the optimization of transparency and durability of self-cleaning glass

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A presentation for the Sigma Xi virtual competition

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  • 1. The Optimization of Transparency and Durability and the Effects of Rigorous and Consistent Use on Superhydrophobic Surfaces

2. Simone Griffith1 and Ekaterina Khlystova1 1North Carolina School of Science and Mathematics 3. Goals Make durable, transparent self-cleaning glass Use Octadecyltrichlorosilane (OTS) to make more hydrophobic glass than Methyltrichlorosilane (MTS) Use multiple layers of alkyltrichlorosilane to improve durability of hydrophobic glass Use sodium hydroxide as a catalyst to make more transparent glass than hydrochloric acid Use 0.05mL of catalyst to make more transparent glass than 0.1mL of catalyst 4. Introduction Millions of dollars and hours of labor are spent every year on window cleaning around the world. Hydrophobic glass is self-cleaning as the water forms droplets and rolls easily off the surface, carrying any particles with it. Self-Assembled Monolayers (SAMs), specifically the alkyltrichlorosilane group which can bond to glass, form a nonpolar layer, and make glass hydrophobic. Longer silane chains, such as those of OTS, yield more hydrophobic glass, whereas short chains, such as those of MTS yield less hydrophobic surfaces. In our project, we seek to optimize the hydrophobicity, transparency, and durability of silane-coated glass in order to make widespread distribution of self-cleaning glass a tangible reality. http://www.sigmaaldrich.com/content/dam/sigma- aldrich/structure1/068/mfcd00000481.eps/_jcr_content/renditions/medium.png 5. Abstract Hydrophobic glass is self-cleaning, meaning a spray with water can easily wash away foreign particles. However, self-cleaning glass is not yet in widespread commercial use. This project focuses on improving the durability and transparency of self-cleaning glass to facilitate future use in public. Basic recipes for the hydrophobic self-assembled monolayers (SAM) methyltrichlorosilane (MTS) and octadecyltrichlorosilane (OTS) were applied onto glass slides with one or two layers, and with varying amounts of either hydrochloric acid or sodium hydroxide, which act as catalysts. Each glass slide was analyzed for transparency using FT-IR spectroscopy. Hydrophobicity was measured by finding the contact angle of a small drop of water on the slides in a homemade goniometer. Durability was tested by measuring hydrophobicity after placing the slides in water and into a shaker for a total of 44 hours. We found that glass slides with two layers of SAM were initially slightly less hydrophobic than their single layer counterparts. Overall, hydrophobicity appeared to decrease with use; however, a second layer of SAM helped prevent deterioration during the first 24 hours with slides catalyzed with NaOH, and we found that OTS SAMs retained their hydrophobicity longer than MTS SAMs. Finally, we observed that SAMs made with NaOH had the highest contact angle to absorbance ratio. The amount of catalyst, however, had insignificant effects on the slide. Our work addressed key issues present in the use of silane-coated glass and provides a foundation for hydrophobic glass to be commercially available for everyday use. 6. Figure 1: Sonicator Figure 2: Solutions Figure 3: Goniometer Figure 4: FTIR Figure 5: Shaker http://upload.wikimedia.org/wikipedia/en/thumb/3/3b/Ncssmlogo.png/150px-Ncssmlogo.png 7. Methods Goniometer We made a goniometer from a cardboard box and other supplies Cleaning Sonicate in water and ethanol; dry in oven Recipes Prepare each of the SAM recipes containing MTS, OTS, and different amounts of NaOH and HCl Application Place 2 glass slides in each vial; cool in refrigerator; rinse slides with toluene, ethanol, a 1:1 mixture of ethanol and water, and water; place 2 layers slides back in refrigerator Contact Angles Photograph water on glass slides in the goniometer; determine hydrophobicity using ImageJ software Transparency Place slides in a FT-IR Spectrophotometer to measure absorbance at a certain wavelength Durability Lay slides in containers with water in a shaker for 24 hours; take contact angle pictures; repeat after 20 hours 8. Results Graph 1 compares the average contact angle of OTS slides treated identically after 0 hours, 24 hours, and 44 hours of shaking to those with an additional layer. It reveals that, when made with NaOH, adding an additional layer generally helps the slides retain hydrophobicity after use. Graph 2 compares the average contact angle of slides with different MTS recipes and layers after 0 hours, 24 hours, and 44 hours of shaking. Slides made with NaOH retained hydrophobicity better after 24 hours. MTS 0.05mL HCl 1 Layer is an outlier in the data. Reproducing these tests in the future will indicate whether this data point is reliable. Graph 3 indicates the optimal ratios of hydrophobicity to FT-IR absorbance; the trends indicate that overall NaOH is the better catalyst to use. Additionally, slides coated with OTS tend to maximize the ratio, with no particular trend indicating the optimal number of layers or amount of catalyst. The contact angle of the MTS 0.05mL HCl 1 Layer slide was unexpectedly high and further study is necessary to determine why this outlier occurred and whether it will occur again in duplicate tests. 