Elsevier

Chemical Physics Letters

Volume 761, 16 December 2020, 138076
Chemical Physics Letters

Research paper
Structural, thermal and surface properties of sticky hydrophobic silica films: Effect of hydrophilic and hydrophobic precursor compositions

https://doi.org/10.1016/j.cplett.2020.138076Get rights and content

Highlights

  • DMDMS:TEOS molar ratio had a significant impact on the hydrophobicity of thin films.

  • Water droplets dropped on DMDMS:TEOS coated glass surfaces stuck to the glass surface.

  • Sticky property comes from the roughness and chemically heterogeneous of solid surface.

  • The hydrophobicity of DMDMS:TEOS silica thin films occurred on a sticky Wenzel state.

Abstract

The synthesis and study of silica thin films’ adhesive properties as a function of dimethyldimethoxysilane: tetraethoxysilane (DMDMS: TEOS (DT)) molar ratio and calcination temperature and its relationship with the silica polymer shape have been conducted. DT molar ratio was very influential on the silica thin film’s hydrophobic nature coated on the glass surface and indicated that the water droplets stuck to the silica-coated glass surface, not rolling down. The sticky property mainly originated from the non-uniformity of functional group composition and material surface roughness. The highest hydrophobic properties were obtained from the balanced composition of the DT molar ratio.

Introduction

Nature provides a method of survival for species that inhabit ecosystems with various features. For example, tenebrionid beetle (Onymacris unguicularis) that lives in dry and semi-arid regions can collect water droplets from the fog to be absorbed by their bodies [1], [2]. The ability to collect water is assumed to originate from the material characteristics of these living things that cause water to stick to their body surfaces [3]. Garrod, et al. [4] tested fog harvesting capabilities and found that rough surfaces attracted more water than smooth surfaces. In their works, any small water droplets formed on the hydrophobic areas were blown across the surface until they reached a critical droplet size for movement. Arguably on smooth surfaces, more hydrophobic surfaces collect more water than hydrophilic surfaces. This contradicts the general idea that hydrophobic surfaces tend to repel water as occurs on lotus leaves. Therefore, the study of surface hydrophobic characters in which water can be attached to it becomes significant for more in-depth study.

Inspired by biological creatures' surfaces with hydrophobic properties, such as lotus leaves, strider's legs, and cricket wings [5], [6], hydrophobicity is revealed to originate from micro or nano-hierarchical structures and materials that have low surface energy. To date, many studies have been carried out to make hydrophobic surfaces artificially, such as through chemical deposition [7], colloid assembly [8], layer by layer deposition [9], and so forth.

In general, the contact angle of water droplets is used to measure surface hydrophobicity. For large contact angles (>90°), the surface is referred to as hydrophobic [10], [11]. Judging from its adhesiveness on the surface, water droplets easily move or shift over the material surface, such as water on lotus leaves. However, there are also water droplets that remain attached to the surface even though the surface is tilted with a high slope angle [12]. Whether water droplets curl or stick to a surface is determined by surface chemical characteristics and surface roughness or topography. Wenzel and Cassie-Baxter proposed two models of the placement of water droplets on the surface of the material.

According to Wenzel [13], water droplets maintain contact at all points with the rough surface underneath. The contact angle in the Wenzel model is expressed by equation (1), with the roughness factor (r) as the ratio of the actual area of the rough surface to the geometric projection area. The roughness ratio, r, is a measure of how surface roughness affects a homogeneous surface. This equation shows that surface roughness increases hydrophobicity because r is always greater than 1 [12], [14].cosθW=rcosθs

θW is the clear contact angle that corresponds to a stable equilibrium state, while θs is the Young contact angle as determined for the ideal surface.cosθs=γSV-γSLγLV

γLV is the liquid-vapor surface tension, and γSL and γSV are solid–liquid and solid–vapor interfacial tensions.

On the other hand, in the Cassie-Baxter model [15], water droplets rest on the top of the surface protrusion, and between water droplets with a surface base, there is space filled with air. The surface area has a solid area fraction φs with the contact angle θs, and the air area fraction φair with a contact angle θair, the Cassie-Baxter contact angle (θCB)is formulated by equation (3) [10].cosθCB=φscosθs+φaircosθair

In Wenzel, an increase in the equilibrium contact angle on a rough surface is escorted by a strengthening effect to deal with perturbations in the contact angle. Conversely, in the Cassie-Baxter model, an increase in equilibrium contact angle on rough surfaces is convoyed by a weakening effect on contact angle perturbations. This theory quantitatively results in a “sticky” surface for the Wenzel type surface and a “slippery” surface for the Cassie-Baxter type surface.

