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Determination of the vapor pressure of Lippia gracilis Schum essential oil by thermogravimetric analysis
Thermochimica Acta 577 (2014) 1–4
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Short Communication
Determination of the vapor pressure of Lippia gracilis Schum essential oil by thermogravimetric analysis Carlos Eduardo Lima de Oliveira a,∗ , Marco Aurélio Cremasco b a b
Chemical Engineering School, University of Campinas, 13083-852 Campinas, São Paulo, Brazil Chemical Engineering School, University of Campinas, 13093-970 Campinas, São Paulo, Brazil
a r t i c l e
i n f o
Article history: Received 21 August 2013 Received in revised form 28 November 2013 Accepted 29 November 2013 Available online 12 December 2013 Keywords: Thermogravimetry Essential oil Lippia gracilis Vapor pressure Antoine equation
a b s t r a c t Thermogravimetric analysis was used to determine the vapor pressure of the Lippia gracilis S. essential oil. The calibration constant value was obtained using thymol as reference compound, due to the fact that compound represents the majority in the essential oil. To check the calibration, the vapor pressure data for carvacrol have been compared with the results reported in the literature and showed a good agreement. 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 and C = 74.23, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The species of Lippia gracilis (Verbenaceae family) are shrubs typical of the Northeast region of Brazil. This species is traditionally used in popular medicine [1] as well as for the treatment of influenza, cough, sinusitis, bronchitis, and nasal congestion [2–4]. This species produces an essential oil whose main component may be thymol [2,5] or carvacrol [6,7]. Thymol and carvacrol (Fig. 1) are isomers with high antimicrobial activity. There are studies that relate the biological and pharmacological activities of these phenols, such as antibacterial [8,9], antioxidant [10], fungicide [11], and acaridae activities [12]. These compounds, as well as their essential oils, are widely employed in the chemical, pharmaceutical and food industries. Due to their wide variety of applications, essential oils have become an inexhaustible source of scientific and technological research, mostly for obtaining their major components. Because they are volatile liquids, vapor pressure data for the essential oils are not only fundamental to the design of the equipment, when one intends to concentrate the major components by distillation, but also for the understanding of the separation process [13].
∗ Corresponding author. Tel.: +55 19 3521 2123; fax: +55 19 3521 3910. E-mail addresses: [email protected] (C.E.L.d. Oliveira), [email protected] (M.A. Cremasco). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.11.023
The most common methods for determining the vapor pressure of a substance (isoteniscope and ebulliometer, for example) require a large amount of sample and need a very long time for carrying out the experimental tests [14]. In this case, thermogravimetric analysis (TG) has been a useful tool for determining this parameter because it is a short test and requires small samples. There are several studies using this technique TG for vapor pressure determination. Wright et al. [15] determined the vapor pressure curves for adipic acid and triethanolamine technique using TG–DTA. Price [16] determined the vapor pressure of plasticizers by TGA. Price and Hawkins [17] used TGA to determine the vapor pressure of various dyes. Butrow and Seyler [18] demonstrated that the vapor pressure of various liquids can be determined by differential scanning calorimetry (DSC). Gomes et al. [19] used the thermogravimetric curves to determine the vapor pressure of alkaloids, warifteine and methylwarifteine. 2. Experimental 2.1. Material and equipment Crystallized thymol (99.5% purity) was purchased from Sigma–Aldrich (Brazil), and carvacrol (purity 99.9% purity) from MP Biomedicals (Brazil). The samples of L. gracilis Schum essential oil were kindly provided by Laboratory of Natural Products (Federal University of Maranhão, Brazil). The chemical composition of the essential oil was identified by gas chromatography/mass
2
C.E.L.d. Oliveira, M.A. Cremasco / Thermochimica Acta 577 (2014) 1–4
Fig. 1. Chemical structures of carvacrol and thymol.
spectrometry (GC/MS). Thymol was present as the major component at a level of approximately 82%. The samples were analyzed in a thermogravimetric system, Shimadzu TGA-50. Rising temperature experiments were conducted at a heating rate of 10 ◦ C min−1 , in nitrogen atmosphere at 50 mL min−1 . In order to obtain thermogravimetric curves all compounds were subjected to a temperature range of ambient up to 400 ◦ C. At this temperature all of the material was completely evaporated. The initial samples mass ranged from 8 to 14 mg, which were placed in a platinum crucible with a cross-sectional area of 0.28 cm2 .
Fig. 2. Thermogravimetric and derivative thermogravimetric (TG–DTG) curves of thymol.
