Adsorption Behavior of n-Hexanol on Ag(lll) from Aqueous 0.05 M KCIO4

Adsorption

Behavior

of ro-Hexanol

on

Ag(lll)

from

Aqueous

0.05

M

_KCIO4

Maria Luisa

Foresti*

and Massimo

Innocenti

Dipartimento di Chimica, Universita’di Firenze, Via G. _Capponi 9, 50121 Firenze,

Italy

Antoinette Hamelin

Laboratoire d’ElectrochimieInterfaciale, CNRS, 1 PlaceA. _Briand,

92195MeudonCedex,France

ReceivedMay9, 1994. In Final Form:September 27, 1994®

A_{computerized chronocoulometric technique}was _employedto_{investigate the adsorption behavior of} n-hexanol(NHEX)on the (111) faceofa silver_singlecrystalfromaqueous 0.05M KCIO*. Thecharge density, <tm, as a function ofthe applied potential, E, andofthe NHEX concentration was analyzed

thermodynamically,yieldingthe Gibbs surfaceexcess andthe standardGibbsenergy ofadsorption,AG°ads,

of NHEX as afunction of Eand ctm· Acomparisonofthe experimental adsorption parameterswith a

molecular model points toaflat orientation oftheadsorbedNHEXmolecules. Thelow valueof—AG°ads

atthe potentialofmaximum adsorption,18.4kJmol™1,isindicativeofthestronglyhydrophilicityofsilver. Introduction

Investigationsoforganicadsorptionon solid metalsare

importantfrom botha_{technological and}a fundamental

point ofview. Thus,these researchesmay allowabetter understanding ofthemechanism of_{electrode processes}

in electrosynthesis and electrocatalysis,as wellasabetter control in theuse of_organicsas corrosioninhibitorsand polishing agents. Moreover, the adsorption ofneutral

organic compounds at electrodes provides valuable in-formation on the structure oftheinterfacial_region.

Quantitative investigations ofthe adsorption of

un-chargedorganicmoleculeson _{singlecrystal}electrodesare

scanty andthemajorityof themwere carried outon metals

withlowmeltingpointssuchas _Bi,1Sn,*12

Zn,3andPb,4on thebasisofdifferential capacity measurements andwith

an _{extrathermodynamic analysis of}the_experimentaldata. Differential capacity measurements often fail to yield

realistic quantitative data5 because ofthelack of equi-librium at the interphase: this isthe main reason _why

these measurements are _hardlyamenableto a thermo-dynamic analysis, which is quite demanding as to the

accuracy andreproducibilityto the data tobe _analyzed.

Results _{accurate enough to} permit a _satisfactory thermodynamicanalysis havebeenmore _recentlyobtained

bychronocoulometry, which allowsreproducible

adsorp-tion measurements even if _adsorption is _{very slow,} providedthat asmall_{potentialrange}is available where desorption is fast and complete. The differences in behavioron differentsinglecrystalfacesare small for the low melting metals, probably due to a _{relatively high}

surfacemobility.6 Conversely,substantialdifferencesare

observedwith Ag and Au. Chronocoulometryhas been employed in the study ofthe adsorption oftert-pentyl

alcohol,pyridine, diethyl ether, andbenzonitrile on Au

®

Abstract published inAdvanceACS Abstracts,December15,

1994.

(1)Paltusova,N. A.;Alumaa,A. R.;Palm,U. V.Elektrokhimiya1979, 15, 1870.

(2)Kuprin, . P.;Grigoryev, N. B.Elektrokhimiya1980, 16, 383. (3)Ipatov, Yu.P.;Batrakov, V. V.Elektrokhimiya1975, 11, 1282. (4)Khmelevaya,L. P.;Chizhov,A.V.;Damaskin,B. B.;Vainblat,T.

I.Elektrokhimiya1980, 16, 257.

(5)Nguyen VanHuong,C.;Hinnen,C.;Dalbera,J. P.;Parsons,R.

J.Electroanal.Chem. 1981, 125, 177.

(6)Parsons,R.J.Electroanal. Chem. 1983, 150, 51.

single crystalfaces andin that ofpyridineon _{Ag single} crystalfaces.7

Quantitativeinvestigationsofthe_{adsorption ofsimple} aliphatic compoundsare _quitelimitedin numberon Au single crystal faces.8 No definite conclusions have yet

beendrawn7astothe effectofthe_{different crystallographic}

orientations ofAu on _teri-pentyl alcohol adsorption.910

Morescarce resultswere _publishedfor_{Ag single crystal}

faces.

For adsorption of aliphatic compounds on _{Ag single} crystal faces, no _{experimental data} accurate enough to allowa_{thermodynamic analysis}are _{presently available}

in the literature. Hinnenetal.11_comparedthedifferential capacity(C)vs _potentialcurves of0.02MNaF_aqueous solutions saturated with ethyl ether on the three low-indexfacesofAgwiththesame curves in theabsenceof

ether. _Ethyl ether does not affect theCvs E curve on

Ag(110),thusindicating that itsadsorptionon thisface is _{either vanishingly}smallor _verysloweven atthe low frequency adopted (15 Hz). However, ethyl ether de-presses thedifferentialcapacityon the

_Ag(lll)

and Ag-(100)facesin theproximityofthe_{potential of}zero_charge,

this depressionbeingslightlygreateron

Ag(lll)

thanon

Ag(100). The lackofa _{desorption peak} on the positive

side ofthecapacityminimum,which contrasts withthe

presence ofa_{desorption peak}on _{the negative side,} was

ascribed by the authors to slow desorptionkinetics,11which is _{particularlypronounced at positive potentials. This}

prevented a _quantitative estimate of diethyl ether

ad-sorptionon _{Ag from}differential_{capacity measurements.}

Vitanovand_{Popovdetermined the adsorption isotherms} of -hexanol,12isobutylalcohol,13andro-pentanol14on

Ag-(111)and_{Ag(100) atthe potential of maximum adsorption} from differential capacity measurements using an

ex-(7)Lipkowski,J.;Stolberg, L. InAdsorptionofMolecules at Metal

Electrodes·,Lipkowski,J.,Ross,P._N.,Eds.;PlenumPress: New York,

1992;p 171.

(8)Lipkowski,J.;Nguyen Van Huong,C.;Hinnen,C.;Parsons, R.

J. Electroanal.Chem. 1983, 143, 375.

(9)Richer, J.; Stolberg, L.;Lipkowski,J._Langmuir1986, 2, 630. (10)Richer,J.;Lipkowski,J.J.Electroanal.Chem. 1988, 251, 217. (11)Hinnen,C.; Nguyen Van Huong, C.;Dalbera,J. P. J. Chim.

Phys.(France) 1982, 79, 37.

