Formation of Monolayers by the Coadsorption of Thiols on Gold: Variation in the Head Group, Tail Group, and Solventr

Long-chain alkanethiols, HS(CH2)nX, adsorb from solut ion onto gold and form oriented. ordered monolayers. Monolayers exposing more than one functional group at the surface can be generated by coadsorption of t* 'o or more thiols from solut ion. In general, the rat io of the concentrat ion of the t\r 'o components in a mixed monolayer is not the same as in solut ion but ref lects the relat ive solubi l i t ies of the components in solut ion and interactions between the tai l groups, X, in the monolayers. Mult icomponent monolayers do not phase-segregate into single-component domains large enough to inf luence the contact angle (a few tens of angstroms across) and also do not act as ideal two.dimensional solutions. In the two-component system HS(CH2)J/HS(CH2)"CH3 in ethanol, where X is a polar tail group such as CH2OH or CN, adsorption of the polar component is particularly disfavored at low concentrations of the polar component in the monolayer. These isotherms may arise from poor solvation of the polar tail groups in the quasi-two-dimensional lkane solution provided by the methyl tail groups. From dilute solutions in alkanes, adsorption of HS(CH2)r9CH2OH is strongly preferred over HS(CHz)roCHr, probably due to the stabilization afforded by intramonolayer hydrogen bonds between the hydroxyl tail groups. The wettability of mixed monolayers is not linear in the composition of the surface. [n a surface comprised of a polar and a nonpolar component, the polar component is more hydrophilic when its concentration in the monolayer is low than when the monolayer is composed largely of the polar component. The formation of oriented monolayer films on a surface by the spontaneous adsorption of molecules from solution has become known as self-assembly. Of all the types of self-assembled monolayers that have been studied,3 two systems have shown the greatest promise as a means of controlling the chemical structure (l) Supported in part by the ONR and by DARPA. The XPS was provided by DARPA through the University Research Initiative and is housed in the Harvard University Materials Research Laboratory, a NSF-funded facil ity. (2) IBM Pre-Doctoral Fellow in Physical Chemistry, 1985-1986. (3) For examples of other systems, see: Zisman, W. A. ln Contact Angle, Wettability, and Adhesion; Fowkes, F. M., Ed.; Advances in Chemistry 43; American Chemical Society: Washington, DC, 1964; pp l-51. Allari, D. L.; Nuzzo, R. G. Langmuir, 19t5, I, 45-52. Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92,2596-2601. Finklea, H. O.; Robinson, L. R.;Blackburn, A.; Richter, B.;Allara, D.;Bright, T. Langmuir, 1986, 2,239-244 and references therein. of organic surfaces: adsorption of organosulfur compounds on noble metals such as goldl-s and silver,e and reaction of alkyltrichlorosilanes with silicon or glass.l0 Our research has concentrated on the first of these systems.ll In a previous paper,s we presented a study of the formation, characterization, and properties of monolayers generated by the adsorption of n-alka(4) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105,4481-4483. (5) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987. 109.3559-3568. (6) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.;Nuzzo, R. G.;Allara, D. L.; Porter, M. D. Langmuir,l988, 4, 365-385. (7) Bain, C. D. Ph.D. Thesis, Harvard, Cambridge, MA, 1988. (8) Bain, C. D.;Troughton, E. B.; Tao, Y.-T.; Evall, J.;Whitesides, G. M.; Nuzzo, R. G. "r. Am. Chem. Soc. 1989, I I I , 321-335. (9) Ulman, A.; Til lman, N.; Littman, J., unpublished results. (10) Sagiv, J. J. Am. Chem. Soc. 19t0, 102,92-98. (l l) For a review, see: Bain, C. D.;Whit'esides, G. M. Angew. Chem.,lnt. Ed. Engl. Adu. Mater. t989, 28,506-512. 0002-7863 l89l15l l -7155$01.50/0 @ 1989 American Chemical Society ' i : 6 . t 1n r Chen t Soc . , Vo l . l l l , No . 