A

csttnramt∼©

K

1

ptfoetfwctp[

0d

Available online at www.sciencedirect.com

Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 194–201

Construction of the homogeneously mixed SAM composed ofoctyltriethoxysilane and octadecyltrichlorosilane by taking

advantage of the molecular steric restriction

Junyong Feng, Guo Hua Xu ∗, Yue An, Xiangxuan ZengDepartment of Chemical Engineering, Zhejiang University, Hangzhou 310027, PR China

Received 9 April 2007; received in revised form 24 August 2007; accepted 1 September 2007Available online 8 September 2007

bstract

By taking advantage of the steric restriction offered by the C8TES molecules, a homogeneously mixed self-assembled monolayer (SAM)omposed of octyltriethoxysilane (C8TES) and octadecyltrichlorosilane (OTS) was constructed by means of the stepwise method on Si(1 0 0)ubstrate. The kinetic formation processes of the pure and mixed SAMs were investigated by the advancing contact angle measurements, and theopographies of the sample surfaces were characterized by AFM. The experimental results showed that by coadsorption method, it was impossibleo construct the mixed SAMs for C8TES and OTS system, since the difference of the chemical activities between C8TES and OTS molecules couldot be simply balanced through the tailoring of the concentration ratios in the coadsorption solutions. However, by taking advantage of the stericestriction caused by the three ethoxyl groups on the heads of C8TES molecule as well as the difference of the chemical activities between C8TES

nd OTS molecules, it was possible to construct both the homogeneously and phase-separated mixed C8TES/OTS SAMs by means of the stepwiseethod. The molecular steric analysis and AFM scanning experiments by AFM cantilever probes with different spring constants also indicated

hat, the structure inside the pure C8TES SAM should be in a loose and unassembled state, which resulted in an ultra-low surface contact angle,20◦, on the C8TES SAM surface. 2007 Elsevier B.V. All rights reserved.

Octylt

owrcSdtttpta

eywords: Mixed self-assembled monolayer (mixed SAM); Steric restriction;

. Introduction

Mixed self-assembled monolayer (SAM) is an importantart in SAM family [1]. Due to its strong ability to regulatehe concentrations of the functional groups on the sample sur-aces, mixed SAM technology is broadly applied in preparationf model surfaces for the studies of some surface phenom-na, such as adhesion, friction and wetting [2,3]. Moreover,he tremendous possibility of the construction of various sur-ace nanostructures also makes the mixed SAM technologyidely used in fabrication of nanostructure devices, such as

hem-/biosensors [4,5] and molecular logic devices [6,7]. Up

o now, coadsorption method is regarded as the easiest and sim-lest route to prepare the so-called homogeneously mixed SAMs8–10]. If one dips a substrate into a mixed solution composed

∗ Corresponding author. Tel.: +86 571 87951742; fax: +86 571 87951742.E-mail address: xugh@zju.edu.cn (G.H. Xu).

Smpu

lb

927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.09.013

riethoxysilane (C8TES); Octadecyltrichlorosilane (OTS); Stepwise method

f two adsorbates in a specific concentration ratio, a mixed SAMill be formed on the substrate surface. Since the chemical

eaction activities of two adsorbates are different, generally theoncentrations of the terminal functional groups on the mixedAM surfaces and the distributions of the nano-scale molecularomains do not linearly depend on the concentration ratios inhe mixed solution. It needs lots of trial and error experiments toailor the preferential adsorption of the components and obtainhe desired concentration ratio [9,10]. Another effective route torepare mixed SAMs is the stepwise method [11,12], in whichhe substrate is dipped into several monocomponent solutions ingiven order, the concentrations of the components in the mixedAMs as well as the sizes and distributions of the nano-scaledolecular domains are mainly controlled by adjusting the dip-

ing time in different solutions. Stepwise method is particularly

seful to construct the phase-separated mixed SAMs.

