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ELSEVIER • I'luclK.iiiiiilyliciil CliJiiiislry 4-t() (l'W7) H3-W

Mechanism ofsolid|liquid interfacial reactions. The reactive dissolution of p-chloranil in aqueous solution as studied by the channel flow cell

with electrochemical detection and atomic force microscopy

Jonathan Booth Giles H.W. Sanders '', Richard G, Compton "*, John H. Atherton Colin M. Brennan ''

rlii'oivliail Clwimiry Uihoniuny. (hfiiiil lliiiirrsiiy. Soiilli I'aiks Kmul. O.xfnnl OX I JQ'/. UK '' '/fiimi IMiiiMl. I'O llo.K-12. lllmkky. Mdiulwslcr M'J.IDA. UK

KcLX'ivi;il 20 NovciiiliLT \W(y, received in revised loriii 6 January I'jy?

Abstract

A t|uuiitita[ivc study of the hydrolytic dissolution of solid p-cliloianii at alkaline pH using a combination of the channel How cell and in situ atomic force microscopy measurements shows that the dissolution is driven by reaction of the substrate with hydroxide ions at, or very close to, the dissolving surface. The dissolution rate equation, deduced from the channel How cell for single crystals of p-chloranil, is rate/molcm 's ' = 3.8 X l()"'[OH ]|| where [OH~ ]̂||/M is the hydroxide ion concentration adjacent to the solid surface. An analogous rate law is derived for pressed pellet substrates. The novel application of the atomic force microscope to make absolute measurements of surface averaged dissolution rates is described. Comparison with the independent channel How cell data reveals good mechanistic agreement and consistency of rate constants provided it is recognised that a thick stagnant diffusion layer can be present under the usual conditions employed for AFM. © 1997 Elsevier Science S.A.

Kr mis: p-Chli)raiiil; S()lid|lic|iil(J inierface: Dissolulioii: Channel flow cell; In silu AFM

1. Introduction

The lundamental study of reactions between solids and lit|uid phiise species is the subject of rapidly growing attention [1.2]. A variety of methods are emerging for kinetie and incehanistic studies, including hydrodynamic (I'orexaiTipie rotating disc) methods, scanning electrochem­ical microscopy (SECM) and atomic force microscopy (AFM). The relative merits of these different techniques hiive recently been assessed in an admirable review [2]. .SECM is able to give spatially-resolved kinetic intbrma-tion whilst the AFM approach is to image reacting or dissolving surfaces under liquid [3-7]. By monitoring the evolution of suitable structural features in real time, such as the translalioii of steps across the surface [3,4,7], the rale of reaction/dissolution may be inferred and approxi­mate estimations of the corresponding rates can be made. Alternatively, a purely kinetic approach is to employ a

Corresponding aullior. Ei-niail: coiiiptonfe'ermine.ox.ac.uk.

channel How cell (CFC) [8,9] to give rate data which, unlike SECM, is effectively averaged over the entire solid surface. The CFC comprises a rectangular duct through which solution is forced under laminar How conditions and the solid substrate of interest is embedded smoothly in one wall of the How cell (Fig. I). An appropriate detector, often electiochemical, is lucated immediately downstream of the solid surface so as to monitor either the release of products or the consumption of reactants. As the reacting surface and the detector arc linked via a well-delmcd hydrodynamic regime, the mass transport between them is calculable from knowledge of the convection and diffusion in the cell. In this manner the separate contributions from mass transport and interfacial chemical phenomena to the detector response may be quantitatively asses.sed and the sought interfacial reaction mechanism identified by mea­suring the detector signal as a function of a wide range of solution ilow rates. The method has been applied to ben­zoic acid dissolution [10], the reaction between solid cya-nuric chloride and an aqueous aromatic amine [11 J, and to

()()22-0728/')7/$l7.()() © 1997 Elsevier Science S.A. All rights reserved, I'll S0()22-072Xiy7)00()50-X

./. Hooih fl ill. / Journal ,il lilci

Solid aiibr.lmlB DHl()i;lnr syalom

/ k [2::f:^5gg*-

Ihc hydrolysis of solid Irilyl chloridL" [12] wliicli proceeds

in iiii aulliLMilically hclcrogeiieoiis fasliion.

