A review on synthesis and application of solvent-free nanofluids
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Nanoparticles have a lot of unique properties because of their unique surface and volume characters, and they have been successfully applied in various fields. The aggregation of the nanoparticles is an inevitable question. To extend the application of nanoparticles, solvent-free nanofluids or liquid-like nanoparticles with a core coated with flexible organic chains had been established by researchers at the beginning of the twenty-first century. Possessing unique properties and functionalities, solvent-free nanofluids can prevent the aggregation of nanoparticles. Besides, they could provide a new platform for common nanoparticles on functionalization and application. This article reviews the development and tendency of solvent-free nanofluids, including synthesis methods and promising applications of different kinds of solvent-free nanofluids. The approaches for synthesis of solvent-free nanofluids with different cores, corona, and canopy will be presented. Applications of these solvent-free nanofluids in a variety of areas comprising gas capture and separation, polymer modification and reinforcement, ionic electrolytes, luminescent materials, and decolorization are also discussed.
KeywordsNanoparticle Solvent-free nanofluids Synthesis Application
In addition, solvent-free nanofluids have favorable tailor ability and functionality in the core, corona, and canopy. Their chemical and physical properties can be readily tailored by varying the structure and chemical composition of the core, corona, and canopy. Several researchers have reported a series of solvent-free nanofluids based on different core, corona, and canopy. The core of solvent-free nanofluids can be roughly divided into two categories. The first category is a series of monocomponent nanoparticles involving SiO2, TiO2, Fe3O4, γ-Fe2O3, DNA, carbon nanotubes, graphene, etc. [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29]. The second category is a series of multicomponent composites involving SiO2@carbon nanotubes, Fe3O4@ carbon nanotubes, Fe3O4@graphene, Fe3O4/PANi nanoparticles, etc. [30, 31, 32, 33, 34]. Corona is usually a class of surface modifier, such as polysiloxane quaternary ammonium, and polysiloxane sulfonate [24, 32]. Typically, one side of corona was grafted onto the surface of core nanoparticles with covalent bonds and the other side forms covalent or ionic bonds with canopy. Canopy is ordinarily a class of flexible organic molecules involving PEG-substituted tertiary amine, poly(ethylene)glycol (PEG)-functionalized sulfonate salt, etc. [30, 32, 33]. The organic shell provides the “solvent” for the dispersion of the nanoparticles, thus imparting the fluidity .
As a large number of preparation methods of solvent-free nanofluids were reported, and the unique properties of solvent-free nanofluids had received increasing attention, a review on this field is necessary and meaningful. This review focuses on the research progress in synthesis approaches and promising applications of solvent-free nanofluids. The article is organized as follows: in the first section, the synthesis approaches of different kinds of solvent-free nanofluids will be introduced. Besides, a summary of the synthesis strategies of solvent-free nanofluids with diverse structures of core, corona, and canopy will be presented. Then, some promising applications in various fields are illustrated. Finally, some suggestions for future study on the synthesis and application of solvent-free nanofluids will be provided.
2 The flow mechanism of solvent-free nanofluids
While researchers had synthesized a variety of solvent-free nanofluids, they had also investigated the flow mechanism of solvent-free nanofluids by establishing several theoretical models, which laid a significant foundation for the synthesis and regulation of new types of solvent-free nanofluids.
Bourlinos et al.  modified silica nanoparticles with the same silane coupling agent and two kinds of counter anions. Dielectric spectroscopy, Brillouin scattering, and shear rheometry were employed to investigate the nanofluids. They figured out that the interaction of cation and anion controlled the glass transition temperature, and the different anions of variable sizes did not affect the local mobility of nanofluids. However, the viscosity of nanofluids was determined by the bulky flexible anions.
Jespersen et al.  synthesized nanoscale ionic mateIrials (NMS) by modifying silica (18 nm) with trihydroxysilylpropylsulfonic acid and Jeffamine M-2070. Nuclear magnetic resonance relaxation and pulse field gradient diffusion had been employed to study the molecular-level dynamics in the NIMS. The T1 relaxation times for methylene carbons of M-2070 suggested that nanoparticles had no effect on fast dynamics of the canopy. Canopy diffusion in the NIMS was slower than bulk M-2070, and faster than the cores, indicating that the canopy molecules constantly exchanged between the cores. This was the main reason that why solvent-free nanofluids have lower viscosity and flow properties.
Yu et al.  had formulated a density functional approach to address the structure of solvent-free nanofluids. The radial distribution function and the static structure factor were solved with models such as point particles and finite hard cores with bead-spring canopy attached. The static structure factor had gone to zero for zero wavenumber in different conditions of core volume fraction as well as canopy radius of gyration. This suggested that each core carried its share of the canopy so that the core and its canopy filled a volume of space that excluded exactly another core.