In this paper, the mixed SAMs composed of octyltriethoxysi-ane (C8TES) and octadecyltrichlorosilane (OTS) were tried toe prepared by means of both the coadsorption and stepwise

ysico

mEsgwmgetscapHbben

2

2

tUacUsT

2

ciswttHwgS

2

OmewipS1

ttaaa

a1

2

bcCasmpg

pc1ttaspospgffitCmd

2

wAtewftosR

J. Feng et al. / Colloids and Surfaces A: Ph

ethods. Our initial interest is to prepare the model surfaces forFM surface potential measurements and MEMS anti-stictiontudies by using the mixed SAM technology. Both the homo-eneously and phase-separated mixed SAMs are needed. It isorthy to point out that, the two chemicals have the same ter-inal group of methyl, but the chain length and the reaction

roups on the molecular heads, trichlorosilane of OTS and tri-thoxysilane of C8TES, are different. Our research indicatedhat for C8TES and OTS system, it was impossible to con-truct a homogenously or phase-separated mixed SAM by theoadsorption method, because the difference of the chemicalctivities between C8TES and OTS molecules could not be sim-ly balanced through the tailoring of the concentration ratios.owever, by taking advantage of the steric restriction offeredy the C8TES molecules, a homogeneously mixed SAM coulde constructed by means of the stepwise method. To our knowl-dge, such kind of the method is reported for the first time andever used by other research groups before.

. Experimental details

.1. Chemicals

Octyltriethoxysilane (C8TES, 97%) and Octadecyl-richlorosilane (OTS, 95%) were purchased from Aldrich Co.,SA and used as received without further treatment. Toluene,

cetone, chloroform, isopropanol, hydrogen peroxide andoncentrated sulfuric acid were all of analytical reagent grade.ltra-pure water with a resistivity greater than 18.2 M� cm was

upplied by UPWS-I-20T (Hangzhou Yongjieda Purificationechnology Co. Ltd., China).

.2. Pretreatment of the Si(1 0 0) substrate surface

Si(1 0 0) chips (1 cm × 1 cm) were used as the substrates, andleaned by sonication in acetone, chloroform and isopropanoln an ultrasonic cleaning tank for 5 min, respectively. After eachonication, the Si(1 0 0) chips were rinsed with plentiful pureater and finally sonicated in ultra-pure water again for 5 min,

hen dried in a stream of nitrogen gas. After the cleaning process,he substrates were oxidized in a piranha solution (70:30 (v/v),

2SO4 (98%)/H2O2 (30%)) at 90 ◦C for 30 min, then rinsedith abundant ultra-pure water and dried in a stream of nitrogenas. A thin layer of SiO2 would be formed on the surface of thei(1 0 0) substrate after the oxidization.

.3. Preparation of the pure OTS and C8TES SAMs

In this experiment, only was the freshly preparedTS–toluene solution used. Since it is well known that OTSolecule is very easily hydrolyzed, if the OTS solution is

xposed to the open-air for a period of time, the OTS moleculeill be hydrolyzed into silanol under the atmospheric humid-

ty, and further form undissolved, three-dimensional disorderedolymerized condensations. The well-cleaned and oxidizedi(1 0 0) substrates were first immersed into the freshly prepared× 10−3 mol L−1 OTS–toluene solution for chemical modifica-

a

cp

chem. Eng. Aspects 316 (2008) 194–201 195

ion. After the modification, the substrates were withdrawn fromhe solution as quickly as possible, rinsed in toluene, chloroformnd isopropanol, respectively, to remove any excess deposits,nd then dried in a stream of nitrogen gas. All reaction temper-ture was controlled strictly at 18 ± 0.1 ◦C in this experiment.

The preparation procedure of C8TES SAM was the sames that of OTS SAM, and only was the freshly prepared× 10−3 mol L−1 C8TES-toluene solution used.

.4. Preparation of the mixed C8TES/OTS SAMs

The mixed C8TES/OTS SAMs were tried to be preparedy means of both the coadsorption and stepwise methods. Inoadsorption method, the mixed toluene solutions composed of8TES and OTS in different concentration ratios were prepared,nd the Si(1 0 0) substrates were immersed into the mixed coad-orption solutions for chemical reaction. After the reaction, theodified substrates were rinsed in toluene, chloroform and iso-

ropanol, respectively, and finally dried in a stream of nitrogenas.