Ill ll.is papLT we bring llic AFM slruL-lural approach and

the Cl-'C kinetic method together to study the liydroiytic

dissolution of solid p-chloranil; the reaction is of indirect

importance in the manufacture of dyestid'I's and fungicides

[13.14]. The mechanism of the reaction is inferred - liie

dissolution is drisen by reaction of hydroxide ions, O i l ,

near the substrate close to the surface - and the kinetics

are deduced first by the established CP'C method and

second, novelly, by using the AFM to give an absolute

measure of the mean position of the surface as a function

of time. Tiie independent approaches applied to pressed

pellets and single crystals wil l be shown to be in excellent

agreement in terms of both qualitative mechanistic and

quantitative kinetic conclusions.

2. ExpLTimuntal

A Topometrix T M X 2010 Discoverer atomic force mi­

croscope, operating in contact mode, was employed to

image the surfaces of solid substrates. A commercial

Topometrix liquid cell was used, without modification, for

in situ AFM imaging. The rates of dissolution of the

surfaces of sinj;le crystals of p-chloranil were measured

usip.g the following procedure. First a single crystal was

freshly cleaved and imaged in air. typically revealing a

smooth (010) surface with a few steps (Fig. 2(a)). The

smallest steps had an average measured height of 1.08 nm.

The same surface was then imaged in an aqueous environ­

ment. After slight initial dissolution and roughening of the

surface in water alone at a nominal pH of 7 (Fig. 2(b)),

several images were taken until any drift on the .v, v and :.

pie/0 scanner tubes was eliminated and their output steady.

Hydroxide solution was then flowed into the liquid cell

and after ca. 10 ml had pas.sed through the cell the flow

was stopped. Images (typified by Fig. 2(c)) of the dissolv­

ing surfaces were recorded continuously, at 50 s intervals,

corresponding to a scan rate of 10 Hz, and a resolution of

200 X 200 data points per image. For quantitative experi­

ments a scan area of 20 X 20 [xm- was employed. In all

cases two sorts of experiment were undertaken: first the

recording of conventional topographical images and sec-

ii:lii;iirliniiiMiy-l-ll>IIW7).S.f iJ.I

ond the monitoring of the absolute ;; pie/o voltage during

scanning. The latter was recorded by scaling down the .:

pie/.o voltage using a potential divider, and feeding it to an

external input channel of the electronic control unit of the

AF'M which permitted an ab.solute voltage map of the

scinned area to be generated. Calibration of the voltage

then permitted real drops or increases in height to be

measured. This calibration was achieved by imaging a lest

grid, with a known precisely defined thickness of 2400 A,

and measuring the r pie/.o voltage changes between topo­

graphical maxima and minima. Using this procedure a

voltage-height "conversion constant' of 0.308 V(j,m ' was

calculated. In this way the height of the crystal could be

measured throughout Ihe di.ssolution period in the presence

of hydroxide.

A diagrani of the CFC is given in Fig. 3: the cell is

composed of a rectangular duct (about 4.5 cm long, 0.1 cm

deep and O.Acm wide) cut in a Perspcx block and closed

by a cover plate. The solid substrate, either in the form of

a pressed pellet or single crystal, was embedded into the

cover plate together with a downstream platinum foil

detector electrode for the amperometric monitoring of the

amount of products released. Solutions were made up

using deioni/.ed water of resistivity 18 M i l cm. The ionic

strength was adjusted to the desired value by adding AR

grade potassium chloride. The solution pH was modified

with volumetric standard potassium hydroxide (Aldrich).

Solution flow rates were obtained in the range 10 ' to

10 ' c m ' s " ' using gravity feed [10-12]. In every experi­

ment, the temperature of the flow system was maintained

at 25 ± 0.5°C using an air-thermostat.

The cover plate was a block of Perspex with a circular

hole (see Fig. 3) which supported a pellet or single crystal.

The solid substrate was masked with thin Teflon tape so

that a known area of the solid was exposed to solution.

The precise geometry of the assembled pellet/detector

system was measured to ±0.002 cm with a travelling

microscope.

