Langmuir 2004, 20, 3593-3600
3593
Neutron Reflectivity and External Reflection FTIR Studies of DL-Aspartic Acid Crystallization beneath Nylon 6 Spread Films Matthew J. Jamieson,† Sharon J. Cooper,*,† Aline F. Miller,‡ and Stephen A. Holt§ Department of Chemistry, University of Durham, UK DH1 3LE, Department of Chemical Engineering, UMIST, Manchester UK, M60 1QD, and ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, UK OX11 0QX Received November 20, 2003. In Final Form: February 20, 2004 The crystallization of DL-aspartic acid beneath nylon 6 spread films has been studied for 150% supersaturated systems using neutron reflectivity and external reflection FTIR. The neutron reflectivity data showed the gradual incorporation of DL-aspartic acid within a nylon 6 spread film layer over a period of 6-8 h, culminating in over 50 vol % of the “film” layer comprising DL-aspartic acid. Accumulation of further DL-aspartic acid material to produce microscopic/macroscopic surface crystals occurred, but on a more limited scale, resulting in ∼1-5% surface coverage of crystals over the same period. External reflection FTIR studies revealed very weak bands attributable to DL-aspartic acid in surface regions devoid of visible crystals, in agreement with the neutron reflectivity studies. In regions with visible crystals, much larger and sharper DL-aspartic acid bands were seen. Changes in the intensity of the nylon 6 NH stretch band were often observed during the visible crystallization and dissolution of DL-aspartic acid and were consistent with the reversible accumulation of nylon 6 around the growing crystals.
Introduction Controlled crystallization has long been a key goal for scientists due to its importance in crystal morphological engineering1-4 and biomineralization.1,2 Landau et al. first reported the ability of a monolayer to induce highly specific crystallization at the air-aqueous interface.5 The resulting crystallization was thought to occur via the ordered surfactant transmitting a corresponding order to the adsorbed crystallizing material. Mann et al.6 reported the importance of stereochemical matching for calcite crystallization beneath monolayers, and others have detailed similar results for biologically important organic species.7-9 Subsequent work10-13 has also shown that the crystallization rate can be maximized under less well-ordered, more compressible films, particularly if there is a lattice mismatch between the film and crystallizing species. * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Durham. ‡ UMIST. § Rutherford Appleton Laboratory. (1) Heywood, B. R. Microsc. Res. Tech. 1994, 27, 376-388. (2) Heywood, B. R.; Hill, S.; Pitt, K.; Tibble, P.; Williams, S. Mater. Res. Soc. Symp. Proc. 2001, M4.5.1-M4.5.12. (3) Weissbuch, I.; Lahav, M.; Leiserowitz, L. Cryst. Growth Des. 2003, 3, 125-150. (4) Dujardin, E.; Mann, S. Adv. Mater. 2002, 14, 775-788. (5) Lahav, M.; Landau, E. M. Nature 1985, 318, 353. (6) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9-20. (7) Lochhead, M. J.; Letellier, S. R.; Vogel, V. J. Phys. Chem. B 1997, 101, 10821-10827. (8) Ma, C. L.; Lu, H. B.; Wang, R. Z.; Zhou, L. F.; Cui, F. Z.; Qian, F. J. Cryst. Growth 1997, 173, 141-149. (9) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. A. ACS Symp. Ser. 1991, 444, 28-41. (10) Cooper, S. J.; Sessions, R. B.; Lubetkin, S. D. J. Am. Chem. Soc. 1998, 120, 2090-2098. (11) Backov, R.; Lee, C. M.; Khan, S. R.; Mingotaud, C.; Fanucci, G. E.; Talham, D. R. Langmuir 2000, 16, 6013-6019. (12) Volkmer, D.; Fricke, M.; Vollhardt, D.; Siegel, S. J. Chem. Soc., Dalton Trans. 2002, 24, 4547-4554. (13) Ouyang, J. M.; Deng, S. P. Dalton Trans. 2003, 14, 2846-2851.
