Triacylglycerol Crystal Growth: Templating Effects of Partial Glycerols

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Triacylglycerol Crystal Growth: Templating Effects of Partial Glycerols Studied with Synchrotron Radiation Microbeam X‑ray Diffraction Stefanie Verstringe,*,† Koen Dewettinck,† Satoru Ueno,‡ and Kiyotaka Sato‡ †

Laboratory of Food Technology and Engineering, Ghent University, Coupure Links 653, 9000 Ghent, Belgium Graduate School of Biosphere Science, Hiroshima University, 1-4-4 Kagamiyama, Higashi-Hiroshima 739-8528, Japan

‡

S Supporting Information *

ABSTRACT: Microstructural investigation of palm oil (PO) containing 8% monopalmitin (MP) was performed using synchrotron radiation microbeam (beam area of 5 × 5 μm2) X-ray diffraction (SR-μ-XRD). The blend was isothermally crystallized at 20 °C and analyzed shortly after reaching the crystallization temperature and after aging at 20 °C. The SR-μ-XRD technique allowed demonstration of the template effect of the monoacylglycerol (MAG) by showing that the PO crystals were orientated by the previously crystallized MP. Interactions through the palmitic acid moiety in PO and MP must have played decisive roles in this template effect as palmitic acid is a major fatty acid in PO. Analysis of aged samples revealed that the MP crystals migrate and concentrate into large spherulitic crystals during long-term storage, probably caused by a process of Ostwald ripening.

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INTRODUCTION Crystallization of lipids is of high significance in food, cosmetic, and pharmaceutical science and technology.1−3 One of the keys to control the crystallization process is the nucleation step. In general, two nucleation mechanisms can be distinguished: homogeneous nucleation and heterogeneous nucleation.4,5 In the former case, nucleation processes occur through molecular interactions among the crystallizing materials without any influences from foreign matters. In the latter case, however, the formation of crystal nuclei is catalyzed by crystallized matters or minor components which are accidentally present or added on purpose in the supercooled liquid of crystallizing materials. Adding such foreign materials in the crystallizing liquid, called templates or additives, is widely applied to modify the heterogeneous nucleation behavior of inorganic and organic substances including lipids.6−11 The necessary conditions for the template/additive to successfully modify the nucleation of lipid crystals are summarized as follows.12,13 (a) Similarity in molecular shape Similarity in molecular shape, such as saturation/unsaturation and chain length of the fatty acid moieties, between the template/additive and the lipid crystals is required. (b) Thermal stability Template/additive materials should not dissolve when they are added to supercooled liquid of lipids. Therefore, the template/additive materials should be thermally more stable than the crystallizing lipids. (c) Optimal supercooling © 2014 American Chemical Society

When the supercooling is high enough to induce spontaneous nucleation, the effects of the template/additive may be minimized because undesired crystals are spontaneously formed without being affected by the template/additives. Many studies have been performed to gain more insight into the effects of templates/additives on the nucleation of lipid crystals.14−17 For example, the effect of the palmitic sucrose ester P-170 on the isothermal crystallization kinetics of a highmelting fraction of milk fat in a mixture with sunflower oil was found to be strongly dependent on the supercooling.18 In addition, 0.1% Tween 60 was found to decrease the induction time of nucleation of a high-melting fraction of milk fat in a mixture with triolein.19 Further, the effect of the molecular shape and concentration of additives of polyglycerine fatty acid esters (PGFEs) on the crystallization kinetics of palm stearin was precisely studied.10 Together with diacylglycerols, monoacylglycerols (MAGs) are considered as the most important group of emulsifiers, representing about 70% of total food emulsifier production.20 Consequently, the effect of MAGs is frequently studied because they are widely used to control lipid crystallization in bulk and emulsion states.7,8,11,16,21−24 For example, the effect of MAGs of hydrogenated palm oil (PO) and of sunflower oil on the crystallization of PO was studied, with observation of a promotion effect for the former and no Received: July 8, 2014 Revised: August 26, 2014 Published: August 29, 2014 5219

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effect for the latter MAGs.8 More recently, we examined the effects of the addition of monopalmitin, both pure and in commercial form, on the crystallization of PO, with the abovementioned conditions for a successful modificiation of the nucleation all fulfilled. It was found that the crystallization process of the MAG/PO blends is initiated by the MAGs and that the crystallization of PO was promoted.14,15 The present study aims at precisely observing the detailed mechanisms of the effects of monopalmitin (monopalmitoylglycerol, MP) on the nucleation of PO crystals. For this purpose, we applied microbeam X-ray diffraction methods (μXRD). μ-XRD analysis can provide microscopic information about crystallized materials on the order of micrometer to submicrometer dimensions.25−27 As for lipid crystals, studies have been conducted on spherulites,28 O/W emulsion,17,29 W/O emulsion,16 granular crystals in fat spread,30 and binary mixture phases of lipids.31 The present work is the first to focus on the heterogeneous nucleation of lipid crystals in neat bulk liquid, demonstrating the interactions between lipids and a high melting emulsifier (MP). In the present study, we confirmed that the morphology of the PO crystals was clearly controlled by the MP additive through template effects.

