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substrate to the surface wetting properties of the self-assembledmonolayer.

EXPERIMENTAL OVERVIEWOrganosilanes react with the terminal hydroxyl (OH) groupson the surface of the silica (glass slide) in a condensationreaction to form a self-assembled monolayer (Figure 1). The

monolayer is further stabilized via condensation reactionsbetween adjacent organosilanes, thus forming a network ofsiloxanes.17 The wetting behavior and the surface free energy ofthe glass surface are controlled by the terminal functionalgroups presented by the SAMs.18 In this experiment, twoorganosilanes are used, 1-octadecyltriethoxysilane (ODTS) and

3-aminopropyltrimethoxysilane (APTMS), which display ahydrophobic alkyl chain and a more hydrophilic aminogroup, respectively (Figure 1). Note that these are tworepresentative reagents of hundreds of commericially availablesilanes. These monolayer modications are directly comparedto that of the terminal hydroxyl groups that are displayed on

base-etched glass substrates. The effect on the surface tensionand wetting behavior of the glass are measured by determiningthe force required to overcome the counteracting forces ofsurface tension and buoyancy to completely immerse thesubstrate. Hydrophobic interfaces such as the ODTSmonolayers have large contact angles (for ODTS, 110),

which allow the formation of an indentation or dimplein the

water, thus requiring a larger sinking force.19 Highly wettingsurfaces such as the bare silica or amine-terminated monolayersdisplay lower contact angles (for APTMS, 50).20

MATERIALSNaOH, hexanes, APTMS, ethanol, and acetone were all ACSgrade reagents and acquired from Sigma Aldrich Corporation

(St. Louis, MA). ODTS was acquired from Alfa Aesar (WardHill, MA). The glass coverslips were acquired from VWR,(West Chester, PA).

HAZARDSThe procedure calls for the use of volatile, ammable solvents(ethanol, hexane, and acetone), irritants (ODTS and APTMS)and a strong base (NaOH). The surface preparation should beconducted in a fumehood whenever possible, whereas the naldemonstration can be conducted in a conventional classroomsetting. Gloves and goggles should be worn when handling theglass coverslips and the chemical reagents.

EXPERIMENTAL PROCEDURE

Three sets of rectangular glass coverslips (22 mm

30 mm, 25mm 25 mm, and 18 mm 18 mm sizes) are etched in 1 MNaOH for 30 min. Two sets of coverslips are removed from thealkaline solution, washed with deionized water, ethanol, andacetone, and dried under a stream of N2. Each set of coverslipsis allowed to incubate in a 10 mM solution of ODTS or

APTMS in hexanes for 1 h. The coverslips are ipped after 30min to ensure complete exposure to the alkylsilane solution.Once the SAMs are formed, the coverslips are sonicated,

washed with acetone, dried under a stream of N2, and gentlyplaced on top of a bath of deionized water. The altered surfaceproperties of the glass can be investigated by determining theforce the coverslip can support before sinking. The coverslipsthat remained in the NaOH solution (hydroxyl terminated)

serve as a control and are washed with deionized water andethanol and dried under a stream of nitrogen gas before beingplaced on the water. Any suitable small weight, such asconventional staples, can be used, but the weight must be easilyplaced on top of the substrate without touching the water. Theglass coverslips were allowed to equilibrate for ve seconds

before the addition of each new mass. The experiment takesapproximately 22.5 h (additional details are available in theSupporting Information).

RESULTS AND DISCUSSIONAt equilibrium, the glass coverslips oat on the surface of water,and the force of gravity, surface tension, interfacial tension, and

Figure 1. Schematic depicting the various surface modications of asilica surface and the corresponding side-view of the watersubstratecontact interface: SAM formed from (A) ODTS and (B) APTMS, and(C) hydroxy-terminated bare glass.

Figure 2. The indentations formed at the watersubstrate interface for 2 mm 30 mm glass coverslips varied according to terminal surfacefunctional groups: SAM formed from (A) ODTS and (B) APTMS, and (C) hydroxy-terminated bare glass.

