Self-Assembled Monolayers of Long-Chain Hydroxamic Acids on the Native Oxides of Metalsl John P

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Self-Assembled Monolayers of Long-Chain Hydroxamic Acids on the Native Oxides of Metalsl John P. Folkers, Christopher B. Gorman, Paul E. Laibinis, Stefan Buchholz, and GeorgeM. Whitesides*

Department of Chemistry, Haruard Uniuersity, Cambridge, Massachusetts02138 Ralph G. Nuzzo*

Departments of Materials Science and Engineering and Chemistry, Uniuersity of lllinois, Urbana-Champaign, (Irbana, Illinois 61801

Receiued September 79, 1994e

Long-chain alkanehydroxamic acids adsorb on the native oxides of metals and formed oriented self- assembled monolayers (SAMs). This study examined SAMs of hydroxamic acids on the native oxides of copper, silver, titanium, aluminum, zirconium, and iron. These SAMs were characterized usingwettability, X-ray photoelectron spectroscopy (XPS), and polarized infrared external reflectance spectroscopy (PIERS). Alkanehydroxamic acids give better monolayers than the corresponding alkanecarboxylic acids on certain basic metal oxides (especially copper(Il) oxide). On the native oxide of copper (which has an isoelectric point greater than the pK" of the hydroxamic acid), the ligand is bound to the surface predominantly as the hydroxamate. The strength of the interaction between copper oxide and the hydroxamate allows incorporation of polar tail groups into the monolayer. On acidic or neutral metal oxides (e.g.,TiO2), the predominant speciesbound to the surface is the hydroxamic acid. Alkanehvdroxamic acids on titanium dioxide bind relatively weakly but. nonetheless.form SAlts that are more stable than those from carboxylic acids (although not as stable as those from alkanephosphonicacids r.

Introduction thoroughly studied.l; SAMs of alkanethiolates on gold and silver are generally the most useful in scientific properties We have examined the of self-assembled applications,l8because of their stability and becausethe (SAMs) monolayers obtained by the adsorption of long- specificity of coordination of the sulfur atom to the metal (general chain alkanehydroxamic acids formula CHs- allows polar groups containing oxygen and nitrogen to be (CHz),CONHOH) onto the native oxidesof freshly evapor- incorporated into the SAM.2'|1're ated frlms of copper, silver, titanium, aluminum, zir- Alkanethiols do not adsorb, however, to the surfaces of conium, and iron and have comparedthese SAMs to others many metal oxides.20Carboxylic acids and phosphonic commonly used with these substrates. We have used acidsadsorb, but the adsorptioncan be weak.7-16These wettability, X-ray photoelectronspectroscopy (XPS ), and polarized infrared externai reflectance spectroscopy . ('hen. Srrr'.1980. 102.92*98. Netzer L.; Sagiv, (PIERS)in characterizingthesemonolayers. We conclude 1983.105. 671- 676. Moaz,R.:Sae-tv, J.J.Colloid that monolayers of alkanehydroxamic acids (or their t00.165 496. R Tao.\'.-T.: Whitesrdes, G. M. Langmuir 1989, anions) are more stable than monolayersobtained by the S, : adsorption of carboxylic acids on all of the surfaces (especiallycopper oxide and silver oxide), but they are less stable than monolayers obtained by the adsorption of alkanethiols to copper or silver. Of the many systems available for the formation of self- assembledmonolayers, monolayers obtained by the chemi- sorption of alkanethiols onto gold and silver,2 amonolayers obtained by the hydrolysis and polymerization of alkyl D. Chem. Phys. Lett. 1990. 167. 198- trichlorosilanes onto hydrated surfaces,5'6and monolayers obtained by the adsorption of carboxylic acidsT-ls or phosphonicacidslG onto metal oxideshave been the most

8 Abstract published in Aduance ACS Abstrocls, January 15, 1995. (1) This research was supported in part by the Office of Naval Research and the Advanced Research Projects Agency (ARPA). XPS spectra were obtained using instrument facilities purchased under the ARPAruRI program and maintained by the Harvard University Materials Research Laboratory. S.B. acknowledges a grant from the Studienstiftung desdeutschen Volkes (BASF-Forschungsstipendium). R.G.N. acknowledgessupport from the National ScienceFoundation (cHE-930095). (2) Nuzzo,R. G.; Allara, D. L. J. Am. Chem.Soc. 1983,105,4481- 4483. Porter, M. D.; Bright, T. B.;Allara, D. L.; Chidsey,C. E. D. J. Am. Chem.Soc. 1987, 109, 3559-3568. Bain, C. D.;Troughton,E. B.; Tao, Y.-T.; Evall, J.; Whitesides,G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989,111,321-335. (3) Walczak,M.M.;Chung, C.;Stole,S. M.;Widrig, C.A.;Porter, M. D. J. Am. Chem. Soc. 1991. 113.2370-2378. (4) Laibinis, P. E.; Whitesides,G. M.; Allara, D. L. ; Tao,Y.-T. ; Parikh, A.N.; Nuzzo, R. G. J. Am. Chem.Soc. 1991, 113,7152-7767.

07 43-74631951241I-0813$09.00/0@ 1995American Chemical Society 814 Langmuir, Vol. 11,No. 3, 1995 Folkers et al. systemsalso cannot be used to incorporate polar groups Scheme 1. Synthesis of Hydroxamic Acids into a monolayer.ll Phosphonic acids have large head groups and can be inconvenient to synthesize. (1) cH3(cH2)^coc,- ----_ cH'{cH2)^coN^.p We have examined hydroxamic acids as an alterna- "r*oP tive to carboxylicacids (3) for self-assembledmonolayers on the surfacesof metal oxidesbecause their complexation constantswith metal ions in aqueoussolution are several ooEDC * H2NoJ > Ho{CH2t,"CoNroJ ordersof magnitude larger than thoseofcarboxylic acids2l Ho{cH2}lscooH and becausethey have similar sizes. Synthesis of long- l-\ chain hydroxamic acids is also straightforward.22'23 Y H2,Pd R(CH2).CONHO--l ---+ R(CH2).CONHOH O O-'M R = CHs,OH tt - ' \ o F.r't # f-ot *Af.o ,Ao, ,Ad" HH was slmthesizedusing 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC) to couple the benzyl-protected 1234 hydroxylamine to 16-hydroxyhexadecanoicacid.23 The same deprotection schemewas used for this compound. When complexedto a single metal ion in solution, the Preparation ofSubstrates andMonolayers. MetaV greater stability of complexQswith hydroxamate relative metal oxide substrates were prepared by electron-beam to those with carboxylate reflects the larger bite size of evaporationof 600-1000 A of the pure metal onto silicon the hydroxamate,which forms five-memberchelate rings wafers that had been precoatedwith 50 A of titanium as with metal ions(2t. Incontrast, carboxyiateforms either an adhesionpromoter. The sampleswere exposedto air strained.four-member chelate rings (4t or monodentate upon removal from the chamber. because we were species.rlr; We had hopedthat this differencein binding interested in studf ing the adsorptionof monolayersto constantsin solutionwould alsobe reflected in the strength the native oxidesof the metals. In this paper,instead of of binding of hydroxamic acids to the ions on the surface writing "metaVmetal oxide" for each reference to a of a metal oxide. In fact, although hydroxamic acids do substrate with a native oxide, we will refer to them simply form more stable monolayersthan carboxylicacids, they as the metal oxide. The oxidation states of the metal in seem to use only one oxygen atom for coordination. the native oxides are given in Table 1 and the Experi- Results mental Section. Solutions of alkanehydroxamic acids and other adsor- Synthesis of Hydroxamic Acids. The long-chain, bates were prepared in either isooctaneor ethanol and methyl-terminated hydroxamic acids were synthesized were not degassed.The nominal concentrationin eachof by reacting the corresponding acid chloride and O- the solutionswas 1 mM, although most of the hydroxamic benzylhydroxylamine,followed by removal of the benzyl acids were not soluble in isooctane to this extent. group by catalytic hydrogenation over palladium on carbon Substrates were left in solution for 1 to 2 days at room (Scheme 1).22The hvdroxvl-terminated hvdroxamic acid temperature. After removal from solution,the samples were rinsed with heptaneand ethanol and then dried in r 18r Applicationsof thiols on goldand silver: Chidsey,C. E. D. Science of nitrogen. r\\'oshrngton.D.Cl.r 1991. 251.9I9 922. Hickman, J. J.; Ofer, D.; a stream Laibinis.P. E.:\\'hitesides. G. !I.:Wrighton, M. S.Science(Washington. Contact Angles on Monolayers of Hydroxamic D.('., 1991.252. 688 691. Prime, K. L.; Whitesides,G. M. Science Acids. Table 1 summarizes advancing and receding r l|'osht rtgtLtrt.D.C ., 1991.252. 1164- 1167. Joyce,S. A. ; Houston,J. E.; contact anglesof rvater and hexadecaneon monolayers )Iichalske.T. A. App/. Pht's.Lett. 1992,60,lI75-7177. Kumar, A.; Biebuvck.H. A.: Abbott. N. L.; Whitesides,G. M. J. Am. Chem. Soc. obtainedby the adsorptionof octadecanehydroxamicacid, 1992.11J. 9188 9189. Abbott,N. L.; Folkers,J. P.;Whitesides,G. M. stearicacid, octadecanephosphonic acid, and 16-hydroxy- SciencerV'ashington, D.C.) 1992,258,1380-1382.Ross, C. B.; Sun, hexadecanehydroxamicacid on the six substratessurveyed L6pez,G. L.; Biebuyck, L.: Clrooks.R. *-1.Langmuir 1993,9,632-636. We also presentcontact angles for mono- H. A. : Frisbie. C. D. ; Whitesides,G. M. Science( Washington,D.C.) 1993, in this study. 260.647 619. Lopez,G. L.; Albers, M. W.; Schreiber,S. L.; Carrol, R. layers of octadecanethiolateon copper.silver, and gold W.:Peralta.E.: Whitesides, G. M. J.Am.Chem. Soc.1993, 115,5877- for comparison,and contact anglesfor surfacesobtained 5878. Kumar. A.; Whitesides,G. M. Appl. Phys. Lett. 1993, 63, 2002 after attempting adsorptionsof the hydroxamic acid onto 2004. t 19t Nuzzo,R. G.; Zegarski,B. R.; Dubois,L. H. J. Am. Chem. Soc. gold. For consistency,all of the data in Table 1 were taken 1987. 109, 733-740. duringthis study, although severalof thesesystems have r20r The exceptionsbeing copper oxide, silver oxide, and possibly been studied previously.'21; other soft metal oxides such as those on platinum, zinc, and lead. For studies on the removal the native oxidesfrom copper and silver using The presenceof a closest-packed,methyl-terminated alkanethiols,see ref 4 and Laibinis, P. E.; Whitesides,G.M. J. Am. monolayeris often indicated by an advancing contactangle -- oQ;2ti2s Chem.Soc. 1992, 114.9022-9028. of water (surface tension Tyv 72 mJlm2 at 25 t21 i E.; R.M. Stability Constants;Plenum Martell, A. Smith, Critical above110o, and an advancingcontactangle ofhexadecane Press: New York, 1977;Vol. 3. For comparison,the binding constants 28 6'8'15 for acetohydroxamatewith Fe3* and Al3* in aqueoussolution are 2.6 (yr": 27 mJlm2)26 above45".2 Nthough contact x 101rand 8.9 x 107M r, respectively,and the binding constants for angles do not provide any structural information about 1, acetatewith Fe3*andAl3* are2.4 x 103and 3.2 x 10t M respectively. monolayers,they are useful as indicators of their quality. t22t Hearns,M. T. W.; Ward, A. D. Ausl. J. Chem.1969,22,16l* 173. Rowe,J. E.: Ward, A. D. Aust. J. Chem. 1968,21,2761- 2767. A low value of contact angle, relative to a structurally Fujii, T.; Hatanaka, Y. Tetrahedron 1973,29,3825-383L. Sun, Y.; well-characterizedsystem such as alkanethiolateson gold, Martell, A. E.; Motekaitis, R. J. Inorg. Chem. 1985,24, 4343-4350. indicates disorder (and/or lower coverage)in the mono- (23 ) Karunatne, V. ; Hoveyda, H. R.; Orvig, C. Tetrahedron Lett. L992, ,33.1827- 1830. (24) Cotton, F. A.; Wilkinson, G.Adua ncedInorganic Chemistry,Sth (26)Jasper,J. J.,J. Phys.Chem. Ref. Data 1972,1,841-1009. -362, Ed.; Wiley-Interscience: New York, 1988. (27) Cognard,J. J. Chim. Phys. 1987, 84, 357 and references {25) Entropy of formation (due to chelation) is the dominant factor therein. rn the strong binding between hydroxamic acids and discrete metal (28) Thesetwo probe liquids can provide different information about ions in aqueous solution. The enthalpies of formation for these the surfaces,because the origins oftheir surfacetensions are different. complexesin water tend to be small and positive. For examples, see The surfacetension of water is dominated by its polar component(;'Lr,.pornr ref 2 1 and Monzyk, B.; Crumbliss, A. L. J . Am. Chem. Soc. 1979, 101, = 50 mJ/m2),and the surfacetension of hexadecaneoriginates entirely 6203-62L3. from disnersive interactions.2T SAMs of Hydroxamic Acids Langrnuir, Vol. 17, No. 3, 1995 815

