Effects of Self-Assembled Monolayers on Selective ... - ACS Publications

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Effects of Self-Assembled Monolayers on Selective Crystallization of Tolbutamide Jinli Zhang,‡ Anyuan Liu,‡ You Han,† Yan Ren,† Junbo Gong,§ Wei Li,*,† and Jingkang Wang§ †

Key Laboratory for Green Chemical Technology of Ministry of Education, ‡Key Laboratory of Systems Bioengineering of Ministry of Education, and §School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, People’s Republic of China

bS Supporting Information ABSTRACT: Selective crystallization of tolbutamide on SAMs with the similar molecule structure of tolbutamide and the functional groups of methyl, trifluoromethyl, and phenyl respectively was studied through characterizations of XRD and crystal morphologies. It is indicated that at low supersaturation tolbutamide (TB) crystallized into form II on the methyl-terminated SAMs and trifluoromethyl-terminated SAMs, whereas the highest bioavailable TB crystals of form IV were obtained on phenyl-terminated SAMs, differing from the precipitated crystals from the solution including TB forms I, II, and III together. The preferential growth orders of the crystallographically important faces of TB forms IIV were assessed on different functionalized SAMs through molecular modeling based on the prediction of equilibrium morphologies of crystals, which is in accordance with the experimental results. Further time-resolved Raman spectra of TB crystals grown on phenyl-terminated SAMs illustrate that the surface functional groups are paramount to adjust the heterogeneous nucleation of tolbutamide polymorphs at low supersaturation. This work provides a feasible approach combining the experimental with molecular modeling methods to understand deeply the relationship between interfacial functional groups of SAMs and molecular packing of crystals, which is fundamental to the rational design of experimental work on selective crystallization of organic crystals.

’ INTRODUCTION Polymorphism, the feature of a substance to exist in two or more crystalline phases that have different arrangements of the molecules in the crystal lattice, has attracted more attention in the field of pharmaceuticals, pigments, agrochemicals, and so on.13 Different polymorphs exhibit distinct physicochemical properties, such as stability, solubility, and bioavailability, etc.4,5 Inspired by nature’s use of self-assembly to construct complex hierarchical structures, LangmuirBlodgett films6,7 and selfassembled monolayers (SAMs),815 especially siloxane SAMs,16 have been considered as promising approaches to control and define crystal nucleation and growth, except for conventional factors influencing crystallization, such as the solvent, the supersaturation, temperature, and cooling rates.17 For instances, Thalladi and co-workers reported that the perfluoroalkyl-terminated siloxane SAMs can promote the exclusive growth of the stable polymorph of indomethacin.18 Swift and co-workers studied the template-directed nucleation and growth of 1,3-bis(m-nitrophenyl)urea (MNPU) on short-chain siloxane SAMs and discovered a new crystal form ε of MNPU on such SAMs.19 Whereas some functionalized SAMs can affect the crystal habit rather than the polymorph, e.g., Myerson and co-workers reported that COOHterminated SAMs crystallized needlelike α-glycine, unlike the bipyramidal crystals of α-glycine that precipitated from the aqueous solution.20 So far it is elusive as to how the functional SAMs affect the growth of crystal polymorphs. r 2011 American Chemical Society

Prediction of crystal morphology has been recently adopted as a useful tool to reveal the interactive mechanism of SAMs toward the selective crystallization of crystal polymorphs. Myerson and co-workers studied the nucleation and growth of L-alanine and DL-valine on SAMs with the functional groups of 40 -hydroxy-(4-mercaptophenyl), 40 -methyl-(4-mercaptophenyl), or 4-(4-mercaptophenyl) pyridine on gold (111) surfaces, and through using molecular modeling methods they explained the influence of the interfacial interactions from the SAMs terminal groups on the growth of different crystal habits.21 Dressler et al.22 utilized molecular dynamics calculations to study the growth of metastable α-L-glutamic acid on SAMs of a phenylalanine derivative, in combination with experimental data, and they showed that the absence of the (011) and (200) facets is crucial to stabilize the α-L-glutamic form so as to prevent the transformation to the stable β-form. Thus, it is fundamental to study intensively the interaction modes of how the functionalizedSAMs affect the growth of crystal polymorphs for the rational design of experimental work on selective crystallization of polymorphic drugs, through the combination of experimental data and molecular modeling methods. Received: August 18, 2011 Revised: November 3, 2011 Published: November 03, 2011 5498

