The surface modification and characterization of SiO2 nanoparticles for higher foam stability

The surfactant and colloidal nanoparticles has been considered for various applications because of interaction of both complex mixtures. The hydrophilic SiO2 nanoparticle could not be surface active behavior at the liquid/air interface. In this study, the SiO2 nanoparticles have been modified with 3-isocyanatopropyltriethoxy-silane (ICP), and the effect of foam stability has been investigated. The physical properties of surface modified SiO2 nanoparticle were analyzed by XRD, TGA, FT-IR, and SEM. After surface modification of SiO2 nanoparticles, the contact angle of SiO2 nanoparticle was also increased from 62° to 82° with increased ICP concentration. The experimental result has shown that SiO2 nanoparticle with ICP was positive effect and improved foam stability could be obtained at proper ICP concentration compared with un-modified SiO2 nanoparticle.

The surfactant and nanoparticles has been considered for various applications because of interaction of both complex mixtures, which could influence the dynamic and static behavior [1][2][3][4] . Silica (SiO 2 ) nanoparticles have been widely used in various applications such as catalyst, biomolecule separations, chromatographic supports 5,6 . SiO 2 nanoparticles are also used as foam stabilizers for the foams and emulsion applications 7,8 . Generally, aqueous foams are thermodynamically unstable because of breaking of the film or irreversible drainage of the liquid. It is reported that colloidal hydrophobic particles are able to significantly improve the foam stability 8 . These approaches are being used for the stable foam applications. However, as synthesized mesoporous silica nanoparticles have hydrophilic properties, which is not adsorb and surface active behavior at the liquid/air interface. Therefore, the use of hydrophilic SiO 2 nanoparticles has been disadvantageous. In order to enhance the foam stability, the surface modification of SiO 2 nanoparticle should be needed. Normally, the modified SiO 2 nanoparticles could stabilized the foam by interfacial elasticity 9 . A number of methods surface modification of SiO 2 nanoparticles have been developed by using organic or in inorganic additives [10][11][12] . The SiO 2 nanoparticle surface has been changed from hydrophilic to hydrophobic properties, resulting from incorporation with functional groups. It is reported that the hydrophobic SiO 2 nanoparticle has enhanced affinity to organic compound 10 .
In this study, the surface modification of SiO 2 nanoparticles has been prepared by copolymerization with organic-silica precursors in the presence of a cetyltrimethylammonium bromide. Our approach has shown in a one-pot synthesis method and did not add any co-solvent or additives during the synthesis of SiO 2 nanoparticles. After modification of the SiO 2 nanoparticle, the morphology and structure were analyzed by SEM and XRD. Foam stability has been improved with SiO 2 nanoparticles modified by 3-isocyanatopropyltriethoxy-silane. Figure 1a shows the small-angle XRD patterns of SiO 2 and SiO 2 -ICP samples synthesized according to the ICP concentration. All SiO 2 -ICP samples show an intense (100) peak and two weak (110, 200) peaks, indicating that the presence of three peaks is common in SiO 2 materials and 2D hexagonally ordered structure. The intensity of d(100) in the XRD peaks were decreased as a function of the ICP concentration. It is indicating that the addition of ICP did not influence the SiO 2 structure, but resulted in well-ordered structure when ICP was added during the synthesis 13 . Moreover, the addition of ICP resulted in the decrease of SiO 2 nanoparticle size, but not in the collapse in the SiO 2 mesoporous structure 13 . The TGA profiles of as-synthesized SiO 2 particles are depicted in Fig. 1b Figure 2 shows the SEM and TEM images of pristine SiO 2 and SiO 2 -ICP particles. As shown in Fig. 2, SEM and TEM image is clearly reveal that the morphology of SiO 2 -ICP nanoparticles was transformed from kidney-bean-shape to spherical shape according to the ICP concentration. It also observed that the particle size of SiO 2 decreased with higher ICP concentration (500 nm < Fig. 2d,h). Moreover, the mesopore structures of SiO 2 are also observed in spite of their different particle size (Fig. 2e-h). From the XRD, SEM, and TEM, the addition of ICP could modify the morphology and size of SiO 2 particles during SiO 2 synthesis.

