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Phase Behavior of Two-Component Self-Assembled Monolayers of Alkanethiolates on Goldt

John P. Folkerc, Paul E. Idbinis, snd George M. Whitesides'Department of Chemistry, Hantard Uniuersity, Cambridge, Massachusetts 02138

Jobn l)eutch'Department of Chemistry, Massachusetts lrstitute of Techrclogt, Cambridge, Massachusetts 02/,39Receiaed: May 17, i,993; In Finol Form: September 14, |993c

This paper examines the relationship between the composition of two-component self-assembled monolayers(SAMs) of alkanethiolates on gold and the composition of the solutions from which they were formed. TheSAMs were prepared by competitive adsorption of a long-chain alkanethiol (HS(CHz)zrCHl) and a short-chainalkanethiol (HS(CHz)rrOH) from solutions in ethanol. Under conditions in which the alkanethiolates in aSAM and the alkanethiols in solution are close to equilibrium, the relationship between the composition of thesolution and the composition of the SAM suggests that the monolayer tends, thermodynamically, to exist asa single phase predominantly composed of either long-chain or short-chain thiolates. A derivation of thethermodynamic relationship betwecn the compositions of the SAM and solution is described that includesintermolecular interactions between components in the SAM; theory and experiment agre,equalitatively. Thisanalysis concludes that, for a two-cornponent system of alkanethiolates on gold well-equilibrated with alkanethiolsin solution, a single phase is preferred at equilibrium; phase-separated, two-component monolayers of the sortextensively studied in Langmuir systems are not observed.

Introduc6on

"Mired SAMs" on gold--elf-assembled monolayers (SAMs)compnsing two alkanethiolates prepared by cochemisorption froma solution containing two alkanethiols-are useful in studyingphenomena involving organic surfaces.2-I3 A number of tech-niques can establish the auerage composition of a mixed SAM.2-I tX-ray photoelectron spectroscopy, XPS, can ascertain thecomposition over an area thesize of the X-rayspot (approximatelyI mm2), and scanning electron microscopy can sometimes detectheterogeneities in composition of a SAM on thedimension of theorder of I prn;tl phase separation has not been observed on thescscales. In short, neither has thc extcnt to which the componentsof a mixed SAM segregate into separate phases within a SAMbeen established nor has the question of the relationship betwecnthe composition of the solution used to form the SAM and itsheterogeneity bcen addressed experimentally or theoretically. Thcproblem is a difficult one experimentally, because there are fewtechniques well adapted for dircctvisualization of phase-separatedregions with small sizes in an organic monolayer, especially onoptically opaque sub,strates. Scanning probe microscopies (es-pecially atomic and lateral force microscopies)17 are techniquesthat are applicable but only to certain types of SAMs.rT-re

In this psper, we investigate the phase behavior of two-component SAMs from both theoretical and experimentalperspectives. The approach we have taken is thermodynamic-toscarch for characteristic signatures of phase separation in thedependencc of average composition on temperature, con@ntration,and tim-rather than spcctroscopic and based on irraging. Wefocused on the question of whether two-component SAMs formphasc-separated domains. In the theoretical section, we derivea thermodynamic relationship between the composition of thesolution and the composition of the sAM in terms of interactionsbctween nearest neighbors within the SAM. This relationsbipprcdicts that phase separation in a SAM by a mechanism involvingtheequilibrium of species in thc sAM with those in solution willbe detectable through a sharp transition between the propertiescharacteristic of one monolayer to those characteristic of thcsecond.

o Abstract publishcd in Adaance ACS Abstrccts, Novembcr I, 1993.

553

ln the Expcrimental Section, we generate two-componentSAMs from HS(CH2)21CH3 (abbreviated Lg for *long") andHS(CH2)1OH (Sh for 'short') under various experimentalconditions. We chose to study SAMs derived from thesecomponents because previous qualitative interpretations of ex-perimentally determined relationships between the compositionof the mixed SAMs and the composition of the correspondingsolutions suggested that mixing of these two components wasunfavorable within the SAM.3'a In this study, we aimed todetermine what the relationship between the composition of theSAMs and the composition of the corresponding solutions wouldbe at equilibrium (if we could indeed reach equilibrium withthese components) and to compare this relationship to thetheoretically determined relationship. The compositions of theSAMs we have formed in this study span the range from kinaicallydetermined to nearly thermodynamically determined.

The spatial distribution of two thiolates in a mixed SAM cannotbe easily determined,l-5'r6 sl1fisugh scanning probe microscopieshave suggested, under some conditions, the existence of phase-separated domains.ls The experiments in this paper are focusedon systems consisting of SAMs in equilibrium with solutionscontaining the corresponding thiols; they do not define the spatialdistributions of thiolates in SAMs that are not ̂ t equilibrium.This work also does not present experimental conditions thatguarantee that equilibrium is reached in these systems. Webelieve, however, that under certain conditions the monolayersdo approach equilibrium.

From this work, we conclude that, when at equilibrium witha solution containing a mixture of two components, a SAM will,in general, comprise a single phase and will not consist of regionsof separate phases. This single phase may contain one alkane-thiolate or a homogenerus mixture of both alkanethiolates. Thestrength of the interaction between molecules will bc the primaryfactor in determining the composition of that phase.

When mixing of the two components in the SAM is unfavorable,a mixed SAM that is caused to equilibrate at constantcomposition-that is, when the SAM is not in contact with thesolution-could presumably form phase-separated islands" Wehave not considered this case.

1994 American Chemical Society0022-36s4/94/2098-0563$04.50/0 @

5& The Journal of Physical Chemistry, Vol. 98, No. 2, 1994

Background

Self-assembled monolayers obtained by the adsorption of long-chain alkanethiols (general formula: HS(CH2)J, where X isthe terminal functional group, or so-called 'tail group') onto

Soldzo and silver2.20'2t are useful model systems with which tostudy the chemistry and properties of organic surfaces. Thesemonolayers are highly ordered's't3,20,22-24 the tail groups areexposed at the monolayer-air interface and strongly influencethe properties of the interface.2-t3,20,21'23'2a Formation of SAMsfrom a single thiol allows a limited degree of control over theproperties of a surface by choosing the tail group X; formationof SAMs from two thiols allows a greater degree of control overthese properties, as both the tail groups and the composition ofthe SAM can be chosen.2-t2'2t Understanding the phase behaviorof two-component self-assembled monolayers is important ininterpreting studies of wetting2-e'r3 and adhesion (and perhapsstudies of other areas such as protein adsorption)8 using two-component SAMs.

In Langmuir monolayers, where lateral diffusion allows forequilibration of the system,2s phase separation has been observedfor both single- and two-component monolayers.26'2? Microscopicphase separation has been predicted theoretically for mixed SAMsderived from alkanethiols with lengths of the alkane chainsdifferent by l0 methylene groups, under the assumption thatlateral diffusion occurs in the plane of the monolayer and thatthe composition of the monolayer is fixed.r5 It is not establishedthat either assumption can be realized experimentally;zs'zr ,5tsecond assumption, in particular, docs not apply to the presentstudy.

A recent study using a scanning tunneling microscope (STM)has inferred the existenceof phase-separated domains within two-component SAMs formed from solutions containing HS(CHz)rrCH3 and HS(CH2)rsCOzCHr, although the exact molecularcompositions of the domains have not been established.lE TheseSAMs were formed from solutions of ethanol with a totalconcentration of I mM in thiol for 4 days at room temperature.A previous study using polarized infrared external reflectancespectroscopy also suggested the existence of domains within mixedSAMs of long- and short-chain alkanethiolates.5'30 From studieson the rates of exchange between long-chain thiolates in the SAMand similar thiols in solution,lo'2l lqgeller with the conclusionsfrom the theoretical section of the present paper, we believe thatSAMs that were shown to contain domains but were formedfrom (and allowed to equilibrate in contact with) solutionscontaining long-chain alkanethiols at room temperature probablywere not at equilibrium with their contacting solutions.

The establishment of equilibrium with mixed SAMs iscomplicated by at least two types of equilibria: (i) equilibriumwithin a SAM containing two components at fixed compositionand (ii) equilibrium between a SAM and a solution containingthiol(s). We cannot presently define conditions necessary toachieve equilibrium, because the mechanisms and rates forformation of a monolayer and for exchange of thiolates in a SAMwith thiols in solution are not completely known.

Several studies on the kinetics of exchange between long-chainthiolates in a SAM and similar thiols in solution have reachedthe same general conclusion: at room temperature, exchange isslow, dependent on the concentration of thiol in solution (but notdependent enough to be first order in thiol) and strongly influencedby defect sites within the monolayer (or on the surface of the goldsubstrate).r0'2r'3r-33 These results imply that part of the surfacestructure is'quenched' and that thermodynamic equilibrium inthese SAMs at room temperature is approached only slowly afterprolonged exposure to solutions with high concentrations of thiol.

