Fabrication of glycerol functionalized silica nanoparticles via pickering emulsion for durable, superhydrophobic fabrics

Stable oil-in-water Pickering emulsions were successfully developed via the simplest approach: amine-terminated polydimethylsiloxane with very high viscosity and low surface free energy was encapsulated by glycerol functionalized silica NPs using a rotor-stator device. The factors affecting the Pickering emulsions performance were investigated. At a certain process (silica NPs concentration: 3 wt%, Oil/water ratio: 1:20, emulsifying speed: 15,000 rpm, emulsifying time: 5 min), it gave smaller and more stable emulsions. Then the Pickering emulsion was applied on plain weave cotton and polyester fabrics through conventional pad-dry-cure method. The coated substrates showed superhydrophobicity and maintained good water-repellency even after 30 standard laundering cycles, which are attributed to the synergistic effect of low surface free energy composite and hierarchical roughness. By contrast, fabrics coated with classical emulsifiers stabilizing Pickering emulsion adopting the same recipes were quickly wetted by liquid droplets.


Introduction
Conventionally, surfactants are used to prevent coalescence in two immiscible liquids, thus stabilizing the emulsions. However, owing to their relatively high cost, impractical recovery, cell damage and potential environmental deterioration, surfactants are gradually replaced by new 'green' stabilizers [1,2]. Recently, solid particles anchored to the oil-water interface, known as 'Pickering emulsion', have been highly investigated. These so-called particle-stabilized emulsions are firstly described by Ramsden and Pickering [3,4]. The Pickering emulsion formulation announces that solid particles contributing to stable emulsions should be wetted by both phases, and the types of emulsions are mainly dependent on the intrinsic surface wetting properties of particles. Normally, O/W emulsion is attributed to the contact angle at the oil-particle-water interface which is lowered than 90°, namely, solid particles are preferable to be wetted by water. Whereas, W/O emulsion is formed when the contact angle exceeds 90°which says solid particles are more likely to be wetted by oil [5]. Besides, the particle morphology, further physical or chemical modification and system surroundings have an influence on the emulsion performance.
To date, various available solid particles have been attempted for stabilizing Pickering emulsions [6][7][8][9][10]. Among all of the popular candidates, silica has gained extraordinary attentions for its humongous commercial availability. Specifically, silica is the most abundant mineral on the Earth. Since 1930 s, silica has been widely used as binders, catalysts, absorbents, densifiers, stabilizers, et al. As it is known that the bare silica nanoparticles are too hydrophilic to stabilize emulsion alone [11,12], selective modifications are necessary to tune the amphiphilicity, thus grafted silica will be spontaneously adsorbed onto oil-water interface, and stable Pickering emulsions are expected. Armes and co-workers utilized an ultrafine anionic silica sol to stabilize active monomers and further used a cationic initiator to (co) polymerize the monomers. In addition, they also reported the formulation of vinyl (co)monomer encapsulated Pickering emulsion by functionalized ultrafine silica sol [12,13]. Their works studied the tentative mechanism for the formation of silica-stabilized Pickering emulsions, so as to provide considerable reference for the future researches. Bjorkegren et al evaluated the emulsification abilities by formulating Pickering emulsions with hydrophilic or hydrophobic groups, or the both functionalized colloidal silica particles [14]. The results revealed that the best emulsification performance was achieved by combining both hydrophobic and hydrophilic functionalization together, and all prepared Pickering emulsions were highly stable towards coalescence. The study offers future researchers very big selection leeway to choose right particles for given applications. Chevalier et al explored the relationship between oil properties and the stabilization of oil-in-water Pickering emulsions which were formed by hydrophilic fumed silica particles [15]. It was found in their study that polar oils, such as short chain mono or diester, isopropyl or isobutyl esters, were favorable for successful emulsification, whereas failures were obtained with poor polar or non-polar oils, like 2-ethylhexyl esters, mineral oil and silicone oil.
In the present work, we report a facile approach of stabilizing O/W emulsions with commercially available glycerol groups functionalized silica sol and aim at their good superhydrophobic performance on textiles (figure 1). Adopting the simplest formulation: oil, water and silica nanoparticles (NPs), yet without any additional auxiliaries like salts, pH buffers and so on, high viscous amine-terminated polydimethylsiloxane (PDMS) is successfully emulsified in the stable Pickering emulsion, which to the best of our knowledge has never been obtained so far [16]. The chosen commercial silicone oil is widely applied to many areas, for example textile and dyeing & finishing, playing a vital role in lubrication, antifoaming, water-repellency and so on. Unlike the expensive fluorinated compounds that may cause seriously environmental concerns, Hozumi and co-workers found that PDMS was water-resistant owing to their low surface tension of 20 mN m −1 at room temperature [17,18]. As a result, PDMS would be a good alternative to fluorinated compounds. We hypothesized that the method would take full advantage of low surface tension polymer PDMS and surface roughness constructer silica NPs when applied in water-repellent surfaces field, specifically in textile industry. Superhydrophobic surfaces with a static water contact angle larger than 150°and low sliding angle have drawn enormous interests for decades [19][20][21][22][23][24][25]. Considering that textiles would inevitably be exposed to the environment, it is necessary to make them liquids-resistant in different kinds of applications. Compared to some previous water-repellent fabrics [26], the coated fabrics herein are superhydrophobic, wear durable as well. What is more, because of the low friction coefficient of PDMS, corresponding to its lubricating nature, the treated fabrics have good softness and smoothness.

