Formation and Reaction of InterchainGarboxylic Anhydride Groups on

Self-Assembled Monolayers 0n GoldLin Yan, Ghristian Marzolin, Andreas Terfort,

and George M. WhitesidesDepartment of Chemistry, Harvard University, 12 Oxford Street,

Cambridge, Massachusetts 021 38

Reprinted fromVolume 13, Number 25, Pages 6704-6712

6704

Introduction

This paper describes a new method of synthesizingmixed, functionalized self-as sembled monol ayers (mixedSAMs) of carboxylic acids and amides by reaction of areactive intermediate-an interchain carboxylic anhy-dride-with amines. The interchain anhydride is easilyprepared by dehydration of a SAM of 16-mercaptohexa-decanoic acid (HS(CH2)IsCOOH) on gold (Scheme 1). Thereare three general strategies to synthesize mixed SAMs:( 1) to synthesize the differently functionalized alkanethiolsand prepare mixed SAMs by coadsorption from a solutionco ntaining two different thiol s ; 1 (2) to prep are a symmetricdialkyl disulfidesl'2 and asymmetric dialkyl sulfidess withrequisite functional groups and then self-assemble themon gold; (3) to introduce the desired functionality into aSAM after its assembly. While the fi.rst two strategieshave been widely used to synthesize a variety of func-tionalized SAMs for a broad range of applications, suchas biocompatibility,a wetting,s adhesion,G corrosion pre-venting,T and micro- and nanofabrication,s'e the lastmethod has been less used. It would be especially valuable

tel , (617) 495-gwhitesides@

inAduance ACS Abstracfs. November 15.

Langmuir 1997, 13, 6704-6712

Fomation and Reaction of Interchain CarboxylicAnhydride Groups on Self-Assembled Monolayers on Gold

Lin Yan, Christian Marzolin, Andreas Terfort, and George M. Whitesides*

Department of Chemistry, Haruard Uniuersity, 12 Oxford Street,Cambridge, Massachusetts 02 138

Receiued July 9, 1997. In Final Form: September 29, 1997@

This paper demonstrates a new method for the synthesis of mixed self-assembled monolayers (mixed

SAMs) by reaction of a reactive intermediate-an interchain carboxyiic anhydride-with alkylamines. Theinterchain anhydride was prepared in high yield from a SAM of l6-mercaptohexadecanoic acid (HS-(CHzhsCOOH) on gold by treatment with trifluoroacetic anhydride. Alkylamines reacted cleanly with theinterchain anhydride to generate a mixed SAM, which comprised a mixture of acids and amides inapproximately 1:l ratio on the surface. The SAMs of the interchain anhydride and the mixed SAM werecharacterized using polarized infrared external reflectance spectroscopy, X-ray photoelectron spectroscopy,contact angle, and ellipsometry. Control of the wettability of the SAMs was demonstrated by allowingthe interchain anhydride to react with alkylamines having different alkyl groups;this model system gavewetting data consistent with earlier studies of mixed monolayers and verified the ability of this methodto manipulate interfacial physical properties. In certain circumstances, this method is experimentailysimpler as a method to produce mixed SAMs than are conventional methods involving coadsorption of twothiols from a mixture in solution. It also assures two other characteristics of the mixed SAM: that thecomposition of the SAM is roughly 1:1in carboxylic acid and amide groups, and that these two groups arewell mixed on the surface.

for generation of patterned SAMs,10'11 fbnnation of three-dimensional multilayers, 12- 14 synthesis of two-dimensionalarrays for combinatorial libraries,15 immobilization of DNAfor rapid sequencing,ls'16 preparation of certain types ofbiosensors and enzymatic microreactors, 1 7 and attachmentof functional molecules and polymers to surfaces.ls 21

Efforts to manipulate functional groups on SAMs havefocused on alkylsiloxanes on SilSiO2, because this class of

SAMs is believed to be (and probably often is) more stablethan SAMs on gold.2z 28 Development of new methods tomodify SAMs on gold is, however, important because: ( 1)many analytical systems and sensors are being developed

W.;Fr isbie. C. D. :Wnghtpn, M. S. l 'angmuir 1993,q

F. J.; Testoff.Rudolph, A. S

M. A . : N ie l sen , T . B . ;Proc. Natl. Acod. Sci.

Stenger, D. A. ;u.s.A. 1994.9.t.

L988,110,6560-

H. A.; Whitesides, G. M. Langmuir lgg4, 19,1825-

Kim, E.;Whitesides, G.M. J. Am. Chem.

Kim, E.; Whitesides, G. Adu. Mater.

R. H.: Mallouk.

1 9 8 9 . 5 . 1 0 1 -

J. Langmuir

R. M. J. Am. Chem. Soc. 1996. /18, 3988-

E . : C rooks . R . 11 . :We l l s ,

1995 . 11 . 3913-

L.; Engelhard, M. H.;Wiacek, R. J.; Lee, L.;

M .

G. K.: Marks. T. J. J. Am. Chem. Soc

Lu . A . T . ;

50743-7463(97)00762-2 CCC: $14.00 O 1997 American Chemical Society

Interchain Carboxylic Anhydride Groups on SAMs

Scheme 1. Schematie Representation of Formationthe Interchain Anhydride and Reaction of the

Interchain Anhydride with an Alkylamine'

RNH2, NMP, r t

Au

" The alkyl chains of the original SAM of the carboxylic acidin these three SAMs are in trans conformation;the methylenegroups near the functional groups may adapt gauche confor-mation and are less ordered. Carboxylic acids in the SAMs ofthe carboxylic acid and in the mixed SAMs of amides andcarboxylic acids involve in hydrogen bonding. The interchainanhydrides orient largely perpendicular to the surface

that are based on eombinations of SAMs on gold andsurface plasmon resonance,zs,30

"l"r1tochemistry,31

-33 mass

measurement using the quartz crystal microbalance,ss'34or surface acoustic wave devices:le (2) SAMs of alkanethi-

(29) Mrksich, M. ; Sigal, G. B.; Whitesides, G. M. Langmuir l9fi.il, 1 1,4383-4385.

(30) Sigal, G. B.; Bamdad, C.; Barberis, A.; Strominger, J.; Whitesides,G. M. AnaL Chern. 1996, 68, 490-497.

(31) Rubinstein,I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagjv,J.Nature1988, 332, 426-429.

