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Aromaticity, electronic structure and molecular dimension in the adsorption of organic compounds on mercury—II. Intermolecular interactions among adsorbed molecules
Electrochimira
PII: S00134686(96)00358-1
Arm, Vol. 42. No. 9, pp. 1373-i 378, 1997 Cmvrieht ii;: 1997 Elwier Science Ltd. Printed in &ea; Britain. All rights reserved 0013-4686/97 $17.00 c 0.00
Aromaticity, electronic structure and molecular dimension in the adsorption of organic compounds on mercury-II. Intermolecular interactions among adsorbed molecules C. Fontanesi and L. Benedetti* University of Modena, Department
(Received 4 January
of Chemistry. Via Campi 183. 41 IO0 Modena, Italy
1996; in revised form 22 July 1996)
Abstract-The Frumkin interaction parameter between aromatic molecules (neutral form) adsorbed from aqueous solution on mercury electrode, is related to the balance of energy dispersion terms (adsorbate-adsorbate) and water-water molecular interaction reflecting the role played by substitution on the electrode surface of water by the aromatic molecule. The interaction among adsorbed aromatics (in neutral form) is always attractive, so that the sign of @ (Frumkin parameter, positive or negative) arises from that kind of balance. For aromatic anions, taking into account a large partial charge transfer from the adsorbate to the metal surface, the change of AGXDsat varying the coverage (0) is mainly due to electrostatic repulsion among charge adsorbed particles.
structure, mercury, Frumkin parameter.
INTRODUCTION The energetic interaction between adsorbed aromatic molecules (in neutral form) and the mercury surface has been successfully rationalised in terms of the intrinsic properties of the “isolated” molecule of adsorbate, ie ionization potential and electron affinity of the species being adsorbed and the corresponding quantities of Hg [I]. The process of substitution of a number of water molecules covering the mercury surface. by the adsorbate. has been revealed to play a fundamenta1 role in determining the energy balance of the overall process. With this in view, an ‘“intrinsic” value of the Gibbs adsorption energy has been obtained for The whole a large number of compounds. procedure takes into account the solvation energy contribution of the pure adsorbate in water and the desolvation contribution of Hg due to the dislodging of water. This “intrinsic” value has been related to
*Author to whom correspondence
should
be addressed. 1373
the molecular properties of the adsorbate, such as “size” structural parameters and “reactivity” indexes
PI. The intermolecular energetic interactions among molecules in the adsorbed layer are now considered: let us focus our attention on the linearized Frumkin isotherm [3] at E = cost: AGiUS = AC;,,, - 2RTu& at constant potential the basic term AGII,,, obtained by extrapolation, is modulated, at increasing coverage, by the Frumkin interaction parameter, L(F. So, for a positive value of &, AGo,,, wilt increase (negative direction) and the interaction will be considered stabilizing and aF attractive. The opposite holds for negative (IFvalues. In fact, or varies with the applied potential and, when large enough and positive, it is generally associated with the occurrence of a phase transition in the adsorbed layer [4] and adhesion of adsorbed molecules. For different compounds studied on the same electrode metal (Hg), at the same potential value (in general Epic) and in the same medium (water and a scarcely electroactive electrolyte) ffr assumes different values and it can be either positive or negative.
1314
C. Fontanesi and L. Benedetti
When the 0 value is fixed, as in the case 0 = I, and the previously cited conditions are fulfilled for an adsorption process without formation of 2D condensed phases, the energetic term E = -2RTac- can be evaluated and discussed on a quantitative basis (value and sign) in relation to the different molecular structures of the organic adsorbates giving rise to different dispersion and electrostatic energy contributions acting among them when lying adjacent on the metal surface. For the sake of simplicity, the adsorbing molecules will be considered unperturbed and arranged in a flat position, at a distance and with a reciprocal orientation so as to give rise to a minimum in the interaction energies (either dispersion or electrostatic). Within the present approach any intervention of solvation of adsorbed molecules by water in the adsorbed layer will be neglected. The value and the sign of & for different molecular structures of adsorbed aromatics is also related to the possible role played by the net electrostatic repulsion between vicinal molecules due to the partial charge transfer from the adsorbate to the metal [5,6]. The case of the adsorption process regarding aromatic anions, for which ar is revealed to be always negative (repulsive), is also considered. CALCULATIONS
AND PROCEDURE
The pair interaction energy, eA ,\, is calculated for the adsorbed neutral molecules taking into account either the dispersion energy term (e) and the electrostatic contribution (Eli). The first one is calculated by using the Buckingham dispersion energy function [7] which, in this case, is limited to configurations featuring the aromatic carbons of different molecules lying on the same plane; the angles 41.2and @i.?were varied with n/4 radiant steps and the distance Yscanned with 0.01 A steps (Fig. 1). Different relative minima of a are so obtained and the absolute one is chosen for the discussion. Assuming the same configuration previously shown and using the same procedure for rotation
8
192
Fig. I. Intermolecular disposition of planar adsorbed molecules used for the calculation of dispersion and electrostatic energy terms of interaction.