Table 1, Advancing and Recedidg Contact Angles of Water and Ileradecane (HD) on Monolayers of Octadecanehydroxamic Acid Stearic Acid, Octadecanephosphonic Acid, l6-Hydroryhexadecanehydroramic Acid, and Octadecanethiol on Copper, Silver, Titaniun, Zirconium, Iron, Aluninum, and Gold" contact angles on various SAMs (advancing,receding)/deg isooctaneb ethanolh

metal adsorbate Hzo HD HzO HD Cr-r/Cu(II) cHs(cHz)rocoNHoH 114.104 47.1r 11.1.101 49,4r CHr(CHz)r6COOH 113,107 49. .1,1 111. 82 46, 38 CHr(CHz)r;POsHz 115,98 43.35 112.76 29,0 CH;r(CHz)r;SH 117.93 62,28 ')7 HO(CHz)TsCONHOH 79,40 0.0 1') 22,0 Ag/Ag{II) CHs(CHzhsCONHOH 113,104 51,46 113.102 5r,44 CHe(CHz)raCOOH 113,107 52,47 81.61 21,0 CHs(CHz)rzPOsHz 115,103 46,2: 82.60 0,0 CHs(CHz)rrSH 113,102 49,43 HO(CHz)TsCONHOH 82, 50 0,0 68,39 0,0 Ti/Ti(IV) cHe(cHz)r6coNHoH 115,105 46.42 110,85 45,29 CH:(CHz)raCOOH 107,71 45,37 50,15 0,0 CH:r(CHz)rzPOgHz 115,100 42,36 110,95 41,36 HO(CHz)TsCONHOH 83,60 0,0 77,5r 0,0 A- ,t,1 AVAI(III) CHs(CHz)ToCONHOH 113,104 .1 ,. ar 111,93 46,40 CH:(CHz)raCOOH 113,105 18.11 110,82 43,35 CHr(CHz)r;POsHz 115,104 ,18.jlE 113,83 37,0 HO(CHz)TsCONHOH 49.29 o. i) 72,36 0,0 .); ZrlZriYl CH3(CH2)T6CONHOH 123,99 JJ 123,95 43.22 ')Q CH rtCH: )r oCOOH r22.100 r: 116.75 23,0 CH3tf11, r1 ,PO,IH: 123.100 :ll. lrr 119. 75 23.0 HO,CH:r;CONHOH ir().20 0.0 Fe,FerIIIt CH,r{CH:rre CONHOH 1j]5.95 l:i. 1; 1rl1. 90 31,16 CHsrf[1,rr.;COOH 1;16.S-1 li. tt 111.65 23.0 CH:trCH: ,1;POrtH: l3;.9; 21.r, 12-1.tr) 20.0 HOtCH2)T5CONHOH ;9. jl-l ().o Au CH3tQ11,)T6CONHOH 106.87 22.tr 8-1.63 0.0 CH;tCHz )r;SH 115,103 49. 39 t n Metals were exposedto air and coveredwitl native oxidesbefore exposure to solutions containing the ligands. Solutionsfrom which monolaverswete formed. " Denotes that monolaverswere not made from these solutions. 2. Decrease in Advancing and Receding Contaet layer; a value comparableto that of the ordered system Table Angles of Water (AOHro) and Hexadecane (AOHD) on does not necessarily indicate the same structure or a Changing Solvent from Isooctane to Ethanol for correspondingdegfee of order. Using contact angles,we Adsorptions of Octadecanehydroxamic Acid, Stearic can infer the quality of one type of SAM relative to others Acid, and Octadecanephosphonic Acid (adv'rec)a with analogous structures on the same metal, but not (CH3(CHz)uPOsHz iCH 3t('H211,;CON HOH (CH3(CH2h6COOH relative to SAMs on different metals, becausethe rough- tII) A{/H:o AOHI ness of the surfacesof the various metals differ.2e'3oTo metalh .$/ilr(' \Fi a0H2o aoHD reduce the variation in roughness of structures for a Cu .2. il -2.' 2 .2,25 3,6 3,22 14,' 35'i -2. '2 ' .2 >-47 33,43 >-46,>28 particular metal, all of the samplesfor each metal listed __h 2. 32.46 37, fi ). 20 . 2. 13 57.56 >-45.>_37 5, 5 <2, <2 from the same evaporation; the '2.1 in Table 1 were taken AI .2. 1I . 3.23 5,9 <2,21 11,>38 $ '2. only exceptions were the monolayer of HO(CHz)ro- Zr . 1 ' 5 6.25 24, 229 4,25 14,::20 CONHOH on silver oxide from ethanol and the two sets Table 1. AO is the difference of monolayers on iron oxide. We infer from Table 1 that " Contact angles rvere taken from in contact angie between those monolayers prepared from isooctane quality ofmethyl-terminated SAMs obtainedfrom each - the and those prepared from ethanol rAl/ : g(isooctane) 0(ethanol)). of the three acids formed from isooctane is greater than b Iron is not included because the substrates examined were not ( that of SAMs formed from ethanol, sincethey have higher prepared in the same evaporation. " <2" is used to indicate that advancingcontact angles and lower valuesofhysteresis.30 changes in their contact angles of 2o or less are within the Table 2 summarizes the decreasein contact angleson uncertainty of the measurements. d ">" indicates that the contact changing the solvent for adsorption from isooctane to angle of the monolayer formed with ethanol was zero, and the greater than that measured ethanol. We use thesevalues as qualitative indicators of increase in surface free energy could be by the decrease in the contact angle. the stability of the monolayersby assuming that ethanol competesfor polar sites on the surfacebut that isooctane does not. In general, differences in the contact angles were lowest for monolayersofthe hydroxamic acid. These (29) The substrates of zirconium/zirconium oxide and iron/iron oxide, as we had prepared them, were much rougher than the other metals: data thus suggestthat monolayers from hydroxamic acids advancing contact angles and values ofhysteresis in the contact angles are more stable than monolayers of either carboxylic acids of water were consistently higher for monolayers on these substrates.30 or phosphonic acids on all of the substrates examined We have not investigated these substrates with the scanning probe in contact microscope,and we are not able to quantitate their relative roughness. except titanium oxide. The smallest changes (30) Hysteresis in contact angles,although poorly understood, is often angles were observedfor monolayers of hydroxamic acids attributed to the chemical heterogeneity or the roughnessofan interface. on copperoxide and silver oxide,suggesting a high stabiiity Since we have attempted to control the relative roughnessesof our for the hydroxamic acid overlayer formed on lhese substratesfor a given metal, we interpret the increasein hysteresisas an increase in the chemical heterogeneity of the surfaces. For examples substrates. By this criterion, of the three acids, the ofexperimental and theoretical studies on hysteresis,see: Joanny, J. carboxylic acid forms the least stable monolayers on all F.; de Gennes,P. G. J. Chem.Phys.1984,81,552-562. de Gennes,P. phosphonic are often - of the substrates. Although acids G. Reu. Mod. Phys. 1985,57, 827 863,and referencestherein. Schwartz, monolayers on L. W.; Garoff, S. Langmuir, 1985, 1.219-230. Chen, Y. L.; Helm, C. used for the formation of self-assembled A.; Israelachvili, J. N. J. Phy.s.Chem. 199L, 95, 10736-1'0747. zirconium oxide, the results in Tables 1 and 2 suggest 8i6 Langmuir,Vol. 11,No. 3, 1995 Folkers et al.