In this study, we have varied the experimental conditions usedto form SAMs in order to study systems having differentrelationships between the composition of the SAM and thecomposition of the solution with which it is in contact. Most, ifnct all, of these SAMs-especially those formed at room

xPs 0'6

L9,SAM 9.4

0.2

AurS(CHj11OH

ArS(CH2)21CH3

0.8

yxPs o'5

ALg,SAM 9.4

0.2

Au-S(CHz)rrOH

0 0.01 0.1 1 10 100 msh

6 [HS(cHz)z rcHr ] Lg

nsorn = ]F_SGFil6F]

Figure l. Data for mixed SAMs derived from HS(CH2)21CH3 andHS(CH2)lrOH adsorbed onto gold from ethanolic solutions. Experi-mental conditions for the individual experiments are listed in the figure,where IRSH]r is the total concentration of thiol in solution: (a) dau forSAMs closest to ideal or purely kinetic behavior (Rsrr.,r = R-r"); (b) datafor SAMS between ideal behavior and equilibrium, showing theprogression toward equilibrium as [RSH]1is increas€d; (c) dau for SAMsclosest toequilibrium. Data are plotted as the mole fraction of the longer

component in the SeU (xf5Ly, determined from the Au(40 X-rayphotoelectron signal) againsf the ratio of the concentrations of the two

thiols in solution. Some data may bave values of xffien ( 0 or

xffi^, > I because of uncertainties in the rneasuicments: thcuncertainty in the intensity of the Au(4f) photoclectron signal usuallyvaries by about *5%. We have left these data outside the rangc

Xffi^*, = 0 and XIF"" = I (rather than moving them to the end points)to ihow the error in the measurements. The dashed curves represcnt themole fraction of tbe SAM if tbe composition of a SAM were the sameas that of the corresponding solution (denoted by Rsev = R'ou in theplot). Lines through the data are presented as guides to the eyc.

temperature-have not reached equilibrium, because exchange

between thiols in solution and thiolates on the surface is slow.l0'2r

We emphasize that the SAMs described here were formed in

contact with solutions containing the corresponding thiols. We

have no evidence for any type of equilibration within the SAM

in the absence of a contacting solution of thiol(s),2tJe although

recent scanning probe studies imply the existence of some lateral

mobility of both surface gold atomsv and alkanethiolates.3s Wehave not examined the influenceof the topologyof the gold surfaceon the rates and positions of equilibria.

Results

Anelysis of the Composition of tbe SAMs: Composition of theSAM as a Function of the Composition of the Solution. FigureI summarizes the range of relationships between the compositionof the SAM and the composition of the solution that we haveobserved when forming mixed SAMs from ethanolic solutionscontaining HS(CH2)2rCH3 and HS(CH2)1OH. The data areplotted as the mole fraction of the longer component in the SAM

Folkers et al.

AtrS(CH2)21CH3

0.8

xPS 0'6

Lg,SAM 9.4

o-2

Au-S(CHdrrOH

Att-S(CHdzrCHg

0.8

.€!lot /

wrI:

l . ltf

[RSH]'= 1 pM

T = 7 0 o Ct = 1 h o u r

o[RSHlr= 1 mMo[RSH] '= 0.1 mMr [RSH],= 0.01 mM

T = 2 5 ' C

o r [ R S H ] - = 1 m Mo [RSH] '= 10 mM

T = 6 0 ' C

Phase Behavior of Alkanethiolates on Gold

(xr-es,cu = [L8]ser'r/([Lg]s^r'r + [Sh]snu)) as determined usingXPS (seethe ExperimentalSection forthe method of determiningXr-es1u using XPS) against the ratio of the concentrations of thetwo thiols in solution (R,on = [Lg]*r"/ [Sh]*rJ. The dashed curvein each of the plots represents the case where the ratio of theconcentrations of the two thiolates in the SAM is the same as thatof the two thiols in the ircrresponding solution; we refer to thiscase as 'Rsnu = Rroln'.

In Figure la, R5ay = Rsoto, although the monolayers are onlyabout 6V70Vo complete.36 The low concentration (total con-centration of thiol in solution, IRSH]r = IHS(CHJTTOH] +[HS(CHt2rCHr] = I pM) and short immersion time (r = I h)limit the amount of exchange that occurs between thiolates onthe surface and thiols in solutions.3T

In Figure lc, Rsay I Rson. The high concentration of thiol([RSH]r = I or l0 mM), long exposure time (r = I day), andhigh temperature (T = 60 oC) in these experiments acceleratethe rate of exchange between thiolates in the SAM and thiols insolution and permit the system to approach equilibrium. Wepropose that the shape of this curvr-an abrupt transition betweenxrgseru = 0 and Xkseu = 1,38'3e that is, a transition betweenSAMs consisting predominantly of either component over a smallchange in the value of R*6-is characteristic of the tendency ofthese systems to exist in a single phase at equilibrium, rather thanas phase-separated islands.

We can form SAMs from these components for which therelationships between Rseu and Rro6 are intermediate betweenthese extremes by varying the experimental conditions (Figurelb). As [RSH]1 is decreased,4 the composition of the SAMbecomes increasingly similar to the composition of the solution;the sharpness of the transition region also decreases withdecreasing [RSH]r. The curve for [RSH]1 = I mM, 25 "C, Iday is important because it represents the conditions mostcommonly used in experimental preparation of mixed SAMS.2-10The compositions of mixed SAMs thus depend on the conditionsunder which the adsorptions are performed.

Efrect of Phese Stnrcture within a SAM on lts Wettability. Wemeasured the contact angles of water on the mixed SAMs as afunction of the conditions for their formation and did not observeany obvious correlation between the wettabilities of the mixedSAMs and the nearness of the SAMs to equilibrium (Figure Iin the supplementary material shows relevant information). Forall of the sets of data, the receding angle of water is practicallylinearly related to the composition of the SAM, regardless of theexperimental conditions under which the SAM was made.4,4rUsing a single set of gold substrates to reduce the variation in theroughnesses of the substratcs, we observed that the hysteresisa2between the advancing and receding contact angles of water onthe mixed SAMs increased as the total concentration of thiol wasincreased in the solution, and thus, as the system moved closerto equilibrium (Figure 2 of the supplementary material). Thistrend can be rationalized by an increase in the 'patchiness" ofthe SAM as it goes toward e{uilibrium.qz'rr

Compooition of the SAMs Formed at 60 oC as a Function ofthe Time in Contect with Solution. In Figure 2, we show theeffect of long and short trrnes of immersion on the compositionof SAMs formed at 60 oC from solutions of ethanol with [RSH]r= I mM. These data demonstrate that the compositions of theSAMs do not change considerably by increasing the time insolution beyond I day, althougb thetransition region does sharpenon going from 18 h to 5 days.

Figure 2 also shows that considcrable exchange betweencomponents in solution and components on the surface occursbefore the monolayer has completely formed. The open circlesin this figure represent SAMs formed for 3 s. Although thesemonolayers had not formed completely in 3 s (approiimately807o complete by XPS), the relationship between the compositionof the SAM and the composition of the solution deviatessignificantly from Rseu = Rsoh. We do not know whether thisexchange occurs during a physisorbed state ofthe thiols or at the

The Journal of Physical Chemistry, Vol.98, No" 2, 1994 555

0 0.01 0.1 10 100oosh

a -Hs(cHz)zrcHgl Lg

.,sorn _E-rcH2);OH]

Figure 2. Data for the 'kinetics' of adsorption at 60 oC in ethanolicsolutions with [RSH]1 = I mM. Closed circles: SAMs formed afterbeing immersed for l8 h. Open circles: incomplete monolayers formedafter being immersed for 3 s. Open squares: SAMs formed after bcingimmersed for 5 days.

0 0.01 0.1 1 10 lfi! oosh

a -[Hs(cHz)zrcHsl Lg

,,so,n _lF_srcFilDTI

Figure 3. Data for exchange experiments in ethanolic solutions at 60 oC

for I day with [RSH]1 = I mM. Closed circles: formation of mixedSAMs on underivatized gold substrates. Open circles: gold substrateswith a monolayer derived from HS(CHz)zrCHr after exchange with asolution of a mixture of thiols with composition given by Rro6. Opensquares: gold substrates with a monolayer derived from HS(CHJTTOHafter exchange with a solution of a mixture of thiols with compositiongiven by Rro6.

state of chemisorbed thiolates in incomplete monolayers (systemsin which exchange is probably more rapid than in completemonolayers, due to the lack of lateral stabilization betweenmolecules in the incomplete SAM).