(12)Yelzen, E.U.T. v.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Anr.Chem. Soc. 1994, .116, 3597-3598.

(33) Marx-Tibbon, S.; Ben-Dov,I.; WiIIner, I.J.Arn. Chem. Soc. 1996,118,4717-4718.

(34) Schierbaum, IL D.; Weiss, T.;Velzen, E. U. T. v.; Engbersen, J.F. J.; Reinhoudt, D. N.; Giipel, W. Sciznce 1994, 265, 1413-1415.

Langmuir, VoL 13, No. 25, 1997 6705

olates on gold are substantially better defrned structurallythan other classes of SAMs.es-sz

Why are there so few methods available for themodification of SAMs on gold after their assembly? Thereare, at present, only few synthetic methods that are used,and are more or less restricted in their generality.Terminal aryl azide groupslO have been activated pho-tochemically to produce nitrenes and perhaps otherreactive intermediates; the chemistry and yield of thistype of coupling are not easy to predict. Terminal aminogroups react with isocyanates in solution.33 Amines onthe surface of SAMs react at significant rates withatmospheric COz and other environmental components,and this chemistry is difficult to control. Three methodsare known for modification of the SAMs of carboxylic acidgroups: formation of acyl chlorides;38 generation ofmixedanhydrides;le in situ activation with carbodiimide reagentsfor amide coupling3e and ester formation.aO In moregeneral terms, many organic reactions do not transposeeasily from solution to the surface for a number ofreasons: the surface is a very sterically hindered envi-ronment and backside reactions (e.g., the Sp2 reaction)and reactions with large transition states (e.g., esterifi-cation or saponification) often proceed slowly. In addition,the technical problems of establishing the identities ofproducts, and their yields, for organic reactions proceedingon the surface remain substantial.

Here we report a new method for preparation of mixedSAMs that has three steps: (1) preparation of a well-ordered homogeneous SAM of l6-mercaptohexadecanoicacid on gold;(2) conversion ofthe terminal carboxylic acidgroups into interchain anhydrides by reaction withtrifluoroacetic anhydride; (3) reaction of the interchainanhydride with an alkylamine to give a mixed SAMpresenting carboxylic acids and amides on its surface. TheSAMs of the interchain anhydride and mixtures ofcarboxylic acids and alkylamides were studied andcharacterized by X-ray photoelectron spectroscopy (XPS),polarized infrared external reflectance spectroscopy(PIERS), contact angle, and ellipsometry. All three ofthese steps take place rapidly and in high yield; the laststep will, we believe, have broad generality. This reactionthus seems a very good sequence to use in introducingorganic functionality into SAMs. The organic aminesrequired are readily available and compatible with mostother organic structures. The one characteristic of thisreaction that will restrict it from some applications is thatit produces a 1:1 mixture of functional groups on thesurface ofthe SAM (-CO2H and -CONHR or -CONRR'),

ratherthan a single species. In some circumstances, whena mixed SAM containing COzH groups is required, thischaracteristic may be an advantage.

Results and Discussion

Preparation of the Interchain Anhydride. SAMsof l6-mercaptohexadecanoic acid were prepared by im-mersing precleaned gold slides in a 2 mM ethanolicsolution of the thiol as described previously.al Terminal

(35) Ulman, A. An Introduction to Ultrathin Organic Films fromLangmuir-Blodgett to Self-Assernbly; Academic Press, Inc.: San Diego,cA, 1991.

(36) Whitesides, G. M.; Gorman, C. B. ln Handbook of Surfa.ceImaging and.Visualization; Hubbard, A.T., Ed.; CRC Press: Boca Raton,FL, 1995; pp 713-733.

(37) IIlman, A. Chem. Rev. 1998,96, 1533-1554.(38) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992,64,337-342.(39) l,eggett, G. J.; Roberts, C. J.; Wilhams, P. M.; Davies, M. C.;

Jackson, D. E.; Tendler, S. J. B. Langmuir 1993, 9, 2356-2362.(40) Hutt, D. A.; Leggett, G. J. Langmuir L997, 13,2740-2748.(41) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; EvaII, J.; Whitesides,

G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111,32I-335.

of

I|

(CF3CO)2O, Et3N, DMF, rt

Y

Au

R

6706 Langrnuir, Vol. 13, No. 25, 1997 Yan et al.

of a unconjugated open-chain aliphatic carboxf i ic anhy-drideaT and suggest that the interchain anhvdride issimilar to an unstrained. ac1'clic aliphatic carboxylicanhydride. Furthermore, the h,rgh intensities of thesebands suggest that the intercharn anhl'dnde groups areoriented largely perpendicular to the surface. The com-plete disappearance of the C:O stretching bands forcarboxylic acid groups on treatment with trifluoroaceticanhydride suggests that the transformation of carboxylicacids at the interface to interchain anhydndes occurs inhighyield (that is, by the critenon of PIERS spectroscoPY,quantitative yield).

The position of the C-H stretching bands of themethylene groups of the alkyl chains indicates the orderof the alkyl chains within s $AI1.+s as In the spectrum ofthe SAM of the carboxylic acid (Figure 1). two absorptionbands at2920 and2852 cm 1 are assigned to aslrnmetricand symmetric C-H stretching bands of the methylenegroups, respectively. The positions of these peaks are thesame as those in crystalline polyethylene and suggest thatmost of the polymethylene chains are fully extended intrans conformations and exist in quasi-crystalline state;they do not, however, give much information about theterminal methylene units, since small shifts in thesegroups are not easily detected against the large back-ground of absorption from the remaining methylene groupsofthe chains. The C-H stretchingbands ofthemethylenegroups of the SAM of the interchain anhydride areindistinguishable from those of the carboxylic acid; thisobsewation indicates that the majority of the polymeth-ylene chains remain trans-extended but again does notdefi.ne the order immediately at the surface.

XPS. The survey spectrum ofthe interchain anhydrideis similar to that of the carboxylic acid (Fieure 2). Thehigh-resolution core-level spectra of these SAMs in the

(47) Dauben, W. G.; Epstein, W. W. J. Org. Chem. f959, 1595-1596.(48) Snyder, R.G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chcm' L982,

86, 5145-5150.(49) Porter, M. D.; Bright, T. B.;Allara, D. L.; Chidsey, C. E. D. J.

Am. Chem. Soc. 1987. 109. 3559-3568.

A.