18 , 1989 nethrt.r is. HSrCHr.tr\ . on gold. In these monolayers, the chemistrr, i tructure. and propert ies of the surface were control led by' varr ing rhc t . r i i g roup. X. The range of proper t ies that can be obta ined I lg6eg€neous monolayers of a single thiol is, however. l imitcd. ( i :e . r ter cont ro l over the s t ructure o f the monolayer is a i iorded br coadsorpt ion o f two or more th io ls that d i f fer in the naturc oi the tai l groupr2 or the length of the hydrocarbon chain.r r Th i : taper is the f irst of two that examine monolayers formed br the coadsorpt ion o f two spec ies that d i f fer in the cha in length. the ta i l group, or the nature o f the head group that b inds thc conrn(lnents oi the monolayer to the gold surface. In th is paper , we present a br ie f survey of organic funct iona l groups. other than thiols, that coordinate to gold and form stablc monolayers. Then we examine monolayers formed from mixturet of nvo thiols with the same chain length but with dif ferent tai l groups. Finally, we address the effect of solvent on the composition of the monolayers and i ts implications for the mechanism tr1. adsorption. In the companion paper,we discuss mixed monolarers ,:omposed of two thiols having dif ferent chain lengths.ra We had three broad aims in this study. First, we wished t ' , understand how the composit ion of a monolayer is related to thc relat ive concentrat ions of thiols in the adsorption solut ion and. in particular, whether the formation of monolayers is under kinetic or thermodynamic control. I f the monolayers are in thermodr namic equi l ibr ium with the adsorption solut ions, i t should bc possible to derive thermodynamic propert ies of the monolarcrs from the adsorption isotherms. A quanti tat ive analysis of the adsorption isotherms is beyond the scope of these papers. Here we discuss qualitatively the impact of excess entropy and enthalpr on the composit ion of the monolayers. ls Second, we wished to elucidate the structure of the mixed monolayers. In part icular, we wanted to establ ish whether the two components in the monolayer segregate into macroscopic. single-component domains and, i f not, to what extent the components do aggregate into small clusters on the surface.16 In this context, we use "macroscopic" to mean sufficiently large such that the properties of the monolayer are dominated by molecules that are in an environment indist inguishable from that in a pure ( i .e.. single-component) monolayer. In the absence of long-range electrostatic interactions, macroscopic probably applies to islands more than a few tens of angstroms across. We wil l present the data in this paper under the assumptions that thermodynamics controls the composit ion of the mixed monolayers and that the monolayers are not phase-segregated and then discuss the evidence that lends support to these assumptions. We will also trv to define more closely the extent of aggregation of the t\ ,r 'o components in the monolayer, Th i rd , rve wished to invest igate ho* the macroscopic phrs ica l properties of an interface-particularll the wettabilin-are related to the microscopic hemica l s t ructure o f the sur face. The general strategy' in these studies was to prepare solut ions containing two thiols. HS(CH2)"X and HS(CH2).Y dif fering in the nature o f the ta i l g roup (X. Y) and/or the cha in length (m, n), in a range of mole fract ions but with the same total concentrat ion of thiol moiet ies in solut ion. Gold sl ides were immersed in these solut ions overnight under ambient condit ions of temperature and pressure. The formation of strong, coordinative gold-sulfur bonds drives the spontaneous assembly of the monolayers. We then used el l ipsometry and X-ray photoelectron spectroscopy (XPS) to determine the composition of the monolayer ( l2) A preliminary communication of this work has been published: Bain, C. D. ; Whi tesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560-6561. (13) Bain, C. D. ; Whi tesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665-3666. Bain, C, D.;Whitesides, G. M. Science (Washington, D.C.) 1988, 240,62-63. ( l4) Bain, C. D.; Whitesides, G. M. J. Am. Chem. .Soc. 1989, I I I , f ollowing paper in this issue. ( 15) For a discussion of excess thermodynamic functions, see: Rowlinson, J.S. Liquids and Liquid Mixtures; Butterworth: London, 1969. (16) To address this problem, we used inferential evidence from contact angles (d), X-ray photoelectron spectroscopy, and ellipsometry. We have not attempted to determine the pair correlation function of the components within the monolayer directly by scattering experiments. Bain et al. and used contact ang les, d , to measure i ts wet tabr l in . We bel ieve8't ; ' t8 16u1 the species ult imately formed on the gold surface by adsorption of thiols from solution is a thiolate {Au-SR)' The mechanism by which an initially physisorbed thiol is converted to a chemisorbed thiolate remains unclear. In this pape r. r're will use phrases uch as *monolayer of an alkanethiol ' to indicate the precursor from which the monolayer was formed, even though thc actual species at the gold surface is probably a thiolate. We u i l l designate monolayers adsorbed