SolidâLiquid Phase Equilibrium and Phase Behaviors for Binary

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/IECR

Solid−Liquid Phase Equilibrium and Phase Behaviors for Binary Mixtures Composed of Tripalmitoylglycerol (PPP), 1,3-Dipalmitoyl-2oleoyl-glycerol (POP), and 1,2-Dioleoyl-3-palmitoyl-glycerol (POO) Chao Lu,†,‡ Bo Zhang,†,‡ Hua Zhang,§ Yun Guo,† Leping Dang,† Zhengan Liu,∥ Qingyan Shu,*,∥ and Zhanzhong Wang*,† †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Key Laboratory of Plant Resources/Beijing Botanical Garden, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, China § Chongqing Institute for Food and Drug Control, Chongqing 401121, China Downloaded by UNIV OF SOUTHERN INDIANA at 07:41:58:401 on May 31, 2019 from https://pubs.acs.org/doi/10.1021/acs.iecr.9b01947.

∥

S Supporting Information *

ABSTRACT: The crystallization and melting behaviors of binary tripalmitoylglycerol (PPP)/1,2-dioleoyl-3-palmitoyl glycerol (POO), tripalmitoylglycerol (PPP)/1,3-dipalmitoyl-2-oleoyl glycerol (POP), and 1,3-dipalmitoyl-2-oleoyl glycerol (POP)/1,2dioleoyl-3-palmitoyl glycerol (POO) were investigated using differential scanning calorimetry (DSC). On the basis of melting phase equilibrium data, solid−liquid phase diagrams of binary systems (PPP/POO, PPP/POP, and POP/POO) were constructed. With respect to PPP/POO and PPP/POP, eutectic and solid solution phase equilibrium were confirmed when PPP content (XPPP) in the mixtures was lower and higher than 90% and 80% (w/w), respectively. For POP/POO, the solid solution and eutectic phase equilibrium were confirmed when the POP content (XPOP) in the mixtures was lower and higher than 80% (w/w), respectively. Interestingly, polymorphic transition behaviors of PPP, POP, and POO at melting stages were also revealed. Finally, melting Gibbs free energy changes based on enthalpy changes for different binary mixtures were predicted by thermodynamics models.

1. INTRODUCTION Melting phase equilibrium is between molten and solid forms of the same chemical species.1 It is far more complex than most phase equilibriums.2 However, knowledge of this complex process is essential for the design and development of separation processes involving crystallization.3 Thus, it has been widely studied and applied in different kinds of fields, such as chemical engineering,4 geology,5 and biochemistry6,7 and in even recent years the food industry,8 especially sugar, amino acids, and kinds of food additives manufacturing, etc.9,10 The melting phase equilibrium data plays an important role in separating fats and further improving product properties.11 It is well known that fractionated oils with different melting points can broaden their use.12 It is also important to make full use of oils and wasted oil resources.13−15 The oil fractionation is to obtain different oils with different melting ranges. The narrower the melting range is, the higher accuracy of the © XXXX American Chemical Society

separation and the stronger the product application performance is.16 Therefore, fractionation is the key node of the top the industrial chain of the oil refining industry. Oil phase equilibrium is a typical example of melting equilibrium, which could exert a significant influence on the application properties of products. For example, mixing cocoa butter substitutes (CBS) with cocoa butter leads to a strong incompatibility in the solid phase, rendering a product softer than expected. Furthermore, some oils used for chocolate fillings are seen to cause oil migration bloom much faster than others.17 Thus, for the practical values applied to the food industry, researchers have carried out some work on the oil melting equilibrium. Received: April 10, 2019 Revised: May 17, 2019 Accepted: May 24, 2019

