Sb(Hg) systems at high surface coverages ag

J.‘EX.rromroL Ekevier

C+nt.

Saquoia

SA;;

138 (1982) La&e

139-153 -

139

Primed

in The Netherlands

INHIR~ORY~~CT OF n-ALIPHATIC ALCOHOLS UPON Cd?/Cd(Hg), -PtJ(OH);/Pb(Hg), Ei”‘/Bi(Hg) AND Sd”/Sb@g) SYSTEMS AT HIGH SURFACE COVERAGES

GIOVANNI

PEZi%TINI,

MARIA

Imritute of Andytical Chemimy, (Received

11th December

A detailed upon

Cd’+.

reduction

investigation

LUISA

Pb(Hg)

under conditions The rate consmnls

GUIDELLI

University of Fiorence, Florence (II&)

1981)

of the inhibitory

Eli”’ and Sb’u reduction

&d

and ROIANDO

FOR-1

THE

oxidation

in alkaline

in which the elcctrwle

of rhe above

effect of n-aliphalic

and Cd(Hg)

oxidation

media, was carried

surface coverage

elecmde

reactions

alcohols,

from n-bulanol

in acidic media, OUI at mercury

and amalgam

9 by Lhe alcohols is almost complete

in the presence

to n-ockmol.

as well as upon

of the various

agree satisfactorily with a theoretical relalionship derived in ref. 5. which accounts energy of adsorption of the iulubltor and for tie lateral-interaction energy. involved solvent molccula by inhibi[or molecules around a single activated complex

alcohols

Pb(OH), electrodes,

(9 > -0.93).

were found

10

for the slandard free in the substimtion of within

the

adsorbed

monolayer_

INTRODUCTION

The inhibitive effect of elcctro-inactive neutral organic surfactants upon electrode reactions has been the subject of several experimental investigations and has been explained in several ways. j1,2]. Recently, on the basis both of surface-phase thermodynamics [3.4] and .of a statistical-mechanical treatment of the adsorbed monolayer [5]. it has been stressed that an electrode reaction taking place on the solvent
0 I&!

Elsevier Sequoia

SA.

140

the surfactant

[5]:

(!) are the numbers

Here nS and np surface

by an adsorbing

of solvent molecules

displaced

from the electrode

of the surfactant and by the reacting particle in the transition state for the electrode reaction, cw and cs are the bulk concentrations of the solvent and of the surfactant, whereas AG,od, is the standard free energy of

adsorption between

molecule

of the surfnctant. contiguous

solvent molecules the adsorbed Equation

Moreover,

adsorbed

A is the energy,

particles.

by surfactant

which

molecules

due to lateral interactions

is involved

around

in the substitutions of

a single activated complex

within

monolayer_ (1) is independent

although

the particular

different

orientations

whether

local

order

of the detailed

form of A depends postulated

for adsorbed

is accounted

model

of the adsorbed

on this model (namely,

monolayer,

on the number

solvent and surfactant

for or if the Bragg-Williams

of

molecules,

on

approximation

is

adopted and so onj. According to this equation, the inhibitory effect consists of a contribution -(no/ns) In(c,/c,), emphasized in Lipkowski and Galus’ work [1,3] and accounting by

for the mass action exerted by the surfactant

the solvent,

of

a contribution

(n,/ns)(AG&/RT)

surfactant

adsorptivity,

Lipkowski.

Galus et al_ [ 1,4,6.7] have venfted

In cs for the reduction alcohols

and

of

of Cuzc,

and acids. in agreement

(n ~ /ns)

the contribution Pb(OH),

--A.

In

the linear dependence and Cd’+

related

a number

the

papers

of In kas=,

in the presence

with eqn. (1). and have deduced

to

of

upon

of aliphatic

the corresponding

values.

