Grafice negre articol DMF: [311338]

A physico-chemical study on the tetrahydrofuran effect in exfoliated graphite nanoplatelets and activated carbon mixtures at temperatures between (293.15 and 308.15) K

Florinela Sirbu1*, Alina Catrinel Ion2, and Luiza Capra3

Present Address:

*Corresponding author:

F. Sirbu1*:

Department of Chemical Thermodynamics, ‘‘Ilie Murgulescu’’ Institute of Physical Chemistry of Romanian Academy, 202 Splaiul Independentei str., 060021 Bucharest, Romania

E-mail: [anonimizat]

Present Address:

A. C. Ion2:

[anonimizat], ‘‘Politehnica’’ University of Bucharest, 1 Polizu str., 011061 Bucharest,

Romania

Present Address:

L. Capra3:

[anonimizat] & Development Institute for Chemistry and Petrochemistry ICECHIM, 202 Splaiul Independentei str., 060021 Bucharest, Romania

Abstract

A [anonimizat]. [anonimizat], and speed of sound of diluted mixed binary solutions mixtures of exfoliated graphite nanoplatelets and activated carbon in tetrahydrofuran were measured from between (0 and 100) (0 to 100) kg m-3 with composition step increments of 20 kg m-3 and at temperatures between (293.15 and 308.15) K from (293.15 to 308.15) K, and at normal pressure. [anonimizat], [anonimizat], space-[anonimizat], at four different temperatures. [anonimizat]. [anonimizat].

Keywords: [anonimizat], tetrahydrofuran, [anonimizat], speed of sound

Introduction

Investigations of thermodynamic and acoustic properties acoustical and optical parameters of binary mixtures solutions in (THF) tetrahydrofuran: (xGnP) exfoliated graphite nanoplatelets xGnP + THF and (AC) activated carbon AC + THF were realised based on their physicochemical behaviour. [anonimizat]-[anonimizat]-based nanostructures in organic solvents (Ubarhande et al. 2011). Single–layer graphene was isolated in 2004 by Novoselov et al. (2004) obtained of Novoselov et al. authors in 2004 year (2004). Graphite-based nanomaterials can be classified based on the dimensions of their sheets, parallel and perpendicular to the layers the sheets of different dimensions, parallel and perpendicular disposed on layers (Pumera 2010). The properties of graphite nanoplatelets (10-100 graphene layers, 3-30 nm thick) also known as exfoliated graphite nanoplatelets, independent of the number of layers show similar electrochemical behavior The graphite nanoplatelets properties of 3-30 nm thick and 10-100 graphene layers are known as exfoliated graphite nanoplatelets presents similar electrochemical behaviour, whatever of the number of layers. (Goh and Pumera 2010). Although the number of articles on xGnP has greatly increased Even though the studies number about xGnP was greatly increased (Zhu et al. 2010), data on their thermodynamic properties, thermo physical behaviour description and molecular modelling properties are still missing. In recent studies, the stabilization of graphene The graphene stabilization dispersed in dimethylsulfoxide (DMSO) and dimethylformamide (DMF) polar solvents by Shih et al. (2010) in recent articles have published was reported. It was observed that DMSO solvent can break polymerized structures of oxygenated compounds It has noted that DMSO solvent can break the ties in polymerised compounds of oxygenated chemical structures (Tamura et al. 1994), in the last years the dispersion of xGnP in DMF and water being also studied (Ion et al. 2013).

The aim of present study is to provide new experimental values on thermo physical properties as refractive indices, speeds of sound and densities in the THF + xGnP, or THF + AC binary sistems for which experimental data are not available. In the present study the density, speed of sound, and refractive index of the pure solvent and mixtures of THF and xGnP, or AC were determined from the point of view of thermo physical properties for which experimental data are not available. The binary mixtures were measured at atmospheric pressure, various compositions of solutes from (0 to 100) kg m-3 between (0 and 100) kg m-3 with increments of 20 kg m-3 and at temperatures of (293.15, 298.15, 303.15, and 308.15) K. The effect of the variation of these parameters on the composition of the concentrations in studied mixtures mixed solutions was evaluated. Based on the understanding of the molecular interactions several applications could be envisaged, such as composite nanomaterials for sensors (Pumera 2010) such in-field sensing systems to monitor environmental stress and sorbents applied for the removal of contaminants or for to allow the administration of drugs (Emerich et al. 2006) enabling the delivery of drugs (Emerich et al. 2006), or the removal of contaminants from mixtures mixed solutions (Farmen 2009). Among the applications of nanotechnologies, new types of sensors and sorbents based on nanomaterials are highly advanced; however, the basic level of knowledge concerning interactions at molecular level is not sufficient (Zaib et al. 2012; Yan et al. 2008; Xu et al. 2012).

