Measurement and Prediction of Isothermal Vapor-Liquid Equilibrium and Thermodynamic Properties of a Turpentine + Rosin System Using the COSMO-RS Model.

The vapor-liquid equilibrium (VLE) of components of a turpentine + rosin system were measured at 313.2 and 333.2 K using headspace gas chromatography. The thermodynamic properties of the turpentine + rosin system such as activity coefficients, total pressure, partial pressure, excess Gibbs energies, and excess enthalpies were calculated using the COSMO-RS model. The results showed that the activity coefficients were greater than 1 for all components of turpentine except for longifolene, indicating a positive deviation from Raoult's law for all components of turpentine except for longifolene. The total pressures were about 1 kPa at 313.2 K and about 3 kPa at 333.2 K. Meanwhile, the excess Gibbs energies G E and excess enthalpies H E of the system were positive, indicating that the mixing of the components of turpentine and rosin was endothermic. Moreover, the hydrogen bonding interaction energy H E(hydrogen bonding) contributed the most for the excess enthalpies H E.

Introduction

Pine oleoresin, a highly viscous resin secreted from the resinous tract of pine trees,[1] is composed of turpentine (mono- and sesquiterpenes) and rosin (diterpenes). Rosin is a nonvolatile solid mixture with a higher boiling point. The main components of rosin are resin acids.[2,3] Rosin is widely used in materials, chemicals, and chemical engineering because of its excellent properties such as anticorrosion, moisture resistance, insulation, adhesion, and emulsification.[4−8] Turpentine mainly consists of monoterpenes and sesquiterpenes. The main components of monoterpenes are α-pinene, camphene, β-pinene, limonene, and p-cymene, which are easily volatile; the main component of sesquiterpenes is longifolene. Turpentine is applied widely in the synthesis of various fine chemicals, such as spices, medicine, and additives.[9−13] Since these pine oleoresin derivatives have various applications in chemical industries, higher oleoresin yields are economically desirable.

Currently, pine oleoresin is often extracted by tapping living trees with the “debarking” (bark-peeling) method, in which bark removal is performed periodically, allowing pine oleoresin to be secreted and collected. However, pine oleoresin tapping is open-ended. As the easily volatile components of turpentine, such as α-pinene, camphene, β-pinene, limonene, and p-cymene, are constantly evaporating into the atmosphere, about half of the turpentine yield is lost. Would the components of rosin (such as abietic acid, palustric acid, neoabietic acid, pimaric acid, sandopimaric acid, dehydroabietic acid, and isopimaric acid) have any effect on the interaction between the components of turpentine (such as α-pinene, camphene, β-pinene, limonene, p-cymene, and longifolene) and thus affect their volatilization? Therefore, vapor–liquid equilibrium (VLE) data of components of turpentine and rosin is worth investigating.

To date, some VLE data of the aforementioned components of turpentine have already been reported. The isobaric VLE data of an α-pinene + β-pinene system was obtained by Wang et al.[14] Sun et al.[15] reported the vacuum VLE data of an α-pinene + β-pinene + p-cymene system at 53.5 and 80 kPa. Ruan et al.[16] provided the isobaric VLE data for a pinane + α-pinene + longifolene system. In their work, the activity coefficients of each component in each binary system were derived, and the excess Gibbs energy and excess enthalpy were obtained. Wu et al.[17] reported the isobaric VLE data of binary systems with camphene, (+)-3-carene, (−)-β-caryophyllene, p-cymene, and α-pinene at 101.33 kPa. They found calculated values by NRTL, Wilson, and UNIQUAC models that were in good agreement with the experimental VLE data. However, fewer reports providing VLE data of systems containing components of rosin were found. Since both turpentine and rosin contain many components which are not easily separated, VLE data for the turpentine + rosin system measured by the usual method of Ellis and Rose using a double-cycle equilibrium kettle seemed difficult. Therefore, the headspace gas chromatography (HSGC) method is used for the determination of VLE because this method has several advantages such as short VLE time, low dosage of reagents, and accurate temperature control.[18−21]

In this work, the isothermal VLE of components of the turpentine + rosin system were measured using the HSGC method at 313.2 and 333.2 K. The thermodynamic properties such as activity coefficients, total pressure, partial pressure, excess Gibbs energies, and excess enthalpies of the system were calculated by the COSMOtherm software based on the COSMO-RS model to provide more basic data for the turpentine + rosin system.

