A review on synthesis and application of solvent-free nanofluids

  • Yudeng Wang
  • Dongdong Yao
  • Yaping ZhengEmail author


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.

Graphical abstract

This article has reviewed the synthesis of various solvent-free nanofluids, and introduced their application in a variety of different fields.


Nanoparticle Solvent-free nanofluids Synthesis Application 

1 Introduction

In the past decades, nanoparticles have attracted researchers’ great attention for their unique mechanical, magnetic, thermal, optical, and catalytic properties [1, 2, 3, 4, 5]. The aggregation, which was caused by the high surface energy and surface activity of nanoparticles, hindered their applications [6]. To solve this problem, new classes of functionalized nanoparticles, which are called solvent-free nanofluids, were first obtained through surface engineering by Bourlinos [7, 8, 9, 10]. As it is shown in Fig. 1, the solvent-free nanofluids consist of a nanoparticle core (which could be organic or inorganic) and flexible organic chains, which was grafted on the surface of nanoparticles. The organic chains could be a single layer as corona or multi layers as corona and canopy. Because of the barrier function of the soft organic chains, the aggregation of nanoparticles was reduced. By increasing the grafting density of organic chains, a liquid-like material could be obtained; the whole system is regarded as a single component and is macroscopically homogenous [10]. Meanwhile, the organic chain grafted onto the surface of nanoparticles enhanced the compatibility and dispersion. Distinguished from conventional colloidal suspensions in a solvent, solvent-free nanofluids are completely solvent-free and zero vapor pressure. Therefore, solvent-free nanofluids could contribute significantly to scientific research and industrial application.
Fig. 1

Schematic illustration of solvent-free nanofluids described in this article. a Solvent-free nanofluids with corona and canopy. b Solvent-free nanofluids with canopy

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 [35].

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. [10] 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. [36] 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. [37] 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. [38] 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.

3.1.1 Polyoxometalates

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 [39]. 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

In 2005, Bourlinoset et al. [7] obtained a functionalized SiO2 nanoparticle, which exhibited liquid-like behavior without any solvent (Fig. 2). In the first step, the nanoparticle was obtained through the modification of SiO2 by condensation of (CH3O)3Si(CH2)3N+(CH3)(C10H21)2Cl- with surface silanol groups. In the next step, the chloride counterion was exchanged with sulfonate anions R(OCH2CH2)7O(CH2)3- SO3-(R=C13-C15 alkyl chain). After the replacement of the chloride, an optical transparent SiO2 solvent-free nanofluid was yielded. It had a high organic content (75 wt%) and liquid-like rheological performance (G″>G′).
Fig. 2

Schematic procedure for the preparation of functionalized SiO2 nanoparticles which exhibited liquid-like behavior without any solvent

Lin et al. [11] also obtained three kinds of SiO2 solvent-free nanofluids with different corona, canopy, and bonding types (Table 1). The first kind of SiO2 solvent-free nanofluids, SN-I-1, had a similar structure with the one mentioned above. The corona (3-(glycidyloxypropyl) trimethoxysilane and 3-(trihydroxysilyl)-1-propane sulfonic acid) were first reacted with the silanol groups of SiO2 nanoparticles, then a monoamine-terminated polyetheramines (ethylene oxide (EO)/propylene oxide (PO) = 31/10) was linked to the surface of corona by ionic bonds. In the case of the second kind of SiO2 solvent-free nanofluids, SN-I-2, it was synthesized without corona. The surface groups of SiO2 nanoparticles were protonated by using HCR-W2 ion-exchange resin. Then, the canopy (ethylenediamine tetrakis(ethoxylate-block-propoxylate) tetrol, M.W.∼ 7200) was attached to the surface of SiO2 nanoparticles by ionic bonds. The third kind of SiO2 solvent-free nanofluids, SN-C-1, was prepared via covalent bond as follow. The corona (3-(glycidyloxypropyl)trimethoxysilane) was first reacted with canopy (polyethylenimine, M.W.∼ 1800), then the resultant was reacted with SiO2 nanoparticles. The physical appearance of the three kinds of SiO2 solvent-free nanofluids at ambient was viscous clear or pale yellow liquid.
Table 1

Classification of SiO2 solvent-free nanofluids obtained by Park and the list of three samples

Moganty et al. [12] 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.