Li et al.  obtained a multicomponent core, which was decorated by the graphene oxide by nanocrystals, and synthesized the nanofluids by grafting organosilanes and polyether amine on the surface of the core. They also established a slipping and fluidity model to reveal that the synergy of nanocrystals and canopy was the critical factor to the fluidity of nanofluids. On the premise of that the nanofluids had the same canopy, the mass fraction and size of the nanocrystals had a significant effect on the fluidity. The equation suggested that the nanofluids with more nanocrystals had higher slipping and fluidity active energy at given condition.
3 The synthesis of solvent-free nanofluids with different cores
There are several synthetic methods for creating solvent-free nanofluids. According to the bonding method, the synthesis methods are mainly divided into the following three categories. The first one is physical adsorption. The hydroxyl-rich inorganic nanoparticles (such as SiO2 and Fe3O4) have a strong physical adsorption effect on the polyoxyethylene-polyoxypropylene ether block copolymer, so they can be used to prepare the nanofluids. The solvent-free nanofluids can also be synthesized by covalent bonding. In this strategy, the canopy molecule can directly react with the hydroxyl or carboxyl groups on the surface of nanoparticles, or can react with the corona molecule, which can be grafted onto the nanoparticles. The last categories of nanofluids are a kind of ionic solvent-free nanofluids, and the typical synthesis involves two steps. The first step is to modify the nanoparticles with a charged corona, and then the organic canopy is grafted onto the nanoparticles by ion-exchange reaction.
In this section, based on the core component, solvent-free nanofluids are divided into two groups: (1) solvent-free nanofluids based on monocomponent core and (2) solvent-free nanofluids based on multicomponent core. Each group can be further classified in terms of different kinds of chemical compositions of core.
3.1 Solvent-free nanofluids based on monocomponent core
Monocomponent nanoparticles, such as SiO2, TiO2, and Fe3O4, were originally used to synthesize solvent-free nanofluids. The core nanoparticles could be coated with one or two layers of different organic chains to form solvent-free nanofluids. Core nanoparticles can be connected to one end of corona and the other end of corona was bonded to canopy, or directly grafted canopy onto the surface of core nanoparticles.
Polyoxometalates (POMs) usually consist of three or more transition metal oxyanions linked by oxygen atoms to form 3-dimensional frameworks. The first solvent-free POM nanofluid with zero vapor pressure and good thermal stability was synthesized by Bourlinos and coworkers in 2004 . A poly(ethylene glycol) (PEG) containing quaternary ammonium salt((CH3)(C18H37)N+[(CH2-CH2O)nH][(CH2CH2O)mH]Cl-, m+n=15) was selected as the canopy to treat an available POM. Through the exchange of the surface protons of POMs, poly(ethylene glycol) (PEG) containing quaternary ammonium cations was attached to POMs as the canopy. The resulting solvent-free nanofluids have a viscosity of 75 Pa s at ambient, which dropped to 0.5 Pa s at 120 °C.
3.1.2 SiO2 and polyhedral oligomeric silsesquioxane
Classification of SiO2 solvent-free nanofluids obtained by Park and the list of three samples
Moganty et al.  obtained a new class of silica ionic liquid by dispersing silica nanoparticles (9 nm) grafted with 1-trimethoxysilyl propyl-3-methyl-imidazolium bis-(trifluoromethylsulfonyl) imide (SpmImTSFI) in a 1-butyl-3-methyl-pyrrolidinium bis-(trifluoromethylsulfonyl) imide (BmpyrTFSI) IL host. Firstly, the IL precursor 1-trimethoxysilyl-propyl-3-methyl imidazolium chloride was synthesized. Then, the IL precursor was added slowly to a silica nanoparticle suspension (2wt%) with stirring for 12 h at 100 °C, and modified silica nanoparticle was obtained by centrifugation and washing. After lyophilization/freeze-drying, SiO2-SpmImCl particle was obtained. Lastly, through an anion-exchange reaction between Cl- anion and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), the SiO2-SpmImCl particle was converted to the desired SiO2-SpmImTFSI. The SiO2-SpmImTFSI had a high grafting density (0.8 ligands/nm2). Remarkably, after the addition (0.1 wt%) of SiO2-SpmImTFSI to BmpyrTFSI, a dramatic affection on the crystallization and melting transitions occurred without any effect on Tg.
In the first step, the hollow silica spheres were prepared by a hydrothermal process. In the second step, a positively charged organosilane was bound to silica shell as a corona at room temperature. In the third step, the surface-modified hollow silica spheres were treated in a poly(ethylene glycol)-tailed sulfonate (PEGS) solution for ion exchange. Finally, the HS liquid was obtained after the removal of the excess of PEGS and drying.
