Elsevier

Applied Surface Science

Volume 253, Issue 14, 15 May 2007, Pages 5972-5979
Applied Surface Science

Long term studies on the chemical stability of a superhydrophobic silicone nanofilament coating

https://doi.org/10.1016/j.apsusc.2006.12.118Get rights and content

Abstract

We have performed extensive studies on the long term chemical stability of a superhydrophobic coating comprised of silicone nanofilaments. Durability was tested by immersion in various liquid media over a period of 6 months. The coating properties were monitored by static contact angle and sliding angle measurements. Changes in surface topography were examined by scanning electron microscopy. The coatings show an exceptional stability in organic solvents, neutral and mildly acidic aqueous solutions and mildly acidic detergent solutions. The superhydrophobic coating properties are stable for several days in mildly basic and strong acidic solutions but deteriorate fast under strong basic conditions.

Introduction

Superhydrophobic surfaces have generated a lot of interest in the last decade due to their significant potential for industrial and scientific applications. Ever since Barthlott and Neinhuis discovered the context between the chemical and structural nature of the lotus leaf surface and its strong water repellent and self cleaning properties [1], a wide variety of methods to mimic this effect on artificial surfaces have been discovered (see reviews [2], [3], [4], [5], [6]). Efforts have been made to theoretically evaluate the design criteria for optimizing the superhydrophobic effect in terms of contact angle and contact angle hysteresis [7], [8], [9], [10], [11], [12]. However, the biggest challenge in this field still remains the facile and cheap production of superhydrophobic coatings with long term stability under the conditions of use [2], [3].

Due to the high surface roughness required for the superhydrophobic effect, the surfaces are generally easily damaged by scratching or abrading. In nature, this can be balanced by regenerative processes [1], a concept that is not easily transferable to artificial surfaces. Fortunately, a number of applications can be envisioned where surfaces are not subject to strong abrasive forces, from large scale outdoor architectural applications like self cleaning facades or window panes to small scale liquid handling devices used in microfluidics. Here the usefulness of a superhydrophobic coating is determined by its chemical stability. A minimum requirement for application is that the surface retains its water repellent properties on contact with the medium itself. This is not trivial, as on some apparently superhydrophobic surfaces, water drops have been observed to slowly seep into the surface after a few minutes [13], [14]. A useful superhydrophobic surface should resist wetting even after prolonged exposure to water. The coating should withstand varying pH values, surface additives or solvents. Although a large number of fabrication techniques for (super) water repellent coatings have been published, stability data, especially in regard to long term stability, are scarcely reported.

In addition to establishing the durability of their coating properties at various temperatures, Erbil et al. observe that their superhydrophobic polypropylene coating did not debond in water, boiling water or heptane [15]. However, no contact angle values are reported to indicate whether the surface properties deteriorate through this treatment. Samples kept at temperatures higher than 30 °C and more than 80% relative humidity showed a decrease in the static contact angle of 10–20°. Feng et al. report superhydrophobic nanostructured carbon films that maintain static contact angles above 150° after 24 h immersion in pure water (pH  7), acidic solution (pH  1) and basic solution (pH  14) [16]. Several groups [17], [18], [19], [20] present superhydrophobic surfaces that are stable under ambient or high humidity conditions for weeks to months. Additional exposition of the surfaces to water overnight [18] or a week [19] are reported not to significantly change the surface properties. Guo et al. report “essentially” stable static water contact angles after soaking their coatings in water, acidic and basic solutions for several hours [20]. Yan et al. [21] report unaltered static contact angles after immersion of super water repellent poly(alkylpyrrole) films in solvents, oils, hot and cold water, without providing immersion times. Recently Wang et al. reported a superhydrophobic coating that retains static contact angles above 150° after a 5 days immersion in acetone, ethanol, toluene, water or hot water [22]. Mael et al. report a superhydrophobic coating with a static contact angle of 150° after more than 60 days of immersion in water [23].

We have developed a novel, simple technique of coating a variety of materials with a dense layer of polymethylsilsesquioxane nanofilaments [24], [25]. The coating exhibits a pronounced superhydrophobicity, is optically transparent, and was already shown to be stable at ambient conditions, at elevated temperatures and for short immersion times in mildly basic and acidic solutions.

In this work we present the results of extensive durability studies on the silicone nanofilament coating. The resistance to organic solvents, aqueous solutions at various pH and aqueous detergent solutions has been evaluated in terms of surface functionality and topography. The change in wettability has been monitored for up to 6 months of full immersion by static contact angle and sliding angle measurements. Changes in surface structure were examined by scanning electron microscopy.

Section snippets

Chemicals and materials

Toluene (p.a.), acetone (p.a.) and chloroform (p.a.) were purchased from Fluka and used as received. pH 0 Solution was produced by diluting hydrochloric acid (p.a., Fluka), pH 3 solution with diluted acetic acid (p.a., Fluka), pH 11 with diluted ammonia solution (p.a., Fluka) and pH 13 with diluted sodium hydroxide (purum, pellets, Fluka). pH measurements were performed with an inoLab pH electrode (WTW, Germany). Ammonia and acetic acid were chosen for the mild pH solutions since they are part

Results and discussion

A major advantage of silicones is their chemical inertness which has led to their widespread application in nearly all areas of life [26], [27]. The resulting chemical stability is well reflected by the stability of the investigated silicone nanofilament coating.

The coating and its properties are not significantly influenced by organic solvents. Fig. 1 shows the progression of water contact and sliding angles as a function of immersion time in the polar solvents acetone and ethanol and the

Summary and conclusion

We have presented the results of extensive studies on the long term chemical durability of silicone nanofilaments in view of their application as superhydrophobic coatings. Both static contact angles and sliding angles were considered in evaluating the superhydrophobic properties.

Annealing considerably increases the stability of the coatings. Annealed samples show a good resistance to organic solvents, neutral and mildly acidic pH solutions and mildly acidic detergent solutions over a period of

Acknowledgements

We thank Reto Hess of the IMPAG AG for supplying the detergent solutions. Also we thank Hanspeter Gautschi of the Electron Microscopy Center of the University of Zürich and Heinz Gross of the ETH Zürich for advice on sample preparation and interpretation of electron microscopy images.

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