that hydroxamic acids form monolayers of even higher the formation of a well-defined hydrophilic structure. The stability on this substrate: this inference remains to be advancingcontact angle of hexadecanewas nonzero;this testedby direct competition. Fortitanium oxide,we infer value is consistent with the formation of a hydroxyl- that the phosphonicacid forms the most stablemonolayers. terminated monolayer,on which a thin, adsorbedlayer of Hvdroxamic acids do not form closest-packedmono- water prevents the spreading of the contacting hydro- lavers on gold, although the measured contact angles carbonliquid.:r6 The measuredthickness ofthis monolayer indicate some partial layer is adsorbedfrom isooctane. was intermediate between that of films formed by The contact angles were only slightly lower than that CHg(CHz)IaCONHOHand CHs(CHz)ToCONHOH(see Fig- expectedfor a methyl-terminated monolayer,but results ure 3 and the accompanyingtext below),suggesting that from X-ray photoelectronspectroscopy (XPS) showed that the adsorption proceedsto high mass coveragesin an the "monolayer" was approximately 11 A thinner than a isostructural manner. Solvent plays a very important monolayerofoctadecanethiolate on gold.3l The adsorption role in this assembly,however. We found, for example, of hydroxamic acids to gold presumably occursby phys- that the hydroxyl-terminated hydroxamic acid forms an isorption on the high-energy gold surface,32as gold does incompletemonolayer on copperoxide when adsorbedfrom not form a stable oxide under ambient conditions; we have isooctane(the coveragewas less than 70Vc). not determined the structure of this layer. Contact Angles on Monolayers of Hydroxamic Adsorption of alkanethiols on coppermodifies its surface Acids as a Function of Chain Length. Figure 1 more than doesthe adsorption of any ofthe acid adsorbates presentscontact angles ofwater and hexadecaneon SAMs used in this study, as judged by the larger values of the of hydroxamic acids with the general formula CHr- contactangle hysteresis measured. Sincethe compositions (CHz),CONHOH on the native oxidesof copper,silver, of the interfacespresent in the two monolayerclasses are titanium, and aluminum as a function of n (the number similar, we attribute the differences in hysteresis to of methylenegroups ). Monolal,'erson copperoxide and variations in the microscopicroughness ofthe interface.;r0 titanium oxide were formed br- adsorption from both This roughening is presumably a direct consequenceof isooctaneand ethanol:monolavers on silver oxide and the removal of the native oxide on copper that occurs aluminum oxrdewere formed onlv using isooctaneas a during the adsorption of the alkanethiol, as has been solvent. shownbyXPS.{ 2( The native oxideis not removedduring The contact angles measured on copper oxide were the adsorption of the three acid adsorbates.33 The independent of n and ofthe solvent usedfor the adsorption. adsorption of alkanethiols on the native oxide of silver Theseresults suggestthat monolayersofhydroxamic acids doesnot roughen the surfacein the sameway, eventhough on copperoxide have high stability. The trend in the data the thin native silver oxide layer is lost during the was similar for the monolayerson silver oxide prepared adsorption.4'34 by adsorption from isooctane. On titanium oxide and The hydroxyl-terminated hydroxamic acid formed in- aluminum oxide,little variation was observedin the chain complete monolayers on most of the substrates studied length dependenceof the contactangles for SAMs formed > here, with the single exceptionof the monolayer formed on by adsorptionfrom isooctanefor n 10; below this chain copperoxide by adsorption from ethanol. We expectedto length, the contact angles decreasedand the hysteresis measure an advancing contact angle of water below 30" increasedmarkedly. Binding of alkanehydroxamicacids for a complete monolayer of a hydroxy-terminated to the surfaces of aluminum oxide and iron oxide thus ligan6''r*'r5 the observedvalue is gur 27' (Table 1). Most appearsto be u'eak. asjudged by the ability of ethanol (or substratesgave advancing contact angles ofwater between impurities suchas rvater)tointerfere with the formation 50" and 80o,suggesting the existenceof nonpolar groups of high coveragemonolavers. at the interface and possiblysome "looping" ofthe hydroxyl The contactangles for the monolayerwith n : 15 on group to the surface. Similar loop structures have been silver oxidewere significantlr'lowerthan thoseof the two observedby Allara et al. with a long-chain cr,a;-dicarboxylic even-carbonchain lengths adjacentto it. The observed acid adsorbateon several oxide surfaces.ll X-ray photo- decreaseis not due to an incompletell'formedmonolayer electron spectroscopyshowed that the mass coveragesof (as confirmedby XPSr. but seemsto be due to a change the hydroxyl-terminated hydroxamic acid monolayers in the structure of the methl'l gloups at the interface (see were 6- 10A lessthan thoseof monolayersof CH:(CHrlru- below). This result is similar to the "odd-even" effects CONHOH (seebelow). in contact angles which have been observedpreviously When a monolayer of the hydroxyl-terminated hydrox- for monolayersof carboxt'lic acidslt and alkanethiolates amic acid was formed on copperoxide by adsorptionfrom on silver.3 We note, however.that the structural origins ethanol, the advancing contact angle of water indicated in the other examples may be very different from those which peftain here.

t31 r A "thinner" or "shorter" monolayeras determinedby XPS implies Formation of Mixed Monolayrs on Copper Oxide. incomplete formation of the monolayer. XPS is used to measure the We formed mixed monolayers by the competitive adsorp- attenuation of a signal from a substrate as a function of the amount of tion of HO(CHz)rsCONHOH and CHs(CH2)rICONHOH on h.l'drocarbonon that substrate; the thickness of the monolayer is then copper oxide (Figure 2). These data demonstrate that determined after making several assumptions about this piocess(see eqs1-3 t. these mixed monolayerscan be used to tailor the wetta- t32 t Weak physisorptionof amphiphilic moleculessuch as dodecanol bility (and, by extrapolation, other properties) of these from solution onto gold has been previously observed;see: Yeo, Y. H.; surfacesand illustrate the utility of hydroxamic acids in NlcGonigal,G. C.; Yackoboski,K.; Guo, C. X.; Thomson, D. J. J. Phys. Chent.f992, 96, 6110 6111. forming complexorganic surfaces on copperoxide without t33 t The hydroxamic acids probably to dissolve some copper lons significantly roughening the surface. These materials from the oxide: when SAMs of hydroxamic acids were formed on silver thus may have a significant advantagefor the modification oxide from ethanolic solutionsthat had beenused previously for copper ofthe surfacesover the previously describedalkanethiolate oxide,copper was evident in the XPS, but sampleson silver made from fresh solutions showed no copper (Folkers, J. P.; Whitesides, G. M. SAMs. Unpubiishedresults). Relative Thicknesses of Monolayers of Hydrox- r34t Laibinis, P. E.; Whitesides,G. M. J. Am. Chem. Soc.lgg2. 114. 1990 1995. amic Acids from X-ray Photoelectron Spectroscopy t35t Bain, C. D.; Whitesides,G. M. J. Am. Chem. Soc. 1988. 110. 6560 6561. Bain, C. D.; Evall,J.; Whitesides,G. M. J.Am. Chem. Soc. (36) Adamson, A. Physical Chemistry of' Surfa<,t:s,4th ed.: Wilev: 1989.111.7155-7\64. New York, 1982; Chapter 3. SAMs of Hydroxamic Acids Langmuir,Vol. 11,No. 3, 1995 817

lsooctane:.6f,o o e:t,o r elo DelD Ethanol: .elt'o oel,o *0lo "el'o

o;- -t- -c - -O o. C-

0.2 -B- 4- s37 532 527 f € 0 $- BindingEnergy in ev

-cos 0 -0.2 120

-F- - r. s- S- -E- { - - +- € + = 30 0

120 _o_ _. _ ._ _ 30 --- -J - 't,: :_ cr F \- 0 -OH 0.2 0.4 0'6 0.8 CHg lAslagol X.rr,.o" Figure 2. Contact angles of water on monolayers formed by -I- the competitrve:rdsorptionof CH:(CHz)r+CONHOHand HO- l- -l - -l -- t_- L-F tCHzTT.,CONHOH tinto plotted E I- copper, againstthe composition n 30 of the SAll presented as the mole fraction of the methyl- 0 terminate-dcomponent rn the SAII t/( It s.\rrt. The composition of the SA\l ri'rrsdett'rmined br.XPS using the oxygen1s signal 120 for the tet'ntinul hr-clr',rxrlgr'oup nn HOTCHTI;CONHOH. o a aot \lonolavers \\'r'l'r,tirrnrecl irt room temperature from ethanolic a solutronsu'rth ii totul concentrationof hydroxamicacid of 1 o - oog m}l frir 5 duvs. Adr-ancingcontact angles are shown as filled ^o crrcles:recedrng contact angles are shownas opencircles. Lines through the data are provided as guides to the eye. The error o m,o;l bars representthe approximateaccuracy of acquiringand fitting o the XPS data. The error in the measurement of the contact angles is smaller than the size of the circles. Above the plot, TIN I XPS spectra are presented for the oxygen ls region of mono- T NDE ! 30 layersof HO(CHz ) l5CONHOH ( left ) and CHs(CHz )14CONHOH 0 (right) on copper. The peak at lower binding energy (-530 eV) is due to the oxide on the copper and the oxygens in the 120 headgroupof the molecules. The peak at higher binding energy aoagoa (-532.5 eVt is due to the terminal OH. 0.2 ooooQ a substrate covered with an alkyl-chain-terminated SAM -cos 0 -0.2 lAyArF;l is given b1' eq 1