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Figure 1. Data analysis of SR-μ-XRD patterns. (a) 2D SR-μ-XRD pattern. (b) Direction of the lamellar plane in a fat crystal. (c) χ extension pattern. Japan. The wavelength of the X-ray microbeam was 0.124 nm, and the focused beam area was 5 × 5 μm2. Samples were placed between Kapton tape films and were moved by an x-y-z stepping motor (1 μm step) while being observed by an optical microscope attached to the microbeam instrument. Application of a temperature profile and temperature control were done using a temperature-controlled Linkam stage (Tadworth, UK). X-ray data were detected by a CCD camera to produce 2D patterns. Technically, it is possible to observe small-angle and wide-angle diffraction patterns simultaneously. However, a lower precision of the long spacing patterns is obtained because of the limited relationship between the distance between the sample and the 2D detector, and the size of the 2D detector. Therefore, only smallangle SR-μ-XRD patterns were observed in this study, so that diffraction peaks corresponding to long spacing values could be obtained with a high enough resolution. Polarized Light Microscopy (PLM). A Leitz Diaplan light microscope (Leitz, Wetzlar, Germany) was used to view the sample microstructure at 20 °C. The details of the experimental setup are reported elsewhere14 (see SI).

EXPERIMENTAL SECTION

Materials. Refined, bleached, and deodorized PO was obtained from Loders Croklaan (Wormerveer, The Netherlands) and was used as received. Pure monopalmitin (>99%, MP) was obtained from NuChek Prep (Elysian, USA). Preparation of the Palm Oil−Monopalmitin Blend. 8% w/w MP was dispersed in melted PO and stirred with a magnetic stirrer at 80 °C until a homogeneous sample was obtained. When the blend was visibly free of dispersed material, it was further mixed for at least 2 h. The blend was stored at −24 °C until analysis. Differential Scanning Calorimetry (DSC). The DSC experiments were performed with a Q1000 DSC (TA Instruments, New Castle, USA). The details of the experimental setup are reported elsewhere14 (see Supporting Information (SI)). Stop-and-return experiments were perfomed by applying the following time− temperature program: holding at 80 °C for 10 min to ensure complete melting and to erase the crystal memory, cooling at −10 °C/ min to 20 °C, holding at 20 °C for a given isothermal period, and heating at 20 °C/min until completely melted. The crystallization process was interrupted during the early stages of isothermal crystallization and after 4 weeks. During the isothermal period of 4 weeks, the DSC pan was stored in a thermostatic cabinet at the crystallization temperature. Synchrotron Radiation Microbeam Small-Angle X-ray Diffraction. An elaborate discussion of the principle of the SR-μXRD technique is reported elsewhere.25 Briefly, local structural information is obtained by scanning a thin section of the sample in two dimensions with an X-ray microbeam. In this micrometerdimension area, two-dimensional (2D) XRD patterns are collected with a 2D X-ray sensitive area detector (Figure 1a). Long spacing values are calculated by the diffraction angle (2θ) extension. In addition, the direction of the lamellar plane of the fat crystals (Figure 1b) can be assessed by measuring the azimuthal angle (χ) extension pattern at a fixed 2θ position (Figure 1c). When all the fat crystals are highly oriented, two sharp 2D diffraction peaks (arc peaks) should appear. In this case, the average direction of the lamellar planes of the fat crystals is directed normal to the direction connecting the two arc peaks. The degree of orientation of the lamellar planes of the fat crystals can be evaluated by the half width of the peaks. A smaller Δχ indicates a higher degree of orientation of the lamellar planes. The SR-μ-XRD measurements were carried out at beamline 4A of the Photon Factory (PF), the synchrotron radiation facility of the High-Energy Accelerator Research Organization (KEK) in Tsukuba,

