Surfactants as

Article pubs.acs.org/jced

Thermophysical Properties of Some Fatty Acids/Surfactants as Phase Change Slurries for Thermal Energy Storage Zhaoli Zhang, Yanping Yuan,* Nan Zhang, and Xiaoling Cao School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, P. R. China ABSTRACT: Capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, and their eutectics are chosen as phase change materials to constitute the phase change slurries for the first time. Under the emulsification of span20/SDS, phase change slurries with 30 wt % fatty acids are then fabricated by high-speed shearing. Thermophysical properties of phase change slurries are characterized by means of DSC. Results indicate that eutectic phase change slurries exhibit lower freezing/melting peak temperatures than corresponding fatty acids slurries. In the melting, peak temperatures from 22.36 °C to 55.61 °C and the freezing peak temperatures from 14.61 to 48.97 °C, latent heat fusion of eutectic phase change slurries are between 36.32 J/g and 51.82 J/g, 34.35 J/g and 48.58 J/g in the endothermic/exothermic processes. Heat fusion has not demonstrated obvious changes under the heating and melting processes, and these values are approximately equal to those of pure fatty acids multiplied by the mass fraction. Additionally, supercooling of phase change slurries are between 0.55 °C and 6.55 °C, and this phenomenon might be induced by the crystal structures formed in fatty acids eutectics.

1. INTRODUCTION Thermal energy storage (TES), which is of great significance to energy saving and the optimized using, can be used to store energy under the condition of mismatched time and space to diminish the increasing global energy demand.1−3 As important organic phase change materials(PCMs), fatty acids have a high energy storage density and constant phase change temperature, which can store or release a large amount of latent heat during their phase transformation process. Therefore, fatty acids have attracted much attention owing to their unparalleled advantages, that is, abundant sources, lower cost, chemical inertness, and nontoxicity, which present tremendous growth potential in TES.4,5 Several researchers all around the world have investigated the physicochemical and thermophsical characteristics of fatty acids as PCMs to boost the heat exchange and thermal storage capacity.6 However, due to the low thermal conductivity, fatty acids occupy a poor heat transfer rate, limiting the promotion in the heat exchange processes. In order to solve this inherent defect, phase change slurries (PCSs) are introduced to enhance the heat transfer rate between PCMs and the surroundings by enlarging the heat-exchange surface.7 Compared to conventional heat transfer fluid, PCSs are a novel type of heat transfer medium, which serves as means of storing energy and enhancing heat transfer. By decreasing the heat transfer area of devices, decreasing pipe size, and picking small devices, PCSs will promote to minify the heat transfer equipment as well as to lower the transmission power.8 However, it is common to perceive the supercooling, which is defined as the phenomenon that PCMs just start to solidify when the temperature is inferior to the freezing point.9,10 © XXXX American Chemical Society

Supercooling will be even more serious when the PCM particles are dispersed in the PCSs, and the smaller the size, the more serious the supercooling. The reason may be that the crystallization of PCMs in PCSs is retarded by the increasing of inactive seed crystal that has emerged in smaller particle size, furthermore enlarging the supercooling extent.11−13 Previous literatures5,4,10,13,14 involved in PCSs can be available where long-chain alkane (tetradecane, hexadecane, and paraffin) as PCMs is the most frequent used. Rather than possessing the conventional advantages of abundant sources, lower cost, chemical inertness, and nontoxicity in long-chain alkane, fatty acids also feature noninflammability, which enable them to be employed at higher temperature. Yet, few researches associated with fatty acids PCSs are available in current literatures. In this case, five types of fatty acids (CA, LA, MA, PA, SA) and their eutectic PCSs (CA-LA, CA-MA, CA-PA, CASA, LA-MA, LA-PA, LA-SA, MA-PA, MA-SA, PA-SA) have been conceived and developed by high-speed mechanical shearing under the emulsification of span20/SDS for the first time, whose thermophysical properties, including enthalpy, melting temperature, freezing temperature, and supercooling, also are investigated in detail in the following sections.