-0.6 1r,\\r: 1,/ exp(-mdll sin 0) (1) !:! 30 -1.0 0 where 1,, is the intensity of the signal without the 8 10 12 14 16 monolayer.l/ scalesthe intensity for attenuation through n in CH3(CH2)nCONHOH the head group of the ligand, m is the number of carbon atoms in the aikyl chain (.m: n * l), d is the thickness Figure l. Advancing (6,,)and receding (9,) contact angles of for a methylene group, z, is the attenuation length (or water and hexadecane(HD) on monolayers derived from the escapedepth) of the photoelectron through the hydro- adsorption of hydroxamic acids of the general formula CH3- carbon layer, and I is the angle betweenthe plane of the tCH2I,CONHOH. where h:8,L0,12,14, 15,16, onto the native 10 oxidesof copper.silver, titanium, and aluminum. Monolayers surfaceand the detector(35o).3? To obtain the value of on copperand titanium were formed from isooctaneand ethanol; d for a set of monolayers,we comparethe intensity of the monolayers on silver and aluminum were formed from isooctane. XPS signal from a SAM to that from the shortest chain As a guide to the eye, the dashed lines in the plot for copper length SAIV[3e''11 representthe averagevalues ofthe contactangles for all values of z and both solvents: 118" for 0^Hro , 102' for O,Hro48' for , t37t 6oHD,and 35" for g.HD.For the monolayerson silver, the contact Bain,C. D.:Whitesides,G. M. J. Phys.Chem.1989,93, 1670- 1673. angles for n:15 were not used to calculatethe averagecontact (38tLaibinis, P. E.; Bain, C. D.; Whitesides.G. M. J. Phys.Chem. angles representedby the dashed lines; these values are ll2" guH:o, 1991..95, 70r 7 702r. for 104'for 0,Hro,51'for O'HD,and 46" for 0,HD. (39)Laibinis, P. E.; Fox, M. A.; Folkers,J. P.; Whitesides,G. M. Langmuir 1991. 47. 3167-3173. Folkers, J. P.; Laibinis, P. E.: Whitesides,G. M. Langmuir 1992,8, f330-1341. (40) (XPS). We have used the attenuation of an XPS signal Seah, M. P. In Practical Surface Analysis by Auger and X-ray PhotoelectronSpectroscopy; Briggs, D., Seah, M. P., Eds.;John due to various (specific Wiley: substrate core levels details are Chichester,1983; Chapter 5. given in the Experimental Section) to estimate relative r4l rCalculationof absolutethicknesses using this methodrequires thicknesses of the SAMs formed by the adsorption of determination of10and /. The primary reasonwe have not determined these quantities is that cleaning metaVmetal oxide hydroxamic acids with 3e substrates by different chain lengths.a'34.37 In sputtering the surfaces with Ar* ions in the XPS chamber often removes this analysis, the intensity, Isana,of an XPS signal from part of the oxide and changesthe measured value of1s. 818 Langmuir,Vol. 11,No. 3, 1995 Folkers et al. - (m - m36)d -A sin 0ln(I-l/*"n) Q)

In eq 2, nt36is the number of carbon atoms in the chain Thickness from XPS 6 of the shortest molecule. We use an empirical equation relative to to estimate the attenuation lengths (A) ofthe photoejected shortest 4 acid (A) electronsthrough the hydrocarbon layer (eq 3) 2 A : 0.0228k + 9.0 (3) -2 where Ep is the kinetic energy (eV) of the photoelectrons; 12 the validity of this equation was establishedfor SAMs of Ag/AgO 10 alkanethiolates on copper, silver, and gold.38 o lsooctane 8 Resultsfor monolayersofhydroxamic acidson the native Thickness d = 1.2A/CH2 from XPS A oxidesof copper,silver, and titanium are summarized in relalive to Figure 3. On copperoxide, a small decreasein thickness shortest 4 acid (A) was observedfor monolayersformed by adsorption from 2 ethanol on to copperoxide relative to those formed using isooctane. Both sets of data yield the same value for the average increasein thickness per methylene group, 1.1 A/CH2. The value expected for a trans-extended alkyl chain oriented perpendicularto the surfaceis 1.36 NCH2.

From this value, we estimate that the averagetilt of the Thickness chainsin a monolayerof hydroxamicacids on copperoxide from XPS relative to is approximately 20o, in agreement with results from shortest infrared spectroscopy(see below). acid (A) On monolayers of hydroxamic acids on silver oxide formedby adsorptionfrom isooctane,we calculatea value of d of 1.2 AJCHzfrom the data. This value suggestsa 10 1'2 tc oH cant angle for the alkyl chain of about 18" from the surface n in HONHOC(CH2)nCH3 normal; the uncertainty in this value is large (*15" or formed from +0.1 A/CH2for d),however, because ofthe limited number Figure 3. Relative thickness of monolayers adsorption of hydroxamic acids of the general formula points of measured. CHr(CHz),CONHOHand HOTCH:)r;CONHOH onto the native On titanium oxide, we observed differences in the oxides of copper.silver. and titanium. All thicknesseswere thicknesses of monolayers formed by adsorption from determinedrelatiye to the monolayerof CH,r(CH:)gCONHOH isooctaneor comparedto those formed using ethanol as from isooctaneusing the attentuation of the XPS signalsfrom (O) the solvent;32the data suggest lower coveragesfor the the substrAtes{se(' t hetext {brdetails i. Filledr O t andopen nrethr'l-tet-minatedmonolavers formed from SAMs prepared using ethanol. These data are poorly fit circlesrepresent isooctaneand ethanol.respectivell'. The thicknessesof mono- by our analytical model and, thus, our ability to estimate layersobtained b1'the acisorption of HOtCHztr;CONHOHfrom a value of d is limited. For the isooctanesamples, the isooctane(O ) and from ethanol tC t are included at the position data are linear (two separatesets of data for isooctaneare of 15 methylene units in the alkyl chain. Lines through the included in Figure 3t, but our analysis leads to an data were determined by least-squares fits to the data and implausible estimated averageincrease in thickness per were determined without the data for the hydroxyl-terminated methylene group of 1.4 NCH2. Several experimental monolayers. issues.especially the roughnessof substrate, can lead to a large error in the calculated value of the attenuation and a second at -398.9 eV.aa The relative intensities of length. Variations in the thickness of the native oxide these peaks varied with the substrate used (as shown in among sampies also are believed to contribute to this Figure 4).aa From these data, we conciude that on copper discrepancy. Similar systematic errors for SAMs on oxide and silver oxide the predominant species bound to aluminum, iron, and zirconium oxides precluded mea- the surface is the monodeprotonated hydroxamate, while surement of values of d on these substrates. on aluminum oxide and titanium oxide, the predominant Deprotonation of the Hydroxamic Acids on the species bound to the surface is the hydroxamic acid. Various Oxides from )(PS. In the XPS data for Data from the C 1s region also suggest varying extents monolayersofhydroxamic acids,the N ls corelevel spectra of deprotonation in the different monolayers (Figure 5).45 prouded the most information on the nature of bonding The peak due to the carbonyl occurs at 287.6 eV for the that occursbetween the hydroxamic acid and the various solid hydroxarnic acid. In the monolayer, this peak is surfaces. Figure 4 presents N 1s spectra for octadecane shifted to287 .0 and 286.5 eV on titanium oxide and copper hydroxamic acid as a solid and for monolayersadsorbed onto the native oxides of copper, silver, aluminum, and (43) Blements in monolayers on thick layers of oxide, such a-sthat titanium. We calibrated the binding energies of these on aluminum, have higher binding energ'lesthan those on thin layers spectra by frxing the C 1s binding enerry of the main of oxide due to the inhomogeneouscharging that occurson the thicker oxide substrate. To correct for these dif'ferences.u'e have referenced Gaussian component (due to the CH2 groups) at 284.6 the binding energiesof elementsin the monolayerrelati ve to C 1s binding eV.12'13The N 1s spectrum of the solid hydroxamic acid energy of the hydrocarbon at 284.6 eV.a: could be frt with a single peak centered at 400.7eV. The G4)The measured binding energiesof the peaks in the N ls region for the monolayer on silver are sigr-ri{icantlvltiu'er than those for the spectra of the monolayers showed multiple components other substrates(400.1 and 398.6eVr Thesepeaks are superimposed in the N 1s region: one centered at -400.5 eV (in on the high-energybackground following the Ag 3d peaks,which might agreement with the peak for the solid hydroxamic acid) complicateaccurate determination of the binding energies. The noise in this spectrum limited our abilitv to determine an accurate ratio of acid to monoanion, although the monoanion is clearly favored. (42iMuilenberg, G. 8., Ed., Handbook of X-ray PhotoeLectron (45) We used monola-n-ersof CHrrrCH:)ICONHOH to improve our Spectroscopy;Perkin Elmer Corp.: Eden Prairie, Mn, 1979. Wagner, sensitinty by decreasingthe attenuation of the signal due tclthe carbonyl C. D.: Gale. L. H.; Raymond. R. H. Anal. Chem. 1979, 51, 466-482. by the alkyl chain. SAMs of Hydroxamic Acids Longmuir, Vol. i1, No. 3, 1995 819