Results and Discussion

Applicability of the COSMO-RS Model

In order to investigate the applicability of the COSMO-RS model to the system in this work, the isothermal VLE data of a β-pinene (1) + p-cymene (2) system in the literature[22] were selected and compared with the values calculated using the COSMO-RS model in this work, as shown in Figure . The average absolute percentage deviation (AAPD) was calculated by eq .where T is the temperature calculated by COSMO-RS and T is the temperature from the literature.[22]

Figure 1

Comparison of calculated and literature data[22] for T–x of the β-pinene (1) + p-cymene (2) system at 100.7 kPa.

The obtained AAPD was 0.04%, indicating that the calculated data with the COSMO-RS model were in good agreement with the literature data. Thus, the COSMO-RS model can be applied to predict the VLE data for the turpentine + rosin system in this study.

Vapor–Liquid Equilibrium Results

The isothermal VLE data were experimentally determined via the HSGC method. In order to verify the reliability of the HSGC method used in this work, the isothermal VLE data of the citral (1) + linalool (2) system at 333.2 K were measured and the experimental data were compared with the literature data.[23] In Figure , the calculated AAPD was 0.85%, indicating that the HSGC method is reliable.

Figure 2

Comparison of experimental and literature data[23] for P–x–y of the citral (1) + linalool (2) system at 333.2 K.

The isothermal VLE data of α-pinene (1), camphene (2), β-pinene (3), p-cymene (4), limonene (5), longifolene (6), and abietic acid + palustric acid + neoabietic acid + pimaric acid + sandopimaric acid + dehydroabietic acid + isopimaric acid (7) at 313.2 and 333.2 K are listed in Table . As shown, with an increasing mole fraction of the components of rosin, the mole fraction of the components of turpentine, such as α-pinene, camphene, β-pinene, p-cymene, and limonene in the liquid phase gradually decreased. However, the mole fraction of the less volatile component longifolene gradually increased. In contrast, in the vapor phase, the mole fraction of α-pinene increased while the other components showed a decreasing trend. Moreover, comparing the two temperatures, it was found that the higher the temperature was, the faster the mole fraction of each component increased or decreased. The isobaric data for α-pinene (1) + β-pinene (3),[14] α-pinene (1) + p-cymene (4),[15] α-pinene (1) + longifolene (6),[16] and α-pinene (1) + camphene (2)[17] have already been reported. However, the pressure and temperature range measured in this work were beyond that available in the literature. Thus, x–y data comparisons with the literature and this work were not possible.

Table 1

VLE Data of α-Pinene (1), Camphene (2), β-Pinene (3), p-Cymene (4), Limonene (5), Longifolene (6), and Abietic Acid + Palustric Acid + Neoabietic Acid + Pimaric Acid + Sandopimaric Acid + Dehydroabietic Acid + Isopimaric Acid (7) Of Turpentine + Rosin System at 313.2 and 333.2 Ka