dx.doi.org/10.1021/cg201083r | Cryst. Growth Des. 2011, 11, 5498–5506

Crystal Growth & Design In this work, tolbutamide [1-butyl-3-(4-methylphenylsulfonyl)urea, TB], which is an oral hypoglycemic agent and used in the treatment of insulin-dependent diabetic patients, was selected as a model system (Figure 1a).23 TB has four polymorphic forms.2426 Form I is stable, has an orthorhombic structure, and belongs to the space group Pna21 with a = 19.626 Å, b = 7.803 Å, c = 9.058 Å, and β = 90o. Form II is a monoclinic structure belonging to the space group Pc with a = 9.087 Å, b = 17.228 Å, c = 17.951 Å, and β = 95.01o. Form III is a monoclinic structure belonging to the space group P21/n with a = 11.735 Å, b = 9.042 Å, c = 13.732 Å, and β = 103.57o. Form IV is a monoclinic structure belonging to the space group P21/c with a = 10.091 Å, b = 15.646 Å, c = 9.261 Å, and β = 100.49o. The melting temperature of TB increases in the order form I (128 °C) > form II (117 °C) > form III (106 °C) > form IV (94 °C),26,27 whereas the bioavailability of TB decreases in the order form IV > form II > form III > form I.28 It is reported that crystals of TB form I, II, and III can be crystallized from solutions but form IV is difficult to crystallize directly from solutions.29 The selective crystallization of TB was first studied by SAMs with the similar molecule structure of tolbutamide and the functional groups of methyl, trifluoromethyl, and phenyl, respectively (Figure 1b). XRD patterns and photomicrographs of crystals indicate that the phenyl-terminated SAMs are beneficial to the growth of highly bioavailable TB forms II and IV. Then through molecular modeling based on the prediction of equilibrium morphologies of crystals, the preferential growth orders of the crystallographically important faces of TB forms IIV are assessed on different functionalized SAMs, which is in accordance with the experimental results. Further characterizations of time-resolved Raman spectra of TB crystals grown on phenyl-terminated SAMs illustrate that at low supersaturation the surface functional groups are paramount to adjust the heterogeneous nucleation of

Figure 1. (a) Molecular structure of TB. (b) Schematic diagram of three kinds of SAMs and their designation in our work.

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tolbutamide. This work provides a feasible approach combining the experimental with molecular modeling methods to understand deeply the relationship between the functionalized SAMs and the selective crystallization of polymorphic drugs.