from pairs of thiols as X/Y I r r tnd icate the pa i r o f ta i l g roups, X and Y, that are exposed at rhc .url 'ace of the monolayers. Thus, for example, the monolayer t 'cp.rrcd bv adsorption of thiols from a solut ion containing HS, ( l l . r C t l rOH and HS(CH2) ' oCH, wou ld be des igna ted ()t i \1c \\ 'e * ' i l l also use terms such as *methyl surface'to refer :, i:rc \L:l.rcc trl'a monolayer that exposes primarily methyl groups . , i i i .e r r , , r , ' l i . l \e r -a i r or monolayerl iqu id in ter face. l : : :u.: :r . i i rcrs. u'e general ly plot contact angles on axes that . : :c . r : . . : r . r r , r : r i . ps1 in 0 i tse l f . cos d is re la ted to sur face f ree i : c : J . e . i l ' , , ^ r l \ r r u n g ' s e q U a t i O n 1 9 ^ , r , C o S d = 7 r u 7 s l ( l ) * : .c:. : , . : :d ^, . l rc the l iquid-vapor, sol id-vapor, and : , , . . d . . J , : . . r i : v ! t l ' c c cne r s i c s . r espec t i ve l y . changes i n cos ".:rr l l . , .r .r .r . : i r ' , : , ' . : lcd trr thC changes in interfacial freeenergies. I : . s e : c : . i l . .L ' t , : ( .1 . : : j ' : .cnec \ , t ln l3Ct angles cannot eas i ly be used for e L . : i ' . .1 . : i r I ( ' , : t i . rc t ang ie : do prov ide s t ructura l in format ion r ] . ' l : : ! . L r l . : s J . r l . i n rono la \e r . The con tac t ang le o f wa te r t \ \ c : \ ' : : . . : , , l i c no l . r r r t r o f t he su r face ; t he con tac t ang le o f f .c r . rdc . . : :c :c : ,c ' i . thc po lar izab i l i ty and, as we shal l show' the d,cr:cc ,: :dr ' : r :r lhc :url-ace. The contact angle of water is more \ : ' : . . : . . : r l . : : :he contact ang le o f hexadecane to the presence of $ , . . : i i rs . i r )n . . . . gr r )ups bur ied be low the monolayerl iqu id inic : : .1 . . \ , ' i t .p . l r i \on o f the contact ang les o f water and hc r , r t t c . . : r ,C g . r r i hus p rov ide cons ide rab le i ns i gh t i n to t he th : cc -d . : r c r r , \ r . : . . t r uC tu r€ o f a mono laye r . Hys te res i s i n t he c i \ r^ : l ;s l r l { .c . : . : r ,s . : i l tL-S ra luable s t ructura l in format ion, but i ts rnrc i ' l :c1 .11. , . ' r . . t . , ' : $c l1-unders tood. In th is paper , we express rhe hr \ ic :c . r : i r . : i .c coo l iCt angle as the d i f ference between the minrnrun'. :L'ccdr:. tg grrnl. ict angle and the advancing contact angle, C \ p f C : : c ' t i . 1 \ a , \ \ l l l C \ C r l : H , C O S d . . r ' Thermodrnamic rersus Kinetic Control over the Formation of \ Iono larers . C)nc, r i ' lhc n lgmas in these s tud ies, which we have not \c t fu l . r rc . , . , l tcd . i : r rhether the composi t ion o f a monolayer adrorbcd l ' f t rnr . : r t r iu l ion conta in ing two or more th io ls is detcrnr rncd br c . lur i rbra t ion between the components in the monoi i rc r . rnd thc prccursors to the monolayer in so lu t ion or bv the k inct rc , t r i adsorpt ion. Th is quest ion is compl icated by exper i mcntal observations that, at f i rst sight, are mutual ly ' inconsistent ' Most of the data in this and the following paper can be rationalized if the compositions of the mixed monolayers were at. or near, the values we would expect from thermodynamic equilibrium between the components of the monolayer and the adsorption solut ions. It is difficult to construct a purely kinetic mechanism that accounts for the observed compositions. For example , a monolayer adsorbed f rom a mix ture o f two l inear th io ls r . r i th the same chain length may be composed almost erclusively oi the minor species in solut ion. even when there rs no obvious kinetic preference for one (17 ) Ba in . C D . B rebuyck . H . A . ; Wh i tes ides , G . M. Langmu i r 1989 , J , 1 2 3 1 2 1 . (18 ) Nuzzo , R . G . ; Zegarsk i . B . R . ; Dubo is , L .H . J . Am. Chem. Soc . t987 . 1 09. 1 33-1 10. (19) Young, T. Phi los. Trans. R. Soc. ( l 'ondon) 1805,9J,65-87. (20) Cassie, A. B. D. Discuss. Faraday Soc. 1948, -1, l l-16. (21) Bain, C. D. ; Whi tesides, G. M. J. Am. Chem. Soc. 1988, 110, 5897-5898 . (22) The maximum advancing and minimum receding contact angles are defined as the angles between the surface and the tangent to the drop at the three-phase line for a drop advancing or retreating quasistatically over a motionless urface. The advancing contact angles reported in this paper were obtained under controlled conditions in which the drop advanced rapidly over the surface, and the contact angles were measured after the drop had come to rest. For a more detailed discussion, see ref 8. l ' a r ra l ron :n thc Head and Ta i l Groups