Molecular Dynamic Simulation of the Patchouli Oil Extraction Process

Dec 31, 2013 - ABSTRACT: Patchouli is a plant which is native in Malaysia. It is an ..... to Universiti Malaysia Pahang and the Ministry of Higher. Ed...
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Molecular Dynamic Simulation of the Patchouli Oil Extraction Process F. Adam,*,† Siti Hana A. B.,† Mashitah M. Yusoff,‡ and S. N. Tajuddin‡ †

Faculty of Chemical and Natural Resources Engineering, and ‡Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, 26300 Lebuhraya Tun Razak, Gambang, Kuantan, Pahang Darul Makmur, Malaysia ABSTRACT: Patchouli is a plant which is native in Malaysia. It is an economic crop, planted for its essential oil. Patchouli oil has a characteristic woody scent and is used commercially as an ingredient in fragrance and cosmetic products. The average yearly consumption around the globe is around one metric ton. A marker compound responsible for the patchouli oil scent is patchoulol (C15H26O). It is the major compound in patchouli oil representing around 40−50% of the essential oil composition. The aim of this study is to simulate the patchouli oil extraction process using patchoulol as a modeled molecule in different solvents, namely acetone, ethanol, and hexane. The simulation aim is to recognize molecular interaction between patchoulol molecules with solvent molecules through hydrogen bonding and also the repulsion forces between them due to the abundance of hydrogen atoms in the patchoulol molecule. The simulation is equilibrated under moles, volume, and energy followed by moles, pressure, and temperature ensembles via molecular dynamics simulation using the Material Studio software package. The interaction in the system is analyzed through the radial distribution function to describe the structure of patchoulol in solvent solution. The rdf trend found that the interaction between patchoulol solutes is through the oxygen atom (O1P) and hydrogen (H1P) atom from the hydroxyl functional group of the patchoulol molecule. In the acetone− patchoulol and hexane−patchoulol systems, the patchoulol solutes tend to self-agglomerate indicated by first neighboring molecules in the range of 4.25 Å and 5.75 Å, respectively, while the first neighboring molecules of patchoulol solutes in the binary ethanol−patchoulol system is located at 7.75 Å. This might suggest that the patchoulol is much more soluble in ethanol then in acetone and hexane. The pattern observed in the simulations is in agreement with extraction yield results obtained from the extraction experiment.

oxidation and 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging assay measurement, while the most recent study by Kiyoharaet al.12 has reported that the patchoulol showed potent in vitro antiviral activity against influenza virus A/PR/8/34 (H1N1). At present the patchouli plant is the only source for the patchoulol compound because there is no developed synthetic process to produce patchoulol for commercial purposes.3 In recent years, the application of molecular dynamic simulation to predict experimental theory has received interest from chemical engineers working to understand the molecular level interaction.13,14 Furthermore molecular simulations also can be used to calculate properties of the individual molecules, and they provide additional tools for observable characterization.15 Adam16 has applied molecular dynamics simulation to investigate any significant structure change in the crystallization solution which leads to the crystallization of 2,6-dihydroxybenzoic acid polymorph. The study16 found different hydrogen bonding types and degrees in the polar (chloroform) and nonpolar (toluene) solvent solutions, which lead to the nucleation of stable and metastable crystal forms. While

Malaysia has variety of native herb plants that produce essential oil of which the estimated value is around RM 200 million per year1 while the worldwide export value for essential oil in 2011 is US$ 16,483 billion.2 One of the herbs which is extracted for essential oil is the patchouli plant or Pogostenum cablin Benth. The patchouli essential oil has a pleasant and woody scent due to the 24 different sesquiterpene3 compositions. Besides being appreciated for the pleasant and woody scent, the patchouli oil has various applications such as an aid in the treatment for depression, an appetite depressant, antifungal aid, and insecticide.4−7 The insecticide properties of the patchouli oil was studied by Betty et al.,8 and it was suggested that patchouli oil has a neurotoxic mode of action toward Formosan Subterranean termites. The odor of patchouli essential oil is dependent on the composition of its marker compound. The marker compound in the patchouli oil is the sesquiterpene patchoulol (C15H26O), which is also the major compound at 40−50% of the composition.9 The patchoulol has been isolated from the patchouli essential oil for use in producing an expensive fragrance component, nor-patchoulenol.10 In addition, the patchoulol also has been used in various studies to investigate, in depth, a few of the patchouli properties such as the antiviral feature. Wei and Shibamoto11 have reported that patchoulol shows high antioxidant activity through inhibited hexanal © 2013 American Chemical Society