(12)Vitanov,T.; Popov, A.Elektrokhimiya1974, 10, 1373. (13)Vitanov,T.; Popov, A.Elektrokhimiya1976, 12, 319. (14) Popov,A.;Velev, O.;Vitanov,T. J.Electroanal.Chem. 1988, 256, 405.

0743-7463/95/2411-0498$09.00/0 &copy; 1995American Chemical Society

Downloaded via UNIV DEGLI STUDI DI FIRENZE on October 6, 2020 at 14:41:46 (UTC).

Adsorption Behaviorof

Ag(lll)

Langmuir, Vol. 11, 2, trathermodynamic approach based on the_two-parallel

capacitormodel. The_adsorptionofn-hexanolfrom0.05 MNa2S04was _investigatedon _{Ag single crystals grown}

electrolyticallyina_{glasscapillary; thesinglecrystal}faces

exhibited a number of screw dislocations which were

partially eliminatedbyelectrodepositingAg+ ionsata 1

mV_{overpotential.12 According}to _{the authors, the}time

requiredto_{attain adsorptionequilibrium}at_{the potential} ofmaximum adsorptionwas about4hforthe mostdilute alcohol solutions and30 min for the saturated solution.

After attaining these _{equilibrium conditions, the} dif-ferentialcapacitywas measuredover thewhole_potential rangewithouthavingtoestablish_{anyfurther equilibrium} conditions. In_{the adsorption measurements}ofisobutyl

alcoholfrom0.05MNa2S04, the Ag single crystal faces

were _{electropolishedanodically}beforeuse:13this_procedure

yielded reproducible positive desorption peaks and ap-parentlyreducedthetime_requiredfor_adsorption equi-librium to 30min forthemostdilutealcoholsolutionsand to 10min for themost concentrated ones. _Comparable timeswere _requiredfor_{ro-pentanol adsorption from}0.1

MKFon _{Ag singlecrystals grownelectrolytically in Teflon}

capillaries.14 TheabsolutevalueoftheGibbs_adsorption free_{energy of}n-hexanol12was foundtobe_{slightlygreater}

on

_Ag(l

₁₁₎thanon _Ag(100)(AG°m= _—24.6kJmol-1and

AG°ioo= _—23.0kJ_{mol-1) whereasan}

oppositetrendwas reportedforn-pentanol14 (AG°m = -15.0 kJ_{mol-1 and} AG°ioo= -15.8 kJmol-1). Theauthors explainedthe slow attainment ofadsorptionequilibriumwhen_shiftingthe potentialfrom negative valuestothe_{potential of}maximum

adsorption by a slow adsorption on the growth _stepsof the silver surface. However, such a _{slow adsorption}is

apparently inconsistentwiththe sharpand_high adsorp-tion-desorptionpeaks reportedby the authors on both

sidesofthe flat differential _capacityminimum.

Experimental

Section

The water usedwas obtained frombottledlightmineralwater bydistillingit once andthendistillingthe water so obtained

from acidic permanganate, while the heads were _constantly

discarded. MerckKC104was _{recrystallized twice frombidistilled}

waterandthendried. Fluka analyticalreagent grade n-hexanol

was distilled inan _atmosphereof_nitrogenbeforeuse. The alcohol concentrationswas varied only from3.46x 10-3to3.46x 10-2

Mbecausefor lowerconcentrationsthe capacitance—potential

curves ofthe base _electrolyte are not altered by addition of n-hexanol(NHEX),andno _higherconcentrationswere studied

because_{of solubility limits in}0.05MKC104. Thesolutionswere

freshly prepared just before the beginning ofeach series of measurements.

The _working electrodesused in this work were _cylindrical

silver single crystals grown accordingtotheBridgeman technique ina_{graphitecrucible, oriented by X-rays and}cut15either inthe Laboratoire d’Electrochimie Interfacialedu ONESin Meudon-Bellevue (France)or in_{Florence, Italy.} Theseelectrodeswere

polishedwith successivelyfinergradesof aluminapower down to 0.05_{µ and} thenannealed in a muffle furnace at normal

atmospheric pressurefor30minat650 °C. Allelectrochemical measurements were carried out in Florence. Before each

experiment the electrode was _{polished chemically} with _CrOa

according to the procedure describedinref16. Afterpolishing, the electrode surfacewas soakedin concentratedsulfuricacid

forabout20minandthen rinsed thoroughly withwater. The hangingsolutionmethod17was _{employed. Thecylindricalsingle}

crystalelectrodewas _{held by}a_{platinum wire}sealedintoa_glass

tube, whichwas secured toa_{movable stand; the}latterwas moved

(15)Hamelin, A. In ModernAspectsofElectrochemistry; Conway,B. E.,White,R. E.,Bockris,J.O’M.,Eds.;PlenumPress: New York,1985;

Vol. 16,p 1.

(16)Hamelin, A.; Stoicoviciu, L.; Doubova, L; Trasatti, S. J. Electroanal. Chem. 1988,244,133.

(17)Dickertmann,D.;Schultze, J. W.; Koppitz,F. D.Electrochim. Acta, 1976, 21, 967.

upor downatan _{adjustable rate by}means ofan _{oleopneumatic}

pistonwhich ensured the complete absence of vibrations. A pressuregauge connected to theoleopneumatic systempermitted

us _{to reproduce theoptimum} rate (1.5 mm/s). The standwas

connected to a _{digital position sensor,} which permitted us to estimatea0.1mm verticalshift ofthestand. Bythissystem the vertical position oftheelectrode, andhencethe meniscusofthe

hangingsolution, couldbe_reproducedwith a _{highaccuracy.}

Thecellwas _{water-jacketed}andthermostatedat25 ± 0.2 °C.

A gold coilwas used as a counter electrode and an external

saturatedcalomel electrode (SCE) servedasthe reference. The

four-electrode potentiostaticsystembyHerrmannetal.18was

employed to minimize the noise. The wholly computerized instrumentation fordifferential_{capacity and charge}

measure-mentsisdescribedinthe next section; positive feedbackcircuitry

was utilized_{to compensate}for_{the electrolyte resistance.} Before _carryingout adsorption measurements, the state of the electrode surfaceincontactwithan _aqueoussolution of0.05