Commercially available p-chloranil (Aldrich, 99%) was

employed without further purification. To produce pressed

pellets a pressure of 5.7 X lO'' nm • was generated by a

screw press to coinpact the powdered material directly into

the Perspex block [15]. Single crystals of p-chloranii - up

to approximately 1 cm in length - were grown using the

slow cooling process [16-19] from saturated .solutions of

p-chloranil in toluene, seeded with smaller pre-selected

crystals. The best results were obtained by slow cooling of

the saturated solution from 55°C to room temperature over

periods of between 168 and 300 h in a 500 ml crystal

growing flask. The solution was kept in motion by a

slowly rotating paddle to ensure solution homogeneity.

The solution cooling rate was controlled by using a home-

built programmable temperature water balh. It consisted of

an electronic controlling system which triggered the i l lumi­

nation of a powerful light bulb (275 W IR reflector, GEC

Electronics), which shone through one side of the water

./. /((/(//// rl (il./.liiiiiiiiil (ij i:i('ilr(iiiii(ilyli((il Clirmisin 4-10 I I'M) .S'.< '>.<

(a) r.

I-iy. 2. Al-M ill I { ) I I l> 1 I 010) I

.sdlulion Tor cii. ( I ) I ( ) I I

and 4.93 niM iCOlK I | H 117) I

165. (v) 220. (v ) 7 I 1 I I I

topographic iiui 100

d m 111 (b) 1 tliloi mil Livst il liter txposiiic to UHILOUS 0 2M KCI

p sLiKL ol I St iliL solution tout imin 0 2 M I ' C I l O l i n M C P T A B

l t l i L C A L i \ s i i l t o t l K ihovc solulioii loi ( i ) i ( i i ) i - i (ill) 110 (iv)

I s c m i i l c IS l ( )H/(200n- in ' ) All mi I-LS iic Idt sh idul I>pii.il

./. Ilodil: fl til. /.hmiimlol Ela lumiiiihlinil Chimisli v •l-l(IIIW7l W- W

(C5}rf

iMg. 2 (conlinucd).

bath lo provide healing. The largest crystals formed were approximately 0.7 X 0.7 X 0.3 cm' in size. Suitably smaller sized crystals prepared in this way were characterized using X-ray diffraction techniques. The unit cell dimen­sions and the unit cell volume were in excellent agreement with the literature [20],

Stopped How measurements were carried out using a Hi-tech Scientific SF-51 stopped How spectroiluorimeter (Zeneca, Huddcrsfield). UV-visible measurements em­ployed a Unicam UV2 spectrometer, with Unicam Prism Software (Unicam Ltd., Cambridge). Platinum and gold rotating disc electrodes (RDEs) were obtained from Ox­ford Electrodes (Oxford, UK). Constant rotation speeds, stable to ±0.01 Hz, were maintained using a motor con­troller employing proportional feedback. All electrochemi­cal measurements emplf^ed a saturated calomel reference electrode (Radiometer, Copenhagen).

Chloranilic acid (Aldrich, > 997f;) and the surfactant dodccyltrimethylammonium bromide (CI2TAB) (Aldrich, 997r) were used without further purification. Solutions for

PTFE masking

'O' ring

I-'ig. 3. Diagram of a practical clianiicl How cell.

Stopped flow experiments were buffered using potassium dihydrogen phosphate (Fisons, > 99%) and sodium hy­droxide (Fisons, 98%), or commercial buffer tablets (BDH). A precise measure of solution pH was made using a pre-calibrated glass combination electrode (ABS) and me­ter (Model 7020, Electronic Instruments Ltd.). All .solu­tions were purged with argon (B.O.C., 99.99%) prior to

3. Results and discussion

Prior to the study of dissolution experiments using the CFC and AFM the solution chemistry and electrochemistry of p-chloranil (CA) in aqueous solution were clarified.

3.1. Sol III ion siH'cUilion

The homogeneous hydrolysis of CA is thought [21-27] to proceed via two sequential nucleophilic displacements of chloride ions by hydroxide, ultimately forming the chloranilatc dianion, as shown in Scheme i, on the basis of potentiometric [21.28] and spectrophotometric [24-27] data. Developing the latter, we conducted stopped How measurements in which solutions of CA in ethanol -t- water (50:50 by volume) were mixed with aqueous buffers in the pH range 7.9 to 9.8 and the CA absorption at 290 nm [23] monitored in time. It was found that the CA loss was first order in both CA and 0H~ with a rate constant of 1.8 X 10̂ M^' s ' . This value suggests that the lifetime of CAat pH 12 is ca, 4/x.s.

