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 proton- ation. 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 homo- geneous,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 inter- phase.* (3) Local pols'acidicinteractions. If the local volumetric density (the equiv- alentof 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 negativecharges into 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 constantas 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