Consequently, we have been investigating the crystallization of amino acids under a variety of spread films, including polymeric species, for which geometric matching does not occur. In this contribution, we report the findings of neutron reflectivity and external reflection FTIR studies on the crystallization of DL-aspartic acid under nylon 6 films spread at the air-aqueous interface. The structure of the nylon 6 film at the air-aqueous interface is expected to be similar to that of the analogous nylon 6 6 film, for which the following information, relevant to these studies, was found. A GIXD study14 at a film concentration of 1.5 mg m-2 revealed the existence of chainfolded hydrogen-bonded sheets similar to those present in the bulk crystal. Within the sheets, the distance in the hydrogen bond direction is considerably longer than that in the chain direction, and hence a fibrillar-type texture is observed. The sheets are parallel to the water surface, with on average 5-6 sheets stacking to produce a multilayer film, corresponding to a thickness of ∼20 Å. External reflection FTIR studies on this system confirmed the presence and orientation of the hydrogen-bonded sheets, while ESEM studies showed the fibrils to be ∼600-800 nm in length and 20 nm wide.15 The fibrils are interspersed by the aqueous phase, but become increasingly close-packed as the film is compressed. However, full fibrillar coverage is only achieved by overcompressing to surface concentrations of ∼4 mg m-2. There is comprehensive literature on the engineering of crystalline architectures at the air-aqueous interface, using techniques such as FTIR,16,17 grazing incidence X-ray diffraction (GIXD),18 fluorescence probe microscopy,19 and Brewster angle microscopy.20 In contrast, there is very (14) Popovitz-Biro, R.; Edgar, R.; Weissbuch, I.; Lavie, R.; Cohen, S.; Kjaer, K.; Als-Nielsen, J.; Wassermann, E.; Leiserowitz, L.; Lahav, M. Acta Polym. 1998, 49, 626-635. (15) Miller, A. F.; Cooper, S. J. Langmuir 2002, 18, 1310-1317. (16) Ahn, D. J.; Berman, A.; Charych, D. J. Phys. Chem. 1996, 100, 12455-12461. (17) Cooper, S. J. Langmuir 2002, 18, 3749-3753.
10.1021/la0361823 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/31/2004
3594
Langmuir, Vol. 20, No. 9, 2004
little information regarding the depth-profiling of crystallizing species beneath monolayers, which neutron reflectivity can provide. A recent study by Weygand et al.21 used a combination of X-ray and neutron reflectivity to provide evidence for a structural reorganization of the phospholipid headgroups during the binding and recrystallization of a bacterial S-layer protein, but we are not aware of other studies. Experimental Section Materials. The materials used were as follows: DL-aspartic acid (H-ASP) (99%, BDH), deuterated DL-aspartic acid (D-ASP) (Aldrich), nylon 6 (H-N6) (Aldrich), and heavy water (Aldrich). Deuterated nylon 6 (D-N6) was polymerized from deuterated caprolactam (Aldrich) by standard methods.22 Preparation of 150% Supersaturated DL-Aspartic Acid Solutions. First, 4.9 g of DL-aspartic acid was dissolved in 250 cm3 of ultrapure (Milli-Q) water by heating. The solutions were passed through 0.22 µm Millipore filters to remove any remaining particles and maintained at least 10 °C above the saturation temperature of 49.6 °C for at least a further hour, before cooling to ambient. This gave solutions of 150% supersaturation at 25 °C, where the percentage supersaturation, µ%, is defined as µ% ) ((s - ssat)/ssat) × 100%, s is the solute concentration, and ssat is the saturation concentration. Nylon 6 Langmuir Film Formation. The nylon 6 films were spread onto ultrapure water and DL-aspartic acid solution subphases contained in a PTFE Nima Langmuir trough of dimensions 30 cm × 20 cm, which was thermostated via a water bath to a temperature