Acid-Base Interactions in Wetting

J. rldlresiortSci. Tetltttol. \irl. -5.No. l. pp.57-69 ( l99l ) o VsP t9el.

Acid-baseinteractions in wetting

GEORGE M. WHITESIDES,*HANS A. BIEBUYCK.JOHN P.FOLKERS andKEVIN L. PRIME Departntent of Chentistry,Harvard Universiry',Cambridge, hlA 02138,USA

Revisedversion received 27 Aueust1990

Abstract-The studv of the ionizationof carboxylicacid groups at the interfacebetween organic solidsand water demonstratesbroad similaritiesto the ionizationsof thesegroups in homogeneous aqueoussolution, but with importantsystematic differences. Creation of a chargedgroup from a neutralone by protonationor deprotonation(whether -NHr* from -NH, or -CO; from -CO,H) at the interfacebetu'een surface-functionalized polyethylene and \r'ateris more difficult than that in homogeneousaqueous solution. This differenceis probably related to the low effectivedielectric constantof the interface(5=9) relativeto water (e=80). It is not known to what extentthis differencein e (and in other propertiesof the interphase,considered as a thin solventphase) is reflectedin the stabilityof the organicions relativeto their neutral forms in the interphaseand in - solution,and to whatextent in differencesin the concentrationof H' and OH in the interphaseand in solution.Self-assembled monolayers (SAMs)-especially of terminallyfunctionalized alkanethiols (HS(CH:),,X) adsorbedon gold-provide model systemswith relativelywell-ordered structures thar are useful in establishingthe fundamentalsof ionizationof protic acidsand basesat the interface betr"'eenorganic solids and water.These systems,coupled r.r'ithnew analyticalmethods such as photoacousticcalorimetry (PAC) and contact angletitration, may make it possibleto disentangle some of the complex puzzlespresented by proton-transferreactions in the environmentof the organicsolid-* ater interphase.

Keyx'ords:Contact ansle titration; photoacoustic calorimetrv: poly'ethylene carboxylic acid: contact angle:polvmer surfaces; u'etting; self-assembled monolavers.

I. INTRODUCTION

Interaction of two condensedphases across an interfaceor interphase(the latter is usually a more appropriateword for organicsolids) dependson the detailsof chemical interactionsatlwithin that interphase.In exploring the details of the influence of the chemistryof the interphaseon its properties,it is particularlv useful to examinefunctional groups capableof undergoingacid-base reactions. Proton transferbetween acids and basesin solution is the best understoodclass of organicreactions [1]. Thesereactions are usuallyfast: Systems consisting of acids and basesare often at thermodynamicequilibrium, and their energiescan thus be analyzedin believabledetail. Manipulation of the propertiesof the solid- water interphasethrough proton transferto and from suitablefunctional groups in the interphasethus offers the opportunity to use simple, well-understood chemistry to influence, cleanly and selectively,one aspect of the molecular composition of the interphase,while leaving others-morphology, many inter: facial mechanicalproperties, the characterof non-acidicand non-basicspecies- largelyor entirelyunchanged.

*To r.r'homcorrespondence should be addressed. 58 George,V. Whitesidesetal.

Understandingthe acidity of organic functional groups at interfaces is important in surface science in at least two contexts. First, it can help to rationalizeand control the propertiesof materials:wetting, adhesion, surface electrical conductivity, tribology, and others of more purely technological interest.Second, it can help to define the fundamentalphysical properties and 'character'of the interphase-as distinctfrom any bulk phasespresent-by using equilibriumproton-transfer reactions as probes.The most relevantproperties of an interphasefrom the vantageof controlling the ionizationof groups contained in it would be the capacity of the interphaseto support charge (either on an ionizablefunctional group or as a diffusiblespecies, especially H* or OH-), the accessibilityof acids and basesin the interphaseto potential proton donors and acceptors,and the stability of the interphaseto changesin its state of protonation. In this paper, we will show how the titration of acidic functionalgroups in the interphaseyields useful information about the properties of the interphase:we will focus on the use of contact angle titration and photoacousticcalorimetry (PAC) to study polyethylene carboxylic acid and its derivatives.We will also illustratethe effectsof titratablegroups in the interphaseupon interfacialrecon- struction.