The Yvap -value can be obtained experimentally by thermogravimetry, and it depends only on the molar mass of the compound under study. Thus, the plot of (p) versus (Yvap ) gives the -value. The value of () was used to determine the vapor pressure curves, first, carvacrol and, after validation in the literature, it was determined the vapor pressure curve of the oil.
2.2. Procedure
3. Results and discussion
Thymol was chosen as the reference material for two reasons. First, it is the major component in L. gracilis S. essential oil. Second, it has its Antoine constants reported in the literature (NIST) [21]. The Antoine and Langmuir equations were used to determine the value of the calibration constant (), from the vapor pressure curve obtained. The Antoine equation [20] is presented as:
Phang and Dollimore [26], as well as Chatterjee et al. [27] used a compound whose Antoine constants are known, and they applied this method to compare the vapor pressure curves found in the literature with those from thermogravimetry. In this work the thymol was chosen as the calibration compound. The thermogravimetric and derivative thermogravimetric (TG–DTG) curves for thymol can be seen in Fig. 2. Derivative thermogravimetry curves (DTG) are fundamental to determining the reaction order [28,29]. Fig. 2 shows that thymol exhibits one simple stage of evaporation that can be observed through an increased rate of mass loss to a maximum value which decreases abruptly to the baseline. This indicates that thymol was undergoing a zero-order process [27]. The essential oil and carvacrol also exhibit a zeroorder kinetics process which can be attributed to the evaporation (Fig. 3).
log (p/Pa) = A −
B T +C
(1)
where p is the vapor pressure; T is the absolute temperature; A, B, and C are the Antoine constants of that particular substance at a given temperature range. Antoine constants for thymol are: A = 5.29395, B = 2522.332 and C = −28.575, valid from 337.5 up to 505 K [21]. The Langmuir equation [19] often used to determine the vapor pressure of various substances is presented as:
1 ˇ .p. kvap = a (mi − mf )
M 2RT
(2)
where a is the area of the container (crucible); M is molar mass of the substance; ˇ is the vaporization coefficient, usually constant with a value equal to 1 [16], but in the presence of a carrier gas the value of this constant tends to be different from unity [22]; R is the universal gas constant. The evaporation coefficient (kvap ) for a zeroorder evaporation process, is given by the rate of mass loss (dm/dt), and it is obtained from thermogravimetry [23,24]. The following modification is described [19]: p=
1√ ˇa
2R
kvap (mi − mf )
T M
= . Yvap
(3)
√ where = 1/(ˇ · a) 2R and Yvap = kvap (mi − mf ) T /M. The variables mi and mf are the initial and final masses in milligrams, respectively [25]. It is important to mention that (ˇ) and (a) values are implicit in the -value.
Fig. 3. Thermogravimetric and derivative thermogravimetric (TG–DTG) curves of carvacrol and essential oil.
C.E.L.d. Oliveira, M.A. Cremasco / Thermochimica Acta 577 (2014) 1–4
3
Fig. 6. Vapor pressure plot for the Lippia gracilis S. essential oil. Fig. 4. Calibration constant () from Eq. (3) using thymol as calibration compound.
The vapor pressure curves for thymol were constructed using the Antoine equation and modified Langmuir equation. The values for the Antoine constants were reported in the NIST webbook [21] as A = 5.29395, B = 2522.330 and C = −28.580 for 337.5 up to 505.0 K. Therefore, the plot (p) against (Yvap ) (Fig. 4) was used to obtain the calibration constant (). The -value obtained was 3.20 × 1016 in SI units system, with R2 = 0.985. The calibration constant has been considered an important parameter in the determination of the vapor pressure by the present method. This parameter is constant and independent of material studied [27]. This was the assumption that must be followed in order to apply the -value to calculate the vapor pressure of unknown substances. 3.1. Vapor pressure curve for carvacrol From the DTG carvacrol curve and -value (3.20 × 1016 in SI), the vapor pressure curve was constructed, for a temperature range of 346 up to 455 K (Fig. 5). A comparison of the vapor pressure curves for carvacrol obtained by the method used in this work with the literature values can be seen in Fig. 5. The Antoine constants for carvacrol were reported [27] as A = 5.34179, B = 2549.860 and
Table 1 Antoine constants of essential oil and carvacrol with their respective standard deviations. Compound
a
Thymol
Essential oil Carvacrol a
Calculated Antoine constants A
B
C
5.29395 10.33519 (0.26228) 8.30643 (0.06409)
2522.33 3156.93 (236.42) 1312.60 (33.72)
−28.58 77.18 (16.84) −132.43 (3.35)
Temperature range (K)
337.5–505.0 329.5–422.6 346.6–455.6
NIST webbook [21].
