Nucleation and Growth of Glycine Crystals with Controllable Sizes and Polymorphs on Langmuir–Blodgett Films Fei Lu,†,‡ Guangdong Zhou,† Hong-Ju Zhai,†,‡ Yi-Bing Wang,† and Hai-Shui Wang*,† Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2654–2657
ReceiVed March 4, 2007; ReVised Manuscript ReceiVed September 6, 2007
ABSTRACT: Control of crystal polymorph and size is very important in many application fields. Herein we demonstrate that Langmuir–Blodgett (LB) films of stearic acid (SA) and octadecylamine (ODA) can serve as templates and generate different polymorphs of glycine crystals. In the neutral aqueous solutions, γ-glycine crystallizes on LB films of ODA while the polymorphic outcome becomes the R-form on LB films of SA. These observed results could be explained by the electrostatic interactions and geometric lattice matching at the LB film/crystal interfaces, respectively. By keeping the appropriate supersaturation, we have successfully controlled the number of crystals grown on LB films; for example, in some certain cases, only one piece of crystal was grown on LB films in solution. Therefore, large crystals of centimeter size could be prepared. These experimental results suggest a new approach to produce an organic crystal with bulk scale. Introduction Polymorphism is the phenomenon of molecules packing in different ways, giving rise to two or more crystal structures. The development of high quality crystal products requires a deep understanding and controlling of polymorphism, because different solid forms may lead to different physical properties (solubility, crystal shape, dissolution rate, etc.), different bioavailabilities, and pharmacological effects.1 In general, only one specific crystalline phase has the required properties of a product. Therefore, a reproducible and reliable crystallization of a desired polymorph is considerably important in industry.2,3 Glycine (H2NCH2COOH) is the simplest amino acid, and it is usually used as a model compound for this kind of research. Glycine is known to exist in three polymorphic forms: R, γ, and β. Among these forms, β-glycine, the unstable form, is obtained by adding ethyl alcohol to a concentrated aqueous solution of glycine;4 it transforms rapidly into the R-form in air or in water.5 Single crystals of γ-glycine usually are grown by slowly cooling aqueous solutions at low pH or high pH.6,7 R-Glycine can be formed spontaneously in a neutral aqueous solution.8 This is because when the pH of the solution is close to its isoelectric point of 5.97, glycine molecules favor the formation of neutral zwitterionic cyclic dimers (+H3NCH2COO-)2, which are the elementary building blocks of the R-polymorph.9 It is well-known that the monolayer at the air/water interface (Langmuir film) and Langmuir–Blodgett (LB) films can act as the sites upon which heterogeneous and indeed epitaxial nucleation may preferentially occur.10 The resulting nucleation is often highly specific, and thus, a particular crystal morphology or polymorph may be obtained by carefully selecting the film materials. Most research works have been concerned with crystal morphology and orientation by the optimization of the structure and chemistry of those monolayer surfaces,11–13 while only a few papers have proposed practical methodologies of the selective crystallization of desired polymorphic crystals. In the present study, glycine crystals with different polymorphs were * To whom correspondence should be addressed. E-mail: hswhsw2000@ yahoo.com.cn. Fax: +0086-431-85698041. Telephone: +0086-431-85262054. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
successfully obtained by the suitable selection of film materials even though they crystallize from the same neutral solutions. The second purpose of this study is to provide a method to prepare crystals with large size. There is a large demand nowadays for single, pure, and defect-free crystals of substances for use in dielectric, piezoelectric, paramagnetic, and optical and laser materials.14 Traditionally, single crystals of organic materials have been grown from the melt,15,16 from vapor,17 or from solution.18 Solution methods offer the potential advantages of much cheaper equipment, since a vacuum apparatus is not needed.19 However, the size of single organic crystals grown from solution is usually limited or requires a very long time.20 For example, the crystal induced under the monolayer at the air/water interface usually only grows to micrometer scale, whether the crystal materials have high21 or low solubility in solution.22 In order to grow large crystals, the temperature, the supersaturation, and the number of grown crystals should be well controlled. In the present work, we show that large crystals of glycine can be obtained on LB films if the crystallization process is carefully controlled. Experimental Section Stearic acid (SA, >99%) and octadecylamine (ODA, >99%) were purchased from Fluka Chemical Co. and used as received without further purification. Glycine (>99%) was obtained from Shanghai Chemical Reagent Co. Ltd. Langmuir–Blodgett (LB) film depositions were accomplished in a commercial two-compartment trough (KSV-5000) by using doubledistilled–deionized water as the subphase. SA and ODA were all dissolved in chloroform (2.0 × 10-3 M) and slowly spread onto the subphase, respectively. The solvent was allowed to evaporate for at least 10 min before compression began. The Langmuir films were slowly compressed to 20 mN m-1 and maintained at that pressure during the film deposition. SA and ODA LB films were deposited on the Si substrates by Y-type stacking for eight layers, respectively. The -COOH group of SA and the -NH3+ group of ODA were all exposed to air. The transfer ratios of SA and ODA were nearly unity. The glycine aqueous solutions were prepared by dissolving 28.125 g of glycine into 100 mL of triple-distilled pure water at 50 °C in a water bath. This provides a 25% supersaturation solution at 25 °C. A LB film was vertically dipped in the solution at the temperature of 50 °C. After 5 hours, the resulting solution was cooled to 27 °C (saturation solution). Then, the solution was cooled to 25 °C slowly and was kept at 25 °C for 3 h.23
10.1021/cg070216d CCC: $37.00 2007 American Chemical Society Published on Web 11/20/2007
Glycine Crystals with Controllable Sizes and Polymorphs
Crystal Growth & Design, Vol. 7, No. 12, 2007 2655
Figure 2. Visualization of the R-glycine structure.