The equilibrium constant for the hydroxide driven inter-conversion of B and C has been reported as 2I0M~'. Spectrophotometric measurements at pH 12 on authentic samples of chloranilate and the species B and C [23,24] (Scheme 2) were made as a preliminary. At 400nm the chloranilate dianion was found to show a negligible ab-sorbance unlike the species B and C which both displayed

./. Iloolh ft (iL/MmiiwI of Elcclmwmlyliail Vlwmisiry -I'll) IIW7I .S.i-'J.i

N' Jx

c i - ' \ / ^ c i

monochloranilic acid anion >- <

STEP B, SLOW

a significanl absorbancc (^/M ' cm ' =2.51 X 10- and 2.63 X 10' respectively). This wavelength was then moni­tored over a period of ca. I h in a solution containing chloranil at concentrations less than 10 ' M, and potas­sium hydroxide, to yield pHs in the range 11.7 to 12.4. Working in this |)H range ensured that there was at least a SO-fold excess of hydroxide over the concentration of the substrate. Analysis ol' the total absorbance at 400 nm was consistent with Scheme 2; that is with B and C in equilib­rium with each other and with C decaying by first or pseudo-first order kinetics. Under this model the total absorbancc decays exponentially in time as was observed experimentally. Analysis of the transient in combination with the measured extinction coefficients of pure B and C permitted first the inference of a value of 200 + 301VI ' for the equilibrium constant describing the intcrconvcrsion of B and C. and second the values for the first order rale constant (A.;,) for the decay of C given in Table I. The latter suggest a weak hydroxide catalysis. The agreement of the equilibrium constant with independent results [23] confirms our interpretation of the spectra and suggests that the detailed homogeneous chemistry of CA given in Scheme 1 may be simplified to the kinetic sequence given in Scheme 2. Considering the latter, wc note that A-, is not readily deduced from UV-visible measurements because of its short lifetime relative to B and C [21,24] and the impossibility of preparing a pure sample to permit the independent characterisation of its spectrum. Instead we turn next to consider the vollammctry of CA solutions with a view to further characterisation of the solution chemistry.

.?.2. Vohamtnetiy of lioiiio^cncous solutions

First species C was generated by adding CA dissolved in a tiny amount of acetonitrilc to a cold (ca. 0°C) aqueous solution of l.OM KOH to give a .solution containing 1% acetonitrilc (by volume) and typically 0,5 niM C [23,24], Voltammetric waves measured using a gold rotating disc electrode showed a two-electron reduction at a half-wave potential of - 0.48( + 0.01) V vs. SCE. Next, acidification of the .stirred solution, by addition of cold l.OM HCl, results in the .solution equilibrium shifting in favour of B and the corresponding voltammctry showed a second wave, with a half-wave potential of ca. -0.33 V vs. SCE in addition to that relating to the reduction of C. The height of both waves was consistent with that seen for C alone under basic conditions, suggesting both processes to be two-electron reductions. This observation was exploited to verily independently the value of k^, measured speciropho-tometrically above. Last, voltammetry using aqueous alka­line solutions of chloranilate and KCI showed no disccni-ablc electroactivity in the potential range +0.2 to - 1.0 V vs. SCE.

3.3. Clmimcl flow cell cxperiinenis

Experiments using a channel flov̂ cell with a platinum detector electrode downstream of pressed CA pellets showed voltammograms such as displayed in Fig. 4 when solutions containing 0.2 M KCI and of pH 11.4 to 12.5 were flowed over the solid. A monitor electrode positioned

Modelling dissolution

"VV" •^°fr- ̂ "A: 4t"fT ^"fx° •0 OH

C(aq) Chloranilate (aq)

./. Ili'nih ,1 III. .'.loiiniiil III l:lri iiniiiiiihliii! Clii'misln IHl I IW7l ,S.< >l.f

[Oil l/niM '" " < , /̂

imiiiL'diaiL'ly uiisliviini ol' llic ilissolsini^ iiitcrl'aci? showL'tl

no ciinvMi ivspuMSL- over ilic coiTCspondinj; polL-iili;il niiiiic.

