Article pubs.acs.org/crystal
Retardation Mechanism of Crystallization of Diacylglycerols Resulting from the Addition of Polyglycerol Fatty Acid Esters Katsuyoshi Saitou,*,† Ken Taguchi,‡ Rika Homma,† Masao Shimizu,† Koichi Yasunaga,† Yoshihisa Katsuragi,† Satoru Ueno,§ and Kiyotaka Sato§ †
Kao Corporation, Sumida-ku, Tokyo 131-8501, Japan Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan § Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima 739-8528, Japan ‡
S Supporting Information *
ABSTRACT: Edible oils containing high concentrations (>80%) of diacylglycerols (DAG oil) have beneficial health effects on obesity and obesity-related diseases; however, at low temperature, undesired precipitation of high-melting fractions in DAG oil can occur. Thus, preventing the precipitation of high-melting saturated fatty acid moieties in DAG oil is crucial for its expanded use. In this study, we investigated the mechanism of retardation of crystallization of DAG oil through the addition of polyglycerol fatty acid esters (PGFEs). We observed the occurrence of birefringence under polarized crossed-Nicols conditions in the PGFE-added DAG oil. We also found that prior to the crystallization of high-melting DAG fractions, PGFE-added DAG oil showed shear-ratedependent changes in viscosity, providing strong evidence for the existence of self-assembled structures that lead to the birefringence. Furthermore, small- and wide-angle X-ray diffraction patterns suggest the formation of a supramolecular assembly comprising DAGs and PGFEs, which is significantly different from the structure of DAG crystals. From these results, we conclude that the retardation of crystallization of DAG oil is caused by the formation of liquid-crystal-like supramolecular complex structures that contain high-melting fractions of DAGs and PGFEs. These complexes may disturb the formation of critical nuclei of high-melting DAG fractions during the prenucleation crystallization stage.
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been studied from basic science12−14 and applications15−20 points of view. DAGs are expected to be used in two different industrial fields; in liquid oil-based products, such as cooking oil, mayonnaise, and dressings,15−17 and as solid fat substitutes for triacylglycerols (TAGs).18−20 For liquid oil-based products, preventing undesired crystallization during storage at chilled temperatures, called “clouding,” is required for practical applications. In the case of DAG oils, the key to inhibition of clouding lies in the nucleation process of high-melting fractions such as 1,3-dipalmitin (1,3-PP) and 1-palmitin-3-olein (1,3PO), as we reported previously.15 We further found that the addition of polyglycerol fatty acid esters (PGFEs), widely used as food emulsifiers, effectively retarded the nucleation of highmelting fractions in DAG oil when the PGFEs contained palmitic and oleic acid moieties;16 however, the mechanism of nucleation retardation by PGFEs has been open to question. Many studies have been conducted to elucidate the effects of additives on the crystallization rates of TAGs in bulk21−26 and
INTRODUCTION Fats and oils are important ingredients in food, cosmetics, and pharmaceutical products. Crystallization kinetics has a strong influence on the overall structures of fat-containing products and inevitably on the physical properties of the final products.1−3 Therefore, it is crucial for manufacturers to have a detailed understanding of and control over the crystallization behaviors of fats and oils used in their products. In general, the crystallization process for fats and oils is divided into nucleation and crystal growth phases.4 During cooling, supercooled molecules pack together and form clusters, eventually leading to the formation of a crystalline nucleus (nucleation). Next, the nuclei grow by incorporating other crystallizing molecules (crystal growth). The nucleation process is considered to be the rate-determining and therefore crucial step in the entire crystallization process. Diacylglycerols (DAGs) are esters of glycerol with two fatty acids that can exist as two isomers, 1,2-(2,3-)DAGs and 1,3DAGs.5 Recently, DAGs have attracted increasing attention because of their health benefits such as suppressing body fat accumulation6−8 and reducing postprandial hypertriglyceridemia.9−11 The crystallization behavior of DAGs has therefore © 2017 American Chemical Society
Received: May 12, 2017 Revised: July 30, 2017 Published: August 21, 2017 4749
DOI: 10.1021/acs.cgd.7b00676 Cryst. Growth Des. 2017, 17, 4749−4756
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Table 1. Chemical Compositions of DAG Oil and Polyglycerol Fatty Acid Ester (PGFE) Additives PGFEs DAG oil a
acylglycerol composition (wt %)
fatty acid moietiesb (wt %)
TAG DAG MAG FFA C16:0 C18:0 C18:1 C18:2 C18:3 others
13.2 85.8 0.9 0.1 4.1 1.9 60.8 19.5 11.6 2.1
b
fatty acid moieties (wt %)
esterification degree (%) polymerization degree
C16:0 C18:1
PGFE 6
PGFE 10
Q-1710S
50 50
50 50
0 100
>80 6
>80 10
>80 10
a
TAG, triacylglycerol; DAG, diacylglycerol; MAG, monoacylglycerol; FFA, free fatty acid. bC16:0, palmitic acid; C18:0, stearic acid; C18:1, oleic acid; C18:2, linoleic acid; C18:3, linolenic acid.
