Surface hydrophobicity: effect of alkyl chain length and network homogeneity
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RESEARCH ARTICLE
Surface hydrophobicity: effect of alkyl chain length and network homogeneity Wenqian Chen (✉)1, Vikram Karde1, Thomas N. H. Cheng1, Siti S. Ramli2, Jerry Y. Y. Heng1 1 Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, UK 2 Department of Food Technology, Faculty of Applied Sciences, Universiti Teknologi MARA (UiTM), Shah Alam, Selangor, Malaysia
© The Author(s) 2020. This article is published with open access at link.springer.com and journal.hep.com.cn 2020
Abstract Understanding the nature of hydrophobicity has fundamental importance in environmental applications. Using spherical silica nanoparticles (diameter = 369 7 nm) as the model material, the current study investigates the relationship between the alkyl chain network and hydrophobicity. Two alkyl silanes with different chain length (triethoxymethylsilane (C1) vs. trimethoxy(octyl)silane (C8)) were utilised separately for the functionalisation of the nanoparticles. Water contact angle and inverse gas chromatography results show that the alkyl chain length is essential for controlling hydrophobicity, as the octylfunctionalised nanoparticles were highly hydrophobic (water contact angle = 150.6° 6.6°), whereas the methylfunctionalised nanoparticles were hydrophilic (i.e., water contact angle = 0°, similar to the pristine nanoparticles). The homogeneity of the octyl-chain network also has a significant effect on hydrophobicity, as the water contact angle was reduced significantly from 148.4° 3.5° to 30.5° 1.0° with a methyl-/octyl-silane mixture (ratio = 160:40 µL$g–1 nanoparticles). Keywords hydrophobicity, surface energy, wettability, alkyl chain network, silica nanoparticle
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Introduction
Due to their low cost, exceptional stability as well as controllable physiochemical properties, silica-based nanostructures have a wide range of environmental applications including oil recovery, carbon dioxide storage and environmental remediation [1–4]. Spherical nanoparticle prepared via the Stöber method is one of the most popular nanostructures due to its high monodispersity in size and shape as well as its versatility for chemical modification [5–13]. The diameter of Stöber nanoparticle
can cover the entire nanometre range (i.e., 10 nm–1 µm) by controlling the synthesis conditions such as pH and temperature [5,6]. Through methods such as co-condensation and grafting, different chemical groups can be attached to the surface of Stöber nanoparticles [14]. For instance, superhydrophobic Stöber nanoparticles can be obtained by attaching fluorinated copolymers onto their surface [15]. The surface chemistry of silica nanoparticles is of fundamental importance for colloidal stability as it affects the interaction among the nanoparticles themselves as well as between the nanoparticles and the surrounding [16]. In the case of Pickering emulsions, it was found that nanoparticles with an intermediate degree of hydrophobicity (i.e., a water contact angle ~90°) could stabilise droplets in water-in-oil and oil-in-water systems [17
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