussed in Section (111.5): AG,,, = - RT ln( c,,,/55.5) With the simple assumption parabola for AGad( E ): AG,, = AG; + yNFE + 1/2((C,

(17) of a linear decrease of the capacity, we obtain a - C,)&)(

E - E,)’

(18)

which will be discussed in Section (IV). For some substances, y shows a complex dependence on I and E. We take three typical substances as examples. Figure 2a shows the potential dependence of y for imidazole [26], which is nearly linear and does not change with the surface concentration. The behaviour of benzoic acid [27] in Fig. 2b differs, showing two y/E functions for small and large coverages, which means that we have to discuss

* -0.4

T‘

-0.2 c)Pyridine

-o.l{ -06

-04

0

02

.

,

-02

0

)

(E-E,YV

Fig. 2. Potential dependence of the electrosorption valency for (a) imidazole, (b) benzoic acid and (c) pyridine. Data derived from refs. 26-28; surface concentrations as indicated.

3.3 3.5

Acetanilide ‘ Cinnamaldehyde ’ Pyrocatecbol

Hydroquinone Phlorogiucinol Pentafluorophenol ’ Phenyltbiourea ’

3.7

Benzotitrile

-0.044

0.28 0.34 0.35 0.08

0.33

-0.10

14.0

3.8

-0.25

-0.20 0.33 -0.18 0.30 0.00 0.50

-0.13

0.21

0.00

-0.04 0.45 0.00 0.90

-

- 0.075 -

-

0.14

- 0.036 - 0.020

/V cm’ mol-’

(aE;/ar)

0.18

0.06 0.12 0.16 0.19 0.09

x~~

-



0.05

0.45 0.35 0.45 0.35 0.50

/v-

(ayl~/ivz)

0.03 0.45

0.01 -0.03 -0.06 -0.10 0.00

v;

10.8 10.0

13.0 10.5

6.0/5.0 5.7 7.0 8.2 S,9/6.3

/uF em-’

c lIl,”

2.8 2.0 4.8

5.2

5.0/5‘4 4.4 5.9 4.5

Benzene Toluene Phenol Aniline r Benzoic acid

/mol cms2

1o’O r m

Adsorbate 0.42 0.50 0.48 0.51 0.56

/rim2

Area

0.43

7.96 d 0.52 7.90 ’ 0.65 * 9.1 b 0.53

8.02 d 0.54

8.39 c 0.66 - 8.4 b 0.71

9.62 b 0.48

9.24 b 8.82 b 8.52 b 7.68 b 9.73 c

Z/eV

Data for the electrosorption of flat adsorbed aromatic and heterocyclic compounds on mercury a

TABLE 1

E; 0.03 0.10 0.14 0.27 0.02

/kJ mol-’

-

- 0.05

0.40

24

f-1 30.5

34 24.5

r=x

20.0

11.1 14.4

6.5

17.7 21.1

19.7

19.7 23.4 10.0 12.3 18.8

AGw,

- AG;, ,&I mold’

24 25 29 r = 2.2 -0.06 25 I‘=1 0.10 0.00 34

/V

47

45 46

44

36

42 43

13

16, 33, 34 33-3s 21, 13, 36, 37 35,38-40 27,41

Ref.

0.11 0.11

_ 0.02 0.30 _ -0.03

6.4

6.1

6.6

7.2

3.2

2-Methylquinohne I-Methylquinohne J-Methylisoquinohne ImidazoIe 0.35

_

-

0.02 0.50

0.10

0.09

- 0.017

-0.006

0.003

0.010

- 0.006

0.007 0.013 -0.004

-0.060 0.006 0.009

9.50 b 0.32

0.70

0.70

0.70

8.54 b 0.62

8.62 b 0.61

0.57 9.30 b 0.40 9.30 b 0.48 9.02 = 8.85 c 0.57

0‘12

-

-0B9

-

-0.06

-0.11 -0.13 -0.08

0.11 -0.12 -0.11

17

-

-

26 33 f =i 2.4 35 i’ = 2.3 -

31 14 20.5

2.0

-

-

-

17.8

7.8 13.2 17.3

20.2 7.2 9.0

26

57

5s

55

5356

28 28 53-55

48,49 28,50-52 28

’ J?, is the maximum surface concentration; C,, is the minimum double-layer capacity; yJ is the electrosorption valency at EN; gy “/iIE is the potential dependence; h” is the ch~ge-tr~f~ coefficient; fJE&/ar is the shift of EN with I’; i is the first ionization potential of a n-electron; EA is the potential of maximum adsorption; AC; is the Gibbs energy of adsorption for r -+ 0 mol cmw2 or for given 1Oro r, values from eqn. (16); AC& is the Gibbs energy of dissohnion 1581,eqn, (17). All potentials are referred to EN. b From ref. 29. ’ From ref. 39. d From ref. 31. ’ From ref. 32. FSee Section (V.l).