x7	x1’	x2’	x3’	x4’	x5’	x6’	y1	y2	y3	y4	y5	y6
T/K = 313.2
0.010	0.844	0.017	0.077	0.018	0.021	0.013	0.883	0.016	0.062	0.014	0.023	0.003
0.020	0.835	0.016	0.076	0.016	0.021	0.015	0.885	0.016	0.062	0.013	0.023	0.002
0.030	0.825	0.016	0.075	0.016	0.021	0.017	0.888	0.016	0.060	0.012	0.022	0.002
0.040	0.816	0.016	0.074	0.015	0.020	0.019	0.888	0.016	0.060	0.012	0.021	0.002
0.050	0.805	0.016	0.072	0.015	0.020	0.022	0.895	0.016	0.057	0.011	0.019	0.001
0.060	0.797	0.015	0.071	0.014	0.020	0.024	0.896	0.016	0.057	0.011	0.019	0.001
0.070	0.786	0.015	0.070	0.013	0.019	0.027	0.898	0.016	0.055	0.011	0.019	0.001
0.080	0.775	0.015	0.068	0.013	0.019	0.030	0.899	0.016	0.055	0.011	0.019	0.001
0.090	0.765	0.015	0.067	0.013	0.019	0.032	0.900	0.016	0.055	0.010	0.018	0.001
0.100	0.755	0.014	0.066	0.012	0.018	0.034	0.902	0.016	0.054	0.010	0.018	0.001
0.110	0.745	0.014	0.065	0.012	0.018	0.036	0.902	0.016	0.054	0.010	0.018	0.001
0.120	0.734	0.014	0.064	0.011	0.018	0.040	0.904	0.016	0.054	0.009	0.017	0.001
T/K = 333.2
0.010	0.843	0.017	0.077	0.016	0.021	0.016	0.892	0.014	0.058	0.014	0.020	0.002
0.020	0.834	0.016	0.076	0.016	0.021	0.018	0.894	0.014	0.058	0.013	0.019	0.002
0.030	0.824	0.016	0.075	0.015	0.021	0.019	0.898	0.014	0.058	0.010	0.018	0.002
0.040	0.815	0.016	0.073	0.015	0.020	0.020	0.899	0.014	0.057	0.010	0.018	0.001
0.050	0.805	0.016	0.073	0.015	0.020	0.022	0.899	0.014	0.057	0.010	0.018	0.001
0.060	0.793	0.016	0.072	0.015	0.020	0.026	0.901	0.014	0.057	0.010	0.018	0.001
0.070	0.783	0.015	0.069	0.013	0.019	0.032	0.900	0.014	0.056	0.010	0.018	0.001
0.080	0.773	0.015	0.068	0.013	0.019	0.033	0.901	0.014	0.056	0.010	0.018	0.001
0.090	0.764	0.014	0.067	0.012	0.019	0.034	0.902	0.014	0.056	0.010	0.018	0.001
0.100	0.754	0.014	0.067	0.012	0.019	0.036	0.902	0.014	0.056	0.010	0.017	0.001
0.110	0.745	0.014	0.065	0.011	0.018	0.037	0.903	0.014	0.056	0.010	0.017	0.001
0.120	0.734	0.014	0.064	0.011	0.018	0.041	0.905	0.014	0.055	0.009	0.017	0.001

Standard uncertainties u are u(T) = 0.1 K, u(x1) = 0.0021, and u(y1) = 0.0023.

Activity Coefficients

The activity coefficient is related to the affinity between solutes and solvents, which is caused by differences in strength and properties between molecules. These differences were caused by local pairwise interactions of surface segments due to electrostatic dislocation, hydrogen bonding, and van der Waals dispersive forces.[24] The activity coefficients of α-pinene (1), camphene (2), β-pinene (3), p-cymene (4), limonene (5), and longifolene (6) of the turpentine + rosin system calculated using COSMO-RS at 313.2 and 333.2 K are presented in Table . As shown in Table , the activity coefficients decreased as the temperature increased. Moreover, when the mole fraction of the components of rosin increased, the activity coefficients of α-pinene, camphene, β-pinene, p-cymene, and limonene showed a gradually increasing trend but that of longifolene gradually decreased. It can also be seen from Table that the activity coefficients were greater than 1 for all components of turpentine except for longifolene (6), indicating a positive deviation from Raoult’s law for all components of turpentine except for longifolene, which is attributed to strong attractive forces in the liquid phase between the components of turpentine. It means that, under the conditions of a mixture of volatile components, for α-pinene, camphene, β-pinene, p-cymene, and limonene, the intermolecular forces of their mixture are less than those of the pure one, as described by Prausnitz et al.[25]