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RESULTS AND DISCUSSION Crystallization Process of 8% Monopalmitin in Palm Oil. The isothermal and non-isothermal crystallization mechanism of PO and PO containing 8% MP has been studied before.14,15 In these previous studies, it was concluded that the crystallization process of MP/PO blends is initiated by the MAG which starts to crystallize in the α polymorph followed by a polymorphic transformation to sub-α, as confirmed with XRD measurements. The prior crystallization of MP caused an earlier crystallization of the PO stearin fraction (stearin crystallization onset temperature was 24.27 ± 0.06 °C for the 8% MP blend versus 18.42 ± 0.24 °C for PO). This suggests a template effect of the MP. To investigate this suggestion, SR-μ-XRD was used to determine whether the previously crystallized MP influences the PO TAG crystal orientation. Therefore, an 8% MP sample was held at 80 °C for 10 min to erase the crystal memory and then cooled to 20 °C at a cooling rate of −10 °C/min. After an isothermal time of 10 min at 20 °C, recording of the SR-μ-XRD patterns was started. The sample was moved automatically within a 2D plane in 20 μm steps. Figure 2 depicts the patterns taken at the different positions, and an enlarged 2D diffraction pattern of the position indicated with an arrow as a typical example. The diffraction angle (2θ) and azimuthal angle (χ) extensions are indicated, providing information on respectively the long spacing and the lamellar plane direction of the crystals present. 5220

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the direction denoted by the broken lines in the figure. The representation of the lamellar plane orientation on the diffraction pattern is shown in Figure 3c. The black line represents the orientation of the PO crystals, while the red line shows the orientation of the MP crystals. Figure 4 depicts the orientation of MP and PO β′ crystals at the different scanning positions, deduced from the χ extension

Figure 2. SR-μ-XRD patterns recorded at all positions in a PO sample containing 8% MP and crystallized at 20 °C for 10 min. An enlarged 2D diffraction pattern of the position noted by an arrow is shown on the right side.

The 2θ extension pattern of this example position is shown in Figure 3a. A peak corresponding with a long spacing of d = 46 Å can be discerned, which corresponds to the long spacing of MP.32 Lutton33 confirmed that all the polymorphs of a given MAG have almost identical long spacings, corresponding with tilted chains in a double chain length structure. Therefore, it is impossible to deduce the polymorphic form affirmatively from only SAXD results. However, as the isothermal crystallization time was limited at the time of measurement, the MP was most probably still present in the sub-α polymorphic form. Next to the MP peak, also a shoulder with a long spacing value of d = 43 Å is present. This value corresponds with the long spacing of PO β′ crystals.34 The χ extension for both crystal species is shown in Figure 3b. Two peaks, 180° separated, are present in both cases at χ = 163° and 343°. No other peaks are present so it can be concluded that, at this scanning position, the lamellar planes of both the MP and the PO β′ crystals are aligned along

Figure 4. SR-μ-XRD patterns recorded at all positions in a PO sample containing 8% MP and crystallized at 20 °C for 10 min with indication of the lamellar plane orientation of PO crystals (black lines) and MP crystals (red lines).

patterns. It can be seen that MP crystals are present at nearly every position and that they show a clear orientation in almost

Figure 3. Patterns of the position denoted with an arrow in Figure 2. (a) 2θ extension pattern. (b) χ extension pattern of MP (...) and PO β′ (__). (c) 2D diffraction pattern with representation of the crystal orientation. The black line represents the orientation of PO β′ crystals, the red line of MP crystals. 5221

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Figure 5. PLM images of PO and PO containing 8% MP recorded directly after reaching the isothermal crystallization temperature (upper images) and after 4 weeks of crystallization at 20 °C (lower images). Scale bars represent 200 μm.

