A Thermodynamics Study on the Tetrahydrofuran Effect in Exfoliated Graphite Nanoplatelets and Activated Carbon Mixtures at Temperatures between 293 . 15 and 308 . 15 K

Department of Chemical ermodynamics, “Ilie Murgulescu” Institute of Physical Chemistry of Romanian Academy, 202 Splaiul Independentei St., 060021 Bucharest, Romania Department of Analytical Chemistry and Instrumental Analysis, Faculty of Applied Chemistry and Material Science, “Politehnica” University of Bucharest, 1 Polizu St., 011061 Bucharest, Romania Department of Analysis, Tests, and Testings, National Research & Development Institute for Chemistry and Petrochemistry ICECHIM, 202 Splaiul Independentei St., 060021 Bucharest, Romania


Introduction
Investigations of acoustical and optical parameters of binary solutions of tetrahydrofuran (THF) with exfoliated graphite nanoplatelets (xGnP) and activated carbon (AC) were realised based on their physicochemical behaviour.
e physicochemical parameters strongly influence the dispersion of xGnP carbon-based nanomaterials compared with the dispersions of AC in THF, offering new information about the structure and interactions of carbon-based nanostructures in organic solvents [1].Graphite-based nanomaterials can be classified based on the thickness, the number, and the disposal of the exfoliated layers [2].Graphite nanoplatelets with dimensions of 3 to 30 nm thick and 10-100 graphene layers, known as exfoliated graphite nanoplatelets, present similar electrochemical behaviour, irrespective of the number of layers [3].Even though the number of articles about xGnP greatly increased [4,5], data on their thermodynamic properties, thermophysical behaviour description, and molecular modelling are still missing.e characterization of graphene dispersed in dimethylsulfoxide (DMSO) and dimethylformamide (DMF) polar solvents was published by Shih et al. [6].It has been noted that DMSO solvent can break the ties in polymerised compounds of oxygenated chemical structures [7].In the last years, the dispersion of xGnP in DMF and water was also studied [8].
e aim of the present study was to provide new experimental values on thermophysical properties such as refractive indices, speeds of sound, and densities in the THF + xGnP or THF + AC binary systems for which experimental data are not available.e binary mixtures were measured at atmospheric pressure, using various compositions of solutes 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. e effect of the variation of these parameters on the concentrations in the studied mixed solutions was evaluated.Based on the understanding of the molecular interactions, several applications could be envisaged, such as sensors with composite nanomaterials [2].
ese infield detection systems are used for monitoring the environmental stress.
e sorbents based on composite nanomaterials are also used for the removal of contaminants, or to allow the administration of drugs [9] from mixed solutions [10].Among the applications of nanotechnologies, new types of sorbents based on carbon nanomaterials are studied and further applied for the removal of environmental contaminants based on their interactions at the molecular level [11][12][13].

Experimental Section
2.1.Materials and Methods.Exfoliated graphite nanoplatelets were purchased from XG Sciences (Lansing, MI, USA) and are characterized by mass fraction of >0.95 carbon, thickness of approximately 15 nm, diameter of 25 μm, and surface area of 50-80 m 2 •g −1 .THF was supplied from Merck and activated carbon (mass fraction > 0.99) from Sigma-Aldrich.ese chemical compounds have been used without further purification, except drying over P 2 O 5 for 72 h, for those with a mass fraction purity of >0.95.In Table 1, the specifications for the chemical compounds used in the working sample preparation are presented.
Working solutions of AC + THF and xGnP + THF with different concentrations have been prepared at 298.15 K using THF of analytical purity (p.a.).e binary mixed solutions have been freshly prepared and kept in airtight bottles by mixing known compositions of stock and pure THF solvent.All precautions have been taken to minimise  2 Advances in Materials Science and Engineering the errors produced by the evaporation of solutions.Specific concentration was expressed in kg•m −3 as measured unit.e initial compositions of the stock mixtures were prepared with an accuracy of ±0.2 kg•m −3 .More details about the refractive indices, speeds of sound, and densities experimental measurements and procedures can be found in previous papers [8,14,15].e refractive index of the 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.
e calibration of the refractometer has been done by measuring the refractive index of deionised twice distilled pure water.e speeds of sound and densities of mixed solutions have been measured with Anton Paar DSA 5000 digital (Austria) equipment under atmospheric pressure.
e precision of the density is of ±0.001 kg•m −3 .
e speed of sound has been measured at a reduced wavelength and a low frequency of approximately 3 MHz [16,17], and the precision is ±0.01 m•s −1 .e temperature was controlled by several Peltier units with a precision of ±0.001K for obtaining the speed of sound and density experimental data.
e densitometer instrument was internally calibrated with air and doubly distilled deionised pure water by determining the speed of sound and density at normal pressure, according to the recommendations of the manufacturer.e 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 values described in the literature [18][19][20][21], with a reproducibility of ±0.000005 g•cm −3 , ±0.04 m•s −1 , and ±0.000005 units, respectively.Uncertainties associated with the experimentally measured data for refractive index,    Advances in Materials Science and Engineering speed of sound, and density were presented under each data table, according to the guide for evaluation of measurement data [22].