2. EXPERIMENTAL SECTION 2.1. Materials. Capric acid (CA, C10H20O2), lauric acid (LA, C12H24O2), palmitic acid (PA, C16H32O2), and stearic acid Received: April 27, 2015 Accepted: July 22, 2015

A

DOI: 10.1021/acs.jced.5b00371 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Chemical Sample Table chemical name

source

purification method

final mole fraction purity

capric acid lauric acid palmitic acid stearic acid myristic acid Sorbitan monolaurate sodium dodecyl sulfate

aladdin aladdin aladdin aladdin guangfu damao kelon

none none none none none none none

0.99 0.98 0.97 0.98 0.98 AR, saponification value =155−175 ⩾0.85

heat, and the content of phase change of the ith fatty acid; and R is the gas constant. The eutectic mixture has a lower melting temperature. Therefore, we can reach the expected eutectic mass ratio when Tm gets the lowest value. Eutectic mixtures of various contents of CA, LA, MA, PA, and SA were stirred in a magnetic stirrer for 30 min at 400 r/min. Meanwhile, fatty acids were heated to a higher temperature to maintain in the form of liquid. Finally, several eutectics were prepared after slowly cooling down to the room temperature. Thermal properties of eutectics at the optimized mass ratio were listed in Table 3. The 100 mL PCSs containing 30 wt % fatty acids, under the emulsification of 1.75 g of span20 and 0.75 g of SDS, were produced by high-speed shearing emulsification machine (C350−S, Shanghai Muxuan Industrial Co., LTD) at 4000 r/

(SA, C18H36O2) purchased from aladdin were selected as PCMs. Table 1 and 2 list used chemical samples and their Table 2. Thermal Properties of CA, LA, MA, PA, and SA at 0.1 MPaa fatty acids

melting temperature °C

J/g

CA

33.15

150.82

LA

MA

PA SA

45.29

55.23

63.98 70.75

latent heat

172.65

184.31

198.84 208.43

RD(T)

reference

RD(H)

reference

0.1510 0.0694

15

17

0.0484 0.0783 0.0411

15

−0.0916 −0.0721 −0.0456 −0.0393 −0.0615 −0.0299 −0.0247 0.0927 −0.0705 0.1335 0.0050 −0.0450 −0.0724 −0.0450 −0.1327 −0.0112 0.0689 −0.0790 0.1427

16

20 16

0.0247 0.0045 0.0421

15

0.0270 0.0188 0.0195 0.0165 0.0180 0.0180

28

23 25

15 31 28 32 15

18 19 21 22 23 19 24

Table 3. Thermal Properties of Fatty Acids Eutectics with Optimal Mass Fraction at 0.1 MPaa

26 27 23 25

fatty acids

latent heat

°C

J/g

62:38

22.96

121.02

0.0352

34

75:25

25.12

134.03

−0.0099

34

82.6:17.4

27.51

129.07

−0.0289

34

92:8

30.18

139.15

−0.0157 −0.0267

35

CASA LAMA

63:37

35.79

142.51

−0.0319

34

74:26

37.01

158.37

−0.0165 −0.0429

36

LAPA

21

30 32

CALA CAMA CAPA

30 33 26 25

a

Standard uncertainties of experimental temperature are u(T)CA = 2.0 K, u(T)LA = 3.0 K, u(T)MA = 2.0 K, u(T)PA = 3.0 K, and u(T)SA = 2.0 K. Standard uncertainties of latent heat are u(H)CA = 12.0 J/g, u(H)LA = 4.0 J/g, u(H)MA = 1.0 J/g, u(H)PA = 12.0 J/g, and u(H)SA = 3.0 J/g. The relative deviation of temperature is RD(T) = (Texp − Tref)/Tref, the relative deviation of latent heat is RD(H) = (Hexp − Href)/Href.