2. THEPHYSICAL.ORGANIC CHE]\IISTRY OF INTERFACIAL ACIDITY Measurementof the acidity of functional groups at the interphasebetween an organicsolid and water is substantiallymore complicatedthan the corresponding measurementin homogeneousaqueous solution. We list below, without greatly detailedelaboration, some of the factors that contribute to this complexity (Fig. 1). For specificity,we will illustratepoints using'polyethylenecarboxylic acid' (PE-COrH), a material about rvhichwe have accumulateda substantialbody of background information t2-101. PE-CO2H is prepared by oxidation of lou'- densitypolyethylene film with chromic acid solution under specifiedconditions.

bulksolid bulkliquid <'I I

A polsed at interface B tethered'in solution" c subsurface D polyacldcfuster E tltratablebut no lnlfuenceon I F In pores G difficultlyaccessible _ by diffusionof OH lnaccessibleto OH-

Interphase

Figure l. Schematicexamples of differentt)'pes of environmentsfor an ionizableacidic functional group(O) in the interphasebet*'een an organicsolid and *'ater. Acid-base interactionsin wetting 59

It has carboxylic acid and ketone or aldehydegroups as rhe only functionalityin the oxidativelytransformed interphase 12, 41.

( 1) The interphaseis a finite, heterogeneousregion. The concept of a homogeneous,well-defined'surface'(in the senseof thegold (111)crystal face) is not a good description of the functionalized region between the bulk polymer and homogeneoussolution. This interfacial region is rough and heterogeneouson manyscales [4].

(2) Local dielectricconstant The dielectricconstant at any point in the inter- phasewill be intermediatebetween that of the bulk polyethylene(-2) and bulk water (801.The local dielectricconstant in the interphasemay, of course,vary substantiallyfrom point to point, dependingon the local structurein the interphase.*

(3) Local pols'acidicinteractions. If the local volumetric density (the equivalentof the concentrationin homogeneoussolution) of carboxylicacid groupsin the interphaseis high,hydrogen bonding betweenthem may serveto stabilizethe protonated form of the system,and thus to increasethe concentrationof base required at equilibrium to remove protons [12, t3l. Similarly, if this density is high, charge-chargeinteractions between carboxylateanions (especiallywhen poorly screenedin a medium of low dielectricconstant) will be unfavorableand may substantialll'hinder the introduction of further negative chargesinto the interphase,once ionization has started t141.

(4) Local dipole arral-s.Ordered regions of the polymer could align dipoles originating in individual functional groups in ways that would be energetically favorable or unfavorable.The ability of the functional groups to relax to con- formations having a lorver energy would depend on the local viscosity.The mobility of functional groups within the interphaseis another parameterthat is not clearly understoodin these systems,although movement of groups between the interphaseand the bulk is well documented[7, 15].

(5) Interfacialwater. Water closeto interfacesappears, in somecircumstances, to have properties sufficiently different from water in the bulk homogeneous liquid phasethat'interfacial water'can be consideredto be a distinctliquid t161. The extent to which water in, for example, a pore in polyethylenelined with carboxylic acid groups rvould have the same dielectric constant as water in the bulk solutionis hardto judge.

(6) Local concentratiortsof hydronium and hydroxide ions. The pK" of an acid in solutionis definedin termsof concentrations[equation ( 1)].

*The low interfacialdielectric constant of the interphasearises from the chemicalheterogeneity in the interphase,which can be seen as a mixed'solution'of functionalizedand unfunctionatized polyethyleneand water. Thus, this effect will be observedirrespective of the actual dielectric constantof the interfacialwater. For experimentalevidence pertaining to the dielectricconstant of the interphasebetween PE-CO,H and water, see ref. [4]. For a discussionof the propertiesof interfacialliquids, see ref. Il I ]. 60 George M. Vthitesideset al.

[H.][A-]. K _ (t) " [HA] The concentrationof hydronium and hydroxide ions in homogeneousaqueous solution is a reasonably rvell-definedquantity over a wide range of concen- 'concentration' trations.The corresponding in the vicinity of a functional group in an interphaseis presentlyimpossible to measure.Any measurementof IH. ] or IOH-] in this type of environmentcan probablyonly be an estimatebased on a relativescale. One can, in principle, assumethe behaviorof a referenceacid in the interphaseand measurethe proton donor ability of another acid of interest relative to that reference.Assuming that the group being examined and the referencegroup experiencethe same local environment-an assumptionthaf is probably incorrect in most cases-it is at least possibleusing this procedure to orderacidities in the interphase. Note that this procedure for comparing aciditiesis substantiallyless certain than that used in constructingacidity functions [1, 17] or in estimatingacidities in non-aqueousor mixed aqueous-organicsolvents [1, 18]. In both of these problemsin measuringacidities in non-aqueousbut still homogeneoussolution, it is possibleto extend the relative scalesof acidity so that they are referenced 'pK^'and 'pH' directly to measurementsof aqueoussolutions in which have their normal meanings. The strategies of using series of overlapping reference indicators(when constructingacidity functions) and of usingthe glassmembrane electrodeas a reproducible,if thermodynamicallyill-defined, reference (as one technique for measuring acidity in non-aqueous solutions) have no direct counterpartin surfacechemistry. It is thus not possibleto measurethe concentration or thermodynamicactivity of hydronium ion or hydroxideion in an interphase.Since both the medium in which the interfacialproton-transfer reactions occur-the interphase-and the concentrations of ions in the interphase are 'pK"' undefined,it is not possibleto define a value of for a functionalgroup in an interphaseusing currently available techniques (Fig. 2). It ma!, however, be possibleto define the value of the solution pH at which the functionalgroup in the interphase is half-protonated [4, 6, 8, 9]. This value-which we call the pK, .',-is a reproducible and useful number, but is less interpretablethan the PK^' ,o o -T | RC.+ H' I' H* +RC T- \ lo-l o- ll r" l{t'l II //o l,o I RC -FRC I bn lo" I ll bulk Interphase bulk solution