Exchange in Complete SAMs of These Components After IIlay at 50 oC with [RSHh = I mM. To clarify the rates ofexchange between thiolates in SAMs and thiols in solution, wehave qualitatively explored exchange reactions at 60 oC startingfrom different initial points (Figure 3): single-component SAMsderived from HS(CH2)r rOH and HS(CHz)zrCHr were immersedin ethanolic solutions containing different mixtures of the thiolswith [RSH]1 = I mM at 60 "C for I day, and the compositionswere compared to those of SAMs formed on underivatized gold.These results demonstrate that removal of the longer componentfrom a SAM is considerably slower than removal of the shortercomponent. It also shows that even a SAM of the shortercomponent-the easier component to remove from the SAM byexchange-is not completely removed by the longer componentover the length of time used to form the SAMs in Figure lc (atleast for [RSH]r = I mM): approximately l|Vo of the originalSAMs seems to survive exchange under these conditions after Iday"

Thermodynamic Besis for the Phase Behevior in SAMs

Wederivea relationship between the mole fractionof thelongerthiolate in the SAM (xr.J and the ratio of the two thiols in solution(R*rn) based on Bragg-Williams solution theory.*'ls In thisderivation, we only include interactions between nearest neighbors.

[RSH]'= 1 mMT - 6 0 " C

o t = 3 s e cr t = 1 8 h o u rt r t = 5 d a y s

[RSH]'= 1 mMT = 6 0 " Ct = 1 d a y

Start with:O NoSAMo Pure Lgtr Pure Sh

566 The Journal of Physical Chemistry, Vol.98, No. 2, 1994

SCHEME I: Pictorid Representation of the Reference endMixed States Used To Derive the Interaction Parameter

Reference State: Two oeparate SAiis

Mked Stals: Mlred SAM wlth lhc sams composltlon ar tho Retcrenca State

We assume that the thiols in solution are dilute and not interacting.For simplicity, we will also assume that the thiolates have nointernal structure, or more precisely, that the internal structureof the adsorbates is the same whether in solution or in a SAM.This assumption does not correspond closely to reality in theseconformationally flexible systems, but it does not limit the utilityof the treatment.

Thermodynamic Reletionship between 14end R36. This theorydescribes the change of state on forming mixed SAMs fromsolution (eq l).

Lg.o,n + Shsoh * Lgseu + ShsAM (1)

The chemical potential of a thiol in solution is

piorn = u[ + krln y,

Folkers et al.

ZIf^Myil2 and lfiAM = Zdn*xi = ZIt^Mx;, where Z isthenumber of nearest neighbors. Using these relationships, we obtain

rn[ = (Z l 2)[otqLg(xyr)' + c.rs61(Xsr)t + 2rro5Xkx51lIntemal Energyof the System

Esr(6)

where i is either Lg or Sh, yr is the mole fraction of the thiol insolution (/r.s * 756 # I because of the mole fraction of the solvent;lulysn= Rsou),{ rrf it tnt chemical potential of the thiol whenit is at infinite dilution (-yi * 0),4? & is Boltzmann's constant, andI is the absolute temperature of the system. The chemicalpotential of a thiolate in the mixed SAM is

p:nt = t i* + kT ln 1, f a(l - xi)2

from eq 4 and

tijl" = Qlz)lo4*eX1g * @smuXsrl (i)

from eq 5, where energy is presented as the energy per molecule:r = r/(Nil" * 4f"). The change in internal energy on going

*il,ff.separate, single-component monolayers to a mixed

o,H;= ,:15 - "*' =(Zl2)[2rr-esu - rLslg - c,rsun]Xr_rxsl (8)

_SAr{ and we define the interaction parameter o asBmura

d = (Zlz)[2<.,tOo - tDLsLg- osnsr] (9)

We now determine the change in Gibbs free energy for theformation of a mixed SAM (Ar. The general equation forforming a SAM composed of Xre and 156 mole fractions of thelong and short thiolates, respectively, is

Lf = xu7'il" - rfj') + xro(df" - ffl (10)When we insert the definitions of the chemical potentials forthiols in solution and thiolates in the SAM (as given in cqs 2 and3, respectively), we obtain

Af = xre?tr,s* - ul.r) - XyrkT ln i/rg =

xsr0rsr* - pL) - X55ftTln /sr + 1ffir (l l)

where the free energy of mixing of the components in the SAMis

A.Afll, = kT(xuln XLe * xsn ln xso) * -xkxsb (12)

The first term in eq12 is the entropy of mixing; the sccond termis the internal energy of mixing from eq 8. Strictly speahng, forGibbE free energy, the second term in eq l2 should bc the enthalpyof mixing, not the internal energy of mixing. The diffcrencebetween these quantities involves contributions from the equationof state of the SAM that are unknown for thcsc systems. Sincewe have assumed structureless particles confined to sites on alattice, we can assume that these terrns will be negligible as longas the monolayers are complete;a we therefore neglect thedifference between enthalpy and internal energy in our treatment.

No Mixing between Components in th€ SAM. [f we considera systemwhereabsolutely no mixing of the two components o@urs,we obtain the change in free energy for the formation of theSAM:

A/= xr.e(A(Ap) - kTlnR*rn) + (psr' - 4ol - kT ln ysn

( t 3 )where

A(Arr) - Lpt_e- Arrsr - (pur* - pL) - (rl.o'- 4o) (14)

To determine the relationship between Rs66 tnd XLg, we minimizethe free energy with respect to 1pr: if k?" ln Ruo ( A(Ap), thenthe free energy will be a minimum when 1s, = 0; if kln ln R'oro> A(Arr), then the free energy will be a minimum when 11, =

l. For this system, a plot of x1, against ln Rro6, therefore, willbe discontinuous at ln R*6 = A(Nr)lkT (Figure 4).

Mixing between Components in tbe SAM. Determination ofthe relationship between Xr-g and Rro6 becomes more complexwhen we allow the two components to mix in the SAM. In thiscase, we minimize eq I I with respect to the mole fraction of the

(2)

(3)

where p;* is the chemical potential of the thiolate in the single-component SAM, 1 is the mole fraction of the thiolate in two-component SAM (in the SAM, Xr-g * xsh = l),6 and ar is theinteraction parameter, which describes intermolecular interactionswithin the SAM.

The interaction pararneter is derived from the difference ininternal energy between a two-component or "mixed' SAM(^ffi[, e.q 4) and two separate, single-component SAMs

(41t, eq S)-that is, the reference state (see Scheme I). In eqs4 and 5,

*5 = trtr'/dfil + rr*04#f + <.,r651Ffi{l (4)

E*1" = (z lz)trrrr/€ft + rroo$f"l (s)

c.rii is the interaction energy between the molecules i and j (positivevalues of <,ri; correspond to repulsive interactiont), N;ot is thenumbcr of nearest-neighbor interactions of a specific type with-in the SAM, and .fAM is the number of molecules of the typei in the SAM. In the Bragg-Williams approximation, Nf;AM =

| ̂ rr*Y

Phase Behavior of Alkanethiolates on Gold

1 .0 1 .0

0.8

0.2

0.0

Xrn,ro" o.o

Rsorn/€$f = t

Ilgue 4. Theoretical relationship between Rro6 and 14 when no mixingbetw^cc-n the components occurs. The x-axis is plotted ai the ratio of R.or,,to rff't; ff't is the value of R,o6 tnat yiins equimolar quantitiesof the two thiolates in the SAM.

longer component in the SAM,

d(Afi ldyyr= 0 = L(Lp.)l kf- h R*,n *rn(TayrlQ - xk)) + (alkT)(l - 21rr) (15)

After rearrangement, we obtain

R,o,n exp(- L(Ap) lkfi =

kttl0 - xr-g)) exp((olkD(1 - 214)) (16)as the relationship between R34s sDd XLs. We plot our results asthe logarithm of Rro6 against 1q:

ln R*,n = ln(xr.g/(l - xre)) + (@ /kn (1 - 21rr) +

A@rr) lkT (17)The two unknowns in these equations are A(4r) and a; R o6 isrhe experimentally controlled parameter, and 14 is the exper-imentally measured parameter.

If Rro6 = 1, the quantity A(Ltr) /kT is related to the shift ofxlr from XLg = 0.5 and reflects the preference of one componentover the other in the SAM relative to the solution. When A(4r)) 0, the center of the transition region (where xLs = 0.5) willoccur at values greater than R,o6 = l; in this case, the drivingforce for having the shorter component on the surfoce relativeto having it in solution is greater than that for the longercomponent. When A(Ap) ( 0, the center of the transition regionwill shift to values less than Rroh = l; the longer component isfavored on the surface. It is important to remember that A(Ap)is independent of the interaction parameter in this derivation andis only dependent on the standard-state chemical potentials ofthe molecules in the solution and in the SAM (see eq l4). In thispqpg-r, we wil l refer to the quantity exp(A(Ap)/kT) asr$f{'r: it is the value of R,o6 that yields equimolar quantitiesof the two thiolates in the SAM (i.e., the value of Rrom for whichRseu = I at equilibrium).ae

Anelysis of the lnteraction Perameter o. In this section, wegive a physical intcrpretation of values of o in units of k2n Thevalue of the interaction parameter will determine the phasebehavior of the SAMs.