in emulsion states;27−30 some additives promote crystallization, whereas others retard it. For example, Cerdeira et al.25 reported that sucrose fatty acid esters delay both crystal nucleation and the growth processes of high-melting fractions of milk fat. Shimamura et al.24 showed that PGFEs promote or retard crystallization of fat depending on the fatty acid moieties, additive concentration, and cooling rate. The promoting effects of additives on crystal nucleation have been understood as “template effects”, where additives act as the starters of crystallization.26 On the other hand, the mechanism of retardation of nucleation, especially declustering effects as mentioned by Shimamura et al.,24 remains unknown. To clarify the nucleation retardation mechanism, it is important to precisely observe changes in macroscopic and microscopic structures of supercooled liquids caused by the addition of crystallization-retarding additives before nucleation begins. The purpose of this study was to investigate the mechanism of retardation of crystallization of high-melting fractions in DAG oil caused by the addition of PGFEs. To achieve this, we studied the optical and rheological properties and X-ray diffraction patterns of DAG oil containing PGFEs, focusing on the early stages of crystallization of the high-melting fractions of the DAG oil. The results suggest that liquid-crystallike structures composed of DAGs and PGFEs form in the supercooled liquid phase of PGFE-added DAG oil and disturb the transformation from clusters to nuclei prior to crystallization.
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Figure 1. Strctures of (A) diacylglycerols and (B) polyglycerol fatty acid esters. Solid Fat Content. The solid fat content (SFC) was measured with pulsed nuclear magnetic resonance (p-NMR) using a Maran SFC instrument (Resonance Instrument Ltd., Witney, U.K.). In the measurement, 3 g samples of DAG oil in the presence (0.2%−1%) and absence of PGFEs were put into glass tubes with an internal diameter of 8 mm. The tubes were heated at 70 °C for 10 min before analysis. Three replicates of each sample were then placed in a thermostated bath at 0 or 6 °C, and SFC readings were taken at appropriate time intervals. Observation of Birefringent Patterns. To observe the birefringent patterns formed in DAG oil, 30 g samples of DAG oil in the presence (0.2%−1%) and absence of PGFEs were put into column-shaped glass vials with an internal diameter of 26.5 mm (SV50; Nichiden-Rika Glass Co., Ltd., Japan) and a box-shaped glass container with dimensions of 36.0 mm × 36.0 mm (TAKARA Co., Ltd., Japan). After the samples were heated at 70 °C for 10 min, they were placed in a thermostated bath, where they were cooled from 20 to 0 °C and then heated from 0 to 24 °C in steps of 2 °C. At each temperature, the sample was held for 30 min, during which the birefringent patterns formed in DAG oil were observed under polarized light (see below). Polarized optical microscopy (POM) was conducted using an Olympus BX50 microscope (Olympus, Japan) equipped with a CCD camera (DP72; Olympus) and a temperature controller (Linkam 10002L; Linkam Scientific Instruments Ltd., U.K.). Samples of DAG oil in the presence (1%) and absence of PGFEs were placed on a glass stage (15 mm diameter) without a cover glass; the stage was heated at 70 °C for 10 min and then cooled to 5 °C at a rate of 5 °C/min. The temperature was kept constant for 60 min, and then the images were recorded. Rheological Measurements. Viscosity measurements were performed with an Anton Paar MCR 501 rheometer with a CP50-1 cone and plate geometry (diameter, 50 mm; cone angle, 1°). The temperature dependence was investigated by measuring the viscosities of DAG oil in the presence (1%) and absence of PGFEs at a constant shear rate (100 s−1). The samples were cooled from 25 to 3 °C in a stepwise manner, holding the sample for 2 min at each temperature, during which the measurement was taken.