angles and distances, the intermolecular term is then calculated:
electrostatic
where .sOis the vacuum permittivity and NA and No are the number of atoms of the two distinguished individual molecules A and B (note that both A and B are the same chemical species), qi and qhare the MO SCF net charges localised on atoms i,k belonging to the distinguished individual molecules A and B respectively. MO SCF net charges of each atom, concerning the unperturbed isolated molecule, are calculated using the MNDO hamiltonian implemented in the AMPAC program [S]. Again, an absolute minimum (which invariably corresponds to a maximised packed disposition) in energy is obtained from several relative minima, and both for ~~1and .sclthey correspond almost to the same configuration between interacting molecules, except for compounds 4, 14, 20 (see Table 1) exhibiting a particular spatial asymmetry. For the different compounds, the equilibrium distance Y ranges between 4 and 7 A. The OF values are obtained from the literature [I, 2,5] and mainly from references cited therein, as well as the values of j,, partial charge transfer coefficient, and AN, fractional number of electrons transferred (in the Pearson’s concept [9]) in the formation of the adduct adsorbed molecule-mercury L51. aF is always related to the adsorption on mercury of the neutral adsorbate from aqueous solution at the potential of zero charge (E,,,,) or close to it: in some cases, the original experimental results have been elaborated to obtain the proper value not explicitly indicated. For anionic aromatic derivatives (Table 2) adsorption parameters have been obtained from [l&14], here again by the elaboration, in some cases, of the original data. RESULTS
AND DISCUSSION
The values of iL and AN (partial charge transfer coefficient and fractional of electrons transferred, respectively), & (Frumkin interaction parameter), and ed and eel (calculated according to the procedure reported in the previous section) are reported in Table I and regard a wide group of aromatics with a variety of molecular structures (-OH, -COOH, -NH2 benzene, mono-, di-, and tri-substituted derivatives and heterocycles). All of them, according to the experimental results reported in the literature, are adsorbed flat on mercury, from aqueous solution, and their adsorption process is well interpreted by the Frumkin isotherm. ar results in being either positive or negative, to a different extent, so that the packing of molecules (liquid-like structure), at increasing 0, the coverage, should lead to either a positive or
Table I. Adsorption
parameters
No. (1) (2) (3) (4) (5) (6) (7) (8) (9) (JO) (11) (12) (13) (14) (15) (16) (17) 08) (19) (20) (21) (22)
Compound
Adsorption
of organic
and calculated
interaction
(neutral)
Benzene Phenol Benzoic acid Cinnamaldehyde Phloroglucinol Phenylthiourea Salicylic acid Pyridine r-picoline Imidazole Cudiacol o-cresol p-OH benzoic acid Tosylglycine Tymine Uracil Adenine 2 chloropyridine 2 aminopyridine L-phenylalanine p-methylphenyl Phtalic acid
acetic acid
compounds
on mercury--II
1375
energies for neutral aromatic compounds
AN”
i.b
0.018 0.025 -.Ol9 -.003 0.024 0.047 0.002 0.001 0.002 0.037 0.021 0.025 -.012 -.050 -.009 -.Ol2 0.022 -.Ol5 0.03 I 0.010 p.005 -.030
0.06 0.16 0.09 0.14 0.35 0.21 0.12 0.00 0.02 0.10 0.16 0.15 0.04 0.11 0.003 0.12 0.16 0.00 0.18 0.07 0.06
flFC
vd
0.70 -.I2 -.40 - I .20 - 2.70 -0.50 -0.60 1.30 1.60 -0.06
3.0 3.4 4.0 5.1 4.6 4.7 4.1 2.8 3.4 2.3 4.0 3.6 4.1 4.4 4.1 4.5 4.4 3.2 3.4 4.3 4.1 4.2
0.30 - 2.0 0.35 0.34 0.45 0.55
0.20 1.00 0.70
-&de (kJ mol-‘) 4.6 5.1 6.6 7.3 5.7 9.2 6.6 5.7 6.9 4.9 3.7 8.1 7.3 17.2 6.9 5.4 27.6 7.5 6.2 8.8 7.3 3.2
;‘Fractional number of electrons transferred from the adsorbate to the metal. hPartial charge transfer coefficient. ‘Frumkin interaction parameter. dNumber of water molecules displaced by one molecule of the adsorbate, when disposition on the surface (A,,,e, z 0.14 nm’). ‘Dispersion interaction energy by Buckingham. ‘Electrostatic interaction energy. negative
contribution
( - 2RTar0)
to
the
adsorption
Table 2. Adsorption
No.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
parameters
Compound
for anionic
aromatic
(anion)
3-indole carboxylic acid 4-indole carboxylic acid 5-indole carboxylic acid Benzoic acid p-amino benzoic acid Tosylglycine Benzoylglycine 4.6 dimethylpyrimidyl benzensulphamide p-methylphenyl acetic acid Nicotinic acid
“Electrovalency. hPartial charge transfer coefficient. ‘Frumkin interaction parameter. dExperimental adsorption Gibbs energy
0.2 6.0 -. 9 - 1.4 - I.1 0.86 -.I6 -1.2 -0.3 -4.7 -0.1 -0.6 -0.4 12.2 2.5 - 11.0 - I.0 I.8 -0.1 0.3 -0.4 -0.1
in a planar
account alone for the extent and sign of the energetic interaction among adsorbed molecules, implying of course always a negative sign of OF. Moreover, the term -~RTuF cannot be merely related to the value of &d. obtained for a mole of adsorbed aromatic molecules lying flat on the surface and covering the whole electrode surface (0 = I): this simply because &dis always stabilizing (ie. negative).
standard
energy at 0 = 0. The positive sign of i as well as that of AN (calculated using Pearson’s approach [I 51, the sensitivity on AN is kO.006 [2]) indicate a tendency of the electrons to flow from the adsorbed molecule towards the mercury, and this fact should always give rise to a repulsive energetic coulombic term (proportional to i’), which cannot Gibbs
adsorbed
I
(kJ t%-l)
compounds
- AG:nsd (kJ molV’)
;‘”
i.b
OF’
a.33 a.25 -0.28 -0.25 a.30 -0.1 I a.06
0.27 0.19 0.21 0.36 0.45 0.00 0.01
-0.95
-1.05
23.2 22.6 22.9 15.0 16.3 16.8 20.0
-0.39
0.42
-0.25
0.40
-0.60 -0.90 -0.50
23.7 29.4 18.0
at 0-O.