In stepwise method, two different dipping orders, namedrocedures I and II, respectively, were tried. In the stepwise pro-edure I, the substrates were first dipped into a freshly prepared× 10−3 mol L−1 C8TES-toluene solution with the reaction

ime from several seconds to 60 min for the first-phase reac-ion. After the first-phase reaction, the samples were rinsed withbundant toluene to remove any excess deposits and dried in atream of nitrogen gas, then dipped immediately into the freshlyrepared 1 × 10−3 mol L−1 OTS–toluene solution for the sec-ndary 30 min reaction. After the second-phase reaction, theamples were completely rinsed in toluene, chloroform and iso-ropanol, respectively, and finally dried in a stream of nitrogenas. In procedure II, the substrates were first dipped into areshly prepared 1 × 10−3 mol L−1 OTS–toluene solution for therst-phase reaction with the reaction time from several seconds

o several minutes, and then immersed into 1 × 10−3 mol L−1

8TES-toluene solution for the secondary 30 min reaction, theiddle and final cleaning processes were the same as the proce-

ure I.

.5. Characterizations

A Multimode AFM (Nano IVa, Veeco Instruments, USA)as used to characterize the topographies of sample surfaces.ll surface images were captured in contact mode in air, and

he height data type was used to collect the image data in thisxperiment. Normally, a silicon nitride triangle cantilever probeith a small spring constant of ∼0.06 N/m (DNP-D, purchased

rom Veeco) was used to capture the AFM images. The usage ofhe small spring constant cantilever could prevent the destructionf the surface structure of the soft SAM samples during thecanning processes. The measurement of the surface roughness,MS, of the sample surfaces and the bearing and grain height

nalyses were performed by AFM off-line software.

Advancing contact angle was measured under an open-airondition by growing small sessile water droplet on the freshlyrepared sample surface at room temperature. A 2.5 �L water

1 Physicochem. Eng. Aspects 316 (2008) 194–201

da1ics

3

3

ocarmrcrHdaSs

Oifaottgaohswm

Fc

apm

trtswFaat

96 J. Feng et al. / Colloids and Surfaces A:

roplet was placed on the sample surface, and the droplet wasdded every 15 s. The procedure was repeated five times (total2.5 �L), and the contact angle was measured by a goniometermmediately after the droplet adding. The reported advancingontact angle is the average of the measured values for eachample, the mean error of the measurement is about ±2◦.

. Results and discussion

.1. Formation kinetics of the pure OTS and C8TES SAMs

Fig. 1 presents the changes of the advancing contact anglesf sample surfaces with reaction time during the formation pro-esses of the pure OTS and C8TES SAMs. For OTS SAM, thedvancing contact angle increases rapidly during the initial 2 mineaction, which implies the adsorption of a great deal of OTSolecules onto the substrate surface. Only after about 5 min

eaction, the advancing contact angle on the sample surfaceould reach ∼106◦ and finally stable at 107◦ after 15–30 mineaction. The results obtained here is consistent with Xu andigashitani [13] several years ago. In that work, they detailediscussed the formation kinetics of OTS SAM on glass surfacend concluded that after 30 min reaction, a well-organized OTSAM could be obtained from a 1 × 10−3 mol L−1 OTS–tolueneolution at 18 ◦C.

The formation regularity of C8TES SAM is similar to that ofTS SAM. As shown in Fig. 1, the reaction rate of the C8TES

s obviously slower than OTS, it need at least 15 min for the sur-ace sample to approach the stable advancing contact angle. Thedvancing contact angle of the finally formed C8TES SAM isnly about 20–21◦, which is much lower than the expected con-act angle on a typical hydrophobic surface. It is well known thathe surface properties of SAMs are determined by the terminalroups of the molecules. The terminal group of both the C8TESnd OTS is methyl (–CH3), a typical hydrophobic group. The-retically, if the formed SAM were orderly close-packed with a

igh quality, the surface property of C8TES SAM should be theame as that of OTS SAM. C8 is regarded as an alkyl componentith the minimum molecular length sufficient to form a stableonolayer [14]. Under a proper thermodynamic condition, such

ptta

Fig. 2. AFM images of the sample surfaces of

ig. 1. Advancing contact angles vs. reaction time during the formation pro-esses of OTS and C8TES SAMs ((©) OTS SAM; (�) C8TES SAM).