'I'lic huge \v;ivc shown in ML;. -I is ii l l i i l i i itcd lo ii nicrjiL'ti

comhinalion of Ihc iwo IWO-CICL-IIOM waves relalini; lo tlie

reduction »!' H ami C. The pie-wave has a hall-wave

polenliai ol' -() .()7V vs. SCii wliieh is ineonsislent wilh

ihe dala uiven ahove I'oi- any ol' V>. C or chloranilale.

Moreover, the iifetime of CA iindei' ihe p l l eondilions

eniployeti is such thai il is loo short lived (ea. I lo I5|j,s)

lo sinvive transit from the solid to the detector electrode.

Aecordiiiuly the pre-wave was altiihiiled to the (two-elec­

tron) reduction ol' species A (Schenies I and 2).

The transport limited currents of hoth voltanmietric

waves were recorded as a I'linction of the solution flow rale

(in the range 10 ' to ()..Bern's ' ) and lor various plis

iietween 11.4 and 12..^. A typical current vs. flow rate

ilependenee is shown in l-'is:. 5. The signal I'roni species A

is seen lo increase steadily with solution flow rate whereas

that rroiii the combination of species U and C is seen lo

rapidly rise lo an a|iproximately constant value.

i^ata such as those shown in Fig. 5 were modelled to

see il" they were consistent wilh various possible tlissolu-

tion niodels. In all cases the homoueneoiis eheniistrv tle-

0.05 0,10 0,15 0,20 0,25 0,30

Volume flow rale/cm'^s'^

:il vs, l l i iw r;ilL' j i rol i l f rci-cirdcil lor lln

•;\ ill ;i,s(i|iili(iiii.-iiiiUiiiiiiii;(),:M KCI

dueed ahove was incorporated to permit the inference of

Ihe detector signals (iliie to A and i i -t C) with solution

flow rale. 'I'lie sleady-stale conveclive diffusion ec|uations

which describe the distribution of the meclianislically sig­

nificant species within the flow cell are as follows.

Species A:

.^lA]

Species 1̂ :

•[A] - / . • , [A] [ ( ) I I ]

0 . . , . ^ ^ - . . ^

I'le. 4. Voli;iiiiiiii>i;r;iiiis iiK-iisuicil ;il :i Ct'C. on |)hilimiMi chjclioilc,-, (wilh

a .saliinili-'cl calciik'! a'TciviiLV clccinuk') l idi i i a ilisMiUiiii; L'hJDranii

prcsscil (VIILM (if Icnjjih (),7S.st-ni ami i lckr l i i r -su l id -uap' of ().l<)2aii. in

llic prcsciKv (.raijiicdus (1,2M KCI al a iioiiiiiial p l l ol' 12,0. ami l l im laU'

i.r (l,()f.2.^L-nr> ' , TIk- imuiiu.r and dolcclur ckvinulcs had nominal

dimensions of 0,.^x(),.Vnr'. •|'ln,- l uo v<illaiiiinLMric waves may I v as-

siijiiod 10 Ilk- ivdiiciion or species ..\ (pie-uave) and bolli I i and ( ' (main

wave).

4 A | [ A ] [ ( ) I I ] -A,,[B][C)1I ] + As|,[C]

Species C:

Species Oil :

-A , [C ' ] - f / > , , [ iS ] [ ( ) I I ]

i[()H A,[A][OI! ]

-A,|[B][011 ]+/v:i,[C]

where the coordinates .v and v are defined in Fig. I. Note

that il is known that AS|/A,|, = 2()()1V1 as deduced above.

A, is reported in Table I. /J,,,, = 5 . 3 X 1 0 ' cm-s '

\2^)]. and 1)^. /J,, and l)f can be estimated (to within

lO'/r) using the established VVilke-Chang correlation [30].

In all modelling il was assumed first that B and C were in

ei|uilibriuiii so that A,, and A-,;, were made very large

( lO 'mo l ' e m ' s ' and 50 s ' respectively) relative to the

rales of transport in the cell, second that Ihe conversion of

A to n is O i l driven wilh a second order rale conslanl

,/. Ilddlli <•/ <(/. /.Ididiwl ofliUrlmiimhtwdt Cliniiisliy -l-ll) ll'WI K.f V.f

(<l)

(ll)

lOI I

2.()1

-l.'Jf.