3.1.3 Nanocarbon materials
Nanocarbon materials, such as carbon nanotube and graphene, have aroused great interest for their specific mechanical properties, high conductivity, and unique structure [40, 41]. Some researchers had synthesized nanofluids with nanocarbon materials as the core to optimize the application of these materials.
There are two steps to obtain the fullerenes nanofluids. In the first step, a series of 2,4,6-tris-(alkyloxy) benzaldehydes were synthesized through using four different kinds of alkyl bromides to react with 2,4,6-trihydroxybenzaldehyde. In the second step, 2,4,6-tris-(alkyloxy) benzaldehydes, C60, and N-methylglycine were refluxed in dry toluene. After purification of crude products, fullerene nanofluids (n = 8) were dark brown solid with a melting point of 147–148 °C; the other fullerene nanofluids (n = 12, 16, 20) were dark brown oil at room temperature.
The ionic solvent-free nanofluids based on multiwalled carbon nanotubes (MWCNTs) were first synthesized by Lei and his coworkers . It had a liquid-like behavior (G″>G′) at ambient; furthermore, an excellent thermal stability was suggested by a high decomposition temperature above 300 °C.
3.1.4 Metal, metallic oxidation, and metal sulfide
Surface modification was an efficient approach to form functional materials with a range of unique properties and potential applications. In the past few years, a novel class of solvent-free nanofluids based on metallic oxidation and medal sulfide was fabricated.
The similar synthesis strategy was employed by Tany and his coworkers to fabricate a series of Fe3O4 solvent-free nanofluids with three different kinds of corona: 3392(1-Decanaminium,N-decyl-N-methyl-N-[3-(trimethoxysilyl)propyl]-,chloride); 6620(1-Octadecanaminium,N,N-dimethyl-N-[3-(trimethoxysilyl)-propyl]-,chloride); and 8415(1-Propanaminium, N, N, N-trimethyl-3-(trimethoxysilyl)-, chloride) (Fig. 10) . It indicated that long alkyl chains of corona can provide nanofluid lower viscosity and better flowability.
3.1.5 Other materials
In addition to the nanoparticles discussed above, DNA, calcium carbonate, and sepiolite could also be introduced to solvent-free nanofluids as the core materials.
In addition to solvent-free nanofluids with a monocomponent core, a new class of solvent-free nanofluids with a multicomponent core has also been developed. Differing from single component materials, synthetic multicomponent materials exhibited remarkable physicochemical properties, which did not exist in the individual component [44, 45, 46]. Some researchers synthesized a series of solvent-free nanofluids based on multicomponent cores, such as SiO2/multiwall carbon nanotubes, Fe3O4/oxidation graphene, Fe3O4/multiwall carbon nanotubes, zinc hydroxystannate boxes (ZHS)/graphene nanosheets (GNS), and Fe3O4/PANI nanoparticles.
4 Application of solvent-free nanofluids
Compared with traditional solid nanomaterials, solvent-free nanofluids possessed many unique and interesting properties such as fluidity in the absence of solvents, zero pressure, excellent compatibility, and their tailorability and functionality in both cores, corona and canopy. Due to these advantages of solvent-free nanofluids, they had been applied in gas separation and capture, lithium-ion batteries, decolorization, lubrication, luminescent materials, magnetic fluids, and resin modification and reinforcement. Then, some promising applications of solvent-free nanofluids will be discussed, specifically, gas separation and capture, resin modification, and reinforcement.
4.1 Gas capture and separation
4.1.1 Solvent-free nanofluids for CO2 capture
Carbon dioxide is one of the greenhouse gases and the impacts of its increasing concentration in the atmosphere have drawn an increasing attention. Therefore, developing an efficient technology for CO2 capture is in urgent need. The most commonly applied materials were amine-based solvents, which reacted with CO2 to form carbamates [48, 49, 50, 51].
To capture CO2, five kinds of solvent-free nanofluids with SiO2 (10–15 nm) as core had been developed by Lin and coworkers . The secondary amines could react with CO2 and the ether groups enhanced CO2 capture via Lewis acid-base interactions. It was also found that the solvent-free nanofluids based on ionic bonds exhibited lower CO2 capture capacity than the solvent-free nanofluids based on covalent bonds because the amine groups of solvent-free nanofluids based on ionic bonds were protonated and unavailable for the reaction with CO2. Moreover, these solvent-free nanofluids with SiO2 as the core exhibited excellent recyclability and had a high selectivity to CO2.