A

DOI: 10.1021/acs.iecr.9b01947 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Costa et al. investigated the melting equilibrium of binary mixtures containing fatty acids and triacylglycerols, and the best modeling of the equilibrium data was obtained by the Margules-3-suffix model.18 With the development of the thermodynamics and the constant accumulation of the experimental data for oils, prediction of the phase equilibrium become more and more accurate by using the thermodynamic model.19 Yui et al.20 took a measurement of the melting equilibrium for binary mixtures of saturated fatty acids (SFAs) or triglycerides (TGs) with hexadecane using DSC. Consequently, all liquidus curves of the nonideal, fatty acid− hexadecane systems were successfully predicted. Palm oil is an important raw material in industry. Its properties and application are subject to fractionation and basic melting phase equilibrium data.21 In addition, since the palm oil contains many kinds of triacylglycerols with different melting points, leading to phase transitions in different temperatures, analysis of the phase equilibrium is extremely complex.22,23 Knoester et al.24 investigated the melting equilibrium of the binary mixtures of six triacylglycerols with palmitic and stearic chains by a microcalorimeter. The results showed that solid miscibility is strongly favored by the presence of an asymmetric triglyceride. Besides, Costa et al.25 successfully determined the solid−liquid equilibrium of tristearin with refined rice bran and palm oils, and the UNIFAC model was used to predict the liquidus line of the system, presenting low deviations in comparison to the experimental data. However, few experimental melting equilibrium data and thermodynamic properties were investigated in the single or binary triacylglycerol systems, especially the main triacylglycerols in palm oil, in which the content of POP, POO, and PPP reaches 27.7%, 24.3%, and 5.1%, respectively.23 For this reason, it is of great importance to predict the thermodynamic properties and phase diagrams for palm oil. Differential scanning calorimetry (DSC) is an extremely versatile thermal analysis technique to characterize the melting equilibrium.26 In this work, taking the major components of palm oil (PPP, POP, and POO) as research objects, phase behaviors of diverse binary triglycerides (PPP/POP, PPP/ POO, POP/POO) originating from palm oil in the cooling and melting stages was first investigated using DSC. Second, phase diagrams were constructed for the binary systems based on those data obtained from the melting phase equilibrium, aiming at offering theoretical instruction for palm oil fractionation. Moreover, polymorphic transition behaviors of PPP, POP, and POO at the melting stages were analyzed. Finally, the melting Gibbs free energy changes were calculated based on the enthalpy changes from the DSC curves using thermodynamic models.

Table 1. Physical Properties and Purities of Chemicals Used in This Work chemical name (alias, abbreviation) tripalmitoylglycerol (PPP) 1,3-dipalmitoyl-2oleoyl-glycerol (POP) 1,2-dioleoyl-3palmitoyl-glycerol (POO)

mole fraction puritya (%)

Mw (g·mol−1)b

formula

source

C51H98O6

Larodan (Sweden) Larodan (Sweden)

>99.0

807.32

>99.0

833.357

Larodan (Sweden)

>99.0

859.395

C53H100O6 C55H102O6

a

Purities were provided by suppliers as determined from GC. bMolar masses were cited from the NIST Standard Reference Data.

weighed by an analytical balance. The binary mixture was prepared according to weight proportions of 0:10, 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, and 10:0. Electronic balances were used in the experiment with a precision of 0.01 mg. 2.3. Differential Scanning Calorimetry Measurement. After the configuration was completed, samples were scanned by DSC. Nitrogen was introduced into the instrument at a flow rate of 150 mL/min. The DSC machine and computer software programs were opened and connected. Samples of particular concentration were weighed to 6.00 mg and loaded into a special aluminum crucible. The crucible was sealed by the instrument and placed in the platform of the DSC equipment, and a sealed blank crucible was set as a control group. The ice machine was turned on before testing to make sure the DSC machine reached the initial temperature of the method. The sample solidifies completely at −5 °C and melts completely at 70 °C. Thus, each sample was melted at 70 °C for 20 min to destroy the crystal structures and dissolve the solid component completely,13,21 then cooled to −10 °C at 2 °C/min, and kept for 20 min to crystallize completely. Then the sample was heated to 70 °C at a rate of 2 °C/min, and the thermogram was obtained. The onset melting temperature and the peak temperature of the melting process in the DSC curves were selected according to Sediawan’s report.27 Futhermore, the melting enthalpy and the crystallization enthalpy were also determined by the integral tools in DSC. The uncertainty of the weight values provided by the analytical balance was 0.01 mg, and the uncertainties of the mass ratios were calculated to be 0.001. The inherent error of temperature and heat flow resulting from different uncertainties of thermocouples and other factors in the calorimeter were calibrated in quintuplicate using the melting properties of calibration substances Bi (271.4 °C, −53.1 J·g−1),28 Zn (419.5 °C, −107.5 J·g−1),29 and CsCl (476.0 °C, −17.2 J·g−1)30 by the supplier of the DSC apparatus, and the thermal properties of each material in parentheses were the values quoted from the references provided by the supplier.28−30 Thereby, the absolute average deviations (AAD) of the inherent error were obtained with ranges from 0.01 to 0.021 K and from 0.004 to 0.008 mW for all of the standards. More than triplicate measurements of the DSC thermal curves for each sample were performed and used to extract the temperature and heat flow with AAD ranging from 0.06 to 0.28 K and 0.01 to 0.12 mW. On the basis of these experimental runs, the uncertainty of the experimental variables can be estimated as not higher than 0.3 K and 0.12 mW. The relative standard uncertainty of enthalpy and melting

