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Notes External Reflection FTIR Studies on Crystallization of L-Asparagine Monohydrate beneath Stearic Acid and an Analogous Polymeric Film at the Air-Water Interface Sharon J. Cooper* Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE, United Kingdom Received September 27, 2001. In Final Form: February 4, 2002
Introduction Film-induced crystallization at the air-water interface represents a model system for the study of many heterogeneous nucleation processes,1 including biomineralization,2 since the well-defined crystallization location (the interface) permits detailed in situ studies, and the density and chemical composition of the monolayer can be systematically varied. In 1996, Ahn et al.3 used external reflection Fourier transform infrared (FTIR) spectroscopy to investigate calcium carbonate crystallizing beneath various small-molecule surfactants. This study found an alteration of film peak intensity as crystallization proceeded, which was shown to be indicative of film reorientation so as to maximize interactions with the growing crystallites. To our knowledge, no other studies have used this technique to probe film-induced crystallization, despite its potential to simultaneously acquire information about the state and orientation of the film and crystallizing species. In this study, we are using external reflection FTIR to investigate the crystallization of L-asparagine monohydrate beneath stearic acid and an analogous polymeric film consisting of a 15% poly(acrylic acid) polyethylene copolymer. Our interest in polymeric films arises from previous studies which showed that a closepacked film was not a prerequisite for film-induced crystallization,4,5 suggesting that a synergic interaction can exist between the film and growing crystallite.3 Hence, we are investigating the role of polymeric versus smallmolecule surfactants, to determine whether chain connectivity enhances adsorption and subsequent nucleation upon the spread film. The effects of aggregation on * Tel: +44 (0)191 374 4638. Fax: +44 (0)191 384 4737. E-mail:
[email protected]. (1) (a) Landau, E. M.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Nature 1985, 318, 353-356. (b) Popovitz-Biro, R.; Wang, J. L.; Majewski, J.; Shavit, E.; Leiserowitz, L.; Lahav, M. J. Am. Chem. Soc. 1994, 116, 1179-1191. (c) Davey, R. J.; Maginn, S. J.; Steventon, R. B.; Ellery, J. M.; Murrell, A. V. Langmuir 1994, 10, 1673-1675. (2) (a) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692-695. (b) Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, D.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993 261, 1286-1293. (c) Xu, G. F.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1998, 120, 11977-11985. (d) Hosoda, N.; Kato, T. Chem. Mater. 2001, 13, 688-693. (e) Buijnstera, P. J. A.; Donners, J. J. J. M.; Hill, S. J.; Heywood, B. R.; Nolte, R. J. M.; Zwanenburg, B.; Sommerdijk, N. A. J. M. Langmuir 2001, 17, 3623-3628. (3) Ahn, D. J.; Berman, A.; Charych, D. J. Phys. Chem. 1996, 100 (30), 12455-12461. (4) Cooper, S. J.; Sessions, R. B.; Lubetkin, S. D. J. Am. Chem. Soc. 1998, 120, 2090-2098. (5) Backov, R.; Lee, C. M.; Khan, S. R.; Mingotaud, C.; Fanucci, G. E.; Talham, D. R. Langmuir 2000, 16, 6013-6019.