every case. In general, it can be stated that when PO crystals are present, they are oriented in the same direction as the MP crystals. These results clearly illustrate that MP orientates the PO crystals, thus demonstrating the template effect of MP. Long-Term Crystallization Behavior of 8% Monopalmitin in Palm Oil. The long-term storage behavior of the 8% MP blend crystallizing at 20 °C was investigated, revealing β formation during aging. This can be seen in Figure 5, showing PLM images of PO and the 8% MP blend recorded directly after reaching the isothermal crystallization temperature and after 4 weeks of crystallization at 20 °C. The MP blend showed a coarser crystal structure at the start of crystallization, as described earlier.14,15 In contrast to PO, formation of large, needle-like crystals could be observed already after 1 week of storage of the MP blend (images recorded after 4 weeks are displayed). This appearance is typical for β crystals.35 The presence of β crystals after 4 weeks of storage was confirmed by wide-angle X-ray diffraction measurements (results not shown) and was clear from the appearance of a DSC melting peak with a very high melting point (>70 °C). This can be seen in Figure 6, which shows the melting profiles of the 8% MP blend recorded after an isothermal crystallization time at 20 °C of 1 min, 30 min, and 4 weeks. A melting peak with peak maximum at 72 °C is present after 4 weeks of storage, indicating a transition to a stable polymorph has taken place. During the melting process of the blend stored for 4 weeks, only the high-melting β crystals are still present when the temperature reaches 50 °C. At this temperature, the large crystal structures are still present as can be seen in Figure 7, showing PLM images recorded at different temperatures during the melting process of the blend stored for 4 weeks. This confirms that the large crystal structures are composed of β crystals. Moreover, β crystals are also present in between the large spherulites, as can be seen in the PLM image recorded at 50 °C.

Figure 6. Melting profiles of the 8% MP blend crystallizing at 20 °C as measured by DSC. The isothermal crystallization was interrupted after 1 min (__), 30 min (__- -) and 4 weeks (- -).

SR-μ-XRD was used to determine whether the observed large β crystal structures are composed of MP, PO, or both. It is known that a β polymorph develops during long-term storage of pure MP.32 Furthermore, aging of a C18 MAG in liquid oil revealed the formation of large β aggregates.36 On the other hand, PO is highly stable in the β′ polymorph because of the diversity in fatty acid chain length.35,37 However, in POcontaining margarine systems, segregation of POP and transition from β′ to β is known to occur.38,39 This phenomenon is favored by the presence of liquid oil, which allows crystal mobility, and is induced by temperature cycling. In PO with 1% MP added, Miura et al.40 observed granular crystals but did not investigate polymorphism of the blends. Basso et al.7 concluded that the addition of 1% of fully 5222

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Figure 8. SR-μ-XRD patterns recorded at all positions in an 8% MP sample crystallized at 20 °C for 1 week and analyzed at 55 °C. Lamellar plane orientation of the MP crystals is indicated. The inset shows the scanned crystal.

Figure 7. PLM images recorded at different temperatures during the melting process of the 8% MP blend crystallized at 20 °C for 4 weeks. Scale bars represent 200 μm.

Figure 9. SR-μ-XRD patterns recorded at all positions in an 8% MP sample crystallized at 20 °C for 1 week and analyzed at 20 °C. Lamellar plane orientation is indicated for PO β′ crystals (black lines) and MP crystals (red lines). The inset shows the scanned area.

hydrogenated palm-based MAGs favored the formation of β crystals in PO. To determine the compositional structure of the large β crystal structures observed in a MP/PO blend, an 8% MP sample, crystallized at 20 °C for 1 week, was heated to 55 °C and held at that temperature for 10 min to melt all unstable crystals. A large β crystal structure was scanned in two dimensions in 15 μm steps. The analysis revealed the presence of only MP crystals (d = 46 Å). Figure 8 shows the scanned crystal, the scanning patterns at the different positions, and the orientation of the lamellar planes of the MP crystals. The orientation pattern is typical for a spherulitic crystal. It can be concluded that the MP present in an 8% MP in PO blend shows a rapid transition to the β polymorph, thereby forming large, needle-like spherulitic crystals. However, as evidenced by the PLM image recorded at 50 °C during the melting process of the aged 8% MP sample (Figure 9), β crystals are also present in between the large spherulitic structures. Therefore, an area of an 8% MP sample, crystallized at 20 °C for 1 week, which did not comprise large spherulitic

crystals, was scanned in two dimensions in 20 μm steps. The scanning occurred at a temperature of 20 °C. It was found that both MP and PO β′ crystals are present at this temperature. The polymorphic form of the MP crystals could not be identified. However, the melting profile after 4 weeks of storage shows no sub-α and α peaks anymore (Figure 6). Therefore, it was assumed that all the MP was transformed to β. Figure 9 shows the scanned area, the scanning patterns at the different positions, and the lamellar plane orientation of the present MP and PO β′ crystals. The spatial distribution of MP and PO crystals after 1 week of crystallization at 20 °C (Figure 9) is different from that after 10 min at 20 °C (Figure 4). In the sample analyzed shortly after reaching the crystallization temperature, MP was detected at nearly every position. However, this is not the case anymore after 1 week of crystallization, where MP is concentrated with randomly oriented PO crystals in between. This is most likely 5223