Planar
-1.15 -1.05 -0.85 -0.40 -1.20
disposition
on the surface.
1376
C. Fontanesi and L. Benedetti
Furthermore, the calculated se1values are in general negative, but what is more they do not fit with the corresponding CI~values, in the sense that they should be always opposite in sign with respect to @r for a compound, and when this happens (only 8 compounds in Table l), the term -2RTar is from 3 to IO times higher with respect to ~~1. So, one can conclude that each of the parameters reported in Table I cannot account alone for the quantitative estimation of QF(sign and value), at least in this ndive approximation used to evaluate the energetic interactions among vicinal adsorbed molecules. However, the adsorption process can be considered as a substitution of water molecules on the surface by the adsorbing substance. So that, at increasing coverage, when the stabilising energy due to the mutual interactions of the adsorbed aromatic particles overcome the even stabilizing interaction energy term among water molecules being substituted, then the AC:,,, B-0 will increase (in the negative sense) with B, or will decrease in the opposite case, so ruling the positive or negative sign of (IF. ~8 and ~~1have been calculated, with the same procedure as for the aromatic compounds, for water molecules lying flat on the surface, and the values are ~d.,~~~,~~ = - 1.56 kJ mol-’ and ~~~~~~~~~~= -3.78 kJ mol-‘. Of course, for each molecule/mole of organic, v molecules/moles of water will be displaced, according to the ratio of their corresponding area occupied on the surface (obtained for the adsorbate by roux, when available; otherwise by the CPK model; A,,,e, = I4 nm?), so that the energy difference between the fI = I and @= 0 states can be expressed as follows: - 2RTar: = &d- YE~.~~,~~,
(1)
which can be considered a shortened version of the relations reported in [I61 and [l7]. In this case, as previously mentioned, the role of the interaction between the adsorbed molecule (in the adsorbed state) and the solvent, &&,&o&ie wBleT, is here neglected. This is because the quantitative value of aF previewed by relation (I), in the absence of that contribution, is in good agreement with the experimental one. Moreover, ~d.adwbate.wstrr (when aging) should greatly affect the energy balance for the different compounds listed in Table 1 by a trend related to the AC;,, values in the bulk (ranging between 2 + 23 kJ mol-’ [5]), and this is not the case. In fact, using the experimental ar values, the calculated a posreriori &d.adSOrbate wa,elcontributions do not show any connection with AC&, (the most soluble compounds of the series, pyridine and related derivatives [I], show the smaller &d.adsorbatc%ter contribution). Finally, it has to be stressed that the balance in energy (relation I) reported above does not refer to a lattice-like model of the mercury surface involving the desorption of individually different clusters of water by the adsorbing aromatic molecule
-1
o! ~. ’ -3
I
I
-2
-1
I
/
0
1
I\
1
2
3
aF Fig. 2. Piot of &values, E = (2RTffF + Ed)/\‘, 1’s the Frumkin parameter a~. Dotted lines correspond to ~d.~~,.-~ = - 1.56 kJ moi-’ and to F.I~.,,~~ = - 3.78 kJ mol-’ respectively.
(each cluster, of I, 2, 3, molecules, contributing to a different extent to the total energy balance); the term %&,a& simply accounts (through v) for the extensive property of energy, and corresponds to the case in which water molecules are present in one state only at the interface [ 181and this state is estimated in the present work by calculating the intermolecular dispersion energy interaction between two water molecules by means of the relevant Buckingham equation. Then, &dhas tentatively been chosen with respect to ccl (for the aromatic compounds and water): at first &dand ccl represent a different approach to the same problem of the energetic interaction, so that they cannot be in any way balanced or mixed. Moreover, the electrostatic interactions due to net charges localised on vicinal molecules should be (not explicitly) just contained in the Buckingham term (Ed) by means of the empirical parametrisation [7]. A consequence of relation (I) is that:
2RTaF + Ed= I’
Ed
.w
a,ef
7
so, the term on the left side should result, for each as constant and corresponding to compound, - I.56 kJ mol-‘. The left-side term (E) has been plotted against @r for the compounds of Table I in Fig. 2. The features of this plot show clearly that, for low values of ar, OF= IO.8 t I], that a simple energetic balance could be reliable. Moreover, at a[: = 0 the perfect energetic balance is attained, E = &&water, thus confirming that the main interaction parameter that has to be considered, at least for water, is correctly &d.w&r (- I .I% kJ mol-‘) and not ~~~~~~~~ {the higher dotted line in Fig. 2), ( - 3.78 kJ mol-‘). Further, the case in which #r = 0 does not represent at all the absence of any attractive (repulsive) interaction among adsorbed molecules, but the situation in which &d= VQ~~,~~~, and this
Adsorption
of organic
compounds
