Stability and Thermal Conductivity Enhancement of Silver Nanofluids

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The stability and thermal conductivity enhancement of silver nanofluids with gemini surfactants Dan Li, Wenjun Fang, Yiyi Zhang, Xianyuan Wang, Meng Guo, and Xiaomei Qin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03347 • Publication Date (Web): 15 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Industrial & Engineering Chemistry Research

The stability and thermal conductivity enhancement of silver nanofluids with gemini surfactants Dan Li1*, Wenjun Fang2, Yiyi Zhang1, Xianyuan Wang1, Meng Guo1, Xiaomei Qin3 1 Department of Chemistry and Chemical Engineering, Weifang University, Weifang 261061, Shandong Province, China; 2 Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang Province, China; 3 Zhengzhou University of Light Industry, Department of Material and Chemical Engeneering, Zhengzhou 450000, Henan Province, China.

*Corresponding author: Tel: +86 536 8785283, E-mail: [email protected]

Abstract: Highly dispersed silver nanofluids were prepared using cationic gemini surfactants (n-6-n) as stabilizers in aqueous solution. The influences of alkyl chain length in the structure of cationic gemini surfactants on the preparation and stability of the silver nanofluids are presented. The thermal conductivity enhancement of the silver nanofluids was determined by means of point heat source method. The effects of polar solvents and temperatures on the stability of the silver nanofluids were investigated.

The size of silver nanoparticles is uniform, and their average

diameter is about 10~20 nm. The siver nanoparticles could be transferred from an aqueous phase into a dichloromethane or chloroform phase, which were still dispersed well. The stability of silver nanoparticles can be ordered as: G16-6-16-Ag > G14-6-14-Ag > G12-6-12-Ag. Keywords: Gemini surfactant; Silver nanofluid; Stablility; Thermal conductivity

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1. Introduction Nanofluid is a stable colloidal suspension and treated as pseudo-homogeneous phase system[1-2]. For the suspended nanoparticles, bulk metals have relatively higher thermal conductivity than their oxides. Silver, copper, iron, aluminum and gold have been the most useful metallic nanoparticles. Since thermal conductivity of fluids suspended metal nanoparticles (nanofluids) is greater than the common fluids, they can be used for heat transfer enhancement applications [3-4]. Nanofluids have their potential applications in catalytic system and energy technologies [5-6]. The preparation and use of nanofluids containing metal nanoparticles still face many problems, including dispersing ability and stability. Several factors affected the stability of nanofluids, such as: type and size of nano-metal, base fluid, surfactants and the conditions like pH and temperature [7-8]. Surfactant was an important factor for the stability of nanofluid. Gemini surfactants have special structure and excellent properties, since they are double-chain amphiphiles and consist of two ionic heads and two hydrocarbon tails. The ionic heads are connected with a spacer. Compared with conventional surfactants with an alkyl chain, gemini surfactants have the advantages of strong solubilization and adsorbability[9-10]. Several kinds of gemini surfactants are good stabilizers in employing for synthesizing and stablizing silver nanoparticles. Xu et al. reported the synthesis of hydrophobic silver nanoparticles stabilized by 18-3(OH)-18. The nanoparticles could be dispersed well in n-heptane [11]. He et al. reported the preparation of silver nanoparticle stabilized by 12-2-12 and their nanosilver had good catalytic activity [12]. Datta et al. investigated the ability of silver nanoparticles stabilized gemini imidazolium surfactants suspending in aqueous media [13]. Bhattacharya et al. prepared silver nanorods with cationic gemini surfactants and investigated the effect of spacer lengths on the formation and growth of nanorods [14] However, the effects of alkyl chain lengths in gemini surfactants’ structure on the formation of nanoparticle were not investigated in these reports. In this work, cationic gemini surfactants (16-6-16, 14-6-14, 12-6-12) were used as stabilizers and


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dispersant. Glucose was used as a reductant. The process allowed us to prepare and obtain silver nanofluids with good dispersion and stability. The effects of temperature and the alkyl chain lengths of cationic gemini surfactants on the particle size and stability were presented and characterized. The effects of addition of polar organic solvents (methanol, ethanol and acetone) on the stability of the nanofluids were studied by UV–vis spectra and zeta potentials. The stability of nanofluids at 120~150 °C was also investigated. The effects of alkyl chain lengths length on thermal stability of nanofluids consisting of silver nanoparticles (G16-6-16-Ag, G14-6-14-Ag, G12-6-12-Ag) were investigated. The thermal conductivity enhancement of the silver nanofluids prepared was presented.