s the existence of the trace water [15] and suitable reaction tem-erature [13–16], a close-packed and well-assembled uniformonolayer could be obtained.According to Brzoska et al. [15–18], a high quality alkyl-

richlorosilane monolayer could be obtained only when theeaction temperature was below a temperature threshold. Ifhe reaction temperature were above the threshold, the cohe-ive interaction between hydrocarbon chains would be relativelyeak, a loose and disordered monolayer would be formed [18].or OTS, the measured temperature threshold was about 28 ◦C,nd the estimated temperature threshold for octyltricholrosilanebout −7 ◦C [15,16]. We do not find the reported temperaturehreshold for C8TES in literature. The 18 ◦C reaction tem-

erature controlled in this experiment is much lower than theemperature threshold of OTS, but obviously much higher thanhat of C8TES, the formed C8TES SAM should be in a loosend disordered structure. Fig. 2 displays the AFM images of

the pure (a) OTS and (b) C8TES SAMs.

J. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 194–201 197

F by a5 surfai nd fo

tibqRiRIdatoeurlbs

3

u(saCtMss1psoi1cbcr

masmtssbsttw

3method

Fig. 4 presents the changes of the advancing contact angleson the sample surfaces with the reaction time during the for-

ig. 3. The structure features of C8TES SAM with 30 min reaction explored�m ×5 �m surface image after the first scan, (b) an extended 10 �m × 10 �m

n the center and (c) a captured 5 �m × 5 �m surface image after several back a

he sample surfaces of pure OTS and C8TES SAMs preparedn this work, respectively, with the reaction times of 30 min foroth samples. As Fig. 2(a) shows, the surface of OTS SAM isuite smooth and close-packed, the measured surface roughness,MS, of OTS SAM was ∼0.105 nm/5 �m × 5 �m. Fig. 2(b)

s the surface image of C8TES SAM (1.5 �m × 1.5 �m), theMS of the sample surface is about 0.092 nm/1.5 �m × 1.5 �m.

n order to find the expected loose and disordered structure inetail, we specially captured an AFM surface image with a rel-tively small size of 1.5 �m × 1.5 �m. Comparing to Fig. 2(a),he surface of C8TES SAM seems somewhat different from thatf OTS SAM, but we cannot clearly point out the distinct differ-nce between the two surface images. However, the unexpectedltra-low surface contact angle on C8TES SAM surface stronglyeminds us that the structure of C8TES SAM should be abso-utely different from that of OTS SAM. The structure featuresuried inside the C8TES SAM will be discussed in the nextections.

.2. Structure features inside the C8TES SAM

The AFM topographic image captured by the normallysed tender cantilever tip with a spring constant of 0.06 N/mDNP-D, purchased from Veeco Probe) could not give us anytructure information inside the C8TES SAM (Fig. 2(b)). Weccidentally revealed parts of the structure features inside the8TES SAM simply by using a relatively stiffer cantilever

ip with a spring constant of ∼0.3 N/m (NSC12/AlBS-E, fromikroMasch). Fig. 3 shows the AFM images captured by the

tiff cantilever tip with the spring constant of ∼0.3 N/m, theample is the pure C8TES SAM with a 30 min reaction in× 10−3 mol L−1 C8TES-toluene solution at 18 ± 0.1 ◦C. Com-aring with Fig. 2(b), after the first-run scanning, the originallymooth sample surface becomes granulated under the rubbingf the stiff cantilever tip (Fig. 3(a)). Fig. 3(b) shows the follow-ng second-run scanning surface image with an extended area of0 �m × 10 �m and the first scanned 5 �m × 5 �m square in the

enter. The AFM images indicate that, with the increase of theack and forth scan times, the grain height increases dramati-ally. After several back and forth scan, the average grain heighteaches 5–6 nm, about three times longer than single C8TES

FttOe

n AFM cantilever probe with a spring constant of ∼0.3 N/m. (a) A capturedce image after the second-run scan with the first scanned 5 �m × 5 �m squarerth scans.