Received: December 15, 2012 Accepted: December 11, 2013 Published: December 31, 2013 183

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Gunther et al.17 used a similar approach to predict the optimum parameter to extract (−)-α-bisabolol, aescin, harpagoside, and stachyose in pure and modified supercritical carbon dioxide solvent. They17 suggested that solutes with heavy molecular weight may have a mass transfer resistance that affect the extraction yield. They17 also concluded that molecular dynamic simulation can be used to predict minimum requirement for the supercritical carbon dioxide modifier to extract both lipohilic and hydrophilic compounds. This paper aims to apply the molecular dynamic simulation technique to study the molecular interaction during the extraction of patchouli essential oil. The commercial patchouli essential oil is produced through a steam-distillation3 technique in which the high temperature may degrade the terpene compounds in the oil.18 Therefore, in this study, the solvent extraction method is the main interest to be simulated. As virtual laboratory, the simulation will be able to visualize the intermolecular interaction between solute and solvent molecules during the extraction process. In principle, the solvent that shows high interaction with the solute (patchoulol) shall produce a higher extraction yield of patchouli essential oil and higher patchoulol solubility. Three types of solvents are used in the extraction experiment, and simulation studies include acetone as the polar aprotic solvent, ethanol as the polar protic solvent, and hexane as nonpolar solvent. According to Suchoki19 and Kolǎŕ et al.20 the specific functional group such as hydroxyl will produce specific interactions such as a hydrogen bond that has a significant effect on the structure of the mixture and the solute solubility. This interaction was believed by Pehlivanoglu et al.21 to produce a higher distribution of polar solvent molecules which will affect the extraction mechanism. Among the chosen solvents, ethanol has been reported by Gironi et al.22 to be capable of reducing the oxidizing reactions and improving the essential oil aromatics and stability characteristics. Mostly the atoms of interest in most of the molecules are the atoms which can contribute to hydrogen bonding. Hydrogen bonding is classified as the strongest intermolecular interaction.19,23 The labeled solute and solvent molecules are shown in Figure 1, and the simulation parameters are tabulated in Table 1. In a nonpolar hexane

system, the chosen atom to be referred is based on the work of Kioupis et al.24 who have simulated the pure hexane system. Table 1. Simulation and Input Parameters to Represent the Patchouli Oil Extraction Process at 298 K and 0.0001 MPa (Yaws25 and Chemspider Database26)

system acetone ethanol hexane patchoulol

box size, A × B × C

g/cm3

Å3

Pure System 0.793 0.780 0.658 1.000 Binary System 1000:20 0.804 1000:20 0.795 1000:20 0.668

1000 1000 1000 50

acetone/patchoulol ethanol/patchoulol hexane/patchoulol a

no. of molecules

densitya

49.556 46.116 60.125 26.421

× × × ×

49.556 46.116 60.125 26.421

× × × ×

49.556 46.116 60.125 26.421

50.537 × 50.537 × 50.537 47.244 × 47.244 × 47.244 60.838 × 60.838 × 60.838

Density of the mix was calculated as follows: ρmix =

MWsolvent + MWsolute (MWsolvent/ρsolvent ) + (MWsolute/ρsolute )

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METHODS Extraction Experiment. The solvent extraction method is used to extract the patchouli essential oil from samples which originate from Syarikat Nilam Suling Sdn Bhd, Miri, Sarawak, Malaysia. Acetone, ethanol, and n-hexane as the solvents were supplied by Merck (99.7% purity). A 10 g sampling of patchoulol leaves was soaked in 100 mL of solvent and left overnight before undergoing the separation process using the Buchi Evaporator (R100), and then the essential oil yield was calculated using eq 1. The extraction experiments were replicated three times for each solvent. yield (%) =

essential oil wt (g) 100 sample wt (g)

(1)

GC−MS Analysis. The essential oils were characterized using an Agilent 7890A Network System gas chromatography, which was attached to a mass spectrometer (Agilent 5975C) with detector in full scan mode under electron impact ionization (EI, 70 eV). The GC−MS is fitted with a capillary column (DB-1MS, i.d. ≈ 0.25 mm; film thickness, ≈ 0.25 μm) with temperatures of the injector and detector set at 250 °C. Each sample was diluted in n-hexane, and 1 μL of this solution was injected in the split mode (ratio 1:20) using helium as carrier gas (1 mL/min). The resulting mass spectra will be compared with the internal mass spectra library to identify the individual compounds. Confirmation of identity was done by comparing the retention indices with those in the National Institute of Standards Technology (NIST) library. Simulation Details. The simulations study of the patchouli extraction process was carried out through the molecular dynamic simulation technique in Acceryls Materials Studio (MS),27 version 5.5, using a HP Z400 workstation. Initially, each of solute and solvent molecules underwent the geometry optimization step prior to the creation of the simulation boxes. In this study the condensed-phase optimized molecular potentials for atomistic simulation studies28 [COMPASS] force field was used to model the system.