Chemicals and materials
Glycerol-functionalized ultrafine silica sol (Bindzil CC40) was supplied by AkzoNobel Specialty Chemicals, The Netherlands. According to the supplier's information, pH of the sol was about 8, the solid content was 37 wt% without any sodium oxide, the particle diameter ranged from 5 to 50 nm with an average of 12 nm, the density and viscosity of the sol were 1.27 g cm −3 and 8 mPa·s, respectively, which were measured under the temperature of 20°C. The amino polydimethylsiloxane (PDMS, Mw = 4000 g mol −1 ) with a high viscosity over 13,400 mPa·s was kindly provided by Momentive Silicone Materials, Shanghai, China. The cotton fabrics and polyester fabrics were purchased from Shaoxing Liuyi Textile Co., Ltd. The commercial prevalently used surfactants ISO-5 and ISO-7 were kindly provided by Sasol (China) Chemicals, Shanghai. Deionized water was used in this study.

Formation of Pickering emulsions
PDMS/silica Pickering emulsions were prepared by homogenizer assisted emulsifying method. Briefly, the Bindzil CC40 silica sol was diluted in water (dilute concentrations from 1% to 10%). After combining the dilute silica dispersion and PDMS [27], the mixture was emulsified immediately with an Ultra-Turrax ® T18 digital rotor-stator device equipped with a S18 N-19 G shaft (IKA, Germany) in an ice bath. The prepared Pickering emulsions were filtrated and allowed to stand for a period of time before characterization. The emulsion type was determined by dropping a few emulsion droplets into either oil or deionized water.

Fabrication of superhydrophobic fabrics
The cotton fabrics of plain weave structure and polyester fabrics were commercially purchased, which were washed three times and then dried direct use. Pretreated cotton fabrics or polyester fabrics were first immersed in the above Pickering emulsions, subsequently padded with two dips and two nips to reach a wet pickup of 70%. Finally, the samples were dried at 80°C for 3 min and cured at 160°C for 3 min.
As a control experiment, the procedure described above was repeated for the emulsion prepared with organic surfactants ISO-5 and ISO-7. Apart from the different emulsifier species, the above two emulsions had the same PDMS concentration, emulsifier concentration, as well as the finishing process.