olecule (Fig. 3(c)). This amazing phenomenon was repeatablend did not be found in pure OTS SAM samples, which demon-trates that even after 30 min reaction, quite a number of C8TESolecules or its hydrolysates are just physically adsorbed on

he substrate surface without any chemical bonding with theubstrate surface or the lateral molecules. Under the heavyishcratching of the AFM tip, these molecules could be pulled upy the roots and form the structure like the floating seaweed ashown in Fig. 3(c). The AFM scanning experiments by the rela-ively stiffer cantilever probe confirms that, the structure insidehe C8TES SAM is indeed loose and incompletely assembled,hich perhaps results in the ultra-low surface contact angle.

.3. Mixed C8TES/OTS SAMs prepared by coadsorption

ig. 4. Changes of the advancing contact angles vs. the reaction time duringhe formation processes of mixed OTS/C8TES SAMs prepared by coadsorp-ion method. (×) OTS:C8TES = 1/10 × 10−3 mol L−1:1 × 10−3 mol L−1; (�)TS:C8TES = 1/50 × 10−3 mol L−1:1 × 10−3 mol L−1; (©) pure OTS (as ref-rence); (�) pure C8TES (as reference).

1 Physicochem. Eng. Aspects 316 (2008) 194–201

mctttStCiatpaSgatcWOfltttabc

3p

t

FStOs

Ft

iaabtos3

98 J. Feng et al. / Colloids and Surfaces A:

ation processes of mixed C8TES/OTS SAMs prepared byoadsorption method. Although the concentration ratios of OTSo C8TES in coadsorption solutions are much lower than 1:1,he formation regularities of the “mixed” SAMs prepared byhe coadsorption method are almost the same as the pure OTSAM. Even the concentration of OTS (1 × 10−5 mol L−1) in

he coadsorption solution was 100 times lower than the that of8TES (1 × 10−3 mol L−1), after 30 min reaction, the advanc-

ng contact angle of the formed “mixed” SAMs was found to bes high as 104–106◦. The contact angle analysis indicates that,here is no essential difference between the “mixed” SAMs pre-ared by the coadsorption method and the pure OTS SAM. Thebsence of the participation of C8TES molecules in the mixedAMs might be attribute to the reactivity difference of the headroups between the ethoxyl groups in C8TES and the chlorinetoms in OTS, as well as the steric resistance caused by thehree big ethoxyl groups on the head of C8TES molecule, whichould result in the preferential adsorption of OTS molecules.

hen a substrate is immersed into the coadsorption solution,TS molecules will quickly occupy almost all the substrate sur-

ace due to its absolute predominance of reaction activity, andeave no space to C8TES molecules to construct its own archi-ecture. The experimental results shows that, it is very difficulto prepare the mixed SAMs for OTS and C8TES system byhe coadsorption method, since the difference of the chemicalctivities between OTS and C8TES molecules cannot be simplyalanced through the tailoring of the concentration ratios in theoadsorption solutions.

.4. Mixed C8TES/OTS SAMs prepared by the stepwise

rocedure I

Fig. 5 displays the changes of the advancing contact angles ofhe final formed mixed C8TES/OTS SAMs with the dipping time

ig. 5. Changes of the advancing contact angles of the mixed OTS/C8TESAMs prepared by the stepwise procedure I with the reaction time in C8TES-

oluene solution in the first-phase reaction. (×) The final samples of the mixedTS/C8TES SAMs (after the secondary 30 min reaction); (©) the midway

amples after the first-phase reaction in C8TES-toluene solution.

tsatmpfoC

Fd

ig. 6. AFM surface image of the mixed C8TES/OTS SAM with the reactionimes of 30 min both in the first and second reaction phase.