Metal Ratio I I HONHOC(CHz)sCHg Cu il I 1:8

Ag o ll/llU2 + tJ 1.1:1.7 HONHOC(CHz)eCHg o .i

G j L 6 tE o o o 5 o o

(cH2)r5cH

2s1 "r:".,::tr";; l' " Figure 5. X-ravphritoeiectrc-rn spectra for the carbon1s region 405 403 401 399 397 395 of monolaversderived from the adsorptionof CH,r(CH2)s- CONHOHonto copper oxide and titanium oxide, and of solid BindingEnergy ineV CHr(CHz)TcCONHOH.Binding energiesof the peaks are Figure 4. X-rayphotoelectron spectra forthe nitrogen1s region referencedto the carbonls peakfor the alkyl chain at 284.6 ofsolidCHe(CHz)TaCONHOH (bottom) and monolayers obtained eV. Thedashed line throughthe spectraallows comparison of by the adsorptionofit ontothe nativeoxides ofcopper, silver, the bindingenergies of the peaksdue to the carbonyl.Relative aluminum,and titanium. Binding energiesof the peaksare intensitiescould not be compareddirectly as they were not referencedto the carbon1s peakfor the alkyl chainat 284.6 referencedto a standard. eV. Relativeintensities could not becompared directly as they werenot referenced to a standard."Ratio" refers to the relative in that we have not performed specific calculations for intensitiesof the peaks ascribed to -CONHOHand -CONHO. eachof theseSAMs but. rather, usedthe well-established structures and optical functions of the alkanethiolate oxide,respectively. The observedline width is verv broad monolaverson copper,silver, and gold as benchmarksfor for the titanium oxide supported monolayersand less so comparison. The C-H stretching region in the infrared for the monolayers on copper oxide. We take this as spectraof hydroxamicacid SAMs on copperoxide dem- present evidenceof a heterogeneousenvironment at the onstrates an intriguing similarity to those of alkanethi- headgroup on titanium oxide and, based on the clear olateson gold. Both the relative and absoluteintensities distinctionsobserved in the N 1s region, attribute this of the methyl and methylenemodes suggest that the alkyl protonation. line width to a heterogeneousstate of chains are cantedfrom the surfacenormal direction.This Orientation and Order in SAMs of Hydroxamic structure involves a chain tilt, a, of -20o (in agreement Acids from Polarized Infrared External Refl ectance with the XPS results presentedabove) and a rotation of (PIERS).'16'47 Spectroscopy Inspection of the PIERS the plane of the trans-zigzag,0,of -50-55". Significant (Figure data 6, Table 3) revealsthat the monola;'ersformed differences in structure are also evident between the two by hydroxamic acidson many of the substratesexamined systems. The frequencyof the antisymmetric methylene (i.e. in this study thoseon copper,silver, aluminum, and stretch for the hydroxamic acid SAM on copper oxide is titanium) are characterized by highl) organized struc- higher than that found for alkanethiolate SAMs on gold tures. Quantitative evaluation, however, indicates that (2921cm I versus2919 cm 1). This observationsuggests eachdiffers in the orientation adoptedby the chain. The that the population of gauche conformations in the alkyl follow mode assignmentsfor the C-H stretching region chains must be relatively high in this system. If so, the from a similar analysis as that describedfor alkanethiolate values of cr and B are at best approximate given that the SAMs on golda and alkanecarboxylic acid SAMs on model is basedon an all-trans conformationof the chain. we aluminum oxide.15 In the discussion that follows, Still, the presenceof a canted chain structure is strongly analyze each system briefly in terms of an average suggestedby the data. The hydroxamic acid SAMs on structure definedby the model shown in Scheme2. This copper oxide differ from the alkanthiolate SAMs on gold model assumesthat a best fit to the data involves a specific in another important respect. The data shown in Figure averageorientation of an all-trans chain. Such a structure 6 show very little, if any, sensitivity to chain length as p, is describeduniquely by the two angles, a and shown evidencedby alternations in the intensities of the methyl qualitative in Scheme2. The discussionsthat follow are symmetric (r-) and in-plane antisymmetric (r, ) stretching models. Alkanethiolate SAMs on gold show pronounced (46) Greenler.R. G. J. Chem.Phys. 1966, 44, 370-315. progressionsin the intensities of (47)Snyder, R.G.;Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, odd-even chain length 86. 5145-5150. these modes,a feature consistentwith a preferred absolute 820 Longntutr, \'ol. 11, No.3, 1995 Fr,lkerset al.

Scheme 2 r@Ttg zl -- I o.oo1 L---} z r lo.oor x --r-/\/\lt HONHOC(CH2)"CH3 n o ',n,\.A (J ,l/\i Vl ,ary'' F -^-nJ'' ! \,.- *-'i €ii rI\ {o ^^/tlJl J\t'" l\ oy

2900 3000 2800 MOx (.x Wavenumbers (cm-1) Cu 120'l 54'-' Ag -14 3B' TE AI I 8'l 47' I.*' |ooo, /1 Ti | /1.4/\ o o "L..-16 at the surface is indeed fluxional, however. The density c *?v GI of gaucheconfbrmations. which we expectto segregateto -o the chain termina,t 1; is sufficiently high in this system o il o to prevent from reaching a definitive conclusion on o us ^^J\td\'^* t5 this point. For example,relaxation of the chain ends via the gauche-trans isomerismmight leadto similar surface il structures (e.g. equivalent average orientations of the methyl groups)in an odd-even chain-length serieswhich *_/A^\^* 14 form a seriesof SAMs characterizedby a single, absolute .-----l-rTr T__-. value of a. 2900 3000 2800 2900 3000 Wavenumbers(cm'1) Structural arguments for hydroxamic acid SAMs on srlver oxideare simpler. The modeassignments for these Figure 6. Polarized infrared external reflectance spectra li)r' SAfIs fbllow similariy from those describedabove. The the C-H stretching region of monolaS'ers of octadecant,-. lori' fi'eque,no'of the d mode Q9IT cm 1) suggestsa heptadecane-, and hexadecanehydroxamic acid on copper. siiver'. structure n'rth a f'eri'gauche conformations. The relative titanium, and aluminum. Scale bars are presented in the figures for comparison. and absoiuteintensrtres of the d- and d modesalso suggest;,i unique structure fbr this SAM: the average Table 3. Frequencies of Peaks in the Infrared Spectra valuesof'the charn r)l'rentirtion parameters, a andp, and for Monolayers of Octadecanehydroxamic Acid on unlike thosefound in unr-class of SAII studied to date. Copper, Silver, Titanium, and Aluminum, Using the modelsdescribed above. we estimatethat a ry (RS and Octadecanethiolate ) on Gold -I4" and p ry 38" in this system. The odd-even progres- (all Frequencies in cm r) sion of intensities of the methyl r,,, and l is pronounced, mode description" Cu Ag Ti Al RS /Au' providing definitive evidence that the bonding geometry CH2,sym id. ) 2851 2851 2850 2851 2851 present at the headgroupsis conserved. The single sign CH3.sym u'- ) 2880 2879 2882 2879 2878 of crand the absenceof signif,rcantsurface reconstructions CHz,asvm rd ) 2920 2917 29t6 2922 2919 due to the population of gaucheconformations must result .'2938 -2937 -2938 -2936 CH3,sym tFRCtr' 2953'1 in a chain length dependenceof the orientations of the CH;l. ?SVITI ll',, )" 2967 2965 297r 2967 2964 methyl groupsthat terminate the chains. It is important b " Assignments of peaks from ref 4. FRC : Fermi resonance to note that hydroxamic acid SAMS on both copperoxide splitting component.l ' For consistency, this spectrum was taken and silver oxide are canted chain structures and that the '1 during the present study. The assignment ofthis peak is uncertain; gaucheconformation densities must dependon more than it may'correspond to an out-of-plane antisymmetric meth5rlstretch. just the presenceofthis structural feature. It seemslikely peak -2935 No is visible for the expected Fermi resonance peak at (which cm 1. '' The frequencies listed are for the in-plane antisymmetric that surfaceroughness we expectto be significantly stretch; the frequencies of the out-of-plane mode (r,6 ) are uncertain higher for the copper oxide surface) must contribute tfor most SAMs except as noted in d) given the weak appearance significantly as well. It is particularly intriguing that of this band as an unresolved shoulder on the r" band at lower the wetting data for the hydroxamic acid SAMs on silver frequencies. oxide (Figure 1t also show odd-even progressions. This observationprovides one ofthe best documentedexamples sign of the tilt angle, cr. The magnitude of the intensity of this structural influence on wetting to date. progressionsis "softened",however, by the presenceof The hydroxamic acid SAMs on aluminum oxide exhibit gaucheconformers at the chain termina. In this respect, structural features that appear to be intermediate between then, the hydroxamic acid SAMs on copperoxide appear those of SAMs on silver oxide and on copper oxide. The to differ significantly from alkanethiolates on gold. One density of gauche populations is likely to be large (d at might assume, based on the model defined above,that -2922 cm 1). Although the average chain tilt is small, the absolutesign ofthe chain cant angle, G,must alternate we estimate an approximate structure with lal ry 8" and in an odd-even series. There exists great uncertainty as fi = 47". This description of the chain structure is, at to whether the head group bonding of the hydroxamate best, approximate becauseof the complex chain confor- SAMs of Hvdroxamic Acids Langmuir, Vol. 11, No. 3, 1995 827

