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Electrochimica Acta 46 (2001) 12771284

On the adsorption of pure ethylene glycol on mercury

Claudio Fontanesi a,*,1, Luca Benedetti1 a, R. Andreoli1 a, Marcello Carla b

a Department of Chemistry, Uni6ersity of Modena and Reggio Emilia, Via Campi 183, 41100 Modena, Italyb Department of Physics, Uni6ersity of Florence, L. go E. Fermi 4, 50125 Florence, Italy

Received 30 June 2000; received in revised form 26 October 2000

Abstract

The standard Gibbs energy of adsorption of pure liquid ethylene glycol (EG) on mercury is assessed to range

between 50 and 60 kJ mol1, about twothree times with respect to water (21 kJ mol1). This evidence is

obtained on the basis of a series of indirect and independent experimental results. Moreover, ab-initio calculations are

performed to account for the interaction energy relating the isolated EG molecule and a cluster of seven (or four) Hg

atoms: three different geometrical approaches of the EG molecule with respect to the cluster are considered (top,

hollow, bridge) as well as three EG conformations (trans, cis, cis-OH, the last one with an intramolecular hydrogen

bond). The deepest minimum in the energy versus (Hgcluster EGmolecule) distance potential energy curve, amounting to

80 kJ mol1, is obtained in the case of the cis conformation of EG. In particular, it is found that this stabilizing

energy is essentially due to the oxygenmercury atoms interaction, giving rise also to an appreciable charge transfer

coefficient from the EG molecule to the Hg cluster. Finally, in this conformation, the EG adsorbed layer is also

stabilized by an attractive energy term effective among vicinal interacting molecules. 2001 Elsevier Science B.V. All

rights reserved.

Keywords: Ethylene glycol; Mercury; Adsorption; Ab-initio calculation

www.elsevier.nl/locate/electacta

1. Introduction

The comparative study of the adsorption of a series

of aromatic derivatives on mercury in the same experi-

mental conditions, but from two solvents, water and

ethylene glycol (EG) [1], has assured the reliability of

the so called Intrinsic adsorption Gibbs free energy,

DGINT a value related to the virtual process [2]:

Adsorbate(pure)+Hg X Hg(Adsorbate)

in that this value is really intrinsic, i.e. it can be fully

related to the molecular properties (experimental and

calculated) of the adsorbed species alone, only when

solvation effects in the adsorbed state are negligible

[3,4]. The electrochemical and thermodynamic proce-

dure to obtain this evidence has, as a direct conse-

quence, that the DGINT of EG on mercury, related

again to the virtual process: EG(l,pure)+Hg X Hg(EG)

should amount to $51 kJ mol1, with no other

direct experimental evidence. Taking into account theadsorption of water on uncharged mercury,

H2O(l,pure)+Hg X Hg(H2O), a value ofDGINT$21

kJ mol1 has been achieved by a direct measurement

from the vapor phase of water and using a classical

thermodynamic cycle [5,6], a confirmation ofDGINT for

EG is crucial to preview, for example, the adsorbability

of EG on Hg from its aqueous solution and to asses its

greater interaction, with respect to water, on a molecu-

lar level, toward the mercury atoms as suggested by the

great and negative DGEG value as previously estimated.

* Corresponding author. Tel.: +39-059-378462; fax: +39-

059-373543.

E-mail address: [email protected] (C. Fontanesi).1 ISE member.

0013-4686/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 7 1 2 - X

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C. Fontanesi et al. /Electrochimica Acta 46 (2001) 127712841278

Table 1

Hardness (p), softness (|) and absolute electronegativity ()

for the three components of the system Hg, H2O, EGa

pcalc | exppexp |calc exp calc

0.18Hg 5.54 4.9

8.7H2O 0.109.5 0.11 3.1 3.5

7.2 0.14EG (cis) 3.9

7.36 0.136EG (cis-OH) 3.957.39 0.135 3.96EG (trans)

a For EG three comformers are considered: i.e. the cis, then

a cis form characterized by an intermolecular OH bond and

trans.

the application of the hard and soft concept [8] to the

competitive three-component system: water Hg EG

(i.e. who is substituted by who on the uncharged mer-

cury surface); (d) a quantum mechanical calculation of

the interaction energy between one EG molecule and a

sheet composed of seven Hg atoms, calculated by the

similar performance of a water molecule on a mercury

layer [9,10].

