the optimization of transparency and durability of self-cleaning glass

The Optimization of Transparency

and Durability and the Effects of

Rigorous and Consistent Use on

Superhydrophobic Surfaces

Simone Griffith1 and Ekaterina Khlystova1

1North Carolina School of Science and Mathematics

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

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-

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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.

Figure 1: Sonicator

Figure 2: Solutions

Figure 3: Goniometer

Figure 4: FTIR

Figure 5: Shaker

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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

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.

0

20

40

60

80

100

120

140

160

Control OTS 0.05mLHCl 1 Layer

OTS 0.05mLHCl 2 Layer

OTS 0.05mLNaOH 1

Layer

OTS 0.05mLNaOH 2

Layer



OTS 0.1mLNaOH 1

Layer

OTS 0.1mLNaOH 2

Layer

Co

nta

ct A

ngl

e (°

)

OTS Treatment

Graph 1: Hydrophobicity in OTS Treated Slides

0 Hours

24 Hours

44 Hours

0

20

40

60

80

100

120

140

Control MTS 0.05mLHCl 1 Layer

MTS 0.05mLHCl 2 Layer

MTS 0.05mLNaOH 1

Layer

MTS 0.05mLNaOH 2

Layer



MTS 0.1mLNaOH 1

Layer

MTS 0.1mLNaOH 2

Layer

Co

nta

ct A

ngl

e (

°)

MTS Treatment

Graph 2: Hydrophobicity in MTS Treated Slides

0 Hours

24 Hours

44 Hours

0

10

20

30

40

50

60

70

80

90

Control MTS,0.1mLNaOH,1 Layer

MTS,0.05mLHCl, 2Layer


OTS,0.05mLHCl, 1Layer

MTS,0.05mLNaOH,1 Layer




MTS,0.05mLNaOH,2 Layer

OTS,0.05mLNaOH,2 Layer



MTS,0.1 mLNaOH,2 Layer



Rat

io o

f C

on

tact

An

gle

to I

R A

bso

rban

cy (

º)

Type of Treatment

Graph 3: Hydrophobicity and Transparency

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.

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.

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?

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