Au

| ,.r,.o,,o

| (czHs)sN

V

s(cH2 lrrc(^s(cHrhscE

cH3(cH2)1oNH2

n, $stcH2)rsco2H5-

S(CHz) TsCONH(cH2)r oCHe

A, Vs(cH2)'uco' H

ws(cH2)15co2 H

B' c-H B c' c=o oc \ l $Nr . : -

iR ,t\/ l

/ \ | I \*-*--/ L .-"."*--/ \"''---"-^-*

N t^ c\l

o N pc . l P :

l on H l l/ \ N A Al \ | t \ / \/ \ A lw \

\^-*r' *-J \|-/vt4^--'-*r$.**

A/ \-*--------"-'

*\--^!----/

olcf)o,c\t

I

C\J

OJ

INrfN

I

e)

I

2750 1900 1800 1700 1600 1500

Wavenumber (cm't ;Wavenumber ( cm-1 )

Figure f. Comparison ofPIERS spectra ofthe SAMs ofthe carboxylic acid, the interchain anhydride, arrd a mixture ofcarboxylicacids and n-uadecylamides on gold: (A) Bchematic representation of the formation of the interchain alhydride and the reactionofttre interchain anhy<lride andn-undecylamine; (B) PIERS Bpectra ofthese SAI{S in lhe C H stretching region; ( C) PIERS spectraof theee SAM8 in the C:O stretching region.

carboxylic acid groups were converted into surface an-hydrides by treatment with trifluoroacetic anhydride andtriethylamine in anhydrous N,lV-dimethylformamide.Since there was no evidence for mixed anhydride incor-porating trifluoroacetyl moieties by XPS, we inferred thatthese surface anhydrides were derived from carboxylicacids on adjacent chains (Scheme 1). Related interchainstructures, such as bridged thioether and disulfide groupsthat connect two adjacent chains of siloxanes in SAMs onSilSiOz, have been reported previously.22

Characterization of the Interchain Anhydride.PIERS. PIERS provides information both about thepresence of IR-active functional groups and also aboutthe order and orientation ofthe hydrocarbon chains withina SAIVI.42 In PIERS, the vibrational modes whose transi-tion dipole moments are perpendicular to the surface showmaximal absorbances; the vibrational modes whosetransition dipole moments are parallel or nearly parallelto the surface show zero or minimal absorbances.a3'u Inthis paper, all the absorption bands are assigned accordingto literature precedent.

Figure 1 shows the PIERS spectra of the SAMs of thecarboxylic acid and the interchain anhydride. Thespectrum of the SAM of the carboxylic acid shows twoC:O stretchingbands at]-744and 1720 cD-1, which arisefrom the free and hydrogen bonded carboxylic acids,respectively.as After treatment of the SAM of the car-boxylic acid with trifluoroacetic anhydride, these two C:Ostretching bands disappeared completely. Two newabsorption bands atL826 and1-752 cm-l appeared instead;this doublet is characteristic of the C:O stretchingabsorption of a carboxylic anhydride, and are assigned toin-phase and out-of-phase stretching modes of the twocoupled carbonyl groups, respectively.46 The position andthe similar intensities of these bands are eharacteristic

(42) Porter, M. D. Anal. Ch.em. 1988, 60,ll43[-1155A.(43) Greenler, R. G. J. Chem. Phys. 19ffi, 44,310-315.(44) Greenier, R. G. J. Chem. Phys. 1989,50, 1963-1968.(45) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Arn. Chem.. Soe.

tggo, 772,558-569.(46) Cooks, R. G. Chern. Ind. 1985,142.

Interchain Carboxylic Anhydride Groups on SAMs

C(1s) region show a similar profrle. The spectrum of thecarboxylic acid exhibits a slightly asymmetric photoelec-tron peak centered at 284.7 eV and a small high-energyphotoelectron peak at 289.5 eV, which arises from thecarbonyl group of the carboxylic acid.al The spectrum ofthe interchain anhydride shows a peak centered at284.7eV and a small high-enerry peak at 289.3 eV, which arisesfrom the carbonyl group of the interchain anhydride. Theposition ofthe photoelectron peak ofthe carbonyl groupof the interchain anhydride is in good agreement with thedata reported for organic polymers containing carboxylicanhydride groups.so

The area under the photoelectron peak of O(ls) (asnormalized to the intensity of the C(ls) peak) of theinterchain anhydride is 0.35 and that of the carboxylicacid is 0.44. The normalized content of oxygen on theSAM of the carboxylic acid is 1.23 times that of theinterchain anhydride (the theoretical value is 1.33, basedon a complete conversion from the carboxylic acid to theinterchain anhydride); these values are compatible withdehydration ofthe carboxylic acid groups on conversionto the interchain anhydride groups. The small difference(ca.7Vo) between observed and theoretical oxygen contentsmay reflect instrumental errors.

The plausible products of the reaction of the carboxylicacid groups and trifluoroacetic anhydride under thereaction conditions are a mixed trifluoroacetic carboxylicanhydride (CFaCOOC(:OXCHz)rsS/Au), an interchaincarboxylic anhydride, or a mixture of both. Since themixed trifluoroacetic carboxylic anhydride contains atrifluoroacetyl goup, which can be readily identified byXPS, we recorded the XPS spectra of the SAM oftrifluoroacetates5l'52 and that ofthe carboxylic acid beforeand afber reaction with trifluoroacetic anhydride (Figure2). The survey spectrum of the SAM of the interchainanhydride shows no observable F(1s) signal. Ifthe newly

(50) Beamson, G.; Briggs, D. HiSh Resolution WS of OrganicPolymers, the Scienta ESCA 300 Database; John Wiiey & Sons: NewYork, 1992.

(51) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G.M. Science lg9l, 254,981-983.

(52) Wasserman, S. R. ; Tao, Y.-T. ; Whitesides, G. M. Larqmuir 1989,5. LO74-t087.

Langmuir, VoL 73, No. 25, 1997 6707

lir

I

A.

U)

I

GE , S

: t $< 5l <

t l

ino

g= ( h

{ =t -

r l

B.N

$rI

d l?r AI /\

II

A/ \t / \

\*Au

nu Sstcn2)r5co2HVs(cH2)15co2H

o//

s (cH2)15c \os (cH2)15c (

o

o, $atarz)rrococFgN-s(cH2)11ococF3

1000 800 600 400 200 0 300 290 280

Binding Energy (eV)Binding Energy (eV)

Figue 2. Comparison ofXPS spectra ofthe SAMs of t.Le carboxylic acid arrd the interchain anhydride and of a model compoundcontaining a trifluoroacetate Eroup on gold: (A) survey sp€ctra of these SAM8; (B) high-resolution spectra of these SAM8 in theC(1s) region.

formed functional group on the surface is an anhydride,and since it is not a mixed trifluoroacetic carboxylicanhydride, it must be an interchain anhydride.