happens for each compound for which &d and v (related to its dimension) satisfies that relation. This feature regarding the existence of an attractive interaction even at or = 0 was already suggested [6, 191 on the basis of a completely different argument, founded on the existence of a positive partial charge transfer for a group of compounds now also considered (giving rise, of course, to electrostatic repulsion among vicinal adsorbed molecules) balancing the attractive interaction itself. One can conclude that even taking into account very different molecular structures, in the Frumkin type adsorption (liquidlike) the energetic interaction among molecules is always attractive, but the sign of or can be positive or negative (and also its value can be estimated) determined in relation to the balance, which accounts for a number of water molecules dislodged from the surface showing always an attractive dispersion energy among themselves. Furthermore, the goodness of the comparison shown here between a Gibbs energy contribution (AG&,, = ,) and a balance of dispersion energy terms (by Buckingham) involves a thermodynamic consequence: the decrease in configurational entropy related to the adsorption of the aromatics (from the solution to the surface) has to be balanced by almost the same amount in entropy gained by a cluster of r water molecules displaced from the surface into the bulk. As a result, AE ( zAH) can be compared to AG and the adsorption equilibrium (Frumkin-type) should depend little on the temperature, as indicated by the few experimental data reported in the literature; in fact, a small effect of temperature (for about 20-30 degrees more or less than 298 K) has been observed for different adsorbed compounds, DMSO, DMF, water itself, pyridine and acetophenone, long chain aliphatic alcohols, polyalcohols and thiourea, and in any case negligible for high surface concentration excess (I) z I) and close to the potential of zero charge or at the value of zero charge [2&24]. Moreover, the isoentropicity of this process (as a whole) accounts for a liquid-like type order of the aromatic adsorbate on the electrode surface. When the adsorption on mercury from aqueous solution (at E,,,,) of a group of aromatic anions is considered (Table 2), the adsorption Gibbs energy values at O-+0, reflecting the interaction between the aromatic anions (still in a planar disposition, with their aromatic moiety on the electrode) and the mercury are very similar to those corresponding to neutral aromatics. Nevertheless, very high partial charge transfer coefficients (obtained by a negative and high experimental value of the electrovalency, 7) are found together always with negative values of aF. The presence of the net charge in the anions should lead to a large and repulsive coulombic interaction among vicinal adsorbed molecules, overcoming any other energy contribution related to the desorbing of neutral water molecules. So that, it is entirely reasonable that UF values result as being high, and
on mercury--II
1377
what is more, negative. So that ar reflects the decrease of AC& at increasing coverage. This can be proved, on a qualitative basis, taking into account that the lower the partial charge transfer, the higher the Frumkin coefficient (negative): indeed, when i is small a higher charge will remain localised on the adsorbed molecule and or will be larger. Figure 3(a,b) illustrates this simple conclusion; Fig. 3(a) shows a quadratic dependence between OF and I - i (or j.). while Fig. 3(b) shows a more linearized one taking into account that the interaction energy among vicinal molecules becomes in some way proportional to the square of the charge (ie, (I - 2.)’ or i.‘). save that in this very simple picture the differences in the symmetry and distances of the charged objects are in this case of minor importance.
CONCLUSIONS The Frumkin interaction parameter (or) can be related, even if still roughly, to the balance of energy dispersion terms regarding adsorbate-adsorbate and water-water interaction reflecting the main role
1.0
0 a
0.9
7
6 1
Ir
0.4 i 1.0
0 7
b
l 6
0.8 ![ N
F 0.6
I
I
70.4 -
0.2 1 0.0
1 I
0.2
1
0.4
I
0.6
I
-
0.8 -a
1.0
1.2
1.4
F
Fig. 3. Relation between: (a) the residual charge on the anion, (b) the square of this residual charge and the Frumkin parameter. The correlation coefficients are. respectively. 0.90 for the least squares second order (parabolic) curve in (a) and 0.83 for the first order (straight line) curve in (b).
1378
C. Fontanesi and L. Benedetti
played by the process of substitution on the electrode surface of water by the aromatic molecule. The interaction among adsorbed aromatics (neutral form) is always attractive, so that the sign of&, as well as its improper meaning of “interaction” parameter, arises from that kind of balance. That the process of substitution on the surface is practically isoentropic (AE can be compared to AG), so that only slight effects of temperature on the formation of the liquid-like adsorbed layer can be previewed. For aromatic anions, the change in AC:, on varying the coverage is mainly due to the electrostatic repulsion among charged adsorbed particles, taking into account the role of a large and dative partial charge transfer from the adsorbate to the metal surface. REFERENCES
4. B. B. Damaskin, 0. A. Petrii and V, V. Batrakov, of
Organic
Conrpound.~ on
Electrodes.
Plenum Press; New- York (197i ). 5. L. Benedetti and C. Fontanesi. Current
dam (1980). 8. QCPE 506, Quantum Chemistry Program Eluchange, Indiana University, Bloomington, USA, (1989). 9. R. G. Pearson, J. Org. Chem. 54, 1423 (1989). IO. L. Benedetti, C. Fontanesi, G. Camurri and G. Gavioli. Collecr. Czech. Chem. Comm. 56, 750 (1991). 1I. S. Quartieri, R. Andreoli, G. Gavioli and L. Benedetti. BioelectrochemBioenerg 12, 543 ( I 984). 12. C. Fontanesi, G. Gavio!i, R. Andreoli and L. Benedetti, J. Electroanal. Chem. 210, IS1 (1986). 13. R. Andreoli, A. Barbieri, L. Benedetti, G. Borghesani and G. Gavioli, E~eetraehjm. Acta 32, 325 (1987). 14. M. Jurkiewicz-Herbich. J. Ele~troanai. Chem. 332, 26.5 (1992).
15. R. G. Pearson, Proc. Nail. Acad. Sci. USA 83, 8440 (1986).
16. P. Nikitas, J. Chem. Sot., Faraday Trans. I 81, 1767 (1985). 17. F. Pulidori, G. Borghesani, R. Pedriali, A. de Battisti and S. Trasatti, J. Chem. Sot., Faraday Trans. 1 74, 19 (1978).