5.8

4.5

0.04 0.04 0.08

Isoquinoline

~~0~~

0.06 0.50 0.07 0.55 0.03 0.51

0.12 0.00 0.02

5.4 5.7 5.8

-0.05 0.45 0.06 0.50 0.07 0.60

2,4-Lutidine 2,4,6-Cohidine

11.5 5.0

3.4 5.9 6.1

salicyhc acid Pyridine a-Picoline

148

two different orientations. The situation becomes more complex if we take pyridine [28], which undergoes a step-like break (Fig. 2~). The arrangement of pyridine depends on both the coverage and the potential. (III) COLLECTION OF DATA

(III. 1) Principles

The principles of data collection are the same as those described in Part I [lo] and refs. 12 and 19. Various aromatic electrosorption systems have been analysed in the literature and presented as sets of data u&E, p), I’(E, p) or others. From these data we evaluated the electrosorption valency y(E, I) as shown in Fig. 2, for example. Such plots yield yN, E,,, (for y = 0) and ay/aE. Further, we obtained aEk/aI’, AGad and, from further calculations in Section (111.2.2), h. As can be seen from the examples shown in Figs. 2b and 2c, we obtained for many substances, two sets of data which will be given in Tables 1 and 2 for the flat and the perpendicular orientation, respectively. In order to determine the molecule’s arrangement, we used the following arguments, as has been shown for phenol as an example [12]. (i) Cti,: We expect a minimum double-layer capacity of C,,& I: 5-6 PF cm-2 and CA, = lo-15 PF cmp2. (ii) I,: The maximum surface concentration should be I’: = 2-3 x 10-i’ mol cmp2; I,’ = 5-6 x lo-” mol cm-‘. (iii) yN: If water desorption dominates, yN is positive. This is always observed for the perpendicular orientation. A negative value, on the other hand, could be due to the replacement of water by rfXh32U~eS possessing a larger dipole term K, > VK, and oriented in the same direction. This does not occur with most of the substances of Table 1. But a charge transfer X > 0 due to “R-electron interaction” causes yN < 0, which is often observed with flat adsorbed aromatics. (iv) Another conclusive argument is the dependence of y on both E and r. Usually the flat orientation is observed at low coverages I < I$, but the perpendicular orientation has to be assumed for I > I’:. Sudden changes of y near Ii indicate a reorientation from flat to perpendicular. The thermodynamic conditions of reorientation will be explained in more detail in Section (IV). (III.2)

Flat orientation

(111.2. I) General data

We will now discuss the general features of the adsorption data given in Table 1. The limiting values of I,,, and C_ are given in the first two columns because some values refer to the perpendicular orientation. The y: values vary in the surprisingly large range of -0.2 to +0.07. This is in clear contrast to flat adsorbed aliphatics (yN = 0.04) and indicates that A > 0, as is shown later.

149

The potential dependence around E, is positive: 0.45 f 0.15 V-‘, but it is a function of yi itself; the more negative the electrosorption valency, the smaller the slope. With regard to eqn. (13), this means that there is a smaller decrease of the double-layer capacity. It will be shown in Fig. 8 that this effect is correlated with the charge transfer X”. According to eqn. (3), y$ and aE&/aI’ should have the same sign. This expectation is fulfilled within the limits of error; deviations occur only near yN = 0. Ionization potentials I c 9 eV show the tendency for charge transfer [12]. The molecular area lies typically between 0.4 and 0.5 nm2 (IL = 4.1-3.3 x lo-” mol cmP2) for one ring, but is 0.6 nm2 (I’: = 2.8 X lo-” mol cmd2) for two condensed rings. The Ek values are mostly on the positive side of E, between -0.1 and +0.4 V. The Gibbs energies of adsorption AG$ range from - 14 to - 34 kJ/mol, clearly more negative than -AGso, (see Section 111.5). (1112.2) Interpretation of the electrosorption ualency yi For flat adsorbed neutral aromatic molecules, eqn. (12) can be simplified, since z = 0. Further, the dipole vector is perpendicular to the field, hence ~4’ = 0. If the molecule does not perform a charge transfer, yi = - VK, should increase linearly with the molecule’s area. Therefore, we plotted y: vs. the area in Fig. 3. The straight line gives the contribution of water desorption. The important feature of Fig. 3 is that most compounds are located in the field below the line. Hence, positive charge transfer takes place from the molecule to the metal. According to eqn. (19) the deviation from the line gives -A” (1 - g) = yj!, +