ContactAngle. Examination ofthe advancing contactangle of water on the SAM of the interchain anhydrideshowed that its surface is more hydrophobic than that ofthe SAM of the carboxylic acid.53 (The stability of theinterchain anhydride in contact with water is suffi.cientto allow the contact angle to be measured;see below.) Theadvancing contact angle of distilled, deionized water (pH: 6.0) on the SAMs of the interchain anhydride is ca.72",whereas it is less than 15" on that of the carboxylic acid.This observation is compatible with the lower polarity ofa carboxylic anhydride compared to that of the corre-sponding acid (or, at neutral pH, a mixture of carboxylicacid and carboxylate anion).sa

The results of PIERS, XPS, and contact angle togetherestablish that an interchain anhydride is formed on thesurface under these reaction conditions and that thetransformation of the carboxylic acid to the interchainanhydride occurs in high yield. Although these interchainanhydrides bridge two adjacent alkyl chains, they aresimilar to open-chain, unconjugated carboxylic anhy-drides, and the underlying alkyl chains are well-orderedand closely packed.

Figure 3 shows a tentative mechanism proposed for theformation of the interchain anhydride on the surface ofthe SAM of carboxylic acid. Triethylamine deprotonatedthe carboxylic acid, and the resulting carboxylate anionreacted with trifluoroacetic anhydride to form a reactiveand perhaps unstable intermediate, a mixed trifluoroaceticcarboxylic anhydride. The mixed anhydride furtherreacted with an adjacent earboxylate anion to produce aninterchain anhydride. It is probable that the trifluoro-

(53) The advancing contact angle of water on the surface of the SAMof the interchain anhydride was stable during the meastrrement. Thehalf-life time for hydrolysis of acetic anhydride in water is about 6 min(Wolfenden, R. ; William s, R. J. Am. C hem. Soc. 1985, I 07, 4345 - 4346).We hypothesize that the hydrolysis ofthe interchain anhydride is sloweron the surface than in solution because the surface has a lower dielectricconstant than water.

(54) Smith, S. G.; Fainberg A. H.; Winstein, S. J. Am. Chem. Soc.1961. 83. 618-625.

o

rI

6708 Langrnuir, Vol. L3, No. 25, 1997

A.

oXoHS-oH EraN

ox*9fo (cF3co)2o/ / - / / -

N\ NisAu

oY.r,oYo-

5-o oYo

YoE t 3 N > / / - / /

ift\s(\\ N\.NS\

B.

Yan et al.

of a monosubstituted amide intrans configuration.se Theamide I band, which generally appears as a relativelystrong absorption band around 1680-1630 cm I due tothe C:O stretchingmode, was absen1.60-62 The transitiondipole moment of this vibrational mode is probablyoriented parallel to the surface and thus is inactive inPIERS. The presence of a strong amide II band and theabsence of an amide I band indicate that the carbonylgroups of the amides are oriented parallel to the surfaceand the amide bonds are in the expected trans configu-ration. This spatial arrangement allows the C:O andN-H groups to participate in hydrogen bonding with theneighboring amide or carboxylic acid groups withoutobvious steric strain.

Absorption bands con'esponding to an imide were notobserved.s8 Efforts to convert the carboxylic acid andadjacent amide groups ofthe mixed SAMs into interchainimi d e s using trifl uoroacetic anhydride were unsucce s sful.This reaction would require initial deprotonation of thecarboxylic acids, followed by reaction of the carboxylateanions with trifluoroacetic anhydride. These processeswould. again, be suppressed by the low dielectric constantin the interior of the monolayer.

After the reaction of the interchain anhydride withn-undecl' lamine. new absorption bands appeared in theC-H absorption region: The band at2967 cm-1 is assignedto the asymmetric C-H stretching vibration of theterminal methyl group;the band at 2886 cm 1 is assignedto the symmetric C-H stretching vibration of the methylgroup ofthe z-undecyl group.ae The meth-'-lene vibrationsat2920 and 2853 cm-1 are significantll'broader than thoseofthe original SAM ofthe interchain anhydride. The low-frequency shoulder peaks at 2932 and 2859 cm I thatappear on the formation of amides are the bands arisingfromthe methylene groups of the n-undecl'l chain.ffi Fromthe peak shapes, we infer that the alkl'l chains of theoriginal SAM remain in trons conformation but that thenew alkyl chains are disordered (Figure 4 i.

>(PS. XPS spectra of the mixed SA-\Is of mixtures ofacids and amides show a photoelectron peak for N( ls) at399.9 eV. The ratio of the intensities of the photoelectronpeaks for N(1s) and O(ls) provides semiquantitativeinformation about the yreld of the reaction. We haveallowed the interchain anhl'dride to react wrth threestructural isomers of butylamine (n-, sec-. and tert-butylamine). The interchain anhydnde groups are in ahighly structured and sterically demanding environment,and we had anticipated large differences in yield of theamides derived from these three amines; in fact, the yieldsare similar.

(58) Williams, D. H.; Fleming, L Spectroscopic Methods in OrganicChemistry, 4th ed. ; McGraw-Hili Book Company ( UK r l,tmited : l,ondon,1989.

(59) In frons configuration of a monosubstituted amide bond, theinteraction between N-H bending and C-N stretching grves rise to astrong amide II absorption band; however, in cls configuration. thereis much less interaction between them and therefore there is noabsorptionbandinthe 1600-1500 cm l region comparable to the amide

II band intrans configuration. For example, lactams TCONH( CHz)^) donot show amide II absorption when n < 9 and do show amide II absorptionwhen n > 9; it is conceivable that the amide bond adapts transconfrguration in large rings and cis confrguration in small rings. Schiedt,U. Angew. Chern. 1954, 66, 609.

(60) l,enk, T. J.; Hallmark, V. M.; Hoffrnann, C. L.; Rabolt, J. F.;Castner, D. G.; Erdelen, C.; Ringsdorf,H.I'angmuir L994, 10,46L0-46t7.