To interpret a, we determine the number of singular points,d ln Rro6/dXLg = 0 (or dpr/d ln Rro6 = - in the way we plotour data), in the relation between ln R o6 and 11, (eq l7)" Wefind the singular points xf, from eq 18.

xiu = t l r* t l r0 -zkTldt /2

The Journal of Physical Chemistry, Yol. 98, No. 2, 1994 X7

X.n,rou o.o

Rsorn/dj|f = tFigure 5. Theoretical relationships betwecn R,o6 and pn determincdusing eq 16. Threc values of the interaction paramcter @ a;re showncorresponding to wcll-mixed SAMs (- = 0), SAMs with critical mixing(o = 2k7'), and SAMs in which mixing of the components is not favored(oo = 3 k7'). The plot of 14 against R1a1 for a = 3& Tsbows the equilibriumrelationship corrcsponding to the minimum energy phascs (solid line)and thecalculated relationship corresponding to metastable statcs (dashedcurve). The minimum was determined using eq I I and the methoddescribed in the text. The x-axis is plotted as the ratio of R1e6 to,ff''.monotonically in the region g ( Xrs ( l; a particular value ofln(R*"/ffiM=r) equals d(xr-J .t only a single value of a (sce,for example, Figure 5 for a = 0). (2) When a = 2kT, there isexactly one singular point at Xie = Xle= 0.5, and the plot of 1qversus Q(xr) becomes vcrtical at XLs = 0.5 (see Figure 5). (3)When w ) 2kT, the plot of 14 versus d(xrJ becomes multivalued(see a = 3kT in Figure 5 including dashed line), and there aretwo distinct -singular points xl, and 11r. Certain values ofm(n*"/4$,1'=r; will cross the-;urve OdL) at these values ofxts.

The physical interpretation of the different regions of a arethese: (l) When the interaction parameter a is less than zero,then interactions between unlike molecules are more favorablethan interactions between like molecules (as might be the case,for example, in a monolayer composed of components containingelectron-donor and electron-acceptor groups). When a is equalto zero, then the mixture is ideal: the components are energeticallyequivalent. When 0 ( o < 2kT, the components prefer to besurrounded by their own kind, but the interaction energy is notsufficient to overcome the entropy of mixing. We, therefore,expectcomplete mixing when a <2kT. (2) If o = 2kT,we havecritical mixing of the two components in the SAM. (3) Whenw ) 2kT, the plot of Xr-g against ln R 66 shows that over a rangeof values of R.oro centered about the midpoint there are threepossible values of xr-s for each value of R ou (Figure 5). Call thepossible values xL, xL, and 1fu. To determine which compo-sition has the lowest free energy, ril€ must evaluate A/(XL),tll31l'1, and A/(1lr) from eq I l. For each value of R,o6, wedetermine the candidate value of xr-g by selecting the value thatyields the lowest free energy. The result is displayed in Figure5 for a = 3kT as the solid line. To sirnplify these calculations,we have assumed that A(Ap) = 0 (i.e., Apq = Apsj, but theconclusion is the same: the equilibrium relationship between lnR3e6_and Xr,g will have a vertical break at the midpoint at R o6= d#-'. The other two compositions (dashed linas in Figurc5) correspond to phases that are metastable; these phases will,therefore, not occur at equilibrium.

In Figure 6, we plot the composition as a function of temperatureto show the calculated phase behavior of this system. In this plot,we take an interaction parameter equal to 3kT at room tem-p€rature (a = 1.8 kcal/mol) as a reference point. We presentthis plot for two reasons: Figure 6 shows the relationship betweenthe interaction parameter and temperature (left and right axes),and it also shows the relationship between the compooitions ofthemost stable phases in the SAM (solid curve) and tempcrature(and, consequently, the value of the interaction parameter). Note

( 1 8 )

There are three regimes defined by values of a. For simplicity,we rewrite eq l7 as eq 19,

ln(R-r/Rfll"M-t) = ln(1rrl(t - xr-s)) + @lkn (l - 2xre)= Q(xyg\ (19)

(l) When o 1 2kT, a plot of Xrg versus d(Xr.J increases

G t = 3 k T a t T = 2 9 5 K

55t The Journal of Physical Chemistry, Vol.98, No. 2, 1994

tl00

T gso

300

2fi

zx)0.0 o.2 oi 0.6 0.8 1.0

/1,Lg,sAM

Figure 6. Phase diagram for two-component SAMs if the interactionparameter is equal to 3kT at room temperature (a = 1.8 kcal/mol at295 K). The left side of the plot shows the temperature in kelvin; theright side shows the corresponding interaction parameter"

that the compositions in the area beneath the curve (the linedarea) of Figure 6 will never be formed at equilibrium when SAMsare formed from solutions containing an excess of the two thiols.

Discussion

It Is Nearly Impossible to Form e Mixed SAM with Phase'Seperated Islands if the SAM Has Resched Equilibrium with aSolution Containing en Excess of the Two Thiols. The mostimportant theoretical conclusion from this paper is that when amixed SAM is in equilibrium with a contacting solution containingan exccss of the two components, two phases will coexist in thatSAM only if R,o,, = 4!,!=I and - > 2kT; for all other valuesof R*1o-regardless of the value of a-only one phase will existin the SAM at equilibrium. When a is greater than the criticalvalue, the predicted range of values of R6n that will permit theexistence of phase-separated domains is so narrow as to beexperimentally unachievable. At equilibrium, formation of phase-separated SAMs over a wide range of values for Rro6, therefore,suggests that the SAMs have not reached equilibrium.

Two-component Langmuir monolayers can contain a mixtureof two phases because the composition of the monolayer can becontrolled exactly and fixed27 and because the mechanism ofequilibration is lateral diffusion of the components in the planeof the monolayer.zs Two-component SAMs could similarly formtwo phases if they were isolated from a thiol-containing solutionand if lateral diffusion occurred by some .rc1t.nitot.2E3e InSAMs, the composition is determined by the interactions withinthe SAM; since the solution generally contains an exc€ss of eachcomponent, only a single-phase SAM will result at equilibrium.

Are the Mixed SAMS in Figure I at F4uilibrium? From previousstudies dealing with the kinetics of exchange in completemonolayers,lo'2l *. conclude that mixed SAMs formed fromethanolic solutions containing HS(CH2)21CHr and HS(CH2) r r-OH at rmm temperature are not at equilibrium, because exchangebetween thiolates in the SAM and thiols in solution is slow andincomplete at room ternperature.l02l Mixed SAMs are oftenformed at room temperature; we infer that none of these SAMsare at equilibrium.

SAMs formed at temperatures higher than room temperature(for the studies in this paper, f > 60 oC) from solutions with highvalues of [RSH]1are probably closest to equilibrium. The SAMsin Figure la are, however, not at equilibrium because of the shortimmersion time and the low concentration of thiol.3? The resultsin Figure la correspond to kinetically trapped, metastablecompositions.

Folkers et al.

One result that suggests that the SAMs in Figure lc may beclose to equilibrium compositions is that the relationship between

Xr-g and Rroln is nearly the same whether the SAMs were formedfrom solutions with [RSH]r = I mM or from solutions with

[RSH]r = l0 mM. This result suggests either that the monolayershave reached stable compositions or that the exchange rate is notdependent on the concentration of thiol in solution. At roomtemperature, the rate-limiting step of exchange between thiols insolution and thiolates in a SAM is predominantly desorption ofspecies from the surface, although there is still a significant(although less than first-order) dependence of the rate of exchangeon the concentration of thiol in solution at room temperature.l0We have also observed a dependence of the rate of exchange onthe concentration of thiol in solution at 60 oC.e (We note that.even after long times of exchange, there could be molecules withinthe SAMs that have not exchanged; kinetically trapped thiolateshave been observed in SAMs at room temperature.lo)

The results in Figures 2 and 3 suggest also that the SAMs inFigure lc (at least those for [RSH]1 = 1 mM) are close to, butprobably have not fully reached, equilibrium. In Figure 2, onlya srnall change in the sharpness of the transition region wasobserved on changing the time in solution from 18 h to 5 daysfor SAMs at 60 oC. Figure 3 shows the thiols in solution do notexchange completely with thiolates in a complete SAM after Iday at 60 "C with [RSH]I = I mM.