C = −32.705 for 346 up to 510 K. There is a very good agreement between 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 the vapor pressure curve for carvacrol (single-component system) can be used to construct the vapor pressure curve for the essential oil (multi component system) if the composition and the average molar mass of this essential oil is known. In this study, we used the GC/MS technique to determine the molar mass of the oil to give 152.837 g mol−1 , which is very close to its major component, thymol (152.220 g mol−1 ). The vapor pressure curve for the L. gracilis S. 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 were calculated using the method of least squares through a non-linear regression. 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 both essential oil and carvacrol, as well as thymol (NIST) are given in Table 1. 4. Conclusion
Fig. 5. Comparison between carvacrol vapor pressure obtained in this work with that one considering Antoine constants from literature (346–455 K).
The essential oil showed order-zero evaporation process. The method of determination the vapor pressure showed to be a good tool obtaining based on the calibration constant associated with the experimental system via thermogravimetric analysis. Therefore, we used thymol as a reference standard, obtaining = 3.20 × 1016 in SI. From the knowledge
4
C.E.L.d. Oliveira, M.A. Cremasco / Thermochimica Acta 577 (2014) 1–4
of this constant as well as the thermogravimetric analysis of the essential oil, it was possible to obtain its vapor pressure curve. This was verified by comparison of the vapor pressure curves obtained by this method with the curves reported in the literature constructed by Antoine equation for carvacrol. References [1] H. Lorenzi, F.J.A. Matos, Plantas medicinais no Brasil: nativas e exóticas, second ed., Instituto Plantarum de Estudos da Flora, Nova Odessa, 2008. [2] F.J.A. Matos, M.I.L. Machado, A.A. Craveiro, The essential oil composition of two chemotypes of Lippia alba grown in northeast Brazil, J. Essent. Oil Res. 8 (1996) 695–698. [3] U.P. Albuquerque, P.M. Medeiros, A.L.S. Almeida, J.M. Monteiro, E.M.F.L. Neto, J.G. Melo, J.P. Santos, Medicinal plants of the caatinga (semi-arid) vegetation of NE Brazil: a quantitative approach, J. Ethnopharmacol. 114 (2007) 325–354. [4] I.A. Neves, J.C.S. Oliveira, C.A.G. Camara, Chemical composition of the leaf oils of Lippia gracilis Schauer from two localities of Pernambuco, J. Essent. Oil Res. 20 (2008) 157–160. [5] S.S. Mendes, R.R. Bomfim, H.C.R. Jesus, P.B. Alves, A.F. Blank, C.S. Estevam, A.R. Antoniolli, S.M. Thomazzi, Evaluation of the analgesic and anti-inflammatory effects of the essential oil of Lippia gracilis leaves, J. Ethnopharmacol. 129 (2010) 391–397. [6] W.J. Silva, G.A.A. Dória, R.T. Maria, R.S. Nunes, G.A. Carvalho, A.F. Blank, P.B. Alves, R.M. Marc¸al, S.C.H. Cavalcanti, Effects of essential oils on Aedes aegypti larvae: alternatives to environmentally safe insecticides, Bioresour. Technol. 99 (2008) 3251–3255. [7] R.M. Neto, F.J.A. Matos, V.S.A. Andrade, M.C.N. Melo, C.B.M. Carvalho, S.B. Guimarães, O.D.L. Pessoa, S.L. Silva, S.F.R. Silva, P.R.L. Vasconcelos, The essential oil from Lippia gracilis Schauer, Verbenaceae, in diabetic rats, Rev. Bras. Farmacogn. 20 (2010) 261–266. [8] M.A. Botelho, N.A.P. Nogueira, G.M. Bastos, S.G.C. Fonseca, T.L.G. Lemos, F.J.A. Matos, D. Montenegro, J. Heukelbach, V.S. Rao, G.A.C. Brito, Antimicrobial activity of the essential oil from Lippia sidoides, carvacrol and thymol against oral pathogens, Braz. J. Med. Biol. Res. 40 (2007) 349–356. [9] A. Nostro, A.S. Roccaro, G. Bisignano, A. Mariono, M.A. Cannatelli, F.C. Pizzimenti, P.L. Cioni, F. Procopio, A.R. Blanco, Effects of oregano, carvacrol and thymol on Staphylococcus aureus and Staphylococcus epidermidis biofilms, J. Med. Microbiol. 56 (2007) 519–523. [10] M. Milos, J. Mastelic, I. Jerkovic, Chemical composition and antioxidant effect of glycosidically bound volatile compounds from oregano (Origanum vulgare L. ssp. hirtum), Food Chem. 71 (2000) 79–83. [11] A. Ahmad, A. Khan, F. Akhtar, S. Yousuf, I. Xess, L.A. Khan, N. Manzoor, Fungicidal activity of thymol and carvacrol by disrupting ergosterol biosynthesis and membrane integrity against Candida, Eur. J. Clin. Microbiol. Infect. Dis. 30 (2011) 41–50. [12] E. Daemon, R. Maturano, C.M.O. Monteiro, M.S. Goldner, T. Massoni, Acaricidal activity of hydroethanolic formulations of thymol against Rhipicephalus sanguineus (Acari: Ixodidae) and Dermacentor nitens (Acari: Ixodidae) larvae, Vet. Parasitol. 186 (2012) 542–545.