thermal properties calculated by differential scanning calorimeter (DSC). Myristic acid (MA, C14H28O2) was acquired from Tianjing guangfu fine chemical institute. Sorbitan monolaurate (span 20, C18H34O6, HLB = 8.6) chosen as a nonionic surfactant was achieved from Tianjin Damao Chemical Reagent Co., Ltd. Another ionic surfactant is sodium dodecyl sulfate (SDS, C12H25−OSO3Na, HLB = 40), which was obtained from Chengdu Kelon Chemical Reagent Co., Ltd. All chemical reagents were analytical reagents and used without further purification. Based on our previous research,3 the theoretical ratio and phase change temperature can be calculated via formula 1 −1 ⎛1 R lnXi ⎞ Tm = ⎜ − ⎟ Hi ⎠ ⎝ Ti

fatty acids: water mass ratio

eutectic melting temperature

29

RD(T)

reference

34

34

LASA

83:17

39.49

153.14

0.1188 −0.0480

64:36

46.85

160.73

−0.0496 −0.0375

35

MAPA MASA PASA

74:26

47.13

163.71

−0.0453

34

62:38

56.71

176.18

−0.0566

30

34

34

a

Standard uncertainties of experimental temperature are u(T)CA‑LA = 1.0 K, u(T)CA‑MA = 1.0 K, u(T)CA‑PA = 1.0 K, u(T)CA‑SA = 1.0 K, u(T)LA‑MA = 1.0 K, u(T)LA‑PA = 2.0 K, u(T)LA‑SA = 4.0 K, u(T)MA‑PA = 2.0 K, u(T)MA‑SA = 1.0 K, and u(T)PA‑SA = 4.0 K. Standard uncertainties of composition are u(x)CA‑LA = 0.04, u(x)CA‑MA = 0.05, u(x)CA‑PA = 0.03, u(x)CA‑SA = 0.03, u(x)LA‑MA = 0.02, u(x)LA‑PA = 0.06, u(x)LA‑SA = 0.02, u(x)MA‑PA = 0.04, u(x)MA‑SA = 0.03, and u(x)PA‑SA = 0.03. The relative deviation of temperature is RD(T) = (Texp − Tref)/Tref.

(1)

where Tm is the phase change temperature of the eutectic mixture; Ti, Hi, and Xi are the phase change temperature, latent B



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min for 30 min. The temperature was maintained above the melting point to ensure the slurries were liquid during the procedure. Then after slowly cooled down to the room temperature, a series of fatty acids PCSs were obtained for further analysis. 2.2. Thermal Properties Analysis. The melting/freezing temperature and latent heat of fatty acids PCSs were detected by the DSC(TA Q20 U.S.A.) at a determined temperature change rate under the N2 pressure of 0.08 MPa. Samples were encapsulated in aluminum crucibles(φ = 5.4, h = 2.6 mm), and the mass was restricted to 5−10 mg for DSC measurements, in the meanwhile, an empty crucible was placed in the apparatus as the standard sample. Results presented in this study were the average value of three samples. The peak value represented melting/freezing temperature in the endothermic/exothermic process. The uncertainty of temperature and latent heat for heat fusion calculated by DSC are, respectively, 0.05 °C and 1.0%. Simultaneously, the obtained PCSs were heated/cooled at 5 °C/min temperature change rate during the DSC operations to detect supercooling.

Figure 2. DSC curves of CA, MA, and CA-MA PCSs.

3. RESULTS AND DISCUSSION 3.1. Phase Change Temperature and Latent Heat of CA-LA, CA-MA, CA-PA, and CA-SA PCSs. CA, LA, PA, SA, and their eutectic PCSs were prepared in this study and DSC curves of them were shown in Figures 1, 2, 3, and 4.

Figure 3. DSC curves of CA, PA, and CA-PA PCSs.

Figure 1. DSC curves of CA, LA, and CA-LA PCSs.

As shown in Figure 1−4, only one peak was observed in the endothermic process, which indicated PCMs were uniformly distributed in PCSs. In this case, the melting peak temperatures are 31.82 °C, 43.56 °C, and 22.36 °C for CA, LA, and CA-LA PCSs; 54.55 °C and 23.54 °C for MA and CA-MA PCSs; 62.52 °C and 26.27 °C for PA and CA-PA PCSs; and 68.83 °C and 30.13 °C for SA and CA-SA PCSs. The freezing temperatures are 26.54 °C, 39.75 °C, and 14.61 °C for CA, LA, and CA-LA PCSs; 49.28 °C and 14.25 °C for MA and CA-MA PCSs; 57.79 °C and 16.47 °C for PA and CA-PA PCSs; and (peak 1, 63.05; peak 2, 45.77) and 15.41 °C for SA and CA-SA PCSs. Results showed that, compared with pure fatty acids, the variations of melting/freezing temperatures of PCSs are almost negligible.