eory and Calculation.
e acoustical and optical thermodynamic properties such as isentropic compressibility (k S ), impedance (Z), space-lling factor (S), speci c refraction (r D ), and relaxation strength (r) at various temperatures and atmospheric pressure were estimated from experimental results of density, speed of sound, and refractive index.
e calculation relations for the derived thermophysical properties have been described elsewhere [8,14,15].

Results and Discussion
Experimental data on densities (ρ), speeds of sound (c), and refractive indices (n D ) as a function of speci c concentration of xGnP or AC in THF solvent at a pressure of 0.1 MPa are reported.e experimental values of these properties at atmospheric pressure for the pure THF solvent in comparison with literature values [23][24][25][26][27][28][29][30][31][32][33][34][35] at di erent temperatures from 293.15 up to 308.15 K are presented in Table 2.
e carbon-based nanoplatelets and activated carbon disperse well in the THF solvent with a high dielectric conductivity because it is known that they are miscible in high dielectric liquids, with low viscosity, low melting point, and high solubility for inorganic salts [36].
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 speci c concentrations at di erent temperatures.
Figures 1-3 present the comparison of the refractive indices, speeds of sound, and densities experimental and ) Table 5: Calculated values of the acoustic impedance Z, adiabatic compressibility k S , space-lling factor S, speci c refraction r D , and relaxation strength r at various temperatures T and speci c concentrations C of AC for the system AC + THF.
where C is the specific concentration (kg•m −3 ) and Y is the measured physicochemical property.e refractive index variation by concentration of the activated carbon and exfoliated graphite nanoplatelet solutes in THF is presented in Figure 1.
e refractive index behaviour decreased in the xGnP + THF or AC + THF mixtures at the same solute composition by raising the temperature.
e refractive index values increase by the temperature in the xGnP + THF system at xGnP concentrations bigger than 90 kg•m −3 .e refractive index values rise very slightly by rising the composition of AC solute in the THF organic solvent at all the studied temperatures, too.In the xGnP + THF mixture, a decrease of the refractive index values by increasing up to C � 60 kg•m −3 (xGnP) composition is noticed, followed by an increase up to C � 100 kg•m −3 .e location of oxygen functional groups at the xGnP edges plays an important role in interactions [37], the number of edges, plane surfaces, defects, and the (xGnP) crystallite dimension, inducing significant differences by comparison with the other studied material activated carbon.
In Figure 2, it is presented the variation of experimental values of speed of sound and its correlation with the solute composition in xGnP + THF and AC + THF studied mixtures.
Figure 2 shows the speeds of sound variation in the AC + THF binary system, which increases by increasing the temperature between 298.15 and 308.15 K.In the same system AC + THF, the speed of sound values decrease up to 60 kg•m −3 and rise at 293.15 K, up to 100 kg•m −3 with a different slope.e speed of sound values in xGnP + THF system varies in a similar way at 293.15 K, increasing up to C � 60 kg•m −3 , then decrease slowly by increasing the concentration.e speed of sound variation at the three studied temperatures 298.15   concentration shows that the existence of powerful solutesolvent interactions [38] via dipole-dipole ones, which in the domain of studied compositions, can produce displacements of nuclei and electrons.In the xGnP + THF mixture, these interactions increase/intensify because the speed of sound slightly decreasing by temperature is much more in the xGnP + THF system in comparison with that in AC + THF. e thermal energy generated by the increase of the temperature contributes to possible bond breakings and weakens the molecular forces in both studied systems.e variation of obtained data for exfoliated graphite nanoplatelets and activated carbon in THF organic solvent as a function of the solute composition is presented in Figure 3.
In the mixed binary solutions of AC or xGnP + THF by rising the temperature at the same solute composition, the density obtained values decrease as it can be observed from Figure 3. e density values in the xGnP + THF and AC + THF mixtures rise by rising xGnP and AC speci c compositions, up to C 100 kg•m −3 .e 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 those between THF and AC compounds.A more pronounced slope of the solute concentration versus density can be mentioned in the xGnP + THF system in comparison with the AC + THF one, based on the interactions occurring between THF and carbon-based nanomaterials, which rise the density by rising the solute speci c composition in the mixed solution.is behaviour can be explained by several opposite e ects.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 [39].
e values of isentropic compressibility empirically evaluated [14], acoustic impedance, space-lling factor, speci c refraction, and relaxation strength were computed from experimental data.