’ EXPERIMENTAL AND MODELING METHODS Experimental Materials. (3-Aminopropyl)trimethoxysilane (g95%), p-toluenesulfonyl chloride (g97%), 4-phenylsulfonyl chloride (g97%), and 4-(trifluoromethyl)benzenesulfonyl chloride (g98%) were purchased from Alfa Aesar. Tolbutamide (g98%) was purchased from Reihefeng Technology & Trade Co., Ltd., and used without further purification. Glass slides (25 76 1 mm) of optical microscopes were purchased from Qinning Glass Co., Ltd. Solvents with a purity higher than 99%, including ethanol, chloroform, 2-propanol, cyclohexane, dichloromethane, and triethylamine, were purchased from Binhai Chemical Plant, Tianjin, China. Modification of Substrates Using SAMs. The glass slides were sonically cleaned by using distilled water, chloroform, and ethanol in turn and dried by a nitrogen purge, followed by the treatment of piranha solution (70% H2SO4:30% H2O2) for 30 min at 80 °C so as to make the substrate surface distributed with hydroxyl groups. The hydroxyl-coated substrate was washed using distilled water, 2-propanol, and distilled water in turn, dried, and then immerged into 0.1 M (3-aminopropyl)trimethoxysilane cyclohexane solution for 2 h at 25 °C to make the substrate surface coated with amino groups. Three kinds of SAMs, i.e., CH3-SAMs, Ph-SAMs, and CF3-SAMs (Figure 1b), were linked covalently onto the dried amino-coated substrates by using respectively the precursor compound of p-toluenesulfonyl chloride, 4-phenylsulfonyl chloride, and 4-(trifluoromethyl)benzenesulfonyl chloride. The substrate was immerged into a freshly prepared 10 mM precursor dichloromethane solution, containing 10 mM triethylamine, for 2 h at 25 °C, and then washed using ethanol and dried by a nitrogen purge. In order to confirm the covalent linkage of SAMs with different functional groups on the surfaces, the SAM-coated substrates were characterized by XPS, ATR-FTIR, and the contact angle measurements, as shown in the section S1 in the Supporting Information. Crystallization on SAM-Functionalized Substrates. Tolbutamide supersaturated ethanol solutions were prepared at 40 °C by ultrasonic dissolving methods, followed by filtration through a 0.45 μm filter, and then introduced into a copper tank installed in the water bath with a programmed temperature controlling system. SAM-modified substrates were placed vertically in the copper tank to let crystals grow onto the surfaces of substrates. The tolbutamide/ethanol solution was cooled from 25 to 15 °C (with the cooling rate of 0.10 °C min1) and then maintained at 15 °C for 2 h. The supersaturation of tolbutamide solution was adjusted in the range of 1.05 (8.11 g/100 mL) to 1.20 (9.27 g/100 mL) at 15 °C,

Figure 2. The schematic of SAMs modeling. (a) Si(100) 8 4 reconstructed surface. The red atoms represent substituted sites and the yellow atoms represent unsubstituted sites. (b) Typical example of a suitable distance 3 3 CF3-SAMs model. (c) The constrained display style of the model; red atoms are fixed and the gray are free. 5499

dx.doi.org/10.1021/cg201083r |Cryst. Growth Des. 2011, 11, 5498–5506

Crystal Growth & Design

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Figure 3. XRD patterns of TB polymorphs grown on CH3-SAMs (a), CF3-SAMs (b), and Ph-SAMs (c) and those precipitated from the solution and on the bare glass (d). (e) Theoretical patterns of TB form IIV based on the crystallographic data of Thirunahari.26 based on the tolbutamide solubility in ethanol of 7.72 g/100 mL at 15 °C, determined via the balance method using a UVvis spectrophotometer.30 The powder X-ray diffraction patterns of crystallized tolbutamide were collected for phase identification on a Rigaku Geigerflex D/MXA 2500 v/ PC diffractometer with Cu Kα radiation at 40 kV. Morphologies of tolbutamide crystals were observed using a polarizing microscope (Sunny Optical Technology XY-P) equipped with a CCD color camera (YESONE, CS080). Raman spectra of crystallized tolbutamide were recorded using a Renishaw series RM2000 spectrometer with a 514 nm excitation wavelength (50 mW) from an argon ion laser. Scans were performed in a spectral window ranging from 200 to 4000 cm1 with a sample exposure time of 10 s and at least 3 accumulations.

Modeling Methods. The crystal faces grow slowly after nucleation in the low supersaturation region, and the effect of supersaturation on crystallization trailed off. In this study, to explore the influence of functionalized SAMs on the selective crystallization of tolbutamide, we adopted crystal habits prediction and binding energy calculations.21,22,31 The crystal habits prediction was performed in the Morphology module, while the binding energy calculation and TB crystal structure optimizations were performed using molecular dynamic calculation (MD) and the minimization method in the Discover module with the COMPASS force field.32 All calculations were carried out using the Material Studio 5.0 package. Morphological Predictions. The morphology of each TB crystal is predicted using the attachment energy method (AE)33 and the BravaisFriedelDonnayHarker (BFDH) model.34,35 The BFDH 5500



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Figure 4. Photomicrographs of tolbutamide crystals precipitated from the solution (a) and grown on the CH3-SAMs (b), CF3-SAMs (c), and Ph-SAMs (d) (all pictures were magnified 100 times).