(61) Tam-Chang, S.-W.; Biebuyck, H.A.;Whitesides, G.M.; Nwzo,R. Langm.uir 1995, 437 I - 4382.

(62) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12,5239-5243.(63) The positions ofthese shoulders were determined by subtracting

the spectrum of the interchain anhydride from that of the mixed SAMof carboxylic acids and ll-undecylamides. Although subtraction ofreference spectra has no mathematical justifrcation, it allowed us toestimate the position of these shoulders.

oY.r,oxoHoxo/ /

\(srs(sAu

--

oYcFtoYoHo).o- o),o

, S

/ / / -N

Figure r. ;;"*.tic representation oi"' t".rtrti ve mechanis mproposed for the formation of the interchain anhydride on thesurface ofthe SAM ofcarboxylic acid: (A) a mixed trifluoroaceticcarboxylic anhydride was generated as a reactive intermediateon conversion of carboxylic acid groups into interchain anhy-drides; (B) during the reaction, both mixed trifluoroaceticcarboxylic anhydrides and interchain anhydrides could migratereversibly on the surface.

acetyl group can translocate reversibly on the surface;the interchain anhydride group can react with an adjacentcarboxylate anion and therefore the interchain anhydridegroup can "walk" on the surface. Such reversible migrationof the trifluoroacetyl group and the interchain anhydrideavoids the statistical isolation of unreacted carboxylic acidor mixed trifluoroacetic carboxylic anhydride on thesurface and also assures completion of the conversion ofcarboxylic acid groups into interchain anhydrides.

Characterization of the Mixed SAIVIs of AmidesandAcids. PIERS. Afterthe reaction ofthe interchainanhydride and n -undecylamine (taken as a representativen-alkylamine), the two C:O stretching bands character-istic of the interchain anhydride disappeared completelyand two new absorption bands appeared atl742 and 1563cm-1 (Figure 1). The former band is assigned to the C:Ostretching absorption of a carboxylic aeid. The positionof this band is between the two C:O stretching bands ofthe SAM ofthe carboxylic acid; this position suggests thatthis carboxylic acid is also hydrogen bonded, perhapsweakly, to the neighboring carbonyl groups of either anamide or a carboxylic acid. No absorption band charac-teristic ofa carboxylate anion was observed around 1420-1300 cm-1.55'56 The carboxylic acids generated duringthereaction are thus protonated, and they appear not totransfer protons to z-undecylamine and form stableammonium salts.57 We suggest that the failure of theformation of the stable salts probably reflects the lowdielectric constant of the interior of the SAM.

The absorption band at 1563 cm-l is assigned to theamide II band.s8 This absorption band is characteristic

(55) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984,707,201-213.

(56) Schlotter, N. E.;Porter, M.D.; Bright, T. B.;Allara, D.L. Chem.Phys. Lett. L986, 132,93-98.

(57) Wells, M.; Dermody, D.L.; Yang, H.C.; Kim, T.; Crooks, R.M.;Ricco, A. Langmuir 1996, /2, 1989-1996.

Interchain Carboxylic Anhydride Groups on SAMs

9HeI

(cHz)r rI

NHcoI

(9Hr)''uISH

Langmuir, Vol. 1-3, No. 25, 1997 6709

0 5 1 0 1 5 2 0

Length of the alkyl chain (n)

Figure 5. Reaction of the interchain anhydride and n-alkylamine (n-CnHzn.rNHz, f l : 4,6, 11, and 18) generates amixed SAM containinga mixture ofcarboxylic acids andamides.The thicknesses of the added hydrocarbon layers were measuredby ellipsometry: Linear regression analysis gives an incrementof the thickness of 0.84 NCH2 of added amine. Error barsrepresent the standard deviation of the mean for at least threeindividual substrates.

thickness (h, cm) of this hydrocarbon layer by eq 1

o : hp lm (1 )

where m (g/mol) is the molecularweight ofthe alkyl chain.This relation holds if we replace the total thickness (h)by

the increment of thickness of the hydrocarbon layer permethylene unit (Lh),and the molecularweight ofthe chain(m)by the molecular weight of one methylene group (Lm).

A similar relation can be written for the surface densityof the alkanethiolates on gold. The ratio of the surfacedensities of the alkyl chains in a well-ordered SAM andin a disordered hydrocarbon layer of the amides of themixed SAM generated by this method is thus given bye q 2

LhucPsc

o . ^-c ;i* c

O >- l

x ( 0. Y f i

F - En(!

f^ll l

tl lo"| |

1cH2) rs

U S HFigure 4. Schematic representation of a mixed SAM ofcarboxylic acids and n-undecylamides ongold. The alkyl chainsof the amides are disordered in liquid-like state; the poly-methylene groups ofthe original SAM ofthe carboxylic acid areclosely packed in quasi-crystalline state. Since alkanethiolatescontaining amides are either positioned next to each other orseparated by one or two alkanethiolates terrrrinated at carboxylicacids, large-scale phase separated domains may not exist.

Table 1. Ratios of N(ls)/O(ls) of Butylamines withDifferent t

T$1t"t#il::: YJl,f"" d c arb oryric

N(1s)/O(1s) relative yreld (%)'

n-C+IIg 0.40s-CaIIs 0.32f-CaHg 0.34

@ The uncertainty in this number is at least t5%.

The reactions of the interchain anhydride with thoseisomeric butylamines were carried out using the sameprocedure as that used for the reaction of other n-alkylamines. Table 1 lists the estimated yields of thesereactions on the basis of the ratios of N(ls)/O(1s) of thesesamples. The relatively high ratios of N(ls)/O(ls) in thesamples generatedbythe reactions of tert-butylamine andsec-butylamine with the interchain anhydride suggest thatalthough these alkyl groups are sterically bulkier thann-butyl, the interchain anhydride readilyreacts \,\rith themin high yield.

