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hort Communication
etermination of the vapor pressure of Lippia gracilis Schum essentialil by thermogravimetric analysis
arlos Eduardo Lima de Oliveiraa,∗, Marco Aurélio Cremascob
Chemical Engineering School, University of Campinas, 13083-852 Campinas, São Paulo, BrazilChemical Engineering School, University of Campinas, 13093-970 Campinas, São Paulo, Brazil
r t i c l e i n f o
rticle history:eceived 21 August 2013eceived in revised form8 November 2013ccepted 29 November 2013vailable online 12 December 2013
a b s t r a c t
Thermogravimetric analysis was used to determine the vapor pressure of the Lippia gracilis S. essentialoil. The calibration constant value was obtained using thymol as reference compound, due to the factthat compound represents the majority in the essential oil. To check the calibration, the vapor pressuredata for carvacrol have been compared with the results reported in the literature and showed a goodagreement. The method was used in the determination of the vapor pressure curve for the essential oil.From vapor curves, the Antoine constants for the essential oil were found to be: A = 10.29230, B = 3116.68
The species of Lippia gracilis (Verbenaceae family) are shrubsypical of the Northeast region of Brazil. This species is tradition-lly used in popular medicine [1] as well as for the treatment ofnfluenza, cough, sinusitis, bronchitis, and nasal congestion [2–4].his species produces an essential oil whose main component maye thymol [2,5] or carvacrol [6,7].
Thymol and carvacrol (Fig. 1) are isomers with high antimi-robial activity. There are studies that relate the biological andharmacological activities of these phenols, such as antibacte-ial [8,9], antioxidant [10], fungicide [11], and acaridae activities12]. These compounds, as well as their essential oils, are widelymployed in the chemical, pharmaceutical and food industries.
Due to their wide variety of applications, essential oils haveecome an inexhaustible source of scientific and technologicalesearch, mostly for obtaining their major components. Becausehey are volatile liquids, vapor pressure data for the essential oilsre not only fundamental to the design of the equipment, when
ne intends to concentrate the major components by distillation,ut also for the understanding of the separation process [13].
The most common methods for determining the vapor pressureof a substance (isoteniscope and ebulliometer, for example) requirea large amount of sample and need a very long time for carryingout the experimental tests [14]. In this case, thermogravimetricanalysis (TG) has been a useful tool for determining this parameterbecause it is a short test and requires small samples.
There are several studies using this technique TG for vaporpressure determination. Wright et al. [15] determined the vaporpressure curves for adipic acid and triethanolamine techniqueusing TG–DTA. Price [16] determined the vapor pressure of plas-ticizers by TGA. Price and Hawkins [17] used TGA to determinethe vapor pressure of various dyes. Butrow and Seyler [18] demon-strated that the vapor pressure of various liquids can be determinedby differential scanning calorimetry (DSC). Gomes et al. [19] usedthe thermogravimetric curves to determine the vapor pressure ofalkaloids, warifteine and methylwarifteine.
2. Experimental
2.1. Material and equipment
Crystallized thymol (99.5% purity) was purchased fromSigma–Aldrich (Brazil), and carvacrol (purity 99.9% purity) from
MP Biomedicals (Brazil). The samples of L. gracilis Schum essentialoil were kindly provided by Laboratory of Natural Products (Fed-eral University of Maranhão, Brazil). The chemical composition ofthe essential oil was identified by gas chromatography/mass
process [27]. The essential oil and carvacrol also exhibit a zero-order kinetics process which can be attributed to the evaporation(Fig. 3).
Fig. 1. Chemical structures of carvacrol and thymol.
pectrometry (GC/MS). Thymol was present as the majoromponent at a level of approximately 82%.
The samples were analyzed in a thermogravimetric system,himadzu TGA-50. Rising temperature experiments were con-ucted at a heating rate of 10 ◦C min−1, in nitrogen atmosphere at0 mL min−1. In order to obtain thermogravimetric curves all com-ounds were subjected to a temperature range of ambient up to00 ◦C. At this temperature all of the material was completely evap-rated. The initial samples mass ranged from 8 to 14 mg, whichere placed in a platinum crucible with a cross-sectional area of
.28 cm2.
.2. Procedure
Thymol was chosen as the reference material for two reasons.irst, it is the major component in L. gracilis S. essential oil. Second,t has its Antoine constants reported in the literature (NIST) [21].he Antoine and Langmuir equations were used to determine thealue of the calibration constant (�), from the vapor pressure curvebtained. The Antoine equation [20] is presented as:
og (p/Pa) = A − B
T + C(1)
here p is the vapor pressure; T is the absolute temperature; A,, and C are the Antoine constants of that particular substancet a given temperature range. Antoine constants for thymol are:
= 5.29395, B = 2522.332 and C = −28.575, valid from 337.5 up to05 K [21].
The Langmuir equation [19] often used to determine the vaporressure of various substances is presented as:
1a
kvap = ˇ
(mi − mf ).p.