2. EXPERIMENTAL PPP, POP, AND POO 2.1. Materials and Apparatus. The standard chemicals tripalmitoylglycerol (PPP, CAS 555-44-2), 1,3-dipalmitoyl-2oleoyl-glycerol (POP, CAS 2190-25-2), and 1,2-dioleoyl-3palmitoyl-glycerol (POO, CAS 65390-75-2) were supplied from Larodan (Sweden). The mass fraction purity of each chemical is more than 99%. Details for the chemicals are listed in Table 1. The DSC instrument and electronic analytical balance (accuracy = 10−4) were kindly provided by METTLER TOLEDO (Shanghai). 2.2. Sample Configuration. Using the experience of Sediawan’s work,27 PPP, POP, and POO were accurately B


Industrial & Engineering Chemistry Research free energy changes calculated using the above datum are all within 5%. All of the averages and deviations for each parameter of different samples are listed in Table 1 and Tables S1−S3. 2.4. Phase Behaviors and Phase Diagram Construction for Binary Systems. Binary phase diagrams of the corresponding mixtures can be obtained from the melting curve of DSC. Accurately speaking, the DSC curves for the binary systems showed mainly two endothermic peaks, corresponding to the solidus (Te) and liquidus (Tl) temperatures.21 The onset point of the first peak was determined as Te, and the peak top temperature of the last peak was determined as Tl.21 2.5. Calculation of Melting Gibbs Free Energy. Calculation of the enthalpy, onset melting temperature, and peak temperature in the melting stage provide enough information for predicting the Gibbs free energy of binary systems1 ΔCp = Cpl − Cps

ln ϕi =

Δm Hi(Tmi) jij 1 1 zy − zzz jj j R T z{ k Tmi

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(6)

ij T yz Δm Gi = −Δm Hi(Tmi)jjj − 1zzz j Tmi z (7) k { The formula is basically strict to predict the Gibbs free energy of samples in the whole stage. Since there was enough information for the calculation, all of the binary systems were studied using this model.

Thus

3. RESULTS AND DISCUSSION 3.1. Phase Behaviors Analysis of Binary System during Cooling Crystallization. 3.1.1. PPP/POO System. Phase behavior data based on crystallization (as shown in Table S1) of the PPP/POO system at the cooling stage were analyzed and are given in Table S1 and plotted in Figure 1.