'J.UI

20,7

[Oi l

2.f)l

4.')f)

O.yi

2').7

l / n i M

1/ii iM

( i / i i i M

I.I

2.3

2.').1

.(.-l.-^

A „ , , A n i s '

MIX 10 '

l.4.SXI() '

O.SSX 10 '

I.OX 10 '

A , / n u l l ' e m ' s '

l . - IXKC

I.-LVXIO^

KX 10'

2..'iXl()^

A , / i m . l ' e m ' s '

I..SX|(I''

1.7X10'

O.SX lO'

\.()fiy.ur

A|. iiiicl third thai the rale of hydroly.sis of CA is such (vide

supra) ihat the species exists only in an extremely thin

reaction layer adjacent to the .solid .so that all .-nodellinj:

considered A as the species resident at the solid|liquid

interlace which controlled the evolution of the oilier species

downstream of the interlace.

The transport equations together with the above as­

sumptions and data permit the inference of the detector

electrode ciuTenl provided boundary conditions are speci­

fied to describe the chemistry at the CA|solutioii interface,

as discussed below. The convcctive-dilTusion problem is

then readily accomplished using the backwards implicit

finite difference method applied in essentially the same

form as described elsewhere [9]. No new conceptual or

computational problems were encountered in this applica­

tion and the reader is directed to the literature for appropri­

ate details. The computations, when made for a known cell

geometry, predict the concentrations of species A, U. C

and OM throughout the flow cell as functions of flow rate

for assumed values of /c, and one other parameter describ­

ing the approprate intcrfacial boundary condition (vide

infra). There arc thus only two adjustable parameters in the

model. The resulting concentration profiles may be used to

predict the current at the detector electrode if the computa­

tion is continued downstream of the solid and further

boundary conditions corresponding (i) at the detector elec­

trode to zero concentrations of species A. B or C and to

zero nux of OH (flux ~ / ; „ „ ;i[OH ] / ; ) v ) . and (i i) in

the gap between the solid and the detector to a zero flux of

any of the four species. In this way the dependence of the

detector signals on flow rate can be identified and the

theoretical data can be related to the experimental results

to give best fit values of A:, and any intcrfacial parameters

by minimising the deviation between the theoretical and

experimental currents in a least squares sense [12].

Three separate possible models were considered for the

intcrfacial chemistry. The first supposed a saturated con­

centration of A at the di.s.solving solid:

[ A ] , „ = a

The second assumed a constant flux of A into the

solution:

Ax ' ' [A ] / ; i . \ \ ,„ = ;8

The third presumed thai the release of A from the interlace was driven by reaction of O i l :

= / ; „ „ i)[OII ] / ; i v , „

where Ain,, describes an effective heterogeneous rate con­

stant. In each case the data was analysed to give best fit

values of A, and of « , f3 or /c,,̂ .|. In the first case

satisfactory fits were obtained but the values of a varied

syslematieally with p l i , increasing with [O i l ] (see Table

2(a)). In ihe second case agreenienl between theory and

experiment, even for a fixed p l l . was poor when consid­

ered over a range of flow rales (see l '̂ig. 6). in contrast the

IhirtI model was found to fit well across the entire range of

p l l studied with no .systematic change in the parameters A,

or A|n.|. The resulting best fit values are summarised in

Table 2(b) and typical fits are given in Mg. 7. The

agreement between experiment and theory using Just two

adjustable parameters is highly satisfactory given lhat the

data are generated over values of [OH ] which change by

more than a factor of ten and over flow rates which vtiry

by over three orders of magnitude, and it may be confi­

dently concluded that the dissolution is hydroxide driven

and that the species A is generated very close to. if not at.

the interface. Fig. 8 shows concentration profiles of the

species A, B. C and OH for the zones of the solid and the

detector electrode computed for the mean best fit values of

A-1 and Ai,̂ .,. The rapid reaction of A clo.se to the solid

surface is apparent together with the partial consumption

of O i l as the solution Hows over the solid. The profiles

of B and C are similar, as expected since the species are in

equilibrium with each other.

The kinetic argiunenis given in this sub-section suggest

that the dissolution of pressed pellets of CA in aqueous

solution is driven by reaction of OH with the substrate.