2. Calculation and procedure

The mercury surface has been simulated using clus-

ters of seven Hg atoms for the on top disposition and

of four Hg atoms for the hollow and bridge site

positions.

Three different conformers for the EG molecule has

been considered in the potential energy surfaces (PESs)

calculations: cis, trans, and cis-OH (i.e. featuring an

intramolecular hydrogen bond).

The geometry of the three EG conformers has been

fully optimized at the RMP2/6-31g* level of the theory.The geometrical parameters of the Hg cluster, inter-

atomic HgHg distance (0.3 nm) and hexagonal sym-

metry, are taken from an experimental X-ray

diffraction study on liquid Hg by Bosio et al. [11]. The

Hg cluster EG molecule PESs calculations were car-

ried out by means of the GAUSSIAN 94 program, using

a LanL2DZ basis set with MP2 (frozen cores) electron

correlation [12]. The PESs were calculated keeping both

the Hg cluster and EG internal geometries frozen, while

varying their reciprocal distance and orientation.

The MNDO Hamiltonian, as implemented in the

AMPAC program [13], was used to calculate Mulliken

atomic net charges and the HOMO and LUMO energy

values of water and EG; the latter to be used in the

application of the hard and soft concept (Table 1).

This choice allows the comparison with previously pub-

lished results [14].

For both EG and water, the interaction energy active

between a couple of molecular species, A A, is calcu-

lated taking into account both the dispersion energy

term (d) and the electrostatic contribution (el). The

first is calculated by using the Buckingham dispersion

energy function [14].

In the case of ethylene glycol, the different trans, cis

and cis-OH conformations are considered with the CCbond essentially disposed coplanar and also with the

two oxygen atoms pointing towards the mercury layer

(Fig. 1).

Variations in EGEG, qEGEG and rEGEG are ofy/4

radiants for the angles and of 0.01 A, for the distance.

For water, the disposition suggested as the more stable

is accounted for: the oxygen atoms facing the Hg

surface, and the dipole moment parallel to the surface

are in someway a little tilted towards the bulk; a

perpendicular disposition is considered also [15,16].

In this paper this attempt is pursued taking into

account the following four independent arguments: (a)

a relation associating the DGINT for a generic adsorbate

to its molecular dimension which is proportional to the

number of solvent molecules (water) dislodged from the

surface of Hg in the adsorption process [3]; (b) a brief

re-examination of the only thermodynamic study [7]

regarding the adsorption of EG from water on Hg; (c)

Fig. 1. Mutual disposition of EG and water molecules for the

calculation of intermolecular energy contributions. The Hg

surface corresponds to the paper plane. Three different confor-

mations of EG are considered, and also two different mutual

disposition of the EG cis conformation. For water two differ-

ent mutual dispositions are taken into account.

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C. Fontanesi et al. /Electrochimica Acta 46 (2001) 12771284 1279

Table 2

Calculated intermolecular interaction energy (d and el) for

adsorbed EG in different conformations and water molecules a

el (kJ mol1)d (kJ mol

1)

EG (cis, planar) 4.9 0.2

5.1 +2.1EG (cis,

perpendicular)

EG (cis-OH) 1.35.20.39.2EG (trans)

3.8Water (planar) 1.6

+1.01.2Water (perpendicular)

a For mutual disposition of the molecules see Fig. 1.

MNDO Hamiltonian implemented in the AMPAC pro-

gram [13].

A minimum value (which invariably corresponds to a

maximized packed disposition) in energy is obtained

from several relative minima. Both d and el reported

values (Table 2) correspond to almost the same configu-

ration between interacting molecules. For the different

compounds, the equilibrium distance r is determined by

the Van der Waals radii.Experimental adsorption data are taken from the

literature and in some cases revised.

3. Results and discussion

The comparative study of the adsorption process of

aromatic derivatives from water and EG solutions

[1,3,17] leads to the conclusion that by itself EG

should be adsorbed on bare mercury, from its pure

liquid phase, with a value of DGEG$51 kJ

mol1

(provided that to the same virtual process, butregarding water, a value of DGw=kJ mol1 has been

assigned). Now for a very large set of organic adsor-

bates, all of them aromatics and reasonably interacting

with the mercury surface in the same mechanistic way,

i.e. through a liquid-like adsorption process and with

planar disposition of the aromatic moiety on the elec-

trode surface [2,5], a good linear relation was obtained

relating the molecular hindrance of the individual ad-

sorbate (proportional to the number, w, of solvent

molecules, in the case water, dislodged in the adsorp-

tion process) and the DGADS,INT value. The relationship

shown in Fig. 2 is highly satisfying, despite the large

variety of molecular structures of the adsorbates (no. 38

compounds, one more point than previously reported

have been included combining the results from Refs.