controlling crystallization, shown by the contrasting behavior of soluble additives that inhibit crystal growth and insoluble additives of related chemical functionality that induce nucleation, are also being investigated by the addition of poly(acrylic acid) and sodium polyacrylate to the aqueous subphase. L-Asparagine crystallizes as the monohydrate with an orthorhombic unit cell of P212121 symmetry and unit cell parameters of a ) 5.593 Å, b ) 9.827 Å, c ) 11.808 Å, and Z ) 4 Å.6 The crystal is held together by a network of hydrogen bonds. The typical aqueous morphology is prismatic elongated along the a-direction, bounded by {012} and {011} faces together with smaller {101}, {111}, {020}, and {002} faces. Previous studies7,8 have shown that the habit can be affected by the addition of tailormade additives at concentrations of ∼2-20 mg per mL. Experimental Section The materials used were as follows: 15% poly(acrylic acid) polyethylene copolymer (Aldrich), stearic acid (98+%, Aldrich), L-asparagine monohydrate (Sigma, 99+%), phenol (99.5+%, Sigma), toluene (99.8%, Aldrich), chloroform (99.8+%, Fluka), sodium polyacrylate (average molecular weight (MW) 2000, Aldrich), and poly(acrylic acid) (average MW 2100, Aldrich). The pH of the asparagine solutions (∼5.5) was altered in the range pH 3-8 by the addition of concentrated HCl and NaOH solutions. There was no noticeable change in the morphology of crystals grown in the absence of additives in this pH range of 3-8, and asparagine remains predominantly in its zwitterionic state within this pH range, since it has pKa values of 2.14 and 8.72 for the COOH and NH3+ groups, respectively. Ultrapure water with a resistivity of 18 MΩ cm was used in all the studies. Attenuated total reflection (ATR) spectra were obtained using a Thunderdome accessory.9 External reflection FTIR studies were undertaken using a Spec-Ac monolayer accessory in the sample chamber of a Nicolet Nexus spectrometer, equipped with a liquid nitrogen cooled HgCdTe detector. The stearic acid and 15% poly(acrylic acid) films were spread from ∼0.5 mg mL-1 chloroform and phenol/toluene (1:3) solutions, respectively. Spectra were obtained on uncompressed films to allow sufficient space for a Wilhelmy plate (attached to a Nima tensiometer) to measure the surface pressure without blocking the infrared beam. A sufficient quantity of film material was spread on a clean subphase surface to achieve a surface pressure of ∼10 mN m-1. Either 128 or 512 scans were collected for each reflection spectrum (R) from 4000 to 650 cm-1 at a resolution of 4 cm-1. The resulting spectra were ratioed against background spectra (R0) taken on the clean subphase surface immediately prior to the film deposition. All of the resulting spectra contain raw data. The small size (∼millimeters) of the infrared beam means that crystallization can only be followed in situ at relatively high nucleation densities. Accordingly, a concentration of 0.47 M (6) Verbist, J. J.; Lehmann, M. S.; Koetzle, T. F.; Hamilton, W. C. Acta Crystallogr. 1972, B28, 3006. (7) Addadi, L.; Berkovitch-Yellin, Z.; Domb, N.; Gati, E.; Lahav, M.; Leiserowitz, L. Nature 1982, 296, 21-26. (8) Black, S. N.; Davey, R. J.; Halcrow, M. J. Cryst. Growth 1986, 79, 765-774. (9) The pressure applied by this device to ensure good sample-ATR crystal contact is not expected to affect the spectrum, since application of up to 0.8 MPa pressure to L-asparagine monohydrate crystals has been shown to have no effect on the Raman spectra other than the disappearance of the band at ∼1080 cm-1; see: Moreno, A. J. D.; Freire, P. T. C.; Melo, F. E. A.; Silva, M. A. A.; Guedes, I.; Mendes, J. M. Solid State Commun. 1997, 103, 655-658.
10.1021/la011492b CCC: $22.00 © 2002 American Chemical Society Published on Web 03/23/2002
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Table 1. Peaks and Assignments for the IR Spectra of Crystalline L-Asparagine Monohydrate and Solvated Asparaginea crystalline transmission data bands/cm-1 from ref 1 3447 3388 3093, 3117 2965 2956 2934 2921 1694, 1673 1647 1594 1580 1534 1435 1428 1409 1362 1304 1299 1237 1142 1102 1077 913 838 807 799
crystalline external reflection bands/cm-1
crystalline ATR bands /cm-1
3452 3379 3098 br 2964 2913 br 1683-1688 1649 1589 1576 1530 ∼1435 sh 1428 1397 1361 1315 1301 1234 1148 1101 1073 908 834 807 799
solute external reflection bands/cm-1
3445 br 3381 3107 2966 2938 br 1680 1640 1590 sh 1577 1521 ∼1435 sh 1428 1401 1361 1315 1305 sh 1236 1150 1103 1075 910 835 806
∼1670 br ∼1590 br ∼1480 br 1390 1350
assignments from ref 13 ν (H2O) νa (NH2) νa (NH3+) νa (CH2) ν (CH) νs (CH2) νs (NH3+) δa (NH3+) ν (CdO) δ (H2O) νa (CO2-) δs (NH3+) δa (CH2) νs (CO2-) δ (CN) δ (CH) δ (CH) ω (NH2) τ (CH2) r (NH3+) r (NH3+) ν (CN) ν (CC) γ (NH2) γ (H2O) r (CH2)
a Notation: ν ) stretch, δ ) bend, ω ) wag, τ ) torsion, r ) rock, γ ) out-of-plane deformation. Subscripts a and s refer to asymmetric and symmetric vibrations, respectively. sh ) shoulder, br ) broad.