Scheme 3. Qualitative Ranking of Strengths of -CHs Adsorption .9 .: Ranking of substrates for affinitres to hydroxamic acids: o 0,8 o A1Ox eox > TLOX >> Au CL E 0.6 o o o 0.4 Ranking of rnrr cr lrror o G 0.2 oxides: 5 a '- !l.rlJl{: :i > PalaCIl - iPf:iI: -oH ".r:i

0 0,01 0.1 1 10 100- ztrconLum, ano Iarsorn - [CH3(CH2)lo'HG'] txb (cnr)rucoruttoxl Figure 7. Competitiveadsorptions of HO(CHz)TsCONHOH againstCH3( CHzhsSH (open circles), against CHs(CHz)r4COOH (opensquares), and against CHa(CHzh+CONHOH (filled circles) rli:l:r: netal oxide ontocopper from ethanol. Ail mixedmonolayers were formed from ethanolicsolutions containing a total concentrationof adsorbateof 1 mM at room temperaturefor 1 day. Surface compositionwas determinedby relating the measuredadvanc- ing contactangle of water to the surfacecompositions deter- CHr(CHz)rr"HG",where the headgroup ("HG")was either minedin Figure2 (seetext). Linesthrough the dataare present COOH or CHzSH; we include "HG" : CONHOH for as guidesto the eye. comparison. We approximated the composition of the SAMs by relating the advancing contact angles of water mations noted above. It initially seemscontradictory that on the monolayersto the surface compositionmeasured the chains are relatively uncantedyet still contain a large byXPS (Figure 2). Although the advancingcontactangles gauchepopulation. Whether surface roughnessor some are not linear in the compositionof the SAM, we assume other defectstructure is responsiblefor this combination that the wettabilities of the mixed SAMs dependonly on of featuresis unclear at present. Nevertheless.the data the amount of eachtail group in the SAM and not on the compela modelof a highly orientedcharn. albeit onewith headgroup ol'the methvl-terminated molecule.ae We have a complex conformational architecture. not establishedn'hether these results reflect kinetic or We cannot estimate the orientation of the alkyl chains thermodvnamrcaffinities. in the monolayersof hydroxamic acidson titanium oxide, Theseresults show that the hydroxamicacid is favored becausethe modesseen in the C-H stretching region are on the surface over the carboxylic acid, by a factor of exceedinglycomplex and are superimposedon a nonlinear approximately 4:1 (this value was determined from the substrate background (by necessity, a gold mirror was compositionsof the solution neededto form monolayers used as a reference). The organization of the alkyl chains with normalized advancingcontact anglesof 0.5). It also in these SAMs is unclear to us. One might assumethat shows that the thiol has a higher affinity for the surface the alkyl chains are predominantly in a trans zigzag of the copper oxide than does the hydroxamic acid; the arrangement, given that the stretching frequency of the Iast data point, for example,is for a solution composition d- band occurs at 29t6 cm-l. The broadness and of 30:1 which gives a monolayer that is approximately complexity of the lineshapes,however, strongly suggest 25% rn hydroxamic acid. that these structures are disordered. Examination of the spectra of the various SAMs in the Discussion low-frequency region below 2000 cm 1 was relatively uninformative with one significant exception. Modes Long-chain alkanehydroxamic acids form stable, ori- corresponding to the stretching vibrations of the carbonyl entedmonolayers on all ofthe substrateswith native metal groups were not observedin any ofthe spectra. This result oxides surveved in this study. Scheme 3 summarizes implies that either the transition moment of the carbonyl rankings of the interactions we have investigated in this group, and therefore the C:O bond, is oriented parallel study. These conclusionswere reachedfrom the various to the surface or that the bond between the carbon and experiments describedin this paper. We discusseach of the oxygenhas substantial single-bondcharacter and as these rankings in turn. a result is shifted to a much lower frequency. For the Basic metal oxides are better substrates for monolayers on silver oxide, a peak is observedat about hydroxamic acids than acidic metal oxides. Hy- 1395 cm 1. By comparisonwith studies on the infrared droxamic acids form monolayers of higher stability on spectra of metal hydroxamates, we assign this band as metal oxides with isoelectricpoints (IEPs) greater than being due predominantly to the C -N stretching motion.a8 the pK^ of the acid (pK. of acetohydroxamicacid:9.3)2t The presenceof this band, in conjunctionwith the absence than on metal oxideswith IEPs lower than the pK, (Table of intensity in the carbonyl reg'ion,suggests that the C:O 4).so-ssMost of the values in Table 4 were not measured bond is oriented largely parallel to the surface and that on native oxides. but thev are nontheless useful for both oxygens are not bound to the surface in the form of a chelate. (49) We are assuming that the distribution of moleculesin the SAM Competitive Adsorption of Hydroxamic Acids and is independent ofthe head group, and, therefore, the structure ofthe Other Adsorbates on Copper Oxide. Figure 7 quali- alkyl chains. This assumption becomesinvalid when the structures and the packing of the alkyl chains of the single-componentSAMs are tatively examines the relative affinities of different head sigaificantly different. groups for copper oxide. We used wettability to follow the (50)Parks, G. A. Chem.r?eu. 1965,65,177-I98. competitive adsorptionsof HO(CH2)15CONHOHagainst (51)Kallay, N.: Torbfc,Z.; Golic,M.; Matijevic. E. J. Phvs. Chem. 1991,95,7028-7032. (52)Kallay, N.; Torbic,Z.; Barouch, E.;Jednacak-Bisan, J. J. Colloid (48) Brown, D. A.; McKeith, D.; Glass,W.K. Inorg. Chim. Acta lg7g, InterfaceSci. 1987, 118,43\-435. 35,57-60. Brown, D. A.; Roche,A. L. Inorg. Chem. Ig83,22,2lgg- (53) Chau, L.-K.; Porter, M. D. J. Colloid Interface Sci.Iggl, 145, 2202. 283-286. 822 Longmuir. \itl. /1. No. 3. 1995 F,ilherset al.