A further test of the validity reaction

with the various

and upon

making

practically

of eqn. (1) consists in inhibiting

members

the reasonable

an amount

A;?‘(ln

the standard

assumption

k9,_,)

equal to (n,/r~~)[~~+‘(aG~~)/RTl,

of experimentai

in the presence

Ar”‘(AG&)/RT. derived from

Values adsorption

from

of A”‘(ln In

kS,,,)by the term (n,/n,), as derived k3,=, vs. In cs plots for the given electrode

of A’:7 ‘(AG$,) measurements of Cd’+

is

minus that of the ith

of the homologous

series, should

provide

in good agreement with those directly of the organic inhibitors were indeed ob-

and Cu2+

electroreduction rates in the presence alcohols [5]. To make this test more general, we investigated a number

of metal ion-metal sponding

term A is

where &i*‘(AGats)

of the (i + I)th member

of any member

tained from measurements of n-aliphatic

that the lateral-interaction

compounds,

of the series. In this case, when passing

free energy of adsorption

the slope

a given electrode

series of ahphatic

the In kBs=, value at constant cs should decrease by

member [5]. Hence, multiplication reaction

of a homologous

the same for all members

the rth to the (i + 1)th member,

from

relative to that exerted directly

inhibited

amalgam

reactions

satisfying

the requirement

waves in the presence of n-aliphatic

alcohols

that~ the corre-

he in the proximity

of the potential of maximum adsorption for these inhibitors_ This allows the bulk surfactant concentration cs to be varied over a satisfactorily large range while maintaining the surface coverage I?, sufficiently close to unity (say. a-O-95) as to permit

a correct

application

of eqn. (1)

The above

requirement

has restricted

the

141

choice of the electrode reactions.investigated Ccl(Hg)

oxidation

oxidation

in acidic mediu.m,-as

in alkaline

to Cd2+,

Bi”’ and Sb’u reductions

well as to Pb(OH),

reduction

and

and Pb(Hg)

medium.

EXPERIMENTAL

All chemicals distilled with

used were Merck

before

use. All

active charcoal_

alcohol avoid

solutions

An

of

of

improvement

any

small

0.22 M and 0.17 M

n-hexanol,

1.3 X IO-‘M

NaOH.

refers

process

followed

sodium

chloride

of

undissolved

soiutions

for n-pentanol,

to

by

HClO,

for n-heptanol.

0.5 M

or. O-05 M H,SO,

saturated

calomel

All

electrode,

by using

The

alcohol

and 0.74 M

and 3.9 X IO-’

3.1 X IO-‘M

for

M for

and 214 X

is that of the given alcohol in

HISO

solutions.

three distillations.

alcohol.

5.5 X 10W2 M

were

treated

value, so as to

are: 0.79 M

where the former concentration

0.01 M

concentration

droplets saturated

water

was. achieved

less than the saturation

and 1.1 X 10-2M

10m3M for n-octanol, O-01 M

reagent grade. Alcohols

from triply distilled

in reproducibility

slightly

these “almost”

n-butanol,

analytical

were prepared

of concentration

the presence

concentrations

or Fluka

solutions

solutions, Mercury

potentials

was

were

whereas

the

purified

by

measured

but are reported

latter a

wet

against

with respect

a

to the

usual SCE. Polarographic Metrohm

measurements

E506 Polarecord

kept constant

by shearing

were

carried

off the drop

dropping

was applied employed

[8,9]_ The same computerized described

mercury

by Anson

drop electrode

25 *0_25”C

a

tapper. Charge

measure-

method employing

a

reservoir a pressure of 12 atm

apparatus,

continuously

with

time was

described

in ref. 8, was also

by the double-potential-step

and co-workers

either

30. The drop

chronocoulometric

upon whose mercury

to measure Pb” adsorption

ric technique hanging

electrode,

at

Electroscan

with an automatic

ments were carried out with a computerized pressurized

out

or with a Beckman

chronocoulomet-

[ 10-121. In this case, a special

renewed

under computer

control

was

employed. RESULTS

To control

the decrease in the surface coverage

potential

curves of the various

saturated

solutions

an

example;

Frumkin’s density

were recorded,

Fig. 1 shows

relation ati.=

in a solution

-density ins the same potential,

alcohols

tYs Gas found

for

n-butanol

c,, charge vs.

of the corresponding

and in 0.01 M HClO,_ in 0.01 M

NaOH.