Figure 2. The estimationof a value of acidity in an interphaserequires knowing both the concentrationsof H* and OH- in the interphaseand the concentrationsof unionizedand ionized functionalgroups. In the absenceof complete information concerningall the speciesinvolved in ionization.it is only possibleto talk about thesolution pH at uhich interfacialionizationoccurs. Acid-base interacIionsin wetting 6l

(7) Surface reconstruction: changesin inte(acial structltre on ionization. The mobility of functional groups in polymers varies with (inter alia) the tempera- ture, the molecular weight and molecular weight distribution of the polymer chains,and the extent to which the systemis plasticizedby low-molecular-weight componenrs including the solvent t151. Parameterssuch as local interfacial viscosity for PE-CO'H and its derivativeswould be expectedto be different in protonated and unprotonated forms t191. Although, in principle, these par- ameterscan be measuredexperimentally, only limited measurementshave been made [3]. The extent to which the positionsof functional groups in the interphase change,or to which the grossmorphology of the interphasechanges, and the influenceof thesechanges on the ionizationand swellingof functionalgroups during acid-basereactions remain to be established.

3. i\IEASURE]\{ENTOF SURFACE ACIDITY A n'ide rangeof experimentaltechniques have been applied with varyingdegrees of successto the measurementof the extent of ionization of groups at surfaces 14-6,8-10, 20-231.These measurementsare often technicallydifficult, as well as difficult to interpret. Any method requiringthe equivalentof a'pH electrode' is probably not practicalfor measuringacidity in the microscopicvolume of the interphase.Other electrochemicalmeasurements are similarlylimited, although techniquessuch as the measurementof electrophoreticmobility can provide some information about the density of fixed chargeon the surfaceas a function of the pH t241.As a result,most measurementsof surfaceacidity have relied on specrroscopicexamination of chromophoric groups [4-6, 23]. Infrared (IR) specrroscopyis, in general,difficult to apply in aqueoussolutions, and Raman spectroscopy(aside from surface-enhancedRaman) doesnot havethe sensitivity required for most studies of interfaces.Ultraviolet, visible, and fluorescence measurementsare complicated by scatteringfrom the interphase,and often by absorption by the bulk phases.The difficulty in identifying and introducing an appropriate chromophore for each spectroscopictechnique often limits the usefulnessof thattechnique. We, and others, have experimentedwith protocols to avoid some of these problems.The experimentswere sometimespractical, but the resultswere often ambiguous.For example,in an effort to apply IR spectroscopyin studying the ionization of interfacial acids, we equilibratedPE-CO2H film againstaqueous solutionshaving knorvn valuesof pH, removedthe film from the water,blotted it unril the majority of the superficialwater had been removed,and measuredthe IR spectrumof the CO'H and COt groups[4]. Horv closelythe ratio of COtH to COt measuredspectroscopically using this techniqueat any value of pH (and especiallyat high and low valuesof the pH) correspondsto that obtained when the film is in contactwith the solutionis difficult to establishabsolutely, although the resultswere in surprisinglygood agreementwith thoseobtained using other methods,when comparisons were possible. We have recentlyexplored two new methodsto try to provide proceduresfor measuringacidity that complementthe existingmethods. Contact angle titration measuresthe contact angle of drops of buffered aqueoussolution on surfaces containingan ionizablefunctionality as a functionof the pH of thesedrops [4-6, 62 George M. Whitesides etal.