Ellipsometry. The thicknesses of the SAMs werecalculated from ellipsometric data that were obtained andanalyzed by procedures described previously.oa Figure 5presents the thicknesses measured by ellipsometry forthe mixed SAMs generated by reaction of the interchainanhydride and four homologous n-alkylamines (n-CnEzn+t-NHz, fl: 4,6, 11, and 18). Linear regressio-n analysis ofthe experimental datayields a slope of 0.84 NCH2and anintercept of 0.65 A. The increment of thickness permethylene unit is approximately 70Va that of the SAMsof carboxylic acid-terminated alkanethiolates (Afr : 1.16A/CHr).nt The PIERS data indicate that the alkyl groupsof the amides are disordered. Assuming that a hydro-carbon layer comprising alkyl chains of the amides ishomogeneous, the surface density (o, moVcm2) ofthe alkylchains is simply related to the density (p, g/cm1) and the

(64) Wasserman, S. R.;Whitesides, G.M.; Tidswell, I. M.; Ocko, B.M.; Pershan, P. S.;Axe, J.D.J.Arn. Chem. Soc. 1989, 1I1,5852-5861.

where subscript "HC" stands for the hydrocarbon layer ofthe amides and "SAM" forthat ofthe SAM ofthe carboxylicacid. Since the z-undecyl groups of the amides aredisordered and liquid-like and the alkyl groups ofthe SAMof the carboxylic acid on gold are highly ordered andcrystalline on the basis of the PIERS data, we assumethat the density of the hydrocarbon layer of the amidesis similar to that ofthe liquid n-undecane (p :0.740 g/cm3)and the density of that of the SAM of the carboxylic acidis similar to that of the crystalline polyethylene (p : 1.00g/cm3),65 then the surfiace density of the alkyl groups ofthe amide is ca. 0.5 times that of the original SAM of thecarboxylic acid. This estimate is compatible with ourinference that the reaction of the interchain anhydrideand alkylamines proceeds in high yield. It also reinforcesthe conclusion that the transformation of the carboxylicacids to the interchain anhydrides occurs in close toquantitative yield.66

(65) We used the density of liquid n-undecane (p : 0.140 g/cm3,Handbook of Chemistry and Physics, 71st ed.; CRC Press: Boca Raton,FL, 1990-1991) for the density ofthe hydrocarbon layer ofn-undecylgroups since both of them are highly disordered. We used the densityof crystalline polyethylene (p : 1.000 g/cm3, Polym.er Handbook, Srded.; John Wiley & Sons: New York, 1989) for that of the ordered (CHr;"region of the SAM, since we infer that it is in the quasi-crystailine state.

(66) A lower yield of this transformation would result in a smalleramount of the interchain anhydride groups and subsequently a lowersurface density ofthe hydrocarbon layer.

1008085

ottc _oseu AhsevrPserr,r

(2)

A A A A A A l A A A

A A A A A A A A A A A 6 A

a a a a a a ' a a a .

o O o O O o O a .

V V Y V V V V

v v v v v v

- - v J _ v _ v _ L

6710 Langmuir, VoL 13, No. 25, 1997 Yan et aL

n-C16H33S/Au

n-C.rrH3rNH,

n-C.r 1H23NH2

n-C6H13NH2

^ HzOu a n-c4HnNH,

cH3NH2

NH:

H02C(CH2)15S/Au

pFt

Figure 6. Dependence ofthe advaocing contact angle (dJ ofbuffered aqueous solutions ofpH on the SAM8 comprising a mixtureof carboxylic acids and amides, generated by reactions ofthe interchain anhydride end alkylanines (n-C,Hz.+rNHz, n :0, 1, 4,6, 11, and 18). The cuwes are labeled by the respective alkylamines on the right side ofthe plot. TVo dashed lines on the top andthe bottom ofthe plot are the reference data for d" of CHr(CH:)rsS/Au and HOzC(CHrtrsVAu as indicated, respectively, on the rightside of the plot.

200

1 412l 0

Contact Angle. Previous, extensive studies of car-boxylic acid functionalized polyethylene films (PE-COzH),67-70 of SAMs terminated in ionizable acids andbases,71 of mixed SAMs of carboxylic acid- and methyl-terminated alkanethiolates,Tz and of SAMs of dialkylsulfidess on gold have established the utility of contactangle titration in charactenzingthe interfaces. We havedetermined the advancing contact angle of water, 0", asa function of pH, for several mixed monolayers obtainedby allowingthe interchain anhydride to react with severalhomologous z-alkylamines (n-C nH2narNHz, ft : O, 1, 4, 6,11, and 18) (Figure 6). These mixed SAMs comprisemixtures of amides and carboxylic acids, with the polarcarboxylic acid groups buried beneath hydrocarbon layersof different thickness.

The values of 0u for the z-undecyl- and n-octadecyl-amine-modified SAMs did not change with pH. Thecarboxylic acid groups of these two amine-modi^fiedsystems were buried beneath ca. I0 and ca. 15 A ofhydrocarbon layers, respectively; these saturated hydro-carbon films thus prevented their direct contact withwater. The values of 0^ for the n-undecyl- and z-octadecylamine modified SAMs are slightly smaller thanthat of a SAM of hexadecanethiolates on gold; the smalldiscrepancy may reflect the disorder of the hydrocarbonlayers of these modified SAMs.

The values of 9. change with pH for mixed SAMscomprising mixtures of carboxylic acids and z-hexyl- andn -butylamides. There are two significant features ofthesedata: First, the titration curves do not reach a plateau athigh pH; the same behavior is observed for the mixedSAMs of carboxylic acid- and methyl-terminated al-kanethiolates on gold prepared from mixtures of HS-

(67) Hoimes-Farley, S. R.; Reamey, R. H.;McCarthy, T. J.; Deutch,J.; Whitesides, G. M. Langmuir 1985, 1,725-740.

(68) Holmes-Farley, S. R.; Whitesides, G. M. Larrymuir 1988, 3 ,62-7 6 .

(69) Holmes-Farley, S. R.; Bain, C.; Whitesides, G. M. Langmuir1988,4,927-937.

(70) Wilson, M. D.; Ferguson, G. S.; Whitesides, G. M. J. Am. Ch.em.Soc. 1990, 112, 1244-1245.

(71) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M.Langmuir lgg4, 10, 7 4I-7 49.

(72) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111,7164-7L75.

(CH2)1oCOOH and HS(CH2)roCHe.7t By contrast, thecarboxylic-acid functionalized material obtained by oxi-dizing polyethylene (PE-CO2H) does achieve plateauvalues at high pH.Gz Second, the onset points of ionizationare approximately pH 10 for the n-hexylamine-modifiedSAM and approximately pH 8 for the n-butylamine-modifred SAM. The values of gB also change with pH formixed SAMs comprising mixtures of carboxylic acids andmethylamide and unsubstituted amides. These titrationcuryes are very similar to that of the SAM of n-butylamidesand carboxylic acids. Both the onset points of ionizationare approximately at pH 7. Such shifts from the expectedvalues on the basis of titration in aqueous solution arealso similar to those observed wrth mixed SAMs termi-nated with carboxylic acid and methyl groups.?3

Contact angle titrations of these n-alkylamine-modifiedSAMs suggest that these systems are more like mixedSAMs of methyl- and carboxylic acid-termrnated al-kanethiolates on gold than they are like PE-CO2H.