As

Using

9,) + uM_,SS, and setting crM_,-, equal to the charge

not containing solution

dilutions

both in 0.01 M NaOH

these curves

aM.a(l -

Ss with decreasing

at different

the surfactant

saturated

with

and

uM_, equal

the surfactant

to decrease. by no more

to the charge

at the same

than 7% over- the potential

applied range

from -00:4 to -0.8 V/SCE. when the. concentration c, of any of the alcohols investigated was decreased from its saturation value csti; to (0.3 X cswlr)_ Equation

.(I)

refers to an electrode

surface

fully covered

by the ‘surfactant

and

142

I

02

I 0.6

I ID --EIVecE)

I 1.4

I 18

Fig. I. (I~, vs. E NI vcs of 0.01 M NaOH containing0.79M (Cl). 0.55M (0). 0.39M (a), n-buranol.

hence,

for

its applicability,

ionic-specific

adsorption

must

be

and 0.24M(A)

avoided.

For

this

NaOH and HCIO, solutions sufficiently dilute (0.01 M) as to exclude appreciable anionic-specific adsorption, even at potentials positive to the pzc. were adopted. In Cd’+ electroreduction. perchlorate ion was preferred to sulphate ion as supporting anion in order to reduce the probability of bulk ion-pair formation with Cd’+ _ On the other hand, the relatively high acidities required by Bi’n and. even more so. by Sb”‘, were more conveniently realized with H,SO,, than with HCLO,. In fact, H,SO, is less strongly adsorbed than HCIO,, and is practically non-specifically adsorbed at potentials negative to -0.45 V/SCE [13]. where Sb”’ and Bi”’ electroreduction in the presence of ahphatic alcohols takes place. Incidentally, Sb”’ in 0.5 M H,SO, is mainly present in the form of the antimony1 ion SbO+ [14]. Bismuth(II1) in acidic solutions is also present in one or more cationic forms, although the nature of these forms is uncertain_ Thus. hydrolytic polymers with charge + I [15;16] or -1-0.65 [17] per monomer unit have been reported, although the existence of cationic hydrolytic monomers has also been postulated [18]. In the case of the validity of eqn. (1) a .gradual decrease in cs within the concentration range in which 9, remains very close to- unity (say, 2~~0-95). is purpose

143

expected to causk the Tafel plot for the electrode potential

axis towards

deer easing over-potentials,

reaction to translate along the without appreciable distortions. In

fact; the &mtities AGz& and A in e@r. (1) are expected to vary only slightly over the relatively narrow potential range covered by this translation. The linear Tafel plots

i Bi’u and Sb”’ reduction

for Cd’+

were actually found to shift without distortions

with decreasing

cs_ The Tafel plots for Pb(OH),

0.01 M

show

NaOH

potential

a curvature

reduction

ing

step

oxidation)

of the Pb(OH),/Pb(Hg)

termining

involves

passage

the uptake

(for

of the first transferring step involves

the uptake

of Fig. 2, relative of n-octanol).

from a mechanism

will undergo

in

Pb(OH), electron

reduction)

or

to a mechanism

or release

a translation

release in which

of the second

without

to Pb(OH),

Such an increase

in which the rate-determin-

distortions

(for

Pb(Hg)

the rate-de-

transferring

Hence, in this case, the linear section of the Tafel plot corresponding two mechanisms

oxidation

a ~resulting increase in slope as the formal is approached closer couple ( - -0_600 V/SCE)

than -0.1 V (see, for instance, the Tafel plots reduction in the presence of different concentrations in slope is due to the gradual

and Pb(Hg)

and

electron.

