0(Accepted - DTIC

THE ANTHRANILATE AMIDE OF "POLYETHYLENE CARBOXYLIC ACID" SHOWSAN EXCEPTIONALLY LARGE CHANGE WITH pH IN ITS WETTABILITY BY WATERMark D. Wilson and George M. Whitesides*Department of ChemistryHarvard UniversityCambridge MA0213800oTechnical Report No. 13 (December 1988)QInterim Technical Report0(Acceptedfor publication in J. Am. Chem. Soc.)PREPARED FOR DEFENSE ADVANCED RESEARCH PROJECTS AGENCY1400 Wilson BoulevardArlington VA22209DEPARTMENT OF THE NAVYOffice of Naval Research, Code 1130P800 North Wuincy Street22217-5000Arlington VAELECTEARPA Order No.:NR 356-856Contract No.:N00014-85-K-0898Effective Date:85 September 01Expiration Date:89 May 31Principal Investigator:2 4 FEB 199George M. Whitesides(617) 495-9430The views and conclusions in this document are those of the authors and shouldnot be interpreted as necessarily representing the official policies, eitherexpressed or implied, of the Defense Advanced Research Projects Agency or theU.S. Government.i" im:A:1re wti"rm been OWmmd MWla IN. 82i24nt .IV

REPRODUCED AT GOVERNMENTEXPENSEECUjR!7Y CLASSiF-CAT!ON OF "I. IS DCGEREPORT DOCUMENTATION PAGEIa.REPORT SECURITY CLASSIFICATION1b RESTRICTIVE MARKINGSUnclassified3 DISTRIBUTiON/AVAiLABLITY2a. SECURITY CLASSiFtCATION AUTHORITYOF REPORTApproved for public release;2b. DECLASS iFCATION / DOWNGRADING SCHEDULEdistributionunlimitedS. MONITORING ORGANIZATION REPORT NUMBER(S)4. PERFORMING ORGANIZATION REPORT NuMBER(S)Technical Report 0136a. NAME OF PERFORMING ORGANIZATIONHarvard University16b.OFFICE SYMBOLI(If applicable)Code 1130P800 North Quincy StreetOffice of Sponsored ResearchHolyoke Center, Fourth FloorArlington VA02138-49938a. NAME OF PUNDINGi SPONSORINGORGANIZATION8b. OFFICE SYMBOL(If apicable)22217-50009. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBERIONR/DARPA8c. ADDRESS (City, State, and ZIP Code)80 NrttretPROGRAMQiny10 SOURCE OF FUNDING NUMBERSPROJECT800 North Quincy StreetArlington VA71Office of Naval Research7b. ADDRESS (City, State. and ZIP Code)6c. ADDRESS (City, State, and ZIPCode)Cambridge MA7a NAME OF MONITORING ORGANIZATION22217-5000ELEMENT NO.NO.85-K-0898NR 356-856ITASKWORK UNiTNOACCESSION NOTITLE (Include Security Claisirication)"The Anthranilate Amide of 'Polyethylene Carboxylic Acid' Shows an Exceptionally LargeChange with pH in its Wettability by Water"12PERSONAL AUTHOR(S)Mark D. Wilson and George M. Whitesides*IInterim14 DATE OF REPORT (Year,Month,Day)13b. TIME COVERED13a. TYPE OF REPORTDecember 1988TOPROM15 PAGE COUNT816. SUPPLEMENTARY NOTATIONFIELD19COSATI CODESGROUPSUB-GROUP18. SUBJECT TERMS (Continue on reve-xe if necessary and identify by block number)polyethylenecontact anglespH-dependant wettabilityinterfacial acid-base behaviorwettabilitypolymer surface modificationABSTRACT (Continue on reverse if necessary and identify by block number)The amide formed from polyethylene carboxylic acid (PE-CO 2 H) and anthranilicacid shows an exceptionally large change in wettability by water with pH. Thevalues of advancing contact angles Oa are Oa(pH 1) - 1100 and9a(pH 12) - 330.Comparison of these values with those for correspondingamides of meta- and para-aminobenzoic acid and aniline suggest that bothconformational mobility of the polar functional group at the solid-waterinterface and surface roughness contribute to this large value.20 DiSTRIBUTION /AVAILABILITY OF ABSTRACTSUNCLASSIFIED/UNLIMITEDC SAME AS RPT22a NAME OF RESPONSIBLE INDIVIDUALDr. Joanne Milliken00 FORM 1473,84 MAR21 ABSTRACT SECURITY CLASSIFICATIONCOTIC USERS22b. TELEPHONE (Include Area Code)83 APR edition may be used untI exhausted.All othter editIons are obsolete.22c. OFFICE SYMBOLSECURITY CLASSIFICATION OF THIS PAGE