2. Experimental section 2.1. Materials D(+)-Glucose, sodium hydroxide (≥ 96 %), silver nitrate (≥ 99.8 %), dichloromethane (≥ 99.5 %), methanol (≥ 99.5 %), ethanol (≥ 99 %), and acetone (≥ 99 %) are analytically pure reagents, which were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The gemini surfactants,

1,6-bis(hexadecyldimethylammonium)

1,6-bis(tetradecyldimethylammonium) 1,6-bis(dodecyldimethylammonium)

hexane hexane

dibromide

hexane dibromide (12-6-12)

dibromide

(16-6-16),

(14-6-14), were

synthesized

and and

recrystallized in our laboratory according to the literature procedures [15]. The synthesized gemini surfactants were characterized by CHN elemental analysis, 1HNMR and FTIR spectroscopic analysis. The chemical structures of the cationic gemini surfactants (16-6-16, 16-4-16, and 16-2-16) of different chain length used in this work were given in Figure 1. 16-6-162+, 2Br-: 1HNMR (600 MHz, CDCl3) δ 0.89 (t, 6H, -CH3), 1.27-1.73 (m, 56H, alkyl chain 2×15CH2), 2.06 (s, 4H, -CH2-CH2-N+-), 3.38 (s, 12H, CH3-N+-CH3), 3.47 (t, 4H, 2-CH2-N+ -), 3.75 (t, 4H,2 -N+CH2-), C,H,N analysis: C 65.0, H 11.6, N 3.6.



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14-6-142+, 2Br-: 1HNMR (600 MHz, CDCl3) δ 0.86 (t, 6H, CH3-), 1.23-1.69 (m, 52H, alkyl chain 2×13CH2), 1.98 (s, 4H, -CH2-CH2-N+-), 3.37 (s, 12H, CH3-N+-CH3), 3.48 (t, 4H, 2-CH2-N+ -), 3.70 (t, 4H, 2 -CH2-N+); C,H,N analysis: C 63.0, H 11.2, N 3.8. 12-6-122+, 2Br-: 1HNMR (600 MHz, CDCl3) δ 0.88 (t, 6H, CH3-), 1.26-1.72 (m, 44H, alkyl chain 2×11CH2), 2.02 (s, 4H, -CH2-CH2-N+-), 3.39 (s, 12H, CH3-N+-CH3), 3.48 (t, 4H, 2-CH2-N+ -), 3.73 (t, 4H, 2 -N+CH2-); C,H,N analysis: C 61.0, H 11.1, N 4.1.

CH3

H3C Br

N

n-C16H33

(H2C)6 Br H3C

CH3

H3C Br

N

n-C14H29

(H2C)6

n-C16H33

N CH3

Br

N

n-C12H25

N

n-C12H25

(H2C)6

n-C14H29

N

Br

CH3

H3C

H3C

CH3

Br H3C

CH3

Figure 1. Chemical structures of the cationic gemini surfactants

2.2. Preparation of Silver Nanofluids Stable silver nanofluids were prepared in an aqueous solution system of gemini surfactants by the reduction of AgNO3. Typically, a clear aqueous solution of gemini surfactants (0.01 M, 20mL) was prepared and heated up to 60°C under vigorous stirring. The solutions of glucose (0.08mol/L, 10 mL) and sodium hydroxide (0.08 mol/L, 10 mL) were added into the above mixed system under vigorous stirring. A freshly prepared AgNO3 solution (0.02 M) was added dropwise into the mixture system. The color of the mixture system became brown after keeping at 60 °C and continuous stirring for 60 min. A silver nanofluid was obtained by one-step synthesis. The silver nanoparticles capped by gemini surfactants could be separated by centrifugation and dried in a vacuum oven at 45°C. They were reserved for other characterization and measurements. 2.3. Characterization