[1,4]). The meaning of this relation sets on the crucial

role, on a energetic ground, played by the process of

substitution of solvent molecules covering the Hg sur-

face worked by the adsorbing species [6,18], much more

than on the fact that all compounds are aromatics;

actually, when aromatics are perpendicularly adsorbed,

their adsorption energy is almost the same [14], inde-

pendent of the aromatic character of the individual

compound.

So, solely referring to the molecular dimension andon the hindrance occupied on the electrode surface, a

tentative extension to non-aromatic molecules of the

relation in Fig. 2 can be attempted.

Therefore, when the adsorption of EG on mercury

with the OH groups interacting with the surface is

considered, a number of 2.1 water molecules (statisti-

cally) has to be replaced, wEG=2.1, taking into account

the following molecular areas, respectively: AEG=0.31

nm2 and Aw=0.14 nm2 ([3,7] and references therein

cited). Then the extrapolation in the plot of Fig. 2 at

Slightly different relative minima of d are so ob-

tained and the absolute one is chosen for the discus-

sion. Using the same reciprocal configurations and

procedure for the rotation of angles and distances, the

intermolecular electrostatic term is then calculated:

el=1

4y0 %NAi=1 %NBk=1 qiqkri,k where 0 is the vacuum permittivity, NA and NB 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 qk are the MO SCF net

charges localized on atoms i, k belonging to the distin-

guished individual species A and B, respectively. MO

SCF net charges of each atom, concerning the unper-

turbed isolated molecule, are calculated using the

Fig. 2. Intrinsic adsorption Gibbs energy of aromatic com-

pounds in relation to their individual area occupied on the

mercury surface, which is proportional to the number (ww) of

water molecules dislodged from the surface itself in the ad-

sorption process.

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wEG=2.1 gives a DGEG of about 50 kJ mol1, as

well as for w=1, a DGw value of about 20 kJ mol1

is obtained, thus corresponding to the process involving

a water molecule (mole) self-replacing at the interface,

according to two different energetic states, the bulk and

the surface. This result, completely independent of that

obtained in Ref. [1], is valuable cause it arises by an

extrapolation procedure. Let us now consider the ad-

sorption process of EG from water, for which thefollowing thermodynamic balance can be considered:

DGEG=DGADS,EG+DGsol,EG+wEGDGw (1)

taking into account the solution of pure EG in water

(DGsol,EG), the experimental measurement of EG ad-

sorbed on Hg from water (DGADS,EG) and the substitu-

tion of wEG=2.1 molecules/moles of water by each EG

molecules/moles, wEGDGw. The only experimental

value of DGADS,EG cited in the literature amounts to

8.4 kJ mol1 [7] (at Epzc, the potential of zero

charge), so that DGEG=DGsol,EG52.5 (kJ mol1).

The value to be ascribed to DGsol,EG depends on the

choice of the standard state regarding the solution,

which has to be the same of that involved for the

evaluation of DGADS,EG. For the solution of EG in

water (not an equilibrium process):

EGpure,liquidEGsolution

DGsol,EG=vEG,solutionvEG,pure,liquid

Moreover:

vEG,solution=vEG,pure,liquid+RT ln(a(X)/aX) (2)

the reference function for the activity is selected as the

molar fraction (X) (the relevant standard state is activ-

ity=X=1) and:

vEG,solution=vEG,solution+RT ln(a(c)/ac ) (3)

In this second case, the reference function for the

activity is selected as the molar concentration (c) (the

relevant standard state is activity=c=1).

Eqs. (2) and (3) correspond to the same physical state

of the system, then:

vEG,pure,liquid(X)+RT ln(a(X)/aX)

=vEG,solution(c)+RT ln(a(c)/ac )

Note that aX=1 as well as ac=1. The use of the

Frumkin isotherm to define the DGADS,EG, as in the

mentioned case [7], involves the choice of activity=c=

1 (molarity), so that

DvEG,sol=vEG,solutionvEG,pure,liquid=RT ln(X,c=1)

(the solution is supposed ideal, aX. When c=1,

X=0.01871 and DvEG,sol=9.85 kJ mol1. In this

way a value of DvEGl=62 kJ mol1 can be

achieved; a value not so far from that previously ob-

tained (sensitivity on wEG90.2 involves the term wEG

DGw, a variation of about 94 kJ mol1).