Table -1. Published Values of Isoelectric Points (IEPs) of and perhapSnlol'e ,,t'dereci tlr rflrr].lvt-l-: I n z:I'C(.)nIumoxide ]letal Oxides and Degree of Deprotonation of and therefore also be r:setul tIl :, ,ntt-':..'.'hn:crtl ripplications Surface-Bound Hvdroxamic Acid" on zirconium oxtde for ri'hrchphL r:Dll,,flle .lclciS have been published IRCONHO 1",'rl used.l6 l'. r.'..i l valuesof IEP tRCONHOHl,,.r Self-assembied monolavers hrrve not been previousiy 9.51' 8 studied on the surface of titanrum u\rde. Our results 10. .1'1 7.7 suggest that on titanium oxide. phosphonlc acids form '2.h 1-7' 0.43 more stable monolayers than either h1'drorirnlic or car- 7- 106. 0.40 boxylic acids (Scheme 3). From the PIERS data. rt seems j(.{' a unlikely that well-ordered monoiayers of alkane h.r'drox- 5-9, amic acids are obtained on titanium dioxide. The contact \'.riu... tirkt'n Ii'om Figure 4. i'Values taken from refs 51 and angles show that their stability is not as great as that of 'l rl \'.rlLrrs trtken fi-om ref 50. Value taken from ref 53. " This phosphonic acid SAMs. Carboxylic acids do not form ', rilut' i. l,r' th€ naturally occurring mineral. Other values listed complete monolayers on titanium oxide from ethanol. in rt.: it trl'ere measured on syntheticZrOz made under strongly ilrt(l .l' :tronglv basrc conditions; the conditions of preparation Section ri|)|)t,.irt,dt() rnfluencethe measured value of the IEP. / Nitrogen 1s Experimental >l)t,r'll'iis'rre not taken for these samples. General Information. Decanoy'lchloride (98ci t, dodecanoyl chloride $8%t. tetradecanoyl chloride (971.rt. hexadecanoyl ratronalizing the trends we have observed.5a According chlorideQ8%), heptadecanoyl chloride (98%), and octadecanol''l to the data in Table 4, the most stable monolayersshould chlorideQgE) were purchasedfrom Aldrich and usedas received; (99+Vc), form on silver oxide and copperoxide. We note that there 16-hydroxyhexadecanoicacid 1-ethyl-3-(3-(dimethyl- hydro- is onlv a ver)'loosecorrelation between values of IEP and amino)prop1'l tcarbodiimide. and O-benzi'lhydroxylamine rvere obtitined from Sigma and used without further the ratios of deprotonatedhydroxamic acid to protonated chloride purification. Steanc acid and palmitic acid were availablein inferred from the N 1s spectra (Figure hr-droxamicacid the laboraton' and u'ere not purified prior to use. Octadeca- 4t. nephosphonicacrd u'as s1-nthesized using the Michaelis-Arbuzov Although thiolates are more stable on copper and reaction.;; Isooctaner99*ci. Aldrich) and hexadecane(anhy- silver, hydroxamic acids present a versatile alter- drous. Aldrich ) were precolatedtwice through neutral alumina native for these metals. Our results indicate that (Fisher); ethanol (Pharmco) was used as received. Copper (Aesar), thiolates form the most stable system of monolayerson (Aldrich), silver (Johnson Matthey Electronics),titanium (Alfa). (Alfa), zirconium (Aldrich) used for ccpperand silr.er.;; Hydroxamic acidsform more stable aluminum iron and evaporations were of 99.99%or higher purity. Single-crystal overlal'ersthan carboxylicacids or phosphonicacids on silicon wafers oriented in the [100] direction were obtainedfrom (Scheme i provide an important these substrates 3 and SiliconSense (Nashua, NH). For infrared spectroscopy,wafers s]'stemfor the formationof thin organicf,rlms on the native u'ere2 in. in diameter and 0.01in. thick; for all other uses,wafers oxidesof copperand silver. Thereal'e two advantagesto u'ere 100mm in diameter and -500 .amthick. the use of h1'droxamicacids rather than thiols on copper ()e n eral Synthes is of O -Benzylhydroxamates. A solution and silver. First, in the case of copper oxide. the ,,t'tht,rre id chloriclt'r20 mmol t. O-benzylhydroxylamine (21 mmol), hydroxamic acid forms a strong bond to the surface but rrnclpr ncirr-rt. ') I tt'tnrol, u'its stirred in dry THF for t h at room doesnot etch the surfaceduring formation as much as the tenlperirt-irt'rutrl -ttIrseclttt'ntlv refluxed for t h. The solution thiol does. Second,hydroxamic acids are not as prone to \\'il:c()n('(.nll'.rl('(l .itlti r.ht' t't'.idut' $-a's redissolved in 200mL of rlt \\'i1s('xtl'ltcted with 100-mL oxidation as thiois and may be more stable to long-term elhvl iicetlrlt' I'it.. ., lLilt twice portionsrrt (-).li \l KI{:r):. ()l-}r'r'u'rth1tlO mL of saturated exposure to ambient conditions.20'56Monolayers of thi- NaHCO,rand onceu-tth l()t)mL brtne. The organiclayer was photooxidizeon the surface,forming labile metal- olates subsequently'driedover Na:SOr. The solr-entwa's evaporated sulfonate linkages with the surface.20':'6The oxidative and the residuewas redissolvedin CH:C1:'hexanet1"1r. Am- stability of the hydroxamic acids suggeststheir use as an monia was bubbledthrough to precipitate tracesof the free acid aiternative to thiols as protectivecoatings. Potential uses as the ammonium salt. The solution was filtered. After of these monoiayers will likely be found in lithography, evaporation of the solvent, the correspondingO-benzylhydrox- corrosion resistance,and tribology.le'20 amate was obtained(92-987c crude yield). 1H (300 Hydroxamic acids present an alternative and N-(Benzyloxy)decanamide. NMR MHz, CDCI3)d (br (br (brs,2 H),2.01(br, 1.6 H), improvement to other organic acids for the forma' 8.17 s,1H),7.35 s,5 H),4.88 1.57(m, 2H),1..23(br m, 12H). 0.85(t. 3 H, J :7 Hz);HRMS tion of self-assembled monolayers on most acidic ' (FAB) calcdfor CrHzxNO: tN{ * H t 278.2120.found 278.2100. metal oxides. Hydroxamic acidsform stablemonolayers N-(Benzyloxy)dodecanamide. rH N\IR t250NIHz. CDCl,r r "acidic" (i.e., with isoelectricpoints on metals oxides those d 8.37(br s. 1 H),7.38 (br s.5 Hr.'1.90rbr s.2 Hr,2.35(br.0.'l lower than the pK, of the hydroxamic acid),:'aalthough H).2.03ft,2H,J :7 Hz),1.59 (m,2 H t.1.25(br m, 14H),0.88 they are bound predominantly as the parent acid. On the (t. 3 H. J :7 Hz); HRNIS(FAB) calcd for CrgHszNOz(M + H+) native oxidesof aluminum, zirconium, and iron, hydrox- 306.2433,found 306.2428.Anal. Calcdfor CrsH:lNOz: C, 74.7 l; amic acids appear to form more stable monolayersthan H. 10.23:N. 4.59. Found: C,74.77;H, 10.64;N,4.51. either carboxylicacids or phosphonicacids (Scheme3). N-(Benzyloxy)tetradecanamide. 1H NMR (400 MHz, The smaller size of the hydroxamic acid comparedto the CDCl,ri)7.95 rbr s. 1H).7.36(br s,5 Ht,4.90(br s,2 H),2.01 (t, : phosphonicacid may allow the formation of more coherent (br.2 Hr. 1.58rm. 2 H ).1.23(brm. 20 H), 0.86 3 H, J 7 Hz); HRMS IFABr c:rlcd{br C:rH:raNO2(M t Na*) 356.2565,found 356.2550.Anal. Calcdftir C:rH'r;NOz:C, 75.63;H, 10'58;N, metal ro.1r The most relevant set of IEPs were determined using 4.2. Found: C, 75.51;H. 10.97;N, 4.21. bead-.b)' measuring the uptake of charged latex particles' although lH NMR (250 MHz, zirconium.iron, and silverwere not studied.rrls2 Jhs IEP of the native N-(Benzyloxy)hexadecanamide. (br (br oxide of silver was measuredby measuring contact anglesas a function CDCIaId 8.02(br s. 1 H), 7.36 s, 5 H), 4.88 s, 2 H), 2.35 o{'the pH of'the drop.53 (br s, 0.4H), 2.01(s, 2 H), 1.60(m, 2}j),I.23 (br m, 24 H), 0.86 r55 r A 1:1solution ofoctadecanethiolateand octadecanehydroxamic (t, 3 H, J :7 Hz); HRMS (FAB) calcdfor CzsHaoNOz(M + H+) acid in ethanol yielded a monolayer on silver oxide that was 9:1 in favor of thiolate by XPS: Folkers,J. P.;Whitesides, G. M. Unpublishedresults' r56 i Monolayers of thiolate are prone to oxidation even on gold. For (57)KosolapoflG. M. J. Ant. Chem. Soc. 1945, 67, 1180-1182. examples on gold, see: Tarlov, M. J.; Newman, J. G. Langm-uir 1992, Bhattacharya,A. K.: Thyagarajan,G. Chem.Reu. 1981, 81,415-430. 7. 1398 1405. Huang, J.; Hemminger,J. C' J. Am. Chem.Soc. 1993, (58) Microanalyseswere performed by Oneida ResearchServices in r I 5. 3342-3343. Whitesboro. NY. 823 SAMs of Hydroxamic Acids Langmuir,Vol. 11, No. 3, 1995 (M 362.3059,found 362.3036. Anal. Calcdfor CzrHrgNOz:C,76.4; calcdfor CraHs+NOz + H') 272'2589,found 272.2573' Anal. H, 10.87;N,3.87. Found: C,76.25:'H, 11.29;N, 3'54. CalcdforCreH3sNO2:C, 70.8;H, 12.25;N,5.16. Found: C,70.74; N-(Benzyloxy)heptadecanamide. 1H NMR (250 MHz' H. 12.48:N, 5.10. 1 CI50MHz, DMSO- CDCI3)6 8.12(br s, 1H),7.36(br s,5 H),4.88(br s,2 H),2.35 N-Hydroxyheptadecanamide. H NMR (s, (s, (s, H),8'64 (s, 1 H), (brs, 0.4 H), 2.01(br,2 H), 1.58(m, 2 H), 1.23(br m,26 H)' 0.86 dn)d 10.3 0.9H). 9.71 0.1H), 8'97 0.1 (t,2IH,,J:7 (m,2H), 1.35- (t, 3 H, J :6Hz);HRMS (EI) calcdfor CzaH+zNOz(M*) 375.3138, 2.23(br.0.2H), 1.91 Hz),1.55-1.4 (br (t, :7 (FAB)calcd for found 375.3126.Anal. CalcdforCz+HarNOz:C,76.75; H, 11.0; 1.15 m. 26 H), 0.84 3 H, J Hz);HRMS * Anal. Calcd N,3.73. Found: C,76.74;H, 11.14;N, 3.65' Cr;HrsNO,:\arN{ Na* }308.2565,found 308'2575. 12.36;N,4.9i. Found:C,7l'72;H, N-(Benrylory)octadecanamide. lH NMR (250 MHz, CDCIg) forCr;H:;NO:: C. 71.53:H. d 8.3?(br s, 1 H), 7.38(br s, 5 H), 4.90(br s, 2 H), 2.03(br, 2 H), 12.35:N. 4.80. 