EXPERIMENTAL SECTION
Materials. The DAG oil examined in this study was prepared as reported previously.15,16 Briefly, fatty acids obtained by the hydrolysis of rapeseed oil and glycerol were esterified under reduced pressure using a 1,3-position-selective immobilized enzyme (Lipozyme RM IM; Novozymes, Chiba, Japan). After esterification, free fatty acids and monoacylglycerols (MAGs) were removed by distillation, and the residue was steam-deodorized. As a consequence, DAG oil containing more than 80 wt % DAGs was prepared. The compositions of the fatty acids and glycerides in the DAG oil (Table 1) were analyzed using a gas chromatography method as described in the previous study.15 Three PGFEs (PGFE 6, PGFE 10, and Q-1710S) supplied by Taiyo Kagaku Co., Ltd. (Mie, Japan) were employed in this study. PGFE 6 and PGFE 10 were almost fully esterified polyglycerols with average degrees of polymerization of the glycerols of 6 and 10, respectively (see Figure 1 for the structure of PGFEs). They contained palmitic and oleic acids in the same proportion as the fatty acid moieties (Table 1). Q-1710S is a commercially available decaglycerol ester containing only oleic acid. 4750
DOI: 10.1021/acs.cgd.7b00676 Cryst. Growth Des. 2017, 17, 4749−4756
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The shear-rate dependence of the viscosities of DAG oil in the presence (0.2%−3%) and absence of PGFEs was measured at 10 °C while the shear rate was increased from 1 to 1000 s−1 logarithmically in 10 steps. A frequency sweep test was performed at 10 °C at a constant strain of 4%, as determined by a strain sweep test. The frequency was decreased from 10 to 0.001 Hz logarithmically in 16 steps. X-ray Diffraction Measurements. X-ray diffraction measurements were performed using Cu Kα1 radiation (λ = 0.15405 nm) with a NANO-Viewer system (Rigaku Co., Japan) operating at 40 kV and 30 mA. A PILATUS 100 K detector (DECTRIS Ltd., Switzerland) was used as a two-dimensional pixel detector. The distance between the sample and the detector was 1002 mm for small-angle X-ray diffraction (SAXD) measurements and 89.7 mm for wide-angle X-ray diffraction (WAXD) measurements. The samples were cooled from 30 to 10 °C and then to 6 °C, holding for 180 min at each temperature. Long spacing values were calculated from the peaks obtained by SAXD, in which the samples were irradiated with X-rays from 30 to 180 min after the samples reached the target temperature. Short spacing values were calculated from the peaks obtained by WAXD, in which the samples were irradiated six times for 30 min at each temperature (i.e., 0−30, 30−60, 60−90, 90−120, 120−150, and 150−180 min). In the SAXD and WAXD experiments, the concentrations of PGFEs added in the DAG oil were increased to 10% to obtain clearer diffraction patterns, since the intensities of diffraction peaks caused by the formation of supramolecular assemblies were too weak when 3% PGFE was added. We believe that the mechanisms underlying the retardation of the crystallization of DAG oil do not change irrespective of whether the PGFE concentration is 3% or 10%.
a reduction in the rate of crystal nucleation processes. This result suggests that the addition of PGFEs with higher degrees of glycerol polymerization is more effective for retarding the crystallization process. The SFC values of DAG oil in the presence of PGFE additives measured at 6 °C did not increase during the 150 min measuring time. Occurrence of Birefringent Patterns in DAG Oil. Figure 3 presents optical images of DAG oil in the presence and
Figure 3. Birefringence observation. (A) Experimental setups for observation of birefringent patterns. (B, C) Optical images taken at 5 °C in the presence and absence of 1% PGFE 10 under (B) nonpolarized and (C) polarized crossed-Nicols conditions.