Figure 5. Predicted morphologies of TB IIV polymorphs by the AE method and the BFDH method, respectively. method relates the crystal habit to the lattice geometry and assumes an inversely proportional relationship between the growth rate and the interplanar distance. The AE method assumes that the growth rate of a surface is proportional to its attachment energy in a vacuum, i.e., the energy released by adding a growth slice to the existing crystal surface. It is known that the crystal habit depends on the relative growth rate of the individual faces. Faces with the slowest growth rate have the greatest morphological importance. In contrast, the fastest growing face has the least morphological importance. Binding Energy Modeling and Calculations. The binding energy studies were carried out for the habit surface of different TB forms and self-assembled monolayer, using minimization techniques.

The well-known Si(100) face36 was chosen as the basement with a monolayer coverage ratio of 50%37 to establish the molecular modeling of SAMs. Figure 2a indicates a 8 4 Si(100) basement and a zigzag packing pattern of substituted sites, which shows a distance of 5.431 Å between two adjacent substituted atoms. Figure 2b is a representative of constructed monolayer model of CF3-SAMs with the distance between two nearest monolayer molecules of about 5.430 and 5.437 Å. After cleaving the morphologically important faces of TB polymorphs, the SAMs molecules were docked on these crystal surfaces for the binding energy calculation. During the calculation, some atoms in the monolayer models close to the silicon substrate are fixed in order to make simplification, as shown in Figure 2c. 5501



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Table 1. Selected Crystallographic Important Faces of TB Forms IIV TB

crystallographically important faces

form I

(200)

(110)

(201)

form II form III

(010) (101)

(011) (101)

(100) (011)

(110)

form IV

(100)

(110)

(011)

(020)

(120)

(301)

The binding energy (ΔEbinding) of each crystal face per SAM molecule was defined as ΔEbinding ¼

1 ðEtotal ESAMs Ecry Þ N

where Etotal is the minimum energy of the SAM molecule interacted with the crystal face, ESAMs is that of the monolayer alone, and Ecry is that of the crystal face, and N is the number of SAM molecules in the monolayer models. A negative value of ΔEbinding indicates preferential binding of the crystallographic face with the monolayer, whereas a positive value suggests an adverse interaction between the crystal face and the monolayer. The more negative the value, the easier binding tendency is.

’ RESULTS AND DISCUSSION Selective Crystallization of Tolbutamide on Functionalized SAMs. Figure 3 shows experimental powder XRD

patterns of crystals grown on SAMs with different functional terminals involving methyl, trifluoromethyl, and phenyl groups and calculated powder XRD patterns of TB forms IIV. It is apparent that under the low supersaturation of 1.05 on the methyl-terminated SAMs (CH3-SAMs) tolbutamide crystallized into metastable form II, as indicated by the diffraction peaks typical for tolbutamide form II crystal, e.g., 2θ = 10.2°, 11.3°, and 19.6°, and similar form II was crystallized on the trifluoromethyl-terminated SAMs (CF3SAMs). In samples where the supersaturation increases from 1.1 to 1.2, diffraction peaks are representative of TB form I (2θ = 8.7°, 12.1°, and 20.0°) growing concomitantly with form II (2θ = 10.2°, 11.3°, and 19.6°) on the methylterminated SAMs; however, on the trifluoromethyl-terminated SAMs there exist peaks of form II concomitantly with those of form III (2θ =15.4°, 25.8°) at higher supersaturation. As for the phenyl-terminated SAMs (Ph-SAMs), tolbutamide form IV (2θ = 10.6°, 18.0°, and 18.8°), the highest bioavailable form, was crystallized at the supersaturation of 1.05, whereas as the supersaturation increases, TB form II turns to be the dominant crystal over form IV. As a control, XRD patterns of the crystals precipitated from the tolbutamide/ethanol solution and those deposited on clean glass substrate were characterized under the low supersaturation of 1.05. Figure 3d indicates that the strongest diffraction peak of the precipitation from the solution is due to TB form III, followed by the peak of TB forms I and II, whereas on the bare glass there is only small peaks corresponding to TB form II and an even smaller one corresponding to TB form I and III. It is suggested that the functional-terminated SAMs can influence the selective crystallization of TB but also the preferential orientation of crystal facets. Different habits corresponding to the tolbutamide crystalline polymorphs are consistent with the XRD patterns, as shown in Figure 4; it is clear that the precipitation from the