Journal of Chemical Education Laboratory Experiment

dx.doi.org/10.1021/ed200832h| J. Chem. Educ. 2012, 89, 154715501548


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buoyancy are in balance. Glass is denser than water, whichindicates that buoyancy alone is not sufficient to allow foroatation of the glass coverslips and that surface tension andinterfacial tension forces play a signicant role. The addition ofa mass load upsets the equilibrium until gravity overcomes theforces of surface and interfacial tension and buoyancy, therebycausing the substrate to sink. Changes in the contact angle

between water and the watersubstrate interface alters themaximum load the substrate can carry before sinking. Aquantitative description of the relationship between contactangle, surface geometry, and oatation has been previouslydetailed, and in summary, this analysis shows that maximumload is related to the perimeter of the object and the three-phase contact angle.14,21,22

To test the effect of SAM functionalization on oatation,three sets of ODTS, APTMS, and based-etched glass substrates

were placed on the surface of water and external masses(staples) were added every 5 s until the substrates sank (Figure2). The maximum force sustained by each substrate wascalculated by summing the mass of the coverslip with the massof the external load and determining the corresponding force ofgravity. The maximum external load that was sustained by each

type of surface functionalization is shown in Figure 3. The

ODTS-functionalized glass coverslips are reported to display a110 contact angle () at the airwater interface.19 Thesesubstrates are extremely hydrophobic and consequently theODTS substrates carried the largest mass loads before nallysinking (Figures2A and3). APTMS-functionalized substrates

show intermediate contact angles (50) and correspondinglysustain intermediate values of maximum load (Figures 2B and3).20 A similar trend is observed for the hydroxyl-terminated

base-etched substrates (0). Importantly, the force of thesurface tension is related to the perimeter of the substrate, andto test the role of this variable on oatation, three different sizesof glass coverslips were tested (Figure3). Larger size coverslips

were found to display larger values for maximum load acrossthe various surface chemistries. For example, the 22 mm 30mm and the 18 mm 18 mm glass coverslips had maximumload to perimeter ratios of 0.209and 0.157 N/m, respectively,

when functionalized with ODTS SAMs and ratios of 0.046 and0.018 N/m, respectively, when left hydroxyl-terminated.

When the substrate is aoat, the factors that need to beconsidered are the force of gravity, buoyancy, surface tension,and the SAMwater interfacial tension. At equilibrium, it ispossible to directly calculate the force of gravity and the surfacetension force (F= (perimeter of substrate) cos() 72mN/m), however, the buoyancy force (see dimpleformed onthe water surface in Figure2) or the interfacial tensioncannot

be easily determined within the scope of this lab.

14,16,22

Nonetheless, molecular functionalization of the substrateprovides for a facile method to alter the contact angle of the

watersubstrate interface, thereby changing the buoyancy forceand surface and interfacial tensions while keeping the masseffectively constant. In fact, if the waterSAM interfacialtension was assumed to be zero, then the magnitude of the

buoyancy force can be experimentally determined from theliterature values ofFand the measured mass of the substrate.For example, the volumes of the indentation of the 22 mm 30mm substrate functionalized with OTDS-formed SAM and

APTMS-formed SAM are calculated to be 1.5 and 0.59 mL,respectively, which is in agreement (within an order ofmagnitude) with qualitative estimates from the photographs

of the dimples in the water (Figure2).

STUDENT EXPERIENCE

Because of the simple setup and the visual nature of the results,this experiment is appropriate for both a rst-year under-graduate general chemistry laboratory class and an upper-levelundergraduate physical chemistry laboratory setting. Arst-yearlaboratory class would focus on the effects of monolayer surfacemodication on the macroscopic properties, whereas a physicalchemistry laboratory class would relate the concepts behind theexperiment to the alteration of wetting behavior, surface freeenergy, and chemical potential. To gauge the student responseto this experiment, 8 rst- and second- year college students

were asked to run the experiment and were then given a survey;

in general, most students were supportive of this experimentbeing incorporated into their chemistry classes and said thatthey gained new knowledge about self-assembled monolayersand their effects. More details of this survey can be found in theSupporting Information.