Furthermore, whether the interaction adsorbate

electrode surface implies a partial charge transfer and/

or a molecular rearrangement, that is the adsorption

process is intermediate between a pure physisorption

and a true covalent bond (see the DGADS,INT values in

Fig. 2), then this kind of interaction could be related to

the concept of absolute hardness, p; this is a quantity

defined by Parr and Pearson [8,19] by an operative

procedure in connection with the ionization potential EIand the electron affinity EA of whatever chemical sys-

tem, molecules, atoms, ions, radicals: p=(EIEA)/2,

the EI and EA quantities can be substituted by the

EHOMO and ELUMO of the corresponding chemical enti-

ties considered as isolated systems. So, hard molecules

show a large HOMOLUMO gap, and on the contrary

soft molecules a lower one: softness=|=1/p. The

interaction between two systems is favored if both of

them are soft: the mercury metal at the point of zero

charge is considered to be relatively soft [20]. The plot

reported in Ref. [4] clearly shows that soft adsorbates

are more active on the surface, at least in terms of the

DGINT value. In this view, let us now consider the most

favorite couple between components of the actual three

components system: H2OHgEG (Table 1). Because

the hardness concept and its value are based on

electron affinity and ionization potential, for liquid

mercury (atomic) they came from the experimental

evidence, while for molecular systems, they have to be

calculated sometimes.

From the data reported (for experimental values see

Refs. [21,22]), mercury turns out to be the most soft

one with respect to both water and EG, but water is

more hard than EG.

For EG, the soft and hard character have been

calculated taking into account three possible conform-

ers: cis, cis-OH (the cis conformer characterized by an

intramolecular OH bond) and trans.

So, an initial system composed by Hg and adsorbed

water (a softhard couple) will change, in the presence

of EG in solution, into a more stable arrangement, Hg

and adsorbed EG, corresponding to a couple with more

softsoft character. On a quantitative ground, EG has

to show an intrinsic adsorption energy higher than

water.

3.1. Theoretical outcomes

To confirm the reliability of this DGEG value, in the

absence of a direct measurement of the adsorption of

EG gas on liquid mercury, a calculation of this energy

of interaction between a molecule of EG and a cluster

of four or seven Hg atoms (anyway simulating a mer-

cury surface) has been performed.

The presence of a minimum in energy (total) of the

whole adduct has been investigated varying both the

distance of the EG molecule from the central mercury

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Fig. 3. Top-1 disposition of a molecule on Hg ( cis conformer)

on a seven atom cluster of mercury. Top view (left) and side

view (right).Fig. 6. Disposition of a EG molecule (trans conformer) on the

seven mercury atoms cluster (top view).atom of the cluster and the disposition of the two

oxygen atoms of EG with respect to Hg atoms.

Five different dispositions have been characterized

and the proper different denominations assigned for

similarity to those regarding a water molecule interact-

ing with a mercury layer. Moreover, three different

molecular conformations of the isolated EG molecule

have been taken into account: cis, trans and a cis-OH.

This because a cis conformation has been suggested by

experiments as the most stable for EG in the pure

liquid bulk and even in the gas phase and the existence

of the intramolecular hydrogen bond has been also

proposed [2325].So, for the cis conformer, the top-1 disposition in-

volves the two oxygen atoms pointing toward the Hg

layer (Fig. 3). In the top-2 arrangement, the two CO

bonds are forced parallel to the metal layer and finally

the top-3 disposition features the two oxygen atoms

pointing away from the mercury layer, toward the bulk.

Furthermore, another two dispositions have been also

considered: the hollow one (Fig. 4a), and the bridge

disposition (Fig. 4b).

Fig. 5 clearly shows that for the cis conformer, the

most stable structure interacting with Hg is that regard-

ing the top-1 disposition. This kind of disposition in-

volves two oxygen atoms of EG pointing towards the

Hg layer and interact, contemporarily, with three Hg

atoms by approaching the cavity among them; the

minimum of the total cluster energy corresponds to

81 kJ mol1.