Experimental Section

Materials and methods. Exfoliated graphite nanoplatelets with a surface area of 50-80 m2 g-1 and a mass fraction > 0.95 carbon, thickness approximately 15 nm, diameter 25 m, were provided from XG Sciences, Lansing, MI, USA. THF was supplied from Merck. Activated carbon with a mass fraction > 0.99 carbon was procured by Sigma Aldrich. These chemical compounds were used have been used without further purification, except drying over P2O5 for 72 h, based on their mass fraction purity > 0.95. The specifications of the chemicals used for sample preparation are presented in the Table 1.

Table 1 Specification of activated carbon, exfoliated graphite nanoplatelets, and THF compounds used in mixtures

aAC = Active Carbon

bxGnP = Exfoliated Graphite Nanoplatelets

cTHF = Tetrahydrofuran

Working solutions of AC + THF and xGnP + THF with different compositions were prepared at 298.15 K using THF of analytical purity (p.a.). The binary mixtures were freshly prepared and kept in airtight bottles by mixing known compositions of stock and pure THF solvent. All necessary precautions were taken to minimise losses due to evaporation during measurements. Specific concentration was expressed in (kg m-3) as measured unit. The initial compositions of the stock mixtures were prepared with an accuracy of ± 0.0002 g cm-3. More details about the experimental procedures for density, speed of sound and refractive index measurements can be found in previous papers (Ion et al. 2013; Ion et al. 2015; Sirbu et al. 2014).

The density and speed of sound of binary mixtures under atmospheric pressures were measured with an Anton Paar DSA 5000 digital (Austria) analyser with a precision of ± 0.000001 g cm-3 for density and of ± 0.01 m s-1 for speed of sound. The speed of sound was measured at a reduced wavelength and a low frequency of approximately 3 MHz (Ebrahimi and Sadeghi 2015; Pal et al. 2015). The temperature for the density and speed of sound measurements was controlled by several Peltier units with a precision of ± 0.001 K. The refractive index in samples was measured with an Anton Paar GmbH Abbe automatic refractometer at a controlled temperature within ± 0.01 K and a precision of ± 0.000001. The densitometer instrument was internally calibrated with air and doubly distilled deionised pure water by measuring the density and speed of sound at atmospheric pressure, according to the recommendations of the manufacturer. The refractometer was calibrated by measuring the refractive index of doubly distilled deionised pure water, too. The density, speed of sound, and refractive index values of water at 298.15 K were measured as 0.99706 g cm-3, 1497.1 m s-1, and 1.33248, similar to the results described in the literature (Riddick et al. 1988; CRC Handbook 2008; Ameta et al. 2013; Arce et al. 1993), with a reproducibility of ± 0.000005 g cm-3, ± 0.04 m s-1, and ± 0.000005 units, respectively. Uncertainties associated for the density, speed of sound, and refractive index experimentally measured were presented under each data table, according to the guide for Evaluation of measurement data (2008).

Theory and Calculation

The acoustical and optical thermodynamic parameters as impedance (Z), isentropic compressibility (kS), space filling factor (S), specific refraction (rD), and relaxation strength (r) at various temperatures and atmospheric pressure were estimated from experimental results of density, speed of sound, and refractive index. The calculation relations for the derived termophysical properties have been described elsewhere (Ion et al. 2013; Ion et al. 2015; Sirbu et al. 2014).

Results and Discussions

Experimental data on densities (ρ), speeds of sound (c), and refractive indices (nD) as a function of specific concentration of xGnP or AC in THF solvent at a pressure of 0.1 MPa are reported. The properties experimental values at atmospheric pressure for the pure THF solvent in comparison with literature values (Du et al. 2001; Singh et al. 2007; Belandria et al. 2009; TRC Data Base 1993; Nain 2006; Nain 2008; Wankhede 2011; Pan et al. 2004; Iloukhani et al. 2005; Kumar et al. 2011; Agarwal and Narwade 2003; Mishra 2011; Anwar et al. 2004) at different temperatures between (293.15 and 308.15) K are shown in Table 2.