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Elemental analyses (Elementar, Vario EL cube, Germany) and 1HNMR (Bruker Advance Ⅲ 600 Hz, Germany) were used to examine the structures of gemini surfactants. X-ray diffraction analysis of the silver nanoparticles were determined using a D8 Advance X-ray diffractometer (Bruker, Germany) with Cu Kα radiation (λ = 1.5405 Å). FTIR measurements were recorded by a 360 FT-IR spectrophotometer (Nicolet, USA). Transmission electron microscope observations were carried out by transmission electron microscopys JEM-2100 (JEOL, Japan). The UV–vis spectra were recorded by a UV-1700 spectrophotometer (Shimadzu, Japan). The zeta potential of the silver-water nanofluids were determined by the Nano ZS Zetasizer (Malvern Instruments Ltd., UK). Each test result was the average value taken from three times measurements. Thermal conductivity of silver nanofluid was measured by KD2 Pro thermal properties analyzer (Decagon Devices, USA). 3 Results and discussion 3.1 Characterization of nanoparticles and the phase transfer Glucose was used to reduce Ag+ ionic in an alkaline system (pH 10-11) to prepare silver nanofluids with three gemini surfactants (16-6-16, 14-6-14, 12-6-12) as stabilizers. In the preparation process, temperature affected the formation of the silver nanoparticles with capping layer. The dispersed stability of silver nanoparticles capped with gemini surfactants is poor when the reaction temperatures is relatively low, and the nanofluids formed easily appeared agglomeration in 24h. The silver-water nanofluids prepared at 60°C are uniform and stable. The XRD patterns of silver nanoparticles (G16-6-16-Ag, G14-6-14-Ag and G12-6-12-Ag) reduced at 60 °C are given in Figure 2. Their XRD patterns of the silver nanoparticles show characteristic diffraction peaks for metallic silver [111], [200], [220] and [311] facets. It indicates that the formation of pure silver and the silver nanoparticles are highly crystalline. There are no impurity phases detected from their XRD



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patterns. Figure 3 shows The FTIR spectra of three gemini surfactants and silver nanoparticles. For example, the strong bands at 2912 and 2846 cm−1 for gemini surfactant 16-6-16 are the result of symmetric and asymmetric stretching vibrations of methylene in the alkyl chains, respectively. The solid G16-6-16-Ag also shows the stretching vibrations of –CH2– at 2913 and 2848 cm-1. The peaks at 930~980 cm−1 are associated with C–N+ vibrations. Some new peaks at 1020, 1054, 1101, and 1143 cm-1 were formed for G16-6-16-Ag, and the intensities of C-N+ bands at 933 and 977 cm-1 decreased. The bands at 1020, 1054, 1101, and 1143 cm-1 are probably due to the stretching modes of the bound C-N+ to the surface nanosilver [16, 17]. It indicates that the surface of the silver cores was covered by 16-6-16. The same changes appeared at the FTIR spectra of G14-6-14-Ag and G12-6-12-Ag.

C

B

(1 1 1 )

A

(2 0 0 ) (2 2 0 )

30

40

50

60

(3 1 1 )

70

80

2 θ /°

Figure 2. XRD pattern a of silver nanoparticles seperated from the silver-water nanofluids: (A) G16-6-16-Ag; (B) G14-6-14-Ag; (C) G12-6-12-Ag


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F E D C

B 2913

1054.5 1101

2848

A

1382 1440

977

2912 2846 4000

3500

3000

2500

2000

1500

Wavenumber/cm

1000

500

-1

Figure 3. The FTIR spectra of gemini surfactant and silver nanoparticles capped by gemini surfactants: (A,C,E) 16-6-16, 14-6-14, 12-6-12; (B,D,F) G16-6-16-Ag, G14-6-14-Ag, G12-6-12-Ag

Shape and size of the silver nanoparticles were characterized with TEM observations. TEM images of G16-6-16-Ag, G14-6-14-Ag and G12-6-12-Ag in water nanofluids and their size distributions were exhibited in Figure 4. The silver nanoparticles were well-dispersed. Gemini surfactants can protect the nanoparticles from aggregation. The influence of the length of alkyl chain in gemini surfactants on the particle size or morphology is not obvious, as shown in Figure 4. The average diameters of G16-6-16-Ag, G14-6-14-Ag and G12-6-12-Ag are about 16.2 nm, 17.7 nm and 19.6 nm. The zeta potential value of silver nanoparticles capped by 16-6-16, 14-6-14 or 12-6-12 is 35(±2) mV, 42(±2) mV and 31(±2) mV, respectively. These results indicated that 16-6-16 and 14-6-14 have more efficient coating and stablizing ability for silver nanoparticles than gemini surfactant 12-6-12.