Characterization
The contact angle was measured according to the sessile-drop method by means of a DSA30 Contact Angle Measuring System (KRUSS GmbH, Hamburg). In order to measure the contact angle with the DSA30, a drop is placed on a sample located on a moveable sample table. The drop is illuminated from one side, and a camera with recommended resolution 1152 × 768, 16 bit color depth, at the opposite side records an image of the drop which is transferred to a computer and shown on the monitor. The DSA software contains time-proven tools for analyzing the drop image with whose help it is possible to calculate the contact angles. The droplet size of the sample was about 5 μl, therefore the gravitational effect could be neglected. The contact angles of each sample were measured at five different places, and the final value reported herein was the average value of five measurements.
The optical micrographs of the prepared Pickering emulsions were taken on transparent glass slides with a Nikon Microscope (Eclipse E100/E100LED MV, Japan). The average size of the emulsion droplets was determined by LS13320 laser diffraction particle size analyzers with one of the highest submicron resolutions, using the Polarization Intensity Differential Scattering (PIDS) technology. The microstructure of the coatings was observed using field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) equipped with a back scattering electron detector and an energy dispersion x-ray spectrometry (EDS; Quantax 400, Bruker).

Results and discussion
3.1. Emulsification and characterization 3.1.1. Effect of silica NPs concentration on the Pickering emulsion Pickering emulsion is affected by various factors, such as silica NPs concentration, emulsifying speed and emulsifying time. As reported in previous researches, bare silica NPs are so hydrophilic that they cannot stabilize Pickering emulsions alone. However, the glycerol-functionalized silica NPs used here are able to improve the emulsifying ability because of surface glycerol groups which are critical for tuning particles wettability. Figure 2 shows the effect of silica NPs concentration on the emulsification performance. It demonstrates that all the samples were of oil-in-water Pickering emulsions. Solid content above 10 wt% was not conducted in this work and this is important, because excess partially hydrophilic silica sol is known to compromise the water-resistant behavior of the coated textiles, which will be discussed in the following section. At a silica NPs concentration of 1 wt%, the emulsion stability was poor owing to the large emulsion droplet average size which was mostly 10 ± 2.5 μm. An increase of solid particles concentration within the certain range results in a remarkable decrease of the average particle size. The average droplet diameters decreased to 4 ± 1.5 μm when the silica NPs concentrations were 3 wt%, further to 3 ± 1.0 μm when increasing the silica NPs concentration to 5 wt% (figure 2(f)). Besides, the emulsions behaved better at higher concentrations, for example, more uniform morphology and greater stability. This could be explained as follows, for the given oil phase, in order to form Pickering emulsion at low solid particles concentration, particles distributed to each oil droplet are less than that at high solid particles concentration, which in turn causes larger droplets size [28]. Further increasing the concentration to 10 wt%, the improvement of emulsion particle size and stability were negligible. It also could be distinguished from the appearance of the prepared Pickering emulsions (figures 2(a)-(e)), too little silica NPs cause particle flocculation. Considering that the glycerol-functionalized silica NPs are critical for hierarchical features while overdosed particles will deteriorate the water-resistant performance of the finished textiles, it is crucial to balance the both essentials.

Effect of silica NPs concentration on the Pickering emulsion
The effect of emulsifying speed on the properties of Pickering emulsion was explored, and results are displayed in figure 3. As the emulsifying speed increased, the average emulsion droplets size was decreased, and the size distribution became more uniform (figures 3(a)-(d)), implying that the Pickering emulsion was more stable. With an emulsifying speed of 5000 rpm, the final size distribution of the emulsion droplet was very large around 15 ± 2.0 μm, showing the poor emulsification. When the emulsifying speed was raised up to 10,000 rpm, the droplets size decreased significantly to 7 ± 1.5 μm. At emulsifying speed of 15,000 and 20,000 rpm, the emulsion droplets possessed nearly the same and smallest diameters, as well as a greater homogeneity in the size and shape (figure 3(e)). Further increasing the emulsifying speed would not affect the emulsion obviously. During the emulsifying process, stronger dispersing power drove the silica NPs uniformly anchor on the PDMS/water interface. This is one of the reasons responsible for stable Pickering emulsions.

