The impact of SiO2 nanoparticles on the dilational viscoelastic properties of water-nonionic surfactant-fuel interface
Graphical Abstract
Introduction
Surfactants as lubricant, anti-wear agent, anti-deposition agent and stabilizer are usually added into the ultra-low sulfur diesel (ULSD) to improve the oil quality. In the engine fuel tank, due to the temperature differences day and night, or during transportation, water is easily mixed into ULSD [1], [2]. In addition, the presence of surfactants increases the stability of the emulsified water in diesel, which leads to the blockage of fuel injectors and engine failure. In order to protect the diesel engine, emulsified water droplets need to be removed by a coalescence separation mechanism, and then separated by gravity in the filtrate. The mechanism of coalescence separation is described as that two water droplets collides and squeezes each other on the coalescence film, and their outermost membrane is deformed to an extreme state, which might break up or return to its original state. If the water droplets break through the barrier of the surfactant film and then merges into larger droplet, this is defined as an effective coalescence process [3], [4], [5]. The key factor of this mechanism not only involves the wettability of the coalescing membrane, but also the rheological properties of the surfactant film at the water-fuel interface.
As discussed in our previous work, the viscoelastic modulus of the surfactant film can be used to evaluate the coalescence effect [6], [7], [8], [9], [10], [11]. The high-elasticity surfactant film can resist interfacial deformation due to the dense arrangement of surfactants, while the low-elasticity surfactant film is easily broken up due to the loose arrangement of surfactants. Moreover, rigid particles are also easily mixed into the diesel tank, and irreversibly absorbed on the water-oil interface like Pickering Emulsion [12], [13], [14], [15]. Commonly, a rigid layer is formed and shows a viscous property, which improves the stability of the emulsion and increases the difficulty for coalescence [16], [17]. Furthermore, for a binary nanoparticle-surfactant emulsion system, the influence of rigid nanoparticles on the rheological properties of surfactant film was less studied, which is worthy of further exploration.
Nonionic surfactants in the oil bulk transfer to the water-fuel interface and arrange as a network[18]. The rheology properties of the surfactant network are measured by the oscillation pendant drop method [19]. When a hanging water droplet in the oil bulk periodically expand and contract, surfactants undergoes a relaxation process: i) transferred from the old interface to the new interface, and rearranged to attain an equilibrium concentration; ii) performing exchange diffusion between the interface and the oil bulk to compensate the interfacial tension gradient [20], [21]. Different surfactants have different responses to the interface deformation, and the parameter to evaluate surfactants’ performance was the viscoelastic modulus(ε), which was measured by the oscillation pendent drop method and defined as follows [22], [23], [24], [25]:where ε was the interfacial dilational viscoelastic modulus, mN/m; γ was the interfacial tension, mN/m; A was the water-oil interface area, m2; represented the elastic part, mN/m; represented viscous part, mN/m; was viscosity, mN∙s/m; i was imaginary part and ω was the oscillation frequency, Hz. The elastic part represented the storage energy to resist deformation, and the viscous part represented the energy loss during the relaxation process [26], [27]. Then, the interfacial rheological properties were calculated by these parameters.
In recent years, solid particles such as SiO2 and TiO2, had been used as emulsifiers to prepare Pickering Emulsions[28], [29], [30], [31], [32]. The nanosized-SiO2 can be modified to adjust its hydrophobicity and hydrophilicity by Pickering Emulsion Polymerization method [33]. Commonly, SiO2 nanoparticles were irreversibly adsorbed on the water-fuel interface [34], [35]. Also, hydrophobic fumed SiO2 nanoparticles preferred to aggregate to a cluster by the interaction of Si-H and Si-O-H, improving the dispersion and the defoaming effect of silicone oil [36], [37], [38]. Therefore, for a binary nanoparticles-surfactant system, SiO2 adsorbed on the water-fuel interface and aggregated to a cluster in the oil bulk. Sharma et al.[39] had proved that SiO2 nanoparticle improved the viscoelastic modulus of polyacrylamide (PAM)-surfactant layer in the oil-in-water (O/W) Pickering Emulsion. Anion surfactant PAM modified SiO2 nanoparticle transferred to the oil-water interface, offering steric resistance against droplet deformation. Simts et. al [40] also proved that positively charged SiO2 and octadecyl amine competitive adsorbed on the oil-water (O/W) interface, and an electrostatic exclusion zone exists around the SiO2 particles, preventing the adsorption of nonionic-surfactant and resulting in an increase in interfacial tension. These researches denoted that at the water-fuel interface, adsorbed SiO2 provided barrier for droplets’ coalescence and there was a competitive adsorption between the SiO2 and the nonionic-surfactant, which disrupted the adsorption of the later on the fuel-water(O/W) interface. Moreover, SiO2 aggregated to cluster and surfactants absorb on their surface, which may also affect the interfacial rheology properties as surfactant concentration decreased. Limage et al.[41] investigated the rheology in diluted mixed particle-surfactant system. For low CTAB surfactant concentration system, nanoparticle preferred to aggregate to cluster, and the interface mainly showed viscous properties. With the concentration of CTAB increased, the repulsion between silica nanoparticles increased as CTAB adsorbed on the SiO2 surface, and the system showed a viscoelastic property. It proved the elastic property of rigid particles in aqueous can be improved by the addition of CTAB with a certain concentration.