Recent Studie Have Suggested tbe Existeoce of Phase'Separated llomains within Mixed SAMs. In other, independentexperiments, we have inferred evidence that SAMs formed under*normal'conditions (room temperature. [RSH]I = i mM, Iday) consist of regions of different phases.5 In these cxpcriments,polarized infrared external reflectance sp€ctrmcopy (PIERS) wasapplied to mixed SAMs containing long- and short-chain thiolatesformed under normal conditions.s0 The frequencies and relativeintensities of the stretching vibrations of the methy'l and methylenegroups in the molecules indicated that the SAMs contained bothregions where the outer parts of the longer chains were in a liquid-like state and regions where these outer parts \rere in a crystal-like state. The observation of crvstalline regions suggests thatthese SAMs consist, in part, of islands.

The results from the PIERS srudy'are conslsrent with imagesof mixed SAMs collected with a scanning tunneling microscope.lESeveral experimental results suggest that the SAVs formed inthese studies were not at equilibrium. Exchange bctween thiolatesin the SAM and thiols in solution is slo* and incomplete at roomtemperature and becomes slower as the alkl l group on the thiolateis lengthened.l0 This exchange is slower on gold substrates thatare flatter than the normal polycrl'stalline samples often em-ployed.33 The effect of the slow exchange couid bc the formationof islands as the SAM go€s toward equilibrium ( both with solutionand internally).

The nature of these islands, and of the processes that formthem, remains undefined. One possibiiity, suggested by thetheoretical conclusion from this work that multiple phases arenot likely to coexist at equilibrium, is that the islands representsingle-phase, single-component *near-equilibrium' regions in asea of a different average composition that is initially formedkinetically. The kinetically formed regions could contain mixturesof the two thiolates, and, as a consequence, would be disordered;the "near-equilibrium" regions would be formed where surfacefeatures (step, defects, ...) would favor exchange of thiolates inthe SAM with thiols in solution.

Conclusions

This study concludes that mixed SAMs formed from theadsorption of a long-chain and a short-chain alkanethiol ontogold, and allowed to equilibrate completely in contact with asolution containing these thiols, will exist as single phases atequilibrium. This single phase may contain both thiolates orpredominantly (or exclusively) one, depending on the magnitude

Phase Behavior of Alkanethiolates on Gold

(:ml$emrbnsFLF

br|trdhblt atLlap.rdlngm|dt'

The Journal of Physical Chemistry, Yol. 98, No. 2, 1994 569

An important question that remains unresolved from this workconcerns completely mixed systems. We have not been able toprepare systems that correspond to kinetically trapped, mixedphases. Although Figure la approximates the behavior expectedfor such as system, the SAMs are incomplete. Why arewe notable to form complete, mixed SAMs? We suggest that formationof the SAM can be considered to proceed in at least three steps.The first is physisorption of alkanethiols (RSH) on the surfaceof the gold.s' Dialkyl sulfides (RSR) adsorb on gold;s2 alkane'thiols should also be able to adsorb to gold without breaking theS-H bond.s2 We expect exchange of alkanethiol in solutionswith alkanethiol physisorbed on the surface to be relatively fast.The second step is conversion of physisorbed alkanethiol tochemisorbed gold(I) alkanethiolate, with loss of the hydrogenfrom thethiol (presumably as H2, although its presence has neverbeen demonstrated experimentally). This step might be expectedto be relatively slow. The third step is exchange of alkanethiolin solution with alkanethiolates on the surface. This process mightproceed by the microscopic reverse of the first and second stepsor by some independent process (such as the desorption ofdisulfidessr or the desorption of gold thiolatesa3). Depending onthe relative rates of thesevarious processes, it would, in principle,be possible to achieve a range of behaviors connecting thecompositiion ofthesolution and thecomposition of the monolayer.We infer that the rates of all three steps are comparable, althoughthe third is probably the slowest. In particular, we hypothesizethat significant equilibration occurs at the stage of physisorbedthiols, before formation of the chemisorbed thiolates.

Experimental Section

Preparation of Substrates. Gold substrates were prepared byelectron-beam evaPoration of -100 A of chromium (Aesar;99.99Vo) followed immediately by evaporation of -2000 A ofhigh-purity gold (Materials Research Corp.: Orangeburg, NY;99.9999Vo\ onto single-crystal silicon (100) test wafers (SiliconSense: Nashua, NH; 100-mm diameter, -500 pm thick).Chromium was used as an adhesive layer between the gold andthe native oxide on the silicon. The metals were evaporated ata rate of 5 A/s onto substrates that were not heated-3l The gold'coated silicon wafers were cut into - I cm x 3 cm slides with adiamond-tipped stylus before being put into solution.

Formation of SAMs. Adsorptions were carried out either in20-mL single-use glass scintillation vials or in 25'mL glassweighing bottles. All adsorptions above room temperature werecarried out in glass weighing bottles. Prior to each experiment,the weighing bottles were cleaned with "piranha solution" (7:3concentrated HzSOn/3}VoHzO) at -90 oC for 30 min, rinsedwith deionized water, and dried in an oven at 200 oC for at least12 h .

WARNING: Piranha solution should be handled with caution;in some circumstances (most probably when it had been mixedwith significant quantities of an oxidizable organic material), ithas detonated unexpectedlY.s3

Docosanethiol and I l -mercaptoundecanol were available fromprevious studies;2-s absolute ethanol (Quantum Chemical Corp.or Pharmco Products, Inc.) was used as solvent for all adsorptionsand was deoxygenated with N2 prior to use. Solutions with

IRSH]I = 10 mM were made by weighing the appropriate amountof each thiol into glass weighing bottles' Solutions with IRSH]1= 0.1 mM or I mM were Prepared by dilutions of appropriateamounts of docosanethiol and I l-mercaptoundecanol from freshlyprepared stock solutions. Solutions with IRSH]r = 0.01 mM orI pM were prepared by dilution of 0.1 mM solutions. Adsorptionsat high temperatures were carried out in a temperature-controlledoil bath; the temperature of the oilbath was controlled to within+1 oC by a Dyna-Sense temperature controller (Cole Parmer).The solutions were equitibrated in the oil bath for at least 2 hbefore addition of the slides. After removal from solution, theslides were rinsed with ethanol and dried in a stream of N2.

crrertqullbdum'lrngrulimo.ro(rt rd|.tbtnl

alttuab.l

o oo

o o j oo -o

ffi:Inltldlttrun.o|Utbo

*cromlSAI'$r'rd|il. !f il!|ovd

trfir roldonlollo'.d byLard 6lth,'bn

t"rgtre 7. Schematic ilhstration of the differencc bctween the predominantmechanisms of equilibration in two-component Langmuir monolayersand twocomponent self-assembled monolayers of alkanethiolates on gold.The top illustratcs equilibration in twocomponent Langmuir films, whichoccurs by lateral diffusion of the molecules in the plane of the monolayer;note that the composition is not change during cquilibration. The bottomillustrates equilibration in two-component self'assembled monolayers ofalkanethiolatcs on gold, which occurs predominantly by exchange withspccies in solution; note that in solution, the composition of the SAMchanges during equilibration. We have implicd a large magnitude of oin favor of the &rkcr spccies for illustrative purposes.

of interactions between *like' and 'unlike" pairs of thiolates inthe SAM. The important point is that (unlike Langmuirmonolayers at the air-water interface) two-component SAMswill not phase separate into islands of different compositions atequilibrium under these conditions.

Since most of our SAMs are not at e4uilibrium, they maycontain two phases: the composition of one phase could be thecomposition obtained at e4uilibrium; the compoaition of the othercould be random, but would probably be closer to an initial,kinetically formed composition. As a SAM approaches equi-librium, islands of the equilibrium composition would grow until,at equilibrium, the whole SAM was that composition.

The essential difference bctween SAMs of alkanethiolates ongold and Langmuir monolayers is the different processes leadingtoward equilibrium. In a Langmuir monolayer, the overallcomposition of the monolayer is fixed when it is first spread anddoes not change thereafter; equilibrium is reached by lateraldiffusion, which is fast within the monolayer on the time scaleof the experiment (FigureT).2s By contrast, SAMs on gold appearto require equilibration with a solution containing alkanethiols,and evidence for any lateral diffusion is still inferential.3s Thus,for Langmuir monolayers, equilibration reaches an equilibriumstate of fixed composition by lateral diffusion; for SAMs,equilibrium is usually approached experimentally by equilibrationwith an infinite reservoir of thiols. It usually involves a changein composition of the monolayer and does not seem to involverapid lateral diffusion (see Figure 7). If Langmuir systems couldbe devised that would equilibrate the monolayer with a solutionreservoir containing its components, the analysis developed hereshould apply to the final equilibrium state; if a method could befound to promote lateral diffusion within an alkanethiolate SAMongold, itcould bc removed fromcontactwith itssolution reservoirof alkanethiols and would reach an equilibrium with phase-separated islands.