Effect of emulsifying time on the pickering emulsion
Emulsifying time is another important factor affecting the stability of the Pickering emulsions, which is illustrated in figure 4. Emulsifying for only 1 min led to a large size and uneven dispersion, which was blamed for the insufficient time for silica NPs moving onto the PDMS/water interface. When the emulsifying time was longer than 3 min, smaller droplets sizes of 3 ± 0.5 μm were observed and the uniform emulsions morphologies could be maintained quite well. Increasing the emulsifying time to 5 min, its average droplets diameter nearly showed the same value and decreased to about 2 μm with further extending the emulsifying time above 5 min ( figure 4(f)). Generally, these results were in reasonable agreement. In addition, the effect of emulsifying time on the Pickering emulsion was not as remarkable as that of emulsifying speed.
Longer emulsifying time meant enough silica NPs to involve in the stable emulsions, which can be told from their appearances. The milk white Pickering emulsions shown in figures 4(a)-(e) indicate that the PDMS was encapsulated by the silica NPs. The emulsion droplets size became smaller as the emulsifying time increased, which attributes to the reason that more time was spared for the silica NPs to transfer from the water phase to the PDMS/water interface, and they could share the oil phase more equally. Besides, unlike surfactant molecules which adsorb and desorb within quite a short time, the solid nanoparticles are irreversibly adsorbed at the interface [29], thus building a solid protection cover for the droplets and contributing to long-term stable Pickering emulsions.

