Investigation of the thermodynamic and thermal properties of clary sage (Salvia sclarea L.) essential oil and its main components

The some physochemical and chemical properties of the clary sage (Salvia sclarea L.) essential oil were determined. The main constituents in the clary sage essential oil (above 2%) were: linalyl acetate (34.62%), β-linalool (17.67%), α-muurolene (8.27%), β-caryophyllene (5.60%), α-ylangene (5.18%), α-terpineol (4.84%), n-docosane (3.00%), and neryl acetate (2.34%).The thermodynamic and thermal properties of essential oil and its main components were investigated.

The thermal properties of borage (Borago officinalis L.) seeds were determined at temperatures ranging from 6 to 20°C and moisture contents from 1.2 to 30.3% [13].
Some authors were reported of viscosities and densities, at T = 298.15 K of the phases formed after diterpenation of bergamot, lemon and mint essential oils, by (liquid + liquid) extraction [14]. In the literature do not found data for determination of thermodynamic parameters of clary sage essential oil as well as its main components which is also the aim of the present study.
2 Material and methods 2.1 Materials. Clary sage oil was provided by Kateco Ltd, Bulgaria. The oil was kept in the refrigerator at 4 °C before analysis. 2.1.5 Thermodynamic and thermal properties of the essential oil and its main compounds. Specific heat capacity is obtained by eq. (1) [18]: where: C p -specific heat capacity, J.mol -1 K -1 ; H -enthalpy, J.mol -1 ; T -temperature, ° K. Dependence between specific heat capacity, thermal conductivity and coefficient of thermal conductivity were determined according to [13,18] by eq. (2).
2.1.6 Statistical analysis. The experimental data were obtained after three replicates. The results are presented as a mean value of the individual measurements with the corresponding standard deviation (SD).

Results and Discussions
The clary sage essential oil presented light yellow with a specific odor and had followed properties: density 0.8623  0.0 and a refractive index ( � 20 ) 1.4635  0.0. The obtained values didn`t exhibited differences with similar data from literature [1,2].
The IR spectrum of clary sage essential oil is presented in Table 1. The analysis of FT-IR of studied oil represents mainly strong vibrations of the components in this oil. Intense line at 3423 cm -1 appears stronger connected with conformation in the β-linalool molecule. In this case the mixture OH-groups lose steric factor.
Another connection with the spectrum of pure β-linalool is the band at 2973 cm -1 and associated with CH 2 -asymmetric vibrations. The probable cause of this behavior can be explained with antagonism with different bands from different components in the investigated oil. The other components does not show strong absorption bands because the limitations of FT-IR method which detects chemical compounds with concentration above 5%.
The experimental FT-IR spectrum of clary sage essential oil is the presented in Fig. 1. Chemical composition of the clary sage essential oil is listed in Table 2.

-Cubebene
Distribution of major groups of aroma substances in the essential oil is shown in Figure  2. The dominant groups in the oil were oxygenated monoterpenes (61.88%); then followed by sesquiterpene hydrocarbons (26.23%), monoterpene hydrocarbons (5.33%), and hydrocarbons (4.54%), oxygenated sesquiterpenes (1.87%), and oxygenated hydrocarbons (0.15%). In Table 3 are presented some chemical properties of the main nine aromatic substances, which formed 82.85% from total composition properties of the essential oil, too. The investigated substances were linalyl acetate, and neryl acetate which are the main components from monoterpenes group, -muurolene, -caryophyllene, hydrocarbons group and n-docosane from functional groups and structure is: hydrocarbons ylangene, and n-docosane), two alcohols (linalool acetate and neryl acetate).
The calculations of thermo physical characteristics of the clary sage essential oil as well as thermal conductivity, specific heat capacity and presented in Table 4. The values are obtained according to the chemical composition presented in Table 2.
The activation energy was calculated about -1796.75 kJ.mol -1 . The negative value specified that processes were non-spontaneous occurring as a result of heat and light. The molar mass of clary sage oil was determined experimentally using GC/MS analysis (190.77 kg.mol -1 ). The thermodynamic parameters of Gibbs energy, enthalpy and entropy of the main components of sage essential oil were calculated. The thermodynamic parameters for salvia oil-ethanol-water solution were found in the literature [20], as the values obtained for the Gibbs energy are from -8968.87 to -10037.77 kJ.mol -1 , for the enthalpy (ΔH) -2490.24 kJ.mol -1 and for entropy (ΔS) from 21.71 to 24.83 kJ.mol -1 K -1 . The differences in the values obtained are probably due to the different composition of the studied essential oils. In pure oils the values of the thermodynamic parameters depend mainly on the chemical composition of the obtained essential oil, and when used in solution the values are also influenced by the solvents. Gibbs energy represents a thermodynamic potential and shows the possibility of the process. The positive values of Gibbs energy mean that the process that takes place in the main components of the essential oil is spontaneous and irreversible. Entropy as a thermodynamic parameter gives a connection about the mechanism of the reaction and the activation complexes. In this case the entropy values are negative, which means that the process is irreversible. The enthalpy values are positive too, which also means that the process is irreversible. In this case the enthalpy of the process is equal to the activating energy with the opposite sign. This means that the process will proceed with the adding of energy. Table 5 presented values of heat capacity and enthalpy of main components of clary sage oil. Composition of the all components are summarised to 100% needs for calculations of the molar heat capacity and enthalpy for each one of main components. Comparison between results given in Table 3 and Table 5 are done for n-docosane and -linalool.
Differences which appeared between literature data [14,19] (Table 3) and experimental values from this work probably connected for different composition of components in the essential oil. It didn`t found data for molar enthalpy of clary sage essential oil at 25 0 C in literature and for this reason no comparison was made of the results obtained in this work.