MKCIO4,previouslypurgedwith argonforabout30min,was checked_{byrecording the}curve ofthedifferentialcapacityvsthe

appliedpotentialat20 Hzofacmodulation. Measurementswere

carriedoutonlyuponascertainingthe_quantitativeagreement ofthesecurves with_{these reported in the literature.19 Each set}

ofdifferential capacity-potential curves and charge

measure-mentsatdifferentalcoholconcentrationswas carriedoutwhile

the singlecrystalfacewas maintained inconstant contactwith the _{solution through} the meniscus. To this end the alcohol concentrationin thecellwas _{increased progressively by adding}

deaerated alcohol froma Hamilton microliter_syringeintothe previously deaerated aqueoussolutionof0.05 MKC104. The plunger ofthe_syringewas fastened_tightlytothe rodofa

digital-display micrometerscrew of0.005mm _{pitch. The}micrometer

screw was held byamovablestandwhich permittedthe syringe needle tobeloweredintothe_{solution during}theadditionand raisedabovethesolutionjust aftertheaddition. Beforeentering thecell,the argonusedforthedeaerationwas bubbledinavessel

containingasolution ofthesame _compositionasthat inthe cell;

this_{prevented both}a_changeinthe alcoholconcentration in the

cell and a decreasein thelevel of the solution surfacewhich wouldhavealtered theshapeofthemeniscus. Aftereachaddition ofthe alcohol, thesolutionwas stirred_mildlywith a_magnetic

stirrer foratime_{long enough to}allow its _completedissolution

(~20 minfor the concentration atsaturation,~4 x 10-2M). For

an accurate estimate of the _stirring time necessary for the dissolution of the alcohol, charge vs _potential curves were

previously recorded on a _pressurized static _{mercury drop} electrode20_{at increasingstirring}timesuntila_{constancy in these} curves was attained. Argonwas bubbledintothesolutionunder studyforafewminutesaftereachadditionofthealcohol,whereas it was flowedover the solution during its_completedissolution

in thesolutionandthe subsequent measurements. Aftereach

addition,thedifferentialcapacity and the capacitive chargewere

recordedin successionas a functionofthe applied_potential.

Results

Differential

Capacity.

Thecurves ofthedifferential capacity(C)againstthe appliedpotential(E)were recorded

assuminga _simple series RC_equivalentcircuit and are

shownin_Figure 1;an ac _signalof20Hz and 5 mVrms was _superimposedon thedc_voltageramp. Inagreement

withthe literature19 thecurve ofthe_{supporting electrolyte}

showsa minimum duetothecontributionofthediffuse layerat-0.746 V. Atthe lowchosen KC104concentration

specific adsorption of this 1,1-valent electrolyte in the

proximity ofthe minimum is _{negligible, and} hence the

potentialoftheminimum correspondsto the_potentialof zero _{charge (pzc).}

The lowest concentrations of NHEX cause a modest

depressionofthedifferentialcapacity inthe neighborhood ofthe_{minimum; thisdepression}isclearlynot compensated

(18)Herrmann,C. C.;Perrault,G. G.;Konrad,D.;Pilla,A.A.Bull.

Soc.Chim. Fr. 1972, 12, 4468.

(19)Valette,G.J.Electroanal.Chem. 1989,269,191.

E/V(SCE)

Figure1. Thefirstcurve, as countedfrom_{top to}bottomover

the intermediate potential region, is theCvs E_{plot for the}

interfacebetween

_Ag(lll)

_{and aqueous 0.05}MKCIO4in the

absence of NHEX; the successive curves are relative to

progressiveadditionsof NHEXfrom3.46x 10'3 to3.46x 10“2M by log(c/M)incrementsof0.1.

for by an _equivalent increase with respect to the dif-ferential_capacityofthe_{supporting electrolyte}on thetwo

sidesoftheminimum,although theCvs Ecurves for all the NHEX _{concentrations explored}merge with that of

the supporting electrolyteat potentials negativeof -1.3

V _{and positive} of -0.3 V. _{Although thermodynamic} analysis couldbedonefrom_{capacitance dataextrapolating} experimental results to 0Hz, some _{qualitative findings} are drawn from_capacitancedataobservedfrom5to150 Hz. If we _{integrate the} Cvs E curves (20Hz) starting

from any ofthesetwo extreme potentials, whereNHEX

desorptionis complete,andwe forcethe_resultingcurves

ofthe_{chargedensity}om vs Etocoincide atone ofthese

twoextremes,theywillnotcoincideon theother extreme. This behaviordenotesan irreversible_{adsorption and/or} an _{adsorption—desorptionstep}whichis slowin thetime

scale of(20 Hz)'1. In what follows, evidence will be

providedbychronocoulometrythatirreversible adsorption

ofNHEXmoleculestakes_{placeonly at the lowest}NHEX

concentrations,whileaslow_{desorption of}thesemolecules at _{positive potentials}occurs atall NHEXconcentrations

explored.

As the NHEX concentration (c) is progressively

in-creased,thedifferentialcapacityminimumbecomeslower

and broader, while _{well-defined desorption} peaks of

increasingheightdevelopon thetwosidesoftheminimum.

The differential capacity minimum in the presence of NHEXis_slightlyshiftedon the negativesidewithrespect

to the pzc in the absence of NHEX. The frequency dispersion is small in the proximity ofthe differential

capacityminimum,whereas it increases _{notably at the}

desorption peaks, as _{expected. Therefore, the capacity} data cannot be usedfor a _{quantitative investigation} of the adsorption of NHEX.

ChargeMeasurements. Capacitivecharge

measure-ments _{by the chronocoulometric technique}were carried out_{byholding the appliedpotential}atan initial value_(E)

atwhich the soluteis adsorbedforatime long_{enough to} attain adsorptionequilibrium;the appliedpotentialwas thenstepped toafinalvalue_(Ef)atwhich desorptiontakes placerapidlyand completely,i.e.thecurrent [¿(i)]following

this potential step is zero after a few milliseconds. Electronic _{integration of this transient yields the} cor-respondingcharge [Q(i)]. The _{experimental strategy in} charge measurements is describedthoroughly in refs21

(21)Foresti,M.L;Moncelli,M.R.;Aloisi,G.;Carla’,M. J.Electroanal.

Chem. 1988,255,267.

and22for measurements ata_{mercury drop and}inref23

formeasurements on solidelectrodes.

Themethodcan be successfullyappliedif no _faradaic

process takes place at theEf,whichisgenerallyan extreme negativeor _{positive potential}chosenoutside_{the potential} rangewherethesubstance_{under study}isadsorbed. Then thecontribution to_chargeis purelycapacitive.

It should be noted that no correction for a faradaic

contributioncan be_regardedas _{entirelysatisfactory}if a

disturbing electrode process yields a _{product which} is adsorbedon theelectrode surface, thus_{interfering with} the adsorption of the substance _{under investigation.}

However, the faradaic complicationcan beovercome not only in thetrivial case in which the_disturbingfaradaic

process is slowin thetimescale adoptedbutalsoin the

more _significantcase inwhich theprocess is notdiffusion

controlled and hence gives rise to a constant current density. In thiscase the_{resultingQ(t) increaseslinearly} in time and a linear _{extrapolation} to t = ₀ corrects

satisfactorilyforthe faradaic contribution, providedthat

thelatter contributionis_{negligibleduringthe very short} time

(1-2

ms)requiredforthe_{imposedpotential}toswitch from theinitial tothefinalvalue.