√M
2�RT(2)
here a is the area of the container (crucible); M is molar mass ofhe substance; is the vaporization coefficient, usually constantith a value equal to 1 [16], but in the presence of a carrier gas the
alue of this constant tends to be different from unity [22]; R is theniversal gas constant. The evaporation coefficient (kvap) for a zero-rder evaporation process, is given by the rate of mass loss (dm/dt),nd it is obtained from thermogravimetry [23,24]. The followingodification is described [19]:
=[
1ˇa
√2�R
][kvap(mi − mf )
√T
M
]= � . Yvap (3)
√ √
here � = 1/( · a) 2�R and Yvap = kvap(mi − mf ) T/M. The vari-
bles mi and mf are the initial and final masses in milligrams,espectively [25]. It is important to mention that (ˇ) and (a) valuesre implicit in the �-value.
Fig. 2. Thermogravimetric and derivative thermogravimetric (TG–DTG) curves ofthymol.
The Yvap-value can be obtained experimentally by thermo-gravimetry, and it depends only on the molar mass of the compoundunder study. Thus, the plot of (p) versus (Yvap) gives the �-value. Thevalue of (�) was used to determine the vapor pressure curves, first,carvacrol and, after validation in the literature, it was determinedthe vapor pressure curve of the oil.
3. Results and discussion
Phang and Dollimore [26], as well as Chatterjee et al. [27] used acompound whose Antoine constants are known, and they appliedthis method to compare the vapor pressure curves found in the lit-erature with those from thermogravimetry. In this work the thymolwas chosen as the calibration compound.
The thermogravimetric and derivative thermogravimetric(TG–DTG) curves for thymol can be seen in Fig. 2. Derivative ther-mogravimetry curves (DTG) are fundamental to determining thereaction order [28,29]. Fig. 2 shows that thymol exhibits one simplestage of evaporation that can be observed through an increased rateof mass loss to a maximum value which decreases abruptly to thebaseline. This indicates that thymol was undergoing a zero-order
Fig. 3. Thermogravimetric and derivative thermogravimetric (TG–DTG) curves ofcarvacrol and essential oil.
ig. 4. Calibration constant (�) from Eq. (3) using thymol as calibration compound.
The vapor pressure curves for thymol were constructed usinghe Antoine equation and modified Langmuir equation. The valuesor the Antoine constants were reported in the NIST webbook [21]s A = 5.29395, B = 2522.330 and C = −28.580 for 337.5 up to 505.0 K.herefore, the plot (p) against (Yvap) (Fig. 4) was used to obtain thealibration constant (�). The �-value obtained was 3.20 × 1016 in SInits system, with R2 = 0.985.
The calibration constant has been considered an importantarameter in the determination of the vapor pressure by theresent method. This parameter is constant and independent ofaterial studied [27]. This was the assumption that must be fol-
owed in order to apply the �-value to calculate the vapor pressuref unknown substances.
.1. Vapor pressure curve for carvacrol
From the DTG carvacrol curve and �-value (3.20 × 1016 in SI),he vapor pressure curve was constructed, for a temperature rangef 346 up to 455 K (Fig. 5). A comparison of the vapor pressure
urves for carvacrol obtained by the method used in this work withhe literature values can be seen in Fig. 5. The Antoine constantsor carvacrol were reported [27] as A = 5.34179, B = 2549.860 and
ig. 5. Comparison between carvacrol vapor pressure obtained in this work withhat one considering Antoine constants from literature (346–455 K).
Carvacrol 346.6–455.6(0.06409) (33.72) (3.35)
a NIST webbook [21].
C = −32.705 for 346 up to 510 K. There is a very good agreementbetween the curves up to 440 K.
3.2. Vapor pressure curve for the essential oil
According to Hazra et al. [28], the method for calculating thevapor pressure curve for carvacrol (single-component system) canbe used to construct the vapor pressure curve for the essentialoil (multi component system) if the composition and the averagemolar mass of this essential oil is known. In this study, we used theGC/MS technique to determine the molar mass of the oil to give152.837 g mol−1, which is very close to its major component, thy-mol (152.220 g mol−1). The vapor pressure curve for the L. gracilisS. essential oil, for 330 K up to 423 K, can be seen in Fig. 6.
3.3. Antoine constant determination
The Antoine constants for the essential oil and carvacrol werecalculated using the method of least squares through a non-linearregression. Origin 6.0 software was used and starting values A = 9.3,B = 2000 and C = −37 were performed according to ASTM methods[30,31], and according to [22,28]. The values of A, B and C, for bothessential oil and carvacrol, as well as thymol (NIST) are given inTable 1.
4. Conclusion
The essential oil showed order-zero evaporation pro-cess. The method of determination the vapor pressure
showed to be a good tool obtaining based on the calibra-tion constant associated with the experimental system viathermogravimetric analysis. Therefore, we used thymol as a refer-ence standard, obtaining � = 3.20 × 1016 in SI. From the knowledge
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f this constant as well as the thermogravimetric analysis of thessential oil, it was possible to obtain its vapor pressure curve.his was verified by comparison of the vapor pressure curvesbtained by this method with the curves reported in the literatureonstructed by Antoine equation for carvacrol.
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