(1)

T

T

∫ Tmi Cps dT + ΔmHi(Tmi) + ∫ Tmi Cpl dT

ΔH =

= Δm Hi(Tmi) +

T

∫ Tmi ΔCp dT

(2)

Clp and Csp are the heat capacity at constant pressure of the sample in liquid state and soild state, respectively. Tmi and T are the melting point and the onset melting temperature, respectively. ΔmHi(Tci) is the melting enthalpy of sample i ΔS =

= Δm Hi(Tci) + +

l T C

s T C

∫ Tmi Tp dT = ΔmSi(Tmi) + ∫ Tmi Tp dT

∫

T ΔCp

∫ Tmi

T

dT =

Δm Hi(Tmi) Tmi

T ΔCp dT Tmi T

(3)

where ΔmSi represents the melting entropy of sample i ij T yzz Δm Gi = ΔH − T ΔS = Δm Hi(Tmi)jjj1 − z j Tmi zz{ k fl T ΔCp T dT = RT ln is + ΔCp dT − T Tmi T Tmi fi

∫

According to Figure 1, peak I is attributed to the freezing of PPP, appearing in the mixtures XPPP (PPP content) ≤ 40%. The onset, peak, and offset temperature of peak I rose with PPP content. With the escalation of PPP content, peak I gradually replaced peak II as the main exothermic peak. Peak II represented the solidification of POO. The temperature parameters (the onset, peak, and offset temperature) of peak II were basically not affected with the changing proportion of diphasic materials. From analysis of peak I and peak II, it could be found that for the mixtures XPPP ≤ 50% the freezing point (onset temperature) of PPP increased with its content. With a higher concentration of PPP (XPPP > 50%) two components coexisted and were frozen together; the freezing of the whole solid corresponded to peak III on the curves. Peak III gradually moved toward the right (high temperature) with the growth of PPP content. This reflects that the rise of PPP elevated the freezing point of the system. As a whole, double peaks were observed in the mixtures iXPPP ≤ 50%, which means PPP and POO crystallized in different temperatures, respectively. A single peak appeared in the mixtures XPPP ≥ 50%, indicating

∫

= −RT ln ϕi

Thus

Figure 1. DSC curves of binary PPP/POO systems at cooling stages.

(4)

Δm Hi(Tci) jij 1 T yzz jj1 − zz − j z RT Tmi { RT k C 1 T p + Δ dT T T R mi

ln ϕi = −

T

∫ Tmi ΔCp dT

∫

(5)

Cp 1 1 T While the value of − RT ∫ T Tmi ΔCp dT + R ∫ Tmi Δ T dT is

much smaller than the front item −

Δm Hi(Tmi) RT

(1 − ), the T Tmi

value of the back one can be omitted reasonably. Thus C


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Industrial & Engineering Chemistry Research PPP solubilized into POO and solidified together for the mixtures XPPP ≥ 50%. 3.1.2. PPP/POP Syystem. Phase behavior data (as shown in Table S1) of the PPP/POP system during the cooling stage were analyzed and are plotted in Figure 2. According to Figure

Figure 3. DSC curves of binary POP/POO systems at cooling stages.

PPP/POO system, simple phase behavior was shown in the POP/POO binary system. Peak I in curves was the result of the freezing of POP and POO, suggesting that, different from other binary systems, POP and POO cocrystallized without the influence of the composition proportion. The temperature parameters with the peak position took an upward trend with increasing POP content, indicating that the freezing point of the system rose with increasing POP content. 3.2. Construction of Phase Diagram of Different Binary Systems. 3.2.1. Phase Transition Behaviors of Pure PPP, POP, and POO at the Melting Stage. Because of polymorphism of oils, analysis of the phase transition behaviors is necessary ahead of investigating phase diagrams. It is generally accepted that α, β, β′, and γ are typical crystal forms for oils.31 As shown in Figure 4 and Tables 2, 3, and 4 the DSC heating thermogram of pure POO exhibited a sharp exothermic peak followed by a sharp endothermic peak. The exothermic peak reveals a melt-mediated α−β′ transformation occurred around 7 °C, and the following endothermic peak reflects the melting process of the β′ form. A similar result of relevant substance was also reported in the literature;32

Figure 2. DSC curves of binary PPP/POP systems at cooling stages.

