Tuning the hydrophobic properties of silica particles by surface silanization using mixed self-assembled monolayers

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Abstract

Here we describe a novel method of preparing hydrophobic silica particles (100–150 nm; water contact angle of dropcasted film ranging from 60° to 168°) by surface functionalization using different alkyltrichlorosilanes. During their preparation, the molecular surface roughness is also concurrently engineered facilitating a change in both the surface chemical composition and the geometrical microstructure to generate hierarchical structures. The water contact angle has been measured on drop-cast film surface. The enhancement in the water contact angle on 3D (curved) SAMs in comparison to that on 2D (planar) surface is discussed using the Cassie–Baxter equation. These silica particles can be utilized for many potential applications including selective adsorbents and catalysts, chromatographic supports and separators in microfluidic devices.

Introduction

In recent years, the use of self-assembled monolayers (SAMs) to impart a desired function to a surface has received extensive attention due to its ease of manipulation of surface energy, and thereby also of such properties like adhesion, lubrication, corrosion and anti-stiction [1]. It is well known that the water contact angle (CA) (θ), θ<90° and θ>90° can indicate surface ‘hydrophilicity’ or ‘hydrophobicity,’ respectively, and when it is above 150°, the word ‘superhydrophobic’ is sometimes used. Nature exhibits this phenomenon in ‘lotus leaf effect’ to harness the roll-off action for self-cleaning of leaves which has been attributed to a combined micro and nanoscale morphology of its surface [2], [3], [4], [5]. In brief, the surface of the lotus leaf is textured with micron-sized hills and valleys (bumps) that are decorated with nanometer sized particles of a hydrophobic wax like material, which prevents the penetration of the water into valleys [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. As a result, water cannot wet the surface and therefore forms nearly spherical water droplets, leading to superhydrophobicity. Achieving superhydrophobicity is of great current interest in view of its diverse applications such as selective adsorbents and catalysts, chromatographic supports, adhesives in paints, fabrics and low-friction surfaces and separators in microfluidic devices [6], [7], [8].

Artificial superhydrophobic surfaces have been prepared using various strategies including the generation of rough surfaces first and then modification with low surface energy molecules or roughening the surface of hydrophobic materials and creating well-ordered structures using micromachining and etching [5], [6], [7], [8], [9], [10], [11], [12]. Various methods have recently been proposed to create superhydrophobic surfaces, including electrochemical deposition, plasma fluorination, sol–gel, UV irradiation, etc. It is well known that, the wetting property of a solid surface is governed by both its chemical composition and geometric microstructure [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. In addition, a few methods have been reported to make the superhydrophobic coating on silica particles by forming polyelectrolyte multilayer films by layer-by-layer process, or by forming the film of silica particles on substrate by Langmuir–Blodgett (LB) technique and subsequent formation of a alkylsilane SAM for fabricating hydrophobic surfaces. It is well known that, the wetting property of a solid surface is governed by both its chemical composition and geometric microstructure [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37].

In this work, we report the enhancement in the hydrophobicity of the SAM formed on the curved surface (silica particles). Our strategy is to optimize the hydrophobicity of silica particles (100nm) using different organosilane molecules (as the chemical compositions determine the surface free energy) and thus have greater influence on wettability. Further, for contact angle measurements we have chosen drop cast films of such functionalized spheres, which automatically render hierarchical micro–nano roughness by their coating on a smooth silicon surface. The molecules employed are: octadecyltrichlorosilane (OTS), octyltrichlorosilane (OTCS), 3-aminopropyltrimethoxysilane (APTMS), 3-[tris-(trimethylsilyloxy)-silyl]-propyl methacrylate (MSMA), poly(MSMA), dodecyltrichlorosilane (DTS), and decyltrichlorosilane (DTCS). Characterization data obtained using CA measurements, FTIR spectroscopy and SEM have been discussed to illustrate the reasons for this superhydrophobic behavior.

Section snippets

Materials

n-Octadecyltrichlorosilane (C18, OTS) (95%), octyltrichlorosilane (C8, OTCS), dodecyltrichlorosilane (C12, DDS), decyltrichlorosilane (C10, DCS), 4-aminopropyltrimethoxysilane (APTMS), 4-[tris-(trimethylsilyloxy)-silyl]-propyl methacrylate (MSMA) and tetraethyl orthosilicate (TEOS) were obtained from Aldrich, while toluene (99.5%), ethanol (99.5%) and ammonia were purchased from Qualigens. Commercially available n-type, one-side polished, silicon wafers of (100) orientation with 0.001–0.007

FTIR spectroscopy

Superimposed FTIR-DRIFT spectra of both pristine and modified silica samples using different silane molecules are shown in Fig. 2A, which gives a clear evidence for surface modification. In the low frequency region, 700–1400 cm−1, fundamental Sisingle bondO bands (bending vibrations at 800cm−1 and stretching vibrations at 1000–1300 cm−1, respectively) are invariant with surface modification, while in the high frequency region certain distinct changes could be seen. Nevertheless, both samples show bands

Conclusions

Surface modification of silica particles (100 nm) using selected self-assembled monolayers of organosilane molecules enables tuning of their hydrophobic/hydrophilic properties by controlling the chain length, the nature of molecule as well as by using a mixture of two or more SAM molecules. The tunability ranges from superhydrophilicity with a CA of 10° to superhydrophobicity with a CA of 168° after surface modification with alkyltrichlorosilane. These results suggest that creating the

Acknowledgements

The authors acknowledge the financial support from Council of Scientific and Industrial Research (CSIR)-New Delhi. S.A.K. would like to thank CSIR for providing research fellowship.

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