Robust porous organosilica monoliths via a surfactant-free high internal phase emulsion process for efficient oil-water separation
Graphical abstract
Introduction
Although water covers 70% of earth surface area, only less than 2 percent can be used for human needs [1]. Water pollutions are threatening the public health and marine ecosystem, which are mainly caused by the increasing amount of industrial toxic wastewater [2], the leakage of organic solvents such as toluene, hexane, dichloromethane, especially, the crude oil spilling [3], [4]. For example, the oil spilling in Gulf of Mexico has caused severe damage to marine life [3], [5]. To work out this issue, various advanced materials and technologies for cleanup oil from water have been developed in recent years [4], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], especially, porous oil sorbent materials exhibit good performances for their oil collection and reuse properties [4], [6]. For porous oil sorbent materials, higher porosity can produce larger absorption capacity [16]. Accordingly, various highly porous oil sorbents such as kinds of aerogels [17], [18], porous carbon materials [19], [20], metal organic frameworks [21], [22], porous organosilicons [23], [24], [25] have been developed. These porous oil sorbents can be devided into two categories: the rigid and the elastic sorbents. The elastic sorbents are much more preferred because they can be easily compressed to collect absorbed oils, while the rigid sorbents could only through some complicated desorption methods, such as washing [26], [27], centrifuging [28], heat treatment [29], or burning [30], etc.
As an alternative, the emerging porous polydimethylsiloxane (PDMS) monolith is considered to be one of the most promising oil sorbents for its excellent recyclability and selectivity, high oil capacity, low fabrication cost and anticorrosion properties [4], [31], [32], [33]. Its outstanding oil absorbing performances mainly attribute to the elastic and swellable property of PDMS. However, the porous PDMS monolith is usually prepared using hard templates like sugar [32], salt particles [33], and porous polycaprolactone (PCL) [31], which need to be removed by subsequent etching or calcination that seriously limits the large-scale fabrication of the PDMS sponges.
High internal phase emulsion (HIPE) that features an internal phase volume fraction of more than 74% is usually used as template to prepare well-defined porous materials [34], [35], [36]. However, this method has rarely been successfully applied to prepare elastic porous PDMS monoliths due to the great capillary force and the flexibility of PDMS molecular chains [37], [38].
Here, we have successfully prepared the superelastic porous PDMS monolith from HIPE templating for the first time. Instead of the conventional HIPEs stabilized by surfactants which are toxic and environmental unfriendly, we utilized a silica precursor polymer, hyperbranched polyethoxysiloxane (PEOS) as stabilizer. PEOS is highly hydrophobic and becomes amphiphilic upon hydrolysis at the oil/water interface, and can further hydrolyze and condense into silica frameworks at the oil/water interfaces and in-situ grow into silica particles in PDMS matrix to strengthen the porous PDMS monoliths. The obtained superelastic porous organosilica monoliths (POMs) are hydrophobic and oleophylic and possess high porosity and robust mechanical strength. They can completely recover to their initial state after compressing for 100 times with strain of 60%. Furthermore, the POMs exhibit outstanding oil-water separation performance and they were applied to set up an oil collection apparatus which could quickly, continually and massively collecting pure oil from the water in oil emulsion.
Section snippets
Materials
Toluene (>99.5%), Tetraethoxysilane (TEOS, 98%), acetic anhydride (ACS reagent, ≥98.0%), titanium ethoxide (33–35% TiO2), were provided by Sinopharm Chemical Reagent Co., Ltd. A common PDMS elastomer (Sylgard 184) was purchased from Dow Corning. Sylgard 184 contains two parts: one is silicone elastomer base (SEB, 5000 cSt) composed of the vinyl end-terminated poly(dimethyl siloxane) oligomers, silica filler (dimethylvinylated and trimethylated silica), and catalyst, another is curing agent (CA,
Preparation of the POMs
As shown in Scheme 1a, a mixture of water and oil is emulsified into a W/O (water in oil) HIPE. The emulsion is then solidified and subsequently dried to obtain the final porous monolith. To prepare a porous polydimethylsiloxane (PDMS) monolith, the silicone elastomer base (SEB) and curing agent (CA) as PDMS precursors were dissolved in toluene (entry 1 in Table S1) and then mixed with water to prepare a mixture that cannot be emulsified owing to the phase separation between water and toluene (
Conclusion
PDMS sponges as flexible substrates are widely used in absorption, separation, photocatalysis, wearable electronics, energy generation, transport and storage [54]. However, the fabrication process is generally complexed or uncontrollable, which seriously limits their large-scale production [54]. In this study, we have demonstrated the facile preparation of superelastic POMs consisting of PDMS and silica particles using surfactant-free HIPEs as templates. The use of PEOS molecules not only role
CRediT authorship contribution statement
Prof. Wu came up with the experimental idea. Doc. Tu did the experiments and finished the data collection under the supervision of Prof. Wu and Prof. Chen. Doc. Tu wrote the original draft.
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.
Acknowledgements
Financial support for this research from National Key Research and Development Program of China (2017YFA0204600) and the National Natural Science Foundation of China (51721002, 51673045) are appreciated.
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