2, in these exothermic peaks peak I was detected in the mixtures 30% ≤ XPPP ≤ 70%, corresponding to the freezing of PPP. With increasing PPP content, peak I became the main exothermic peak in the system. For the mixtures 30% ≤ XPPP ≤ 40%, the onset, peak, and offset temperatures remained stable at around 37, 33, and 28 °C, respectively. Interestingly, peak I shifted to the right when PPP content was higher than 40%, and the onset, peak, and offset temperatures remained constant at around 40, 37, and 31 °C, respectively. The different positions of peak I at these two stages results in crystallization of two different PPP crystal forms during the cooling stage. Peak II, appearing in the mixtures XPPP ≤ 70%, was attributed to the freezing of POP. For the mixtures XPPP ≤ 50%, the temperature parameters and peak position of peak II were not affected with the PPP/POP ratio. As to the mixtures XPPP > 50%, the position of peak I shifted to the right. The changes of peak I and peak II suggest that the increasing PPP content affected the freezing point of PPP and POP. Peak III represented the freezing of the whole system (PPP with POP), which reflects that PPP and POP cocrystallized in the mixtures XPPP ≥ 80%. Peak III shifted to the right with increasing POP content. As a whole, for the mixtures XPPP ≤ 20%, there was a single exothermic peak (peak II) appearing at about 14 °C. It should be noticed that in the curve of XPPP = 20% there was a tendency of being double peaks in the system. Then in the range of 20−80% PPP content, double exothermic peaks (peaks I and II) were observed. The curves reverted back to single peaks (peak III) in the mixtures XPPP ≥ 80%. This indicates that the minor components were solubilized into the major components at concentrations of PPP and POP lower than 20%. 3.1.3. POP/POO System. Phase behavior data (shown in Table S1) of the POP/POO system during the cooling stage were analyzed and are plotted in Figure 3. Compared with the

Figure 4. DSC curves of pure PPP, POP, and POO at the melting stage. D


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Table 2. Peak Informations (Tonset, onset temperature; Tpeak, peak temperature; Toffset, offset temperature; Tm, melting point) for Pure POO in Melting Stage at Pressure p = 0.1 MPaa peak I

peak II

α → β′

β′ melting

sample name

Tonset (°C)b

Tpeak (°C)b

Toffset (°C)b

Tonset (°C)b

Tpeak (Tm) (°C)b

Toffset (°C)b

heating rate

ref

POO

6.72 ± 0.16 −15.64

9.42 ± 0.14 −12.21

11.78 ± 0.14 −9.86

11.79 ± 0.12 17.46

19.64 ± 0.15 22.87

22.23 ± 0.14 29.21

2 °C/min 5 °C/min

this work 29

Standard uncertainty for pressure, u(p), is 5 kPa. bThe standard uncertainties u(T) = 0.2 °C.

a

POO melted at different temperatures for the mixtures XPPP < 90%. Furthermore, two endothermic peaks in these mixtures reveal that the PPP/POO binary system formed a eutectic system. On the other hand, the single peak in the mixtures XPPP ≥ 90% indicates that POO was solubilized into PPP, and the eutectic system transformed into the solid solution when the PPP content was higher than 90%. In summary, from the phase diagram the results of the eutectic system (for the mixtures XPPP < 90%) and the solid solution (for the mixtures XPPP ≥ 90%) were demonstrated. 3.2.3. Phase Behaviors and Phase Diagram of the PPP/ POP System at the Melting Stage. Figure 6a depicts DSC heating thermograms of the PPP/POP mixtures at various ratios. Temperature parameters of peaks shown in Figures 4 and 6a were used to construct the phase diagram (Figure 6b) for exhibiting the phase behavior of the PPP/POP system. Combining Figure 6a with 6b, peak I corresponded to the melting of γPOP with the melt-mediated crystal form transformation of POP (γ → β′). Although the peak temperature of peak I was not affected by component proportion, peak I became smaller with increasing PPP content, even disappearing when the PPP content reached 60%. Peak II represents the melting of β′POP and αPPP with the transformation of αPPP to βPPP in the mixtures XPPP ≤ 10%. For the mixtures XPPP > 10%, peak II represented the melting of β′POP only and the peak temperature remained stable; however, with increasing PPP content, the temperature parameters of peak II had the same changes as peak I and disappeared when the PPP content was higher than 50%. Peak III was attributed to the melting of αPPP with the transformation of αPPP to βPPP. For the mixtures XPPP ≤ 40%, the temperature parameters of peak III reamined stable, while in the mixtures XPPP > 40%, peak III became smaller and shifted to the right, which indicates that the growth of PPP content raised the melting point of αPPP. Finally, peak III disappeared when the PPP content was higher than 70%. Peak IV represents the melting of βPPP in the mixtures XPPP < 80%, and the peak position shifted to the right with increasing PPP content, illustrating the rise of PPP elevated the melting point of βPPP. When the PPP content was higher than 80%, peak IV represents the melting of the whole binary system, suggesting that POP was solubilized into PPP when the PPP content was higher than 80%. The onset temperature of peak IV remained constant within 70% PPP content and had an increase when the PPP content was above 70%. The peak and offset temperature of peak IV increased with increasing PPP content in the mixtures XPPP ≤ 40% and remained constant in the range of 50−70%. Compared with the system XPPP ≤ 70%, a certain amount of increase was observed in the peak and offset temperature of peak IV when the PPP content was higher than 70%, and in this range, these two temperatures remained stable. Therefore, for the mixtures XPPP < 80%, multiple peaks were shown in the DSC curves, which suggests