I. L. Benedetti, G. Gavioli and C. Fontanesi, J. C/rem. Sec.. Faraday Trans. 88, 843 (1992). 2. L. Benedetti and C. Fontanesi, Electrochlm. AC/a 39, 737 ( 1994). 3, U. Retter and H. Lohse, J. Efectroanat. Chem. 134,243 (1982). A~orption
Weak ~ntermole~uiar interactions in Cllernjstr~~ and Biology, Studies in Physical and Tlzeoreti~a~Chemistry 3, Elsevier, Amster-
7. P. Hobza and R. Zaharadnik,
Tomes in
Electrochemistry 3, 6 I (1994).
6. D. Rolle and J. W. Schultze, J. Electroanal. Chem. 229, I41 (1987).
IS. P. Nikitas, J. Chem. Sot., Faraday Trans. I 82, 977 (1986). 19. D. Rolle and J. W. Schultze, Electrochhim. Acta 31, 991 (1986). 20. B. E. Conway and L. G. M. Gordon, J. Php. C/rem.
73, 3609 (1969). 21. B. B. Damaskin, A. A. Survila and L. E. Rybalka, Ele~tra~~l~rniya3, I46 (1967). 22. G. Hills and F. Silva, Can. J. Chem. 59, 1835 (1981). 23. F. Silva and S. Rodrigues, Port. Elertrochim. Acta 5, 257 (1987). 24. Z. Borkowska and J. Stafiej, J. Eiectroanal. Chem. 226,
283 (1987).
PII: S00134686(96)00358-1
Arm, Vol. 42. No. 9, pp. 1373-i 378, 1997 Cmvrieht ii;: 1997 Elwier Science Ltd. Printed in &ea; Britain. All rights reserved 0013-4686/97 $17.00 c 0.00
Aromaticity, electronic structure and molecular dimension in the adsorption of organic compounds on mercury-II. Intermolecular interactions among adsorbed molecules C. Fontanesi and L. Benedetti* University of Modena, Department
(Received 4 January
of Chemistry. Via Campi 183. 41 IO0 Modena, Italy
1996; in revised form 22 July 1996)
Abstract-The Frumkin interaction parameter between aromatic molecules (neutral form) adsorbed from aqueous solution on mercury electrode, is related to the balance of energy dispersion terms (adsorbate-adsorbate) and water-water molecular interaction reflecting the role played by substitution on the electrode surface of water by the aromatic molecule. The interaction among adsorbed aromatics (in neutral form) is always attractive, so that the sign of @ (Frumkin parameter, positive or negative) arises from that kind of balance. For aromatic anions, taking into account a large partial charge transfer from the adsorbate to the metal surface, the change of AGXDsat varying the coverage (0) is mainly due to electrostatic repulsion among charge adsorbed particles.
structure, mercury, Frumkin parameter.
INTRODUCTION The energetic interaction between adsorbed aromatic molecules (in neutral form) and the mercury surface has been successfully rationalised in terms of the intrinsic properties of the “isolated” molecule of adsorbate, ie ionization potential and electron affinity of the species being adsorbed and the corresponding quantities of Hg [I]. The process of substitution of a number of water molecules covering the mercury surface. by the adsorbate. has been revealed to play a fundamenta1 role in determining the energy balance of the overall process. With this in view, an ‘“intrinsic” value of the Gibbs adsorption energy has been obtained for The whole a large number of compounds. procedure takes into account the solvation energy contribution of the pure adsorbate in water and the desolvation contribution of Hg due to the dislodging of water. This “intrinsic” value has been related to
*Author to whom correspondence
should
be addressed. 1373
the molecular properties of the adsorbate, such as “size” structural parameters and “reactivity” indexes
PI. The intermolecular energetic interactions among molecules in the adsorbed layer are now considered: let us focus our attention on the linearized Frumkin isotherm [3] at E = cost: AGiUS = AC;,,, - 2RTu& at constant potential the basic term AGII,,, obtained by extrapolation, is modulated, at increasing coverage, by the Frumkin interaction parameter, L(F. So, for a positive value of &, AGo,,, wilt increase (negative direction) and the interaction will be considered stabilizing and aF attractive. The opposite holds for negative (IFvalues. In fact, or varies with the applied potential and, when large enough and positive, it is generally associated with the occurrence of a phase transition in the adsorbed layer [4] and adhesion of adsorbed molecules. For different compounds studied on the same electrode metal (Hg), at the same potential value (in general Epic) and in the same medium (water and a scarcely electroactive electrolyte) ffr assumes different values and it can be either positive or negative.
