Demonstration pubs.acs.org/jchemeduc
Preparation of Transparent Superhydrophobic Glass Slides: Demonstration of Surface Chemistry Characteristics Jessica X. H. Wong and Hua-Zhong Yu* Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada S Supporting Information *
ABSTRACT: A demonstration for undergraduate teaching in upper-division physical chemistry and materials science courses is described. A simple protocol was developed to prepare superhydrophobic and hydrophobic glass by treating standard microscope slides with methyltrichlorosilane and octadecyltrichlorosilane, respectively. The unique wetting, optical, and self-cleaning properties of the modified surfaces can be demonstrated to students in class. Octadecyltrichlorosilane forms a closely packed, methyl-terminated, selfassembled monolayer that changes the glass surface from hydrophilic to hydrophobic; treatment with methyltrichlorosilane yields 3-dimensional polymethylsiloxane networked nanostructures, which leads to a superhydrophobic surface, that is, water droplets sit atop in the Cassie−Baxter state. In both cases, the glass slides maintain optical transparency despite remarkable changes in the surface wettability. A classroom demonstration of superhydrophobicity and self-cleaning using these surfaces, along with a brief explanation, motivates students to apply their basic knowledge of chemistry to study natural phenomena and practical applications. KEYWORDS: First-Year Undergraduate/General, Upper-Division Undergraduate, Demonstrations, Physical Chemistry, Hands-On Learning/Manipulatives, Materials Science, Physical Properties, Surface Science
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hemistry topics taught to students from high school to undergraduate levels revolve around teaching students fundamental techniques and basic principles of chemistry, often with little or no discussion of practical applications. Superhydrophobicity, the so-called “lotus effect”, is a fascinating phenomenon in nature: lotus leaves exhibit extremely high water repellency and self-cleaning abilities. It is of great interest to students because of the antisticking, self-cleaning properties that can be applied to the industrial production of everyday materials (e.g., water-repelling windshields and clothing). The preparation and characterization of these novel materials require a basic understanding of organosilane chemistry, which is not typically included in the undergraduate curriculum today. Hydrophobicity is the surface property of being waterrepellent; a surface is called hydrophobic when the water contact angle, θ, is greater than 90° and superhydrophobic when it exceeds 150°. The contact angle measures the wettability of a surface, determined by the thermodynamic equilibrium of the interfacial tensions between the solid−vapor (γSV), solid−liquid (γSL), and liquid−vapor (γLV) interfaces at the three-phase contact line (Figure 1A), as given by Young’s equation:1 cos θ =
Figure 1. Contact angle of a water droplet (A) on a hydrophobic solid surface and (B) on a superhydrophobic, nanostructured surface.
thereby reduce spreading and wetting of the surface) are used to prepare superhydrophobic surfaces in conjunction with the creation of nanostructures on naturally flat substrates.2 Surface roughness at the sub-micrometer scale aids in creating cavities that trap air pockets to give a composite solid−air−liquid interface (Figure 1B), known as the Cassie−Baxter state, with the effective contact angle (θc) given by3 cos θc = f (1 + cos θ) − 1
where f is the fraction of the solid−liquid interface and θ is the water contact angle on the solid surface. The challenge of creating surface roughness to bring about a superhydrophobic surface is that hydrophobicity and transparency (an ideal property for applications in household or automotive glass) are two opposing properties; increasing roughness enhances the hydrophobicity to the point where transparency is reduced due to light scattering. It is therefore
γSV − γSL γLV
(1)
Low-surface-tension coatings (i.e., coatings that render the surface with a lesser degree of adhesion force between the liquid and solid than the cohesion forces within a liquid, and © 2013 American Chemical Society and Division of Chemical Education, Inc.
(2)
Published: June 28, 2013 1203
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deionized water and ethanol, and deionized water, prior to drying in an oven at 100 °C for 5 min.