however, in Zhang’s study, the exothermic peak corresponding to α−β′ transformation in the literature tends to be at lower temperature for the possible reason that the chemical used in these two studies is not totally the same (1,3-dipalmitoyl-2oleoyl glycerol and 1,3-dipalmitoyl-2-oleoyl-sn-glycerol). Regarding the melting peak of the β′ crystal form, compared with our work, it emerged in a higher temperature, which is attributed to a different heating rate. As for POP, the first exothermic peak emerged at 10.33 °C, which represents the transition of α−γ. It is noticed that there were two successive peaks. The sharp big endothermic peak was caused by the γ form melting with the transformation of γ−β′, and the peak at higher temperature represented the melting of the β′ form. Due to the different heating rates (2 and 5 °C/min) in our data with that of the literature,32 there is about a 2 °C temperature deviation in peaks. The curve of pure PPP was more complex than the others; there existed an exothermic peak between two sharp endothermic peaks. Specifically, pure PPP crystallized in the form of an α crystal form and melted at about 42 °C, corresponding to the first melting peak. The exothermic peak indicates a solid−solid transition of α to β forms. Through this process the α form first transformed into the β′ form and then further transformed to the thermodynamically more stable β polymorph via a solid−solid transition. Finally, βPPP totally melted at around 65 °C, which caused the high-temperature endothermic peak. This result demonstrates a general agreement to the work of Bhaggan.33 3.2.2. Phase Behaviors and Phase Diagram of the Binary PPP/POO System at the Melting Stage. Figure 5a depicts DSC heating thermograms of the PPP/POO mixtures at various ratios. On the basis of Table S2 and Figures 4 and 5a, the onset, offset, and peak temperatures of peaks were used to construct the phase diagram shown in Figure 5b. The phase behaviors in the PPP/POO binary system at the melting stage were investigated combining the data in Figure 5a and 5b; peak I could be attributed to the melting of POO in the β form, appearing in the mixtures XPPP < 90%. The peak and offset temperatures of peak I remained steady without the influence of the composition proportion, while with increasing PPP content, the onset temperature of peak I had a slight increase and the value became smaller. Peak II was as result of the melting of βPPP. The increasing PPP proportion affected the position of peak II moving toward higher temperatures and elevated the peak value temperature of peak II, which reflects that the increase of PPP proportion elevated the melting point of βPPP in the system. These two endothermic peaks were observed in the mixtures XPPP < 90%. With increasing PPP content, peak II gradually replaced peak I as the main endothermic peak. When the PPP content was higher than 90%, peak I disappeared. Only a single endothermic peak (peak II) could be observed in the curves. The changes of the endothermic peaks in this binary system indicate that PPP and E