For the EG trans conformer, taking into account the

role played by the interaction of the two oxygen atoms

of EG with Hg, only the disposition shown in Fig. 6

has been investigated because any other arrangement of

the EG molecule should be pointing at least one oxygen

atom toward the bulk. In this case the minimum in

energy of the cluster plus EG corresponds to about

43 kJ mol1 (Fig. 7). In our calculations, the dipolemoment of the Hg7 EG cluster depends linearly on the

distance between the mercury plane and the oxygen

atoms, this result indicates that the HgEG energetic

interaction is mainly of electrostatic nature. In fact, the

oxygens of EG bear a negative net charge which origi-

nates a negative electrostatic potential field more local-

ized (intense) than the positive one; such a negative

electrostatic potential field can account for the leading

role played by the orientation of the oxygens with

respect to the mercury. This view is also supported by

Fig. 4. Hollow (left) and bridge (right) disposition of a

molecule of EG (cis conformer) on a cluster of four mercury

atoms.

Fig. 5. Total electronic energy of the Hg clusterEG molecule

adduct (cis conformer) at varying the distance from the CC

axis of EG from the mercury cluster.

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Fig. 7. Total electronic energy of the Hg clusterEG molecule

adduct (trans conformer) at varying the distance from the CC

axis of EG from the mercury cluster.

the fact that none of the occupied MOs show a signifi-

cant bonding nature between the Hg and the EG

molecule and is in line with theoretical studies concern-

ing the adsorption of water on mercury [26].

Finally, the cis conformation of EG with the in-

tramolecular hydrogen bond has been studied, finding a

shallow minimum in energy, 0.5 kJ mol1 for the

disposition set out in Fig. 8.

The energetic scheme for the different types of inter-actions is summarized in Fig. 9.

The process simulated by using quantum mechanical

methods is then:

EG(g)+Hg(l)Hg(EG) DU

and the relevant calculated quantity results a DU value,

which is not directly comparable to the DGEG value

previously obtained; it can be transformed into a DH

value with the assumptions that EG(g) is a pure ideal

gas (P=1 bar, T=298 K) and that the volume of a

mole of EG adsorbed on mercury with respect to the

molar volume of the gas is negligible; so,D

H=D

URTDn=DU2.5 kJ mol1.

Now, when the gas liquid phase change is consid-

ered for EG [27], it can be obtained the relation (3):

EG(l)EG(g) (a)

EG(g)+Hg(l)Hg(EG) (b)

EG(l)+Hg(l)Hg(EG) (4)

suitable combination of the relevant thermodynamic

function variations yields:

and considering T=298 K:

DG(1)

=DH(1)

TDS(1)

(5)

DG(1)=DUTDS(1)+62.4

provided that the main entropic contribution is due to

the process (a)2; then:

DG(1)=DU+15.8 (6)

Fig. 8. Disposition of a EG molecule (cis-OH conformer) on

the seven mercury atoms cluster (top view).

Fig. 9. Energy diagram (three different EG conformers) for the

formation of the EGHg cluster adduct.

2 Regarding the EG adsorption, as suggested by experi-

ments, no close packing of adsorbed molecules can be re-

vealed, so that a not negligible degree of freedom of the

adsorbed species on the metal surface has to be accounted for.

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This relation gives rise now to a value of DG1 of

about 65 kJ mol1 only when DU for the top-1

disposition without an intra-molecular hydrogen bond

is considered in the interaction with the mercury clus-

ter. This result is fully comparable with the outcome

characterized by the previously reported methods in the

first section of this paper.

3.2. The interaction between EG adsorbed molecules

In a crude view, the interaction energy among ad-

sorbed and then vicinal EG molecules can be repre-

sented by the values of d and el (see Section 2)

reported in Table 2 for different conformations of EG

and also for at least two different mutual dispositions

of EG molecules on the surface, planar and perpendicu-

lar, but anyway with the two oxygen atoms pointing

toward the metal surface. This last arrangement is

however the one that gives rise to the deepest energy

minimum when the interaction of EG with Hg in the

top-1 disposition is considered.The dispersion energy term, d, is always negative,

pointing out an attractive interaction among EG

molecules, while the electrostatic one, el is in some

cases positive (smaller than d).

These two terms represent a different approach to the

same problem of the energetic interaction, because the

electrostatic ones, due to the net charges localized on

vicinal molecules which give rise to el, are just con-

tained (but not explicitly) in the empirical parameteriza-

tion of the Buckingham term d [14].