ForeachNHEXconcentration_investigatedtwodifferent chronocoulometric measurements were carried out in

sequence: withboth measurements theinitial potential

(E)was varied between-1.3 and-0.3 Vby20mV_steps,

butthefinal potential(Ef)was setalternativelyequalto

-1.2 and -0.1 V. The time _{required for attaining}

adsorptionequilibrium at£ under_{mild stirring}was found

tobelessthan10sover the whole rangeof_potentialsand NHEX _{concentrations investigated.} Hence each mea-surementwas carriedoutby

stirring

the solutionat each E value for 10 s, blocking the

stirring

2 s before the

potentialstep£ —

Ef,holding thepotentialatEffor100 ms, andthen steppingthe_potentialback to thenew initial valueof£. Thiscyclewas _repeatedunder_{computer control}

untilthe_{whole range}ofEvalueswas covered. _{The charge}

Q(E,t)was _{sampled at} 1 ms intervals startingfrom the

instant t = ₀of_each

potentialstep(E —

Ef). The time

requiredforattainingadsorptionequilibriumismuchless thanthat_reportedbyVitanov andPopov12for thesame

system.

By properadjustment ofthepositivefeedbackcircuitry,

the charge Q(E,t) following the potential step E —

Ef

attained a _{practicallytime-independent value in} a few millisecondswhen Efwas_{set equal to}

_-0.1V;

thisindicates that no detectable faradaicprocessesoccur at this final

potential in the 100 ms time window (see Figure 2). Conversely,the Q(E,t)vs tcurves recordedbysettingEf

= _—1.2 V exhibit _a

slight constant slope due to a mild hydrogen evolution whichoccurs at-1.2 V. At_{any rate,}

the correction for this faradaic contribution _by linear

extrapolation ofQ(E,t) to t = ₀is

quite accurateand is entirelyjustified in view ofthelackof hydrogenadsorption on silver. The_{extrapolation to}t= ₀of_theQ(E,t)values

sampledover thetime interval fromt= ₅₀to100_ms after

thepotential stepE—·

Efwill be _{denoted by Q(E}—

Ef).

Figure3showsthe Q(E—* _Ef)vs E_curves obtained in

0.05 M KCIO4 by setting Ef equalto —0.1 and —1.2 _V,

respectively. Both curves were converted into ctm vs £ curves _{by settingQ(E}— _Ef)

equalto zero at the _pzc as estimated from the minimum of_{the corresponding}

dif-ferential_capacitycurve. It isapparentthatthetwocurves overlapexactlyover abroadintermediate potential_range,

while_theydiffersomewhatattheextreme _{potentials. In}

(22)Foresti,M.L.;Pezzatini,G.;Monteagudo,J.C. G.J.Electroanal.

Chem. 1990,295,251.

Adsorption Behaviorof

Ag(lll)

Figure2. PlotsofAQvs tfor0.02MNHEX in0.05MKC104.

Thecurves refertodifferent initial potentials,E, andone final

potential,Ef= -0.1 _{V. Dashedcurves} refer_to

potentialsat which hydrogen evolution interferes andwere therefore dis-cardedin our _analysis.

E/V(SCE)

Figure3. Plotsofctmvs Efor0.05MKC104intheabsenceof

NHEXobtainedas describedin thetext atan

Ag(lll)

_single

crystal electrodewhichhad been_{working for}a _{long time by}

settingEf equal to-0.1 V(dashedcurve) and to—1.2V(solid curve).

particular,the absolute valuesofom are _{greater for Ef}= -0.1

Vthanfor£f=

-1.2V at themore _{negativepotentials,} whereas theyare smaller at themore _{positivepotentials.} This behaviorcan be_{explained by considering the way}in

whichtheseOmvalueswere obtained. Whenthe electrode

isheld at themore _negativesvaluesfor10s,adetectable amount ofhydrogen is formedat its surface: if we _step thepotentialto afinal value Ef= _—1.2V,this_hydrogen

will remainunaltered,but if we _stepthe_potentialtoEf

= _{—0.1 V, it} will _be

instantaneously reoxidized, thus increasing slightly the positive charge Q(E— -0.1 V)

involved this step with respect to its pure capacitive

contribution. In addition, the high OH™ concentration createdattheelectrodesurfaceentailsan increasein the

measured charge, due toOH™_{adsorption.24} Inthis_respect theay¡values atthe more negativepotentials are more reliableif theyare obtainedbysettingEf= _{—1.2V. On}

the otherhand,whenthe electrode is_{kept at}more _positive E values for10s,a_slightamount of silveroxideisformed on itssurface: if we_{stepthe potential to}E¡= -0.1 V,this

oxide_{will remain unaltered; in addition,}more oxidewill

be formed_{duringthe holding time} at —0.1V (100 ms).

However, if we _step the_potential to Ef = _{—1.2 V,} this

oxide will be _{instantaneously}reduced,with a _resulting

slight increase in _{the negative} charge Q(E— _—1.2

V) involved in this stepwith respect to its pure capacitive contribution. In this _respectthe ctmvalues at the more

(24)Hamelin,A.;Morin,S.;Lipkowski,J.J.Electroanal.Chem.1991, 304, 195.

Langmuir, 11,No. 2,

positivepotentialsare determined conveniently_bysetting

Et=

-0.1 V.

When_startingfroma _{freshlyprepared single crystal}

electrode or from an electrode _entirely restored by

mechanical polishing and annealingofthe_{singlecrystal} face,we have_constantlyobservedthat the_{discrepancies} pointedout in Figure3increasewith the repeateduse of theelectrode_{in adsorption}measurements ofthealcohol.

Thus, a _{freshly prepared} electrode or an electrode

em-ployedin onlya fewseriesofadsorptionmeasurements

wouldyieldom vs Ecurves ofthe_{supporting electrolyte} as obtained by thetwodifferent_procedureswhich practi-callycoincidedover the whole_{potentialrange from}—1.3

to-0.3 V: thetwocurves in Figure3 were indeedobtained atan electrodewhich had been used for several setsof

measurements. The above _{experimental observations}

stronglysuggestthatthe repeateduse ofasilver_single crystalelectrodein measurementsof_{specificadsorption} increases _{the density} ofsurface defects which are not completelyremovedby the chemicalpolishingwith CrOg and which constitute preferential sites for _hydrogen

evolution and surface oxide formation. Therefore, the experimental data reported herein where obtained on

electrodeswhichwere either freshly preparedor hadbeen employedinno more thanthreeor four seriesof_adsorption

measurements.