Table 2 Comparison of experimental densities (ρ), speed of sound (c), and refractive indices (nD) of pure tetrahydrofuran with literature values at various temperatures

AC and xGnP are miscible with high dielectric liquids, thus these materials are completely miscible with THF, a high dielectric solvent with low viscosity, low melting point and high solubility for inorganic salts (Koniosa et al. 2014).

Tables 3 and 4 present experimental results for the same thermophysical properties of AC and xGnP in THF solvent measured as a function of their specific concentrations at different temperatures.

Table 3 Experimental values of the density ρ, speed of sound c, and refractive index at various temperatures T and specific concentrations C of AC, for the system AC + THFa

aC/kg·m-3 is the specific concentration of AC in the THF solvent. Standard uncertainties u are u(T) = 0.001 K for ρ and c; u(T) = 0.01 K for nD and the combined expanded uncertainties Uc are Uc(ρ) = 0.01 kg·m-3, Uc(c) = 0.05 m·s-1; (level of confidence = 0.95, k = 2) and Uc(nD) =0.00001.

Table 4 Experimental values of the density ρ, speed of sound c, and refractive index at various temperatures T and specific concentration C of (xGnP), for the system xGnP + THFa

aC/kg·m-3 is the specific concentration of xGnP in THF solvent. Standard uncertainties u are u(T) = 0.001 K for ρ and c; u(T) = 0.01 K for nD and the combined expanded uncertainties Uc are Uc(ρ) = 0.01 kg·m-3, Uc(c) = 0.1 m·s-1; (level of confidence = 0.95, k = 2) and Uc(nD) =0.00001.

The density, speed of sound and refractive index values as a function of specific concentration of activated carbon (AC) and exfoliated graphite nanoplatelets (xGnP) are illustrated in Figs. 1, 2, and 3, respectively, by comparison between both binary AC + THF and xGnP + THF mixtures, along with polynomial correlated values.

The polynomial equation used for the correlations of the measured and calculated properties in both systems is of the following type:

(1)

where, Y represents the measured properties and C represents the specific concentration, [kg m-3].

Figure 1 shows comparative experimental and correlated data of the density as a function of the solute concentration (activated carbon and exfoliated graphite nanoplatelets) of binary systems AC + THF and xGnP + THF.

The density values of the binary (AC or xGnP) + THF systems decrease by increasing the temperature at the same composition of the solute, as it can be observed from Fig. 1. The density values increase by increasing AC and xGnP concentrations, up to C = 100 kg m-3 in AC + THF and xGnP + THF mixtures. The density of the system xGnP + THF increases in comparison with that of the AC + THF system, probably due to stronger interactions between THF and xGnP compounds than in between THF and AC compounds. A more pronounced slope of the solute concentration vs. density can be mentioned in xGnP + THF system in comparison with the AC + THF one, based on the interactions occurring between carbon-based nanomaterials and THF, which increases the density by increasing the concentration of solute in the mixture. This behavior can be explained by several opposite effects. It can be assumed that dipole-dipole interactions, dispersion forces and hydrogen bonds are the most important forces that can appear between these unlike molecules (Chen et al. 2015).

The dependence of the speed of sound on the composition of AC + THF and xGnP + THF is presented in Figure 2, along with the polynomial correlated values.

As shown in Fig. 2, the speed of sound values of the binary AC + THF systems increase by increasing the temperature between (298.15 and 308.15) K. In the same system AC + THF, the speed of sound values decreased up to 60 kg m-3 and increase at 293.15 K, up to 100 kg m-3 with a different slope.

The speed of sound values in xGnP + THF systems varies in a similar way at 293.15 K, increasing up to C = 60 kg m-3, then decrease slowly by increasing concentration. The speed of sound variation at the three temperatures (298.15, 303.15, and 308.15) K is similarly as in binary AC + THF system. The speed of sound values of xGnP + THF mixtures differ than those obtained for the system AC + THF. In what concerns the slope, this strongly decreases at (298.15, 303.15, and 308.15) K. The behavior of speed of sound values by increasing concentration of the solute AC or xGnP indicates the presence of powerful solute-solvent interactions (Venkatramanan et al. 2015) via dipole-dipol ones, which can produce displacements of electrons and nuclei in the range of specific concentrations.