Table 2

Activity Coefficients of α-Pinene (1), Camphene (2), β-Pinene (3), p-Cymene (4), Limonene (5), and Longifolene (6) of the Turpentine + Rosin System at 313.2 and 333.2 Ka

x1’	γ1	γ2	γ3	γ4	γ5	γ6
T/K = 313.2
0.844	1.002	1.001	1.002	1.122	1.029	0.994
0.835	1.005	1.003	1.003	1.120	1.029	1.000
0.825	1.008	1.006	1.005	1.119	1.029	1.008
0.816	1.012	1.009	1.008	1.119	1.031	1.016
0.805	1.017	1.013	1.012	1.119	1.033	1.024
0.797	1.022	1.017	1.016	1.121	1.036	1.033
0.786	1.027	1.021	1.020	1.123	1.039	1.042
0.775	1.033	1.025	1.024	1.125	1.042	1.052
0.765	1.038	1.030	1.028	1.127	1.046	1.061
0.755	1.044	1.035	1.033	1.130	1.050	1.071
0.745	1.050	1.040	1.038	1.133	1.054	1.081
0.734	1.056	1.045	1.043	1.136	1.058	1.091
T/K = 333.2
0.843	1.001	1.001	1.002	1.110	1.026	0.991
0.834	1.003	1.002	1.002	1.106	1.025	0.997
0.824	1.006	1.004	1.004	1.104	1.025	1.003
0.815	1.009	1.006	1.006	1.103	1.025	1.009
0.805	1.013	1.009	1.008	1.102	1.026	1.016
0.793	1.017	1.012	1.011	1.103	1.028	1.024
0.783	1.021	1.015	1.014	1.104	1.030	1.032
0.773	1.025	1.019	1.017	1.105	1.032	1.040
0.764	1.030	1.022	1.021	1.106	1.035	1.048
0.754	1.035	1.026	1.025	1.107	1.038	1.056
0.745	1.040	1.030	1.028	1.109	1.041	1.065
0.734	1.044	1.035	1.032	1.111	1.044	1.074

Standard uncertainties u, u(γ1) = 0.0051.

Total Pressure and Partial Pressure

The total pressure and partial pressure of α-pinene (1), camphene (2), β-pinene (3), p-cymene (4), limonene (5), and longifolene (6) of the turpentine + rosin system calculated using COSMO-RS are shown in Table . As shown in Table , the total and partial pressure were increased as the mole fraction of the rosin components and the temperature increased. The total pressure was about 1 kPa at 313.2 K and 3 kPa at 333.2 K. Moreover, α-pinene and β-pinene have the highest contribution to the total pressure since their molar fractions in the initial prepared samples were greater than those in the other components of turpentine.

Table 3

Total Pressure and Partial Pressure of α-Pinene (1), Camphene (2), β-Pinene (3), p-Cymene (4), Limonene (5), and Longifolene (6) of the Turpentine + Rosin System at 313.2 and 333.2 Ka

x1’	P/kPa	P1/kPa	P2/kPa	P3/kPa	P4/kPa	P5/kPa	P6/kPa
T/K = 313.2
0.844	1.176	1.062	0.018	0.075	0.010	0.011	0.000
0.835	1.166	1.054	0.018	0.074	0.009	0.011	0.000
0.825	1.156	1.046	0.017	0.073	0.009	0.011	0.000
0.816	1.147	1.038	0.017	0.072	0.009	0.011	0.000
0.805	1.136	1.029	0.017	0.071	0.008	0.010	0.000
0.797	1.128	1.023	0.017	0.070	0.008	0.010	0.000
0.786	1.118	1.015	0.016	0.069	0.008	0.010	0.000
0.775	1.108	1.006	0.016	0.068	0.007	0.010	0.001
0.765	1.098	0.998	0.016	0.067	0.007	0.010	0.001
0.755	1.090	0.990	0.016	0.066	0.007	0.010	0.001
0.745	1.080	0.983	0.016	0.065	0.007	0.010	0.001
0.734	1.069	0.974	0.015	0.064	0.006	0.009	0.001
T/K = 333.2
0.843	3.206	2.888	0.049	0.208	0.027	0.033	0.001
0.834	3.176	2.863	0.048	0.205	0.027	0.032	0.001
0.824	3.145	2.837	0.047	0.202	0.026	0.032	0.001
0.815	3.120	2.816	0.047	0.200	0.025	0.031	0.001
0.805	3.092	2.790	0.046	0.198	0.025	0.031	0.001
0.793	3.056	2.758	0.046	0.196	0.024	0.031	0.002
0.783	3.020	2.733	0.044	0.189	0.022	0.031	0.002
0.773	2.996	2.712	0.043	0.187	0.021	0.030	0.002
0.764	2.972	2.691	0.043	0.186	0.021	0.030	0.002
0.754	2.946	2.668	0.043	0.184	0.020	0.029	0.002
0.745	2.922	2.649	0.042	0.180	0.019	0.029	0.002
0.734	2.890	2.622	0.041	0.177	0.019	0.029	0.003