Furthermore, we also demonstrated the highly elastic surfactant films can be weaken by adding complex structures and large molecular weight surfactants for competing adsorption [26]. Wang et al. [42] investigated the relaxation processes of AM/POEA (acrylamide/2-phenoxylethyl acrylate) block copolymer without and with SDS (sodium dodecyl sulfate) at the octane-water interface. The added SDS surfactant either compete with the copolymer at the interface or combine with it to form micelles, which both have influence on the interfacial rheology properties. Also, Zhou et al. [43] had investigated that the dense array of surfactants can be weakened by the insertion of active components at the crude oil/water interface. However, there were few studies on the influence of SiO2 nanoparticles on non-ionic surfactant films in the fuel (W/O system), which was the driving force of this work.
In conclusion, rigid SiO2 particles or densely packed surfactants provided steric hindrance, which was not conducive to droplets’ coalescence, but the effect of SiO2 nanoparticles on the viscoelastic modulus of nonionic-surfactant in the water-fuel (W/O) system was rarely studied. In this paper, the interfacial rheological properties of SiO2-nonionic-surfactant composite layer was studied in the water-ULSD(W/O) system. Chevalier et al. [44] found that aggregated SiO2 in the water-alcohol system increased the system viscosity, and when the volume concentration of 22 nm SiO2 was below 0.1%, the relative viscosity of SiO2-ethanol nanofluids was below 1.1, proving the viscosity of the system had no obvious change. Therefore, in this work, the concentration of SiO2 was chosen as 0.01 wt%~0.1 wt%. By adjusting the oscillation period, the concentration of SiO2 and the particles size of SiO2, the interfacial rheological properties of the water-SiO2-nonionic surfactant-ULSD systems were deeply investigated, trying to provide some ideas for reducing the emulsion stability and eliminating the coalescence barriers.
Section snippets
Materials
ULSD was purchased from a diesel station in Beijing. Active clay for ULSD-treatment was purchased from Huangshan Baiyue Activated clay Co., Ltd. The activated clay was often used for decolorization, adsorption and purification of oil products. Liposoluble nonionic surfactants monoolein (MW: 356.5 g/mol, purity: 50%) and Pentaerythritol Oleate (PETO-B) (MW: 1193.9 g/mol, purity: 99.8%) were respectively supplied by Dalian Meilun Biotech Co. Ltd. and Liaocheng Ruijie Chemical Co., Ltd. Compared
Interfacial tension
Nonionic surfactant has a polar group as a hard head and an alkyl chain as a soft tail, which arranges at the water-fuel interface by forming an elastic network through intermolecular interaction. As SiO2 particles do not have soft and mobile alkyl chains, SiO2 layer at the water-fuel interface has low elastic modulus. Therefore, different interface layers have different interfacial rheological properties.
As shown in Fig. 2, the interfacial tension of ULSD, PETO-B-ULSD, monoolein-ULSD was
Conclusions
The influences of SiO2 on the interfacial rheological properties of the water-monoolein-ULSD layer and the water-PETO-B-ULSD layer were investigated. The smaller the size of SiO2 had greater influence on the viscoelastic modulus. After adding with 20 nm SiO2, the interfacial tension of monoolein-ULSD increased from 16.0 mN/m to 21.4 mN/m, and the viscoelastic modulus decreased from 17.1 mN/m to 10.9 mN/m. The results proved that adding nanosized SiO2 particles could reduce viscoelastic modulus
CRediT authorship contribution statement
Qian Zhang: Conceptualization, Methodology, Writing – original draft, Writing – review & editing, Data curation. Zhiwei Guo: Conceptualization, Methodology. Yujie Yang: Software. Yanxiang Li: Visualization. Chuanfang Yang: Conceptualization, Methodology. Wangliang Li: Conceptualization, Methodology.
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
This work is supported by the National Natural Science Foundation of China (22078347), National Natural Science Foundation of China (21961160745), Science and Technology Program of Guanshanhu ([2020]13), Key Research and Development Program of Hebei Province, China (20374001D; 21373303D), Program of Innovation Academy for Green Manufacture, CAS (IAGM2020C04).
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