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Visual signs of concentration/migration were already visible after 1 day of crystallization at 20 °C. Moreover, after 2.5 h of crystallization, the melting peaks of the MP already showed a shift to higher temperatures (Figure 11). Therefore, it can be

due to migration of the MP crystals. This suggestion is further reinforced by the examination of the PLM images recorded during the melting process of an aged 8% MP sample (Figure 7), where it can be seen that the area surrounding the large spherulite is depleted of MP (PLM image at 50 °C). It is likely that MP crystals from this area have migrated to form the large spherulitic crystal. This observation supports the assumption that MP is concentrated during long-term storage through migration, with the large spherulites as extreme example of this segregation process. Eventually, the segregation process comes to an end when all MP crystals have gathered in spherulite crystals. This statement is supported by the PLM images shown in Figure 10, recorded during melting of an 8% MP blend aged

Figure 11. Melting profiles of the 8% MP blend crystallizing at 20 °C as measured by DSC. The isothermal crystallization was interrupted after 1 min (__), 30 min (__- -), and 150 min (- -).

assumed that the migration process is probably linked to the transformation of MP to the β polymorphic form. In MAG/ water systems, the transformation from α to β or from the α-gel state to the coagel state is believed to occur via a two-stage mechanism.41,42 First, the D- and L-isomers of the MAG are separated in the bilayers of the α-gel. In the second stage, growth in the third dimension occurs by the stacking of the bilayers, thus forming β crystal plates with all the water expelled.41 This second stage has been allocated as an example of Ostwald ripening, proceeding through a mechanism of monomeric transport.42 Similarly, we assume that a process of Ostwald ripening is responsible for the observed migration of MP in a blend with PO. The solid fat content of the 8% MP blend has been determined and is around 30%. Probably, a substantial amount of liquid oil is a necessary prerequisite for the segregation process to occur. Further research is necessary to determine if the segregation process of MP is prevented in systems that contain less liquid oil.

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CONCLUSIONS Microstructural investigation of PO containing 8% MP using SR-μ-XRD gave more insight into the crystallization process of this blend as summarized in the following. (1) MP, which crystallized prior to the PO TAGs, promoted PO crystallization and orientated the PO crystals. (2) During long-term storage, the MP crystals migrated and concentrated into large spherulitic crystals. (3) PO and MP crystals separated during aging, which was probably caused by Ostwald ripening of the MP crystals. Using SR-μ-XRD, the template effect of a MAG could thus be demonstrated. As palmitic acid is a major fatty acid in PO, interactions through the palmitic acid moiety in PO and MP must have played decisive roles in this template effect. Such interactions between the template/additive and lipid crystals were also documented in PGFE/palm stearin systems and a PO system with added MAGs of hydrogenated PO.8,10 However,

Figure 10. PLM images recorded at different temperatures during the melting process of an 8% MP blend crystallized at 20 °C for 8 months. Scale bars represent 200 μm.

at 20 °C during 8 months. At a temperature of 50 °C, big spherulitic β crystals are present, similar to the melting process after 4 weeks. In contrast, after 8 months, no β crystals are present anymore in between the spherulitic crystals, as opposed to the melting process after 4 weeks (Figure 7). All MP crystals have thus migrated to the large spherulites. 5224