Figure 1. Schematic labeling of patchoulol (a), acetone (b), ethanol (c), and hexane (d) molecular structure. 184

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Sun (1998) 28 detailed the bonded and nonbonded interaction model of the COMPASS force field. The van der Waals part of the nonbond interaction is modeled using the Lennard-Jones (LJ) 9-6 function which is softer to the repulsion region compared to LJ 12-6 function as per eq 2, whereas eq 3 represents the Coulombic part which models the electrostatics of the nonbond interaction. ⎡ ⎡ R ⎤9 ⎡ R ⎤6 ⎤ E VDW = Do⎢2⎢ o ⎥ − 3⎢ o ⎥ ⎥ ⎣ R ⎦ ⎥⎦ ⎢⎣ ⎣ R ⎦

(2)

⎡ qiqj ⎤ ECol = ⎢ ⎥ ⎣ εR ⎦

(3)

Figure 2. Comparison of rdf for O1A···O1A in pure acetone liquid system from this study and Liang et al.30 work using the COMPASS and OPLS force field, respectively.

where D is the energy parameter, R is the radius, C is the unit conversion factor, ε is the relative dielectric constant, and q is the partial charges. The COMPASS force field is suitable in the simulation of organic molecules, inorganic gas molecules, and common polymers.28 The simulation work employed the Verlet velocity algorithm integrator with an atom-based summation method as the cutoff method for long-range interactions. The cutoff used for both van der Waals and electrostatics is 15.5 Å with cubic spline truncation. The atom-based summation method is a simple direct method to calculate the long-range nonbond interaction, where the interaction beyond the cut off is ignored. This cheaper calculation mode is used in this study to reduce the simulation time period. The simulation began with equilibration of the system under the constant number of moles, volume, and energy (NVE) ensemble for 100 ps. After the equilibration, the systems were run in the NPT ensemble which is constant number of moles, pressure, and temperature for a total simulation time of 5 ns with 1 fs stepsize. In the NPT ensembles, the pressure of all systems is controlled at 1 atm by coupling the system with a Berendsen barostat with a decay constant of 0.1 ps, whereas a Nose thermostat is used for the temperature control with a fictitious mass, Q, ratio of 1.0. The simulation temperature is at 298 K to represent the extraction experiment which was carried out at room temperature. The trajectory file produced from the dynamic simulation was analysed through calculation of the rdf which is a structural property that can be correlated to the probability of finding the nearest neighbor atom, gxy (r). This probability can be described in the following expression.16,29 gxy(r ) =

is carried out through comparison of the O···O interaction between this study with Liang et al.30 and Saiz et al.31 data, respectively. Both graphs show the difference in the rdf pattern. The difference in rdf trend might be caused by the different force field and summation method used and number of molecules simulated. Liang et al.30 simulated 512 acetone molecules using the OPLS [optimized potentials for liquid simulations] force field, and Saiz et al.31 simulated 125 ethanol molecules up to 10 ns with the OPLS force field and applied the Ewald summation method for the electrostatic interaction. The OPLS force field is suitable for simulation of the organic and biomolecular systems.28 Meanwhile this study applied the COMPASS force field in the simulation of 1000 acetone and ethanol molecules for 5 ns with the atom based-summation method for both van der Waals and electrostatic interaction. Atom-based summation requires less expensive calculations compared to the Ewald summation in the Material Studio Package. A literature32 concluded that the “atom-based summation method is an inferior cutoff method” which is not suitable to calculate the electrostatic interaction due to large errors from forces discontinuity. In addition, a literature33 evaluation of the Ewald summation method has concluded that if the number of molecules N ≤ 1000, the standard Ewald summation method is efficient to describe the electrostatic interaction in a polar molecule such as acetone and ethanol. This would explain the difference of the rdf pattern in Figure 2 and the intensity of g(r) obtained in Figure 3. The validation of the use of the COMPASS force field for the pure hexane structure is shown in Figure 4. The CH3···CH3

⟨Ny(r , r + dr )⟩ ρy 4πr 2dr

([4])

where r is the spherical radius, ρy is the density of the y atom, Ny (r, r + dr) is the number of y atoms in a shell of width Δr at distance r, and x is the reference atom.