8-10, 20-221. Photoacoustic calorimetry (PAC) measures the photoacoustic signalfrom a surfaceas a function of the pH [23,251. Neither techniqueis a panacea, but each provides valuable information. Contact angle titration is amenable to a wide variety of surfaces, and is independent of the optical properties of these surfaces.PAC circumvents many of the optical problems associatedwith light scatteringat rough surfaces. The origin of contact angle titration was the elementaryhypothesis that when functional groups ionize at a solid-water interface,their hydrophilicity (and the hydrophilicity of the interphase)increases. This changewould be reflectedin a changein the solid-liquid interfacialfree energy, /sr-,and thus, via the familiar Young's equation,in a changein cos 0 [equation (2)i /sv and y,-uare, respec- tively,the interfacialfree energiesof the solid-vapor and solid-liquid interfacesl. /sv-/sr-. cosg- e) Yw A plot of 0 (or of cos 0) vs. pH does,in fact, show an inflectionresembling that expectedfor the titration of a functional group in solution (Fig. 3). This value of the solution pH at rvhich the inflection is centeredhas, in severalinstances, been correlatedwith ionization of the interfacialfunctional group [4-6,8, 20J. Thus, contact angle titration is, at least, a useful qualitativetechnique for examining certaintypes of ionizations. The most important generalizationto emergefrom applicationof contactangle titration to derivativesof PE-CO'H is that the apparent aciditiesof functional groups in theseinterphases differ markedly from those in solution [4-6, 8]. This differenceis uniformly in the senseexpected if transformationof a neutral to a chargedgroup were more difficult in the interphasethan in solution.For most of the systemsexamined, the absolutevalue of the differencebetween pKr,, and pK" is 2-4 units, although in related studies of self-assembledmonolayers, it

Figure 3. Contact angle titrations of three organic solids having interfacialCO:H groups: (l) : 'pH' PE-CO:H; (0): Si/O:Si(CH2)r..CO:H;(O) : AuS(CH,)',CO:H.The in thisplot is the pH of the '4' bufferedaqueous solution in contact*,ith the sample,and the is the advancingcontact angle. Although it is not possibleto titratea SAM of HS(CH: ) ,,CO:H on gold underair, it is possibleu'hen the sampleis immersedin a non-polarsolvent, e.g. cyclo-octane. Acid-base interactionsin wetting 63 appearsthat carboxylicacids presentin relativelylow'concentration'in a non- polar surfacemay not ionizeuntil the pH of the aqueoussolution is greaterthan t0lzt ,221. As our exploration of the phenomenacontributing to contact angle titration has continued,the subject has become more complex. It appears,for example, that the plateauin the titration curve observedfor polyethylenecarboxylic acid may be due to a limit in the hydrophilicityof the material(Fig.4) t8l. This limit, in turn, is probably dependenton the procedure used for derivatization.We still have not resolved the relative contributions of thermodynamic and kinetic factorsto the observedcontact angle.For example,is the value of the advancing contactangle set, at leastin part, by reactivespreading? That is, does the edgeof a drop of alkaline water expanding on PE-CO2H (or a carboxylic acid- terminatedSAM) continuebeyond some hypothetical'equilibrium'value, driven by reaction of hydroxide ion with the carboxylic groups? The advancingand recedingcontact anglesof water on PE-CO'H and its derivativesand on carboxylicacid-terminated SAMs on gold havedifferent values [8,9,21]: What is the origin of this hysteresis,and rvhat information can be derived from it? How important is the presenceof a condensedwater film on that part of the surface not coveredby thedrop [8]? PAC is a spectroscopictechnique that has characteristicsof potentiallygreat valuein surfacechemistry. In this technique,the sampleis irradiatedwith a short pulseof light,and the dissipationof the energyabsorbed by a chromophoreinto the solutionas heatis detectedin the form of an acousticwave in the liquid film using a pressuretransducer (Fig. 5) t251. PAC is very sensitive,especially for systems show'ingu'eak absorbances due to sample against a very weakly

CsH170H

ea Bo

-2 IE

Figure 4. A plot of the advancingcontact angle, 0", of water on variousfunctional derivatives of PE-CO:H againstthe hydrophilicityof the functionalgroups shows a limit at high valuesof hydrophilicity.The hydrophilicityis given by the Hansch parameter,z. This parameteris defined as a^:log P*-log P". where P" is the partition coefficientof the parentcompound, R-H, befir'een two solvents,and P* is the partitioncoefficient of the substitutedcompound, R-X. The valuesshow'n here u'erederived using benzene as the parent compound,and water and l-octanol as the solvents (seeref. t27l). 6{ Ceorge lll. Whitesidesetal.