Figure 4. DSC curves of CA, SA, and CA-SA PCSs.

This phenomenon may be explained by the physical combination of PCMs and water. C



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The thermal properties of LA-MA, LA-PA, and LA-SA PCSs are shown in Figures 5−7. The melting peak temperature is

Additionally, latent heat fusion which are calculated in endothermic processes of Figure 1 are 44.66 J/g, 51.51 J/g, and 36.48 J/g for CA, LA and CA-LA PCSs; 55.31 J/g and 42.92 J/ g for MA and CA-MA PCSs in Figure 2; 59.67 J/g and 38.91 J/ g for PA and CA-PA PCSs in Figure 3; and 62.63 J/g and 41.98 J/g for SA and CA-SA PCSs in Figure 4. In the exothermic process, however, it is clear to find that there exist a great number of branched peaks, making it difficult to calculate the freezing temperatures and heat fusion. As seen from Figure 1, the heat fusions are 44.12 J/g, 50.13 J/g, and 34.35 J/g for CA, LA, and CA-LA PCSs, 52.07 J/g and 40.56 J/ g for MA and CA-MA PCSs, 58.17 J/g and 38.72 J/g for PA and CA-PA PCSs, and 59.69 J/g and 39.61 J/g for SA and CASA PCSs. The appearance of branched peaks in the freezing process may be caused by the phase transition of fatty acids.37 The changes in band intensity and frequency of CH2 bending, CH2 twisting, skeletal C−C stretching, and CH3 rocking revealed both transitions when using fatty acids. The changing of sequence and process of these vibrational modes is induced from those changes of band intensity and frequency. Then these vibrational modes will exert influence on fatty acids molecular structure, leading to the phase transition and appearance of branched peaks finally. 3.2. Phase Change Temperature and Latent Heat of LA-MA, LA-PA, and LA-SA PCSs. LA-MA, LA-PA, and LASA PCSs, emulsified by span20 and SDS, were acquired by high-speed shearing. Thermal properties of these PCSs will be discussed in this section. The DSC curves of LA, MA, and LA-MA PCSs are manifested in Figure 6. Some obvious shifts are observed among three PCSs and LA-MA PCSs possess the lowest melting and freezing temperatures. These shifts are in conformity with those of LA, MA, and LA-MA PCSs presented in Tables 4 and 5.

Figure 5. DSC curves of LA, MA, and LA-MA PCSs.

Table 4. Thermal Properties of Fatty Acids PCSs at 0.1 MPa PCSs

melting peak temperature

latent heat

°C

J/g

CA LA MA PA SA

31.82 43.56 54.55 62.52 68.83

44.66 51.51 55.31 59.67 62.63

freezing peak temperature °C 26.54 39.75 49.28 57.79 Peak 1 63.05, peak 2 45.77

latent heat J/g 44.12 50.13 52.07 58.17 59.69

Figure 6. DSC curves of LA, PA, and LA-PA PCSs.