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(13) Sato, K.; Bayés-García, L.; Calvet, T.; Cuevas-Diarte, M. À .; Ueno, S. External factors affecting polymorphic crystallization of lipids. Eur. J. Lipid Sci. Technol. 2013, 115 (11), 1224−1238. (14) Verstringe, S.; Danthine, S.; Blecker, C.; Depypere, F.; Dewettinck, K. Influence of monopalmitin on the isothermal crystallization mechanism of palm oil. Food Res. Int. 2013, 51 (1), 344−353. (15) Verstringe, S.; Danthine, S.; Blecker, C.; Dewettinck, K. Influence of a commercial monoacylglycerol on the crystallization mechanism of palm oil as compared to its pure constituents. Food Res. Int. 2014, 62, 694−700. (16) Wassell, P.; Okamura, A.; Young, N. W. G.; Bonwick, G.; Smith, C.; Sato, K.; Ueno, S. Synchrotron radiation macrobeam and microbeam X-ray diffraction studies of interfacial crystallization of fats in water-in-oil emulsions. Langmuir 2012, 28 (13), 5539−5547. (17) Arima, S.; Ueno, S.; Ogawa, A.; Sato, K. Scanning microbeam small-angle X-ray diffraction study of interfacial heterogeneous crystallization of fat crystals in oil-in-water emulsion droplets. Langmuir 2009, 25 (17), 9777−9784. (18) Huck-Iriart, C.; Candal, R. J.; Herrera, M. L. Effects of addition of a palmitic sucrose ester on low-trans-fat blends crystallization in bulk and in oil-in-water emulsions. Food Biophys. 2009, 4 (3), 158− 166. (19) Litwinenko, J. W.; Singh, A. P.; Marangoni, A. G. Effects of glycerol and Tween 60 on the crystallization behavior, mechanical properties, and microstructure of a plastic fat. Cryst. Growth Des. 2004, 4 (1), 161−168. (20) Moonen, H.; Bas, H., Mono- and diglycerides. In Emulsifiers in Food Technology, Whitehurst, R. J., Ed.; Blackwell Publishing Ltd.: Oxford, 2004; pp 40−58. (21) Goldstein, A.; Marangoni, A.; Seetharaman, K. Monoglyceride stabilized oil in water emulsions: an investigation of structuring and shear history on phase behaviour. Food Biophys. 2012, 7 (3), 227−235. (22) Cheong, L.-Z.; Zhang, H.; Xu, Y.; Xu, X. Physical characterization of lard partial acylglycerols and their effects on melting and crystallization properties of blends with rapeseed oil. J. Agric. Food Chem. 2009, 57 (11), 5020−5027. (23) Martini, S.; Herrera, M. L. Physical properties of shortenings with low-trans fatty acids as affected by emulsifiers and storage conditions. Eur. J. Lipid Sci. Technol. 2008, 110 (2), 172−182. (24) Ghosh, S.; Tran, T.; Rousseau, D. Comparison of pickering and network stabilization in water-in-oil emulsions. Langmuir 2011, 27 (11), 6589−6597. (25) Ueno, S., New method to study molecular interactions in fats synchrotron radiation microbeam X-ray diffraction. In Cocoa Butter and Related Compounds; Garti, N.; Widlak, N. R., Eds.; AOCS Press: Urbana, 2012; pp 339−363. (26) Ice, G. E.; Budai, J. D.; Pang, J. W. L. The race to x-ray microbeam and nanobeam science. Science 2011, 334 (6060), 1234− 1239. (27) Paris, O. From diffraction to imaging: New avenues in studying hierarchical biological tissues with x-ray microbeams (Review). Biointerphases 2008, 3 (2), FB16−FB26. (28) Ueno, S.; Nishida, T.; Sato, K. Synchrotron radiation microbeam X-ray analysis of microstructures and the polymorphic transformation of spherulite crystals of trilaurin. Cryst. Growth Des. 2008, 8 (3), 751−754. (29) Shinohara, Y.; Takamizawa, T.; Ueno, S.; Sato, K.; Kobayashi, I.; Nakajima, M.; Amemiya, Y. Microbeam X-ray diffraction analysis of interfacial heterogeneous nucleation of n-hexadecane inside oil-inwater emulsion droplets. Cryst. Growth Des. 2008, 8 (9), 3123−3126. (30) Tanaka, L.; Tanaka, K.; Yamato, S.; Ueno, S.; Sato, K. Microbeam X-ray diffraction study of granular crystals formed in water-in-oil emulsion. Food Biophys. 2009, 4 (4), 331−339. (31) Bayés-García, L.; Calvet, T.; Cuevas-Diarte, M. À .; Ueno, S.; Sato, K. Heterogeneous microstructures of spherulites of lipid mixtures characterized with synchrotron radiation microbeam X-ray diffraction. CrystEngComm 2011, 13 (22), 6694−6705.

the present study has provided much clearer evidence of the template effect by using the SR-μ-XRD technique to show the molecular level interaction.

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary text describing the details of the experimental setup for carrying out differential scanning calorimetry (DSC) experiments and polarized light microscopy (PLM) experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 0032/ 9.264.61.68. Fax: 0032/9.264.62.18. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.V. is a research assistant of the Fund for Scientific Research − Flanders (F.W.O. − Vlaanderen). The authors acknowledge the financial support of F.W.O. − Vlaanderen through a travel grant for a long stay abroad. The experiments were performed with the approval of the Photon Factory Program Advisory Committee (Proposal Nos. 2012G165 and 2012G166). The authors gratefully acknowledge the help of Prof. A. Iida, Station Manager of Beamline 4A at Photon Factory (KEK Institute, Tsukuba, Japan).

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dx.doi.org/10.1021/cg5010209 | Cryst. Growth Des. 2014, 14, 5219−5226