Zhang et al. [13] synthesized a novel class of solvent-free nanofluids, which was a liquid porous material by surface modification of hollow silica nanoparticles (Fig. 3). For preserving the hollow structures in this nanofluid, they chose the hollow silica spheres with mesoporous shell that could block species larger than 1.9 nm as the core, and selected an organosilane ((CH3O)3Si (CH2)3N+(CH3) (C10H21)2Cl-), whose molecular size was about 2.0 nm, as the corona.
Fig. 3

Schematic procedure for the preparation of SiO2 solvent-free nanofluids by modification of hollow silica nanoparticles

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.

In 2006, Tsuyoshi et al. [15] first reported the preparation of a series of fullerene solvent-free nanofluid (Fig. 4). Four kinds of 2,4,6-tris-(alkyloxy) benzaldehydes with different chain length (n = 8, 12, 16, 20) were synthesized as canopy, and N-methylglycine was the corona.
Fig. 4

Synthesis of 4 kinds of fullerene solvent-free nanofluids

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 [16]. 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.

Three steps were included in the synthesis procedure (Fig. 5). The first step produced shorter, well-dispersed, open-ended pipe multiwall carbon nanotubes and modified their surface with polar hydrophilic groups (COOH, C=O, OH) by acid oxidation [42]. In the second step, a polysiloxane was grafted at the surface of MWCNTs through the reaction between the silanol group with OH or COOH of MWCNTs. In the third step, an ion exchange occurred between modified MWCNTs and sulfonate salts to obtain the MWCNT solvent-free nanofluids. An organic layer, which is about 6.5 nm, was discovered in the surface of MWCNTs by high-resolution TEM.
Fig. 5

Synthesis of multiwalled carbon nanotube ionic solvent-free nanofluids

Zhang et al. [18] obtained a multiwalled carbon nanotube covalent liquid with only canopy. The material is waxy solid at room temperature, which melts and behaves like a liquid at 45 °C. A typical synthesis procedure concluded two steps (Fig. 6). In the first step, the carboxylic MWCNT was dispersed in the pluronic copolymer (PEO-b-PPO-b-PEO, Mn = 14600, PEO = 82.5 wt%) aqueous solution by sonication, then the precipitate was removed through centrifugation to result into a homogeneous black solution. In the second step, the black solution was dried at 70 °C, and the residual material was rinsed with water and dried.
Fig. 6

Synthesis of multiwalled carbon nanotubes covalent solvent-free nanofluids

They also fabricated a MWCNT solvent-free nanofluid with a canopy of hyperbranched poly(amine-ester) (HPAE). It was a black viscous liquid at room temperature, and the MWCNT content was about 16.8 wt% [19]. The method of fabrication included two steps (Fig. 7): the first step is the preparation of hyperbranched poly(amine-ester) (HPAE). The reaction of methyl acrylate and diethanolamine and the N, N-diethylol-3-amine methylpropionate was obtained. Then, HPAE was yielded by the reaction of trimethylolpropane (as a core) and N, N-diethylol-3-3 amine methylpropionate (as branched monomer) at a molar ratio of 1:3. The second step is the synthesis of MWCNT solvent-free nanofluids. Acidified MWCNT was treated with HPAE for 3 h, then the resulting mixture was centrifuged and dried to a black viscous liquid.
Fig. 7

Synthesis of multiwalled carbon nanotube covalent solvent-free nanofluids with HPAE

To expand the gallery of solvent-free nanofluids, the graphene solvent-free nanofluid was first fabricated by Tang and his coworkers (Fig. 8) [20]. Graphene core was synthesized by oxidation natural graphite powder according to a modified Hummers method. In the second step, hydrazine monohydrate and VBL were added to the graphene oxide (GO) aqueous dispersion to obtain a VBL-modified graphene. In the last step, a M2070 aqueous solution (10 wt%) was added into the VBL-modified graphene aqueous solution. Then, the reactant was dried for several days to yield graphene-based ionic solvent-free nanofluids.
Fig. 8