Figure 6. The binding energy values of the major faces of TB IIV polymorphs grown on SAMs with the terminal group of (a) CH3, (b) CF3, and (c) Ph.

solution includes form I being platelike, form II being needlelike, and form III being cylindrical, while on the CH3-SAMs the crystals are the platelike form I and the needlelike form II, on the CF3-SAMs they are the needlelike form II, and on the PhSAMs they are the microfibril form IV and the needlelike form II. These results illustrate that the terminal functional groups of SAMs play an important role in polymorph selectivity, which promotes us to investigate the effects of the functional groups on the crystal packing modes of tolbutamide so as to 5502



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Figure 7. Morphologically important faces of TB form II: (a) face (010), (b) face (011), and (c) face (100). (d) The specific interactions between the SAMs terminal groups and the crystal face (100), where yellow-boxed inset displays the hydrogen bondings between CF3 and molecules in the face (100) and the cyan-boxed insetshows the ππ interactions between Ph and the face (100).

Figure 8. Molecular packings of face (100) (a) and face (100) (b) of TB form IV, and the ππ interactions of the calculated optimal structures of face (100) (c) and face (011) (d) interacted with the Ph-SAMs. (e) The geometric space matching interaction between face (100) of TB form II and CH3SAMs similar to the ππ interaction.

understand deeply the molecular interactive mechanisms during the selective crystallization. Molecular Interactions between TB Crystal Faces and Functionalized SAMs. In order to disclose the reasons that tolbutamide polymorphs are discriminated by the functional groups of the surfaces, it is required to determine the dominant crystal faces of each TB crystal form. On the basis of crystallographic data of Tan and co-workers,26 the distinct crystal habits of TB were predicted using the AE and the BFDH method, respectively, as shown in Figure 5. Either prediction method exhibits similar morphologically dominant faces which are in

accord with most of the preferentially oriented faces reflected by XRD patterns in Figure 3, besides the face III (120) and III (301) on the CF3-SAMs. Thus, further molecular modeling calculations were performed on these faces, as listed in Table 1, following the extension to a 2 2 supercell through cleaving the morphologically important faces. The binding energy of each crystal face per SAM molecule, obtained from the minimization calculations, is useful to assess the interactive mode between the dominant faces of TB polymorph with the terminal functional groups of SAMs. Figure 6 displays the binding energy values of morphologically dominant 5503



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Figure 9. Time-resolved Raman spectra of TB crystals grown on Ph-SAMs at the supersaturation of 1.05 (a), the magnified spectra of the aromatic ring deformation (b), and the asymmetric stretching of sulfonyl groups (c and d).

faces of TB forms IIV on three kinds of SAMs. It is indicated that on the methyl-terminated SAMs, the binding energy of face (100) of TB form II is the most negative, followed by face (201) of TB form I and face (101) of TB form III, while the binding energies of faces of TB form IV are positive or negative values with the absolute value close to zero (Figure 6a). The more negative the binding energy, the more preferentially the crystal face grows on the functionalized SAMs. Thus, it is illustrated that the face (100) of TB form II is superior to grow on the methylterminated SAMs, competing with the face (201) of TB form I and face (101) of TB form III. Similarly on the trifluoromethyl-terminated SAMs, it is suggested that the first preferential is the crystal face (100) of TB form II, followed by faces (301) and (110) of TB form III and face (201) of TB form I (Figure 6b), through comparing the binding energy values. Whereas on the phenyl-terminated SAMs, the first preferential is the crystal face (011) of TB form IV, followed by face (100) of form IV and face (100) of TB form II (Figure 6c). Significantly, in Figure 3, the calculated and experimental X-ray diffraction reveals that TB form II is mainly preferentially oriented with its (100) face parallel to the three