n C8TES-toluene solution in the first-phase reaction, and thedvancing contact angles of the corresponding midway samplesfter the first-phase reaction in C8TES-toluene solution preparedy the stepwise procedure I. With the increase of the reactionime in the first-phase reaction, the advancing contact anglesf the mixed OTS/C8TES SAMs decrease obviously. It is reallyurprising that, for the samples with the reaction time longer than0 min in the first-phase reaction in C8TES-toluene solution,he advancing contact angles of the final formed mixed SAMamples can still reach 101–102◦. But the advancing contactngle measurements of the corresponding midway samples showhat the sample surfaces should be fully covered by the C8TES

olecules after the first-phase reaction. It seems in the second-

hase reaction, the OTS molecules can penetrate the C8TES filmormed in the first-phase reaction, and construct a new structurever the C8TES film. Fig. 6 is the AFM surface image of a mixed8TES (30 min in first-phase reaction)/OTS (with 30 min reac-

ig. 7. Molecular models and sizes of (a) OTS, (b) C8TES and (c) octyltrihy-roxysilane, the hydrolysate of C8TES.

J. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 194–201 199

ring the formation of the mixed OTS/C8TES SAM.

tdmaeStamet

btStsCwotawCIfwptwtd(yia6do

ts

lsAlyifinally formed mixed SAMs should be like the model displayedin Fig. 9. Although there is no obvious difference between theAFM surface images of the mixed SAM (Fig. 6) and the pure

Fig. 8. Steric analysis of the kinetic process du

ion in second-phase reaction) SAM sample. There is no distinctifference between Figs. 2(a) and 6, even the surface RMS of theixed SAM in Fig. 6 is ∼0.105 nm/5 �m × 5 �m, just the same

s that of the pure OTS SAM. The one and only measured differ-nce is the advancing contact angle, for the mixed C8TES/OTSAM, the advancing contact angle is 102◦, about 5◦ lower than

hat of the pure OTS SAM. The 5◦ gap in the advancing contactngle cannot be explained by the measurement error, since theean error of the contact angle measurements is ±2◦ in this

xperiment, which should imply some special structure insidehe mixed C8TES/OTS SAM.

The secret buried inside the mixed SAM may be revealedy the steric analysis during the kinetic formation process ofhe mixed SAM. According to the formation mechanism ofAM proposed by Brzoska et al. [15], in the first-phase reac-

ion, after being immersed into C8TES-toluene solution, theurface of the substrate is first covered by the physically adsorbed8TES molecules, and these physically adsorbed moleculesill be hydrolyzed into silanols by the trace water adsorbedn the substrate surface, the silanol molecules then react withhe hydroxyl groups on the substrate surface and the heads of thedjacent silanol molecules to construct the self-assembled net-ork. Fig. 7 displays the molecular models and sizes of OTS,8TES and octyltrihydroxysilane, the hydrolysate of C8TES.

f all the C8TES molecules were adsorbed on the substrate sur-ace perpendicularly, each C8TES molecule would hold an areaith a diameter about 9.213 A as Fig. 7(b) shows. Suppose thehysically adsorbed C8TES molecules were closely packed,he distances between the centers of the adjacent moleculesere about 9.213 A (Fig. 8(a) and (c)). After being hydrolyzed,

he C8TES molecules change to octyltrihydroxysilane, and theiameter of the hold area of each molecule shrinks to 4.961 AFigs. 7(c) and 8(b)). Finally, in the center of four octyltrihydrox-silane molecules, a space with a diameter of ∼8.066 A comesnto being (Fig. 8(b) and (c)). This space could barely acceptn OTS molecule with the diameter of the molecular head about.733 A (Fig. 7(a)), but could not be inserted by C8TES moleculeue to the steric restriction caused by the little bigger head size

f the C8TES molecules.

The steric analysis indicates that, even for 30–60 min reac-ion in the first-phase, the C8TES molecules on the sampleurface cannot be closely packed and form a well-assembled

FSOmi

Fig. 9. A proposed structure model of the mixed OTS/C8TES SAM.

ateral network between neighbor molecules due to its hugeteric restriction caused by the three ethoxyl groups on the head.fter the hydrolyzation of the physically adsorbed C8TES, the

eft spaces in the centers of the adjacent four octyltrihydrox-silane molecules can accept the inserting of OTS moleculesn the second-phase reaction, the structure cutaway view of the

ig. 10. Changes of the advancing contact angles of the mixed C8TES/OTSAMs prepared by the stepwise procedure II with the reaction time inTS–toluene solution in the first-phase reaction. (©) The final samples of theixed OTS/C8TES SAMs; (+) the midway samples after the first-phase reaction

n OTS–toluene solution.