B

A

C

0.40 0.30

0.20

0.35

a

0.25

b

0.30

c

0.15

0.15

0.25

Frequency

Frequency

0.20 Frequency

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0.20 0.15

0.10

0.10

0.10 0.05

0.05

0.05

0.00

0.00

0.00 8

10

12

14

16 d/nm

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20

22

24

10

15

20

25

30

d/nm

14

16

18

20

22

24

26

28

30

d/nm

Figure 4. TEM pictures and size distributions of silver nanoparticles in water phase: (A, a) G16-6-16-Ag; (B, b) G14-6-14-Ag; (C, c) G12-6-12-Ag

The obtained silver nanoparticles capped by gemini surfactans (16-6-16, 14-6-14 or 12-6-12) were stable in water medium for long time. The used gemini surfactans were very important for formation and stabilization of the silver nanoparticles. Gemini surfactants provide additional electrostatic stabilization and passivate the surface of silver nanoparticles formed. There are two amphiphilic moieties (hydrophilic headgroups with alkyl chains) in the structure of cationic gemini surfactants, which are combined by a spacer group. The molecules of gemini surfactans formed on the surface of silver nanoparticles in aqueous solution are bilayer structure [12, 14]. The bromine ion can absorb on the surface of silver nanoparticle directly, and the cationic head groups of gemini surfactants formed ion pairs with bromine ion. The cationic head groups surrounded bromine ion coating layer by electrostatic interactions [18, 19]. This is the inner layer of the gemini surfactants molecules. The hydrophobic carbon tails of this layer are outward. Because of the intense hydrophobic force, the gemini surfactants molecules formed other layer of ion pairs outside. The


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cationic head groups were outward and surrounded by bromine ion. Therefore, the surface of the silver nanoparticles is hydrophilic. Gemini surfactants can offer good capping ability and stability to silver nanoparticles because of their structure and high charge density. Thus the nano-silver could be stabled in water phase for a long term of storage. It was found that the silver nanoparticles (G16-6-16-Ag, G14-6-14-Ag and G12-6-12-Ag) could successfully transfer from water phase into dichloromethane phase. 2 mL A silver-water nanofluid stablized by gemini surfactants is placed in a vial and 2 mL dichloromethane is added. The silver nanoparticles are gradually transferred from water to dichloromethane and the nanoparticles are stably dispersed in dichloromethane. After 72 hours, most of the silver nanoparticles are successfully transferred into dichloromethane phase. The lower dichloromethane phase changed from colorless to brown and the upper aqueous phase becomes primrose yellow or colorless, as shown in Figure 5. G16-6-16-Ag, G14-6-14-Ag and G12-6-12-Ag can automatically transfer to the chloroform phase, dichloromethane phase, or toluene phase (The process of phase transfer from water to toluene needed agitation). It was found that the G12-6-12-Ag partly agglomerated and deposited in the process of phase transfer as shown in Figure 5. The nanoparticles of G16-6-16-Ag and G14-6-14-Ag in CH2Cl2 remain highly dispersed. Comparing the ability of phase transfer of the silver nanoparticles, the ability of phase transfer of G16-6-16-Ag and G14-6-14-Ag is better than that of G12-6-12-Ag. Figure 6 gives the TEM pictures of silver nanoparticles in CH2Cl2 after phase transfer. A

B a

b

c

C a

0h

b

c

24 h


a

b

c

72 h


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Figure 5. Silver nanoparticles capped by gemini surfactant before and after phase transfer (from water to dichloromethane): A, 0 h; B, 24 h; C, 72 h (a, G16-6-16-Ag; b, G14-6-14-Ag; c, G12-6-12-Ag)

A

B

C

Figure 6. TEM pictures of silver nanoparticles in CH2Cl2 after phase trasfer: (A) G16-6-16-Ag; (B) G14-6-14-Ag; (C) G12-6-12-Ag