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RESULTS AND DISCUSSION Retardation Effects of PGFEs on the Crystallization of DAG Oil. Figure 2 shows the time variations in the SFC values
absence of 1% PGFE 10 taken at 5 °C. As shown in Figure 3A, birefringent patterns were observed with a glass vial containing the DAG oil, an optical path of at least 25 mm, and two polarizers through which light was passed. Figure 3B shows the optical images taken under nonpolarized light. A white image resulted for the DAG oil without PGFE as a result of crystallization, whereas a transparent image was observed for the DAG oil with 1% PGFE 10 because of the retardation of crystallization. Crossed-Nicols polarized images of the same samples are shown in Figure 3C. White patterns were still observed for the sample with no additive, whereas birefringent patterns were detected for the sample with 1% PGFE 10. This means that certain anisotropic birefringent structures are formed in the supercooled DAG oil containing PGFE 10 without the occurrence of DAG crystals. Such a birefringent pattern was not detectable by POM using a sample with an optical path of less than 1 mm. These results indicate that a liquid-crystal-like structure was formed in supercooled DAG oil, which produced such a weak birefringence pattern that a long optical path was needed for observation. A possible reason for the weakness of the birefringence is the sparsity of structured domains present in the DAG oil, as discussed later. Observation of the dynamic behavior of the birefringent patterns in the DAG oil under shear was conducted to examine the dynamic properties of the liquid-crystal-like structure. The bright/dark portions of the birefringent patterns in the DAG oil containing PGFEs dynamically varied in their areas and positions under shear. To illustrate this effect, videos for DAG oil with 1% PGFE 10 at 5 °C under polarized crossedNicols conditions were recorded, one under static conditions (Video S1) and the other under stirred (150 rpm) conditions (Video S2). A snapshot was taken from each video (Figure 4). In Figure 4A (static conditions), bright areas were observed in the central portion and dark areas were observed in narrow channels, with no regular distribution of bright and dark areas. It must be noted that different birefringent patterns were
Figure 2. Solid fat content (SFC) values of DAG oil in the presence and absence of two types of PGFE additives measured at 0 °C. Values are presented as mean ± standard deviation (SD) (n = 3).
of DAG oil in the presence and absence of PGFE additives measured at 0 °C. The SFC values measured at 150 min were the same for all of the samples, but the addition of PGFEs up to 1% remarkably retarded the crystallization of DAG oil, as revealed in the delayed increases in the SFC values in the early stages of isothermal crystallization. In particular, PGFE 10 showed stronger retardation effects compared with PGFE 6; PGFE 10 remarkably lengthened the time before the SFC values started to increase (the induction time), which indicates 4751
DOI: 10.1021/acs.cgd.7b00676 Cryst. Growth Des. 2017, 17, 4749−4756
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Figure 4. Polarized optical images taken (A) under static conditions (from Video S1) and (B) under stirring at 150 rpm (from Video S2).
Figure 5. Optical images of DAG oil containing 1% PGFE 10: (A) nonpolarized image taken after crystallization at 0 °C; (B) polarized crossed-Nicols image after heating to 8 °C.
viewed when the angles were changed, suggesting spatial inhomogeneity of the oriented molecules (Video S1). Under gentle stirring (150 rpm), an interference color was observed, indicating that the molecules became more oriented in the direction of the flow (Figure 4B and Video S2). The patterns have an extinction position along the central axis of the swirling. From these observations, we can conclude that optically anisotropic structures showing birefringence are formed in the PGFE-added DAG oil and that such structures change their positions and directions under shear. Tables 2 and 3 show the temperature dependence of the occurrence behavior of the birefringent patterns in the DAG oil examined in the glass vials. During the cooling processes (Table 2), the high-melting fractions of DAG oil start to crystallize at 12 °C without PGFEs. In the presence of PGFEs, the crystallization was retarded, and birefringent patterns were observed before the crystallization. The temperature at which the birefringence occurred increased with increasing concentration of PGFEs; PGFE 10 showed slightly stronger birefringent patterns than PGFE 6. At 0 °C, regardless of the presence of PGFEs, DAG oil solidified because of crystallization of its main components (e.g., 1,3-diolein) (Figure 5A). During the heating processes (Table 3), the crystals started to melt at 6 °C, and the birefringent patterns reappeared and coexisted with the precipitated crystals (Figure 5B). This result clearly shows that the birefringent patterns were caused by optically anisotropic structures that are different from the structures of DAG crystals because they continued to appear after melting of the crystals. It would be worth noting that the temperature at which the birefringent patterns disappeared during the heating process was higher than the temperature at which they appeared during the cooling process. For example, the birefringent patterns started to appear at 12 °C in the DAG oil containing 3% PGFE 6 and PGFE 10 on cooling, whereas they disappeared at 22 °C on heating. These results suggest that DAG oil containing PGFEs forms optically anisotropic structures that exhibit a remarkable thermal hysteresis.