Synthesis of graphene ionic solvent-free nanofluids

In addition, a carbon black (CB) solvent-free nanofluid based on ionic bonds was prepared by Li and his coworkers [21]. It had a relatively high organic fraction of 81 wt%, and behaved as a liquid at room temperature. The preparation concluded three steps (Fig. 9): oxidation of CB, grafting by polysiloxane quaternary ammonium salt, and ion exchange of a poly(ethylene)glycol (PEG)-functionalized sulfonate salt. In the first step, CB nanoparticles were oxidized in H2SO4/HNO3 mixture. Subsequently, the charged polysiloxane quaternary ammonium salt DC5700 [(CH3O)3Si(CH2)3N+(CH3)2(C18H37)Cl-] was grafted on the surface of oxidized CB as the corona. Finally, the canopy, poly(ethylene)glycol (PEG)-functionalized sulfonate salt C9H19C6H4O(CH2CH2O)10SO3K+, was connected to modified CB nanoparticles through an ion-exchange reaction.
Fig. 9

Synthesis of carbon black ionic solvent-free nanofluids

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.

An ionic Fe2O3 solvent-free nanofluid was first reported by Bourlinos and his coworkers (Fig. 10) [7]. Firstly, Fe2O3 nanoparticles were modified by organosilane ((CH3O)3Si(CH2)3N+(CH3)(C10H21)2Cl-). Then, the chloride counterions were exchanged with the sulfonate anion R(OCH2CH2)7O(CH2)SO3-(R=C13-15) to obtain a viscous liquid. Bourlinos et al. [9] also synthesized a TiO2 solvent-free nanofluid, which took poly(ethylene)glycol (PEG)-functionalized sulfonate anion C9H19-C6H4-O(CH2CH2O)20SO3- as canopy (Fig. 10).
Fig. 10

Synthesis of Fe2O3, TiO2, and Fe3O4 ionic solvent-free nanofluids

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) [24]. It indicated that long alkyl chains of corona can provide nanofluid lower viscosity and better flowability.

A kind of Fe2O3 solvent-free nanofluid-based covalent bond was developed by Lin and his coworkers (Fig. 11) [26]. The covalent-bonded solvent-free nanofluids could be more advantageous than the ionic solvent-free nanofluids, because the covalent bonds are more stable in aqueous medium. Two steps were involved in the synthesis process. In the first step, the corona 3-glycidyloxypropyl-trimethoxysilane and the canopy polyetheramine (Jeffamine M2070) reacted with each other to produce the flexible organic chains. In the second step, the resulting organic chains were grafted on the surface of Fe2O3 to obtain a dark viscous fluid.
Fig. 11

Synthesis of Fe2O3 covalent solvent-free nanofluids

Most metals have a high melting point, and they cannot flow unless they are heated above 1000 °C. The high melting point severely limited the application of metals. Metal nanofluids have a liquid-like behavior at room temperature. Some solvent-free nanofluids based on metal nanoparticles, such as Au, Pt, and Ag, were developed [22, 43]. For example, Zheng et al. [22] reported a facile method of making gold solvent-free nanofluids (Fig. 12). The synthesis method is based on a two-step process. In the first step, carboxylate-terminated alkanethiol (11-MUA) [HS-(CH2)10-COOH] was grafted on the surface of Au nanoparticles via chemisorption. In the second step, PEG-substituted tertiary amines ([(C18H37)N(CH2CH2O)nH(CH2CH2O)mH],m+n=25) self-assemble on the modified gold surface via an electrostatic self-assembly interaction between the surface carboxyl groups and the amine groups to form the canopy.
Fig. 12