Figure 1. Optical microscopy images of glycine crystallized (a) in aqueous solution or (b) on LB films of stearic acid. (c) The sketched structure and (d) X-ray powder diffraction pattern for R-glycine nucleated on stearic acid LB films.
Figure 3. (a) Two-dimensional drawing of the (010) face of the R-form of glycine. (b) Two-dimensional packing of the head groups of stearic acid LB films. (c) Schematic of the matching between glycine molecules and LB film head groups.
The crystals were examined by optical microscopy and with a D/Max-RA X-ray diffractometer. Infrared spectra of LB films were measured at 4 cm-1 resolution with a Bruker IFS-66 vacuated spectrometer.
Results and Discussion Glycine crystals with the R polymorph usually grow from solutions in the pH range 3.8-8.9.24 Figure 1a shows an example of an R glycine crystal grown from a neutral solution. It has the expected prismatic morphology.25 Glycine crystallizes on a LB film of SA from aqueous solution also in the R-form, as seen in Figure 1d. However, the crystal grown on LB films shows a slightly different morphology, as given in Figure 1b. Figure 1c is a sketched structure of a glycine crystal on a LB film template. It is clear that a crystal with the (010) basal face attached to a LB film of SA was developed. It is well-known that the electrostatic interaction, the H-bond interaction, and the lattice matching are important factors responsible for the controlled crystallization under the LB template. The CdO stretching band of SA in LB films appears at about 1700 cm-1 in the FT-IR spectrum, and no carboxyl salt bands (COO-) appear in the spectrum. Those results reveal that the COOH groups of SA are almost undissociated. Therefore, the electrostatic interactions between a LB film of SA and glycine do not seem to be an important factor for the controlled crystallization of a glycine crystal. According to the literature,26 R-glycine is composed of centrosymmetric bilayers formed by strong NH · · · O hydrogen-bonding interactions between cyclic hydrogen-bonded zwitterionic molecular pairs. These bilayers are related along the b axis by glide symmetry through weak CH · · · O interactions between (010) layers (see Figure 2). Considering the crystal structure of R-glycine (Figure 1b) and because of H-bonding as well as van der Waals interactions between the glycine methylene groups and the surface carboxyl groups of SA LB films,27 we proposed that the (010) faces are located at the crystal/film interface. Moreover, the formation of R-glycine can be rationalized on the basis of a potential lattice matching relationship at the template/crystal interface. As discussed above, the face of glycine at the template/crystal interface is the (010) face. What aspects of the (010) face could be simulated by the interaction
Figure 4. Glycine crystals nucleated on octadecylamine LB films: (a) optical microscopy image; (b) X-ray powder diffraction pattern.
between the SA LB films and R-form glycine crystals? The R-form of glycine is a monoclinic crystalline lattice with the lattice contants a ) 0.510 nm, b ) 1.20 nm, c ) 0.546 nm, and β ) 111.7°.28 The drawing of the (010) face of the R-form of glycine is shown in Figure 3a. According to Petty et al.,29 the main cell of the c-form of stearic acid LB films is monoclinic with the lattice parameters a ) 0.936 nm, b ) 0.495 nm, and c ) 5.07 nm. The two-dimensional lattice of the head groups of SA LB films is deemed to be the same as the projection on the ab plane (Figure 3b). The lattice constant b (0.495 nm) of SA LB films is close to the lattice constant a (0.510 nm) of the R-form of glycine. From Figure 3c, it is clear that the lattice constant a (0.936 nm) of the SA is approximately twice the c sin β (c and β are the lattice contants of R-form glycine crystals), with a misfit of only 8%. The good lattice matching between the ab plane of SA LB films and the (010) face of the R-form of glycine may account for the formation of the R-form of glycine on LB films of SA. The interfacial chemical functionalities and interfacial properties of the organized films will influence the heterogeneous nucleation and growth of glycine crystals and determine the polymorphs and morphologies. When an ODA LB film was dipped in an aqueous solution with pH ) 6.0, the crystal crystallized on the ODA LB film appeared as short trigonal prisms (Figure 4a). The XRD pattern of the crystals shown in Figure 4b indicates that the crystal is the γ-form. As mentioned in the Introduction, glycine crystallizes from neutral solution usually in the R-form. Therefore, it is somewhat surprising that we got γ-glycine crystals on ODA LB films. It
2656 Crystal Growth & Design, Vol. 7, No. 12, 2007
Figure 5. Visualization of the γ-glycine structure showing the polar c-axis, the growing NH3+ end (+c), and the growing COO- end (-c).