1314
C. Fontanesi and L. Benedetti
When the 0 value is fixed, as in the case 0 = I, and the previously cited conditions are fulfilled for an adsorption process without formation of 2D condensed phases, the energetic term E = -2RTac- can be evaluated and discussed on a quantitative basis (value and sign) in relation to the different molecular structures of the organic adsorbates giving rise to different dispersion and electrostatic energy contributions acting among them when lying adjacent on the metal surface. For the sake of simplicity, the adsorbing molecules will be considered unperturbed and arranged in a flat position, at a distance and with a reciprocal orientation so as to give rise to a minimum in the interaction energies (either dispersion or electrostatic). Within the present approach any intervention of solvation of adsorbed molecules by water in the adsorbed layer will be neglected. The value and the sign of & for different molecular structures of adsorbed aromatics is also related to the possible role played by the net electrostatic repulsion between vicinal molecules due to the partial charge transfer from the adsorbate to the metal [5,6]. The case of the adsorption process regarding aromatic anions, for which ar is revealed to be always negative (repulsive), is also considered. CALCULATIONS
AND PROCEDURE
The pair interaction energy, eA ,\, is calculated for the adsorbed neutral molecules taking into account either the dispersion energy term (e) and the electrostatic contribution (Eli). The first one is calculated by using the Buckingham dispersion energy function [7] which, in this case, is limited to configurations featuring the aromatic carbons of different molecules lying on the same plane; the angles 41.2and @i.?were varied with n/4 radiant steps and the distance Yscanned with 0.01 A steps (Fig. 1). Different relative minima of a are so obtained and the absolute one is chosen for the discussion. Assuming the same configuration previously shown and using the same procedure for rotation
8
192
Fig. I. Intermolecular disposition of planar adsorbed molecules used for the calculation of dispersion and electrostatic energy terms of interaction.
angles and distances, the intermolecular term is then calculated:
electrostatic
where .sOis the vacuum permittivity and NA and No are the number of atoms of the two distinguished individual molecules A and B (note that both A and B are the same chemical species), qi and qhare the MO SCF net charges localised on atoms i,k belonging to the distinguished individual molecules A and B respectively. MO SCF net charges of each atom, concerning the unperturbed isolated molecule, are calculated using the MNDO hamiltonian implemented in the AMPAC program [S]. Again, an absolute minimum (which invariably corresponds to a maximised packed disposition) in energy is obtained from several relative minima, and both for ~~1and .sclthey correspond almost to the same configuration between interacting molecules, except for compounds 4, 14, 20 (see Table 1) exhibiting a particular spatial asymmetry. For the different compounds, the equilibrium distance Y ranges between 4 and 7 A. The OF values are obtained from the literature [I, 2,5] and mainly from references cited therein, as well as the values of j,, partial charge transfer coefficient, and AN, fractional number of electrons transferred (in the Pearson’s concept [9]) in the formation of the adduct adsorbed molecule-mercury L51. aF is always related to the adsorption on mercury of the neutral adsorbate from aqueous solution at the potential of zero charge (E,,,,) or close to it: in some cases, the original experimental results have been elaborated to obtain the proper value not explicitly indicated. For anionic aromatic derivatives (Table 2) adsorption parameters have been obtained from [l&14], here again by the elaboration, in some cases, of the original data. RESULTS
AND DISCUSSION
The values of iL and AN (partial charge transfer coefficient and fractional of electrons transferred, respectively), & (Frumkin interaction parameter), and ed and eel (calculated according to the procedure reported in the previous section) are reported in Table I and regard a wide group of aromatics with a variety of molecular structures (-OH, -COOH, -NH2 benzene, mono-, di-, and tri-substituted derivatives and heterocycles). All of them, according to the experimental results reported in the literature, are adsorbed flat on mercury, from aqueous solution, and their adsorption process is well interpreted by the Frumkin isotherm. ar results in being either positive or negative, to a different extent, so that the packing of molecules (liquid-like structure), at increasing 0, the coverage, should lead to either a positive or
Table I. Adsorption
parameters
No. (1) (2) (3) (4) (5) (6) (7) (8) (9) (JO) (11) (12) (13) (14) (15) (16) (17) 08) (19) (20) (21) (22)
Compound
Adsorption
of organic
and calculated
interaction
(neutral)
Benzene Phenol Benzoic acid Cinnamaldehyde Phloroglucinol Phenylthiourea Salicylic acid Pyridine r-picoline Imidazole Cudiacol o-cresol p-OH benzoic acid Tosylglycine Tymine Uracil Adenine 2 chloropyridine 2 aminopyridine L-phenylalanine p-methylphenyl Phtalic acid
acetic acid
compounds
on mercury--II
1375
energies for neutral aromatic compounds
AN”
i.b
0.018 0.025 -.Ol9 -.003 0.024 0.047 0.002 0.001 0.002 0.037 0.021 0.025 -.012 -.050 -.009 -.Ol2 0.022 -.Ol5 0.03 I 0.010 p.005 -.030
0.06 0.16 0.09 0.14 0.35 0.21 0.12 0.00 0.02 0.10 0.16 0.15 0.04 0.11 0.003 0.12 0.16 0.00 0.18 0.07 0.06
flFC
vd
0.70 -.I2 -.40 - I .20 - 2.70 -0.50 -0.60 1.30 1.60 -0.06
3.0 3.4 4.0 5.1 4.6 4.7 4.1 2.8 3.4 2.3 4.0 3.6 4.1 4.4 4.1 4.5 4.4 3.2 3.4 4.3 4.1 4.2
0.30 - 2.0 0.35 0.34 0.45 0.55
0.20 1.00 0.70
-&de (kJ mol-‘) 4.6 5.1 6.6 7.3 5.7 9.2 6.6 5.7 6.9 4.9 3.7 8.1 7.3 17.2 6.9 5.4 27.6 7.5 6.2 8.8 7.3 3.2
;‘Fractional number of electrons transferred from the adsorbate to the metal. hPartial charge transfer coefficient. ‘Frumkin interaction parameter. dNumber of water molecules displaced by one molecule of the adsorbate, when disposition on the surface (A,,,e, z 0.14 nm’). ‘Dispersion interaction energy by Buckingham. ‘Electrostatic interaction energy. negative
contribution
( - 2RTar0)
to
the
adsorption
Table 2. Adsorption
No.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
parameters
Compound
for anionic
aromatic
(anion)
3-indole carboxylic acid 4-indole carboxylic acid 5-indole carboxylic acid Benzoic acid p-amino benzoic acid Tosylglycine Benzoylglycine 4.6 dimethylpyrimidyl benzensulphamide p-methylphenyl acetic acid Nicotinic acid
“Electrovalency. hPartial charge transfer coefficient. ‘Frumkin interaction parameter. dExperimental adsorption Gibbs energy
0.2 6.0 -. 9 - 1.4 - I.1 0.86 -.I6 -1.2 -0.3 -4.7 -0.1 -0.6 -0.4 12.2 2.5 - 11.0 - I.0 I.8 -0.1 0.3 -0.4 -0.1
in a planar
account alone for the extent and sign of the energetic interaction among adsorbed molecules, implying of course always a negative sign of OF. Moreover, the term -~RTuF cannot be merely related to the value of &d. obtained for a mole of adsorbed aromatic molecules lying flat on the surface and covering the whole electrode surface (0 = I): this simply because &dis always stabilizing (ie. negative).