Kinetics of Formation of the Interchain Anhy-dride and Its Reaction with n-Undecylamine. Weestimated the rate offormation ofthe interchain anhydrideand its reaction with alkylamine, in order to make thissystem useful in surface modification.

Since the anhydride surface was hydrophobic (0u:75")and the carboxylic acid surface was hydrophilic (0" < 15"),we estimated the formation of the interchain anhydrideby measuring the advancing contact angles of water onthe anhydride surfaces as a function of time of reaction.No observable hydrolysis ofthe anhydride occu:red duringthe measurement (that is, over 1-2 rnin). Since no F(ls)was identified by )(PS during the course of the reaction,and since the reaction conditions were anhydrous and theformation of the interchain anhydride on the surface wasirreversible, we infer that the only two moieties on thesurface are the interchain anhydride and the unreactedcarboxylic acid and therefore that the hydrophobicity ofthe anhydride surfaces was mainly due to the generationof the interchain anhydride. Figure 7A shows that theformation ofthe surface anhydride is very rapid: Withinthe initial 30 s ofthe reaction, the advancing contact angle

A.

Interchain Carboxylic Anhydride Groups on SAMs Langmuir, Vol. 13, No. 25, 1997 67LI

interchain anhydrides is rapid and complete;all the acidsare transformed into the interchain anhydrides and thereare no mixed trifluoroacetic carboxylic anhydride groupsdetectable by XPS. Subsequent reaction ofthe interchainanhydride and alkylamine proceeds rapidly and cleanlyto generate a mixture ofacids and amides in approximately1:1 ratio. The PIERS studies show that the underlyingpolymethylene groups of the SAMs of the interchainanhydride remain well-ordered; this order seems to bepreserved in the mixed SAMs, although the alkyl groupsof the amides are disordered.

This method provides a useful new synthetic route toprepare mixed SAMs, especially those comprising 1:1mixture of acids and amides. It provides an easier andquicker route than de nouo synthesis of the alkanethiolsthat contain amide groups. One important feature of themixed SAMs generated by this method is that they areunlikely to contain large-scale phase separated domains.Such domains may complicate the interpretation of datain many types of experiments.

We believe that this method is the basis for a simpleand versatile procedure for constrrrction ofcomplex surfacestructures. Other functional groups can also be readilyincorporated into SAMs using this route. For example,we have allowed the interchain anhydride to react withcysteamine (HSCHzCH2NH2) to generate a surface thatpresents thiol groups; this type of surface could not begenerated by reaetion of the thiol (HSCH2CH2NHCO-(CH2)rbSH) with gold.Ta It is likely that other functionalgroups that react rapidly with anhydride (e.g., alcohols,enamines, Grignard reagents, hydrazines) can be used tointroduce a variety of functionalities into the interface.T5This method also provides a simple route for graftingpolymers to the surface. Finally, we also believe that thismethod can be readily applied to modification of SAMsand construction of two-dimensional patterns or three-dimensional structures on other metallic or semiconductorsubstrates. We are currently investigating applicationsof this new method.

Experimental Section

General Materials. 5(100) wafers were obtained fromSilicon Sense Inc., Nashua, NH. Absolute ethanol (PharmcoProducts, Inc.), anhydrous N,"lf-dimethylformamide (HPLCgrade, Aldrich), anhydrous 1-methyl-2-pyrrolidinone and tri-fluoroacetic anhydride (Aldrich), hydrochloric acid, sodiumphosphate, citric acid, acetic acid, sodium acetate, boric acid,and sodium hydroxide (Fisher), and glycine ($igrna) were.usedas received. Triethylamine was dried over 3 A molecular sieve,and methylene chloride was distilled over calcium hydride. 16-Mercaptohexadecanoic acid and 1 l-hydroxylundecanethiol wereprepared according to the literature.al

Preparation and Characterization of SAlVIs. The goldsubstrates were prepared by e-beam evaporation of 5 nm oftitanium and 200 nm of gold onto a silicon wafer as previouslydescribed.T6 The gold-coated wafers were cut into co. I cm x 2cm pieces and washed with absolute ethanol before adsorptionof alkanethiol. SAMs were prepared by overnight exposure ofthe freshly prepared substrates to a2 mM ethanolic solution of16-mercaptohexadecanoic acid at room temperature in a clean20-rr;'L scintillation vial.al

PIERS spectra were obtained in single reflection mode usinga dry air purged Digilab Fourier transform infrared spectrometer

(74) XPS shows a photoelectron peak at 163.6 eV, and contact anglemeasurement shows an advancing contact angle of water of 60 oC. Seeref 22.

(75) Tleatment of the interchain anhydride with n-hexanol and4-dimethylaminopyridine gives a surface having an advancing contactangle of ca.85".

(76) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Zongmuir 1992,8. 1330-1341.

0.05

" t "@ l @' t ' ^d ld

I

;o

-0.05

3

T i m e l m i n )

o ^

5 3 c uo 6

s -e 4.0c oY E

F 6 J . U

1 . 0

0.00 1 0 2 0 3 0 4 0 5 0 6 0 7 0

Time (min)

Figure 7. (A) The surface of the SAMs of the interchainanhydride is more hydrophobic than that ofthe carboxylic acid.Formation of the interchain anhydride was followed by contactangles. (B) The reaction of the interchain anhydride andn-undecylamine in anhydrous 1-methyl-2-pyr:roiidinone formeda hydrocarbon layer of the amide at the surface of the originalSAM ofcarboxylic acid. The change ofthe thickness of the layerwas measured by ellipsometry. The inset is a piot of ratios ofthe thickness at time f and the final thickness during the initial30 min. The solid circle (O) indicates that the concentration ofn-undecylamine is 10 mM; the open circle (O) indicates that theconcentration is 1 mM.

of water had almost reached its frnal value, and thereaction was complete within 5 min. These resultsestablish that the formation of the interchain anhydridefor the SAM terminated in carboxylic acid group byreaction with trifluoroacetic anhydride is a fast reaction.