1HNMR (250MHz, DMSO- 1.60(m, 2H), L25 (br m, 28 H), 0.88(t, 3 H, J :7 Hz); HRMS N-Hydroxyoctadecanamide. r, 1s. Hr.8.97 (s, 0.1 H), 8.64 (s, 1 H), (FAB) calcdfor Cz;H++NOz(M + H+) 390.3372,found 390.3358. da)d 10.31{-s. 0.9 H 9.71 0.1 (br,0.2 r. rt. : 7 Hz),1.6-1.1(br m, 30 H), Anal. Calcdfor Cz;H.rsNO2:C, 77.07;H,11.12;N,3.59' Found: 2.22 H 1.91 1.8H. J (t. : tFAB t calcdfor CraHgeNOz(M + C.77.17:H. 11.09;N,3.49. 0.84 3 H. J 7 Hz); HRMS found 300.2895.Anal. Calcdfor CrsHazNO2:C, Synthesis of 16-Hydroxy'N'(benzyloxy)hexadecana' H*) 300.2902. N,4.68. Found: C.72.23;H, 12.68;N,4.6. mide. 16-Hydroxy-.1/-(benzyloxy)hexadecanamidewas prepared 72.19:H.12.45: 1H NMR (300 MHz' by a slight variation of a method for synthesizingtrihydroxamic l6JV-Dihydroxyhexadecanamide. d 3.43 (t, 2 H, J : 7 Hz), 1.97 (t, 2 H, J : 7 Hz'), acidsthat can tolerate hydroxyl groups.23The pH of a solution MeOH-d+) (m,4 H), 1.3-1.1(br m, 22H); HRMS (FAB)calcd for of 1 g (3.67 mmol) of 16-hydroxyhexadecanoicacid and 0.604 g 1.55-1.35 (M -r H*) 288.2539,found288.2541. Anal. Calcdfor (3.85 mmol) of O-benzylhydroxylaminehydrochloride in THF/ CroHaaNOa 66.86;H, 11.57;N, 4.87. Found: C, 66.95;H, lHzO(.2:l) was adjusted to 4.8 by adding 1 N NaOH. A 1.41-9 CroHssNOs:C, (7.34 mmol) portion of 1-ethyl-3-(3-(dimethylamino)propyl)- 11.58;N, 4.75. Substrates. The copper,silver, zirconium, carbodiimide was added in small amounts to the stirred solution Preparation of films (1000A; thinner films, generally600- and the pH was held approximatelyconstant by adding 1 N HCl. iron, and aluminum becauseof the high powerneeded After being stirred for an additional 6 h, the reaction mixture 800A. were usedfor aluminum, metal) were prepared by"evaporation of the was extracted twice with 200-mL portions of ethyl acetate. The to evaporate the wafers primed with 50 A of titanium as an combinedorganic phaseswere extracted,washed with 150 mL metals onto silicon titanium {1000Atrvas evaporateddirectly onto of saturatedNaHCOg, 150 mL of 0.5 M citric acid. 150 mL of adhesioniaver: nietalswere evaporated at a rate of 3 A/s. u'ater.and 150mL ofbrine. and dried over NazSO.r' The solution the siliconu-afers, All n'ere performedin a cr1'ogenicallypumped \vas concentratedand the residue rvas purified bv column The evaporatr(rns basepressure < 1 x 10-; Torr). After chromatographr',silica: hexaneethf i acetatemethanol 7 6 2 electron-be:tI'r^e\-ilporatol ri'as backfilled with nitrogen. The Yield: 0.8g.60e. rH NllR 250\IHz. CDCI, ,)7.95 br s. 1 H . cvaporatlonthe chrlmber --; through air to the solutionsof the 7.38(br s,5 H .4.90,brs.2 H . 3.63t. 2 H. J Hz . 2.35br. san-rples\\'ere tritnsf'erred ri'ithrn5-10 min. 0.4H ).2.02rbr. 1.6 Hr. 1.59 rm,4 H'. 1.25'brm. 22H: HR\IS hvdroxamicacids Monolayerswere formed from (FAB)calcd for CzsHroNOsrII - H- ' 378.3008.found 378.2983. Preparation of Monolayers. or ethanol with concentrationsof Anal. Calcdfor CzsHseNOs:C, 73.I7 H. 10'41:N. 3.71' Found: solutions in either isooctane The hydroxamic acids,however, were not C, 73.01;H, 10.29;N, 3.57, 1 mM in adsorbate. in isooctaneat 1 mM, but enough dissolved to form General Synthesis of Hydroxamic Acids. The O-benzyl- soluble Monolayersfor infrared spectroscopywere formed hydroxamates(20 mmol) were dissolvedin 200 mL of ethanol monolayers. in glass Petri dishes, which had been cleaned with "piranha and hydrogenatedfor 6-12 h at a H2 pressureof 1 atm using 50 'C (7:3 concentratedHzSO,tl3j% HzOz) at -90 for 30 mg of Pd on carbonas catalyst. The solutionswere filtered over solution" rinsed with deionizedwater, and dried in an oven at 125 Celite and concentrated,yielding 95-I00Vc of the free acid.Upon min. prior to use. recrystallization from CHzClzlhexane(1:1) 70-80Vc of the hy- "C Piranha solution should be handled with droxamic acid were recovered. WARNING: caution; it has detonated unexpectedly.6o For all other uses, N-Hydroxydecanamide.ss 1H NMR (400 MHz, DMSO-do) monolaverswere formed in single-useglass scintillation vials d 10.32(s, 0.9 H), 9.67(s, 0.1 H), 8.97(s, 0.1 H), 8.65(s, 0.9 Hl, r20mL r. The slrdeswere removed from solutionsafter 1-2 days, 2.25(t.0.2H, J =7 Hz),L92(t, 1.8H, J: 7 Hil,I.47 (q,2H. nnsed u'ith ethanol and heptane, blown dry with N2, and J : 7 Hz), 1.3- 1.1(br m, 12 H), 0.86(t, 3 H, J : 7 Hz t; HRNIS charactenzed. The method for forming solutions for mixed (FAB)calcd for CroHzzNOztM * H*t 188.1650,found 188.1626. monolavershas beendescribed previously'34 35'3e Anal. Calcdfor CroHzrNOz:C.64.13: H. 11.3:N. 7.48 Found: Measurement of Contact Angles. Contact angles were C, 63.91;H, 11.38;N, 7.4. measured on a Rame-Hart }lodel 100 goniometer at room N-Hydroxydodecanamide. 1HNIIR r250 llHz. DIISO-d6 r temperatureand ambient humidity. Water for contactangles d 10.32(s,0.9 H),9.71 (s,0.1 Hl,8.97 rs.0.1 Hr.8.65 rbr s.0.9 was deionizedand dtstilled in a glass and Teflon apparatus. H),2.24(br,0.2 H), 1.92(t, 1.8H,J : i H2,.1.55-1.4tm,2 Ht, Advancing and recedingcontact angleswere measuredon both 1.3-1.1(br m, 16H), 0.86(t, 3 H, J : i Hz r:HRIIS rFABr calcd sidesof at leastthree dropsof eachliquid per slide;data in the for CrzHzoNOz(M + H+)216.1963, found 216.1948. Anal. Calcd figures represent the average of these measurements' The for CrzHzsNOz:C, 66.93;H, 11.7;N, 6.5. Found: C. 67.00:H' followingmethod was usedfor measuringcontact angles: A drop 11.37;N, 6.4. approximately1-21rL in volume was grown on the end of the lH NX'IRt250 N{Hz.DMSO- N-Hydrorytetradecanamide. pipette tip (Micro-Electrapette syringe; Matrix Technologies: 10.3(s,0.9 H),9.74 (s,0.1 H),8.97 (s.0.1 Ht.8.64 (s,0.9 de)d Lowell, MAt. The tip was then lowered to the surfaceuntil the H),2.23(br,0.2 H), 1.91(t, 1.8H. J: 7 H2t.1.55-1'4(m,2 H), drop came in contactwith the surface. The drop was advanced 1.35-1.15(br m,20 H),0.84(t.3 H, J = i Hz);HRMS (FAB) by slowly increasing the volume of the drop (rate -l pUs)' for Cr+HgoNOz(M -f H- I244.2276,found 224.2261.Anal' calcd Advancing contactangles of water were measuredimmediately Calcd for Cr+HzgNOz:C, 69.09;H, 12.01;N, 5.75. Found: C' after the front of the drop had smoothly moved a short distance 68.95;H, 11.88;N, 5.60. acrossthe surface. Recedingangles were taken after the drop IH NMR (250 MHz, DM SO- N-Hydroxyhexadecanamide. had smoothly retreated across the surface by decreasingthe (s, (s,0.1H), 8.96(s, 0.1 H), 8'64(s, 0.9 do)d 10.3 0.9H), 9.71 volume of the drop. (br, (t, H, J : 7 Hz), 1.55-7.4(m, 2 H), H),2.22 0.2H), 1.90 1.8 X-ray Photoelectron Spectroscopy (XPS). X-ray photo- (br H),0.84 (t, 3 H, J : 7 Hz); HRMS (FAB) 1.35-1.15 m, 24 electron spectra were collectedon a Surface ScienceSSX-100 spectrometerusing a monochromatizedAl Kctsource (hv:1486.6 lH (59) Two setsof peaksin the NMR are observedfor the hydrogens eVi. Th" spectrawere recordedusing a spot sizeof 600 ,amand on the oxygen and nitrogen in the hydroxamic acid and the hydrogens a pass energy on the detectorof 50 eV (acquisitiontime for one in the methylene group o. to the hydroxamic acid. These two sets correspondtoE andZ conformationsof the hydroxamic acid. For more detail. see: Brown, D. A.; Glass, W. K.; Mageswaran, R.; Girmay, B. (60) Several warnings have appeared concerning "piranha solu- Magn. Reson.Chem. t988,26,970-973. Brown, D. A.; Glass,W. K.; tion": Dobbs,D. A.; Bergman, R. G.; Theopold.K. H. Chem.Eng.I'{ews (26),2. Mageswaran,R.; Mohammed, S. A. Magn. Reson.Chem. 1991,29, 40- 1990.68(17\.2. Wnuk, T.Chem.Eng.News1990,68 Matlow. (30). 45. S. L. Chem. Ens. News 1990, 68 2. 824 Langmuir, Vol. 11,No. 3, 1995 Folkers et al. scan was approximately 1.5 min). For the monolayers,spectra eV for Zr(Nl Fe 2plz, 2pnat 706.5eV and 719.5eV for Fe(0), u'erecollected for carbon,nitrogen, and oxygenusing the 1speaks and at 710 and 723.5eV for Fe(IIIt.12The binding energiesfor iit 285. ,400.and 530 eV, respectively;the binding energiesfor the substrateswere not standardizedto a referencesample. All element,sin the monolayer were referencedto the peak due to spectra were fitted using an 807r Gaussian/20%Lorentzian peak hvdrocarbonin the C 1s region, for which we fixed the binding shape and a Shirley background subtraction.6l e.nerglat 284.6eV.a2 Spectra for the solidhydroxamic acid were Infrared Spectroscopy. The experimental apparatus for 8 collectedusing an electronflood gun of 4.5 eV to dissipatecharge taking infrared spectra has been describedpreviously.a The in the sample. The following sigaals were used for the sub- angle between the incident beam and the surface normal was strates: Cu 2p,r.z at 932eV for Cu(O),and at -934 eV for Cu(II); approximately 85". Ag 3d;,. 3d:;:at 368and 374eV for all oxidationstates of silver; L49407 47r Ti 2p'r,.2pr: at 454and 460 eV for Ti(O),and at 459 and 464.5 eV fbr TirI\rr: Al 2p at 73 eV for Al(0), and at 75 eV for AI(III); Zr i)d;,.3d,r:at 179and 181eV for ZrQ), and at 183and 185.5 (61r Shirley, D. A. Pfrv.s.Rer'. B 1972.5,4709 4774