(corresponding to a concentration 2.19 greater than that of a saturated solution in water at 26 °C) was used so that this was generally possible. However, crystallization also occurred on the poly(tetrafluoroethylene) (PTFE) trough base at this supersaturation level, precluding quantitative studies on the nucleation rate.10 The supersaturated 0.47 M L-asparagine monohydrate solutions were prepared by hot-filtering the solutions and heating the solutions for a further 1-2 h at 75 °C. The solutions were then left to cool slowly to room temperature (26 ( 2 °C). At the end of each experiment, the trough was moved carefully so as to access other surface regions to determine, using the same external reflection FTIR technique, the approximate relative proportions of the different crystal faces growing beneath the films. This movement did not cause any change in the crystal face uppermost on the surface as verified by eye and by performing a similar movement with the trough under an optical microscope. In situ optical micrographs were obtained on a Nima trough fitted with a glass window on its PTFE base that was mounted onto an Olympus microscope.
Results and Discussion The L-asparagine monohydrate crystalline peaks observed with the external reflection technique match those obtained via attenuated total reflection, as shown in Table 1, although the former exhibit negative absorption peaks in accordance with the 30° incidence angle.11,12 Furthermore, the peaks are generally within 5 cm-1 of bands observed in a recent Raman polarized transmission study on L-asparagine monohydrate crystals13 (see Table 1). Consequently, we can be confident of the assignments proposed in Table 1, with the exception of the proposed CN stretch and CH bend at 1397 and 1315 cm-1, respectively, which show deviations greater than 10 cm-1 from the transmission data. The external reflection and ATR data are typically not able to resolve the CH and CH2 (10) A thermostated glass trough is being constructed that will inhibit this crystallization and accurately control the temperature, enabling quantitative evaluation of the relative nucleation abilities of the films. (11) Mielczarski, J. A. J. Phys. Chem. 1993, 97, 2649-2663. (12) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373-1379. (13) Moreno, A. J. D.; Freire, P. T. C.; Guedes, I.; Melo, F. E. A.; Mendes-Filho, J.; Sanjurjo, J. A. Braz. J. Phys. 1999, 29, 380-387.
Figure 1. External reflection spectra of the {020}, {012}, {011}, and {101} faces of L-asparagine monohydrate from 1800 to 1000 cm-1.
stretches and the NH3+ symmetric stretch, for which typically only broad peaks at ∼2913 and ∼2938 cm-1 are observed, respectively. However, the wealth of other peaks observed indicates that the external reflection technique is suitable for studies on L-asparagine monohydrate crystallization at the air-water interface. Figure 1 shows the external reflection spectra obtained by floating L-asparagine crystals with their {020}, {012}, {011}, and {101} faces uppermost at the air-saturated DL-asparagine interface;14 these faces are the ones that crystallize under the films and conditions studied. The differences observed in the spectra are in accordance
Notes
Langmuir, Vol. 18, No. 9, 2002 3751 Table 2. Main Differences between External Reflection Infrared Spectra of {020}, {012}, {011}, and {101} L-Asparagine Monohydrate Crystal Facesa relative strength of band 1687 1649 1530 1361 angle between crystal cm-1 cm-1 cm-1 cm-1 crystal face and dhkl/ + + face Å δa (NH3 ) ν (CO) δs (NH3 ) δ (CH) {020} face/deg {020} {012} {011} {101}
s w m m
w s s-m s-m
vw m m-w s-m
m m s-m w
0 59.0 39.8 90
4.91 5.06 7.55 5.05
a Notation: s ) strong, m ) medium, w ) weak. Bold type indicates the main distinguishing features of a particular crystal face spectrum.