These interactions are improved in the xGnP + THF mixture, because the speed of sound slightly decreasing by temperature is much more in xGnP + THF system in comparison with AC + THF one. As the temperature increases, the thermal energy contributes to possible bonds breakings and weakens the molecular forces in both studied systems.

The refractive indices variation by concentration of the activated carbon and exfoliated graphite nanoplatelets solutes in THF is presented in Fig. 3.

The refractive index behavior in the binary AC + THF or xGnP + THF system decreased by increasing the temperature at the same concentration of the solute for both mixtures. The refractive index values increase by the temperature in xGnP + THF system, at xGnP concentrations bigger than 90 kg m-3. The refractive index values very slightly increase by increasing AC concentration in the AC + THF mixture at all studied temperatures, too. In the xGnP + THF mixture, a decrease of the refractive index values by increasing concentration of the xGnP up to C = 60 kg m-3 is noticed, followed by an increase up to C = 100 kg m-3. The location of possible oxygen functional groups at the edges of xGnP plays a major role (Kim et al. 2012), the amount of edges, planes and defects and the crystallite size of xGnP inducing major differences compared to the simpler carbon material (AC).

On the basis of the measured properties, the derived thermo physical parameters as acoustic impedance, isentropic compressibility empirically evaluated (Ion et al. 2015), space filling factor, specific refraction, and relaxation strength values as function of specific concentration fraction C of AC in THF solvent at various temperatures T have been computed and presented in the Table 5.

Table 5 Calculated values of the acoustic impedance Z, adiabatic compressibility kS, space filling factor S, specific refraction rD, and relaxation strength r at various temperatures T, specific concentration C of (AC), for the system AC + THF

Similarly, Table 6 presents the same thermophysical parameters, computed for binary system of xGnP in THF solvent under the same conditions.

Table 6 Calculated values of the acoustic impedance Z, adiabatic compressibility kS, space filling factor S, specific refraction rD, and relaxation strength r at various temperatures T, specific concentration C of (xGnP), for the system xGnP + THF

Figures 4 and 5 illustrate comparative representations between binary AC + THF and xGnP + THF systems for the estimated and correlated values of isentropic compressibility kS and relaxation strength r values as a function of specific concentration of the solute (activated carbon and exfoliated graphite nanoplatelets).

As it can be observed from Fig. 4, the isentropic compressibility values of the binary AC + THF system increase by increasing the temperature at the same concentration of the solute. Also, the isentropic compressibility values decrease by increasing AC concentrations, up to C = 100 kg m-3 in both systems. A different trend of variation of the concentration in the AC + THF system at 293.15 K was noticed, till 40 kg m-3.

The isentropic compressibility values of the binary xGnP + THF system are smaller than the values of the binary AC + THF system all over the solute concentration range.

Differences were observed between the two systems, indicating that the nano exfoliated graphite solute changes the structure of THF + xGnP system strongly than of THF + AC one. This behavior of the estimated isentropic compressibility may be the results of several opposite interactions.

Dipole-dipole interactions usually improve the dispersion of the structures in a better way than by breaking of molecular clusters. The behavior of isentropic compressibility in the xGnP + THF system is less than in the AC + THF system, the repulsive interaction resulting from the compression of the C-H vertical bonds between the nanosheet surfaces (Chen et al. 2015; Vaisman et al. 2006). In the mixture of the AC dispersed in THF solvent, the THF molecules are limited at a single-layer structure on the surface of AC particles, possibly losing some of the solvent-solvent interaction energy, due to the smaller number of adjacent solvent molecules. The isentropic compressibility empirically estimated based on the obtained experimental data seems consistent, having the same sign in both studied binary mixtures (Ion et al. 2015).

In the xGnP + THF and AC + THF binary mixtures, the relaxation strength increased by increasing the temperature up to C = 40 kg m-3 and C = 60 kg m-3, respectively, for 293.15 K, as in the Fig. 5. In AC + THF it decreases at the following temperatures (298.15, 303.15, and 308.15) K.