[13] M.A. Cremasco, N.P. Braga, Análise termogravimétrica do óleo essencial de pimenta longa (Piper hispidinervium C. DC), Acta Amazon. 41 (2011) 275–278. [14] M. Casserino, D.R. Blevins, R.N. Sanders, An improved method for measuring vapor pressure by DSC with automated pressure control, Thermochim. Acta 284 (1996) 145–152. [15] S.F. Wright, D. Dollimore, J.G. Dunn, K. Alexander, Determination of the vapor pressure curves of adipic acid and triethanolamine using thermogravimetry analysis, Thermochim. Acta 421 (2004) 25–30. [16] D.M. Price, Vapor pressure determination by thermogravimetry, Thermochim. Acta 367/368 (2001) 253–262. [17] D.M. Price, M. Hawkins, Calorimetry of two disperse dyes using thermogravimetry, Thermochim. Acta 315 (1998) 19–24. [18] A.B. Butrow, R.J. Seyler, Vapor pressure by DSC: extending ASTM E 1782 below 5 kPa, Thermochim. Acta 402 (2003) 145–152. [19] A.P.B. Gomes, F.D. Freire, C.F.S. Aragão, Determination of vapor pressure curves of warifteine and methylwarifteine by using thermogravimetry, J. Therm. Anal. Calorim. 108 (2012) 249–252. [20] V. Majer, M. Svoboda, J. Pick, Heats of Vaporizations of Fluids, Elsevier, Amsterdam, 1989. [21] National Institute of Standards and Technology, Chemistry Web Book, NIST Standard Reference Database Number 69, 2011, Available in http://webbook.nist.gov (accessed in 25.05.12). [22] K. Chatterjee, D. Dollimore, K.A. Alexander, New application for the Antoine equation in formulation development, Int. J. Pharm. 213 (2001) 31–44. [23] D. Dollimore, C. O’Connell, A comparison of the thermal decomposition of preservatives using thermogravimetry and rising temperature kinetics, Thermochim. Acta 324 (1998) 33–48. [24] K. Chatterjee, A. Hazra, D. Dollimore, K. Alexander, An evaporation study for phthalic acids – a rapid method for pharmaceutical characterization, J. Pharm. Sci. 91 (2002) 1156–1168. [25] P.J. Haines, Thermal Methods of Analysis: Principles, Applications and Problems, Blackie, London, 1995. [26] P. Phang, D. Dollimore, The calculation of the vapor pressures of antioxidants over a range of temperatures using thermogravimetry, Thermochim. Acta 367/368 (2001) 263–271. [27] K. Chatterjee, D. Dollimore, K. Alexander, Calculation of vapor pressure curves for hydroxy benzoic acid derivatives using thermogravimetry, Thermochim. Acta 392/393 (2002) 107–117. [28] A. Hazra, D. Dollimore, K. Alexander, Thermal analysis of the evaporation of compounds used in aromatherapy using thermogravimetry, Thermochim. Acta 392 (2002) 221–229. [29] A. Hazra, K. Alexander, D. Dollimore, A. Riga, Characterization of some essential oils and their key components: thermoanalytical techniques, J. Therm. Anal. Calorim. 75 (2004) 317–330. [30] Annual Book of ASTM Standards, Designation: E 1719-05, Standard Test Method for Vapor Pressure of Liquids by Ebulliometry, West Conshohocken, PA, USA, 2005. [31] Annual Book of ASTM Standards, Designation: E 1782-08, Standard Test Method for Determining Vapor Pressure by Thermal Analysis, West Conshohocken, PA, USA, 2008.