substrate surfaces. While TB form IV is mainly preferentially oriented with its (100) and (011) faces parallel to Ph-SAMs surface only. It is worthwhile to note that the crystal face (010) of TB form II has positive binding energy on all the three functionalized SAMs, although the face (100) of TB form II preferentially grows on the functionalized SAMs. Figure 7 illuminates the distinct molecular packings of faces (010), (011), and (100) of TB form II. The faces (010) and (011) mainly consist of methyl groups that can hardly interact with terminal groups of SAMs. However, the face (100) has plenty of oxygen and nitrogen atoms and phenyl groups, through which hydrogen bonds (F 3 3 3 HN) occur between fluorine atoms in the terminal of CF3-SAMs and imino groups in the face (100) of TB form II, as well as the ππ stackings generated between phenyl-terminated SAMs and the aromatic rings of the face (100) of form II. Figure 8 indicates that the interactions between the crystal faces (011) and (100) of TB form IV and the phenyl-terminated SAMs are also attributed to the ππ stackings. In the case of CH3-SAMs, there exist special packing structures between the face (100) of TB form II and the benzyl 5504


Crystal Growth & Design terminals of the SAMs (Figure 8e), which can be defined as the geometric space matching.12,13 In addition, it is notable that on three kinds of SAMs the binding energies of major faces of forms I and III are almost negative with comparable absolute values, except for the face (101) of form III. However, no diffraction peaks of TB forms I and III were observed on the Ph-SAMs. According to the molecular packing modes of the morphologically important faces of TB form I (Figure S5, Supporting Information) and form III (Figure S6, Supporting Information), it is illustrated that for TB form I the face (110) is comprised of NH and OdSdO groups the faces (200) and (201) are NH and CH3 groups, while for TB form III the faces (011) and (110) are comprised of CH3 and OdSdO groups and the faces (101), (301), and (120) are mainly CH3 groups. Therefore, it is impossible for such groups to form influential interactions with the phenyl terminals on the SAMs, so that it is hard for TB forms I and III to grow on the Ph-terminated substrates. On the other hand, owing to the formation of hydrogen bonds between the NH group of TB with the terminal group of CH3, and NH group with the terminal group of CF3, it is reasonable to observe the diffraction peak of TB crystals of form I on CH3-SAMs (Figure 3a) and form III on CF3-SAMs (Figure 3b) at higher supersaturation. These molecular modeling calculations together with the experimental results illuminate that the phenyl-terminated SAMs are beneficial to the growth of TB crystals of form IV owing to the significant ππ stackings, and the CF3-SAMs are superior to TB crystals of form II owing to the hydrogen bondings, while the CH3-SAMs has a weaker advantage of TB crystals of form II owing to molecular geometric space matching. This is in accordance with the above XRD patterns; i.e., crystals of TB form II are crystallized on CH3- and CF3-SAMs while crystals of TB form IV crystallized on the Ph-SAMs at the low supersaturation of 1.05. Time-Resolved Raman Spectra of Crystals on Ph-SAMs at Low Supersaturation. The influence of interfacial functional groups on the heterogeneous nucleation of tolbutamide was further revealed through comparing the time-resolved Raman spectra of TB crystals grown on Ph-SAMs at the supersaturation of 1.05. Figure 9 displays that the crystals grown in the first half hour display distinct peaks at 797, 1144, 1161, and 1437 cm1, and as time goes on to 2 h all these peaks experience enhanced intensities. However, after 3 h, there occur obvious peak shifts from 797 to 798 cm1, accompanying an emerging peak at 810 cm1; from 1144 and 1161 cm1 to 1147 and 1158 cm1; and from 1437 to 1436 cm1, accompanying with an emerging peak at 1450 cm1. It is known that in Raman spectra the peaks around 796810 cm1 are attributed to the aromatic ring deformation, while those around 11441161 and 14361450 cm1 are due to the asymmetric stretching of sulfonyl group of TB molecules.38 In combination with XRD patterns of TB crystals, it is illustrated that the close interactions between phenyl groups with tolbutamide molecules at the initial crystallization stage are beneficial to the crystal growth of metastable TB form IV, whereas as time goes on the functional groups of SAMs are covered by TB crystals so as to make the interactions toward tolbutamide molecules weakened, and then the following crystallized TB crystals gradually change to be like form I, as those directly deposited in the solution.