At this level of the discussion, any interaction of the

first EG layer toward the bulk (i.e. the presence of asecond or a third layer of oriented EG) is completely

neglected.

Anyway, the most stable disposition of EG on the

mercury layer, i.e. a cis-conformation in the top-1

configuration, shows to be stabilized also by an attrac-

tive interaction among vicinal molecules, mainly on the

basis of the d value. On the reliability of this value,

which now is crucial, one can get some answers devel-

oping the conclusions obtained by Trasatti [7] on the

adsorption process of EG from water on mercury. For

that process, a substitution of water molecules ad-

sorbed on the Hg surface by the EG molecules can besuggested:

EG(solution)+Hg(water)

X Hg(EG)+water(solution)

In terms of energetic contributions, taking into ac-

count the linearized Frumkin isotherm and the relations

discussed in the Refs. [28,29] one can suggest that:

2RT AF(EG)=EGwEGwater (7)

where AF(EG) represent the Frumkin interaction

parameter of the macroscopic adsorption process, EGand water have the usual meaning till now discussed

(they can be represented by d or el and wEG=2.2 is

obtained taking into account a molecular area of 0.31

nm2 for EG and 0.14 nm2 for water [1,3,7].

In fact, wEG corresponds to the number of molecules/

moles of water displaced by the adsorption of one

molecule/mole of EG.The experimental evidence shows an adsorption be-

havior of EG from water [7] of Langmurian type, i.e.

with AF(EG)=0.

Of course this does not imply that an energetic

interaction between two nearest EG adsorbed molecules

is not effective; on the contrary, it let the reliability of

d or el for EG to be estimated from the corresponding

value for water, and this procedure can be checked for

different conformations of EG and different EG dispo-

sitions on mercury and water as well, taking into ac-

count the data reported in Table 2 and the relation (7)

with an AF(EG)

value=0.

The ratio EG/water should result equal to wEG=2.2,

and from Table 2 it can be observed that for the

couples d,EG cis(planar and perpendicular) and d,water(planar and perpendicular) a value for the ratio range

between 3.06 and 4.18, not so far from the value of 2.2.

A value of 2.3 is anyway obtained when el is used for

the d,EG cis( perpendicular) and d,water(perpendicular),

but unfortunately those two terms are individually pos-

itive, i.e. indicating a repulsive interaction (moreover,

the pure perpendicular disposition of water is not con-

sidered the most favorable one).

All other combinations of EG and water give rise to

values corresponding to a ratio far from 2.2.

It is worth mentioning that this rigid modeling and

crude view of the interaction regarding EG and water

molecules among themselves only partially and poorly

account for the disposition suggested in Refs. [7,15], in

which the EG molecules are considered to be in some

way tilted toward the bulk as well as for water, so that

an intermediate disposition should be much more

reasonable.

3.3. Some conclusions

Data based on independent approaches involvingindirect experimental procedures and classic thermody-

namics lead to the evaluation of the Standard Gibbs

energy of adsorption of pure liquid EG on mercury: it

should range between 51 and 62 kJ mol1.

The value is confirmed taking into account the quan-

tum mechanical simulation of the interaction of a

molecule of EG gas on a cluster of mercury atoms, but

only for the cis conformation involving the pointing of

the two oxygen atoms towards the mercury layer. In

this case, a partial charge transfer coefficient from EG

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to mercury can also be calculated (0.077 electrons)

for the top-1 disposition.

This theoretical result suggests that the trans confor-

mation of EG can be not favored in the interaction

with mercury as well as the cis one but showing the

intramolecular hydrogen bond, cause the hydrogen

bond should prevent the stabilizing allocation of each

of the oxygen atoms among the Hg atoms of the

cluster.Moreover, the calculated dispersion energy term for

EG-cis confirms a stabilizing effect among vicinal inter-

acting molecules and suggesting at least a pseudo-per-

pendicular disposition of each molecule in the adsorbed

layer on mercury.

Acknowledgements

Planning and development of the studies here pre-

sented form a part of an Italian National Research

Project entitled Elettrocatalisi ed Elettrosintesi, Coor-dinatore Scientifico Professor Rolando Guidelli. The

authors would like to thank the Italian Ministero dell

Universita e della Ricerca Scientifica e Tecnologica

(Programmi di Ricerca Scientifica di Rilevante Interesse

Nazionale) for financial support.

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