Table 5. Thermal Properties of Fatty Acids Eutectics PCSs at 0.1 MPa PCSs

melting peak temperature

latent heat

freezing peak temperature

latent heat

°C

J/g

°C

J/g

CA-LA CA-MA CA-PA CA-SA LA-MA LA-PA LA-SA MA-PA MA-SA PA-SA

22.36 23.54 26.27 30.13 35.48 37.12 38.93 46.59 47.54 55.61

36.48 42.92 38.91 41.98 39.96 46.60 46.77 46.17 49.06 51.82

14.61 14.25 16.47 15.41 30.43 28.14 23.43 39.72 39.54 48.97

34.35 40.56 38.72 39.61 41.56 42.12 44.94 45.70 48.75 48.58

Figure 7. DSC curves of LA, SA, and LA-SA PCSs. D



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35.48 °C for LA-MA PCSs, 37.12 °C for LA-PA PCSs, and 38.93 °C for LA-SA PCSs. Their corresponding latent heat fusions are 39.96 J/g, 46.60 J/g, and 46.77 J/g in the exothermic process, respectively. During the freezing process shown in Figures 5−7 and Table 5, the freezing temperatures display the same trend as the melting ones, and the peak temperature is 30.43 °C for LA-MA PCSs, 28.14 °C for LA-PA PCSs, and 23.43 °C for LA-SA PCSs. Besides, latent heat fusion of three PCSs in the freezing process which can be calculated by the DSC curves are 41.56 J/ g, 42.12 J/g, and 44.94 J/g. Obviously, the latent heat fusion nearly has no change in the exothermic/endothermic processes and these PCSs will promote the application of PCMs in heat exchange and thermal storage. 3.3. Phase Change Temperature and Latent Heat of MA-PA and MA-SA PCSs. These DSC curves shown in Figures 8 and 9 present the effects of temperatures on heat flow

corresponding heat fusions are 46.17 J/g and 49.06 J/g, respectively. A certain supercooling, which is figured out from the melting temperature subtracted by the freezing temperature, can be found in these DSC curves. The freezing peak temperature is 39.72 °C for MA-PA PCSs and 39.54 °C for MA-SA PCSs. At the same time, as illustrated in the Tables 4 and 5, latent heat fusions in the freezing process are 45.70 J/g and 48.75 J/g for MA-PA and MA-SA PCSs. 3.4. Phase Change Temperature and Latent Heat of PA-SA PCSs. Thermal properties of PA-SA PCSs, which are of great significance to TES, are demonstrated in Figure 10. During the process of heating up, there are merely one absorption peak and the maximum is 55.61 °C. In the meantime, the corresponding enthalpy in this process is 51.82 J/g.

Figure 10. DSC curves of PA, SA, and PA-SA PCSs. Figure 8. DSC curves of MA, PA, and MA-PA PCSs.

Similarly, during the process of cooling, a few of branched exothermic peaks are observed and the exothermic process is divided into branched peaks. This phenomenon may be caused by different crystallization and phase transition. The freezing peak temperature of PA-SA PCSs is 48.97 °C, whereas the corresponding latent heat is 48.58 J/g. It is obvious to figure out that the latent heat fusion exhibit no significant changes in the melting/freezing processes. 3.5. Supercooling. PCSs are produced by distributing PCMs into heat transfer fluid, with the high-speed method, forming stable dispersed solutions.38 Under the condition of isothermal and constant pressure, the Gibbs free energy ΔG can be expressed as eq 2, according to the thermodynamics when the system changes from State 1 to 2 ΔG = ΔH − T ΔS

(2)

where ΔG is Gibbs free energy (in J), ΔH is enthalpy (in J), and ΔS is entropy (in J/K). Although the temperature reaches the phase change temperature like melting point Tm, the system will reach a balance where ΔG is 0. In the case, the Gibbs free energy ΔG can be indicated as eq 3

Figure 9. DSC curves of MA, SA, and MA-SA PCSs.

for MA-PA and MA-SA PCSs. As illustrated in the melting process, the single melting peak temperature is 46.59 °C for MA-PA PCSs and 47.54 °C for MA-SA PCSs. The

ΔG = ΔH − TmΔS = 0 E

(3) DOI: 10.1021/acs.jced.5b00371 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Thus, at any temperature T of the unbalanced condition, provided that ΔH and ΔS do not vary with temperature, the following (eq 4) of ΔG can be deduced from eq 2 and eq 3 ΔG = ΔH − T

T −T ΔH ΔT = ΔH m = ΔH Tm Tm Tm

process is greatly influenced by the length of the carbon main chain and the number of the branched chains. PCSs with different crystal structures of PCMs have different supercooling degrees in the heating/cooling cycles. Furthermore, PCMs are dispersed into the distilled water in this investigation, establishing a dispersive emulsion systems. There are several kinds of materials in the system, including fatty acids as PCMs, water as the continuous phase, and span20/SDS as emulsifiers. In this case, the interactions among PCMs, water and emulsifiers may play a significant role in the melting/freezing processes. Thus, the supercooling should be considered to be caused by the composite effects of PCMs crystal configuration and interaction among PCMs, water, and emulsifiers.39