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’ CONCLUSIONS Selective crystallization of tolbutamide on SAMs with the similar molecule structure of tolbutamide and the functional groups of methyl, trifluoromethyl, and phenyl, respectively, was studied through characterizations of XRD and crystals morphologies. It is indicated that highly bioavailable TB forms II and IV preferentially grow on phenyl-terminated SAMs, and the influence of functionalized-SAMs on the heterogeneous nucleation of tolbutamide is well-explained by molecular modeling in combination with time-resolved Raman spectra. This work provides a feasible approach combining the experimental with molecular modeling methods, not a pure trial-and-error approach, to understand deeply the relationship between the functionalizedSAMs and the selective crystallization of polymorphic drugs. ’ ASSOCIATED CONTENT

bS

Supporting Information. Characterizations of SAMmodified substrate surfaces including contact angle, XPS, ATRFTIR, the XRD patterns of TB raw materials, and molecular packings in morphologically important faces of TB forms I and III. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +86-22-27890643. Fax: +86-22-27403389. E-mail: liwei@ tju.edu.cn.

’ ACKNOWLEDGMENT This work was supported by NSFC (20836005, 21076141) and the RFDP. ’ REFERENCES (1) Brittain, H. G. Polymorphism in Pharmaceutical Solids; Marcel Dekker: New York, 1999. (2) Singhal, D.; Curatolo, W. Adv. Drug Delivery Rev. 2004, 56, 335–347. (3) Paulus, E. F.; Leusen, F. J. J.; Schmidt, M. U. CrystEngComm 2007, 9, 131–143. (4) Huang, L. F.; Tong, W. Q. Adv. Drug Delivery Rev. 2004, 56, 321–334. (5) Sheth, A. R.; Bates, S.; Muller, F. X.; Grant, D. J. W. Cryst. Growth Des. 2004, 4, 1091–1098. (6) Wang, H. S.; Lu, F.; Zhou, G. D.; Zhai, H. J.; Wang, Y. B. Cryst. Growth Des. 2007, 7, 2654–2657. (7) Rapaport, H.; Kuzmenko, I.; Berfeld, M.; Kjaer, K.; Als-Nielsen, J.; Popovitz-Biro, R.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. J. Phys. Chem. B 2000, 104, 1399–1428. (8) Carter, P. W.; Ward, M. D. J. Am. Chem. Soc. 1994, 116, 769–770. (9) Flath, J.; Meldrum, F. C.; Knoll, W. Thin Solid Films 1998, 327329, 506–509. (10) K€uther, J.; Tremel, W. Thin Solid Films 1998, 327329, 554–558. (11) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500–4509. (12) Hiremath, R.; Basile, J. A.; Varney, S. W.; Swift, J. A. J. Am. Chem. Soc. 2005, 127, 18321–18327. (13) Cox, J. R.; Dabros, M.; Shaffer, J. A.; Thalladi, V. R. Angew. Chem. Int. Ed. 2007, 46, 1988–1991. (14) Pokroy, B.; Chernow, V. F.; Aizenberg, J. Langmuir 2009, 25, 14002–14006. 5505



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