AnincreaseintheNHEX concentrationcauses amodest butappreciableincreasein therate ofhyrogenevolution at E values negativeofca. —1.15_V, as indicated_bythe

increaseintheslopeofQ(t)vs t curves _{obtained by stepping}

to these_potentials. A_gradualincreasein c also causes an _incipientincrease andthena decreasein the rate of oxideformation atE_{values positive}ofca. —0.4V. Thus, the_chargedensitiesQ(E—· -12 V) obtainedatthelatter

E values first increase _slightly over the corresponding

charge values for the supporting electrolyte alone and

thendecrease: thisbehaviordenotesan increaseandthen adecreasein the faradaiccontributionto Q(E— _—1.2

V)

due to surface oxide formation. Since at _potentials

negativeof—1.15Vand_positiveof—0.4Vthedifferential

capacitycurves of Figure 1_pointtoan almost _complete desorptionofNHEX, theaboveeffectsare _probablytobe ascribed to the catalytic action of alcohol molecules irreversibly adsorbedon the surface defects existingon the electrode surface. _{This hypothesis} is supported by

the observation that the lowest NHEX concentrations

depressthe differentialcapacitycurves in the region of theminimum (seeFigure 1),while theydonot increase

themon thetwo sidesoftheminimum: this _suggeststhat

the lowest NHEX concentrations tend to saturate the

surface defects _{almost completely,} while _adsorption on the terracesisstill _quitelow. Furtherevidencein favor

ofthis_{interpretation}isprovided by theeffectofthefirst

NHEX_{additions, performed by regularlogarithmic} incre-ments logc ^ 0.1,on thecurves ofthe_{absolute charge}

density(om)against E andon the correspondingcurves

oftheinterfacialtension_(y)againstE: thefirst addition of NHEX causes a _change in um (see _Figure 4) and a

depressionin ywhichare _greaterthan_{those producedby} thesubsequentaddition. Thishasledus to discardthe

ctm vs E curve of the supporting electrolyte in the thermodynamic analysis of the experimental data.

We _already stated that the charge densities

Q(E— _{—1.2V)are not}

complicatedbyhydrogenevolution

at extreme negative potentials, whereas the charge

densities_Q(E— _—0.1

V)are not _{complicatedby surface}

oxideformation at extreme positivepotentials. In view ofthese observatons,the following strategywas _adopted

to obtain reliable vs Ecurves at the differentNHEX concentrations. The two curves of_Q(E— _—0.1

Figure4. In passingfrom the steepest totheflattest curve over theintermediate potential_region,thefirstcurve istheau vs E plot for the interfacebetween

_Ag(lll)

and aqueous 0.05 MKCIO4in theabsenceof NHEX; the successivecurves are

relativeto progressiveadditions of NHEX from4.35 x 10-3to 3.46 x 10~2Mby log(c/M)increments of0.1.

and ofQ(E—* -1.2 V)_vs E relativeto the_{same c} value

were first shifted along theverticalaxiswith _respectto

eachotherso asto_{put them in}coincidenceover thebroad

intermediate_{potentialrange from}ca. —1.0toca. —0.6V.

Atpotentials negativeof_{this range,}where thetwocurves

diverge,only theQ(£—· _—1.2

V)vs Ecurve was _retained;

conversely, at potentials positive of this rangeonly the

Q(E— _—0.3

V)vs Ecurve was retained. _{The Q(E)}vs E curve for the supporting electrolytesoobtainedwas then convertedinto the corresponding curve ofthe absolute

chargedensityom vs EbysettingQ(E)equaltozero atthe

pzc,asestimated from theminimum in_{the corresponding}

differential capacitycurve. All other Q(£) vs E curves

relativeto thevariousNHEXconcentrationswere _finally

made to coincidewith that ofthe _{supporting electrolyte} at themore _{negativepotentials,}whereNHEXis_completely

desorbed. _Figure4shows awholesetofum vs E curves obtained_bythis_procedure. Allthesecurves intersectat

a_{single point,}whosecoordinateslocatethe_charge,om=

—1.65

µ

cm-2, and the potential, Em = _—0.788

V, of maximum adsorption. Unfortunately,even _byfollowing

this _procedure, thecurves do _{not merge} at —0.3 _V, but rathertheyrun almost_parallelover theextreme_positive potentialrange explored. Thisagreeswiththedifferential

capacity curves of Figure 1 _coinciding over the same

potentialrange. This behaviorisprobably ascribable to a slow_{desorption in the time} scaleof100 ms as we _step

to the positivefinal potential Ef= _—0.1V.

Incidentally,

aslow desorptionat_{potentials positive}tothe adsorption regionhas been_reportedfor_{ethyl ether}on

Ag(lll)

and

Ag(100)byHinnenet al.11 and_{for ferf-pentyl}alcoholon Au(111)byRicheretal.9 Sincethe vs £curves obtained

with E{= _—1.2

V, at which potential desorptionis fast, are almost exactly superimposableon thecurves obtained

with

£f

= -0.1 V_over the potential range from_ca. -1.0

to ca. —0.6 _V, we are led to conclude that the rate of desorptionfollowinga_{potentialstep}£— _—0.1V_decreases

progressively andrapidly with a decreaseinthewidthof

this step. For the above reasons, results obtained at

potentials positiveto-0.6 V willnotbetaken intoaccount

in the further datatreatment.

Thermodynamic Analysis.

The _{thermodynamic}

analysis ofthe_experimentalcurves ofomvs £was carried

out by an _entirely numerical _{procedure25 using molar} concentrations in _{place of activities;} such a tactic is expected to produce a _{practicallynegligible}error at the (25)Carla’,M;Alois!,G.;Foresti, M.L;Guidelli,R.J.Electroanal.

Chem. 1986, 197, 123.

relativelylowNHEXconcentrations_{investigated.26 The}

surface excesses _F(£,c), were _{obtained by numerical} differentiationwith_{respect to In}coftheinterfacialtension

valuesy(£,c). Thesecurves were obtained by_integration

with_{respect to}£ ofthe_directlymeasuredom(£,c) values. In_{calculating the isotherms}at constant charge, Parsons’

function

_{( , )}

=

+ ay¡Ewas first obtained. The surface excesses _T(um,c) were then calculated _by numerical

differentiation of|(ctm,c)with respect toInc.