Lateral diffusion of the thiolates could only affect the finaldistribution of thiolates in a monolayer if the SAM were removedfrom solution before it had reached equilibrium with the solution.The SAM could then reach an equilibrium distribution analogousto that observed in Langmuir monolayers by way of lateraldiffusion. For a SAM in contact with a solution containing thiol,the equilibrium structure will always contain one phase even iflateral diffusion is fast, because no matter what lateral diffusionleads to, thc system can lower its energy by becoming a singlephase (Figure 7).

o ooO O ^

L'', \'/

" o- f f i .

PhD-af,ailldn{Dds lb.rl.tlybb'd

o e oO O n

\-, v

o - a' m

Tgullbdsm- SAI olFS/Au tom.d trom

I

570 The Jourrul of Physical Chemistry, Vol.98, No. 2, 1994

Ctencterization of SAMs. All SAMs were characterized usingcontact angles and X-ray photoelectron sPectroscopy (XPS). Theapparatus and method for the measurement of contact angleshave been de.scribed previously.lso

X-ray photoelectron spectra were taken on the same slides asused for the contact angles within several days after removalfrom solution; slides were stored in wafer trays (Fluoroware)prior to characterization by XPS. Spectra were collected on anSSX-100 spectrometer (Surface Science Instruments) usingmonochromatic Al Kc X-rays. The'spot size" was either 600or 1000 pm, and the pass energy on the detector was either 50or 100 eV. For quantitation of the individual elements in theSAMs, we used the 4f doublet for gold ('Au(4f)" at 84 and 87eV for 4f7p and 4f 5p, resfictively) and the ls peala for carbonandoxygen ('C(ls)" and'O(1s)" at 284and 532eY ,respectively).Two scans were taken for both gold and carbon, and 10 scanswere taken for oxygen (acquisition time for one scan wasapproximately 2 min). Spectra were fitted using an 80VoGaussian/20Vo Lorentzian peak shape.

Determination of the Composition of the SAMs Using XPS. Todetermine the compositions of the mixed SAMs, we used thenatural logarithm of the Au(4f) pcak because it is directly relatedto the thickness of the hydrocarbon layer (ds6) of a two-componentSAM through e2l.st Equation 21 comes directly from eq 20,which models the inelastic scattering of the photoelectrons

Au(4|)rn" = Au(40d exp(4r"1(I sin d)) (20)

ln Au(4f)rAM = -dncl(), sin 0) * constant (21)

through the hydrocarbon layer as an exponential decay. In theseequations, Au(4f)o is the intensity of the Au(afl peaks for a baregold substrate, p is a term for attenuation due to the sulfur atomon each thiolate, tr is the inelastic mean free path of thephotoclectrons through the hydrocarbon layer (40 Al,sr'ss and 4is the angle between the plane of the substrate and the detector(35') .

In terms of XLs, we write eq 2l as

ln Au(4f)rAM = -(xyr(drr- dsJ + dsh)/(I sin {) +

constant (22)

where dk and ds5 are the thicknesses of the hydrocarbon layersfor the single-component SAMs. After relating dygand dsl toln Au(4f)r, and ln Au(4f)s1, respectively, using eg,zl, andsubstituting these relationships into q,22, we obtain

xr, = (ln Au(a0sAM - ln Au(4f)*)/(ln Au(4f)t, -

ln Au(4f)ru) (23)

as the relationship between Xr-g and the experimentally measuredquantities.

Acknowledgment. We thank our colleagues Nicholas Abbott,Hans Biebuyck, and Mathai Mammen for useful discussions andcomments. Professors David Allara and Paul Weiss of PennState provided us with a copy of ref 18 and with valuablediscussions of phase separation.

Supplementary Material Aveilablq Figure I showing plots ofadvancing and receding contact angles of water and Figure 2showing the hysteresis between the advancing and receding contactanglesof water as a function of the composition of the monolayersfor the six sets of SAMs shown in Figure I (4 pages). Orderinginformation is given on any current masthead page.

References and Notes

( I ) This rescarch was supported in part by the Oflice of Naval Research,the Dcfensc Advanced Research hojccts Agency. XPS spcctra were obtainedusing instrumcnt facilitics purchascd undcr the DARPA/URI program andmaintaincd by thc Harvard University Matcrials Rescarch Laboratory.

Folkers et al.

(2) Au: Bain, C. D.; Evall, J.; Whitcside, G. M. J. Am. Chem. Sor.l9t9,IIl,7155_7164. Cu, Ag, Au: kibinis, P. E.; Whitcsidcs, G. M. J. Am.Chem. Soc. 1992, I 1 4, 199|u_.1995.

(3) Bain, C. D.; Whitcsidcs, G. M- J. Am. Chem. Soc. 19tt, 1i,0,3655-3666. Bain, C. D.; Whitcsidas, G. M. Science (Washington, D.C.) 19|&i.,240,62-63. Bain, C. D.; Wbitcsidcs, G. M. J. Am. Chem..Soc. 19t9, II l,7164-'1r75.

(4) Folkers, J. P.; Laibinis, P. E.; Whitcsidcs, G.M. Iangmuir 1992,8,l33 r l34 l .

(5) Laibinis, P. E.; Nuzzo, R.G.; Whitcsidcs, G. M. J. Phys. Clvm.t992, 96,5097-5 I 05.

(6) Bertilsson, L.; Licdbcrg, B. Langmuir 1993, 9, l 4l-149.(7) Bain, C. D.; Whitesidcs, G. M. Langmuir l9t!1, J, 1370-1378.(8) Palc-Grosdemange, C.; Simon, E. S.; Primc, K. L.; Whitcsides, G.

M. J. Am. Chem. Soc. 1991, I13, 12-20. Prime, K. L.; Whitcsidcs, G. M.Science (Washington, D.C.) 1991, 252, I 16+1167.

(9) Ulman, A.; Evans, S. D.: Shnidman, Y.; Sharma, R.; Eilers, J. E.;Cbang, J. C. J. Am. Chem. Soc. 1991, /lJ, 1499-1506. Sanassy, P.; Evans,S. D. I-angmuir 1993, 9, 102+-1027.

(10) Chidscy, C.E. D.; Bcrtozzi, C. R.;Putvinski, T. M.; Mujscc, A. M.J. Am. Chem. Soc. 1990, II2,43014306. Collard, D. M.t Fox, M. A.Langmuir 1991, 7, I192-1 197.

( I I ) Hickman, J. J.; Ofcr, D.; Laibinis, P. E.: Whitsidcs. G. M.; Wrigbton,M. S. Sciezce (Washington, D.C.) 1991, 252,68H91. Chailapakul, O.;Crooks, R. M. I^angmuir 1993,9, 88zt-888.

(12) Rowc, G. K.; Creagcr, S. E. I-angmuir 1991,7,230'7-2312.(I3) Forreviews,se€: Bain, C. D.; Whitcsidcs, G. M. Angau. Chem., hu.

Ed. Engl.19t9, 28,506-512. Whitcsidcs, G. M.; Laibinis, P.E. I-angmuir1990, 6, 87 -96. Ulman, A. An I nt rod uc t i on t o U I t r at h i n Argani c Fi I ms F romLangmuir- Bldgett to S elf- As sembly;Acadcmic hess; San Diego CA, I 99 I ;Chapter 3.

(14) Although mixcd SAMs have not bccn systcmatically studicd rsingthe SEM, sevcral reccnt studics havc shown that the SEM can bc uscd todetect lithographed patterns of different thiolates in SAM on small lengthscalcs; see: lJip.zc. P.; Bicbuyck, H. A.; Whitcsidcs, G. M. Langmuir 1993,9, l5l3-1516. Wollman, E. W.; Frisbie, C. D.; Wrighton, M. S. Langmuir1993, 9, I 5 I 7-l 520. L6pea G. P.; Biebuyclq H. A. ; Frisbic, C. D.; Whitcsidcs,G.M. Science (Washington D.C.) 1993, 2&,647 449. l(r'rna1, A.; Whitesides,G.M. Appl. Phys. Iztt., acccpted for publication.

(15) A Monte Carlo simulation indicates that mixcd SAMs containingmixtures of a thiolates with long chains ({(CHr)reCHr) and thiolatcs withshort chains ({(CHreCH3) will phasc separate on Bold if allowcd to cquilibratefully by lateral diffusion in the plane of the monolayer: Sicpmann, J. I.;McDonald, l. R. Mol. Phys. 1992, 75, 251259.