Figure 2. Parts a-d show the different crystal planes, 020, 012, 011, and 101, of L-asparagine monohydrate, respectively. The 020, 012, and 011 planes are viewed looking down the a-axis, while the 101 plane is viewed down the b-axis. Bonds to hydrogen atoms are shown in white. The carbonyl and carboxylate bonds are shown by double lines, one of which is dashed for the carboxylate bonds.
with the 30° incidence angle,11,12 the peak assignments listed in Table 1, and the molecular orientation in the crystal (see Figure 2a-d). In particular, the molecules are aligned approximately lengthways in the 100 plane with the average orientation of the C-NH3+ and CdO bonds along the b-direction, and the C-CO2- bonds oriented on average along the c-direction. This orientation ensures that the reflection spectra of the {020}, {012}, {011}, and {101} faces are easily differentiated; the most obvious distinguishing features of each reflection spectra are listed in Table 2. External reflection FTIR can also be used to probe the supersaturation level of the asparagine zwitterions in aqueous solution, by comparing the reflection spectra of the solution to those of known solute concentration. The external reflection spectra of a 0.47 M L-asparagine solution (which corresponds to a 0.84% mol fraction ratio and ∼3.8% volume fraction ratio), obtained by ratioing against a pure water spectrum background, is shown in Figure 3a. The main peaks observed are listed in Table 1 along with tentative assignments based on previous studies15,16 and comparison with the crystalline data. (14) Saturated DL-asparagine solutions were used as these resemble spectroscopically the supersaturated L-asparagine solutions used in the growth studies, without causing additional growth onto the floating crystals. (15) Rahmelow, K.; Hu¨bner, W.; Ackermann, T. Anal. Biochem. 1998, 257, 1-11.
Typically, only the asymmetric and symmetric methylene stretches at 2918 and 2850 cm-1, respectively, were observed for the films at π ∼ 10 mN m-1; the water vapor peaks are sufficiently intense to obscure the region in which the weak film CdO and CO2- stretching bands would be expected. The methylene stretching bands tend to be slightly broader in the polymeric films, showing a larger distribution of conformational states. Once nucleation occurs, the methylene stretching bands become obscured by those of the growing crystals, precluding investigations into whether film reorganization occurs during crystallization. Both the stearic acid and 15% poly(acrylic acid) films promoted asparagine monohydrate crystallization, with the specific crystal faces developing beneath the film dependent upon the pH range. In particular, on pure asparagine solution (pH 5.5), {020} and {101} growth occurred predominantly in the ratio ∼2:1, with a minority of {012} growth also observed. At pH 3, the predominantly carboxylic acid films promoted {011}, {012}, and {020} growth mainly with a minor amount of {101} growth. At pH 8, the carboxylate anion films induced {012}, {020}, and {101} nucleation and growth. These results show that the films were not specific nucleators, but rather they enabled the growth of a subset of low-energy faces. Typical FTIR spectra obtained during the course of crystallization are shown in Figure 3b,c. Representative optical micrographs of the faces found growing beneath the films are shown in Figure 4. Note the surface irregularities and Hopper growth features typically found for these faces, whereas all other faces remain smooth on this length scale. These features are consistent with the misfit dislocations expected for heterogeneous nucleation and a diffusionlimited growth mechanism for these faces.4 The crystallization behavior beneath the carboxylic acid films may be contrasted with the effect of adding poly(acrylic acid) and sodium polyacrylate to asparagine monohydrate supersaturated solutions. The addition of 2-10 mg per mL of poly(acrylic acid) inhibits crystal growth on the {012} and {011} directions, so that platelets elongated in the a-direction are observed (see Figure 4d). In contrast, sodium polyacrylate at these concentrations (leading to solution pHs of ∼7-7.5) has no effect on the observed crystal habit or the typical crystal faces found nucleating beneath the films (see Figure 4c). The poly(acrylic acid) adsorbs most strongly onto the {012} and {011} crystal faces, and yet the 15% poly(acrylic acid) copolymer induces crystallization of the {020}, {012}, {011}, and {101} faces, although growth of the latter is suppressed at low pH when the film is predominantly in its undissociated acid form. This finding highlights the different controlling factors for crystal growth and nucle(16) Pearson, J. F.; Slifkin, M. A. Spectrochim. Acta 1972, 28A, 24032417.