We used ellipsometry to estimate the rate ofthe reactionofn-undecylamine and the surface anhydride (Figure 7B).The formation of the amide is characterized by twodifferent phases: Within a few minutes the thicknesseshave reached about 70-80Vo of their maxima; completionof the reaction occurs slowly. We hypothesize that thisresult reflects the decreasing reactivity of the functionalgroups on the surface as the reaction approaches comple-tion. As the formation of amide groups on the surfaceproceeds toward completion, the unreacted anhydridegroups become more buried by the long alkyl chains ofthenewlyformed amides and, therefore, become less accessibleand less reactive.

Conclusions

This paperdemonstrates a simple, two-step method thatgenerates mixed SAMs of carboxylic acids and amidesfrom a well-ordered SAM of carboxylic acids on gold:dehydration of terminal carboxylic acids to form interchainanhydrides, and reaction ofthe interchain anhydrides withalkvlamines. Conversion of the carboxvlic acids into the

0 . 1 0

B.8.0

6.0 o1 . 0

o 0.8

, 0 .6- t a

- l !

0.4

w . a

0 . 00 5 1 0 1 5

O = 1 0 m MO = 1 m M

6712 Langmuir, Vol. 73, No. 25, 1997

(BioRad, Cambridge, MA).?? The p-polarized light was incidentat 80" relative to surface normal of the substrate. A narrowband mercury-cadmium - telluride (MCT) detector, cooled withIiquid nitrogen, was used to detect the reflected light. A spectrumof a SAM of z-hexadecanethiolate-dse on gold was taken asreference. Typically, 1024 scans were averaged to yield spectrawith excellent signal-to-noise ratios at a resolution of 2 cm-l.

XPS spectra were collected on an SSX-100 spectrometer(Surface ,Science Instrument) using monochromatic Al Ka X-rays(,1 : 8.3 A). The spectra were referenced to Au(4hd at 84.00 eV.Survey spectra were recorded with a 150-eV pass energy, l-mmspot size with an acquisition time of 3 min. High-resolution corelevel spectra were recorded with a 50-eV pass energy, 300-mmspot size with an acquisition time of 30 min. All spectra werefitted by using 907o Gaussian/IlVa Lorentzian function in thecomputer system of the spectrometer.

Ellipsometric measurements were made on a RudolfResearchType 43603-200E ellipsometer using a He-Ne laser (,1 : 632.8nm) at an incident angle of 70.0' relative to the surface normalof the substrate. At least three separate spots were measuredon each of three individually prepared substrates, and thereadings were then averaged. The thickness of the monolayerwas calculated usinga program written byWasserrnan accordingto the algorithm of McCrackin et al.6a and a refractive index of1.45 for caiculation.

Advancing contact angles, d., were determined on a Ram6-Hart Model 100 goniometer under ambient iaboratory conditionson static drops that were applied using an Electra-Pipettedispenser (Matrix Technologies, Lowell, MA).52 Contact angleswere measured on both sides of the static drop. Data presentedin this paper were averages of the measurements of at leastthree spots on each of three individually prepared samples. Theconcentrations of all the buffers used were 0.1M. The followingbuffers were used for the titration: UCI (pH : 1), glycine (2,l0),sodium phosphate and citric acid (3, 4,6,7), acetic acid (5), boricacid (8, 9), sodium phosphate (11, 12), and sodium hydroxide(13) .

Preparation of the Interchain Anhydride. In a 20-mLscintillation vial, a 10-mL solution of 0.1 M trifluoroaceticanhydride and 0.2 M triethylamine in anhydrous N*lf-dimeth-ylformamide was prepared. Precleaned substrates of the SAMof the carboxylic acid were immersed in this freshly preparedsolution?s without stirring for 20 min at room temperature,removed from the solution, rinsed thoroughly with CH2CI2, and

(77) Laibinis, P. E.; Bain, C. D.;Nuzzo, R. G.;Whitesides, G. M. J.Phys. Chem. 1995, 99,7663-7676.

Yan et al.

dried in a stream of nitrogen. The substrates of the interchainanhydride were used immediately.

Reactions of the Interchain Anhydride with Alkyl'amines. A 10-mL solution of 10 mM alkylamine in anhydrous1-methyl-2-pyrrolidinone was prepared in a 20-mL scintillationvial (the solution of z-octadecylamine was above its saturationconcentration at 10 mM, and used as a suspension ). Precleanedsubstrates of the freshly prepared interchain anhydnde wereimmersed in this solution without stirring for 30 min at roomtemperature, removed from the solution, rinsed wrth ethanol,and dried with a stream of nitrogen.

Kinetic Measurement of Formation of the InterchainAnhydride and Its Reaction with Alkylamines. Formationof the interchain anhydride was monitored b1'advancing contactangles of distilled, deionized water. Substrates of the SAMs ofthe carboxylic acid were immersed in a freshll' prepared solutionof trifluoroacetic anhydride and triethylamine in anhydrous l/,lV-dimethylformamide at room temperature. They were removedfrom the reaction mixtures after different periods of time andquicklv nnsed with CHzClz. The contact angles of water weremeasured immediately by the procedure described above.

Reaction of n-undecyiamine and the interchain anhydride wasmonitored by ellipsometry. The thicknesses of the SAM of thecarboxylic acid of individual substrates were measured prior tothe reaction. The substrates were then immersed in a freshlyprepared solution of trifluoroacetic anhydride and triethylaminein anhydrous Nf-dimethylformamide at room temperature.After 20 min. the substrates were withdrawn from the mixture,quickly rinsed with CH2CI2, and immersed in a 10-mM solutionof n -undecyl amine in anhydrou s 1 -methyl -2-pyrrolidinone. Thesubstrates were removed from the amine solution after differentperiods of time, rinsed with absolute ethanol, and blown dry ina stream of nitrogen. Ellipsometric measurements were takenimmediately according to the procedure described above.

Acknowledgpent. This work was supported by theOf;fice of Naval Research and the Defense AdvancedResearch Projects Agency. We thank Professor PaulLaibinis for generously allowing us to use the FTIRinstrument in his laboratories at MIT. We thank YuanZ.Lu for useful discussions concerning XPS measurements.

r,A970762G

(78) The reaction solution has to be prepared freshly because tertiaryamines can be oxidized by trifluoroacetic anhydride: Schreiber, S. L.Tetrahedron Lett. L980. 2 1. IO27 -1030.