200 J. Feng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 194–201

F he stei

OSwc

trCstmb

3p

octcassOpSt5tbts

toAb

4

trSsatstCasmTbiwC

A

DUiF

R

ig. 11. Captured AFM images of the mixed OTS/C8TES SAMs prepared by tn OTS–toluene solution in the first-phase reaction.

TS SAM (Fig. 2(a)), the density of OTS molecule in the mixedAM should be somewhat lower than that of pure OTS SAM,hich might be the reason of the 5◦ decrease of the advancing

ontact angle for the mixed SAM.In the light of the above discussion, even the controlled reac-

ion temperature lower than the temperature threshold, the stericestriction caused by the three ethoxyl groups on the head of8TES molecule could still result in the loose and unassembled

tructure inside the C8TES SAM. However, by taking advan-age of this steric restriction, it is possible to construct a much

ore homogeneously mixed SAM for C8TES and OTS systemy means of the stepwise procedure I.

.5. Mixed C8TES/OTS SAMs prepared by the stepwiserocedure II

Fig. 10 presents the changes of the advancing contact anglesf the mixed C8TES/OTS SAMs prepared by the stepwise pro-edure II with the reaction time in OTS–toluene solution inhe first-phase reaction, the advancing contact angles of theorresponding midway samples after the first-phase reactionre also drawn in the same figure as a reference. As Fig. 10hows, the advancing contact angle on the sample surfaceteps up gradually with the increase of the reaction time inTS–toluene solution in the first-phase reaction. Fig. 11 dis-lays two captured AFM images of the mixed C8TES/OTSAMs prepared by the stepwise procedure II with the reac-

ion times of 5 s (Fig. 11(a), 6 �m × 6 �m) and 10 s (Fig. 11(b),�m × 5 �m) in OTS–toluene solution in the first-phase reac-

ion, respectively. The isolated OTS adsorption islands cane observed on the mixed SAM surfaces, the phase separa-ion of OTS and C8TES molecular domains can be achieveduccessfully.

According to Fig. 7, the OTS molecule is about 1 nm longerhan C8TES molecule, and about 1.3 nm than the hydrolysatef C8TES. Both the bearing and grain height analyses of theFM image of Fig. 11(a) show that, the average height distanceetween two separated domains is about 1.3 nm.

pwise procedure II with the reaction times of (a) 5 s and (b) 10 s, respectively,

. Conclusion

Both the coadsorption and stepwise methods were employedo construct the mixed C8TES/OTS SAMs. The experimentalesults showed that, it was impossible to construct the mixedAMs for C8TES and OTS system by the coadsorption method,ince the difference of the chemical activities between the OTSnd C8TES molecules could not be simply balanced throughhe tailoring of the concentration ratios in the coadsorptionolutions. However, by taking advantage of the steric restric-ion caused by the three ethoxyl groups on the heads of the8TES molecules as well as the difference of the chemicalctivities between the OTS and C8TES molecules, it was pos-ible to construct both the homogeneously and phase-separatedixed C8TES/OTS SAMs by means of the stepwise method.he molecular steric analysis and AFM scanning experimentsy a relatively stiffer cantilever probe proved that, the structurenside the pure C8TES SAM was indeed loose and unassembled,hich would result in an ultra-low surface contact angle on the8TES SAM surface.

cknowledgments

We are thankful to Prof. Tang Ruikang, Dr. Pan Haihua andr. Jiang Wenge in the Department of Chemistry at Zhejiangniversity, for their helps and useful discussions in AFM exper-

ments. This work was supported by Chinese National Scienceoundation (No. 20276057).

eferences

[1] A. Ulman, An Introduction to Ultrathin Organic Films fromLangmuir–Blodgett to Self-assembly, Academic Press Inc., Boston, 1991.

[2] J.F. Kang, S. Liao, R. Jordan, A. Ulman, Mixed self-assembled monolayers

of rigid biphenyl thiols: impact of solvent and dipole moment, J. Am. Chem.Soc. 120 (1998) 9662–9667.