to either of the

along

the potential

Fig Lng T~s,-I vs_ E ploo for I X 10m4 M Pb(OH), reduction from 0.01 M NaOH solutions containing 3.1 X lop3 M (a), I.86 X IO-’ M (b) and 9.3 X lOpa M (c) n-octanol. Here. as well & in the. CoUowing figurer. x0.x 1 is the dimensionlus kinetic parameter (12#/7D)‘/’ k,== ,. where I is the drop time and D

z

is the diffusion coefficient oi the reactarM_

144

I

5

I

I

0

I

I

I

lo

I o-

Fig. 4. Log X,,=, vs. -log c-Splots for 1X 10_a M Cd(Hg) oxidation at E = -0.540 V in 0.01 M HCIO, solutions containing various n-aliphatic alcohols. The dashed plots are corrected for diffuse-layer effects.

~~~~4~~~ . l

D-

L

.

.’

-1

.

.

~, i

I :x0.

I

I ,.’

:_--“._ -.

-20

lois.’

\

I -ID

\

\ . . \

\ I -L5

Y

L

i

\

\

\

\ l. .

\r

\

-a5 I Q5.

.m=cJ.

for‘ I X 10:. & Pb(OH), mjuction ai E =_-0.740 V from 0.01 M Fig_ 5. LAJ&$~_=, ,vsI’l@g cs p NaOH s&rion~ mnlain;ng various n-$ipha& alcohols. Tj16 dash&l plots are corr&Ied for diffuxc-layer cfkA5_‘- .~-.-

‘~

\ \

to ‘.

\

‘t

vs. log c,

Fig. 6. Log $+=, solutions

containing

~101s

for

wxrious n-aliphatic

I

I

X IO-. alcohols.

M

Pb(Hg)

I

I

oxidation

The dashed plolr

I

at E = -0.470 V in 0.01 M NaOH

arc corrected

for diffue-layer

effects.

I

I 4

0

\

I

I

-30

-25

I -20

I -1.0

I

-15

IogC, Fig. 7. Log solutions

zr,=,

con:aining

M-

log cz plol~ for

various

n-alIphatic

I X 10m4 M Sb”’ alcohols.

reduction

I -as

~. at E = -0:450 v from~0.5 M

H,SO,

-25

-30

-20

-1.5

Fig. 6. L.og &,=, vs..log cs plots for 5X IO-’ M IS” solulions wnrtining~various n-aliphatic alcohols.

axis

only within

shown

the potential

-1.0

-05

raluction aL E = -0.700 V from 0.05 M H,SO,

range in which the given mechanism

is operative,

as

in Fig. 2.

In Figs. 3-8 the cathodic rate constants &=, electroreduction. electro-oxidation, various

for Cd_‘+,

as well as the anodic rate constants as measured

n-aliphatic

alcohols

at constant

potential

under conditions

Pb(OH), , Sb”’ and Bi”’ kaS=, for Cd(Hg) and Pb(Hg) in saturated

solutions

in which 9, is approximately

of the >0.93,

are plotted -against log c,. Solid curves in these figures are not corrected for diffuse-1,ayer ef&ts_ On the other hand, the dashed curves iri Figs. 3-6, which refer to the Cd’+/Cd(Hg) log K&, correction

valuek and [(z,

+ aiQ&,,/(2_303RT)]

charges of the oxidized~and charge-tra&eF

r&luced

was obtained

charge ~densities uM obtainekfrom &=,

-w&e- obtaind.~ + z;F&RT)

againsL. F(Ealtihdls

to the log ks,,, reactants, &nd

coefficients’ and q, is the average

plane. The latter potential &&es

systems, are log kas,,

and Pb(OH);/Pb(Hg)

vs. log ci plots corrected for diffuse-layer effects. As -was performed by adding [(zo - a$&,J(2.303RT)]

from -the aget

q&)/R-T

-F(E-

for a&.. relative

For the Cd’%/Cd(Hg)

:for.:th&..bk

a’ are the cathodic at the outer

via .the Gouy-Chap&n

and anodic Helmholtz

theory upon using

measurements_

The (I and &

of corrected Tafei plots [i.e. plots of (ln q,))/RT f or G.-and (ln k6,_, + z,Fq+JRT) to the saturakd

solutions

of

the various

system, z ,wassetequalto.+2andz,equaltoO_A

detailed I&e& kkstig&.&~f fhe pb(Oe),/Pb(Hg) lO_*M; to.b~~d~cribed’e~~~~e~[.lP]ii;as shown @c&ok&d.%