After phase transfer to dichloromethane phase, the surface structure of the capping layer of silver nanoparticles has changed dring the process of phase transfer. The capping layer of nanoparticles is hydrophobic after the phase transfer, which can dispersed well in dichloromethane phase. The gemini surfactant layer of the nanoparticles becomes a single layer structure from the double layer with the hydrophilic tail chains outward. Thus, the dispersion stability of nanoparticles in dichloromethane or chloroform is enhanced [11, 20]. It is possible to separate the nanoparticles by evaporating the dichloromethane or chloroform, redissolving the nanoparticles in other non-polar organic solvent. The phase transfer is ensured that the silver nanoparticles are more soluble in nonapolar solvents. The silver nanoparticles prepared in aqueous phase transfer to the organic solvent phase, and the oil-based nanofluid will be obtained. It is important for the preparation of the oil-based nanofluid. The Uv-vis spectra of the nanoparticles (G16-6-16-Ag, G14-6-14-Ag and G12-6-12-Ag) were tested before and after phase transfer to dichloromethane (Figure 7). The


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spectra of silver nanoparticles before and after phase transfer show the characteristic SPR band at 400∼420 nm [11]. The absorbance of the transferred nanoparticles appears to be unchanged upon transfer from water to dichloromethane.

1.0

1.2

A

1.0

B

0.8

Absorbtion

0.8 0.6

C

0.8 Absorbtion

1.0 Absorbtion

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0.6

0.4

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0.4 0.2

0.2 0.0 300

400

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700

0.0 300

0.2

400

Wavelength/nm

500

600

700

Wavelength/nm

0.0 300

400

500

600

700

Wavelength/nm

Figure 7. Uv-vis spectra of silver-nanoparticles in water (black lines) and after phase transfer in dichloromethane (red lines): (A) G16-6-16-Ag; (B) G14-6-14-Ag; (C) G12-6-12-Ag

3.2 Thermal conductivity of silver nanofluids In this work, k is the thermal conductivity of the nanofluid, and k0 is the thermal conductivity of the base fluid (H2O). k/k0 represents the thermal conductivity ratio. The effects of temperature on thermal conductivity ratio (k/k0) and effects of mass fraction of the silver nanoparticles on k/k0 were studied. There is an increase for thermal conductivity ratios of nanofluids with G16-6-16-Ag, G14-6-14-Ag, and G12-6-12-Ag with increasing temperature as shown in Figure 8A. The thermal conductivity of nanofluid has no increase at 25 and 30 °C. When the measured temperature was rose above 35 °C, the thermal conductivity increases. The surface of silver nanopaticles was capped by gemini surfactants and not naked. The surface capping layer can affect the thermal properties of the silver nanofluids[3]. Therefore, there is no increase for the thermal conductivity at 25 and 30 °C. This behavior can be explained by Brownian motion of nanofluids. Figure 8B shows k/k0 of silver nanofluids with 0.4, 0.8 and 1.2 mass% G16-6-16-Ag. With increasing mass fraction of G16-6-16-Ag, k/k0 of silver nanofluid were increased nonlinearly.



1.5

1.5 16-6-16 Ag 0.4% 14-6-14 Ag 0.4% 12-6-12 Ag 0.4%

1.3 1.2 1.1

A

1.0 25

30

35

1.2%16-6-16 Ag 0.8%16-6-16 Ag 0.4%16-6-16 Ag

1.4 Thermal conductivity ratio

1.4 Thermal conductivity ratio

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40

45

50

1.3 1.2 1.1

B

1.0 25

Temperature/°C

30

35

40

45

Temperature/°C

Figure 8. Thermal conductivity ratios of silver nanofluids: (A) thermal conductivity ratios at different temperatures; (B) Thermal conductivity ratios of nanofluids with 16-6-16-Ag at different concentrations.