(4)

From eq 4, the phase change process will happen spontaneously under the condition of ΔG < 0, meaning ΔT = Tm − T > 0. In other words, the crystallization continues to process if there exists a certain supercooling. On the other hand, the PCMs particles embodied in slurries are generally in the micrometer level,13,38 when the mechanical shearing force is strong enough. On the basis of the crystallization kinetics, supercooling will be much larger when the smaller particle sizes are dispersed in the PCSs. However, there will be more effective contact areas that are produced by the curve surface areas of small PCMs particles. As known, smaller particles in normal shape possess larger specific surface, enhancing the heat transfer applied in the industrial heat exchange and thermal storage system. To analyze supercooling of prepared PCSs, these PCSs were heated/cooled at the temperature change rate of 5 °C/min during the DSC operations. Supercooling degree illustrated in this part is defined as the deviation between the onset melting and onset freezing temperature. Table 6 lists the supercooling of PCSs at 5 °C/min temperature change rate.

4. CONCLUSIONS PCSs containing 30 wt % eutectic prepared under the emulsification of span20/SDS exhibit the lower freezing/ melting temperatures than two kinds of corresponding fatty acids slurries. These eutectic PCSs, with the melting peak temperatures from 22.36 °C to 55.61 °C and the freezing peak temperatures from 14.61 °C to 48.97 °C, will demonstrate a superiority in the low temperature phase change field. Furthermore, the latent heat fusions of eutectic PCSs are between 36.32 J/g and 51.82 J/g, 34.35 J/g and 48.58 J/g in the endothermic/exothermic process. It is obvious to conclude that no significant changes of latent heat fusion are detected in the heating/melting processes and these values approximately equal latent heat fusion of pure fatty acids timed by the mass fraction. Supercooling of PCSs span from 0.55 °C to 6.55 °C, at the 5 °C/min temperature change rate. And the crystal structures formed in fatty acids eutectics might be accounted for this phenomenon. Additionally, there are still a great number of researches in such fields, as morphology, particle size, and rheology, which need to be carried out in the following studying.

Table 6. Supercooling of PCSs at 0.1 MPa PCSs

Supercooling

CA LA MA PA SA CA-LA CA-MA CA-PA

2.63 1.76 1.89 2.19 2.12 2.36 4.20 4.49

PCSs

Supercooling

CA-SA LA-MA LA-PA LA-SA MA-PA MA-SA PA-SA

2.28 0.55 4.54 6.55 1.87 4.05 2.43

°C

°C

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (0086 028) 87634937. Fax: (0086 028) 87634937.

As shown in Table 6, eutectic PCSs containing CA demonstrate the higher supercooling and the supercooling values of CA CA-LA, CA-MA, CA-PA, and CA-SA PCSs are between 2.28 °C and 4.49 °C, at the 5 °C/min temperature change rate, whereas CA-SA PCSs appear to have a lower supercooling value (2.28 °C). The increasing trends can be found in PCSs containing LA and MA. For eutectic PCSs containing LA, the supercooling degree span from 0.55 to 6.55 °C at the 5 °C/min temperature change rate. LA-SA PCSs appear to possesss the largest supercooling degree (6.55 °C), though LA-MA PCSs have the smallest one (0.55 °C). In the same manner as eutectic PCSs containing CA, the supercooling values of eutectic PCSs containing MA are between 1.87 and 4.05 °C, at the 5 °C/min temperature change rate. Table 6 also indicates that PA-SA PCSs exhibit the supercooling of 2.43 °C when heated/frozen at 5 °C/min temperature change rate. This increasing phenomenon may be induced by the crystal structures formed in fatty acids eutectics. In the order of CA, LA, MA, PA, and SA, the latter has more carbon atoms than the former. The crystal configuration formed in the freezing

Funding

This research was financially supported by the Natural Science Foundation of China (51378426), Sichuan Province Youth Science and Technology Innovation Team of Building Environment and Energy Efficiency (2015TD0015), and Fundamental Research Funds for the Central Universities (2682015CX038). Notes

The authors declare no competing financial interest.

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REFERENCES

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