Standard uncertainties u, u(P)= 0.0089 kPa.

Excess Gibbs Energies and Excess Enthalpies

The excess Gibbs energies GE and excess enthalpies HE of the system were calculated using COSMO-RS as shown in Table . As can be seen, GE and HE of the system were positive, indicating that the mixing process of the components of turpentine and rosin was endothermic. Moreover, both the GE and HE values increased with the decreasing mole fraction of turpentine components and increasing temperature. This may be attributed to the spatial effect. Since the components of rosin with a tricyclic phenanthrene skeleton are larger than the components of turpentine, with the mole fraction of the components of rosin increasing, the spatial effect makes the mixing of more rosin and the components of turpentine take more heat. Similar spatial effects are observed for mixtures of p-cymene + β-caryophyllene + 3-carene reported by Yao et al.[26]

Table 4

Excess Gibbs Energies and Excess Enthalpies of the Turpentine + Rosin System at 313.2 and 333.2 K

x1’	HE/(J/mol)	HE(MF)/(J/mol)	HE(HB)/(J/mol)	HE(VDW)/(J/mol)	GE/(J/mol)
T/K = 313.2
0.844	139.687	66.868	73.230	–0.411	52.484
0.835	252.077	109.193	143.621	–0.736	91.372
0.825	344.371	144.623	200.509	–0.761	126.443
0.816	421.626	175.008	248.069	–1.451	158.291
0.805	500.745	201.564	300.718	–1.538	187.366
0.797	556.500	224.904	333.831	–2.234	213.990
0.786	607.553	245.866	364.592	–2.905	238.610
0.775	652.879	264.752	391.701	–3.574	261.415
0.765	693.206	281.775	415.676	–4.245	282.523
0.755	729.123	297.129	436.922	–4.928	302.054
0.745	761.294	311.069	455.805	–5.581	320.182
0.734	790.429	323.875	472.710	–6.156	337.130
T/K = 333.2
0.843	159.984	65.078	95.287	–0.381	45.268
0.834	273.442	109.734	164.436	–0.728	79.733
0.824	369.570	148.036	222.487	–0.953	111.133
0.815	442.107	181.450	262.219	–1.562	139.943
0.805	514.904	211.135	305.507	–1.738	166.544
0.793	578.999	237.748	343.631	–2.381	191.140
0.783	635.750	261.505	377.321	–3.075	213.783
0.773	686.354	283.000	407.221	–3.868	234.950
0.764	731.748	302.485	433.905	–4.641	254.621
0.754	772.651	320.216	457.812	–5.377	272.932
0.745	809.532	336.402	479.220	–6.090	290.010
0.734	843.303	351.370	498.625	–6.692	306.051

As shown in Table , HE(VDW) is negative, indicating that the interactions between homogeneous molecules are smaller than that between heterogeneous molecules. This is because of the heterogeneity between the components of turpentine and rosin, which is confirmed by the σ-profiles analysis. According to the distribution of σ-profiles in Figure , the surface shielding charges of the components of turpentine were distributed in the weak polar region of [−0.01,+0.01] e/Å2, indicating that the components of turpentine were weakly polar molecules. Obviously, two broad peaks were observed in the [−0.02, – 0.01] e/Å2 hydrogen bond donor region, which were attributed to hydroxyl oxygen for the components of rosin, while one broad peak in the hydrogen bond acceptor region of [+0.005, + 0.015] e/Å2 was attributed to carbonyl, reflecting the fact that the components of rosin were the polar compounds with both hydrogen bond donors and hydrogen bond acceptors.