Synthesis of Au ionic solvent-free nanofluids

In addition, a solvent-free ionic molybdenum disulfide (MoS2) nanofluid was synthesized by Gu and his coworkers (Fig. 13) [27]. Solvent-free ionic MoS2 nanofluid was prepared by acid-base reaction. At first, 3-(trihydroxysilyl)-1-propane sulfonic acid (SIT) was grafted to the surface of MoS2 sheets as the corona, then the modified MoS2 nanoparticles were treated through an ion-exchange column to ensure complete replacement of Na+ ions by protons. Finally, the modified MoS2 was tethered with canopy PEG-substituted tertiary amine ((C18H37)N(CH2CH2O)mH)((CH2-CH2O)nH, Ethomeen, Mw = 930 g/mol).
Fig. 13

Synthesis of MoS2 ionic solvent-free nanofluids

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.

Bourlinos and his coworkers reported the synthesis of DNA solvent-free nanofluids employing a PEG-tailed quaternary amine [(CH3)(C18H37)N(CH2CH2O)nH(CH2CH2O)mH, n + m =15] as the canopy (Fig. 14) [9]. It was prepared by ion-exchange reaction between sodium counterions of DNA, which was obtained by neutralizing the free acid using NaOH, and PEG-tailed quaternary amine [(CH3) (C18H37)N(CH2CH2O)nH-(CH2CH2O)mH, n + m = 15].
Fig. 14

Synthesis of DNA ionic solvent-free nanofluids

Moreover, calcium carbonate solvent-free nanofluid was obtained by Li and his coworkers (Fig. 15) [28]. A two-step procedure was adopted in the synthesis of calcium carbonate solvent-free nanofluids. In the first step, the calcium carbonate was modified by a charged polysiloxane quaternary ammonium salt DC5700 ((CH3O)3Si(CH2)3N+(CH3)2(C18H37)Cl-) in a pH of about 10.5. In the second step, PEG-functionalized sulfonate salt (C9H19-C6H4O(CH2CH2O)10SO3K+) was connected to the surface of the modified calcium carbonate nanoparticles through an ion-exchange reaction. A similar approach was adopted by Zheng and her coworkers to synthesize a sepiolite solvent-free nanofluid (Fig. 15) [29]. First, the core sepiolite was modified by another charged polysiloxane quaternary ammonium salt ((CH3O)3Si(CH2)3N+(CH3)(C10H21)2Cl-). Then, chloride was replaced by the canopy, PEG-functionalized sulfonate (C9H19-C6H4O(CH2CH2O)10SO3-).
Fig. 15

Synthesis of a calcium carbonate ionic solvent-free nanofluids and b sepiolite solvent-free nanofluids

3.2 Multicomponent

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.

In 2009, Zhang fabricated a solvent-free nanofluid with a core of MWCNTs decorated with SiO2 [30]. The solvent-free nanofluids had a reversible process of melt and solidification, which was a wax solid at ambient and exhibited liquid-like behavior at 45 °C. The main synthesis method has been showed in Fig. 16. At first, the SiO2 nanoparticles, which were modified by 3-(trimethoxysilyl)-1-propanethiol to restrain the flocculation, were deposited on the surface of carboxylic MWCNTs. In the second step, the copolymer (Mn = 14600, PEO = 82.5 wt%) reacted with the COOH, C=O, and OH on the surface of carboxylic MWCNTs to form the canopy.
Fig. 16

Synthesis of SiO2/MWCNT covalent solvent-free nanofluids

Li et al. [31] synthesized a solvent-free nanofluid based on graphene@Fe3O4 through attaching organosilanes SIT8738.3 and Jeffamine M2070 on the surface of graphene@Fe3O4. The weight fraction of graphene@Fe3O4 was about 13.78 wt%. Moreover, it was a superparamagnetic fluid material at ambient. The synthesis process was shown in Fig. 17. Firstly, the Fe3O4 nanoparticles were deposited on the surface of GO sheets by chemical precipitation of FeCl2 and FeCl3 to obtain the graphene@Fe3O4 hybrid. Secondly, the organosilane SIT8738.3 was attached onto the surface of graphene@Fe3O4 hybrid to form the corona. Then, the sulfonate groups of functionalized graphene@Fe3O4 hybrid were protonated. Finally, the canopy Jeffamine M2070 was grafted onto the functionalized graphene@Fe3O4 hybrid by ionic bonds.
Fig. 17