Coating and performance
Numerous studies reported that both the surface chemical components and geometrical structure contribute to the wettability of the solid sample surface [30][31][32]. In the present work, glycerol-modified ultrafine silica NPs exhibit two main roles. On the one hand, silica NPs act as stabilizers hindering the oil-water phase separation. On the other hand, silica NPs roughen the coating surface enhancing the micro-nano structures. After immersed, padded and dried, both cotton and polyester fabrics were coated with PDMS and solid particles composite films, and their water-resistant performance was evaluated by water contact angle system (shown in    figure 5(c)). Specifically, the effect of silica NPs concentration on the water-resistance of treated fabrics was illustrated in figure 5(a), at a low silica NPs concentration (1 wt%), the treated fabrics with a water contact angle of 143°, 147°respectively showed hydrophobicity. Since low silica NPs concentration is insufficient to fully encapsulate the PDMS phase or form a stable Pickering emulsion. What is worse, the lack of enough solid nanoparticles doesn't build well-defined roughness profiles. While the treated fabrics exhibited excellent water-resistance when the silica NPs concentration was around 3 wt%. Their static water contact angles were all above 150°, coupled with low sliding angles around 10°. It predicts that at the silica NPs concentration around 3 wt%, the final Pickering emulsion with desired properties could take good advantage of silica NPs as being the stabilizer. In addition, the nanoparticles were enough to bring roughness structures while not compromising the effect of water-resistance, owing to their ability of being partly wetted by water phase ( figure 5(b)). This property is also responsible for the following water-repellent deterioration of treated fabrics as increasing the silica NPs concentration to 5-10 wt%. Meanwhile, These results demonstrate the vital role of both the low surface tension PDMS and surface roughness contributer silica NPs on the wetting performance of the treated fabrics.
For a control experiment, fabrics were treated with surfactants ISO-5 and ISO-7 stabilized PDMS emulsion in the same formulations except for the different stabilizer species. As depicted in figure 5(b), there was not obviously visual difference in the textiles appearances before and after coating treatments. What is more, the fabrics coated with PDMS emulsion had good softness and smoothness. The former fabrics coated with PDMS Pickering emulsion showed a considerable improvement in liquid repellency compared to the latter coated with surfactants stabilized PDMS emulsion. The water, red dye, juice, Coca Cola, tea, coffee and milk droplets (5 uL) all maintained their round shapes on the former treated fabrics, while wetting the latter treated fabrics less than a few seconds. Besides, these testing droplets did not pin on the PDMS Pickering emulsion treated fabrics surfaces and generally the fabrics had low sliding angles around 10°. Both the used silica NPs and ISO surfactants are partially hydrophilic, the results indicate that the roughness profile provided by used silica NPs plays a vital role in generating the superhydrophobicity.
One critical issue for treated fabrics is their laundering durability when being used in practical applications, which is not always reported in the related articles [33][34][35]. The water-repellent durability of the treated cotton and polyester fabrics was evaluated in accordance with ISO 105-C10: 2007 standard method, using 5 g l −1 soap and 2 g l −1 sodium carbonate for each washing cycle. Figure 6(a) illustrates the water-resistant performance of treated cotton and polyester fabrics before and after standard laundering. The water contact angles changed very slightly even after 30 washing cycles. The treated fabrics exhibited mechanical robustness and still maintained their good water-repellent performance due to the strong covalent bonding between glycerol groups functionalized silica NPs and amine-terminated polydimethylsiloxane, as well as the large amounts of hydroxyl groups on the cotton fabric surface or the hydroxyl groups at the end of polyester chains, which could also be confirmed by the FE-SEM results ( figure 6(b)). The uncoated cotton fibers in figure 6(b) (I) were partly peeled and have a large micro-scale roughness morphology because of the pretreatment process, especially the bleaching action. While after treated with PDMS Pickering emulsion, this damage caused by bleaching was made up. The treated cotton fibers were fully covered with PDMS layer, so did the silica NPs and aggregates. Seen from figure 6(b) (II), the silica NPs and its aggregates were shallowly embedded in the PDMS coating, resulting in the micro-nano structured cotton fibers which were especially effective in achieving superhydrophobicity. Amazingly, even after 30 times washing tests, the coated cotton fabric was also highly hydrophobic, which rivaled those involving the cross-linkers. The superhydrophobicity behavior was mainly related to the firm residence of high viscous yet low surface energy compound PDMS and roughness-induced silica NPs (figure 6(b) (III)).
When applying the same approach to polyester fabrics, their water contact angles were a little bit lower than that of treated cotton fabrics. It is plausible considering the linkage between polyester fabric and the used coating is inferior to that between cotton fabric and the used coating. However, the coated polyester fabric still presented good water-resistance after 30 standard washing cycles. Figure 6(b) (IV) depicted the morphology of uncoated polyester fabric. Quite a lot of pits were clearly revealed in each polyester fiber and it was totally ascribed to the previous alkali peeling process. It is generally known that most polyester fiber and fabric would undergo alkali peeling process before landing on the market, in order to improve their wearability. The profile of treated polyester fabric before and after washing was presented in figure 6(b) (V), figure 6(b) (VI) respectively. The holes in the polyester fiber were stuffed by the adopted PDMS Pickering emulsion, and new nano/micro hierarchical roughness was formed owing to the contribution of silica NPs. Thus, air was allowed to be trapped underneath the testing liquid droplets, implying the coated fabrics were rendered good liquid repellency.

Conclusion
In summary, we have fabricated high viscous PDMS-in-water Pickering emulsions via the most facile method without any extra additives or complex technology. In this work, tuning the relevant parameters (silica NPs concentration, emulsifying speed, emulsifying time) allowed the average droplets diameter to be as low as 3 ± 1 μm, Figure 6. (a) CA changes with 30 washing times. The inset is the photo of water droplets on the treated cotton (black) and polyester (red) fabrics; (b) FESEM images of uncoated cotton (I) and polyester (IV) fabrics, coated cotton (II) and polyester (V) fabrics before washing, coated cotton (III) and polyester (VI) fabrics after 30 washing cycles. which was confirmed by the optical microscope analysis. In practical application, the low surface free energy composite together with silica NPs has proven to be essential to render the coated fabrics long-term superhydrophobicity. The Pickering emulsion treated fabrics herein exhibited not only excellent waterrepellency, but durable behavior even after 30 standard washing cycles as well. The environmental-friendly method shows good potentiality and compatibility in indoor and outdoor water-resistant fields.