values. Here, z. and zR are the

potential

chronocoulometric slopes

vs. log c, and

is customary, this to the log kas=,

ion is. :2;i,.~h&eas .,-. _:

system at .kn ionic strength of that -the q+odic e&qko+erpicaI the ,kx&sponding

ancklic reaction

148

order

is + 1. This

implies

that the reactant

in .Pb(OH),

reduction

is &OH)+,

whereas

the reacting particles in Pb(Hg) oxidation are (Pb’ + OH-): In view of theseresults, z. was set equal to + 1 and zR equal to - l_ Uncorrected as well as corrected

Tafel

plots for Pb(OH),

Fig. 2)

reduction

and hence are characterized

consist of two roughly by two different

tions hold for the Tafel plots relative to Pb(Hg) introduced into a)fGpJ(2_303RT)] Tafel

H,SO,

potential

values for Sb”’ and Bi”’

although

reduction

of Pb” from 0.01 M NaOH

kinetics of the Pb(OH), by additions

* +‘=-

I

z.

values,

the v,.’ values in 0.05 M

are small.

solutions containing KF

electrode

of 0.09 and 0.99M

-ma

Moreover.

the effect of an increase Pb(Hg)

correct the

because~ the corresponding

almost certainly positive, are uncertain.

In an attempt to determine

duced

considera-

Hence, the G and C%values

of interest_ No attempt was made.to

and, even more so, in 0.5 M H,SO,,

Ahorption

oxidation.

the correction terms [(z. - a%&.J(2303RT)] and [(zR + of Figs. 5 and 6 are the slopes of the corresponding corrected

plots at the applied

log G.,,,

linear segments (see e.g.

G values; analogous

KF.

1

reaction

I

upon

in 0.01 A4 NaOH.

it was observed

-Cl70

-0.65

in ionic strength

the

as pro-

that the Tafel

plots

I

-07

EIVCTCE)

Fig. 9. Log Xds=, vs. E ~101sfor I X IO-’ M Pb(OH), reduction from (0.01 M NaOH+O.G n-ocCmol_~ solutions conmining 1.9X IO-‘M (a). 1.3X IO-’ M (b) and 810X IO-‘&f(c)

.-.

M KF)

_‘,

._-

~.

149

,.

06tainecl::iti th&.: &+a undergo notable, chege$ in slope as ~cs is gradually de&&d @I&, for @st@ce;- -Fig:9j- This ,apparently anomalous .behaviour.- which &xitras~- ~th~~tixpectati~ns as :w&ll as ‘k-i& the ‘behaviour of Fig. 2, is explained by -. Pb”~$sor$on ix,,ihe:ti&&y el~trcide. SF&an adsorption was detected with the ~dck~le$ot+&l-stepchrooocoulometric technique [lo-l21 by stepping the applied potenti&from:an initial value: E;, -which wti. gradually varied: over ~the potential range-in which Pb(OH), is electro-inactive, to a fixed t&al value E, on the plateau of the Pb(?H),- r+ction wave Iti the absence of KF tid of aliphatic alcohols, Pb” adsorption.in:O_Ol M NaOH starts-to be detected at reactant concentrations c,, a- 5 X lgd5M. Hciwevex, -upon saturating the 0.01 M NaOH solution with any of the n-aliphatic alcohols h&in empl&d, no apprekiable Pb” adsorption is detected even at _cO= 5 X 10L4~M_-Addition of KJ? to 0.01 M,NaOH in~kases the absorptivity of Pb”.both in the absenCe a#d in the presence of-n-aliphatic.alcohols. This is clearly apparent from Fig. 10, which show.s 2FlY& VS. Ei plots as obtained from 5 X lOF5 M Pb(0I-I); solutions in .O.OlM NaOH t.0.99 M ICI? containing different amounts of I

I

I

Fig; Id. PI& & 2fr&v~ &for 5X iO_I5 M Pb(bH)< reduckon fro& (0.01M NaOH+O.W M KF) &h~k+&m_+i~g 1.14~ IO-” ti (a>.-7.6~ 16-j M @.b’). 5.7x iO-4 M (c,c’).and 3.8~ 10-J M (d,d’) ~n+xanol. Thc~risr time ciFthe liaiging meriuj drop electrode at Eiwai30 s for curves a, b, c and d. and