VK,

A”

=

-

( $!J

where g = 0.2 is assumed [12,19]. The value depends on the substance;

Requtted

area

of the molecule

+

VK,)/0.8

09)

h” dominates for the benzene derivatives

AR/nm’

Fig. 3. Electrosorption valency yi of different compounds as a function of the required area of the molecule. Deviation from the straight line gives the charge-transfer coefficient. Data from Table 1.

0.03 0.01 0.05 0.02 0.08

YNI

0.35 0.40 0.30 0.35 0.40 0.35 0.25 0.25 0.40 0.40 0.25 0.30

0.35

0.38 0.30 0.40 0.33 0.40

(8Y ‘/8E) /v-r

0.05 0.07 0.07 0.06 0.13 0.12 0.09 0.10 0.26

(aEk/gF) /Vcm2 mol-’

7.33 8.67 6.50 8.40 8.00

7.40 6.50 1.45

1.19 4.83 5.10 5.36

0.00

10% /Cm m,

0.82 0.70 0.12 0.78 0.78 0.70 0.70 0.72 0.78 0.72 0.98

0.70 0.78 0.81 0.80 0.88

Length /nm

’ mp is the dipole moment; I, is the mokcule length; K~ ’ is the dipole term of the adsorbate; account [30,59). For the meaning of the other symbols and references, see Table 1. b See Section (V.2).

(a) k! (b) 0.06 Pyridme b 0.11 ar-Picoline 0.13 2,4_Lutidine 0.13 2,4,6-Co&dine ’ 0.05 Quinoline b 0.15 Isoquinoline b 0.13 2-Me~ylqu~o~e 0.14 ~Me~ylq~o~e 0.08 3-Methylisoquinolme ’ (a) 0.09 (b) 0.11

Pyrocatechot b Pentafluorophenol b

Bem#Xle Toluene Phenol Aniline Benwic acid

Adsorbate

K:

0.000

0.010 0.040 0.040 0.038

cos

‘p

0.065 0.077 0.058 0.065 0.070 0.050

0.066 0.057 0.060

- 0.04

it:

0.000 0.050

0.065 0.057

- 0.04 0.04 0.066 .” 0.057 -

- 0.010 0.003 - 0.028 0.038

0.000

i$ cosg,

-0.12 - 0.05 -0.16

- 0.08 -0.04

0.00 0.00 -0.15 -0.37 - 0.35 -0.30 -0.17 - -0.4 - -0.5 - 0.22 -0.15 - 0.36 - 0.37

G /v

- 22

18 18 F = 3.2 12 14 15

r-4

19 17 19 25

- AG,I, /kJ mot-’

is the dipole term taking the dipole angle into

Data for the electrosorption of perpendicular adsorbed aromatic and heterocyclic compounds on mercury a