Figure 3

σ-Profiles of components of (A) turpentine and (B) rosin.

Moreover, as can be seen from Table , HE(HB) contributes most to the excess enthalpies HE, which is probably due to the fact that the oxygen atom of the carboxyl group in the components of rosin can be considered as a hydrogen bond acceptor and can readily form hydrogen bonds with the methyl and methylene groups in the components of turpentine. This is consistent with the σ-profiles analysis results from Figure , where the peaks of +0.007 and +0.012 e/Å2 are attributed to the oxygen atom of the carboxyl group in the components of rosin.

Conclusions

The isothermal VLE of the components of the turpentine + rosin system at 313.2 and 333.2 K were measured by headspace gas chromatography. The thermodynamic properties of the system such as activity coefficients, total pressure, partial pressure, excess Gibbs energies, and excess enthalpies were calculated using COSMO-RS model. The results showed that the activity coefficients were greater than 1 for all components of turpentine except for longifolene, indicating a positive deviation from Raoult’s law. The total pressures were about 1 kPa at 313.2 K and 3 kPa at 333.2 K. Meanwhile, GE and HE of the system were positive, indicating that the mixing process for the components of turpentine and rosin was endothermic. The interactions between homogeneous molecules were smaller than those of heterogeneous molecules and resulted in negative HE(VDW). Moreover, HE(HB), which was caused by the hydrogen bonding interaction, contributed the most for the excess enthalpies HE.

Computational Methodology

COSMO-RS Model

The COSMO-RS (Conductor-like Screening Model for Realistic Solvation) model[27−34] allows prediction of thermodynamic properties of mixtures without relying on experimental data. The basis of the COSMO-RS model calculation is the surface shielding charge distribution σ. The intermolecular interaction can be described by the shielding charge interaction between the small pieces of molecules in contact with each other on the molecular surface. The energy of intermolecular interactions is given by eqs –4.[28]where EMF is the asymmetric energy of the shielding charge, EHB is the energy of the intermolecular hydrogen bond, EVDW is the energy of the reference state in solution, σ and σ’ are the surface shielding charge density of the two interacting blobs, aeff is the effective contact area, α’ is an interaction parameter, cHB is the hydrogen bond strength, σHB is the threshold for hydrogen bonding, and τVDW and τ’VDW are element specific adjustable parameters.

The calculations using the COSMO-RS model occur over two steps. The first step is the quantum chemical calculations for the various substances involved in the system. These quantum chemical calculations for the COSMO-RS model are performed by the Turbomole software based on density generalization theory, which can generate cosmo files to describe the molecular surface shielding charge σ in a short time. The next step is the calculations of the microscopic intermolecular forces or the chemical potential to obtain thermodynamic properties by inputting the liquid phase composition and temperature using the COSMOtherm software.

Activity Coefficients

The activity coefficients for a given finite concentration can be calculated using the COSMO-RS model, in which the molar or mass fraction of the target compound is set as the desired value in the solvent composition. The activity coefficient of component i of the mixture at molar fraction x is calculated by eq .[35]where μ(x) is the chemical potential of component i in the mixture, μ(p) is the chemical potential of the pure component i, and R is the gas constant.

Pressure

The total pressure of the VLE is calculated by eq .where P is the partial pressure of component i, P0 is the vapor pressure of component i, x is the molar fraction of component i in the liquid phase, and γ is the activity coefficient of component i in the liquid phase. P0 and γ are both calculated by the COSMOtherm software.

Excess Gibbs Energies

The excess Gibbs energies at a given temperature and mixing conditions are calculated by eq .where x is the molar fraction of component i in the liquid phase, γ is the activity coefficient of component i in the liquid phase calculated by the COSMO-RS, and R is the gas constant.