JA882292TREVISEDThe Anthranilate Amide of "Polyethylene Carboxylic Acid" Shows an ExceptionallyLarge Change with pH in Its Wettability by WaterMark D. Wilson and George M. Whitesides*Harvard UniversityDepartment of ChemistryCambridge MAAbstract.02138The amide formed from polyethylene carboxylic acid (PE-CO H) andanthranilic acid shows an exceptionally large change inwettability by waterwith pH.The values of advancing contact angles are ,B(pH 1) I1(IpH 12) 339.andComparison of these values with those for correspondingamides of meta- and Dara-aminobenzoic acid and aniline suggest that bothconformational mobility of the polar functional group at the solid-waterinterface and surface roughness contribute to this large value.QAecession ForNTIS GRA&I-DTIC TABUnannounced]IJustil oationByDistribution/Availability CodesDs i anSd/orDistSpecialA ,l

2The Anthranilate Amide of "Polyethylene Carboxylic Acid" Shows an Exceptionally1Large Change with pH in Its Wettability by WaterMark D. Wilson and George M. Whitesides*Harvard UniversityDepartment of ChemistryCambridge MA02138"Polyethylene carboxylic acid" (PE-CO 2H) is a material prepared from lowdensity polyethylene film (PE-H) by generating carboxylic acid groups in a thin( 2 nm) interfacial region using chromic acid solution as oxidant. 2 -6Thismaterial is a convenient substrate with which to explore the physical-organicchemistry of organic surfaces.One interesting and analytically usefulcharacteristic of PE-CO 2 H is that its wettability by water depends on pH:forpH 4, the carboxylic acid groups are protonated and the surface relativelyhydrophobic (the advancing contact angle, 8a, is -550); for pH10, thecarboxylic acid groups are present as the more hydrophilic carboxylate anions,and the contact angle drops to 9a 200.Other acidic, surface-functionalizedderivatives of PE-CO 2 H also show a pH-dependent contact angle.In most casesthe magnitude of the changes in cos 8a with pH are limited by the facts thatderivatives of PE-CO 2 H seldom show a value of a less than -200 (independent ofthe relative hydrophilicity of the functional groups present at the polymerwater interface) 6 and that most potentially ionizable derivatives are7relatively hydrophilic at low values of pH.

3Here we report that the material 2 formed by reaction of PE-COC1 withanthranilic acid 8 shows a very large change in contact angle with pH:from a - 1100 (more hydrophobic than unfunctionalized PE-H) at low pH to ea33'at high pH (Figure 1).9000PE-/PE4N/NNHHH 3CCO 2 HCO 2 H2 (ortho)3 (meta)4 (para)150N,H-OHN067The value of 8a - 1190 for the reference material 1 is independent of pH,and is also higher than that of PE-H.The difference in a for I and PE-Hcannot be interpreted directly in terms of hydrophobicity, since the surface of2 is rougher than that of PE-H.Very hydrophobic derivatives of PE-CO 2H showvalues of a a 1300 (for example, PE-OSi(CH 3 )3 , Oa - 1300; PE-CONHC 18 H3 7 ,9a - 1320).Nonetheless, 1 is unexpectedly hydrophobic.We interpret the

I0PE-OSi(CH 3 )3 '-0.520fPEH-"120PPE-H"PEPE00.0-0-90NH-DCO 2 HC0 2 8101214pHFigure 1. Variation in the advancing contact angle9aof water as a functionof pH on several anilide derivatives of "polyethylene carboxylicacid", PE-CO 2H. Data for PE-CO 2H, PE-H, PE-CONHC 6H5 , andPE-OSi(CH 3 )3 are given for reference.