Advances in Materials Science and Engineering
e computed physicochemical parameters in the xGnP + THF mixed binary solutions are presented in the same conditions in Table 6.
Figures 4 and 5 illustrate the compared variations of xGnP or AC + THF binary mixtures of the estimated and correlated values of isentropic compressibility k S and relaxation strength r of the solute composition.
e isentropic compressibility values of the binary AC + THF system (Figure 4) increase by rising the temperature to the same solute composition.In both systems, the isentropic compressibility values decrease with rising the solute compositions up to a C value of 100 kg•m −3 .A different trend of variation of the concentration in the AC + THF system at 293.15 K was noticed, till 40 kg•m −3 .e values of the isentropic compressibility for the xGnP + THF mixture are smaller than the AC + THF compressibility values all over the solute concentration range.e two systems present different values of isentropic compressibility: nanoexfoliated graphite modifying the xGnP + THF system strongly than the AC + THF system. is behaviour of the estimated isentropic compressibility may be the result of several opposite interactions.Dipole-dipole interactions usually improve the dispersion of the structures in a better way than by breaking of molecular clusters.e values of isentropic compressibility in the xGnP + THF system are smaller in comparison with those in the AC + THF system, the repulsive interaction resulting from the compression of the C-H vertical bonds between the nanosheet surfaces [39,40].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.e isentropic compressibility empirically estimated based on the obtained experimental data seems consistent, having the same sign in both studied binary mixtures [14].
In the xGnP + THF and AC + THF binary mixtures, the relaxation strength rises by temperature up to a concentration values of 40 kg•m −3 and 60 kg•m −3 , respectively, for 293.15K, Relaxation strength values in the xGnP + THF system decrease up to a solute composition of approximately 60 kg•m −3 at the temperatures from 298.15 up to 308.15 K and rise by raising the solute composition, which reflects a predominance of the molecular interactions [41].e computed slope and intercept of simple polynomial equation with "statistical functions" from Excel for all properties from the presented figures are shown in Tables 7 and 8.
e values for the absolute average percentage deviation (AAD%) obtained from the correlated of equation ( 1) of ρ, c, n D , k S and r as a function of composition have been computed.
e following relationship has been used for computing the absolute average percentage deviation (AAD%): where N is the experimental data number.e subscripts "Expt."and "Calc."are the experimental and calculated property values, respectively.
Fitting parameters A i and AAD% values of all the studied temperatures are summarized in Table 9 for AC + THF binary solutions.
Tables 9 and 10 present the calculated values of the A i parameters, correlation coefficient R 2 obtained for density ρ, speed of sound c, isentropic compressibility k S , and relaxation strength r [8,14,15], together with the AAD% calculated from (2) for AC + THF and xGnP + THF binary mixed solutions.
e absolute average percentage deviation for the physicochemical properties: refractive index, speed of sound, density, isentropic compressibility, and relaxation strength are less than 0.0002, 0.011, 0.009, 0.001, and 0.003% for the AC + THF mixture and less than 0.119, 0.002, 0.003, 0.003, and 0.002% for the xGnP + THF mixture, being well correlated for both systems, as it can be seen from Tables 9 and 10.
e isentropic compressibility (k S ) rises by the temperature, but it decreases when the concentration rises.
As shown in Tables 9 and 10 and in Figures 1-5, the values of the thermophysical properties experimentally obtained on the basis of polynomial relation (1) have been correlated with good accuracy.
e thermophysical behaviour of studied carbon-based nanomaterial-mixed solutions is well described by using Table 9: Fitting parameters A i and correlation coefficient R 2 obtained for density ρ, speed of sound c, refractive index n D , isentropic compressibility k S , and relaxation strength r along with the absolute average percentage deviation (AAD%) for binary AC + THF mixtures.a     Advances in Materials Science and Engineering three correlation parameters with polynomial expression (1).e experimental property and computed property values have been then statistically interpreted by analysis of variance "two-factor without replication" by method ANOVA.
e obtained values for the both binary mixtures of AC and xGnP in THF solvent are given in Tables 11 and 12, respectively.
e ANOVA results suggest that the model coefficients are significant if the F value is higher than F crit value with P value < 0.05.In ANOVA statistical analysis, for a 95% confidence level, the "alpha" significance level was used as 5%.In Table 11, the F values are greater than the corresponding F crit values of density, speed of sound, refractive index, isentropic compressibility, and relaxation strength in binary AC + THF mixtures at different compositions and temperatures, while the P values are much smaller than the "alpha" value for each property (P < 0.05) [42][43][44].In Table 12, the values of F < F crit , and P > 0.05 demonstrates an insignificant effect of concentration on the speed of sound and relaxation strength in binary mixture with xGnP.e ANOVA results show that some parameters have a significant influence on the thermophysical properties in the binary mixtures AC and xGnP with THF solvent.