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Short Communication
Determination of the vapor pressure of Lippia gracilis Schum essential oil by thermogravimetric analysis Carlos Eduardo Lima de Oliveira a,∗ , Marco Aurélio Cremasco b a b
Chemical Engineering School, University of Campinas, 13083-852 Campinas, São Paulo, Brazil Chemical Engineering School, University of Campinas, 13093-970 Campinas, São Paulo, Brazil
a r t i c l e
i n f o
Article history: Received 21 August 2013 Received in revised form 28 November 2013 Accepted 29 November 2013 Available online 12 December 2013 Keywords: Thermogravimetry Essential oil Lippia gracilis Vapor pressure Antoine equation
a b s t r a c t Thermogravimetric analysis was used to determine the vapor pressure of the Lippia gracilis S. essential oil. The calibration constant value was obtained using thymol as reference compound, due to the fact that compound represents the majority in the essential oil. To check the calibration, the vapor pressure data for carvacrol have been compared with the results reported in the literature and showed a good agreement. 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 and C = 74.23, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The species of Lippia gracilis (Verbenaceae family) are shrubs typical of the Northeast region of Brazil. This species is traditionally used in popular medicine [1] as well as for the treatment of influenza, cough, sinusitis, bronchitis, and nasal congestion [2–4]. This species produces an essential oil whose main component may be thymol [2,5] or carvacrol [6,7]. Thymol and carvacrol (Fig. 1) are isomers with high antimicrobial activity. There are studies that relate the biological and pharmacological activities of these phenols, such as antibacterial [8,9], antioxidant [10], fungicide [11], and acaridae activities [12]. These compounds, as well as their essential oils, are widely employed in the chemical, pharmaceutical and food industries. Due to their wide variety of applications, essential oils have become an inexhaustible source of scientific and technological research, mostly for obtaining their major components. Because they are volatile liquids, vapor pressure data for the essential oils are not only fundamental to the design of the equipment, when one intends to concentrate the major components by distillation, but also for the understanding of the separation process [13].
∗ Corresponding author. Tel.: +55 19 3521 2123; fax: +55 19 3521 3910. E-mail addresses: [email protected] (C.E.L.d. Oliveira), [email protected] (M.A. Cremasco). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.11.023
The most common methods for determining the vapor pressure of a substance (isoteniscope and ebulliometer, for example) require a large amount of sample and need a very long time for carrying out the experimental tests [14]. In this case, thermogravimetric analysis (TG) has been a useful tool for determining this parameter because it is a short test and requires small samples. There are several studies using this technique TG for vapor pressure determination. Wright et al. [15] determined the vapor pressure curves for adipic acid and triethanolamine technique using TG–DTA. Price [16] determined the vapor pressure of plasticizers by TGA. Price and Hawkins [17] used TGA to determine the vapor pressure of various dyes. Butrow and Seyler [18] demonstrated that the vapor pressure of various liquids can be determined by differential scanning calorimetry (DSC). Gomes et al. [19] used the thermogravimetric curves to determine the vapor pressure of alkaloids, warifteine and methylwarifteine. 2. Experimental 2.1. Material and equipment Crystallized thymol (99.5% purity) was purchased from Sigma–Aldrich (Brazil), and carvacrol (purity 99.9% purity) from MP Biomedicals (Brazil). The samples of L. gracilis Schum essential oil were kindly provided by Laboratory of Natural Products (Federal University of Maranhão, Brazil). The chemical composition of the essential oil was identified by gas chromatography/mass
2
C.E.L.d. Oliveira, M.A. Cremasco / Thermochimica Acta 577 (2014) 1–4
Fig. 1. Chemical structures of carvacrol and thymol.
spectrometry (GC/MS). Thymol was present as the major component at a level of approximately 82%. The samples were analyzed in a thermogravimetric system, Shimadzu TGA-50. Rising temperature experiments were conducted at a heating rate of 10 ◦ C min−1 , in nitrogen atmosphere at 50 mL min−1 . In order to obtain thermogravimetric curves all compounds were subjected to a temperature range of ambient up to 400 ◦ C. At this temperature all of the material was completely evaporated. The initial samples mass ranged from 8 to 14 mg, which were placed in a platinum crucible with a cross-sectional area of 0.28 cm2 .
Fig. 2. Thermogravimetric and derivative thermogravimetric (TG–DTG) curves of thymol.