TABLE 2

151

whereas it can be almost neglected for the pyridine compounds. We summarize here only the important conclusions and correlations of the detailed paper [12]. The tendency to perform charge transfer depends on the energetic location of the electronic density of states function of the adsorbed molecule relative to the Fermi energy of the metal. To understand the different influence of A” on the electrosorption valency, we correlated X” with the first ionization potential Z of the a-electron. For substances with Z < 9 eV, charge transfer becomes important, with a slope of dX”/dZ= -0.40 V-‘. The same tendency should be observed for the oxidation potentials E,,. The more easily a compound becomes oxidized, or in other words the more negative E,,, the more charge transfer should appear. We could prove this for some phenol derivatives; the slope yielded dh”/dE,, = -0.35 V-‘. It should be stated that absolute values of A” may be uncertain by up to 30% due to various assumptions. Nevertheless, the analysis shows that partial charge transfer takes place from flat adsorbed aromatic compounds and can be understood qualitatively. Moreover, it dominates for many systems and influences various macroscopic values such as AC:: (Fig. 6), ay “/aE (Fig. 8) and the lateral repulsion (Fig. 12), as will be discussed later. (111.3) Perpendicular orientation (ZZZ.3. I) General data As already pointed out, some substances undergo reorientation from flat to vertical. In Table 2 we list again the electrosorption valency y;, its potential dependence and the shift of E,. The values of yk and i!IEA/aI’ are always positive, which is similar to the aliphatic compounds but different to flat adsorbed substances. Furthermore, dipole moments m, and the values of K: are given. The dipole vector is parallel to the field for most molecules; if this is not fulfilled, the term K,‘cos cp has to be taken into account. Usually only the absolute value of m, is known. Its direction in space can be fixed either from the symmetry group of the molecule or experimentally from the Stark effect in microwave spectroscopy. For example, for phenol m,, = 4.05 x 10P3’ C m and m, = -0.443 X 10P3’ C m were found [60]. According to Fig. 1, the dipole term is calculated to be K,’ cos ‘p = 0.003. Hence, in spite of the well-pronounced dipole moment m = 4.06 x 10e3’ C m, yk is determined mainly by water desorption. Ei is always on the negative side of E N; the Gibbs energies of adsorption are smaller than those for the flat orientation and range from - 12 to - 25 kJ/mol. (ZZZ.3.2)Interpretation of the electrosorption valency yk As with the aliphatics, we will test the validity of eqn. (12) and plot yk vs. the dipole term K,’ cos cp. Figure 4 features a good correlation, especially for the benzene derivatives; the ordinate segment is given by water desorption UK,. From molecular models we calculate that Y= 2. Correspondingly, the line for aromatic

152

Fig. 4. Dependence

of the electrosorption

valency

~4

on the dipole term K,‘- cos cp. Data from Table 2.

molecules intersects the ordinate at 0.03 as for aliphatics [lo]. There is no negative are separated from deviation, i.e. AL = 0, which is reasonable since the s-electrons the metal surface in the perpendicular case [12]. Thus we obtain y,+ =

K$

-

VK,

=

KS’-

+

0.03

(20)

However, the heterocyclic compounds show a certain offset of about 0.015 (pyridine) to 0.04 (quinoline) with respect to the theoretical value. In contrast to the benzene derivatives, the heterocyclic compounds are readily soluble in water because of H-bonding of water with nitrogen. Therefore, hydrogen bridges should be present at the interface too; this has already been pointed out by Conway et al. [51]. Because the nitrogen atom faces the electrolyte, dimers could be built up, linked together by a water molecule. The dipole term of such an ensemble Moreover, water desorption for the slightly exceeds those of two monomers. associate might be greater, hence the offset in the yk values becomes explicable. (III. 4) Ionic systems Comparative analysis of the adsorption data of the ions is more difficult because only a few results are available and, moreover, the composition of the double layer becomes complex. It was shown in the fundamental papers by Blomgren and Bockris [61] and Conway and co-workers [62,63] that not only adsorption of the ion takes place, but also that of the neutral molecule and above all that of inorganic ions of the electrolyte. The adsorption of aromatic cations at positive potentials was attributed to n-electron interaction while adsorption at negative charges is due to coulomb forces. In the case of coadsorption of S (e.g. RH+) and X (e.g. Cll), IX,ad changes with Is,ad. To describe such mixed systems the coupling factor p [64] was defined: P = (arx,,,/ars.,,

h+

(21)

153 TABLE Data

3

for the electrosorption

of ionic aromatic

Adsorbate

and heterocyclic

YN

compounds

on mercury



10’0 r m /mol cm-’

h”

Ref.


0.05 0.05 0.08 0.05 0.09 - 0.01

61 61 61 61 61 61

1.6 1.8

0.36 0.45

65 66

Calions Anilinium 2-Methyl-anilinium 2,3-Dimethyknilinium 2,6-Dimethyl-anilinium Quinolinium Pyridinium

0.04 0.04 0.02 0.05 0.00 0.07

Amons Benzoate CAminobenzoate

-0.25 -0.30

a For the meaning of the symbols, see Table NaClO, +O.Ol M NaOH (pH 12) for anions.