Excess Enthalpies

To further investigate the nature of intermolecular interactions in solution, another important thermodynamic property, excess enthalpies HE, is calculated by the COSMO-RS model based on eqs –11.where HE(MF) is the misfit interaction energy in the mixture, HE(HB) is the hydrogen bonding interaction energy in the mixture, HE(VDW) is the van der Waals interaction energy in the mixture, and HE is computed as the expectation value of the partition sum of the microscopic segment–segment interaction enthalpies.[36] As shown in eq , the excess enthalpies HE are composed of HE(MF), HE(HB), and HE(VDW).[37]

Experimental Section

Materials

Turpentine, provided by Guangxi Chaoyan Guoyou Forestry, mainly contains the following six components analyzed by gas chromatography: α-pinene, camphene, β-pinene, p-cymene, limonene, and longifolene. Rosin, provided by Guangxi Wuzhou Richeng Forestry Chemical Company, mainly contains the following seven components analyzed by gas chromatography: abietic acid, palustric acid, neoabietic acid, pimaric acid, dehydroabietic acid, sandopimaric acid, and isopimaric acid. The results of gas chromatography of turpentine and rosin are listed in Table . The molecular structures of the main components of turpentine and rosin are shown in Figure . Methanol (mass fraction, 99.9%, CAS, 67-56-1) and tetramethylammonium hydroxide solution (mass fraction, 25% aqueous solution, CAS, 75-59-2) were purchased from Shanghai Macklin Biochemical Company.

Table 5

Mole Fraction and CAS Numbers of the Main Components of Turpentine and Rosin Used in This Study

	components of turpentine
	α-pinene	camphene	β-pinene	limonene	p-cymene	longifolene
mole fraction	0.851	0.017	0.078	0.018	0.022	0.014
CAS	80-56-8	79-92-5	127-91-3	138-86-3	99-87-6	475-20-7

Figure 4

Molecular structures of the main components of turpentine and rosin used in this study.

Apparatus and Procedure

The isothermal VLE data of components of the turpentine + rosin system at 313.2 and 333.2 K were measured by HSGC, which consists of a gas chromatography (GC) system (Agilent 7890B) and a headspace sampler (HS) (Agilent 7697A). GC included a HP-5 (30 m × 0.32 mm × 0.25 μm) capillary column and a flame ionization detector (FID), while HS included an electro-pneumatic sampling system and a precision thermostat with an accuracy of 0.1 K. The carrier gas of GC was N2 (99.999%) with a constant flow rate of 25 mL/min. The column temperature was 323.2 K at the beginning, and ramped to 343.2 at 5 K/min, ramped to 403.2 at 10 K/min, then ramped to 483.2 at 5 K/min and finally 523.2 at 20 K/min. The injector and detector temperature were 523.2 K.

Sample Preparation

Samples were prepared in appropriate mass ratios using an analytical balance (Mettler Toledo) with a precision of 1 × 10–4 g. The rosin was first poured into the headspace vial (equilibrium cell, 20 mL), and the others were injected with a pipet gun. Then, the headspace vial was sealed with an aluminum cap and placed on a sample tray, followed by heating for 60 min in the thermostat. Since the sample mixture (approximately 5 mL) and equilibrium cell were too small, 60 min was sufficient for the vapor and liquid phase to equilibrate with each other. Finally, the vapor phase in the headspace vial was automatically sampled and transported to the GC by the electro-pneumatic sampling system to obtain vapor phase composition. The liquid phase sample after vapor–liquid equilibrium was analyzed using GC following methyl esterification to obtain the liquid phase composition.

According to the properties and components of turpentine and rosin, six main components such as α-pinene (1), camphene (2), β-pinene (3), p-cymene (4), limonene (5), and longifolene (6) were selected for the VLE data measurements. The vapor–liquid phase components are all calculated by using eq .where x’ is the mole fraction of the six main components of turpentine in liquid phase without rosin, y is the mole fraction of the six main components of turpentine, A is peak area of component i in the vapor–liquid phase in gas chromatography, M is the molar mass of component i, and n is the six main components of turpentine.