critical to control the surface roughness such that both needs are satisfied; transparency can be maintained when microscopic structures of the films are not greater than 100 nm.4−10 Although various methods of preparing superhydrophobic substrates have been proposed in the past two decades, only a few of them retain optical transparency.5−10 Typically these multistep preparative procedures require specialized equipment or controlled environments.11−23 A simplified method using relatively inexpensive materials and equipment to make the surfaces of normally hydrophilic microscope slides superhydrophobic is described herein. The chemicals required are, however, sensitive to moisture. The equipment used in the preparation step may not be readily available in all laboratories. Although it is difficult for students to prepare the superhydrophobic slides in the laboratory due to these limitations, the concept can be demonstrated after preparation of the slides by the instructor. This allows students to explore and learn concepts for surface modification and to understand different organosilane reactions. The demonstration consists of using glass slides modified with two organosilanes, methyltrichlorosilane and octadecyltrichlorosilane, under varied reaction conditions, to obtain different hydrophobicities. The water contact angles of the surfaces can be tested by students or by the instructor; then the self-cleaning effect of the superhydrophobic glass can be demonstrated and even quantified.24 The modification with methyltrichlorosilane is an adaptation of a method by Gao and McCarthy,25 which we have optimized to treat glass and retain transparency.26 Measurements of contact angles are typically conducted with a digital contact angle goniometer (the AST VCA system was used in these experiments). Because this commercial instrument may not be available, contact angles can also be determined with a simple setup designed by Lamour et al.27 Provided neither setup is available, students should be able to visualize the difference between hydrophilic, hydrophobic, and superhydrophobic surfaces after a brief introduction of contact angles; smaller difference in contact angles (especially between hydrophobic and superhydrophobic surfaces) may not be as differentiable to the naked eye.
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In-Class Demonstration
Following the drying, the slides can be used in the demonstration. Students or an instructor deposited water droplets of 1−5 μL on the surface using a syringe or micropipet. A visual difference between the three slides was seen immediately before optional measurement of contact angles was performed by students using a contact angle goniometer or digital camera system. The transparencies of the modified slides, relative to a quartz plate, were shown by the instructor, who measured the transmittance of UV−visible light. This result can also be shown to students graphically or through visual observation by students. The self-cleaning behavior of the modified surfaces was demonstrated by an instructor by sprinkling silicon powder (or any fine powder) on the surface of the slide followed by deposition of a water droplet with a syringe, which students may also attempt. The exercises take between 15 and 45 min to complete depending on the degree of students’ involvement.
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HAZARDS Methyltrichlorosilane and octadecyltrichlorosilane are airsensitive, flammable, and corrosive; they can irritate the skin and eyes on contact and should be kept under an inert gas. Concentrated hydrochloric acid is corrosive and causes burns to tissues. Toluene can be a skin and eye irritant and should not be inhaled due to its toxicity. Silicon powder is flammable and can be an irritant. UV light can damage the eyes and skin, potentially resulting in cataracts and skin cancer. All solutions should therefore be prepared and handled in a fume hood in a wet laboratory (prior to the demonstration); the appropriate protective equipment including gloves, safety goggles, and lab coats should be used.
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OBSERVATIONS AND DISCUSSION
Contact Angles
The water contact angles of modified glass slides were measured with AST’s VCA digital contact angle goniometer by determining the contact line of the water droplet with the surface and measuring the inside angle of the drop with its tangent. Visual differences between the contact angles should be evident, but a setup using an optical lens and camera to magnify the image is also appropriate.27 The measured contact angles indicate that the two alkyltrichlorosilanes have reacted with the hydroxylated glass surface. Whereas both slides become hydrophobic upon modification, a clear difference can be observed between the two slides (Figure 2). A side view of the slides should be sufficient for students to observe the effects. The reaction of octadecyltrichlorosilane and methyltrichlorosilane with trace amounts of water (humidity of air) adsorbed onto the glass surface leads to the formation of a silanol, which then undergoes condensation reactions with the hydroxylated glass surface and with neighboring silanol molecules.11 The difference between the two reactions stems from the different reactivities of the two alkyltrichlorosilanes (different alkyl chain lengths). Octadecyltrichlorosilane forms a well-ordered methyl(Me-) terminated monolayer at the surface (Figure 3A)28 that changes the polarity of the glass (giving water contact angles of 112 ± 2°), whereas methyltrichlorosilane forms a 3-dimensional polymethylsiloxane network that has the ability to trap
DEMONSTRATION OVERVIEW
Slide Preparation
The instructor prepared the slides before the demonstration. At least three glass slides should be used in the demonstration: one unmodified slide (hydrophilic) and two modified (hydrophobic and superhydrophobic) slides. A standard microscope glass slide was rinsed with deionized water, sonicated in ethanol, and then sonicated in water. It was then dried and placed in a UV−ozone cleaner for at least 15 min, but depending on the size of the cleaner, more than one slide can be cleaned at a time. The slide was then immersed in a glass vial containing 10 mL of toluene to cover the entire surface of the glass. Using a syringe, 95 μL of methyltrichlorosilane and 0.25 mL of concentrated hydrochloric acid were added before the vial was capped and stored at 4 °C (or 20 °C for a room temperature comparison) for 3−5 h. A second slide was treated the same way with 0.35 mL of octadecyltrichlorosilane and 0.25 mL of concentrated hydrochloric acid and stored at 4 °C (or 20 °C) for 1.5 h. After the required reaction time, both slides were rinsed three times each with toluene, ethanol, a 1:1 mixture of 1204
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Figure 2. Side-view images of water droplets on the glass slides: (A) untreated glass, (B) glass treated with octadecyltrichlorosilane at 20 °C, and (C) glass treated with methyltrichlorosilane at 4 °C. The dashed horizontal lines indicate the contact lines between the water droplet and the glass surface; the portion above the line is the water droplet and the portion below the line is a reflection of the droplet. The contact angles, measured by an AST VCA system, are (A) 19 ± 3°, (B) 112 ± 2°, and (C) 152 ± 3°.