ls/Eel IS/Epl'nrr

0.4 PH Figure 5. Representativetitration curves for chromophoricacids and basesat a polyethylene-water interphase,and comparison u'ith curves for the same groups in water. The vertical axis is the normalizedintensity of the photoacousticsignal. The inset shou'srepresentative photoacoustic \\'avesarising from dansyl groups at low (-) and high (------) valuesof pH, and from the background(- - - -). The upper inset shou,ssignals arising from dansylgroups attached to poly- ethy'lene;the los'er insetshou's signals arising from dansylgroups in aqueoussolution. The valueof pH is the pH of the bufferedaqueous solution in which the derivatizedpolyethylene is immersedor in u'hichthe dansvlderivative is dissolved.S is the integratedintensity of the observedpressure wave (shownas the cross-hatchedregion in the inset);Eo is the po\\'erof the laserpulse.

absorbing backgroud. PAC is thus ideal for examining a strongly absorbing functional group located in the interphasebetween optically transparentpoly- ethyleneand $'ater,because the techniquehas an intrinsicallylow backgroundin this systemand is also insensitiveto scatteringof light at the solid-liquid interfaceor by crystallineheterogeneity in the polyethylene. We believe that PAC, when applicable, provides the best spectroscopic techniqueno\\' availablefor analyzingthe acidity of interfacialfunctional groups. Application of PAC to acidic and basicderivatives of PE-CO2H has given values of pK, ,rin agreementwith thosefrom contactangle titration 1,231. PAC has,so far, been applied to a limited rangeof problemsin surfaceacidity. Acid-base interactionsin wetting 65

Nonetheless,in severalcases, the normalizeddependence of the PAC signalon the pH for similarfunctional groupsin the interphasebetween polyethylene and water and in homogeneousaqueous solution has also suggestedtwo additional, I important facts.First, the titratablegroups seem to comprisea singlepopulation: they fit a singletitration curve with a singlevalue of pK, ,rl23l. Second,a signifi- cant, non-zero PAC signal is observed for values of pH where no signal is expected,based on our previousobservations [5]. The origin of this background is not presentlyunderstood.

4. THE INFLUENCEOF FUNCTIONAL GROUP ACIDIT}'ON SURFACE RECONSTRUCTION

One of the interestingcharacteristics of PE-COrH and its derivativesis that the surface of these materials reconstructson heating (and probably also recon- structs on mechanicaldeformation or on swelling with organic solvents) 1,71. When the temperatureof PE-COrH is raisedto a valueclose to the meltingpoint of the polymer (T = 100"C), functional groups migrate from the region of the interphasethat determinesthe contactangle into deeper regionsof the polvmer [7, 15]. In these regions, the functional groups may still be observed by penetrating forms of spectroscopy(IR [4, 6J, fluorescence[5], and to some extentXPS [4]), but they no longerinfluence wetting by water. This reconstrutionof the surfaceinvolves migration of a functionalgroup from a near-surfaceregion to the interior of the polymer. Particularly when the functional group is polar and in contactwith water, this movementmight be anticipatedto be energeticallyunfavorable, mainly owing to the loss of solvation energv u'hen the functional group moves from water to hydrocarbon. As expected.using this argument,the rate of thermalreconstruction of PE-CO,H is much slou'er u'hen the sampleis in contact with aqueousalkaline solution and the carboxylicacid groups are presentin ionizedform than when the carboxylic acidsare protonatedand the sampleis dry [7, 15]. A remarkableexample of the coupling of wetting to the ionization of functional groups is presentedby the amide formed by reaction of PE-COCI with anthranilicacid (PE-anthranilamide)(Fig. 6) t201. In contact with acid, this material is verl' hydrophobic;in contactwith base,it becomesvery hydrophilic. The differencebetween these two statesof the material appearsto be a small conformationalchange in the anthranilamidemoiety (Fig. 7). At high pH, the surfacearranses the anthranilamidegroups to exposeCOl moieties;at lou, pH, only C-H bondsare exposed.

5. ACIDIT}'OF INTERFACIAL GROUPS. RESULTS FROM STUDIES OF PE-CO2H AND DERIVATIVES, AND COMPARISONS WITH S ELF-ASSEMB LED MONOLAYERS Protonation or deprotonation of functional groups at interfaceschanges the atomic/molecular properties of these groups, and consequentlychanges the macroscopic properties of the surfaces.To what extent is it now possible to predict the acidity of functional groups in interphasesfrom their propertiesin solution? Can one predict the influenceof protonation or deprotonationon a macroscopicproperty suchas wetting? 66 George Il. Whitesideset al.