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RESULT AND DISCUSSION Simulation Validation. The analyzed rdf of pure acetone, ethanol, and hexane liquid solvent is validated and compared to the literature data.24,30,31 The purpose of validation is to check that the force field is able to work well in reproducing the rdf property in such a pure system, and therefore the force field should work well in the binary system. Both Figure 1 and Figure 2 show the validation for the pure acetone and ethanol simulation, respectively, through comparison with literature data available. The validation for both pure acetone and ethanol

Figure 3. Comparison of O1E···O1E in pure ethanol liquid system from this study and Saiz et al.31 work using COMPASS and OPLS force field, respectively. 185

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Table 2. Simulation Results for Patchouli Oil Extraction Process average simulated density system pure acetone pure ethanol pure hexane pure patchoulol binary: acetone− patchoulol binary: ethanol− patchoulol binary: hexane− patchoulol

Figure 4. Comparison of CH3···CH3 interaction in pure hexane liquid system from this study and Kioupis et al.24 work using COMPASS and TraPPE force field, respectively, which shows a satisfactory agreement in rdf.

average simulated temperature

average simulated pressure

g/cm3

K

atm

0.795 0.804 0.695 0.952 0.807

297.993 316.0 298.0 298.0 297.998

270.0 2187.0 1.0 1.0 214.0

0.801

305.4

2023.2

0.706

298.0

1.0

In this work, the solute−solute interaction between O1P and H1P atom as the reference intermolecular interaction which represents the hydrogen bonding for pure patchoulol and solvent-patchoulol systems is the main interest for the potential of solubility control. The rdf pattern in Figure 5 follows the

interaction of this study produced a similar pattern to the rdf pattern in the work of Kioupis et al.,24 even with different total simulation time, number of molecules, and force field used. Kioupis et al.24 ran the molecular dynamic in the range of 2−5 ns with the transferable potentials for phase equilibria (TraPPE) force field. They simulated a range of molecules number of hexane from 20 to 200 molecules but did not further explain the method used to describe the long-range interaction. The TraPPE force field can describe the properties of linear alkanes through the optimization of the LJ methyl and methylene parameters.34 Similar to pure acetone and ethanol systems, this study simulated 1000 hexane molecules using the COMPASS force field with the atom-based summation method to calculate the nonbond interaction with a total simulation time of 5 ns. The similar rdf pattern between this study and Kioupis et al.,24 may suggest that the simulation of pure hexane has a minimal long-range interaction effect toward the structure produced and does not require the Ewald summation method to model the long-range interaction in a nonpolar system. Comparisons of validation graphs indicate that polar molecules such as ethanol and acetone require a more accurate summation method to reproduce an accurate rdf pattern. Nevertheless the rdf pattern for both pure acetone and pure ethanol is still reliable to describe the position of the neighboring atom r, especially the ethanol rdf pattern. Kim et al.35 also produced a similar rdf pattern for the carbon (polysulfon membrane)−oxygen (water) interaction. Simulation Results. The comparison of simulated data and the setting parameters are tabulated in Table 2. Pure hexane and binary hexane−patchoulol systems have shown the highest deviation from the setting value of density with 5% absolute error. However the percentage error is still in an acceptable range such as reported by Sun.28 His work simulated about 150 organic structures using COMPASS force field with a maximum absolute error of 6% for the simulated density. The reason for the deviation in the simulated temperature and pressure for systems which contain ethanol and acetone molecules might because of the simple calculation of the atom-based summation method. These findings are similar in Kim et al.36 in their simulation studies of benzene, toluene, and p-xylene. They have suggested that the calculation using the atom-based summation method may not have properly addressed the polarization issue in ethanol and acetone which may affect the molecular movement as written by Leach.37

Figure 5. The solute−solute interaction in the pure patchoulol system and binary systems with O1P and H1P as reference hydrogen bond interaction.

solid phase rdf trend because the isolated patchoulol preferably exists as a solid crystal at room temperature.3 Therefore the presence of these kind of interactions indicates a solid phase trend. It shows clearly that the O1P···H1P interaction in binary ethanol−patchoulol shifted to 7.75 Å from 1.75 Å in the pure patchoulol system. The change reflects a significant effect to the solute−solute interaction with the presence of solvent molecule in the binary system. The change may indicate that the solubility of patchoulol in the solvent occurs through the breaking of O1P···H1P hydrogen bonding. Therefore it is expected that ethanol will extract the highest yield of patchouli oil followed by hexane and acetone from the solvent extraction experiments. The stronger interaction between ethanol and patchoulol may occur because both patchoulol and ethanol molecules have an O atom and H atom (O1E···H1P and O1P···H1E) which can establish a higher number of degree of hydrogen bond (synthons) (Figure 6), while for the hexane−patchoulol interaction, only the London dispersion forces can be established, which are usually found between polar and nonpolar molecules and the London dispersion force has been classified as the weakest intermolecular force.38 Never186

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Figure 6. The solvent−solute intermolecular interaction in a binary acetone−patchoulol system, ethanol−patchoulol system, and hexane− patchoulol system with O and H atom from both solvent and solute as reference atom.