We next turn to consider the dissolution of single crystals

of CA. CFC experiments were conducted using freshly

DO 0.05 0.10 0.15 0.20 0.25 0.30

Volume flow rate / cm^s'''

; poor iigrccnient bcluecii ihc licsl I'll otit i i i i ial lor ihc l l i i

ramclcrs were iisetl; H = 2>

A-, = 1.4: lO 'mol

,/, Ihiilli CI ,il./J(iiiiv(il iifHIcrlmwKilyliail (Iwinixiry -l-IO II'WI S.f-').<

c 20|-

E E 5

0.10 0.15 0.20 0.25 0,30

Volume flow rate /ernes' '

0.05 0.10 0.-

Volume flow rate / cm^s"

3.00 0.05 0.10 0.15 0.20

Volume flow rate / cm^s'^

c) /o

60

< 3- 50

0} 40

3

O) 30 c

E 20

10

/ ' • "

•

-

• '•-.-^

%

J^

• • - -

.^__^—^^^

0.10 0.15 0.20 0.25

Volume flow rate / cm^s'^

I'lg. 7. Kcprc.scnialivc Ills of Iho hydro.xiclL- driven cli.ssolulion nioili,'! lo CI-'C clala ohtiiiiicti u.sinj; a prcs.scil pc l ld and ai|iici)us 0.2 M KCI al (a) p l l 1 1.4

with a pL-llcl Icnyili ()t'().72ycm and dclcctor lo solid scparalion (ir().24()cm. (h) p l l I 1.7 vvitli a pcllel Icimlli or0.784c-ni and dclccior lo solid .separation

orO.iO.'Scm, ( c ) p l l 12.0 wilh a pcllel lenglli oro.7«5cm and dcleclor lo solid soparalion ol '0.154cni. (d) pM 12.5 wii l i a pcllel lenylh o r 0 . 7 y i c m and

dclcclor lo solid separation of ().20.Sein. In all cases a plalinuin. sqnare delcclor elcclrodc of side 0..1cin was employed.

cleaved 010 phines [20], but otherwise a.s for pressed

pellets. Reproducible detector electrode voltammetry was

obtained only in the presence of" millimolar CI2TAB

surl'actant, otherwise nejiligible dissolution took place. The

cationic surfactant may improve the .solution Vv-etting of the

hydrophobic CA crystal surface. Fig. 9 shows representa­

tive experimental data together with the best fit theoretical

points generated from the hydroxide driven dis.solution

model. Experiments conducted over the same range of pH

conditions as for the pres.scd pellets gave values of A,,̂ ., =

3 . 8 X 1 0 ' em s ' and A, = 1.25 X IO' mol ' e m ' s " ' .

The good agreement with the value of A, obtained for the

pressed pellets further vindicates the proposed model: the

lower value for A,,̂ ., is not unexpected in the light of the

comparative surface roughnesses of the pellet and crystal

substrates.

.^.4. AlinnicJune wiiroscojiy

In situ dissolution experiments were conducted using

solutions containing 0.2 M KCi in the pH range 11.7 to

12.7. in the presence of a small concentration ( I mM) of

C I2TAB, Fig. 2 shows the evolution of the surface with

lime from (Fig. 2(a)) a freshly cleaved surface, to (Fig.

2(b)) a surface somewhat roughened through equilibration

under water (containing 0.2 M KCI) for ca. 5min. and

finally (Fig. 2(c)) a sequence of images taken after expo-

.sure to aqueous 0.2 M KCI + 4.9.1 mM KOH + 1 miVl

C12TAB for up to ca. .5min. Notice that the hydroxide

solution induces pitting of the surface and that the surface

appears to dissolve reactively through the rapid evolution

of the pits.