No birefringence was observed with Q-1710S (data not shown), which contains only oleic acid moieties and showed no retardation effects on DAG crystallization, as reported previously.16 These results indicate that the presence of the palmitic acid moiety of PGFEs is a prerequisite for the occurrence of birefringence. Furthermore, the results indicate that the appearance of birefringence is closely related to the retardation effects on the crystallization of DAG oil. Changes in Rheological Properties of DAG Oil with PGFE Additives. Figure 6 shows variations in the viscosity values of DAG oils in the absence and presence of 1% PGFE 10 with decreasing temperature at a constant shear rate of 100 s−1. At 25 °C, the viscosity of PGFE-added DAG oil was slightly higher than that of PGFE-free DAG oil. The difference in the viscosities of PGFE-added and PGFE-free DAG oil became larger with decreasing temperature. The viscosity of PGFE-free DAG oil rapidly increased at 9 °C because of crystallization; however, the viscosity of the DAG oil with PGFE 10 did not show a rapid increase until 3 °C since no crystallization occurred under these conditions. Therefore, the viscosities of the two types of DAG oils were reversed at 7 °C. Figure 7 shows the variation in the viscosity of DAG oil with shear rate measured at 10 °C. At this temperature, the difference between DAG oils in the presence and absence of 1% PGFE 10 is the largest at a shear rate of 100 s−1 (Figure 6). The viscosity of TAG-based oil, such as rapeseed oil, is known to be shear-rate-dependent to a certain extent; a shear-thinning behavior of the oil is revealed at very low shear rates ( 005 > 001.34 In the case of the DAG oil containing PGFE 6, which has smaller polyglycerol groups than PGFE 10, the SAXD peaks were the same as those from DAG oil with PGFE 10 added (Figure 9D). This result suggests that the size of the structures
Figure 10. X-ray diffraction patterns of DAG oil (A) without additives and (B) with 3% PGFE 10 taken at 6, 10, and 30 °C. The diffraction patterns were calculated by subtracting the data of the first irradiation (0−30 min) from those of the last (150−180 min) after an intensity correction. 4754
DOI: 10.1021/acs.cgd.7b00676 Cryst. Growth Des. 2017, 17, 4749−4756
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Figure 11. Schematic models of (A) a cluster of 1,3-PO DAGs and (B) a supramolecular assembly of 1,3-PO DAGs and PGFEs containing palmitic and oleic acid moieties. In the supramolecular assembly model, the polar groups of DAGs and PGFEs form one unit layer, and the fatty acid moieties are aligned differently to form a triple-chain-length structure.
understood as follows. When the PGFE-added DAG oil is cooled, liquid-crystal-like structures may start to aggregate by incorporating 1,3-PO fractions; however, they do not form rigid crystals because of the bulky polyglycerol groups. The formation of the liquid-crystal-like structures results in a decrease in the concentration of 1,3-PO, thus reducing the probability of nucleus formation. The incorporation of highmelting DAG fractions into the liquid-crystal-like structures is supported by the disappearance of X-ray diffraction peaks of 4.65 nm for DAG crystals and the emergence of new diffraction peaks of 4.19 and 8.37 nm (Figure 9). In addition to the molecular interactions of the fatty acid moieties, the polyglycerol groups of PGFEs play an important role in the retardation effect. As a noncrystalline part, polyglycerol groups can prevent DAGs and PGFEs from crystallizing and maintain them in a liquid-crystal state. In addition, the optimal size of polyglycerol groups may exist in PGFE 10, since it showed somewhat stronger birefringence than PGFE 6 (Table 2, 3). In fact, PGFE 10 retarded the crystallization of DAG oil more effectively because the attractive polar interactions between DAG and PGFE may increase with the increase in the degree in polymerization from 6 to 10 (Figure 2).