Figure 7. The repulsion phenomena in a binary acetone−patchoulol system and ethanol−patchoulol system.

specification as suggested by Singh et al.39 Beside, GC−MS data also suggest that the solvent extraction method can produce a higher quality of patchouli essential oil compared to the hydrodistillation technique. The patchoulol compound then can be further isolated for the production of hydroxypatchoulol, which has similar odor to a valuable fragrance, norpatchoulenol.9 Besides the patchoulol compound, the analysis detected α-patchoulene, β caryophyllene oxide, caryophyllene, spathulenol, and three new unknown compounds. In this study, both simulation work (molecular properties) and extraction experiment (bulk properties) complement and link to each other.

theless, this kind of interaction still plays a significant effect on the solute−solute interaction as revealed in Figure 5 (binary hexane−patchoulol system). As a polar aprotic solvent, acetone can act as a hydrogen-bond acceptor. However the repulsion interaction of the O1A···O1P presence has weakened the O1A···H1P hydrogen bonding in acetone solution. The repulsion force presence in the binary ethanol−patchoulol system through the O1E···O1P interaction indicates less structure and a weak interaction pattern. To further confirm this simulation results, an experimental work using a similar solvent type has been carried out. The extracted essential oil is then analyzed by the GC−MS to confirm the percentage of patchoulol presence in the oil as the key component to determine the oil quality.39 The extraction yield and percentage area calculated by GC−MS for each sample being extracted by different solvents are summarized in Table 3. Interestingly, the extraction experiment is in

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CONCLUSION The molecular dynamic simulation approach has successfully described the structural property of patchoulol solute in the application of the solvent extraction process. A higher yield of extraction in ethanol has been recognized due to the hydrogen bonding arrangement (synthons) between the solute−solvent molecules. The synthons is able to increase the solute solubility in ethanol compared that in hexane and acetone. The finding is useful for a chemical engineer to obtain insight at the molecular scale for controlling the extraction process parameters. This study should be further extended to understand other process parameters such as temperature, pressure, and concentration on the structure and synthons of the extraction process. The molecular diffusivity also should be measured to describe mass transfer behavior during the extraction process. The Ewald summation method is suggested to improve the model of structural properties in the polar solvent system.

Table 3. The Extraction Experiment Yield for Patchouli Oil Extracted Using the Solvent Extraction Method average patchouli oil yield

average patchoulol identified by GC− MS

solvent

wt/wt %

% area

acetone ethanol hexane

8.34 30.99 13.15

58.04 66.17 64.55

agreement to the simulation results pattern in which ethanol has produced the highest oil yield and acetone produced the lowest oil yield. A stronger molecular interaction between patchoulol and ethanol molecules has attracted more patchoulol molecules to diffuse out from the leaves matrix into the solvent, while the lower ability of acetone to extract the patchouli essential oil result is probably due to the presence of O1A···O1P repulsion between acetone and patchoulol molecules. The repulsion has a significant effect toward the solute−solvent interaction such as seen in Figure 7. In the hexane−patchoulol system, the interaction of O1P−H3C or London dispersion is responsible for the higher extraction of patchouli oil compared to that in acetone. It may be concluded that the repulsion between the abundant hydrogen atoms in hexane and the patchoulol molecules is not as strong as the O1A···O1P repulsion in the acetone−patchoulol system. In Table 3, GC−MS has detected a high percentage of patchoulol composition in all of patchouli oil samples. The range indicates that the oil samples meet the market

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +609-549 2824. Fax: +609-549 2889. Funding

The authors would like to express high appreciation for support to Universiti Malaysia Pahang and the Ministry of Higher Education, Malaysia, through Fundamental Research Grant Scheme (FRGS - UMPRDU 110105). Notes

The authors declare no competing financial interest.

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dx.doi.org/10.1021/je3013292 | J. Chem. Eng. Data 2014, 59, 183−188