O.t O.O 5 ""o*r 3 o o o ffl 000

8 pH

Figure 6. RepresentativepH titration curvesfor a number of aminobenzoicacid derivativesof PE-CO,H. The verticalaxis is proportionalto the cosineof the advancingcontact angle of u'ater, cos 0.(H,O), on thesesurfaces. This measure,and not simplythe contactangle 8", is used because, from Young'sequation, cos d is directly proportionalto the net free energyof interactionof the contactingliquid and the polymerinterphase. The sizeof the sy'mbolsrepresents the variabilityin the contactangle measurements. The horizontallines represent the contactangles of severalanalogous but non-titratablederivatives of polyethylene.The solid curve is the contact-angletitration curve obtainedfrom PE-CO.H.

Q .-LL _.- \rHO

'polyethylene-anthranilamide' Figure 7. Hypothesizedchange in conformationof on going from lo*' to highpH.

In broad terms,it is possibleto predict trends and approximatemagnitudes. It is not possibleto predict accuratevalues for pKt,z or contact angle based on a knorvledgeof pK^, or even to rationalize satisfactorilymany of the trends. Deprotonationof a carboxylic acid by aqueoushydroxide ion and protonation of an amineby hydronium ion both require higher concentrationsof the base or Acid- baseinteractions in w,etting 67

acid by two to four ordersof magnitudewhen the functionalgroup is presentin the interphasebetween PE-CO2H or its derivativesand water than when that I functionalgroup is simply dissolvedin water [4-6,8, 20]. This effectcould be due to a relativelylow local dielectricconstant in the interphase,or to a low concentrationof hydroxideion (hydroniumion) in the interphase(both relative to bulk solution),or to a combinationof the two. Many of the propertiesof interfacial groups on PE-CO'H suggestan interfacialdielectric constantof approxi- mately 10 t4l. This value would tend to discriminateenergetically against any chargedspecies-whether covalently incorporated into the interphaseor entering it by diffusion-relativeto thebulk solution. Polyacidityin the interphasecould take two forms. First, during ionization of carboxylic acids by base there could be an accumulation of concentrationsof weakly screenednegative charges sufficiently high that unfavorable coulombic interactions would require increased concentrations of hydroxide to achieve complete ionization. Second, the carboxylic acid groups might be sufficiently close togetherthat hydrogenbonds would stabilizethem in protonated form and thus, also, u'ould increase the difficulty of removing protons. Although the majority of carboxylicacid groups in PE-CO2H are clearly hydrogen bonded to one another(by infraredanalysis) [4], polyacidityprobably does not contribute to the apparent low acidity of the carboxylic acid groups in this material. Increasingthe ionic strengthof the aqueoussolution in contactwith PE-CO2H does not seemto changethe curvesobtained by contactangle titration [a]. This observationsupports the assertionthat polyacidity is not a major contributor to the acidity of carboxylic acid groups on PE-CO2H. This traditional tesr is, hos'ever, one that has been developed for use in homogeneoussolution with solubleions, and its applicabilityto the interphaseis open to some questionfor t\\'o reasons.First, the solutionsused in contact angle titrations are buffered and are alreadyat a relativelyhigh ionic strength(especially at low and high valuesof pH).Second, added metal halide saltsmight themselvesbe excludedfrom the interface, and might even be excluded preferentially relative to hydronium or hydroxideion. Comparisonof the highlydisordered solid-water interphase presented by PE- CO:H and the more ordered interfacespresented by self-assembledmonolayers sugqeststhat the two are governedby similarrules. The carboxylicacid groupsin SAMs also require contact with a stronglybasic solution to accomplishtheir ionization [21]. (Correspondingstudies with amine-terminatedSAMs are complicated by the high reactivity of the amino group, which leads to ready con- taminationof the surfaces[26].) It is not possibleto definea value of pK, ,rf.or CO'H groupsincorporated into SAMs, sinceno plateauin the curve of cos g vs. pH is observedat high valuesof pH f,2ll. The magnitude of the shift in onsetof ionization can, however,be defined, and can be larger for SAMs than for PE- CO'H. Sincethis shift toward high valuesof pH becomeslarger as the carboxylic acid groups becomemore dilute at the interfacel2l| the most probable explan- ation for the low apparent acidity of these groups is a low local dielectric constant at the monolayer-water interface. To what extent the low local dielectricconstant influences the energeticsof convertingCO2H to COl groups, and to what extentit influencesthe partitioningof hydroxide ion or hydronium ion betweenthe interface and the bulk solution,remains to be established. r