[3] P. Gupta, A. Ulman, S. Fanfan, A. Korniakov, K.J. Loos, Mixed self-assembled monolayers of alkanethiolates on ultrasmooth gold do notexhibit contact-angle hysteresis, Am. Chem. Soc. 127 (2005) 4–5.

ysico

[

[

[

[

[

[

[

[

J. Feng et al. / Colloids and Surfaces A: Ph

[4] C. Boozer, J. Ladd, S. Chen, Q. Yu, J. Homola, S. Jiang, DNA directed pro-tein immobilization on mixed ssdna/oligo(ethylene glycol) self-assembledmonolayers for sensitive biosensors, Anal. Chem. 76 (2004) 6967–6972.

[5] S. Basurto, T. Torroba, M. Comes, R. Martınez-Manez, F. Sancenon, L.Villaescusa, P. Amoros, New chromogenic probes into nanoscopic pocketsin enhanced sensing protocols for amines in aqueous environments, Org.Lett. 7 (2005) 5469–5472.

[6] S. Nitahara, N. Terasaki, T. Akiyama, S. Yamada, Molecular logic devicesusing mixed self-assembled monolayers, Thin Solid Films 499 (2006)354–358.

[7] S. Nitahara, T. Akiyama, S. Inoue, S. Yamada, A photoelectronic switchingdevice using a mixed self-assembled monolayer, J. Phys. Chem. B 109(2005) 3944–3948.

[8] J.P. Folkers, P.E. Laibinis, G.M. Whitesides, Self-assembled monolayersof alkanethiols on gold: comparisons of monolayers containing mixtures ofshort and long-chain constituents with CH3 and CH2OH terminal groups,Langmuir 8 (1992) 1330–1341.

[9] P.E. Laibinis, R.G. Nuzzo, G.M. Whitesides, Structure of monolayers

formed by coadsorption of two n-alkanethiols of different chain lengthon gold and its relation to wetting, J. Phys. Chem. 96 (1992) 5097–5105.

10] J.P. Folkers, P.E. Laibinis, G.M. Whitesides, Phase behavior of two-component self-assembled monolayers of alkanethiolates on gold, J. Phys.Chem. 98 (1994) 563–571.

[

chem. Eng. Aspects 316 (2008) 194–201 201

11] K. Shimazu, T. Kawaguchi, T.J. Isomura, Construction of mixed mer-captopropionic acid/alkanethiol monolayers of controlled composition bystructural control of a gold substrate with underpotentially deposited leadatoms, Am. Chem. Soc. 124 (2002) 652–661.

12] I. Choi, Y. Kim, S.K. Kang, J. Lee, J. Yi, Phase separation of a mixedself-assembled monolayer prepared via a stepwise method, Langmuir 22(2006) 4885–4889.

13] G.H. Xu, K. Higashitani, Formation of OTS self-assembled monolayer onglass surface investigated by AFM, J. Zhejiang Univ. 1 (2000) 162–170.

14] P. Silberzan, L. Leger, D. Aussere, J.J. Benattar, Silanation of silica sur-faces: a new method of constructing pure or mixed monolayers, Langmuir7 (1991) 1647–1651.

15] J.B. Brzoska, I.B. Azouz, F. Rondelez, Silanization of solid substrates: astep toward reproducibility, Langmuir 10 (1994) 4367–4373.

16] J.B. Brzoska, N. Shahidzadeh, F. Rondelez, Evidence of a transition tem-perature for the optimum deposition of grafted monolayer coatings, Nature360 (1992) 719.

17] R.R. Rye, Transition temperatures for n-alkyltrichlorosilane monolayers,

Langmuir 13 (1997) 2588–2590.

18] K. Iimura, Y. Nakajima, T. Kato, A study on structures and formationmechanisms of self-assembled monolayers of n-alkyltrichlorosilanes usinginfrared spectroscopy and atomic force microscopy, Thin Solid Films 379(2000) 230–239.

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页 文献云下载 图书馆入口 外文数据库大全 疑难文献辅助工具