is well-known that γ-crystals grow elongated along their polar c-axis. One end of the crystal is terminated by a carboxylaterich end (-c direction), and the other end is terminated by a NH3 +-rich end (+c direction),30 as seen in Figure 5. The value of Kb for a long chain alkylamine is about 5 × 10-4; therefore, in the neutral solution, the majority of the alkylamine molecules exist as RNH3+. It was reported that ODA monolayers transferred at pH ) 7.5 contain mainly alkylammonium ion.31 In the present study (pH ) 6.0), the amine groups of ODA LB films should be almost protonated, and this prediction was also demonstrated by the infrared spectrum of ODA LB films (see Supporting Information). The strong bands at 1660 and 1560 cm-1 correspond to the antisymmetric and symmetric stretching modes of the NH3+, respectively. So, the outer functionality of ODA LB films is the NH3+ group, and the electrostatic interactions between the COO- group of zwitterionic cyclic dimers (+H3NCH2COO-)2 and the NH3+ functionalities of LB films will drive glycine binding to the protonated surface of ODA LB films. The electrostatic interactions would align the glycine molecules appropriately for the observed interfacial crystallization of the polar γ structure. The electrostatic interactions are probably the main factor for controlled crystallization of γ-glycine crystals. It should be noted that only the +c direction of the γ structure is available for growth in the present study. Growth along the -c direction will be rooted on the LB films as seen in Figure 5. Towler et al.24 have verified that the -c direction growth of the γ structure is inhibited at low pH. Therefore, it is quite reasonable that the morphology of the crystals in Figure 4a is similar to the morphology of γ-glycine that was made in acidic solutions.30 The monolayer at the air/water interface (Langmuir films) and LB films have been used to selectively nucleate and grow a number of crystals, but their use as templates for large crystals has been somewhat limited. Crystals attached to Langmuir films are usually micrometer-sized because heavy crystals could not survive at the air/water interface. The crystals attached to LB films do not usually extend much outside the range of hundreds of micrometers because of the limitations imposed by traditional experimental techniques (evaporating method). In order to grow large crystals, the amounts of crystalline materials in solution should be enough and the number of growing crystals should be limited. In the present study, by changing the solution temperature from 50 to 25 °C, we could obtain a large enough quantity of glycine in the original solution because the solubility of glycine decreases rapidly with temperature: cooling from 50 to 25 °C would produce about 3.125 g of glycine in the solution. The activation energy for nucleation on LB films was lowered, and the crystallization became easier than that in bulk solution.32 Therefore, the nucleation and growth of crystals will become
Lu et al.
easier on LB films than in solution. In fact, glycine crystals were observed to nucleate on LB films while there are no crystals formed in the solutions. Many factors, such as solvent, temperature, and supersaturation, may have a considerable influence on the number of crystals grown on LB films. In the present study, LB films were dipped into an initial unsaturated solution (the solubility of glycine is about 40 g at 50 °C); at that temperature, there are no crystals formed on LB films. When the temperature goes down slowly, crystal nucleation and growth occur at a controlled rate only on the LB films when the solution becomes supersaturated; spontaneous nucleation in solution is avoided because the system is never allowed to become labile. The limitation of number of crystals on LB films will result in large size crystal products. It should be noted that, in some cases, there is only one piece of crystal formed on LB films. Our experimental results showed that centimeter-sized glycine crystals can be easily obtained by the present cooling process. Conclusions Langmuir–Blodgett films of stearic acid and octadecylamine have been prepared and used as templates for the nucleation and growth of glycine crystals. Glycine crystallizes in the R-form or γ-form dependent on the surface functionalities of LB films. The selectivity of polymorphism is rationalized on the basis of geometric lattice matching and electrostatic interactions at the LB films/crystal interface. The nucleation of crystals on LB films occurs at a lower supersaturation than in solution; therefore, no crystals were observed in solution. Large crystals with centimeter size can be obtained by a slow controlled cooling of the solution. In such a process, the number of crystals on LB films is limited and the development of large crystals becomes possible. These findings reveal a new approach to producing single organic crystals with bulk size. Acknowledgment. This work was supported by The National Natural Sciences Foundation of China (No. 20273068 and No. 90306001). Supporting Information Available: Infrared spectra of stearic acid and octadecylamine Langmuir–Blodgett films. This information is available free of charge via the Internet at http://pubs.acs.org.
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CG070216D