Synthesis of Fe3O4/oxidation graphene ionic solvent-free nanofluids

Furthermore, Zheng et al. [32] obtained a solvent-free nanofluid utilizing MWCNTs@Fe3O4 as a core by attaching another organosilanes SID3392 and poly(ethylene glycol)4-nonylphenyl-3-sulfopropyl ether and potassium salt (PEGS) on the surface of the MWCNTs@Fe3O4 hybrid (Fig. 18). A similar synthesis method was employed. Firstly, the MWCNTs@Fe3O4 hybrid was obtained through chemical precipitation of FeCl2 and FeCl3, then the MWCNTs@Fe3O4 hybrid was modified by organosilane SID3392. At last, the modified MWCNTs@Fe3O4 hybrid was treated by PEGS.
Fig. 18

Synthesis of Fe3O4/MWCNT ionic solvent-free nanofluids

Recently, Li et al. [33] obtained a covalent solvent-free nanofluid with graphene nanosheet (GNS)@ zinc hydroxystannate boxes (ZHS) hybrid as core. The synthetic procedures are shown in Fig. 19. In the first step, ZnSO4·7H2O and Na2SnO3·3H2O were added into GO suspension with a molar ratio of [Zn]/[Sn] = 1:1 under continuous stirring to synthesize GNS@GO hybrid. In the second step, γ-(2,3-epoxyproxy) propyltrimethoxysilane (KH560) was used to modify the GNS@GO hybrid. Then, the modified GNS@GO hybrid was treated by Jeffamine M2070 to obtain the nanofluid.
Fig. 19

Synthesis of zinc hydroxystannate boxes (ZHS)/graphene nanosheets (GNS) covalent solvent-free nanofluids

In 2016, Bai et al. [34] first fabricated a magnetic solvent-free nanofluid based on Fe3O4@PANI nanoparticles. The core of the Fe3O4@PANI solvent-free nanofluids is the Fe3O4 nanoparticle, and PANI was coated on the surface of the Fe3O4 nanoparticle to form the corona. The canopy structure is flexible chains synthesized by adipic acid and Jeffamine M2070. As shown in Fig. 20, the synthetic procedures comprised three main steps. In the first step, the core Fe3O4 was synthesized through a chemical precipitation of FeCl2 and FeCl3. In the second step, the corona PANI was coated on the surface of Fe3O4 nanoparticles via an in situ emulsion polymerization according to the literature [47]. In the third step, adipic acid was grafted on the surface of corona by the reaction of -COOH groups of adipic acid and -NH- of PANI, and the resulting product was treated by Jeffamine M2070 to complete the synthesis of the canopy.
Fig. 20

Synthesis of Fe3O4 /PANI solvent-free nanofluids

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 [11]. 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.

In other works, a class of solvent-free nanofluids with MWCNTs as the core was synthesized by researchers and the CO2 capture properties of these nanofluids were explored by the authors [52, 53, 54]. The CO2 capture mechanism is illustrated in Fig. 21, -NH- groups and ether can interact with CO2, and the hollow structure of MWCNTs also contributed to the increasing of CO2 capture capacity. Furthermore, the viscosity also influenced the capacity of CO2 capture, a higher viscosity made it harder for CO2 to charge into the potential space and reduce the contact between CO2 and -NH- groups.
Fig. 21

The CO2 capture mechanism of MWCNT nanofluids

4.1.2 Gas separation

A solvent-free nanofluid containing empty cavities has been fabricated by surface modification of hollow SiO2 nanoparticles with flexible organic chains. Zhang et al. [13] took advantage of this solvent-free nanofluids with hollow SiO2 nanoparticle as the core as a separation medium for N2/CO2 separation. Figure 22 presents the structures of this solvent-free nanofluids and the separation mechanism of N2/CO2.
Fig. 22

The structure of hollow SiO2 solvent-free nanofluids and the separation mechanism of N2/CO2 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. [55] 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. [56] 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.