and 1,3-PO). This satisfies the requirement for strong molecular interactions between the additives and fats and leads to strong retardation of the high-melting fractions of DAG oil with PGFE additives. Our results demonstrate that liquid-crystal-like structures form in DAG oil containing PGFEs in supercooled states, as shown by the polarized optical observations as well as rheological and X-ray diffraction measurements. The next focus of our study was to determine what types of liquidcrystal-like structures form and how they disturb nucleation of DAG oil. Figure 11 illustrates the structural model of the liquid-crystallike structures whose formation causes retardation of the nucleation of the high-melting fractions of DAG oil. The DAGs and PGFEs possess polar moieties and similar fatty acid chains. Specifically, DAG oil contains 1,3-PO as a major crystallizing component,15 and the PGFEs contained approximately even amounts of palmitic and oleic acid moieties. When DAG oil crystallizes without additives, 1,3-PO must form double-chainlength structures with a lamella distance of 4.6 nm, as illustrated in Figure 11A. This structure was confirmed for 1-stearoyl-3oleoylglycerol.41 In contrast, in the case of liquid-crystal-like structures composed of PGFEs and DAG, the glycerol backbones in DAG and the polyglycerol groups in PGFEs may form large polar group moieties as a result of polar interactions, and the fatty acid moieties may be aligned in the separated layer. In particular, palmitic and oleic acid moieties are arranged differently to form a triple-chain-length structure with two polar groups present in one unit layer (Figure 11B). Such types of triple-chain-length structures were widely observed in TAG crystals containing saturated and unsaturated moieties.33,34 The triple-chain-length structures of Figure 11B are liquid-crystal-like structures, not crystal structures. It is possible to estimate the layer thickness of the liquidcrystal-like structure in Figure 11B, composed of two palmitic lamella plus one oleic acid lamella (P−O−P type) or one palmitic lamella plus two oleic acid lamella (O−P−O type) and two polar groups. In the case of triple-chain TAG crystals such as POP and OPO, the lamella thickness is about 6 nm.42 In the present case, the layer thickness must be greater because of the flexibility of the fatty acids and polar groups as well as the liquid-crystal-like properties, likely leading to a layer thickness of 8.37 nm. Moreover, other high-melting fractions in DAG oils with similar chemical structures, such as 1-stearin-3-olein (1,3SO), might be partially incorporated through aliphatic and polar interactions, which may also contribute to an increase in the lamella thickness. The retardation mechanism of the nucleation of DAG crystals by the formation of liquid-crystal-like structures can be
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CONCLUSION We have investigated the mechanism for retardation of crystallization of DAG oil with PGFE additives. Birefringence occurred in DAG oil with PGFEs containing palmitic and oleic acid moieties, and the crystallization was significantly retarded. The changes in rheological properties and drastic changes in the X-ray diffraction patterns compared with those of DAG crystals suggested that liquid-crystal-like structures formed before crystallization. From these results, it can be considered that the retardation of crystallization of DAG oil is caused by the formation of liquid-crystal-like structures in which the highmelting fractions of DAGs are incorporated. To the best of our knowledge, this is the first report of a nucleation retardation mechanism. We can assume that similar phenomena may occur in TAG and other fats with other types of additives.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00676. Video of DAG oil with 1% PGFE at 5 °C under polarized crossed-Nicols conditions (quiescent) (AVI) Video of DAG oil with 1% PGFE at 5 °C under polarized crossed-Nicols conditions (stirred) (AVI) 4755
DOI: 10.1021/acs.cgd.7b00676 Cryst. Growth Des. 2017, 17, 4749−4756
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Katsuyoshi Saitou: 0000-0001-5952-0017 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Taiyo Kagaku Co., Ltd. (Mie, Japan) for supplying the PGFEs and Mr. Naoto Kudo and Mr. Yuki Mitsui of the Kao Corporation (Tokyo, Japan) for their technical advice and helpful discussion.
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ABBREVIATIONS POM, polarized optical microscopy; SAXD, small-angle X-ray diffraction; SFC, solid fat content; WAXD, wide-angle X-ray diffraction
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REFERENCES
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DOI: 10.1021/acs.cgd.7b00676 Cryst. Growth Des. 2017, 17, 4749−4756