The hydrophobicity of dimethyldimethoxysilane: tetraethoxysilane (DMDMS: TEOS, which for the next term is abbreviated as DT) silica thin film in the studies of the functional group has been extensively discussed in our previous paper [16]. In that paper, the relationship between the surface hydrophobic character and the functional group forming the silica skeleton was studied. It was concluded that there is a close relationship exists between the hydrophobic properties and the functional group composition, especially in the Si-OH and Si-CH3 groups.

Further research on these thin films showed that this material has sticky natures. Although studies on the silica hydrophobicity have been carried out intensively, there are still many things that need to be revealed in the phenomenon of sticky hydrophobic silica. To our knowledge, there has been no study of the relationship between the composition of hydrophilic precursor (in this case, TEOS) and the hydrophobic one (in this case, DMDMS) on the hydrophobicity of silica thin film. Therefore, in this study herein, we intend to observe the influence of the DT molar ratio on the sticky hydrophobic phenomena, especially on structural, thermal, pore, and surface properties. To explore the effect of silica precursor composition, several compositions of DT molar ratio were selected, as reported in our previous study [16].

On the other hand, the composition of the precursor solution to the solvent and the catalyst was carefully adjusted. Water droplet and slope tests were used to assess the hydrophobic character of the silica thin film coated on the glass surface. In this study, the effect of surface roughness on the adhesiveness of thin films was examined. The surface morphology of DT modified silica thin films was analyzed by a scanning electron microscope (SEM) in place of atomic force microscopy (AFM), which cannot be obtained due to limited equipment. The relationship between hydrophobicity and thermal properties as well as pore and surface morphology was also examined, which analyzed using thermogravimetric analysis (TGA) and gas sorption analysis (GSA) to review the character of silica thin films for consideration of further applications such as thin-film membranes or adsorption.

Section snippets

Materials

Hydrophobic silica thin films were synthesized using two silica precursors, i.e., dimethyldimethoxysilane (DMDMS) (98% pure) from Aldrich and tetraethoxysilane (TEOS) (99% pure) from Merck. The solvent used in the hydrolysis and condensation steps was ethanol from Merck (99% pure) with an ammonia solution (25%) from Merck as the catalyst. Distilled-deionized water and glass slide with a thickness of 2 mm (Sail Brand) were used in this experiment.

Preparation of DT xerogels and thin films

Silica thin films were synthesized by the sol-gel

Sticky hydrophobic properties

The coating of the silica DT sols on the glass surface produces a clear transparent thin film in which the film wettability depended on DT composition. The silica thin film shows a strong adhesion to water droplets where the water droplets do not roll due to gravity even though the glass is tilted (Fig. 1b), upright vertical (Fig. 1c), or even reversed (Fig. 1d). This is like what happens on rose petals’ surface, where droplets still pin when the petals are turned upside down. This character is

Conclusions

This work shows that silica thin films derived from dimethyldimethoxysilane -tetraethoxysilane (DMDMS: TEOS) on glass surface have a considerable influence on the sticky hydrophobic phenomenon, especially on the DT composition of 50:50. The water droplets do not move after the tilting test on the glass surfaces coated by DT thin films from slope angles of 45°, 90°, and 180°. Micro-sized asperity textures are formed on silica thin films. The strong adhesiveness and convexity of thin-film

CRediT authorship contribution statement

Adi Darmawan: Conceptualization, Methodology, Visualization, Investigation, Writing - review & editing. Riza Eka Saputra: Data curation, Investigation, Writing - original draft. Yayuk Astuti: Supervision, Validation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Adi Darmawan and Yayuk Astuti gratefully acknowledge financial support from the Ministry of Research, Technology, and Higher Education, Republic of Indonesia, via the Penelitian Kompetensi (Hikom) (Number: 343-12/UN7.5.1/PP/2017). Adi Darmawan, in particular thanks to FIM2lab at the University of Queensland, for supporting all TGA and GSA analyzes.

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