1. The electrolyte

was 1 M HCI for cations

and

1 M

Coadsorption and competitive adsorption can be distinguished. by the value and sign of p; for ion pairing (e.g. RHCl), p should be positive and an integer. The main question is whether charge transfer is observed. Of course, it should be greater for anions than for cations. We restrict ourselves to yN, which is given in Table 3. From the I,,, values the flat orientation is obvious. For the aliphatic cations, 0.1 < yN < 0.3 was found [lo]. The term gz dominates yN and g = 0.2 was deduced. The corresponding values for the cations given in Table 3, however, are relatively small, yN EZ0.07. HCl was used as the supporting electrolyte for all the bases. According to ref. 61, the adsorbed species in the vicinity of the zero potential consists of an ion pair. With yN(Cl-) = -0.2 [19] and p = 1, one finds for anilinium: y,(mix)

= Y&NH:)

0.04 = YN(RNH:) yN (RNH:)

+

PyN("-)

- 0.2

= 0.24 = gz - A”(1 - g) -

(22b) YK,

(224

Thus a small charge transfer h” = 0.05 can be estimated. It is reasonable that X” is much smaller than that for neutral aniline (h” = 0.19). Benzoic acid as well as 4-aminobenzoic acid was investigated in an aqueous solution of NaClO, and NaOH, pH 12. The charge-transfer coefficient of the benzoate (X” = 0.36) is larger than that for the neutral benzoic acid (A” = 0.09) due to the larger electron density. (111.5) Gibbs energy of adsorption at the point of zero charge In a classical paper, Bockris and co-workers [13] calculated the Gibbs energy of adsorption for some different butyl, phenyl and naphthyl derivatives and correlated these values with the Gibbs energy of dissolution, AG,,, (eqn. 17) given by the

154

30

7.

.O”l”OlrnP

/

E 20 $

10

blperpendxular

2i)

lb

tb AL&tkJ

moi' ~‘

Fig. 5. CorreIation of the Gibbs energy of adsorption at EN for (a) the ~endi~lar orientation, with the Gibbs energy of dissolution. Data from Tables 1 and 2.

and (b) the flat

concetitration c, of the adsorbed molecule in a saturated aqueous solution. As an extension, we took into account a few more aliphatic compounds [67] and reproduced the correlation of Bockris and co-workers; the extrapolation to AG,,, = 0 kJ

b 0

01

02

03

00 i"

01

02

03

Fig. 6. Correlation of the Gibbs energy of adsorption (a) and (b) the difference (- A(& + AGaol)with the charge-transfer coefficient. Data from Table 1.

155

mol-’ gave AGad= -6 kJ mol-‘. The same is observed in Fig. 5b for the aromatic compounds adsorbed perpendicularly. Benzene deviates from the line; this may be due to experimental difficulties as a result of the easy vaporization already pointed out by Baikerikar and Hansen [34]. Flat adsorbed molecules, however, give a weak correlation, seen in Fig. 5a. Bockris and co-workers [13] explained the ordinate segment of - 14.7 kJ mol-’ for AG,,, = 0 kJ mol-’ as a constant contribution due to the n-bond. The differences in AG,‘:,were attributed to the different solubilities. The situation, however, is more complex. In Fig. 5a a rough tendency in accordance with ref. 13 is observed, thus poor solubility means good adsorbability, but the influence of h” varies. The alternative correlation between the Gibbs energy of adsorption and h”, however, is disappointing and approximate again (Fig. 6a). The AG,! values include not only the chemisorption AG, but also the influence of solubility AC=, and water desorption AG,,,. The last term was taken constant for the different compounds of Fig. 6a. Hence the difference - AGjL + AG,, should reflect the degree of AG,,. In fact, Fig. 6b gives a better correlation; the slope yields dAG/dX” = -50 kJ mol-‘. This is a reasonable value for a chemisorption caused by the r-bond. (IV) REORIENTATION

(IllI)

PROCESSFiS

Change of orientation with potential

So far, we have discussed only yN, and now wish to extend the analysis to the whole potential scale and the reorientation problem. Therefore, we will now discuss the potential dependence of AGti and its impact on the orientation since the minimum of AGad governs the orientation. The next two questions refer to the position of the AG,, parabola on the potential scale and its width. Both the increase of 1AG,, 1 and the potential of maximum adsorption E,,, (y = 0) depend on yN, i.e. the dipole term k,‘- and the charge-transfer coefficient h” , respectively (with vk, = constant). In Fig. 7 we plotted E, vs. yN for the flat (a) and the perpendicular case (b). E,’ is more negative than EA, as was derived from Tables 1 and 2. According to eqn. (4), a straight line should be the result of a constant quotient (C, - C,)/lY,,,. Figure 7b fulfils this quite well, giving C, - Cl1 = 22 PF cm-’ from the slope with I’,’ = 6 x lo-” mol cmv2. In Fig. 7a we can roughly distinguish two areas: a flatter one for EA > 0.1 V resulting in Co - Cf = 10 PF cme2 and a steeper one with a decrease of the capacity of 15 PF cmm2 (IL = 3 x lo-” mol cmm2 was assumed for both slopes). The previous paragraph has dealt with the position of the parabolas. Its width should be a function of ay/k?lE. Figure 8 gives a plot of the slope of the potential dependence 3y “/aE vs. A”. The quadratic influence of the electric field decreases with increasing X”, and the linear term of eqn. (18) becomes dominant. For a strong charge transfer in a polarizable bond, the capacity Cr” should exceed that of the blank solution Co. This should be reflected in a negative lly“/aE, which was found