The Yvap -value can be obtained experimentally by thermogravimetry, and it depends only on the molar mass of the compound under study. Thus, the plot of (p) versus (Yvap ) gives the -value. The value of () was used to determine the vapor pressure curves, first, carvacrol and, after validation in the literature, it was determined the vapor pressure curve of the oil.
2.2. Procedure
3. Results and discussion
Thymol was chosen as the reference material for two reasons. First, it is the major component in L. gracilis S. essential oil. Second, it has its Antoine constants reported in the literature (NIST) [21]. The Antoine and Langmuir equations were used to determine the value of the calibration constant (), from the vapor pressure curve obtained. The Antoine equation [20] is presented as:
Phang and Dollimore [26], as well as Chatterjee et al. [27] used a compound whose Antoine constants are known, and they applied this method to compare the vapor pressure curves found in the literature with those from thermogravimetry. In this work the thymol was chosen as the calibration compound. The thermogravimetric and derivative thermogravimetric (TG–DTG) curves for thymol can be seen in Fig. 2. Derivative thermogravimetry curves (DTG) are fundamental to determining the reaction order [28,29]. Fig. 2 shows that thymol exhibits one simple stage of evaporation that can be observed through an increased rate of mass loss to a maximum value which decreases abruptly to the baseline. This indicates that thymol was undergoing a zero-order process [27]. The essential oil and carvacrol also exhibit a zeroorder kinetics process which can be attributed to the evaporation (Fig. 3).
log (p/Pa) = A −
B T +C
(1)
where p is the vapor pressure; T is the absolute temperature; A, B, and C are the Antoine constants of that particular substance at a given temperature range. Antoine constants for thymol are: A = 5.29395, B = 2522.332 and C = −28.575, valid from 337.5 up to 505 K [21]. The Langmuir equation [19] often used to determine the vapor pressure of various substances is presented as:
1 ˇ .p. kvap = a (mi − mf )
M 2RT
(2)
where a is the area of the container (crucible); M is molar mass of the substance; ˇ is the vaporization coefficient, usually constant with a value equal to 1 [16], but in the presence of a carrier gas the value of this constant tends to be different from unity [22]; R is the universal gas constant. The evaporation coefficient (kvap ) for a zeroorder evaporation process, is given by the rate of mass loss (dm/dt), and it is obtained from thermogravimetry [23,24]. The following modification is described [19]: p=
1√ ˇa
2R
kvap (mi − mf )
T M
= . Yvap
(3)
√ where = 1/(ˇ · a) 2R and Yvap = kvap (mi − mf ) T /M. The variables mi and mf are the initial and final masses in milligrams, respectively [25]. It is important to mention that (ˇ) and (a) values are implicit in the -value.
Fig. 3. Thermogravimetric and derivative thermogravimetric (TG–DTG) curves of carvacrol and essential oil.
C.E.L.d. Oliveira, M.A. Cremasco / Thermochimica Acta 577 (2014) 1–4
3
Fig. 6. Vapor pressure plot for the Lippia gracilis S. essential oil. Fig. 4. Calibration constant () from Eq. (3) using thymol as calibration compound.
The vapor pressure curves for thymol were constructed using the Antoine equation and modified Langmuir equation. The values for the Antoine constants were reported in the NIST webbook [21] as A = 5.29395, B = 2522.330 and C = −28.580 for 337.5 up to 505.0 K. Therefore, the plot (p) against (Yvap ) (Fig. 4) was used to obtain the calibration constant (). The -value obtained was 3.20 × 1016 in SI units system, with R2 = 0.985. The calibration constant has been considered an important parameter in the determination of the vapor pressure by the present method. This parameter is constant and independent of material studied [27]. This was the assumption that must be followed in order to apply the -value to calculate the vapor pressure of unknown substances. 3.1. Vapor pressure curve for carvacrol From the DTG carvacrol curve and -value (3.20 × 1016 in SI), the vapor pressure curve was constructed, for a temperature range of 346 up to 455 K (Fig. 5). A comparison of the vapor pressure curves for carvacrol obtained by the method used in this work with the literature values can be seen in Fig. 5. The Antoine constants for carvacrol were reported [27] as A = 5.34179, B = 2549.860 and
Table 1 Antoine constants of essential oil and carvacrol with their respective standard deviations. Compound
a
Thymol
Essential oil Carvacrol a
Calculated Antoine constants A
B
C
5.29395 10.33519 (0.26228) 8.30643 (0.06409)
2522.33 3156.93 (236.42) 1312.60 (33.72)
−28.58 77.18 (16.84) −132.43 (3.35)
Temperature range (K)
337.5–505.0 329.5–422.6 346.6–455.6
NIST webbook [21].