9. 0 20 40 60 80 100 120 140 160 Control OTS 0.05mL HCl 1 Layer OTS 0.05mL HCl 2 Layer OTS 0.05mL NaOH 1 Layer OTS 0.05mL NaOH 2 Layer OTS 0.1mL HCl 1 Layer OTS 0.1mL HCl 2 Layer OTS 0.1mL NaOH 1 Layer OTS 0.1mL NaOH 2 Layer ContactAngle() OTS Treatment Graph 1: Hydrophobicity in OTS Treated Slides 0 Hours 24 Hours 44 Hours 10. 0 20 40 60 80 100 120 140 Control MTS 0.05mL HCl 1 Layer MTS 0.05mL HCl 2 Layer MTS 0.05mL NaOH 1 Layer MTS 0.05mL NaOH 2 Layer MTS 0.1mL HCl 1 Layer MTS 0.1mL HCl 2 Layer MTS 0.1mL NaOH 1 Layer MTS 0.1mL NaOH 2 Layer ContactAngle() MTS Treatment Graph 2: Hydrophobicity in MTS Treated Slides 0 Hours 24 Hours 44 Hours 11. 0 10 20 30 40 50 60 70 80 90 Control MTS, 0.1mL NaOH, 1 Layer MTS, 0.05mL HCl, 2 Layer MTS, 0.1mL HCl, 2 Layer OTS, 0.05mL HCl, 1 Layer MTS, 0.05mL NaOH, 1 Layer OTS, 0.1mL HCl, 1 Layer OTS, 0.05mL HCl, 2 Layer MTS, 0.1mL HCl, 1 Layer MTS, 0.05mL NaOH, 2 Layer OTS, 0.05mL NaOH, 2 Layer OTS, 0.1mL NaOH, 2 Layer OTS, 0.05mL NaOH, 1 Layer MTS, 0.1 mL NaOH, 2 Layer OTS, 0.1mL NaOH, 1 Layer MTS, 0.05mL HCl, 1 Layer RatioofContactAngletoIRAbsorbancy() Type of Treatment Graph 3: Hydrophobicity and Transparency 12. Discussion OTS-coated slides had higher contact angles than the MTS-coated slides immediately after application. OTS slides also retained more hydrophobicity after 24 hours and after 44 hours than their MTS counterparts. Although an additional layer of silane initially seems to decrease the hydrophobicity of the glass slide, it appears to be more durable and retain hydrophobicity more than the single-layer slides after 24 hours in the shaker for only slides made with NaOH. This trend is not present after 44 hours in the shaker. The contact angle to absorbance ratio was highest when OTS and NaOH were used, indicating these substances improve transparency and hydrophobicity. There were some inconsistencies and outliers in the data we collected. Duplication of the tests performed will help determine if this data is reliable and why the outliers occurred. 13. Conclusion Our questions for this project were: will a second layer of SAM withstain heavy use better than a single layer on glass? Will the amount and type of catalyst improve transparency? We found that: o Slides made with OTS were more hydrophobic than those made with MTS, which supported findings from readings. o A second layer of alkyltrichlorosilane initially decreases the hydrophobicity of glass slides. o In the silanes catalyzed with NaOH, the second layer of silane increases glass hydrophobicity after 24 hours of shaking, before rapidly decreasing in hydrophobicity after 44 hours. o Slides made with NaOH catalyst generally had a higher ratio of contact angle to absorbance. These slides were the most hydrophobic and the clearest. o The amount of catalyst used was inconsequential. 14. Future Work The following questions have yet to be answered and will be addressed in our future work in this subject area: If our tests are duplicated, will the outliers still exist in the data? Why does adding a second coat initially decrease the contact angle but exhibit a trend of greater contact angles after testing for NaOH slides? How can the mechanism behind additional layers of alkyltrichlorosilanes be analyzed further using scanning electron microscopy (SEM)? Is it possible to make one layer of alkyltrichlorosilane as durable as multiple layers? What other methods of testing could model everday use, and how would these methods affect hydrophobicity? Is NaOH the best catalyst available for low absorbance and high hydrophobicity and why was it better than HCl? Is there a more effective and less labor-intensive alternative to coating glass with alkyltrichlorosilanes? How can we reduce error and inconsistencies? 15. Acknowledgements Dr. Myra Halpin, North Carolina School of Science and Mathematics Preparation of Transparent Superhydrophobic Glass Slides: Demonstration of Surface Chemistry Characteristics, Jessica X. H. Wong and Hua-Zhong Yu, Journal of Chemical Education 2013 90 (9), 1203-1206 Contact Angle Measurements Using a Simplified Experimental Setup. Guillaume Lamour, Ahmed Hamraoui, Andrii Buvailo, Yangjun Xing, Sean Keuleyan, Vivek Prakash, Ali Eftekhari- Bafrooei, and Eric Borguet: Journal of Chemical Education 2010 87 (12), 1403-1407 Performance and durability of octadecyltrichlorosilane coated borosilicate glass, Kirkpatrick, R., and C. L. Muhlstein, Journal of Non-Crystalline Solids, 2007, v. 353, p. 2624-2637. References

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