60sTorcurvcsb’;‘c~.andd’_..~

:

~:.

:- .. _:i

.-.

:

150

is negligible at n-octanol concentrations > 60% of the saturation value, but becomes notable at lower concentrations. Under all conditions, the chronocoulometric plots for 50th forward and backward potential steps (cf-.ref. 11) were found to be satisfactorily linear. with standard deviations C 1%. Adsorption increases with the rest time of the hanging mercury drop electrode at the rnmal potential Ei, without showing any tendency to attain a maximum limiting value (see e.g. Fig. IO). This behaviour is consistent with an initial two-dimensional precipitation of some Pb” salt [possibly Pb(OH),] on mercury, similar .to that observed frcm lead bromide and iodide solutions [20]. followed by growth of three-dimensional nuclei. It is interesting to observe that, for a given rest time of the hanging mercury drop electrode at Ei, Pb” adsorption shows a minimum in the proximity of -0.4V. Since neither OHnor F- ions are specifically adsorbed, anion-induced adsorption cannot be invoked in the present case. In view of the above results. additions of KF to Pb(OH), solutions in 0.01 M NaOH with the aim of increasing the ionic strength were avoided, in order to exclude reactant adsorption. n-octanol.

The Pb” adsorption

DISCUSSION

The log Las=, and log Las=, values reported in Figs. 3-8. even those corrected for diffuse-layer effects. are still uncorrected for compact-layer effects. The latter effects should be felt whenever, as in the present case, the reactant penetrates into the compact layer before charge transfer_ Quite probably, the major contribution to compact-layer effects stems from the dipole surface potential due to the close-packed. two-dimensional array of -OH groups of the adsorbed molecules of aliphatic alcohols. If we make the reasonable assumptions that, a: 9, = 1. (1) all n-aliphatic alcohols investigated have their -OH group directed towards the solution with the same dipole-moment normal component, and (2) the reacting particles in the transition

state are closer

to the electrode

than

the -C;I

group

of the adsorbed

molecules, then the correction for the dipole surface potential is expected to be practically the same for all alcohols. Hence, with these assumptions. differences between log kas=, values for the same electrode reaction in the presence of different n-aliphatic alcohols are expected to be only slighty affected by compact-layer effectsTable 1 summarizes (np/ns) values as obtained from the slopes of the log kB,=, vs. log c, plots in Figs. 3-8_ The n,/ns values for Cd’+ reduction are similar to those obtained by Golwnowski et al. (71 in 0.5 M Na,SO,, although slightly smaller. However, [email protected]’s values for Cd(Hg) oxidation in 0.5 AU Na,SO_, are decidedly smaller than the corresponding values for Cd’+ reduction, whereas we have not observed such a difference in 0.01 M HCIO,. Hence, we must coriclude that, at least in 0.01 M HCIO,. the activated complex for Cd’* reduction and that for Cd(Hg) oxidation have approximately the same cross-sectional area, and consequently are almost equally hydrated. As distirict from the Cd’+/Cd(Hg) system, all other metal ion-metal amalgam reactions herein investigated are characterized by n+ /ns values ranging from 1.2 to 1.6, and hence by activated complexes appreciably alcohol