C = −32.705 for 346 up to 510 K. There is a very good agreement between 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 the vapor pressure curve for carvacrol (single-component system) can be used to construct the vapor pressure curve for the essential oil (multi component system) if the composition and the average molar mass of this essential oil is known. In this study, we used the GC/MS technique to determine the molar mass of the oil to give 152.837 g mol−1 , which is very close to its major component, thymol (152.220 g mol−1 ). The vapor pressure curve for the L. gracilis S. 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 were calculated using the method of least squares through a non-linear regression. 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 both essential oil and carvacrol, as well as thymol (NIST) are given in Table 1. 4. Conclusion
Fig. 5. Comparison between carvacrol vapor pressure obtained in this work with that one considering Antoine constants from literature (346–455 K).
The essential oil showed order-zero evaporation process. The method of determination the vapor pressure showed to be a good tool obtaining based on the calibration constant associated with the experimental system via thermogravimetric analysis. Therefore, we used thymol as a reference standard, obtaining = 3.20 × 1016 in SI. From the knowledge
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of this constant as well as the thermogravimetric analysis of the essential oil, it was possible to obtain its vapor pressure curve. This was verified by comparison of the vapor pressure curves obtained by this method with the curves reported in the literature constructed by Antoine equation for carvacrol. References [1] H. Lorenzi, F.J.A. Matos, Plantas medicinais no Brasil: nativas e exóticas, second ed., Instituto Plantarum de Estudos da Flora, Nova Odessa, 2008. [2] F.J.A. Matos, M.I.L. Machado, A.A. Craveiro, The essential oil composition of two chemotypes of Lippia alba grown in northeast Brazil, J. Essent. Oil Res. 8 (1996) 695–698. [3] U.P. Albuquerque, P.M. Medeiros, A.L.S. Almeida, J.M. Monteiro, E.M.F.L. Neto, J.G. Melo, J.P. Santos, Medicinal plants of the caatinga (semi-arid) vegetation of NE Brazil: a quantitative approach, J. Ethnopharmacol. 114 (2007) 325–354. [4] I.A. Neves, J.C.S. Oliveira, C.A.G. Camara, Chemical composition of the leaf oils of Lippia gracilis Schauer from two localities of Pernambuco, J. Essent. Oil Res. 20 (2008) 157–160. [5] S.S. Mendes, R.R. Bomfim, H.C.R. Jesus, P.B. Alves, A.F. Blank, C.S. Estevam, A.R. Antoniolli, S.M. Thomazzi, Evaluation of the analgesic and anti-inflammatory effects of the essential oil of Lippia gracilis leaves, J. Ethnopharmacol. 129 (2010) 391–397. [6] W.J. Silva, G.A.A. Dória, R.T. Maria, R.S. Nunes, G.A. Carvalho, A.F. Blank, P.B. Alves, R.M. Marc¸al, S.C.H. Cavalcanti, Effects of essential oils on Aedes aegypti larvae: alternatives to environmentally safe insecticides, Bioresour. Technol. 99 (2008) 3251–3255. [7] R.M. Neto, F.J.A. Matos, V.S.A. Andrade, M.C.N. Melo, C.B.M. Carvalho, S.B. Guimarães, O.D.L. Pessoa, S.L. Silva, S.F.R. Silva, P.R.L. Vasconcelos, The essential oil from Lippia gracilis Schauer, Verbenaceae, in diabetic rats, Rev. Bras. Farmacogn. 20 (2010) 261–266. [8] M.A. Botelho, N.A.P. Nogueira, G.M. Bastos, S.G.C. Fonseca, T.L.G. Lemos, F.J.A. Matos, D. Montenegro, J. Heukelbach, V.S. Rao, G.A.C. Brito, Antimicrobial activity of the essential oil from Lippia sidoides, carvacrol and thymol against oral pathogens, Braz. J. Med. Biol. Res. 40 (2007) 349–356. [9] A. Nostro, A.S. Roccaro, G. Bisignano, A. Mariono, M.A. Cannatelli, F.C. Pizzimenti, P.L. Cioni, F. Procopio, A.R. Blanco, Effects of oregano, carvacrol and thymol on Staphylococcus aureus and Staphylococcus epidermidis biofilms, J. Med. Microbiol. 56 (2007) 519–523. [10] M. Milos, J. Mastelic, I. Jerkovic, Chemical composition and antioxidant effect of glycosidically bound volatile compounds from oregano (Origanum vulgare L. ssp. hirtum), Food Chem. 71 (2000) 79–83. [11] A. Ahmad, A. Khan, F. Akhtar, S. Yousuf, I. Xess, L.A. Khan, N. Manzoor, Fungicidal activity of thymol and carvacrol by disrupting ergosterol biosynthesis and membrane integrity against Candida, Eur. J. Clin. Microbiol. Infect. Dis. 30 (2011) 41–50. [12] E. Daemon, R. Maturano, C.M.O. Monteiro, M.S. Goldner, T. Massoni, Acaricidal activity of hydroethanolic formulations of thymol against Rhipicephalus sanguineus (Acari: Ixodidae) and Dermacentor nitens (Acari: Ixodidae) larvae, Vet. Parasitol. 186 (2012) 542–545.