Result and discussion
The surface modification of SiO 2 particles was investigated by FT-IR. Figure 3 shows the FT-IR spectra of SiO 2 and SiO 2 -ICP samples. The peak at 1060 cm −1 was attributed to the Si-O-C asymmetry stretching vibration and Si-OH stretching vibration 14 . After modification of ICP, the new peaks were observed at 2974 and 3700 cm −1 , which is assigned to C-H stretching vibration in CH 2 and CH 3 and the 1387 cm −1 peak was corresponding to the vibration of C-H, which was not observed in SiO 2 particles but were in SiO 2 -ICP particles 15 . This is the fact that the ICP was effectively grafted on the SiO 2 surface. The intensity peak of at 1060 cm −1 was decreased as a function of ICP concentration, indicating the surface modification of SiO 2 particles by ICP.
For the foam stability measurement, the aqueous foam was generated and its stability was recorded by using Foamscan analyzer. The N 2 gas was used to produce the foam volume of 200 ml. After that, the gas flow was stopped and foam volume was monitored according to time. Figure 4 shows the decay of the foam volume of surfactant which contained 1 wt% SiO 2 particles (EM 100, 1 wt%) at pH 2. All the samples show the similar foam volume properties and the foam stability of SiO 2 -ICP particles and exhibit higher foam stability than SiO 2 particles without ICP. The foam stability was improved with increasing ICP concentration. It is considered that the hydrophobic particles can be attached on the liquid-gas interfaces and stabilize the foam bubbles in surfactant-free diluted suspensions [16][17][18][19] . Moreover, it is reported that the smaller particle leads to higher viscosity of the suspension under identical particle geometry and type 20 . Therefore, the high ICP concentration could be favor the foam stability because of modification of wettability of SiO 2 particles at the air-water interface and aqueous solution.
In order to clarify the improvement of foam stability of SiO 2 nanoparticles, we have evaluated the contact angles of SiO 2 -ICP samples in air. The particle-stabilized silica nanoparticle depends on significantly on the particle hydrophobicity at the water-air interface in terms of the contact angle 21 . It is reported that the hydrophilicity/hydrophobicity have been determined by contact angle method, which is important evidence for affecting properties of the particle surface 22 . Figure 5 shows the contact angle of SiO 2 nanoparticles with increasing ICP concentration. As shown in Fig. 5, in the absence of ICP, the contact angle is 62°. However, the ICP concentration is increased, contact angle was also increased from 62° to 82°. The result of contact angle of SiO 2 -ICP nanoparticle matched well with the foam stabilities. The SiO 2 nanoparticle surface thus undergo transition more hydrophobic wettability, indicating that more ICP molecules are adsorbed on the SiO 2 nanoparticle surface. It can be concluded that the organic groups of ICP affect the surface properties in the SiO 2 nanoparticles.

Conclusion
In this study, we synthesized SiO 2 nanoparticles through sol-gel process and modified its surface by adjusting ICP concentration. The surface modification of SiO 2 nanoparticles was analyzed by XRD, TGA, SEM, TEM, FT-IR and contact angle. The morphology of silica nanoparticle transformed from kidney-bean-shape to spheres according to the ICP concentration and contact angle was increased as the ICP amount was increased, which became more hydrophobic. The results of TGA and FT-IR have shown the interaction between the SiO 2 nanoparticle surface and ICP after the surface modification. The foam stability was gradually increased with increasing ICP amount, which means that the hydrophobic properties of SiO 2 nanoparticle affect the foam stability and foam stabilization.

Methods
The SiO 2 nanoparticles were synthesized by following a published procedure 23 . Typically, the cetyltrimethylammonium bromide (CTAB, 2.0 g) and 2 M sodium hydroxide solution (NaOH, 7.0 ml) were added in deionized  3 ml) and desired 3-isocyanatopropyltriethoxy-silane (ICP) were added to the solution via rapid injection. The withe precipitation was observed after 3 min and the solution was maintained at 80 °C for 2 h. After the reaction time, the as-synthesized particles were washed by water and methanol, and dried under vacuum oven. The morphology of SiO 2 nanoparticles were analyzed by a field emission scanning electron microscopy (FE-SEM, Hitachi), a high resolution transmission electron microscope (HR-TEM, JEOL). Small angle X-ray diffraction (XRD, PANalytical) analysis was conducted by using Cu Kα radiation (λ = 1.5405) in a range of