0) flat

b) perpendwlor

.4-Methyl-qulnollne

0

-02

-04

02

E. IV

Fig. 7. Correlation of yN with the potential of maximum adsorption perpendicular (b) orientation. Data from Tables 1 and 2.

E,

for the flat (a) and the

for phenylthiourea. The width of the parabolas of the perpendicular fraction varies little because the quotient (CO- CIL)/r, is nearly constant. To explain the reorientation at intermediate potentials, we show in Fig. 9 some

04.

flat orlentotlo”

lhloroglucinol

lHydroqulnone lPyrocotechol Anlllne

0 3- \

=r< 02 ::

Totue”e~~~~~~~~~~~“,

1 e

03

Ok (di‘/dE)

0.5 V-’

06

Fig. 8. Correlation of the charge-transfer coefficient for the flat orientation with the potential dependence of y ‘I. Data from Table 1.

157

Fig. 9. Tharetical potential dependence of the change of the Gibbs energy of adsorption for the perpendicular (a) and the flat (c) arrangement, according to eqn. (18). Resulting electrosorption valency for the perpendicular (b) and the flat (d) orientation. Intersecting parabolas of AGad (e) and potential dependence of y (f) for the two alignments for explanation of r~~entation processes.

model calculations with typical yN values. Figures 9a and 9b refer to the perpendicular orientation, Figs. 9c and 9d to the flat one. The combination in Figs. 9e and 9f shows that the flat orientation prevails at positive potentials and reorientation takes place at EC, = -0.2 V. In the following, two special substances are discussed. (IV.I.I)

a-Picolint?

In Fig. 2c a step-like jump was observed in the potential d~endence of y for pyridine. As can be seen from Fig. lOa, a-picoline behaves similarly. With the aid of the Langmnir isotherm, we calculated the Gibbs energy of adsorption (r,/ = 3.3 X mol cm-*). The transition can be explained, 10 -lo mol cm-*; I’,* = 6.3 X lo-” because for potentials more negative than -0.48 V 1AGiL 1 is smaller than 1AGai 1, hence reorientation to the perpendicular alignment sets in, In addition, we gave the y/E function for I’ = 4 x 10-i” mol cmm2, which exceeds rz. Hence, the flat orientation is impossible. Therefore, y/E is nearly linear, and the dependence of the Gibbs energy of adsorption on the potential is an almost perfect parabola. The increase of 1AG& 1 with increasing I’ is due to attractive interaction between the adsorbed molecules.

In contrast to the relatively sharp transition observed for a-p&line, the reorientation of phenylthiourea is featured in a somewhat broader potential scale. The molecule changes the ~~gement in such a way that for negative potentials the benzene ring is next to the surface, while it rotates, as the potential becomes

158

12’

b)

.

-0 6

-08

-04 (E-E&

-0.2

0

0.2

Fig. 10. Potential dependence of the ekctrosorption valency (a) and the Gibbs energy of adsorption for a-picoline [28]. Surface concentration as indicated.

I

Benze”e

/

o/-----O

0’ Thiourea D

(b)

\.

r.15

Id’“~~l~~m

\

_

Ph&hiourea

a:

f , Phenylthiourea

/”

bl

Fig. 11. Potential dqmdencc of the electromption vakncy (a) and the Gibbs energy of adsorption (b) for phenylthiourea [47]. Benzene [33] and thiourea [lo] are given for comparison; surface concentration as :_A:--r^A