The_{parameters for}NHEX_adsorptionon

Ag(lll)

from

aqueous 0.05MKC104are summarized in Table1,where

they are _compared with the parameters for NHEX adsorptionon _{Hg fromaqueous}0.1MNaF.27 The charge,

am, andthe potential,£m,ofmaximum adsorptionwere determinedfrom thecoordinatesofthecommon intersec-tion point ofthe om vs £ plots. The maximum surface concentration, Fm, was determinedfrom the adsorption isotherm at£m, whichshowsadistinct

_limiting

value(see

Figure 5). The differential _{capacity Ci} at maximum

coveragewas obtainedfrom the interceptofplotsofthe differential_capacityCat£ = _£m

against on the = _Fm

axis, as describedin ref28. The_{potential of}zero _charge

at maximum coveragewas obtained_{bydetermining the} interceptson thectm= ₀ axisofthelinear_sectionsof_the

vs £ plots about £ = _£m at alcohol concentrations corresponding to =

I7Tm values greater than 0.5, by plottingthese_{intercepts against andbymeasuring the}

interceptofthe_{resultingplot}on the 0=1 axis. Theshift

£n ofthe_potentialofzero _chargewhen passing from =

0 to 1 was then obtained _{by subtracting} the _potential

charge at maximum coverage so obtained from the

potential ofzero _charge,-0.746 _V/SCE,in theabsenceof the surfactant.

The standard Gibbs energy of adsorption at zero

coverage,AG°ads,as a functionofthe_{appliedpotential}£

was _{obtained by}

_fitting

the experimental isotherms at constant _potentialto the Frumkin isotherm:

553 expl~

-RT)=

expfoil1 (1)

whereaistheFrumkininteractionfactor. ValuesofAG°ads anda were thus obtained from the intercepts and_slopes

ofthe_{least-squaresfittings}toa _{straight line}of

ln[c(l

—

0)/0]vs plots at constant £, over the range from0.2 to0.7. _Figure6showsa_plotof_{AG°adS vs £,}whereasthe

AG°adsanda valuesat £ = _£mare

reported in Table 1.

This table also _{reports values} ofthe coefficients fcE =

3(AG°ads/RT)/d(E

-£m)2 for the quadraticdependenceof

the standardGibbsenergyofadsorption upon(£

-£m),

the superscripts + and —

denoting whether the

least-squares

fitting

toa_{straight line}was made at_negativeor positive valuesof(£—

£m). This

_fitting

was confinedto thelower(£

-£m)2values,whenever appreciable

devia-tions fromlinearbehaviorwere observed at_highervalues.

The_{adsorption isothermat constant charge} obtained

at = _am is

practicallyidentical to_{the corresponding}

isothermat constant_potentialobtainedat£ = _£m. This

is the _{obvious consequence} of the independence ofthe

intersection pointoftheomvs £ curves _upon thealcohol concentration, which causes the potential £ to remain

practicallyconstant at£mwhenauiskeptconstant atom. (26)Moncelli, M.R.;Foresti, M.L;Guidelli,R.J.Electroanal.Chem. 1990,295, 225.

(27)Foresti, M.L;Moncelli, M.R.J.Electroanal.Chem.1993,348,

429.

(28)Moncelli, M.R.;Foresti, M.L.J.Electroanal.Chem. 1993, 359, 293.

Adsorption Behaviorof

Ag(lll)

Table 1. Parameters for NHEX_Adsorptionon

Langmuir, Vol. 11, No. 2,

Ag(lll)

from Aqueous0.05MKCIO4and on _{Hg from Aqueous}0.1M NaF parameter

Ag(lll)

(this work) Hg27 parameter

Ag(lll)

(this work) Hg27

10"10rm/mol cm"2 2 3.4 (AG°ada)min/kJmol"1 -18.4° -23.8“

A/nm2 0.49 GE(E—

Em) ~ -2.9 -2.6

am/pCcm"2 -1.65 -1.9 ¿>E+/V"2 35 10.8

EJV(SCE) -0.788 -0.520 Z>e"/V"2 28 8.1

Ci/µ cm"2 14 6.2 6t7+/cm4pC~2 0.037 0.029

Fn/V 0.060 0.185 ba~!cm4 pC~2 0.023 0.021

0_{Thestandardstate for the solventisthe pure solvent, and that for the soluteisthe hypothetical ideal puresubstancein bothbulk}

andadsorbedstates.

Figure5. Plotsof vs _logcatconstant ErelativetoNHEX adsorptiononAg(111)fromaqueous 0.05MKCIO4 forEvalues equal to-0.780 (a),-0.860 (b),-0.700 (c),-0.660 (d),-0.940

(e), -0.620 (f), -0.980 (g),and -1.020 V(h).

Figure7. Plotsof vs _logcat constant ctmrelativetoNHEX

adsorptionon _{Ag( 111)}from aqueous 0.05MKCIO4 foromvalues

equal to-2 (a),0(b), -4 (c),1 (d),2 (e), -6 (f), 3_(g),-7 (h),

—8_{(i), and} -10

_µ

cm 2 (j). 1 -15 -i -19 ¡-1-1- -!-1--1 -0.8 -0.6 E/V(SCE)

Figure6. PlotofAG°adsvs EforNHEX adsorptionon

Ag(lll)

from_{aqueous 0.05}MKCIO4.

TheAG°adsanda values atthe constant_charge =

om as obtained from the _interceptand slope ofthe

ln[c(l

)/ ]

vs plotat om are thereforepractically identicalto

the corresponding values at the constant potentialE =

Em. However,as we _{depart from the}chargeofmaximum

adsorption, am, the vs _logc _plots at constant _charge densitytendtoan _{apparentlimiting}valuewhichdecreases progressivelywithincreasing|ctm

—

om|,as shownin Figure

7. This behaviorofthe_{adsorption isotherms at constant} chargeas _{compared to}that oftheisotherms at constant

potential is observedfor_{practically all} simple aliphatic compounds (see, for instance, the vs _log c curves at

constantsandamfor n-pentanol adsorption fromaqueous

0.5 M Na2SC>4 in Figures 3 and 4 of ref 26). As a

consequenceof this behavior, the

ln[c(l

—

)/ ]

vs plots

at constantchargeas obtained by_setting = _r/Tm,with

Fmderived from the adsorption isothermatam= _am,show increasing deviations from linear behavioranda

progres-sive decreaseinthe averagevalue of their slopewithan

increasein |ctm —

am|. Due to theselarge deviations,no

attemptwas madetoestimateFrumkininteractionfactors

aatconstant chargeforchargesam ^ am. Moreover,AG°ads

values at constantamwere obtained bylinear extrapolation

Figure8. PlotofAG°adsvs amforNHEX adsorption on

Ag-(111) from aqueous 0.05 M KCIO4. The dashed curve was

calculatedas describedinthe text. to = ₀of_thelower portionof ln[c(l

)/ ]

vs plotsat

constant_{charge, i.e.}the portion from = _{0.1to0.4. Figure}

8 showsa_plotofAG°adsvs crM. Table1 _{reports values}of

the coefficients ba = _{d(AG°a¿s/RT)/d(am}

-am)2 for the quadratic dependence ofthe standard Gibbs energy of adsorptionupon(am~

om),where the superscripts+ and

— denote

whether the least-squares

fitting

toa _straight

linewas madeat negativeor _{positive values}of

(

-am).

