The surface chemical structure of PM
The FTIR-ATR spectra of PM and silica/PMs were shown in Fig. 2. Compared with PM, the characteristic peaks of SiO2 are all observed in silica/PMs, including 457 cm−1 Si–O–Si bending vibration, 810 cm−1 Si–O–Si symmetric stretching, 1075 cm−1 Si–O asymmetric vibration and 1635 cm−1 H–O–H bending vibration. These peaks indicated silica was fabricated on PM successfully.
Morphologies of silica and silica/PM
As shown in Fig. 3a–c, silica nanospheres C-SiO2-1, C-SiO2-2, C-SiO2-3 were obtained using CTAB and TMB as templates. The size of the nanospheres increased by increasing the content of TMB. The diameters of the nanospheres are in the range of 30–80 nm, 60–100 nm, 0.5–1.5 μm, respectively. When 1 mL TMB was added, many pinholes are found on the surface of silica sample C-SiO2-1 (Fig. 3d), the pore size are about 1 nm. When TMB were increased into 3 mL, both the hollow nanostructures and radical pores are observed on the surface of the sample C-SiO2-2 (Fig. 3e). The pore size is about 3.5 nm. The diameter of silica sphere increased dramatically after adding 6 mL TMB and the walls of the shells collapsed. The hollow nanostructures and pinholes are also observed on the surface of the sample C-SiO2-3 (Fig. 3f). The diameter, pore size and thickness of C-SiO2-3 are about 0.5–1.5 μm, 6 nm and 25 nm, respectively. In the reaction system of CTAB/TMB, the amount of TMB plays an important role in the forming various nanospheres. When a small amount of TMB is added, it dispersed in the hydrophobic micelle center of CTAB, and further swelled the size of the micelle. Therefore, as the content of TMB increases, the diameter of the obtained silica spheres increases gradually. While the content of TMB is large enough, the TMB oil droplet is used as template, CTAB is adsorbed on the surface of the droplets and further adsorbed TEOS through electrostatic interaction. After the template is removed, a larger hollow silica sphere is obtained (Peng et al. 2014). It was found that silica nanospheres were uniformly embedded into the pores of PM (Fig. 3g–i). SEM images of C-SiO2-1/PM, C-SiO2-2/PM and C-SiO2-3/PM are shown in Fig. 4a, d and g, respectively. EDS mapping images of C-SiO2-1/PM; C-SiO2-2/PM; C-SiO2-3/PM are shown in Fig. 4b, c, e, f and g–i, respectively. Red dots represent element of silicon. EDS mapping images of the silica/PMs were also confirmed that the silicons (red dots) were uniformly dispersed on the surfaces of PMs (Fig. 4).
As shown in Fig. 5, worm-like silica nanotubes and nanospheres L-SiO2-1, L-SiO2-2, L-SiO2-3 were obtained using L-16AlaPyPF6 and different count of TMB as templates. For L-SiO2-1, worm-like silica nanorods with two ends of spherical structures were obtained after adding 0.5 mL TMB (Fig. 5a). TEM image confirmed they were hollow nanotubes (Fig. 5d). The length and diameter of the nanotubes are about 130–350 nm and 30–45 nm. When TMB was increased to 1.5 mL, uniform silica nanotubes L-SiO2-2 with worm-like structures were found in Fig. 5b, e. The length and diameter of L-SiO2-2 are about 100–325 nm and 47–63 nm, respectively. However, for sample L-SiO2-3, only silica nanospheres were identified in Fig. 5c after adding 3.0 mL TMB. As shown in Fig. 5f, the spheres are hollow structures with shells collapsing. The diameter is about 58–600 nm and many mesopores are found on the surface of the nanospheres. Chiral small molecules assembled a bundle-like structure through hydrogen bonds. When the content of TMB is low, the self-assembly of chiral small molecules is used as a template. Worm-like structure is obtained by removing the template. With the content of TMB increasing, TMB oil droplets serve as template. L-16Ala5PyPF6 is adsorbed on the surface of the oil droplets, and silica is adsorbed on L-16Ala5PyPF6 by electrostatic action. The hollow silica sphere can be obtained by removing the template. As shown in Fig. 5g–i, worm-like silica nanotubes and nanospheres were uniformly fabricated on PM. Due to the large number of hydroxyl groups on the surface of these silicas, they have very good hydrophilicity. Therefore, spraying these silicon on the PM film can significantly improve the hydrophilicity of PM. SEM images of L-SiO2-1/PM, L-SiO2-2/PM and L-SiO2-3/PM are shown in Fig. 6a, d and g, respectively. EDS mapping images of L-SiO2-1/PM; L-SiO2-2/PM; L-SiO2-3/PM are shown in Fig. 6b, c, e, f and g–i, respectively. Blue dots represent element of silicon. As is shown in Fig. 6, the silicas prepared by L-16Ala5PyPF6/TMB are uniformly distribute on the surface of the films.
Changes of WCA contact angles over time
The hydrophilicity of the different membranes’ surfaces were monitored by water contact angle (WCA) tests which are shown in Fig. 7. It took about 96 s when the contact angle of the PM from 79.5° to 0°. The embedded image in Fig. 7 is an enlarged view of the contact angle of silica/PMs within 5 s, from which we can see that the slowest silica/PM took about 1.5 s when the contact angle became 0°, while the fastest film took only 0.5 s. It can be concluded that the hydrophilicity was improved after PM embedded silica nanospheres with radical pores, hollow silica nanospheres and worm-like silica nanotubes. Since there are a large number of hydroxyl groups on the surface of silica, and there are a large number of pores inside these silica balls or silica tubes, the hydrophilic properties of the membrane can be greatly improved. It provides the possibility for the separation of oil and water membrane. EDS mapping images of the silica/PM were also confirmed that the silicas were uniformly dispersed on the surface of PMs (Figs. 4 and 6).
Oil/water separation
As shown in Fig. 8a, we put the prepared C-SiO2-2/PM into a beaker filled with water, and then dropped the chloroform dyed with 1-[[4-[(dimethylpheny)azo]dimethylphenyl]azo]-2-Naphthalenol on the surface. Because chloroform is denser than water, when pink chloroform oil drops onto the surface of the underwater film, which showed hydrophobic characteristics on the surface of the membrane. When we took n-hexane oil droplet dyed with 1-[[4-[(dimethylpheny)azo]dimethylphenyl]azo]-2-Naphthalenol, which is less dense than water, it’s found that the pink n-hexane oil drops below the membrane also exhibited hydrophobic properties. It is worth noting that when we suck the chloroform away from the membrane with a straw (Fig. 8c, d), no oil droplets are left on the membrane (Fig. 8e), so the membrane can be used as an oil repellent. Due to the super-hydrophilicity in air and super hydrophobic oil underwater (Fig. 8a, b), C-SiO2-2/PM can be used in oil—water separation. The separation property of the C-SiO2-2/PM is further studied for n-hexane/water mixtures. N-hexane and water (dyed by erioglaucine disodium salt) (1:1 v/v) are put it in the oil and water separator. Since the density of n-hexane is lighter than water, n-hexane is distributed above the water. As a result, the water penetrated through the SiO2/PM while the oil is intercepted (Fig. 8f, g). Therefore, silica/PM can effectively separate oil and water mixture through gravity-driven filtration. Therefore, we carried out the study of oil–water separation with six kinds of films prepared above. It’s found that the membranes can also effectively separate n-hexane/water, silicone oil/water and peanut oil/water mixtures. The separation efficiencies are listed in Fig. 9. The results show that all the silica/PMs exhibit high separation efficiency (> 98%) for oil/water mixtures.