In the xGnP + THF binary mixture, the relaxation strength values decreased up to a solute composition of approx. 60 kg m-3 at the temperatures (298.15 to 308.15) K and increased by increasing the solute concentration, suggesting the predominance of molecular interactions (Baluja et al. 2009). Correlations of ρ, c, nD, kS and r as a function of composition (Eq. 1) along with the absolute average percentage deviation (AAD) were investigated. The (AAD %) absolute average percentage deviation was computed using the following relationship:

(2)

where N is the number of experimental points. The subscripts “Expt.” and “Calc.” are the experimental and calculated values of property.

Fitting parameters Ai and absolute average percentage deviation results are summarized in Table 7 for binary AC + THF solutions at all the studied temperatures.

Table 7 Fitting parameters Ai, correlation coefficient (R2) obtained for density ⍴, speed of sound c, refractive index, isentropic compressibility kS and relaxation strength r along with the absolute average percentage deviation (AAD %) for binary AC + THF mixtures.a

aAi and R2 were obtained from Eq. 2.

Tables 7 and 8 present the calculated values of the Ai parameters, correlation coefficient (R2) obtained for density ⍴, speed of sound c, isentropic compressibility kS, and relaxation strength r (Ion et al. 2013; Ion et al. 2015; Sirbu et al. 2014), together with the absolute average percentage deviation (AAD %) calculated from (Eq. 2) for AC + THF and xGnP + THF binary mixtures.

The absolute average percentage deviation for the calculated thermodynamic properties: density, speed of sound, refractive index, isentropic compressibility and relaxation strength are less than (0.009, 0.011, 0.0002, 0.001, and 0.003) %, for the binary AC + THF system and less than (0.003, 0.002, 0.119, 0.003, and 0.002) % for the binary xGnP + THF, being well correlated for both systems, as it can be seen from Tables 7 and 8.

Table 8 Fitting parameters Ai, correlation coefficient (R2) obtained for density ⍴, speed of sound c, refractive index, isentropic compressibility kS and relaxation strength r along with the absolute average percentage deviation (AAD %) for binary xGnP + THF mixtures.a

aAi and R2 were obtained from Eq. 2.

The isentropic compressibility (kS) increases by increasing the temperature, but it decreases by increasing the concentration.

As shown in Tables 7 and 8 and in Figs. 1 to 5, the values of the thermophysical properties experimentally obtained were correlated with good accuracy based on polynomial (Eq. 1). It is observed that the polynomial expression (Eq. 1) good reproduces the studied thermophysical properties using three correlation parameters in order to describe the mixtures behaviour.

Conclusions

In the present investigation, the density, refractive index and speed of sound in binary mixtures of activated carbon AC + THF and exfoliated graphite nanoplatelets xGnP + THF over a composition range from (0 to 100) kg·m-³ and at temperatures between (293.15 and 308.15) K were measured. A comparison between the two studied systems, activated carbon AC + THF and exfoliated graphite nanoplatelets xGnP + THF show that the values of the absolute percentage deviations are comparable for the calculated thermodynamic properties. The absolute average percentage deviation for the calculated thermodynamic properties of the binary AC + THF system, as density, speed of sound, refractive index, isentropic compressibility, and relaxation strength are less than (0.009, 0.011, 0.0002, 0.001, and 0.003) %, being be well correlated by this equation. For binary xGnP + THF mixture, the absolute average percentage deviation for the same calculated thermodynamic parameters are less than (0.003, 0.002, 0.119, 0.003, and 0.002) %, respectively.

The theoretical methodology presented in this study might explain the ability of the organic solvent THF to disperse xGnP and to stabilize the xGnP + THF mixture in comparison with the AC + THF one. THF, which disperses carbon-based nanostructures is one of the best solvents, its behavior based on fundamental principles and practical methods being emphasized in this work, too.

Acknowledgements The present study was carried out within the research programme Chemical Thermodynamics of Ilie Murgulescu Institute of Physical Chemistry, which was financed by the Romanian Academy of Sciences. Support from the EU (ERDF) and Romanian Government, which allowed for the acquisition of the research infrastructure under POS-CCE O 2.2.1 Project INFRANANOCHEM-Nr. 19/01.03.2009 is gratefully acknowledged.

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