In addition to the MWCNT solvent-free nanofluids, multicomponent MWCNT/Fe3O4 solvent-free nanofluid was obtained by Zheng and her coworkers and then it is used to improve impact toughness and heat resistance of epoxy resin [32]. A liquid-like MWCNT/Fe3O4 derivative was first prepared by attaching organosilane SID3392 and sulfonic acid PEGS onto the surface of MWCNTs/Fe3O4. When the content of MWCNT/Fe3O4 solvent-free nanofluids was 1 wt%, the impact toughness of epoxy was increased by 94%. For comparison, the impact toughness of the epoxy composites containing equal weight fraction of MWCNTs was improved by 51.7%. The significant increase in impact strength is mainly due to the better dispersion of solvent-free nanofluids. As shown in Fig. 23, solvent-free nanofluids can generate more microcracks in matrix, so the nanofluid/epoxy composite can absorb more energy than MWCNT/epoxy composite when the matrix receives the impact. In addition, when the content of MWCNT/Fe3O4 solvent-free nanofluids is 1.5 wt%, the Tg of the epoxy was improved by 16.9 °C. It was impressive that the MWCNT/Fe3O4 solvent-free nanofluids could improve the impact toughness and Tg simultaneously.
Fig. 23

Illustration of impact failure mechanism. a MWCNT/epoxy composite. b Nanofluid/epoxy composite

Apart from MWCNT solvent-free nanofluids, graphene solvent-free nanofluid was obtained and employed to enhance flame-retardant property of epoxy composites [33]. Compared with pure epoxy, the peak heat release rate (PHRR) of the epoxy, which added GNS-ZHC solvent-free nanofluids, decreased by 34.5%. The PHRR of GNS-ZHC/epoxy composite was 567.8 Kw m−2, which was higher than the composite of GNS-ZHC solvent-free nanofluids and epoxy with the same weight fraction of GNS-ZHC. Meanwhile, the composite of GNS-ZHC solvent-free nanofluids and epoxy presented lower total heat release (THR), total smoke release (TSR), and FIGRA values (Fig. 24). It was probably because of the synergism between the catalysis effect of ZHS and the adsorption effect of GNS. This work illustrated a potential application of graphene solvent-free nanofluids for improving the flame-retardant property of epoxy matrix.
Fig. 24

The comparison of flame-retardant property between EP, GNS-ZHS composite, and GNS-ZHS nanofluid composite

4.3 Miscellaneous application of solvent-free nanofluids

Additionally, as shown in Fig. 25, solvent-free nanofluids with different compositions could have other applications such as ionic electrolytes, luminescent materials, decolorization, and magnetic fluids. For example, Lynden et al. [23] reported an ionic solvent-free electrolyte, which is synthesized by grafting ILs to inorganic ZrO2 nanoparticles. The fluid electrolytes exhibited excellent thermal stability and redox stability. It could form long-term stable interface in the presence of lithium anode. The solvent-free electrolyte presented outstanding ionic conductivities when it was doped with lithium bis-(trifluoromethylsulfonyl)-imide (LiTFSI) salt. At room temperature, the ionic conductivity value of LiTFSI-doped ionic ZrO2 solvent-free nanofluids was up to 10−5 S m−1. Additionally, the mechanical characterization also indicated that the elastic modulus in the limit of zero strain was 4.5 MPa, which was much larger than that of its ionic liquid precursors.
Fig. 25

The miscellaneous applications of solvent-free nanofluids. a Application in ionic electrolytes. b Application in luminescent materials. c Application in decolorization

Another potential application of the solvent-free nanofluids went to luminescent materials [57]. Sun et al. [58] 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 [26]. 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.

Suggestions for future work to study on the solvent-free nanofluids cover two areas:
  1. (1)

    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.

  2. (2)

    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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Authors and Affiliations

  1. 1.School of Natural and Applied SciencesNorthwestern Polytechnical UniversityXi’anPeople’s Republic of China

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