19.59 ± 0.16 25.84

26.61 ± 0.18 28.04


17.23 ± 0.16 19.68

Tpeak (°C)b 29.11 ± 0.14 30.75

Toffset (°C)b 29.11 ± 0.15 31.07

Tonset (°C)b 31.23 ± 0.16 32.67

Tpeak (Tm) (°C)b

β′ melting

peak III

33.62 ± 0.12 33.86

Toffset (°C)b 2 °C/min 5 °C/min

heating rate

ref this work 29

F

a

44.72 ± 0.14 49.02

44.90 ± 0.11 49.02

45.64 ± 0.18 53.13

Tpeak (°C)b


44.01 ± 0.22 46.89

42.45 ± 0.16 44.65

PPP

Tonset (°C)b

α→β Toffset (°C)b

α melting

Tpeak (°C)b

Tonset (°C)b

sample name

peak II

peak I

51.17 ± 0.11 56.32

Toffset (°C)b

60.84 ± 0.14 63.60

Tonset (°C)b

65.53 ± 0.12 66.87

Tpeak (°C)b

β melting

peak III

67.01 ± 0.17 71.84

Toffset (°C)b

2 °C/min 5 °C/min

heating rate

this work 30

ref

Table 4. Peak Information (Tonset, onset temperature; Tpeak, peak temperature; Toffset, offset temperature; Tm, melting point) for Pure PPP in the Melting Stage at Pressure p = 0.1 MPaa

a

14.91 ± 0.12 18.42

10.28 ± 0.2 15.83

POP

Tonset (°C)b

γ melting Toffset (°C)b

α→γ

Tpeak (°C)b

Tonset (°C)b

sample name

peak II

peak I

Table 3. Peak Information (Tonset, onset temperature; Tpeak, peak temperature; Toffset, offset temperature; Tm, melting point) for Pure POP in the Melting Stage at Pressure p = 0.1 MPaa

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Figure 5. (a) DSC curves of melting thermograms of the PPP/POO systems. (b) Phase equilibrium diagram of the PPP/POO system.

Figure 6. (a) DSC curves of melting thermograms of the PPP/POP systems. (b) Phase equilibrium diagram of the PPP/POP system. Melting point of γPOP (black), βPOP (blue), αPPP (green), βPPP (red), and β′POP (pink).

the system was under the state of a eutectic system. On the other hand, when the PPP content was higher than 80%, there were only single peaks, illustrating that the binary system formed solid solutions. 3.2.4. Phase Behaviors and Phase Diagram of POP/POO System at the Melting Stage. Figure 7a depicts DSC heating thermograms of the POP/POO mixtures at various ratios. The temperature parameters of the peaks shown in Figures 4 and 7a were used to construct the phase diagram (Figure 7b) for exhibiting the phase behavior of the PPP/POP system. Combining the data in Figure 7a and 7b, the exothermic peaks (peak i, existing in the mixtures XPOP (POP content) ≤ 40%) corresponded to the transition of POO (α−β′). The temperature parameters of peak i remained constant in the mixtures XPOP ≤ 30%. For mixtures with higher POP content (XPOP > 30%), the peak position slightly moved toward the right. As reported,32 peak i existed in the mixtures XPOP ≤ 60%, remaining constant in the mixtures XPOP ≤ 20% and shifting to higher temperature in the mixtures XPOP > 20%. As for the endothermic peaks, peak I was attributed to the melting of β′POO and β′POP. For the mixtures XPOP ≤ 30%, the peak temperature of peak I remained constant while the onset and offset temperature of peak I increased with POP content. In the range of 30−80% POP content, the onset and offset temperatures were not affected with increasing POP content.