151 TABLE n +

/ns

1 values as dccm-mined from the slope of log xdsE,

Cd’+

-Cd(Hg) Ccf(H&dd*+ Pb(OHh_ - Pb(Hg)_ Pb(Hg)Pb(OH), Sb”’ 4 Sb(Hg) Bi”’ -. Bi(Hg)

vs. log cs plots n-hcptanol

n-oclanol

a-butiol

Il-penliUl01

n-hcxanol

-

1.88

2.03

220

2.10

l.il 1.13 -

1.93 1.37 I .26 I.14 1.29

2.24 I.21 121 1.27 1.23

2.25 1.39 1.42 1.26 1.34

2.32 I .60 I.53 I.23 1.39

less hydrated than those for Cd2+ reduction and Cd(Hg) oxidation_ It should be noted that np/ns klues between - 1.4 and - 1.6 were also recently reported by Pyzik and Lipkowski [4] for Cu2* electroreduction at mercury electrodes fully covered by n-aliphatic alcohols. This serves to stress the particularly large size of the activated complexes for Cd2+ reduction and Cd(Hg) oxidation. According to the statistical-mechanic treatment of ref. 5, ns and n,+ denote the numbers of adsorbed water molecules displaced by one adsorbing molecule of the surfactant and by one of the activated compleq independent of whether water molecules are H-bonded within the adsorbed monolayer and of whether such an H-bonding causes surfactant adsorption to satisfy the Frtunkin isotherm [21] (cf. ref. 7 for a contrasting statement). Hence, noting that one adsorbed molecule of n-ahphatic alcohol with the hydrocarbon chain normal to the electrode surface displaces from 2 to 3 water molecules from the adsorbed monolayer, we may conclude that u+ ranges from 3 to 4 for all electrode reactions investigated, with the exception only of the Cd2+/Cd(Hg) system where n+ is more lilely to be close to 5. The plots of Figs. 3-8 were employed to derive the differences Ai+’ In kss=, at constant c, for any given electrode reaction in passing from the ith to the (i + 1)th member of the homologous series of n-ahphatic alcohols (i stands for the number of carbon atoms in the alcohol molecule). It should be noted that these ki*’ In kSs=, values do not depend critically upon the choice of cs since the slopes of the plots in gigs. 3-8 are rougbIy independent of the chain length for a given electrode reaction (cf. Table 1). Each AiT’ ln kBszlvalue was then divided by the arithmetic mean of the slopes of the two consecutive ln ksS+ vs. In-c, plots whose distance along the vertical axis yields the given A[+ ’ In k,S= I value. In view of eqn. (1) the resulting ratio should be a measure of &i+‘(A($!o,)/RT. ln ka,,lns/n+) for all electrode reactions Table2 summarizes valueS of -(Ai?’ and for all pairs of successive n-aliphatic alcohols investigated. Values obtained from In k,,=,vs.Incsplots corkted for diffuse-layer effkts are reported in parentheses. With a few exceptions, the valties in Table2 range from 1.2 to 1.6. These values should be compared with the -B~*‘(Ai;,O,)/RT values obtained from independent adsorption ~measurements of n-aliphatic alcohols. The -Bi+‘( AC&)/RT values

-.

152

-

+

+

A i-

+

‘:.

:

.obtaiiied by Dgkskin