(16) In a recent study of the polymcrization of styrene-psulfonatc onammonium-terminatcd monolayers. phasc scparation of twlcomponentmonolayers dcrived from an ammonium-tcrminatcd disulfidc and a pcrflu-orinatcd thiol was inferred from the inability of the mixcd SAMs to blockelectron transfer betwecn Fe(CN)o| in solution and the undcrlying goldelectrode after polymerization: Niwa, M.; Mori, T.; Nigashi, N. J. Mater.Chem. 1992, 2,245-251. In this study, thc authors polymcrizcd the styrenederivative on top of the SAMs using UV photoinitiation and examincd thcability of the films to block elcctron transfer. Whcn thc mixcd SAMs wcrcuscd, they observed that incrcascd photolysis dccrcascd thc ability of films toirnpcde electron transfcr. Thc results in another reccnt papcr sug,g6t that thcobservations of Niwa et al. may not bc due to phase separation but rathcr topartial removal of the SAM by UV-promoted oxidation of the thiolate groups:Huang, J.; Hemminger, J. C. J. Am. Chem..Soc. 1993, II5,3342-3343.

(17) Domains in phase-scparatcd Langmuir-Blodgctt films have becnimaged using the atomic force microscope (AFM) and a frictional forccmicroscopc (FFM), a modification of an atomic forcc microscopc. AFM:Chi, L. F.; Andcrs, M.; Fuchs, H.; Johnston, R. R.; Ringsdorl H. Science(Washington,D.C.)1993,259,213-216. FFM: Ovcrney, R. M.; Meyer, E.;Frommer, J.; Brodbeck, D.; Luethi, R.; Howald, L.; Guenthcrodt, H. J.;Fuijihira, M.; Takano, H.; Gotoh, Y . Nature (London) 1992, 359,133-135.

(18) Stranick, S. J.; Wciss, P. S.l Parikh, A. N.; Tao, Y.-T.; Allara, D.L., private communication.

(19) Haussling, L.; Michel, B.; RinSsdorl H.; Rohrcr, H. Angav.Chem.,Int. Ed. Engl.1991, 30,569-572.

(20) Au: Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 19t3, /05,4481-4483. Portcr, M. D.; Bright, T. B.: Allara, D. L.; Chidscy, C. E. D. J"Am. Chem. Soc. 19t7, j,09,3559-3568. Bain, C. D.; Troughton, E. B.; Tao,Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 19t9,I I1,321-335. Ag: Walczak, M. M.; Chung, C.; Stolc, S. M.; Widrig, C.A.;Porter, M. D. J. Am. Chem. S oc. 1991, I I 3, 237 0-237 8. Cu, Ag, Au: Laibinis,P. E.; Whitcsides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo,R. G. J. Am. Chem. Soc. l99l , 113,1152- '1161.

(2 I ) Laibinis, P. E.; Fox, M. A.; Folken, J. P. : Whitcsidc, G. M. I-angmuir199r, 7,3167-3173.

(22) Transmission clectron diffraction: Strong, L.; Whitcidcs, G. M.I-angmuir 198t, 4, 546-55t. Helium atom diffraction: Chidsey, C. E. D.;Liu, G.-Y.; Rowntrec, P.; Scoles, G. J. Chem. Phys.19t9,9/,,4/21423.Infrarcd spectroscopy: Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem.Phys. 1990, 93,767-773. Infrarcd spcctroscopy and clectron diffraction:Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G../. Chem. Phys.1993,98,678-6E8. X-ray diffraction: Samant, M. G.; Brown, C. A.; Gordon, J. G., II.Langmuir 1991,7,437439. Fcnter, P.; Eiscnbcrgcr, P.; Li, J.; Camillonc,N., III; Bernaseh S.; Scole.s, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir1991,7,2013-2016. Fenter, P.; Eiscnbcrger, P.; Liang, K.S. Phys. Rea.Izu.1993,70,2447-2450. Scanning tunncling microscopy: Widrig, C. A.; Alves,C. A.: Portcr. M. D. J, Am. Chem. ̂Soc. 1991, I13,2805-2E10. Atomic forcc

Phase Behavior of Alkanethiolates on Gold

micrccopy: Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc-l9tt2, It4, t222-t227.

(23 ) For studies on the structures and propcrtics of monolayers with varioustail groups, scc: Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem.Soc. 199t0, 112,558-569. Dubois, L.H.;Zrgarski, B. R.; Nuzzo, R. G. "/.Am. Chem. Soc. 1990, I12, 57G579. Chidsey, C. E. D.; Loiacono, D. N.Langmuir 1990, 6, 682491.

(24) For molecular dynamics calculations on the structurcs and propcrticsof singlc-component SAMs, scc: Hautman, J.; Klein, M. L. J. Chem. Phys.ly$r, 9 I, 4994-500 l. Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989,5, | 147 -ll 52. Hautman, J.; Bareman, J. P.; Mar, W.; Klcin, M. L. J. Chem.Soc., Faraday Trarc. 1991, 87, 2031 -2037. Hautman, J.; Klcin, M. L. Phy s.Rea. Iztt. 1991, 67, 17 63-17 66.

(25) For a diffusion coefficient of 2 x l}? cm2ls for a molecule in aLangmuir monolayer (determined electrochcmically at 28 oC for the diffusionof N-octadecylfenocenccarboxamide at the air-water interfacc in a monolayerwith a void volume of 4 Az/molccule; see: Charych, D. H.; Landau, E. ilf.;Majda, M. J. Am. Chem. Soc.l99l, I13,334$-3346),molecules diffusc 0.1pm in 250 ps.

(25) Phasc-separated domains within Langmuir monolayers have bcenobscrved dircaly with the use of the cpifluorescence microscope: VonTscharncr, V.; McConnell, H. M. Biophys..f. l9tt, 36,409419. Losche,M.; Sackmann, E.; M6hwald, H. Ber. Bunsenges. Phys. Chem.1983,87,84E-E52. McConnell, H. M.; Tamm, L. K.;Weis, R. M. Proc. Natl. Acad.Sci. U*S.l. t964,8I, 32413253. Weis, R. M.; Mc€onnell, H. M. Nature(London) lJn4,310,4749. Heckl, W. M.; M6hwald, H. Ber. Burcenges,Phys. Chem. lgtff.,90,3249-3253. Chi, L. F.; Johnston, R. R.; Ringsdorf,H. Langmuir 1991, 7,2323-2329.

(27) Phasc scparation has been inferred for two-component Langmuirfilmscontaininga phcphatidylcholine anda small mole fraciion of cholesterol.For cxamples, scc: Pagano, R. E.; Gcrshfeld, N. L. J. Phys. Chem.1n2,76,1238-1243. Gcnhfcld, N. L.; Pagano, R. E. J. Phys.Chem.l972,76,t24+-1249. Tajima, K.; Gcrshfeld, N. L. in Monolayers;Goddard, E. D.; Ed.; ACSAdvanccs in Chemistry Series 144; American Chemical Society: WashingtonD.C., 1975;pp 165-176. Subramaniam, S.;McConncll, H. M. I. Phys. Chem.19t7,91, 1715-1719. Hcckl, W. M.; Cadenbead, D. A.; Mohwald, H.Itngmuir,l9tt, 1352-1358. Ricc, P. A.; McConnell, H. M. Proc. Natl.Acad. Sci. US.A. 19t9, 86, 64/,54448.

(28) Although the lateral diffusion coefficients for self-assembled mono-layen of alkancthiolatcs on gold have not becn determined experimcntaily,lines of alkanethiolatc on gold as small as 0.10 pm wide show no evidence ofspreading over a period of several hours at scales of 0.1 pm (Abbott, N. L.;Biebuyck, H. A.; Kumar, A.; Lopcz, G. P.; Whitesides, G. M., unpublishedobscrvations). From comparisons of thesc obs^ryations with those charac-terizing the latcral diffusion cocfficients in langmuir monolaycrs,s we inferthat latcral diffusion is probably not thc predominant mechanism ofcquilibration in mixcd SAMs of alkanethiolates on gold.

(29) In a rec€nt study, self-asscmbled monolayers derived from octa-decancthiol wcre uscd as masks for lithography with thc scanning tunnelingmicrccopc (STM). Pits with dimensions of 25 nm x 25 nm were etched inthe monolayer with the STM; thesepits'were dimensionally stable forar leasrseveral days", implying that lateral diffusion ofthe thiolates on the gold is veryslow (unless thc ctchcd areas wcre damaged during thc etching process): Ross,C. B.; Sun, L.; Crook, R. M. Langmuir 1993,9, 632436.

(30) In rcf 5, mixcd SAMs were formcd at room temperature from solutionscontaininS mixtures of HS(CHz)21CH3 and HS(CD2[rCD3 and solutionscontaining mixtures of HS(CDJ;7CD3 and HS(CH2),1CH3. Since thesecomponcnts havc lcngths similar to thce used in the prescnt study, we wouldexpect thcir bchavior to be analogous to the behavior of the SAMs formedundcr similar conditions in this studv.