standard
energy at 0 = 0. The positive sign of i as well as that of AN (calculated using Pearson’s approach [I 51, the sensitivity on AN is kO.006 [2]) indicate a tendency of the electrons to flow from the adsorbed molecule towards the mercury, and this fact should always give rise to a repulsive energetic coulombic term (proportional to i’), which cannot Gibbs
adsorbed
I
(kJ t%-l)
compounds
- AG:nsd (kJ molV’)
;‘”
i.b
OF’
a.33 a.25 -0.28 -0.25 a.30 -0.1 I a.06
0.27 0.19 0.21 0.36 0.45 0.00 0.01
-0.95
-1.05
23.2 22.6 22.9 15.0 16.3 16.8 20.0
-0.39
0.42
-0.25
0.40
-0.60 -0.90 -0.50
23.7 29.4 18.0
at 0-O.
Planar
-1.15 -1.05 -0.85 -0.40 -1.20
disposition
on the surface.
1376
C. Fontanesi and L. Benedetti
Furthermore, the calculated se1values are in general negative, but what is more they do not fit with the corresponding CI~values, in the sense that they should be always opposite in sign with respect to @r for a compound, and when this happens (only 8 compounds in Table l), the term -2RTar is from 3 to IO times higher with respect to ~~1. So, one can conclude that each of the parameters reported in Table I cannot account alone for the quantitative estimation of QF(sign and value), at least in this ndive approximation used to evaluate the energetic interactions among vicinal adsorbed molecules. However, the adsorption process can be considered as a substitution of water molecules on the surface by the adsorbing substance. So that, at increasing coverage, when the stabilising energy due to the mutual interactions of the adsorbed aromatic particles overcome the even stabilizing interaction energy term among water molecules being substituted, then the AC:,,, B-0 will increase (in the negative sense) with B, or will decrease in the opposite case, so ruling the positive or negative sign of (IF. ~8 and ~~1have been calculated, with the same procedure as for the aromatic compounds, for water molecules lying flat on the surface, and the values are ~d.,~~~,~~ = - 1.56 kJ mol-’ and ~~~~~~~~~~= -3.78 kJ mol-‘. Of course, for each molecule/mole of organic, v molecules/moles of water will be displaced, according to the ratio of their corresponding area occupied on the surface (obtained for the adsorbate by roux, when available; otherwise by the CPK model; A,,,e, = I4 nm?), so that the energy difference between the fI = I and @= 0 states can be expressed as follows: - 2RTar: = &d- YE~.~~,~~,
(1)
which can be considered a shortened version of the relations reported in [I61 and [l7]. In this case, as previously mentioned, the role of the interaction between the adsorbed molecule (in the adsorbed state) and the solvent, &&,&o&ie wBleT, is here neglected. This is because the quantitative value of aF previewed by relation (I), in the absence of that contribution, is in good agreement with the experimental one. Moreover, ~d.adwbate.wstrr (when aging) should greatly affect the energy balance for the different compounds listed in Table 1 by a trend related to the AC;,, values in the bulk (ranging between 2 + 23 kJ mol-’ [5]), and this is not the case. In fact, using the experimental ar values, the calculated a posreriori &d.adSOrbate wa,elcontributions do not show any connection with AC&, (the most soluble compounds of the series, pyridine and related derivatives [I], show the smaller &d.adsorbatc%ter contribution). Finally, it has to be stressed that the balance in energy (relation I) reported above does not refer to a lattice-like model of the mercury surface involving the desorption of individually different clusters of water by the adsorbing aromatic molecule
-1
o! ~. ’ -3
I
I
-2
-1
I
/
0
1
I\
1
2
3
aF Fig. 2. Piot of &values, E = (2RTffF + Ed)/\‘, 1’s the Frumkin parameter a~. Dotted lines correspond to ~d.~~,.-~ = - 1.56 kJ moi-’ and to F.I~.,,~~ = - 3.78 kJ mol-’ respectively.