4.1.2 Gas separation
To prevent the flexible chains from filling the empty cavities, a hollow SiO2 sphere with microporous shell that blocked molecules larger than 1.9 nm was chosen as the core. Meanwhile, the organosilane with a molecular size of about 2.0 nm was employed to modify the core as the corona and the canopy PEGS were grafted on the surface of the modified SiO2 sphere by replacing the chloride counter-anion of corona. When N2/CO2 mixed gas passed through the solvent-free nanofluids, the ether groups of the canopy enhance the CO2 solubility via Lewis acid/base interactions, and the empty cavities provided free volume for gas to pass through the solvent-free nanofluids. Therefore, CO2 could diffuse faster in canopy and empty cavities to go through the liquid. The gas separation test showed that the CO2 permeability is determined as 158 barrier, higher than the permeability of N2. Hence, the solvent-free nanofluids with hollow SiO2 nanoparticle as the core were a promising choice for gas separation.
4.2 Polymer reinforcement and modification
Solvent-free nanofluids are considered as efficient reinforcement and modification materials for their excellent dispersion and compatibility. Li et al.  reported a simultaneous reinforcement and plasticization on polyamide 11 (PA11) through using MWCNT solvent-free nanofluids. In the case of surface-functionalized MWCNTs, it presented as fluid at room temperature without any solvents and the flowability of MWCNT solvent-free nanofluids was favorable for the homogeneous dispersion. As the weight fraction of MWCNT nanofluids in polyamide 11 varied from 0 to 2 wt%, the value of tensile modulus increased gradually and then came to an invariant level. The fracture elongation was maintained within the range of 140–260% at the same time.
In addition, the MWCNT solvent-free nanofluids also exhibited great mechanical reinforcement efficiency in epoxy resin. Yang et al.  synthesized an amino-functionalized MWCNTs by attaching 2,2′-(ethylenedioxy)diethylamine to MWCNTs. The MWCNTs could be homogeneously dispersed in epoxy resin by a solvent-free processing. In the curing reaction, covalent bonds would be created by the reaction of epoxy group and the amino groups of MWCNT solvent-free nanofluids. By adding 0.5 wt% of MWCNT solvent-free nanofluids, the Young’s modulus, storage modulus, tensile strength, failure strain, and toughness of neat epoxy were increased by 28.4%, 23.8%, 22.9%, 24.1%, and 66.1% respectively.
4.3 Miscellaneous application of solvent-free nanofluids
Another potential application of the solvent-free nanofluids went to luminescent materials . Sun et al.  obtained the QDs ionic liquid by attaching organic chains onto the surface of PbS nanoparticles. The resulting amphiphilic QDs ionic liquid exhibited liquid-like behavior at ambient without any solvent. The optical absorption, photoluminescence, and absolute quantum yield were tested to investigate their properties. The lowest energy absorption peak and photoluminescence emission peak of PbS ionic liquid were red-shifted by about 65 nm compared with those of normal PbS QDs. The quantum yield from PbS ionic liquid was about 22%, which is lower than the normal PbS QDs. The most interesting thing was that the PbS ionic liquid had an excellent photostability. There was no virtual decrease in photoluminescence intensity of the PbS ionic liquid.
The solvent-free nanofluids could also be applied in decolorization. A covalent-bonded solvent-free nanofluid with Fe2O3 nanoparticle as the core was fabricated by Lin and his coworkers and its capability of dye decolorization in water was examined . The decolorization capability was demonstrated by removing Nile Blue dye from the water. Nile Blue can be attracted by the corona and canopy via the interaction between the quaternary amine, ether group, and the hydrogen bonding. The Langmuir model was employed to estimate the maximal adsorption capacity. It was found that the maximal adsorption capacity of the covalent-bonded solvent-free nanofluids was 38 mg g−1, which was significantly higher than that of traditional solid materials.
5 Conclusion and future outlook
To conclude, the structure features, synthesis methods, and promising application of solvent-free nanofluids had been summarized in the paper. These solvent-free nanofluids were synthesized by modifying the organic chains on the surface of nanoparticles via covalent or ionic bond. These strategies preserve the agglomeration of nanoparticles, so that the application of nanoparticles in various fields could be improved. However, the organic chains, which were modified on the surface of nanoparticles, also have some negative effect on the nature characters of the cores, such as electrical conductivity and magnetism.
More choices for the cores of nanofluids should be explored. Apart from the nanomaterials mentioned in this paper, there are many other nanomaterials that can be used to prepare nanofluids, and the properties of these nanofluids are worthy of being studied.
The relationship of core and organic chains (corona and canopy) should be figured out. It is important to work out a method of synthesizing nanofluids while maintaining the nature characters of nanoparticles.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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