68 George,\1. ll'hitesideset al. \ The only macroscopicproperty that correlateswell with ionizationof inter- facialgroups is wettabilityby water.As expected,converting neutral to charged groupsat the interfaceincreases the wettabilityof the interfaceby water.In very generalterms, the contact angleof rvateron derivativesof PE-COrH correlates with the hydrophilicity of the functional groups (as reflectedin their Hansch z values-group additivity parametersrelated to the free energy of transferof the group betweenwater and octanol) [27,28]. The correlationis, however,odd, in the sensethat the influence of functional group polarity on wetting seemsto 'saturate'at high valuesof polarity [8]. Although increasingthe polarity of the group X in a series of materials having the composition PE-X decreasesthe contact angle for groups of low and intermediate polarity, a number of highly polar groups give rise to similar, non-zero values of the contact angle [8]. We have offered a hypothesisto explain these data basedon the idea that, for highly polar interfacialfunctional groups and at high relativehumidity, the surfacewill consistmostly of adsorbedrvater, independent of the polarity of the groupsat the interface.We havenot, so far, deviseda direct testof this hypothesis.

6. SUMMARY The behaviorof acidic and basic functional groups at interfacesis an apparently simple subjectwith many complex features.As with many simple reactions,it is difficult to disguise ignorance. We know that these functional groups follou' patterns of reactivity similar to those observed in solution, but that there are important systematic differences betu'een behavior in the interphase and in solution.There is no sound theoreticalunderstanding of thesedifferences, and, at the moment, no experimentalmethod applicableto the measurementof those properties-interfacial thermodynamic activities of hydronium and hydroxide ion, microscopic dielectric constant and solvating capacity of the interphase. structure,properties, and thermodynamic properties of the three-phasesolid- liquid-vapor region at the edge of a reactive liquid drop on an acidic or basic surface-that u,ouldbe most helpful in resolvingthese issues. Nerv experimentaltechniques are, horvever,now appearing that promise to renew the study of interfacial acid-base reactions.SAMs are structurallyu'ell- defined systemsthat will serve as tractable models for thesestudies 121,221. PAC and contact angle titration offer new approachesto determiningthe extent of ionization,and to correlationsof ionization and wetting.Microfabrication may' make possible the construction of microelectrodes,and the study of local concentrations1,29).X-Ray techniquesare offering increasingresolution in the studyof thestructure of interfacesbetu'een condensed phases [30, 31]. The simplest subjects may teach the most profound lessonsin science.\\/e hope that study of simple acid-base reactions at solid-water interfaces rvill providefundamental information concerningthese ubiquitous systems.

Acknowledgentents This work was supported in part by the Office of Naval Researchand the DefenseAdvanced ResearchProjects Agency. The u'ork summarizedhere is the result of researchcarried out by colleagueswhose names are given in the references. Acid- base interactions in w,ettittg 69