Conclusions
e refractive index, speed of sound, density of exfoliated graphite nanoplatelets, and activated carbon dispersed in THF solvent were measured at different temperatures over the 0 to 100 kg•m −3 concentration domain.
e derived physicochemical properties from measured experimental data were calculated, and the absolute average percentage deviation (AAD%) values are comparable for both mixtures.In the xGnP + THF mixture, the AAD% values for the physicochemical parameters such as refractive index, speed of sound, density, isentropic compressibility, and relaxation strength are less than 0.119, 0.002, 0.003, 0.003, and 0.002%.Also, in the AC + THF mixture, the AAD% values for the same physicochemical parameters are less than 0.0002, 0.011, 0.009, 0.001, and 0.003% being well correlated by polynomial relation with three correlation parameters.Also, the effects of different parameters such as the temperature and concentration in the AC/xGnP + THF binary mixtures have been statistically investigated by the ANOVA method, and the obtained results, in generally, demonstrate their higher significance influence on of the studied thermophysical properties.e theoretical methodology presented in this study might explain the ability of the organic solvent THF to

Table 1 :
Specifications of exfoliated graphite nanoplatelets, activated carbon, and THF chemicals used in mixtures.

Table 2 :
Comparison of experimental densities (ρ), speeds of sound (c), and refractive indices (n D ) of pure tetrahydrofuran with literature values at various temperatures.

Table 3 :
Experimental values of the densities ρ, speeds of sound c, and refractive indices n D at various temperatures T and specific concentrations C of AC for the system AC + THF. a

Table 4 :
Experimental values of the densities ρ, speeds of sound c, and refractive indices n D at various temperatures T and specific concentrations C of xGnP for the system xGnP + THF. a xGnP: open diamond, 293.15; open circle, 298.15; open triangle, 303.15; open square, 308.15; for AC: solid line, dashed line; polynomial correlated values.
Figure 1: Comparative representation of the refractive indices of binary xGnP + THF and AC + THF systems versus concentration of the solute at various temperatures, T (K): lled diamond, 293.15; lled circle, 298.15; lled triangle, 303.15; lled square, 308.15; for xGnP: open diamond, 293.15; open circle, 298.15; open triangle, 303.15; open square, 308.15; for AC: solid line, dashed line; polynomial correlated values.) C (kg•m -3 ) Figure 2: Comparative representation of the speeds of sound of binary xGnP + THF and AC + THF systems versus concentrations of solute at various temperatures, T (K): lled diamond, 293.15; lled circle, 298.15; lled triangle, 303.15; lled square, 308.15; for , 303.15, and 308.15K is similar as in AC + THF binary mixture.e values of speed of sound in xGnP + THF mixtures differ than those obtained for the AC + THF system.With regard to the slope, this strongly decreases at 298.15, 303.15, and 308.15 K. e behaviour of speed of sound values by rising xGnP or AC solute

Table 6 :
Calculated values of the acoustic impedance Z, adiabatic compressibility k S , space-filling factor S, specific refraction r D , and relaxation strength r at various temperatures T and specific concentrations C of xGnP for the system xGnP + THF.

Table 7 :
e slope and intercept parameters obtained for density, speed of sound, refractive index, isentropic compressibility, and relaxation strength in binary AC + THF mixtures.

Table 8 :
e slope and intercept parameters obtained for density, speed of sound, refractive index, isentropic compressibility, and relaxation strength in binary xGnP + THF mixtures. 10

Table 10 :
Fitting parameters A i and correlation coefficient R 2 obtained for density ρ, speed of sound c, refractive index n D , isentropic compressibility k S , and relaxation strength r along with the absolute average percentage deviation (AAD%) for binary xGnP + THF mixtures.a

Table 11 :
Analysis of variance two-factor without replication for density, speed of sound, refractive index, isentropic compressibility, and relaxation strength in binary AC + THF mixtures.

Table 12 :
Analysis of variance two-factor without replication for density, speed of sound, refractive index, isentropic compressibility, and relaxation strength in binary xGnP + THF mixtures.Advances in Materials Science and Engineering 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, and its behaviour based on fundamental principles and practical methods is being emphasized in this work, too.