Next, mcasuremenis of the average surface height as a

function of time were made and Fig. 10 shows typical

results: the mean height, as plotted on the v-axis. was

obtained from averaging the ,- piczo voltage within a

single .scan as described above. The approximately linear

decrease in height suggests a steady dissolution tlux from

the surface. The slope of plots such as Fig. 10 were u.sed to

calculate a dissolution flux from the equation

llux/molcm - s ' = [dV/dt}{dh/dV]{ p/M]

./, IUioihctiil./.limmiliifElc~ uilyllnil Clwimin -Nil (IW7) H.rvj

I'i{;. H. Concciilnilioii prDlllcs in ihc channel, ycncralcci using llic liyilroxidi, driven clissolu

Icnylli O.XOcni, solid to detector separation () . l5cni, deteelor dimensions O l x O l t n i ' /

Af/= I()(){). How rate 0.01 em 's ' and p l l 12,00, The I'iyure shows the concLntntions in the

ihe diagram, l-'or clarity only one c)iiarier of the cell depth (2//) is shown.

n model (set text) with the lollowir

=-0 'JSy 10 ' A, - I 4 / lO ' 2/i

pi inc with the cr\st il loc ilcd on the

pellet

12(100.

0.05 0,10 0.15 0.20

Volume flow rate / cm"^s"

to C i C I'll;, y. A typical lit ol the hydro.\ide driven dissouilion

data ohlained iisiny a sinule crystal (010 plane) ol' CA at pM 12.0 lor a

solution conlainini: I.OOm.Vi C I2TAB and 0.2.VI KCI. lixposed length of

crystal ()..^4';cni: detector to solid separation ().2fi.'<cin.

10. The variatio r the average surface height with time

.- pie/o voltage (see text) in the presence ol

lai^in^ 0 . 2 M " K C I aiiid I.OI mM CI2TAB. al

./. Ildiiili (•/ III. /.Idiirniil (if Elccimiimlyiwiil Clwmisliy 'l-ID (IW7> <S.f-W

[OH-]/M

Fig. 11. The rule of dissoliilion. as inlcrrecl from Ihc slope of plots riti. y, with the hydroxide concentration in solution.

4. Conclusions

The use of AFM lo provide qiiantilativc mcasiircmcnls of fates of dissolution averaged over the surface is de­scribed. For the case of p-chloranil dissolved in aqueous base, such measurements permit the inlcrcncc that the dissolution is driven by reaction of hydroxide ions with the substrate at the interface. This conclusion is in complete agreement with independent kinetic measurcincnts made using a channel flow cell. Comparison of the kinetic data shows that significant changes in solution concentrations can occur in diffusion-only AFM cells. It follows that quantitative intcrfacial kiitctic inferences should be drawn with caution in the latter type of experiment. At the same time the complementary nature of structural AFM ob.serva-tions and quantitative kinetic flow cell experiments merits emphasis.

where <iV/dt is the measured slope, d/(/dV is obtained by calibration of the :. piczo voltage using a grid of known pitch. f)/gcm'^ is the density of the solid and M is its relative molar mass (gmor ' ) . The results of such analy-.ses made as a function of [OH'] are shown in Fig. 11. The good linear dependence again suggests that the dissolution is driven by reaction of hydroxide with CA at the solidlliquid interface. This mechanistic conclusion is iden­tical with that resulting from the CFC kinetic data. The slope of Fig. 10 pennits the inference of the rate law for the dissolution process:

rate/molcm"-s"^' =4 .3X 10~"[OH-]h„n

where [OH'lhun/M relates to the bulk concentration of hydroxide (see below). Analogous experiments conducted in the total absence of surfactant gave a rate constant of 4.13 X 10~'*dm-'cm"' s~', suggesting that the surfactant has little influence on the rate of reactive dissolution.

We next consider the rate law obtained above from the AFM studies with that obtained for single crystals using the channel flow technique:

rate/molcm" = 3.8X 10'^[OH-]„

where [OH~]()/M relates to the surface concentration of hydroxide. Both techniques clearly show the mechanistic involvement of the OH" ion. However they can be quanti­tatively consistent if there is significant depletion of hy­droxide from the bulk concentration near the crystal sur­face in the diffusion-only environment of the AFM cell in the form of a Nernst diffusion layer [31]. These kinetic observations therefore point to appreciable depletion ef­fects operating in the stagnant, diffusion-only conditions of the AFM cell. We are actively developing AFM flow cells with well-defined and modellable hydrodynamics to permit meaningful kinetic measurements in the AFM environ­ment.

Acknowledgements

We thank Zeneca for financial support through their Strategic Research Fund, and the EPSRC for a studentship for JB. Stimulating and illuminating di.scu.ssions with Paul Mullins, Emma Hill, and Barry Coles are gratefully ac­knowledged.

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