REFERENCES

l. R. P. Bell, Tlte Proton in Chentisrrl'.Cornell UniversityPress, Ithaca, New York (1959); C. H. Bamford and C. F. H. Tipper (Eds), ContprehensiveChernical Kinerics,vol. 8. Elsevier, Amsterdam (1911): H. L. Finston and A. C. Rychtman,A Nen' View of CurrentAcid-Base Theories.John Wiley',Nerv York (1982); M. Eigen, Angew.Chem. Inr. Ed.3, I (1964); W. P. Jencks,Acc. Chern Res.9, 425 (1976). 2. J.R. Rasmussen,E. R. Stedronskl'andG. M. Whitesides,J. Ant. Chem.Soc. 99. 4736(1977\. 3. J. R. Rasmussen,D. E. Bergbreiterand G. M. Whitesides,"/. Ant. Chem.Soc. 99,4746 (1977|l. .1. S. R. Holmes-Farlel',R. H. Reamey,T.J. McCarthy,J. Deutchand G. Nt.Whitesides, Langntuir r,725 (l985). 5. S.R. Holmes-Farleyand G. M. Whitesides,Langntuir 2,266 ( 1986). 6. S.R. Holmes-Farlevand G. M. Whitesides,Langntuir 3,62 ( 1987). 7. S. R. Holmes-Farley',R. G. Nuzzo, T. J. McCarthy and G. M. Whitesides,Langntuir 3, 799 u987). 8. S.R. Holmes-Farler'.C. D. Bainand G. M. Whitesides,Langmuir 4,921 (1988). 9. G. S.Fersuson and G. M. Whitesides,Chemtracts l, l7l (1988). 10. G. M. Whitesidesand P. E. Laibinis,Langmuir 6,87 ( 1990). I l. J. N. Israelachrili. lntermolecularand SurfaceForces. Academic Press, London (1985). 12. J.Rebek,Jr.. R.J. Duff andW. E.Gordon,J.Ant. Cltent.Soc. 108,6068 (1986). 13. C. L. Perrin.T. J. Du'ver.J. Rebek.Jr. and R. J. Duff, J. Ant. Chent.Soc. I I 2, 3l 22 (1990). 14. H. P.Gregor and lr1.Frederick, J. PolS'm..Sci.23,.154 (1957). 15. G. S. Ferguson and G. M. Whitesides, in: illodern Approaches to Wettabilitl':Theory and Applications.G. Loeb and M. Schrader(Eds). Plenum Press, New York (in press). 16. B. \'. Derjaeuinand N. V. Churaev, in: Fluid IttterfacialPltenometra. C. A. Croxton (Ed.), pp.663-738. John \\'iley', Neu'York ( 1986). 17. C. H. Rochester.Acidiry Futtctiorts:A. T. Blomquist,Ser. Ed; Organic Chemistry,Vol. l7; AcademicPress: Neu York (1970);C. H. Rochester.Q. Rev,.Chern. Soc.20,5ll (1966);R. A. Cox andK. Yates.Cott. J. Chent.6l,2225 ( 1983). 18. K. Ivl.Drumaev and B. A. Korolev,Russ. Chent. Rev.49,l02l (1980);W. S. Matthervs,J. E. Bares.J. E. Bartmess,F. G. Bordwell, F. J. Cornforth, G. E. Drucker, Z. Margolin, R. J. N{cCallum.G. J.N{cCollum and N. R. Vanier,J. Ant. Chem.Soc.97,7006(1975). 19. C.L. Riceand R. Whitehead,J. Phys. Chem.69.40l7 (1965). 20. N{.D. \\'ilson.and G. M. Whitesides,"/. Am. Chenr..Soc. I10, 8718 (1988). 21. C. D. Bainand G. M. Whitesides,Langmuir 5,1370 (1989). I2. S.R. \\asserman,Y.-T. Tao and G. M. Whitesides.Langrnuir 5, 1074 ( 1989). 23. L.Zhang.N{.A.Shulman,G.M.WhitesidesandJ.J.Grabou'ski."/. Ant.Chenr.Soc. 112,7069 (1990). 21. S. S. Dukhin and B. V. Derjaguin,in: Surfaceand Colloid Science,r'ol. 7, pp. l-336. Inter- science,Neu York (197a); A. M. James,in: Surfaceand Colloid Science,R. J. Good and R. R. Stromberg(Eds), vol. ll, pp. l2l-185. PlenumPress, New York (1979). 25. L.J.Rothberg,J.D.Simon,M.BernsteinandK.S.Peters,J.Am.Chern.Soc.105,3454(1983); S. E. Braslavsky,R. M. Ellul, R. G. Weiss,H. Al-Ekabi and K. Schaffrier,Tetraltedron 39, 1909 (1983);T.J. Burkey, M.Majeu'ski and D. Griller,J. Anr. Chenr..Soc.108,2218 (1986). 26. K. L. Primeand G. M. Whitesides,unpublished results ( 1990). ?7. C. Hansch,A. Leo, A. S. M. Ungar, K. H. Kim, D. Nikaitaniand E. J. Lien, J. Med. Chem. 16. t207 0 973). 28. M. D. Wilson,G. S.Ferguson and G. M. Whitesides,J.Am. Chem..Soc.I12,1244 (1990). 29. P. E. Laibinis,J. J. Hickman, M. S. Wrightonand G. M. Whitesides,Science (lUashington, DC) 245, 815 (1989);J. J. Hickman,C. Zou,D. Ofer,P. D. Harvey,M.S. \\/righton,P. E. Laibinis,C. D. Bainand G. M. Whitesides,J. Am. Chem..Soc. lll,7Z7l ( 1989). 30. S. R. Wasserman,G. N'!.Whitesides, I. M. Tids*'ell, B. M. Ocko, P. S. Pershanand J. D. Axe, ./. Am. Chem.Soc. 111,5852 11989);I. M.Tids*ell, B. M. Ocko, P.S. Pershan, S. R. Wasserman, G. M. Whitesidesand J. D. Axe, Phys.Rev. B 41, I I I I (1990);M. V. Baker,I. M. Tidsu'ell,T. A. Rabedeau,P. S. Pershan and G. M. Whitesides,Langntuir (submitted). 31. M.J. Bedzyk,M.G. Bommarito,M. Caffrey'andT. L. Penner,Science (V'ashington, DCI 248, 52 ( 1990).