Theratios (6±e/5±0)1/2are in the range from 31to35_µ¥ cm-2andare therefore_relativelyclosetothedifferential

capacity Co ofthe Ag(111_{)/electrolyte interface in the} absenceofsurfactants, inaccordancewith _{predictions.26}

Discussion

TheTmvalueforNHEXon

Ag(lll)

is_appreciablyless

thanthe value, «3.95 x 10~10molcm"2,correspondingto full coverageofNHEX, calculatedformoleculesin a flat

orientation, area = _{0.42 nm2, on} the basis ofa

space-filling

model.26 In contrast, the experimentalTm value forNHEXadsorptionon _mercury as _{reported in Table}1

Table2. Standard GibbsEnergyofAdsorption (kj mol-1) ofNHEX at Metal/Water Interfaces at the

Potential of Maximum _Adsorption

Hg Bi Sn In(Ga) Cd

_Ag(lll)

Zn Ga

-23.8 -21.2 -21.1 -20.2 -19.4 -18.4 -18.3 -17.6

water moleculeswhichremainadsorbedon silverwhen

reachesits

_limiting

_{value points to}a_higher

hydrophi-licity

of this metal with respect to mercury. Further

evidence in favor of this conclusion is provided by the muchmore _{negative standard}Gibbsenergy ofadsorption

ofNHEXon _{Hg, -23.8} kJ mol-1.27 As a matter of_fact, the standard Gibbs energy ofadsorption, AG°adS, of a

neutral _compoundwhich interacts weakly with metals can beusedas ameasure oftheir_{hydrophilicity,}sincethe main difference in AG°adswhen passing fromone metal

toanothercan thenbe ascribedto the difference in the energyrequiredtodesorbthe watermoleculeswhich must makeroom for the adsorbing_compound. Table2 sum-marizesAG°adSvaluesforNHEXadsorptionfrom aqueous

solutions on various sp metals as well as on

Ag(lll).

Taking intoaccountthatanincrementinthehydrocarbon chain length ofre-aliphatic alcoholsbyone

-CH2-

_group

is accompaniedbya _roughlyconstant increment in the negative valueofAG°ads,theAG°adsvaluesforNHEXon

spmetalsinTable2were obtained_bylinear_{extrapolation}

ofthe_{corresponding values for re-propanol, re-butanol, and} re-pentanolavailable intheliterature (seeref 29forGa, ref27 _{for Hg, and}ref30forall othersp metals). It can

be seen _{that, according}to this hydrophilicity scale,

Ag-il

11)isnearlyas_hydrophilicas _gallium,andhenceturns out to be one ofthe most _hydrophilic metals. A more detailed discussion is _{given in} ref31 for the (100) and (110)silver faces.

Thedifference inbehaviorofthe_{adsorption isotherms} at constant _{chargein Figure} 7 with _respectto those at

constant potential in Figure5agreeswiththe predictions of a three-dimensional lattice model of TIP4P water molecules andofpolar dimeric solutemolecules _against

a _{charged wall, recently developed} in our _{laboratory.32}

The model, which does not make use of _molecules, represents solute molecules as _{dimers consisting} ofa nonpolarsegment,which simulates a_{very short} hydro-carbon chain, and ofa _{polar segment,}which simulates the polar head. _{This model predicts} that the surface

concentration,

,

at constant charge increaseswithlogc, tendingtoan ill-defined apparent

_limiting

valuelessthan

Tm, whichislower the more the _{chargedeparts from its} value ofmaximum adsorption, am. This behavior32 is explainedby the factthatthe localelectric_{field anchoring} the water molecules totheelectrode surface consists of

theexternal field 4 plusthe additional polarization field

created by the neighboring water molecules,whichacts

in the direction_{opposite to}theexternal electric field.As

the surface coverage increases, the polarization field decreasesaccordingly,andhencethe localfieldatconstant

charge increases in absolute value, approaching 4 . Hence,theresidualadsorbed water moleculeswillbeheld in the adsorbed state _{by the local}fieldmore

_tightly

the higheris6;this explains theattainment ofan _apparent

limiting

valueofthe _{surface coverage,albeit ill-defined,}

whichdecreaseswith increasing| |. No suchabehavior

ispredicted when it is_{the applied potential E which}is

(29)Pezzatini, G.; Moncelli, M. R.; Innocent!, M.; Guidelli, R. J. Electroanal.Chem. 1990,295,275.

(30)Trasatti,S.InModernAspectsofElectrochemistry, Conway, B.

E.;Bockris,J.O’M.,Eds.;PlenumPress: _{New York,}1979;Vol13,p81.

(31)Foresti, M. L.; Innocenti, M.;Guidelli,R.J. Electroanal.Chem. 1994, 376, 85.

(32)Guidelli,R.;Aloisi,G.J.Electroanal.Chem. 1992, 329, 39.

heldconstant. Infact,theimposedconstancy ofErequires a_gradual decreasein theabsolutevalueoftheexternal electric field4 withincreasing ,inorderto_compensate for _{the gradual} decrease in the absolute value ofthe counterfieldproducedby theadsorbedwater molecules. Therefore, at constant E _{the progressive} increase in

doesnotcause the residualadsorbedwater molecules to

bemore _stronglyanchored to the surface as in the case

ofconstant_om, andhenceFmturns outtobe_practically

independentofE. The_{penalty which}one hasto pay in using theaboverefined three-dimensionallatticemodel,

whichdoes notmakeuse of_{adjustableparameters,32 is} the _{difficulty of predicting} the adsorption behavior of

re-aliphaticcompoundshavinga_{long hydrocarbon chain,}

due to the notable increase _{in the complexity} ofthe

statistical mechanical treatment ofthe model with an

increaseinthenumberofmonomeric segments composing

the solute molecule.