3.3 Effects of addition of polar solvents on stability of nanofluids The silver nanoparticles in nanofluids show a homogeneous and stable dispersion. No turbidity, precipitation and color variation were observed after storing for 60 days. UV–vis spectra (at 300-700 nm) and zeta potentials of the silver nanofluids were recorded after 60 days. The position and shape of the absorption peak of the nanofluids and their zeta potential did not change significantly. The plasmon absorption peak at about 410-430 nm is the characteristic peak of silver nanoparticles[19-22]. The effects of the polar solvents (methanol ethanol or acetone) addition on the stability of the silver nanofluids were characterized by UV–vis spectra (Figure 9). The absorption maximum of G16-6-16-Ag was 1.12 at 415 nm initially, as shown in Figure 9a. After the addition of methanol, ethanol or acetone (20 %) for 10 min, the shape and position of the absorption peak has no obvious change. The wavelength maximum (λmax) did not appear red shift. The absorption maximum of G14-6-14-Ag without adding protic solvent was 0.615 at 414 nm. After the addition of the polar solvents, the wavelength maximum (λmax) of the absorption peak has no obvious change (Figure 9b). The absorption maximum of G12-6-12-Ag without adding polar solvent was 0.81 at


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420 nm. After the addition of ethanol, methanol or acetone to G12-6-12-Ag nanofluids, the absorbance of the absorption peak appears obvious decrease, the wavelength maximum (λmax) appears red shift and the absorption band is broadened. It can be seen that the absorbance of G12-6-12-Ag is strongly affected by adding polar solvents. A strong decrease (from 0.81 to 0.352 for methanol, respectively) in absorbance and the wavelength maximum (λmax) shifts from 420 nm to 463 nm. This effect of adding of methanol and acetone was more significant than that of adding ethanol, as shown in Figure 9c. Based on the absorbance spectra change of adding polar solvents in three silver nanofluids, the stability of silver nanoparticles in nanofluids can be ordered as: G16-6-16-Ag > G14-6-14-Ag > G12-6-12-Ag. The formed adsorbed layer of gemini surfactant 12-6-12 is less stable than that of 16-6-16 and 14-6-14. The silver nanofluid stabilized by 12-6-12 shows poor stability. The effects of the polar solvents addition on the stability of the silver nanofluids were characterized by zeta potential. The zeta potential values of water-based nanofluids with G16-6-16-Ag, G14-6-14-Ag or G12-6-12-Ag were measured. Figure 10 shows the evolution of zeta potentials of the silver nanofluids with changing the volume fraction of polar solvents (methanol, ethanol and acetone). The zeta potential of nanofuids was decreased with increasing of the addition amount of the polar solvent. Acetone affected more strongly than ethanol and methanol for the more variation of zeta potential, as shown in Figure 10. From the zeta potential values, the stability of silver nanoparticles was varied with the addition amount of the polar solvents. The hydrophobic character of the polar solvents could be considered for explain the effect of polar solvents [22]. The effect on adsorption increases with hydrophobic character of the solvents: acetone > ethanol > methanol. Although there is no obvious change of the UV spectra analysis for G16-6-16-Ag and G14-6-14-Ag within 30 min, but zeta potential test results show that the nanofluids with G16-6-16-Ag and G14-6-14-Ag are beginning to be unstable after adding polar solvent. The capped layer on their surface is beginning to be destroyed by polar solvents slowly. Therefore, the



absorption peaks appear obvious decrease and redshift after 24 h (Figure 9A and 9B). The capped layers of gemini surfactants protected the nanoparticles and inhibited them from agglomeration. The addition of protic solvents broke the stability of the capping layer of silver nanoparticles, and resulted in the agglomeration of nanoparticles.

0.9 without polar solvent +20% methanol +20% ethanol +20% acetone +24 h

0.7 Absorbance

Absorbance

1.0 0.8 0.6

0.6 0.5 0.4

0.6

0.4

B

0.2 0.2

without polar solvent +20% methanol +20% ethanol +20% acetone

0.8

0.3

A

0.4

1.0

without polar solvents +20 % methanol +20 % ethanol +20 % acetone 24h

0.8

Absorbance

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C 0.2

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400

500

600

0.0 300

700

400

500

600

700

0.0 300

400

500

Wavelength/nm

Wavelength/nm

600

Wavelength/nm

Figure 9. Effect of protic solvents on the Uv-vis spectra of silver nanoparticles: (A) G16-6-16-Ag; (B) G14-6-14-Ag; (C) G12-6-12-Ag. 40

50 methanol ethanol acetone

35

35

methanol ethanol acetone

40

30

methanol ethanol acetone

30

25 20 15 10

Zeta potential/mV

25 Zeta potential/mV

Zeta potential/mV

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20 15 10

10

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5 0 0.00

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C

5

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0 0.00

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Figure 10. The evolution of zeta potentials of the water-based silver nanofluids as a function of the volume fraction of polar solvents: (A) G16-6-16-Ag; (B) G14-6-14-Ag; (C) G12-6-12-Ag.