[13] M.A. Cremasco, N.P. Braga, Análise termogravimétrica do óleo essencial de pimenta longa (Piper hispidinervium C. DC), Acta Amazon. 41 (2011) 275–278. [14] M. Casserino, D.R. Blevins, R.N. Sanders, An improved method for measuring vapor pressure by DSC with automated pressure control, Thermochim. Acta 284 (1996) 145–152. [15] S.F. Wright, D. Dollimore, J.G. Dunn, K. Alexander, Determination of the vapor pressure curves of adipic acid and triethanolamine using thermogravimetry analysis, Thermochim. Acta 421 (2004) 25–30. [16] D.M. Price, Vapor pressure determination by thermogravimetry, Thermochim. Acta 367/368 (2001) 253–262. [17] D.M. Price, M. Hawkins, Calorimetry of two disperse dyes using thermogravimetry, Thermochim. Acta 315 (1998) 19–24. [18] A.B. Butrow, R.J. Seyler, Vapor pressure by DSC: extending ASTM E 1782 below 5 kPa, Thermochim. Acta 402 (2003) 145–152. [19] A.P.B. Gomes, F.D. Freire, C.F.S. Aragão, Determination of vapor pressure curves of warifteine and methylwarifteine by using thermogravimetry, J. Therm. Anal. Calorim. 108 (2012) 249–252. [20] V. Majer, M. Svoboda, J. Pick, Heats of Vaporizations of Fluids, Elsevier, Amsterdam, 1989. [21] National Institute of Standards and Technology, Chemistry Web Book, NIST Standard Reference Database Number 69, 2011, Available in http://webbook.nist.gov (accessed in 25.05.12). [22] K. Chatterjee, D. Dollimore, K.A. Alexander, New application for the Antoine equation in formulation development, Int. J. Pharm. 213 (2001) 31–44. [23] D. Dollimore, C. O’Connell, A comparison of the thermal decomposition of preservatives using thermogravimetry and rising temperature kinetics, Thermochim. Acta 324 (1998) 33–48. [24] K. Chatterjee, A. Hazra, D. Dollimore, K. Alexander, An evaporation study for phthalic acids – a rapid method for pharmaceutical characterization, J. Pharm. Sci. 91 (2002) 1156–1168. [25] P.J. Haines, Thermal Methods of Analysis: Principles, Applications and Problems, Blackie, London, 1995. [26] P. Phang, D. Dollimore, The calculation of the vapor pressures of antioxidants over a range of temperatures using thermogravimetry, Thermochim. Acta 367/368 (2001) 263–271. [27] K. Chatterjee, D. Dollimore, K. Alexander, Calculation of vapor pressure curves for hydroxy benzoic acid derivatives using thermogravimetry, Thermochim. Acta 392/393 (2002) 107–117. [28] A. Hazra, D. Dollimore, K. Alexander, Thermal analysis of the evaporation of compounds used in aromatherapy using thermogravimetry, Thermochim. Acta 392 (2002) 221–229. [29] A. Hazra, K. Alexander, D. Dollimore, A. Riga, Characterization of some essential oils and their key components: thermoanalytical techniques, J. Therm. Anal. Calorim. 75 (2004) 317–330. [30] Annual Book of ASTM Standards, Designation: E 1719-05, Standard Test Method for Vapor Pressure of Liquids by Ebulliometry, West Conshohocken, PA, USA, 2005. [31] Annual Book of ASTM Standards, Designation: E 1782-08, Standard Test Method for Determining Vapor Pressure by Thermal Analysis, West Conshohocken, PA, USA, 2008.