For a _{quantitativecomparison}between_experimental behavior and modelistic predictions, we shall adopt anothermodel ofadsorbed water monolayer and solute

moleculesin whichthe water moleculesinteract witheach

othervia H-bondsand dipole—dipoleforces,whereas solute molecules interact with each other and with water

molecules via _{dipole—dipole forces} only.33 This model makesuse oftwo_{adjustableparameters}tofit_experimental

results, namely the number(re) ofadsorbedwater

mol-ecules displaced by one adsorbing solute molecule and

thenormal component(mn) ofthe dipole momentofthe

adsorbed solute molecules. Theunknownvaluesofreand

µ are obtainedfrom the experimental valuesofamand

ba~bymakinguse ofacalculated_plotofthe_{µ /re}ratiovs omtoobtain

µ /

andthenofcalculated plotsofµ /revs ba~ at differentrevaluestoobtainre.26-34 Inthe present

case (NHEX) the estimated valueofthe

_µ^/

ratioisclose to -0.3 D. _Confining ourselves to re values which are

multiples of0.5,theresultingrevalue_equals4,andhence

coincideswiththe valueestimatedforNHEX_adsorption

on _Hg26 on the basis ofthe same model. This re value

compliessatisfactorily with the surface area, 0.42 nm2,

estimatedforaNHEXmoleculeinaflat orientationwhen we considerthatthearea _{projected by}one adsorbedwater

moleculeis closeto 0.10 nm2. Thisresult supports the

point ofviewthat theNHEXmolecules are adsorbedin a flat orientationon

Ag(lll)

aswellas on _Hg,atleast at low surface coverages; moreover, their dipole moment

normalcomponent(mn)isapproximatelythesame on both

electrodematerials. Thenegativevalue

_ofµ

_impliesthat

theNHEXmoleculeis adsorbedwiththe positiveendof

itsdipoletowardthe metal;thisjustifies thepositive value of60mV fortheshiftEnofthe pzcwhen passing from = ₀to_1,alsoinviewof_thefactthatthewater dipoles_are

expected toturn theirnegative endtoward the metal in the proximity ofthe_pzc.

Themodel _adopted herein forthe estimate of theµ andrevalues33_predictsan _{adsorption behavior at constant}

chargewhichisin_fairlygoodagreementwith theFrumkin isothermofeq1,atleast for < 0.7. Hence,

fitting ln[c(l

—

)/ ]

vs plots calculated from the modelto a _straight

lineover the _range from0to0.7_yieldsan _intercepton the6= ₀axiswhich_measures the standardGibbs_energy

of_adsorption at zero _coverage, AG°adS, apart from an additive constant;moreover, theslopeofthisstraightline

providesavalue for theinteractionfactorotobe_compared

with _{the experimental}one. The dashedcurve in Figure

8 isa_{plot of}AG°adsagainsta calculated from themodel

forµ /re= _—0.3Dand_re= _4;this _{curve was} shifted_along (33)Guidelli,R.;Foresti, M. L.J.Electroanal.Chem.1986,197,103.

(34)Moncelli, M.R.;Pezzatini,G.;Guidelli,R.J.Electroanal.Chem. 1989, 272, 217.

Adsorption Behaviorof

Ag(lll)

Langmuir, 11, 2, 1995

theverticalaxisso as toobtainthebestoverlapwiththe correspondingexperimentalcurve. Theavaluecalculated

from the model,

-3.7,

is _{somewhat greater} than the corresponding experimental value.

Thedifferential_{capacity Ci} at maximumcoverage on the

_Ag(lll)

face, 14_pF cm™2, is relatively high when

comparedwith the corresponding valueon _mercury, 6.2 pF cm™2.27 This _{higher C\ may} be due partly to the

presenceofsurface defectswhichincreasethe realarea

oftheelectrode surfacewith_{respect to thegeometric}area. However, theratio ofthesetwo areas is expected tobe hardlygreater than l.l.35 Afurthercause oftheincrease

ofCi in passing from Hg to Agis due tothefactthatthe

water content intheadsorbedfilmatmaximumcoverage is_greateron _Agthanon _{Hg. Thus, thepresence}ofwater

in the adsorbedmonomolecular film may increase the

distortional dielectric constant (e) ofthefilm36·37and/or

decreaseitsaveragethickness(ó):22·38botheffectstendto

increase Ci in view ofthe well-known expression C =

e/(4jrd) for the capacity of a _{parallel-plate capacitor.}

Anotherfactorfor_{Ci being greater}on _{Ag than}on _{Hg may}

beascribed to the factthatthe negativecontribution, CefS from electron spillover to the reciprocal (Co™1) of the

differentialcapacityat zero _{coverage isprobably greater} for Ag than for _{Hg. Roughly speaking,} this negative

contributionis_{greater thelarger}thepolarizabilityofthe

“electronictail”,whichinturn is_{expected to increase}with

an increase in the bulk _density offree electrons. The

problem of the effective density of free electrons is

complicated in the case of sd metals, because the d

electrons in these metals can neither be considered as

freenor asbound. SchmicklerandHenderson39estimated (35)Foresti, M.L;Guidelli,R.;Hamelin, A.J.Electroanal.Chem. 1993, 346, 73.

(36)Damaskin,B. B.;Survila, A. A.; Rybalka, L.E.Elektrokhimiya,

1967, 3, 146.

(37)Schumann,D.J.Electroanal.Chem. 1986,201,247. (38)Carla’, M.;Aloisi,G.;Moncelli, M.R.;Foresti, M. L.J.Colloid InterfaceSci. 1989, 132, 72.

the electron densityof_Agtobemuchcloser to thehigh

value for Ga than to the _relatively low value for _Hg.

However,it must beconsideredthattherelative_weight oftheCei”1contributiontoCr1iscertainlylessthanthat

to Co™1,since

Cr1

> Co”1.

Sofardiscussionwas basedon themodel described_by

Guidelli andco-workers.32·33

Recent_{experimentalfindings about ordering}ofwater

at the

Ag(lll)/NaF

interface40_byin-situ SXRS (surface

X-rays scattering)suggestnot only reorientation ofwater dipoleswhen the signofthe_chargeon the metal_changes

butalsoa_changeofareal densityinthefirst _{layer from}

1.1 at -10 _{pC cm”2 to} 1.8 at 25 _pC cm™2. Our base

electrolytewas KCIO4, notNaF, thereforethesefindings cannotbe_directlyintroduced inour _{results;furthermore,}

it is difficult to foresee what would be the influence of NHEX_adsorptionon these_{largefirst-layer}water

densi-ties. These SXRSobservationsdemonstratethat thereis

no _changein the atomicsurface structure

ofAg(lll)

from

—10to_{25 pC}cm”2, i.e.no _{reconstruction,}but_onlyasmall contraction between the top and second _{layer of silver} atoms at the more _{positive charges.} Therefore, the

potentialstepsare _performedall_alongour _experimenton the same _{superficial atomic structure, it} isnot the case for faces which reconstruct, low-index gold faces for instance.

Acknowledgment.

The authors wish to thank Mr. FerdinandoCapolupoand Mr.AndreaPozziforvaluable

technicalassistance. The_{financial support of}theMURST

and ofthe

Italian

CNRis_{gratefullyacknowledged.} LA9403789

(39)Schmickler, W.; Henderson,D.J.Chem. Phys. 1986, 85, 1650. (40) Toney,M.F.;Howard, J. N.; Richer,J.; Borges, G.L;Gordon,

J. G.;Melroy,O. R.;Wiesler, D. G.; Yee, D.; Sorensen,L.B.Nature