3.4 Thermal stability of Silver Nanofluids Temperature is a very influential factor of the stability of nanofluids. Heating at high temperature leads to agglomeration and settlement of nanoparticles. The stability of the nanofluids with G16-6-16-Ag, G14-6-14-Ag or G12-6-12-Ag was tested against temperature and monitored by Uv-vis spectroscopy. The nanofluids were considered to be stable and uniform when there was no


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turbidity and precipitation and the absorption value at λmax did not decrease and no red-shift. Decrease of the absorbance value indicated the silver nanoparticles partly agglomerated and deposited. Figure 11 depicts the change of thermal stability times for three water-based silver nanofluids with temperature increasing. It is shown that the nanofluid with G16-6-16-Ag could be stable and no agglomeration at 120 °C for 16 h. The nanofluid with G14-6-14-Ag and G12-6-12-Ag can be just stable for 10 h and 7.5h at 120 °C. The thermal stability time of G16-6-16-Ag is the longer than that of G14-6-14-Ag and G12-6-12-Ag at the same temperature of heating. As the chain length of alkyl group in the gemini surfactant is relative longer, the nanofluid shows better stability. This result is agreed well with that from the effects of the polar solvents addition on the stability of the nanofluids with G16-6-16-Ag, G14-6-14-Ag or G12-6-12-Ag.

18

G16-6-16 Ag G14-6-14 Ag G12-6-12 Ag

16 14 Thermal stability time/h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12 10 8 6 4 2 0 120

130

140

150

160

Temperature/°C

Figure 11. Thermal stability of water-based nanofluids with G16-6-16-Ag, G14-6-14-Ag or G12-6-12-Ag

4. Conclusion Cationic gemini surfactants (16-6-16, 14-6-14 and 12-6-12) are used as capped stabilizers to synthesize stable water-based silver nanofluids. They are good stabilizers for preparation and stabilization of monodisperse silver nanoparticles because of their structure and high charge density.



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The prepared silver nanoparticles could gradually transfer from water to dichloromethane and the nanoparticles are stably dispersed in dichloromethane and chloroform phase. The effects of adding polar solvents and temperatures on the stability of the silver nanofluids were studied. The thermal conductivity ratios of the nanofluids increase nonlinearly with the increase of mass fraction of surface capped nanoparticles. Based on the results, the stability of silver nanoparticles can be ordered as: G16-6-16-Ag > G14-6-14-Ag > G12-6-12-Ag. Gemini surfactants 16-6-16 and 14-6-14 are better for stabilizing silver nanoparticles than 12-6-12.

Acknowledgements The authors are very grateful for the financial supports from Natural Science Foundation of Shandong Province, China (No. ZR2016BQ40) and National Natural Science Foundation of China (No. 21103129).

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[19] Sondi, I.; Goia, D. V.; Matijević, E. J. Preparation of highly concentrated stable dispersions of uniform silver nanoparticles. J. Colloid Interface Sci. 2003, 260, 75. [20] Li, X.; Shen, J.; Du, A.; Zhang, Z.; Gao, G.; Yang, H.; Wu, J. Facile synthesis of silver nanoparticles with high concentration via a CTAB-induced silver mirror reaction, Colloid. Surface. A. 2012, 400, 73. [21] Roldán, M. V.; Scaffardi, L. B.; Sanctis, O.; Pellegri, N. Optical properties and extinction spectroscopy to characterize the synthesis of amine capped silver nanoparticles. Mater. Chem. Phys. 2008, 112, 984. [22] Khan, Z.; AL-Thabaiti, S. A.; Obaid, A. Y.; Khan, Z. A.; Al-Youbi, A. O. Effects of solvents on the stability and morphology of CTAB-stabilized silver nanoparticles. Colloid. Surface. A. 2011, 390, 120.


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Stability and Thermal Conductivity Enhancement of Silver Nanofluids

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