et _al_~ 1221 in 0.1 AUNaF at -9.434 V/SCE from .&patiit~~&tiureme~ts range from 1125.to 1.4, the average-value being agr&r+t :b&we+ these dir*ily measured - ti,*‘( AGf&)/RT values deduced: ind&ctly- from’ the magnitude .of. the inhibitory effect of alc&ho~~~upoti Sever+ +e@l ion-metal amalgam reactions with the aid

differential 1.3. Hence, and those n-aliphatic of eqp. (1)

-.can be regarded ti satisfactory. It should be noted that the -(tiiW’ In kag_,n,/n,) valu&ig Table 2 refer ‘to .&differenti&& strengths, depending on the electrode reaction i&!$gated, and. that the A’,?‘(A&,“,,) values obtained by Damaskin et al. [22] refer to a yet differeikionic strength. Now, the.adsorptivity of aliphatic alcohols increases &h an increase in ionic strength, owing to the salting-out effect. Fortunately, this. incre&e in adsorptivity cti be satisfactorily accounted for via an increase in the activity coefficient of the alcohol in the bulk solution, this activity &efficient being pra&+ly independent of the alcohol concentration cs [23]. Hence, even though the absolute-value of the standard free energy of adsorption AG,“5 of any giveri alcohol increases Hiith an increase in ionic strength, differences fii+‘( ACT’,,) as derived at a given ionic strength are expected to be practically independent of the ionic strength ado&d. In conclusion, the preceding results show unequivocally the dependence of the inhibitory properties of organic surfactants upon their free energy of adsorption, and demonstrate the validity of eqn. (1) in interpreting this dependence. REFERENCES I For a review see J. Lipkowski and Z. Galus. J. Eleclroanal. Chem., 61 (1975) I I. 2 For a review see B.B. Damaskin and B.N. Afanas’ev. Elcktrokhimiya. 13 (1977) 1099. 3 J. Lipicowski and Z. Galus, J. Electroar@ Chem., 98 (1979) 91. 4 G. Pyzik and J. Lipkowski. J. Elec~roanal. Chem.. 5 R Guide114 M.L. Foresti and M.R

6 J. tipkowski, E Kosikka. ( 1975) 344

123 (1981) 351.

Moncelli.

J. Electroanal. Chem. 113 (1980) 171. &f. Golcdzinowski. J. Nicnicwska and Z. Galus. J. Electroanal. Chem., 59

7 M. Golgdzinowski, L. I&o&_ J. Lipkowski and Z. Galus, J. Elcctroanal. Chem.. 95 (1979) 43. Monalli and R. Guidelli. J. Uectrcanal. Chcm.. 109 (1980) I. 9 M-L. Fore&. G. Paratini and R. Guidelli. J. Eleztroanal. Chem.. 109 (1980) 15. 10 F.C. Anson, J.H. C&stie and RA Osleryoung. J. Uectroanal. Chem.. 13 (1967) 343. 1 I J.H. Christie. RA. Cklcryoung and F.C. Ansoo, J. Eleclroanal. Chem, 13 (1967) 236. 12 G. Laueri R. Abel and F-C. Anson. Anal. Chcm.. 39 (1967) 765. 13 R. Payne, J. Elecrroanal. Chcm.. 60 (1975) 183. 14 J.L Da&m. J. Wilkinson and M-1. Gillibrand. J. Inorg. Nucl. Chem.. 32 (1970) 501. I5 RW_~Holmb&g_ KA_ Klaus and J.S. Johnson. J. Am. Chem. Sot., 78 (1956) 5506. 16 .R.S. Tobias, 1. Am. C+n_ Sot. 82 (1960) 1070: 8 M.L. Foresti, M.R

17 R-S. Tobias and_S.Y. Tyre Jr.. J. Am. Chcm. Sot.. 82 (1960) 3244. 18.F. G-&r. A,. Olin and L.G. Sill&n, Acta Chcm. Stand., 10 (1956) 476. 19 M-I- F&IL G. Penatini &d R Guidclli. unpublished &sulrs. 20 H&. Herman. RL McNeely, P. Sum&, CM. Elliott and RW. Murray. Anal. Chem., 46 (1974) 1274. 21 R Guidelli; J. Electraanal. Chcm..-I23 (1981) 59. 22 B.B_ Dama&&. AA. Survila Ad L-E Rybalka. Elek&okhimiya. 3 (1%7) 146_ 23 B.B. Dan&p. AA. Survila and LE. Rybalkq Elcktrokhimiy& 3 (1967) 927.

.. ~-