159

positive, to an alignment with the stdphur next to the metal, analogous to that of thiourea. For low surface concentrations (I’ < 3 X lo-” mol cmm2) and E - EN < -0.1 V, the variation of y with the potential is similar to that of benzene (0.45 V-l), which is seen in Fig. lla. For E - E, > 0.1 V, however, the slope becomes negative and comparable to that of thiourea (- 0.1 V-l), indicating the dominant contribution of X to yN_ The coverage 8 which is needed for the evaluation of AGad was calculated with differing maximum surface concentrations, depending on the potential Figure llb gives the result. The parabola-like behaviour of AGad at negative potentials is relieved by a continuous increase of 1AGad1 at positive potentials. The y values derived from Fig. llb are in good agreement with those of Fig. lla. (ZK2) Change of orientation with the surface concentration In this section we discuss the influence of the surface excess at constant potential Instead of the Gibbs energy of adsorption, we will now discuss the overah energy AGaJ. AGad is calculated again for flat and vertical alignment separately. Then AGadI’ is plotted vs. I. For the Langmuir isotherm we expect two straight lines originating from the zero point up to I,,.,. Reorientation wiII occur if 1r * AG,; 1 > I r ‘IAGA I. In Fig. 12a, the energies for phenol at 0.2 V are given. The perpendicular orientation satisfies Langmuir behaviour (a = 0) approximately while AGiL deviates. With the aid of the Frumkin isotherm, one can fit the energies with a parabola (a = - 1.0). For I > 3 x 10-‘“, the transition to vertical ahgnment takes place.

Phloroglucmol E-E,=OV

a) 2

4

lotor

Fig. 12. Surface energy lO”AG,_,T as a function of the surface concentration for (a) flat and perpendicular adsorbed phenol at E - EN = 0.2 V (data from ref. 36) and (b) phloroglucinol (repulsion energy from eqn. 23; data from ref. 45).

160

acid .

Sal~cyl~r

l

Benzene Pyrrdme

-5

-L

-1

interaction

0

parameter

a-?tol~ne

?

2

0

Fig. 13. Correlation of the charge-transfer coefficient with the interaction parameter a of the Frumkin isotherm. corrected

The parabola was calculated with the aid of eqn. (23) for by a factor of 0.07. Data from Table 1.

r = lo-”

mol cmm2

and

A”

Much work has been done for the determination of CI, especially in the Soviet school [3]. Repulsive behaviour of the molecules is typical for aromatic compounds at positive potentials and attractive behaviour on the negative side. In contrast to most of the papers, we calculated a separately for the orientations. It is interesting enough that a is a function of X”. The greater X” and correspondingly the positive charge of the adsorbate, the more negative is the Frumkin parameter, which is seen in Fig. 13. In a simple model we calculated the repulsion energy originating from charge transfer: V, = lf2fe,2X2AN,)/(4~~*~~~,)

= - RT2aB

U = U, + U, = l/2( e,2h2AN,l.575)/(4nD,D,r,)

(23) (24)

The repulsion U was calculated as the sum of the part from the next neighbours (U, ) and the second but one (U,). The number of molecules A is six for both. The distances r, are obtained from basic geometry and D, was taken as 6. One would expect repulsive behaviour as h” > 0. Figure 13, however, demonstrates a certain attractive offset: a = 0 for h” = 0.07. Increased charge transfer neutralizes the attraction and causes repulsion. As an example, we took phloroglu~nol, which was treated as a circular molecule with a reduced diameter of 0.4 nm because the partially positive charge is concentrated only on the benzene ring. Within a hexagonal structure these discs occupy the centres of hexagons whose extension F depends of course on the surface concentration, F = l/IN,. In Fig. 12b we plotted AGaL I for phloroglucinol and the straight line of pure Langmuir behaviour (hi’ = 0, a = 0). The calculated repulsion with X”(corr) = X” - 0.07 fits the decrease of 1AG:: I’ / .