(each cluster, of I, 2, 3, molecules, contributing to a different extent to the total energy balance); the term %&,a& simply accounts (through v) for the extensive property of energy, and corresponds to the case in which water molecules are present in one state only at the interface [ 181and this state is estimated in the present work by calculating the intermolecular dispersion energy interaction between two water molecules by means of the relevant Buckingham equation. Then, &dhas tentatively been chosen with respect to ccl (for the aromatic compounds and water): at first &dand ccl represent a different approach to the same problem of the energetic interaction, so that they cannot be in any way balanced or mixed. Moreover, the electrostatic interactions due to net charges localised on vicinal molecules should be (not explicitly) just contained in the Buckingham term (Ed) by means of the empirical parametrisation [7]. A consequence of relation (I) is that:
2RTaF + Ed= I’
Ed
.w
a,ef
7
so, the term on the left side should result, for each as constant and corresponding to compound, - I.56 kJ mol-‘. The left-side term (E) has been plotted against @r for the compounds of Table I in Fig. 2. The features of this plot show clearly that, for low values of ar, OF= IO.8 t I], that a simple energetic balance could be reliable. Moreover, at a[: = 0 the perfect energetic balance is attained, E = &&water, thus confirming that the main interaction parameter that has to be considered, at least for water, is correctly &d.w&r (- I .I% kJ mol-‘) and not ~~~~~~~~ {the higher dotted line in Fig. 2), ( - 3.78 kJ mol-‘). Further, the case in which #r = 0 does not represent at all the absence of any attractive (repulsive) interaction among adsorbed molecules, but the situation in which &d= VQ~~,~~~, and this
Adsorption
of organic
compounds
happens for each compound for which &d and v (related to its dimension) satisfies that relation. This feature regarding the existence of an attractive interaction even at or = 0 was already suggested [6, 191 on the basis of a completely different argument, founded on the existence of a positive partial charge transfer for a group of compounds now also considered (giving rise, of course, to electrostatic repulsion among vicinal adsorbed molecules) balancing the attractive interaction itself. One can conclude that even taking into account very different molecular structures, in the Frumkin type adsorption (liquidlike) the energetic interaction among molecules is always attractive, but the sign of or can be positive or negative (and also its value can be estimated) determined in relation to the balance, which accounts for a number of water molecules dislodged from the surface showing always an attractive dispersion energy among themselves. Furthermore, the goodness of the comparison shown here between a Gibbs energy contribution (AG&,, = ,) and a balance of dispersion energy terms (by Buckingham) involves a thermodynamic consequence: the decrease in configurational entropy related to the adsorption of the aromatics (from the solution to the surface) has to be balanced by almost the same amount in entropy gained by a cluster of r water molecules displaced from the surface into the bulk. As a result, AE ( zAH) can be compared to AG and the adsorption equilibrium (Frumkin-type) should depend little on the temperature, as indicated by the few experimental data reported in the literature; in fact, a small effect of temperature (for about 20-30 degrees more or less than 298 K) has been observed for different adsorbed compounds, DMSO, DMF, water itself, pyridine and acetophenone, long chain aliphatic alcohols, polyalcohols and thiourea, and in any case negligible for high surface concentration excess (I) z I) and close to the potential of zero charge or at the value of zero charge [2&24]. Moreover, the isoentropicity of this process (as a whole) accounts for a liquid-like type order of the aromatic adsorbate on the electrode surface. When the adsorption on mercury from aqueous solution (at E,,,,) of a group of aromatic anions is considered (Table 2), the adsorption Gibbs energy values at O-+0, reflecting the interaction between the aromatic anions (still in a planar disposition, with their aromatic moiety on the electrode) and the mercury are very similar to those corresponding to neutral aromatics. Nevertheless, very high partial charge transfer coefficients (obtained by a negative and high experimental value of the electrovalency, 7) are found together always with negative values of aF. The presence of the net charge in the anions should lead to a large and repulsive coulombic interaction among vicinal adsorbed molecules, overcoming any other energy contribution related to the desorbing of neutral water molecules. So that, it is entirely reasonable that UF values result as being high, and
on mercury--II
1377
what is more, negative. So that ar reflects the decrease of AC& at increasing coverage. This can be proved, on a qualitative basis, taking into account that the lower the partial charge transfer, the higher the Frumkin coefficient (negative): indeed, when i is small a higher charge will remain localised on the adsorbed molecule and or will be larger. Figure 3(a,b) illustrates this simple conclusion; Fig. 3(a) shows a quadratic dependence between OF and I - i (or j.). while Fig. 3(b) shows a more linearized one taking into account that the interaction energy among vicinal molecules becomes in some way proportional to the square of the charge (ie, (I - 2.)’ or i.‘). save that in this very simple picture the differences in the symmetry and distances of the charged objects are in this case of minor importance.
CONCLUSIONS The Frumkin interaction parameter (or) can be related, even if still roughly, to the balance of energy dispersion terms regarding adsorbate-adsorbate and water-water interaction reflecting the main role
1.0
0 a
0.9
7
6 1
Ir
0.4 i 1.0
0 7
b
l 6
0.8 ![ N
F 0.6
I
I
70.4 -
0.2 1 0.0
1 I
0.2
1
0.4
I
0.6
I
-
0.8 -a
1.0
1.2
1.4
F
Fig. 3. Relation between: (a) the residual charge on the anion, (b) the square of this residual charge and the Frumkin parameter. The correlation coefficients are. respectively. 0.90 for the least squares second order (parabolic) curve in (a) and 0.83 for the first order (straight line) curve in (b).
1378
C. Fontanesi and L. Benedetti
played by the process of substitution on the electrode surface of water by the aromatic molecule. The interaction among adsorbed aromatics (neutral form) is always attractive, so that the sign of&, as well as its improper meaning of “interaction” parameter, arises from that kind of balance. That the process of substitution on the surface is practically isoentropic (AE can be compared to AG), so that only slight effects of temperature on the formation of the liquid-like adsorbed layer can be previewed. For aromatic anions, the change in AC:, on varying the coverage is mainly due to the electrostatic repulsion among charged adsorbed particles, taking into account the role of a large and dative partial charge transfer from the adsorbate to the metal surface. REFERENCES
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