(31) Thc morphology of thc gold substratcs should affect thc properriesof a SAM and may affect the distribution of thiolates in mixed SAMs thathave not reached cquilibrium. In this study we used substrates formed by theevaporation of gold onto silicon wafcrs that had been primed with t00 A ofchromium as an adhcsion layer betwecn the gold and the oxide layer on thesilicon. Substrates formcd under these conditions consist of mounds of goldthat are approximately 100 nm across and 2G-30 nm high.a

(32) Topology could affect the rate of exchange between thiolates in theSAM and thiols in solution. Anncaling the gold during (or after) cvaporationqanlead toggld substrates with smoothertopologies: Halhnar( V. M.,Chiang,S.; Rabolt, J. F.; Swalen, J. D.; Wilson, R. J. Piys. Rea. Lctt.lgt7, 5r,2879-2882. Chidsey, C. E. D.; Loiacono, D.N.; Sleator, T.; Nakahara, S.Szy' Sci. 19|dli., 200,4546. hrtnam, A.; Blackford, B. L.; Jericho, M. H.;Watanabe, M. O. Sul Sci. 19t9, 217,27G28E. Vancca, J.; Reiss, G.:Schncider, F.; Bauer, K.; Hoffmann,H. Surf. Sci. 19t9, 218,108-126.

(33) Exchange bctwcen thiolates in tbe SAM and thiols in solution isslower for SAMs formed,on gold substrates that werc ctched with aqua regiaprior to formation of thc SAM than on similar substrates that were not etchld;this effect was attributed to a lower numbcr of defects in thc ctched goldsubstrates: Crcager, S. E.; Hockctt, L. A.; Rowe, G. K. Langmuir lg9r,8,85fE6l.

(34) Examplcs of studies showing the diffusion of atoms of Au on annealedsurfaccs: Jaklcyic, R.C.; Elie, L. Phys. Reo. Lett.19tt,60, l2Ft23. Emch,!1; !.{9ggmif .; Dovck, M. M.; Lang, C. A.; Quatc, C. F. J. Appl. Phys.19t9,65,7944. Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Flrys. Reu. Iztt.19t9,62,929-932. Sommerfeld, D. A.; Cambron, R. T.; Bccbe, T. P., Jr.J. Phys. Chem. 1990, 94,89264932. Holland-Moritz, E.; Gordon, J., II;Borgcs, G.; Sonncnfcld, R. Iangmuir 1991,7, 301-306. Gimzewski, J. K.;Bcrndt, R.; Schlittlcr, R. R. Phys. Reu. B 1992, 45, 68444957.

The Journal of Physical Chemistry, Yol. 98, No. 2, 1994 S7l

(35) Wciss, P. S.; Allara, D. L., private communication.(36) Wc havc dctcrmined the 'completeness' of a SAM from thc diffcrence

in the thickncss bctween thc two single-componcntSAMs asdctermincdusingxPs.4,r

(37) If we leave the gold substratcs in thesc solutions for longcr than I h,the SAMs begin to favor the longer componcnt and the transition regionbegins to sharpen.

(38) Whcn R.ou ) - 0.2, the monolaycr is composcd largcly of the longer{{Pone nt (X' s "ru

= I ); when &"b < 0.07, the SAMs are compccd largelyol the sbortcr component (Xra55r = 0)._ (1? The individual sets of data in Figure lc for [RSHJT = I mM and

IRSH]r = l0 mM are considerably sharpcr than the two scs appcar to bcwhen combincd. we combined the two sets because they were closc enoughto believe that the difference was only due to a differcncc in the prcparationof the solutions.

(401 We hayg not included data for SAMs for at room tempcrature with[RSH]1 = I pM bccausc after I day all of the SAMs had not formcd complctclyenough to give a consistent set of XPS data.

(4 I ) Folken, J. P.; Laibinis, P. E.; Whitesidcs, G. M. J. Adhes. S ci. Technol.1992, 6. 1397-1410.

(42) Hysteresis in contact anglcs remains a poorly undcrstood phcnomcnon,although it is commonlyattributed tothe chemical hetcrogcneityand roughnessof an interface. For a conceptual reyiew of contact angles and hystcrcsis, scc:de Gennes, P. G. Reu. Mod. Phys. lgti, 57,821463. For other studics onhysteresis, see: Johnson, R. E.,Jr.; Dettre, R. H. In Coztact Angle,Wettability,and Adhesion; Gould, R. F., Ed.; ACS Advances in Chemistry Scrics 43;American Chemical Society: Washington, D.C., 1964; pp I l2-135. Dcttre,R. H.; Johnson, R.8., Jr. InContact Angle,Wettability,and Adhesion;Gould,R. F., Ed.; ACS Advances in Chemistry Scrics 43; Amcrican ChcmicalSociety: Washington, D.C., 1964; pp l3Gl44. Joanny, J. F.; dc Genn6,P. G. J. C he m. P hy s. l%4, 8 I, 5 52-562. Schwarta L. W .; G aroff,S. Langmuir,1985, /, 219-230. Andrade, J. D.; Smith, L. M.; Gregonis, D. E.In Surfaceand Interfocial Aspects of Biomedical Polymers; Andradc, J. D., Ed.; PlcnumPress: New York, 1985; Vol. l, pp 249-292. Chen, Y. L.; Hclm, C. A.;Israclachvili, J. N. J. Phys. Chem.1991,95,1073G10747.

(43) We havc not eliminated the possibility that thc exchange proc6s6arc roughening the surfaccs by dissolution of gold atoms on the surface. Arecent study using the scanning tunneling microscnpe has shown that goldsurfaccs are roughcned after cxposure to thiol fron :hanol. Gold was alsodetected in the solution after adsorption for 24 h; tL. -mount of gold foundin solution corresponded to approximately 50Vo of a monolayer: Edinger, K.;G6lzhiuser, A.; Dcmota, K.; W6ll, Ch.;Grunzc, M. Langmuir1993,9,4-8.

(44) Brage, W. L.;Williams, E.J. Proc. R. Soc. IatdonlgK,I45,699-730.

(a5) The Bragg-Williams approximation has becn uscd to model thcadsorption isotherms of single- and twecomponent gas6 and surfactants. Forexamples, see: Jaroniec, M.; Piotrowska, J. Colloid Polymn. Sci.lg|d/0., 258,977-979. Retter, U. J. Electroanal. Chem.19t7,236,21-30. Cases, J. M.;Villicras, F. Iangmuir 1992, 8, 125l-1264.

(46) In eqs 2 and 3 we are making the approximation that thc mole fractionsof the component in solution and in the SAM and thcir activitics are thesame-that is, we are assurning activity cocfficicnts of l.

(47) This "standard state' is for theoretical purposcs only. It is a commonstandard state used for solutes because it extrapolates the chcmical potentialof the solute to infinite dilution rather than to the pure state, which would notbe related to its chemical potential as a solute. The gencral equation for thisstandard state is

/rlou," = r-.lla, ft.d - kTln/rou..)

For further explanation, see: Kirkwood, J. G.; Oppenheim, l. ChemicalThermodynanics; McGraw-Hill: Ncw York, 196l; Chaptcr I l.

(a8) As long as both thc rcference and mixcd SAMs are completc (sccScheme I), we are working at a constant pressure within thc monolayers.Following the formalism developed for monolayers at the air-water intcrfacc,the enthalpy and internal energy are effcctivcly equal as long as we assumethat the change in pressure upon mixing is zero. For cxamples of thc cquationof state for two-component, insoluble monolayers at the air-watcr interfacc,sec: Lucassen-Reynders, E. H. J. Colloid Interface Sci. 1972,41, 156-167.Gaines, G. L., Jr. J. Chem. Phys. 197t, 69, 92+-930. A&mson, A,. PhysicalChemistry of Surface,4th ed.; Wiley: New York, 1982; pp l3Fl43 andreferenccs thcrein.

(a9) The valucs of .t'$M'r in ref 4 were detcrmined on systems that wcrenot at cquilibrium. We thereforc cannot relatc thosc values to A(A#).

(50) Folkers, J. P.; Whitcsides, G. M., unpublished obscrvations.(51) Nuzzo, R. G.; Zeganki, B. R.; Dubois, L. H. J. Am. Chem. Soc.

yw, 109,733-740.(52) Troughton, E. B.; Bain, C. D.; Whitcsidcs, G. M.; Nuzzo, R. G.;

Allara, D. L.; Porter, M. D. Langmuir l9tt, y', 365-385.(53) Scveral warnings have recently appeared concerning'piranha

solution": Dobb6, D. A.; Bergman, R. G.; Theopold, K.H. Chem. Eng. News1990, 68 (17),2. Wnuk, T. Chem. Eng. News lg$t,68 (26),2. Matlow, S.L. Chem. Eng. News 1990, 68 (30), 2.

(54) Bain, C. D.; Whitesides, G. M. J. Phys. Chem.l9t9,93,l67Ll673.(55) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J- Phys. Chcm.1991,

95.7017-702t.