Figure 4. UV−vis spectra showing the transmittance of glass slides: untreated (black trace); hydrophobic (red line); superhydrophobic prepared at room temperature (green line); superhydrophobic glass prepared at 4 °C (blue line). The inset image shows a water droplet on a transparent superhydrophobic slide.
began to slide, the particles were picked up and trapped; this can be seen by the “trail” of cleaned glass surface behind the water droplet (Figure 5). Depending on their size and the
Figure 3. Schematic representation of a normal hydrophilic glass surface after treatment with (A) octadecyltrichlorosilane that results in the formation of a highly ordered self-assembled monolayer28,30(hydrophobic) and (B) methyltrichlorosilane that results in the formation of a 3D polymethylsiloxane network (superhydrophobic). The inset shows a SEM image of the superhydrophobic surface.
air pockets when water droplets are deposited on the slide (Figure 3B).29 The air pockets, according to the Cassie−Baxter equation, increase the contact angle of the water droplet and thereby make the surface superhydrophobic (>150°). It was also observed that low-temperature reactions (at 4 °C) resulted in slightly lower contact angles (152 ± 3°) than room temperature reactions (156 ± 2°), which can be explained by different surface morphologies (see the Supporting Information). In contrast, octadecyltrichlorosilane exhibited no significant difference in contact angles when prepared at different temperatures (112 ± 3° at 4 °C).
Figure 5. Image showing water droplets on a transparent superhydrophobic glass slide covered with solid particles. Particles are trapped by the water drop as it rolls along the surface, leaving a “trail” of cleaned glass (highlighted with red rectangles).
superhydrophobicity of the surface, the water drops would roll or slide off the surface at very low tilt angles. A movie is provided in the Supporting Information to show the selfcleaning effect on a superhydrophobic glass. On the other hand, the octadecyltrichlorosilane-treated surface did not allow water droplets to trap particles, and tilting the slide to an angle of up to 90° failed to make the droplets slide off (with or without any solid particles). The different behaviors are believed to be the result of distinct contaminant−surface and contaminant−water interactions.31,32
Glass Transparency
The UV−vis spectra confirm that hydrophobic glass (octadecyltrichlorosilane-treated) has nearly the same level of transparency as untreated glass, regardless of reaction temperature (Figure 4). Superhydrophobic samples prepared at room temperature exhibit decreased transmittance due to increased light scattering. On the other hand, improved optical transparency was observed for low-temperature reactions of methyltrichlorosilane, which can be attributed to reduced reflection of light.
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Self-Cleaning
ASSOCIATED CONTENT
S Supporting Information *
To demonstrate the self-cleaning ability, water droplets on a methyltrichlorosilane-treated surface were shown to entrap solid particles. On a glass slide covered with tiny particles, several water droplets were deposited on the surface; as a drop
Notes for the instructor with detailed experimental procedures and a video demonstration of self-cleaning.This material is available via the Internet at http://pubs.acs.org. 1205
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(26) Wong, J. X. H.; Asanuma, H.; Yu, H.-Z. Thin Solid Films 2012, 522, 159−163. (27) Lamour, G.; Hamraoui, A.; Buvailo, A.; Xing, Y.; Keuleyan, S.; Prakash, V.; Eftekhari-Bafrooei, A.; Borguet, E. J. Chem. Educ. 2010, 87, 1403−1407. (28) Chan, C. J.; Salaita, K. J. Chem. Educ. 2012, 89, 1547−1550. (29) Campbell, D. J.; Andrews, M. J.; Stevenson, K. J. J. Chem. Educ. 2012, 89, 1280−1287. (30) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151−256. (31) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1−8. (32) Adamson, A. W. Physical Chemistry of Surfaces; Wiley: London, 1990.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Catherine Rong, Terry Tran, Mandy Yang (undergraduate students), Hidehiko Asanuma (graduate student), and Juan Shi (research associate) are thanked for their participation in testing the reproducibility of the experiment, and Eberhard Kiehmann for editing the manuscript. Natural Science and Engineering Research Council (NSERC) of Canada is acknowledged for financial support.
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