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Figure 3. (a) External reflection spectra of 0.47 M L-asparagine monohydrate solution from 1800 to 1000 cm-1. Parts b and c show representative spectra from 1800 to 1000 cm-1 taken prior to and during crystallization of 0.47 M L-asparagine monohydrate. (b) Spectra taken at 0, 75, 85, and 88 min beneath the 15% poly(acrylic acid) film. {020} growth is observed from 85 min onward. Depletion of asparagine in solution due to crystallization elsewhere results in very weak positive asparagine solute peaks observed at 75 min. (c) Spectra taken at 0, 10, 19, 23, 26, and 29 min beneath the 15% poly(acrylic acid) film in the presence of 10 mg per mL poly(acrylic acid) additive. A particularly high nucleation rate is observed due to the growth inhibitory effect of poly(acrylic acid). Combined {012}, {011}, and {020} nucleation and growth result in the spectra observed from 19 min onward.
ation processes. For surface-controlled growth, the growth rate of a face increases with its attachment energy, Eatt (defined as the interaction energy per molecule between a slice (hkl) and the crystal face (hkl)).17 Faces with the lowest Eatt dominate the habit, and the value of Eatt is mediated18,19 by the extent of additive-crystallite adsorp(17) Hartman, P.; Bennema, P. J. Cryst. Growth 1980, 49, 145-156.
Notes
Figure 4. Optical micrographs showing typical crystal faces growing beneath the films: (a) {020} growth beneath the 15% poly(acrylic acid) film, (b) {101} growth beneath the 15% poly(acrylic acid) film, (c) {012} growth beneath the 15% poly(acrylic acid) film in the presence of 10 mg per mL sodium polyacrylate, and (d) growth of {011} and {012} platelets beneath the 15% poly(acrylic acid) film in the presence of 2 mg per mL poly(acrylic acid).
tion upon the kink sites (sites of favorable growth), which is governed by the interaction energy between the crystal face and additive. The nucleation rate depends on the interfacial tension, γI, between the film and crystal nuclei, given by γI ) γ1 + γ2 + E12 where γ1 and γ2 are the surface tensions of the crystal face and film, respectively, and E12 (18) Berkovitch-Yellin, Z. J. Am. Chem. Soc. 1985, 107, 8239-8253. (19) Clydesdale, G.; Roberts, K. J.; Lewtas, K.; Docherty, R. J. Cryst. Growth 1994, 141, 443-450.
Notes
is the interaction energy between the two. γ1 and Eatt may be related by γ1 ≈ -Eattdhkl/2v where dhkl is the interplanar spacing of plane hkl and v is the molecular volume.17 So we might expect nucleation promotion and crystal inhibition of the same crystal faces for insoluble and soluble additives with a particular functional group if the faces have similar d spacings (see Table 2), since the determining factor will be the strength of the interaction between the crystal face and film/additive. This correlation is largely observed in the present study, except that nucleation promotion is less specific than the corresponding growth inhibition effects. This may arise from the flexibility of the spread films which enables the film to reorganize so as to minimize γI, irrespective of the particular face nucleating.20 Computational simulations are in progress to model the different nucleation promotion and crystal growth inhibition effects of the carboxylic acid and carboxylate functionalities. (20) Cooper, S. J.; Sessions, R. B.; Lubetkin, S. D. Langmuir 1997, 13, 7165-7172.
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Conclusions External reflection FTIR can be used to monitor the relative supersaturation of L-asparagine solutions and enables identification of the crystal faces growing beneath spread films. Asparagine monohydrate crystallization was promoted beneath the stearic acid and analogous polymeric film; however, the growth was not specific and a combination of {012}, {011}, {020}, and {101} growth was observed depending upon the solution pH. The ability of the acid functional group to act as a nucleation promoter when in an aggregated state and as a crystal growth inhibitor in a molecularly dispersed state is demonstrated. However, the particular faces affected by the soluble and insoluble forms of the additive do not correlate entirely. Acknowledgment. S.J.C. acknowledges ICI for the kind provision of her lectureship. LA011492B