However, the peak value temperature of peak I rose with increasing POP content, which indicates that increasing POP content had effects on the melting point of the system. When the POP content was higher than 80%, peak I separated into two endothermic peaks, peak II and peak III. Peak II corresponded to the melting of β′POO. As for peak III, the offset and peak temperature of peak III remained stable all of the time, while the onset temperature of peak III became higher with increasing POP content. The changes of peaks reflect that for the mixtures XPPP < 80% the system was in the state of solid solution, while for the mixtures XPPP ≥ 80%, the system became a eutectic state. Compared with the report mentioned above,32 peak I emerged in the mixtures XPPP < 70%. The peak temperature basically rose with the increasing content of POP, which is consistent with the tendency found in our study. For the mixture XPPP ≥ 70%, peak II and peak III gradually appeared with increasing POP content, which was also in accordance with the tendency revealed in our study. 3.3. Calculation of Melting Enthalpy and Gibbs Free Energy Changes. From Figures 1 and 2 it was found that PPP/POO and PPP/POP mixtures exhibited obvious double peaks at the cooling stage, and these three binary systems G


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the Gibbs free energy fluctuated at around 270 J·g−1 in the mixtures XPOP < 80% and increased in the mixtures XPOP ≥ 80%.

4. CONCLUSIONS Taking advantage of DSC, the melting phase equilibrium data for binary mixtures (PPP + POO, PPP + POP, and POP + POO) were determined. On the basis of phase behavior analysis and phase equilibrium data, phase diagrams were constructed. The PPP/POO system formed a eutectic system (for the mixtures XPPP < 90%) and transformed into a solid solution (for the mixtures XPPP ≥ 90%). The PPP/POP system satisfied eutectic characterization for the mixtures XPPP < 80%, while solid solution phase equilibrium was observed in the mixtures XPPP ≥ 80%. Differently, the POP/POO system transformed from a solid solution (for the mixtures XPOP < 80%) into a eutectic system (for the mixtures XPOP ≥ 80%). The polymorphic transformation behaviors were revealed in different systems. In the PPP/POP system, crystal form transitions of γPOP → β′POP and αPPP → βPPP were found, while transitions of αPOP → γPOP → β′POP and αPOO → β′POO were observed in the POP/POO system. Finally, based on the enthalpy obtained from DSC, melting Gibbs free energy changes of binary systems were predicted.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01947. Temperature parameters of peaks for binary mixtures at the cooling stage at a cooling rate of 2 °C·min−1 with pressure p = 0.1 MPa; temperature parameters of peaks for binary mixtures at the melting stage at a heating rate of 2 °C·min−1 with pressure p = 0.1 MPa; total of enthalpy changes and melting free energy changes (ΔfusG) for binary mixtures at pressure p = 0.1 MPa (PDF)

Figure 7. (a) DSC curves of melting thermograms of POP/POO systems. (b) Phase equilibrium diagram of the PPP/POP system. Melting point of αPOO (blue) and αPOP (cyan), and the solidus and liquidus lines are represented in black and red line, respectively.

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showed double or multiple peaks at the melting stage. Generally, the crystallization enthalpy for oils was not associated with melting enthalpy on values, which is attributed to the extremely different phase behavior between the cooling and the melting stages. During the cooling stage, in the PPP/ POO binary system, high crystallization enthalpy was exhibited when the PPP content was higher than 50%. For the PPP/POP system, a high crystallization enthalpy was shown in mixtures XPPP = 40% and 90%. For the POP/POO system, the crystallization enthalpy roughly increased with increasing POP proportion. In the melting stage, when the PPP content was above 70%, the PPP/POO binary system showed higher melting enthalpy. For the PPP/POP system, higher melting enthalpy was shown when the PPP content was at 10% and 60−90%. As for the POP/POO system, the values of the melting enthalpy were closer at different ratios. On the basis of the data, the Gibbs free energy changes in the melting process were determined by eq 7 and are listed in Table S3; on the basis of the data it was found that in the PPP/ POO binary system when the PPP and POP content reached 90%, the Gibbs free energy was much less than other proportions. Meanwhile, for the PPP/POP binary system, a relatively lower Gibbs free energy was found when the PPP content was above 80%. As for the POP/POO binary system,

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chao Lu: 0000-0001-5364-2136 Leping Dang: 0000-0003-1713-5422 Zhanzhong Wang: 0000-0002-9151-3308 Author Contributions ‡

C. L. and B.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (21676196) and Tianjin Municipal Natural Science Foundation (17JCYBJC20400).

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