40a 1320).Nonetheless, 1 is unexpectedly hydrophobic.We interpret thehydrophobicity of this material as indicating that its phenyl groups aresufficiently large to shield the polar amide moieties from contact with water.Introduction of a carboxylic acid moiety into the position ortho to theamide group of I produces only a modest decrease in the value of Oa at lowvalues of pH.At high pH, however, Oa is much lower for 2 than for 1. Theamides of meta- and para-aminobenzoic acids (3 and 4) have hydrophilicitiessimilar to that of 2 at high pH, but are more hydrophilic at low pH and thusshow less striking changes in 0 a with pH.We have not rigorously identified the origin of the large change in Oawith pH for 2, but we offer the hypothesis that it lies in a change inconformation of the acyl anthranilate moiety with respect to the surface(represented schematically by 6 -1).We suggest that at low pH, 2 adopts aconformation that buries both the -CONH- and -C02 H moieties; in thisconformation, 2 resembles 1 from the vantage of the contacting water.At highpH, the carboxylic acid group ionizes and the free energy of solvation of theresulting carboxylate ion induces a change in conformation that exposes theC02 - group to water; in this conformation, the surface of 2 resembles that ofPE-C0 2 -. This hypothesized conformational change rationalizes the large changein Oa with pH, but also leaves unanswered a question:orientation suggested by 6?Why should 2 adopt theBy burying the carboxylic acid, conformation 6sacrifices the polar interaction between the carboxylic acid and the contactingwater.The similarity between the wettability of 1 and 2 at low pH does,however, argue for 6 or a similar conformation for 2.Intramolecular hydrogenbonding between the NH group of the amide and the carboxylic acid (carboxylate

5anion) may be important, since the behavior of 5 (Oa(pH 1) 84', 8a(PH 12) 301) resembles that of 3more closely than that of 2.The large change in 8a with pH for 2 suggests that the functional groupsat the polyethylene-water interface are conformationally mobile with respect tothat interface, and that this mobility, by burying or exposing polar functionalgroups, can strongly influence macroscopic properties of the interface such aswettability.The high hydrophobicity of 2 at low pH reemphasizes theeffectiveness with which small, non-polar organic groups at an interface can10shield underlying polar functionality from contact with water.References1)The work was supported in part by the Office of Naval Research and theDefense Advanced Projects Research Agency.MDW held an IBM PredoctoralFellowship in Polymer Science (1986-87).2)Holmes-Farley, S.R.; Reamey, R.H.; McCarthy, T.J.; Deutch, J.; Whitesides,G.M. Lanamuir 1985, 1, 725-740.3)Holmes-Farley, S.R.; Whitesides, G.M. Lanqmuir 1986, 2, 266-281.4)Holmes-Farley, S.R.; Whitesides, G.M. Lanqmuir 1987, 3, 62-76.5)Holmes-Farley, S.R.; Reamey, R.H.; Nuzzo, R.; McCarthy, T.J. Lanqmuir1987, 3, 799-815.6)Holmes-Farley, S.R.; Bain, C.; Whitesides, G.M. Langmuir 1988, 4, 921-937.7)Cos 8a is proportional to interfacial free energy by Young's equation (TLVcos 0 - 1SV - ISL) and is thus a more useful parameter in physical-organicstudies of wetting than is 8a: Adamson, A.W. Physical Chemistry ofNew York, 1967.Surfaces; Wiley Interscience:,iilI ImId

68.PE-COCI was placed in a flask containing a saturated solution (-0.2 M) of2-aminobenzoic acid in dry diethyl ether under an atmosphere of N2 . After-12 h the film was removed and soaked for 10 min alternately in acetoneand water (four alternations).before measuring9.0aThe material was dried in air ( 30 min).All of the surfaces studied here show pronounced hysteresis.Values forreceding contact angles Or are (at pH 1) PE-H 850; 1, 150; 2, 00; 3, 00;4, 0; 5, 0'.Although the origin of this hysteresis remainsuncertain, 3 ,5 ,6 it is clear that the values of Oa do not reflectinterfaces at thermodynamic equilibrium.10.Bain, C.; Whitesides, G.M. J. Am. Chem. Soc. 1988, 110, 5897-5898.

7CaptionsFigure 1. Variation in the advancing contact angle ea of water as a functionof pH on several anilide derivatives of "polyethylene carboxylicacid", PE-CO 2 H. Data for PE-CO 2 H, PE-H, PE-CONHC 6 H5 , andPE-OSi(CH 3 )3 are given for reference.