161 (V) APPENDIX

(V, 1) Special remarks for the compounds of Table 1 The value of I’,,, for phenylthiourea is that at E,. Moreover, we used this value for the calculation of the molecule’s area. A remarkable slope of ay/aE is found for cinnamaldehyde (0.9 V-i). The reason for this is not clear, but incomprehensibly Rueda and co-workers [43] calculated y only in a very narrow potential range, - 0.1 < E - EN < + 0.1 V. The ionization potential of 8.4 eV was estimated from similar compounds like styrene. For the flat arrangement, the dipole moment is usually perpendicular to the field, thus IC~COS‘p = 0. However, this is not valid for aniline [68], which shows an out-of-plane moment of m, = 3.63 x lo- 3o C m; its direction in the interface, As the latter value however, can be up or down, giving K: = 0.03 or ~4’ = -0.03. seems more plausible, it was taken into account when calculating X”. Damaskin stated [38] that the peak at positive potentials in the capacity is due to a change in orientation but not to desorption of the molecule. In the isotherm he limited the coverages to positive potentials. Conway and Barradas [40], on the other hand, gave coverages for four potentials only. Analysis of the small coverages becomes impossible because the curves merge together for I towards zero. As aniline performs that change in the orientation, y; was obtained from extrapolation of the potential dependence of y ” to E,. The ionization potential of pentafluorophenol was estimated from the differences of the values of pentafluorobenzene and benzene. Acetanilide is another remarkable compound of Table 1 because its adsorption was measured in solutions of NaH,PO,. The inorganic anion is specifically adsorbed at positive charges [69]. With the y values for H,PO;, we calculated p = - 1 for E - E, = 0.2 V, which indicates displacement of the anion by the aromatic ring. (V. 2) Special remarks for the compounds of Table 2 Unfortunately, no m, was found for pyrocatechol. Fixing the molecule’s arrangement becomes almost speculative because H-bonding with water is certainly important. For low surface concentrations, the molecule changes orientation from perpendicular to flat as E - E, > -0.25 V. Pentafluorophenol is curious because for low surface concentrations there seem to be two changes in the alignment. In accordance with Damaskin [46], we interpret The molecule performs two changes from yN around E, with the flat orientation. flat to perpendicular as the potential becomes positive ( y L “) or negative ( y L h). The reorientation on the negative side is not quite certain from the experimental data, but is within the bounds of probability. If one interpolates the y I/E functions to E,, one obtains yk A= 0.00 and yd h = 0.06. One can explain these values with a dipole term of K, = 0.04, estimated from 4-fluorophenol, water desorption and cos cp= +1 or -1.

162

Pyridine is one of the most investigated and best described compounds. Damaskin et al. compared the papers published up to 1966 [52]. Because the most detailed data are given by Ntirnberg and Wolff [28], we used their data for the calculation of y. Comparing the pyridine derivatives, it is obvious that 2,4,6-collidine is an exception. Although it should possess a dipole moment of the order of pyridine, y$ is relatively small. This peculiarity has already been noticed by Ntirnberg, who explained this by the formation of polylayers. Hence, we exclude it from the correlations. Like pyridine, the related quinoline is analysed in detail. Unfortunately, early work by Bordi and Papeschi [55] was carried out with KNO, as the supporting electrolyte; coadsorption of the anion cannot be neglected. In a series of papers, Cl. Buess-Herman and L. Gierst and co-workers investigated the behaviour of three quinolines very carefully [54,56,57]. It included the kinetics of the two-dimensional phase transition which in turn resulted in a general theory based on the classical theory of nucleation [70]. Worth mentioning is a paper by Humphreys and Parsons [53] which proves impressively the adsorption isotherm of quinoline with the aid of ellipsometry. The yi+ values derived from refs. 55 and 54 differ only slightly (Ay,’ = 0.01); the potential dependence in KNO, is somewhat steeper (0.3 V-‘) than that in Na,SO, (0.25 V-‘). The situation becomes more complicated with isoquinoline because, besides the flat orientation, two vertical alignments are formed. To identify these, the ring atoms are numbered in the common way: thus, nitrogen occupies position 2. 5-6 orientation means an arrangement with carbon atoms 5 and 6 facing the metal. At negative potentials (E - E, < -0.3 V), an extremely sharp transition from the 5-6 to the 6-7 phase takes place. In the isotherm I =f( E), one observes a step-like jump to a second maximum surface concentration, I,l b = 8.1 X lo-” mol cm-‘. Because of this, no y& values for the 6-7 phase could be derived. A 4-5 variant analogous to quinoline is not observed because of a relatively small dipole moment of about 3 X 10p3’ C m parallel to the field. To make things more complicated, multilayer adsorption is found for high concentrations ( > 1.5 x lo-* M) and positive potentials (E - EN > - 0.1 V). As a result of experimental difficulties, accurate data for 3-methyl-isoquinoline could be achieved only with concentrations > 1 X 10e3 M. Therefore, we did not obtain any yi. For the vertical alignment, we again have to distinguish between two orientations, a 4-5 variant (a) and a 6-7 variant (b). ACKNOWLEDGEMENT

This work was supported by the Erich Rabald Stipendium der Max Buchner Stiftung (Fonds der chemischen Industrie), which is gratefully acknowledged. REFERENCES 1 A.N. Frumkin, 2. Phys., 35 (1926) 792. 2 E. Gileadi (Ed.), Electrosorption, Plenum

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