POSSIBLE EXPERIMENTAL EXTENSIONS

This experiment can be further expanded to highlight certainaspects of molecular self-assembly. For example, mixedmonolayers consisting of a binary mixture of adsorbates can

be used to generate a range of water contact angles andmaximum loads.3,18 This highlights the ability to tune themolecular composition of the monolayers and their macro-

scopic properties. The value of water surface tension can bemodied by altering the surface pKa23 or by altering the

solution pH, ionic strength, or by addition of surfactant andthese variables may be introduced as additional challengeproblems in discussing this demonstration within the class-room.

ASSOCIATED CONTENT

*S Supporting Information

Student information; instructor notes; concept questions forstudents; results from a sample experiment; student response.This material is available via the Internet athttp://pubs.acs.org.

Figure 3.Plot of the maximum force of gravity required to completelyimmerse the different monolayer-functionalized glass substrates. Thisreported mass includes the mass of the coverslip as well as the externalmass that was added to the substrate. Each error bar represents onestandard deviation of two independent experiments.


http://pubs.acs.org/http://pubs.acs.org/


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Notes

The authors declare no competing nancial interest.

ACKNOWLEDGMENTSWe thank Keith Berland, and Yoshie Narui for discussions.

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nanolithography.Nat. Nanotechnol. 2007, 2 (3), 124155.(2) Stabley, D. R.; Jurchenko, C.; Marshall, S. S.; Salaita, K. S.

Visualizing mechanical tension across membrane receptors with afluorescent sensor. Nat. Methods 2012, 9, 6467.

(3) Ulman, A. Formation and structure of self-assembled monolayers.Chem. Rev. 1996, 96(4), 15331554.

(4) Narui, Y.; Salaita, K. Dip-pen Nanolithography of OpticallyTransparent Cationic Polymers to Manipulate Spatial Organization ofProteolipid Membranes. Chem. Sci. 2012, 3, 794799.

(5) Atkins, P., de Paula, J. Physical Chemistry. 8th ed.; W. H. Freemanand Company: New York, 2006.(6) Ellis, A. B. Elements of curriculum reform: putting solids in the

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(7) Okumura, M.; Beauchamp, J. L.; Dickert, J. M.; Essy, B. R.;Claypool, C. L. Projects supported by the NSF division ofundergraduate educationUndergraduate laboratory for surfacescience. J. Chem. Educ. 1996, 73 (2), 142143.

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(9) McKenzie, A. J.; Huffman, L. M.; Parent, K. E.; Hutchison, J. E.;Thompson, J. E. Patterning self-assembled monolayers on gold Green materials chemistry in the teaching laboratory. J. Chem. Educ.2004, 81 (4), 545548.

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(17) Sagiv, J. Organized monolayers by adsorption 0.1. formation andstructure of oleophobic mixed monolayers on solid surfaces. J. Am.Chem. Soc. 1980, 102 (1), 9298.

(18) Salaita, K.; Amarnath, A.; Maspoch, D.; Higgins, T. B.; Mirkin,C. A. Spontaneous phase separationof patterned binary alkanethiolmixtures. J. Am. Chem. Soc. 2005, 127(32), 1128311287.

(19) DePalma, V.; Tillman, N. Friction and wear of self-assembledtrichlorosilane monolayer films on silicon. Langmuir1989,5(3), 868872.

(20) Zeng, X.; Xu, G.; Gao, Y.; An, Y. Surface wettability of (3-aminopropyl)triethoxysilane self-assembled monolayers. J. Phys. Chem.

B 2010, 115 (3), 450454.(21) Vella, D.; Lee, D.; Kim, H Sinking of a horizontal cylinder.

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