of 25 ( 0.2 °C. Surface pressure measurements were carried out using a Nima tensiometer by the Wilhelmy plate method. A full compression cycle of the subphase was completed to ensure a clean surface, prior to the addition of the nylon 6 solution. Next, 30 µL of the nylon solution, which contained ∼1 mg mL-1 of nylon in a 2:3 (v/v) formic acid/ chloroform solution, was added dropwise to the subphase surface. After the solvent had evaporated (∼5 min), the trough barriers were driven together to give a surface pressure, π ) 3 mN m-1, corresponding to a surface concentration of ∼1.8 mg m-2. The films were relatively stable, equilibrating at π ≈ 2.5 mN m-1. Neutron Reflectivity Measurements. Neutron reflectivity data were collected on the SURF reflectometer at the ISIS pulsed neutron source at Rutherford Appleton Laboratories, Didcot, UK. SURF is a time-of-flight station with a horizontal geometry specifically designed for studies of liquid surfaces.23 A rectangular NIMA trough was mounted on an anti-vibration table and enclosed by a Perspex box to minimize contamination, evaporation, and H/D exchange between the subphase and atmosphere. The well-collimated neutron beam entered and exited the enclosing box via quartz windows. Data were acquired at fixed incident angles of 0.8° and 1.5°. The wavelength range, 0.55-6.8 Å, results in a scattering vector, Q, range of 0.026-0.60 Å-1, because Q ) 4π sin θ/λ, where λ is the wavelength of the neutrons and θ is the scattering angle. Data at the two incident angles were combined into a single reflectivity profile and calibrated with respect to D2O data. Four systems were studied: system 1, H-N6 on D2O; system 2, D-N6 on saturated H-ASP in D2O; system 3, D-N6 on 150% supersaturated H-ASP in D2O; and system 4, D-N6 on 150% supersaturated H-ASP in null reflecting water (NRW). (18) 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. (19) Mowald, H. Rep. Prog. Phys. 1993, 56, 653-685. (20) Li, B.; Liu, Y.; Lu, N.; Yu, J. H.; Bai, Y. B.; Pang, W. Q.; Xu, R. R. Langmuir 1999, 15, 4837-4841. (21) Weygand, M.; Kjaer, K.; Howes, P. B.; Wetzer, B.; Pum, D.; Sleytr, U. B.; Lo¨sche, M. J. Phys. Chem. B 2002, 106, 5793-5799. (22) Carraher, C. E., Jr. J. Chem. Educ. 1978, 55, 51-52. (23) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899-3917.
Jamieson et al. Data for each reflectivity profile were collected for ∼2 h. One reflectivity profile was obtained for systems 1 and 2, while four and three profiles were collected for systems 3 and 4, respectively. Data Analysis. Only a brief synopsis of the data fitting method employed is given here. Further details of this and other data fitting methods are available elsewhere.24-26 We are concerned only with the specular reflection of a neutron beam that is incident on a surface. The variation of the reflectivity () ratio of reflected intensity to incident intensity) with scattering vector, Q, is determined by the change in neutron refractive index normal to the interface. The neutron refractive index is controlled by the scattering length density (SLD), F, of the layer: F ) ∑ bini, where ni and bi are the number density and the coherent scattering length, respectively, of species i in the layer. The reflection properties of each stratified layer that models the composition distribution are contained within a characteristic optical matrix, and the product of these matrices produces an overall matrix from which the reflectivity can be calculated.27 The matrix for each individual layer is incorporated with a Gaussian roughness.28,29 Different numbers of layers are tested by the model until an acceptable fit to the experimental data is obtained by varying the roughness, thickness, and SLD of each layer. The composition of the layers is then determined from the SLD and thickness values. Determination of Layer Compositions. The SLD, F, of any given layer is the sum of the total scattering length of the components present; that is, for this system, F ) φN6FN6 + φWaterFWater + φAspFAsp, where φi is the volume fraction of component i and for this system, φN6 + φWater + φAsp ) 1. Assuming the insoluble nylon 6 is entirely contained in the near surface layer, these equations can be rearranged to:
φAsp )
F - φN6FN6 - FWater(1 - φN6) FAsp - FWater
φWater )
F - φN6FN6 - FAsp(1 - φN6) FWater - FAsp
The SLD values for the components in each system are listed in Table 1. In calculating these values30 for D-N6, H-N6, D-ASP, and H-ASP, the nature of the subphase had to be considered. For hydrogenated species in D2O, the labile protons were expected to exchange with deuterium. For deuterated species on null reflecting water (NRW), 92.051% of the labile protons were expected to exchange with 1H, corresponding to the composition of the subphase. The labile protons for the DL-aspartic acid were those contained within the carboxylic acid, carboxylate, and ammonium groups; for nylon 6, the amide proton is labile. The SLD values for 100% crystalline and 100% amorphous nylon 6 will differ slightly, because of their different densities. For instance, H-N6 has densities of 1.18 and 1.11 g cm-3 in its crystalline and amorphous regions, respectively. Consequently, both the nylon 6 crystalline and amorphous SLD values are included in Table 1, with the amorphous values given in brackets in italics. The SLD value of the subphase was also affected slightly by the solute concentration. However, at 150% supersaturation, the quantity of solute represents only ∼2% of the total mass of solution, and so this has been neglected in the subsequent modeling as it was within experimental error. External Reflection FTIR Studies. 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. Nylon 6 and nylon 6 6 films were spread from ∼0.5 mg mL-1 formic (24) Penfold, J.; Thomas, R. K. J. Phys.: Condens. Matter 1990, 2, 1369-1412. (25) Jones, R. A.; Richards, R. W. Polymers at Surfaces and Interfaces; Cambridge University Press: New York, 1999. (26) Lovell, M. R.; Richardson, R. M. Curr. Opin. Colloid Interface Sci. 1999, 4, 197-204. (27) Born, M.; Wolf, E. Principles of Optics; Pergamon Press: New York, 1980. (28) Nevot, L.; Croce, P. Phys. Appl. 1980, 15, 761. (29) Pynn, R. Physica B 1992, 45, 602-612. (30) Sears, V. F. Neutron News 1992, 3, 29-37.
Studies of
DL-Aspartic
Acid Crystallization
Langmuir, Vol. 20, No. 9, 2004 3595
Table 1. Scattering Length Densities, G, for the Materials Comprising Systems 1-4a
system 1: H-N6 on D2O F/10-6 Å-2
system 2: D-N6 on sat H-ASP in D2O F/10-6 Å-2
system 3: D-N6 on 150% supersat H-ASP in D2O F/10-6 Å-2
system 4: D-N6 on 150% supersat H-ASP in NRW F/10-6 Å-2
8.00 (7.47)
8.00 (7.47)
7.39 (6.92)
5.54 6.39
5.54 6.39
2.46
D-N6 H-N6
1.52 (1.42)
H-ASP D2O NRW a
6.39
0.00
The values in brackets in italics correspond to amorphous nylon 6 material. Table 3. SLD, Roughness, Uniform Layer Thickness, and Volume Fractions, φ, Derived for D-N6 on Saturated H-ASP in D2O (System 2)a timepoint 0-2 h
F/10-6 roughness/ thickness/ Å Å Å-2 7.15
10
27
φN6
φWater
φAsp
0.53 0.45 0.01 (0.56) (0.62) (-0.18)
a The values in brackets in italics correspond to amorphous nylon 6 material.
Figure 1. Acquired data and best-fit curve for H-N6 on D2O (system 1). Table 2. SLD, Roughness, Uniform Layer Thickness, and Volume Fractions, φ, Derived for H-N6 on D2O (System 1)a timepoint F/10-6 Å-2 roughness/Å thickness/Å 0-2 h
3.7
13
30
φN6
φWater
0.54 0.46 (0.57) (0.43)
a The values in brackets in italics correspond to amorphous nylon 6 material.
acid/chloroform (2:3 v/v) solutions. Sufficient quantity of the film material was spread on a clean subphase surface to achieve a surface pressure of typically ∼3 mN m-1. Next, 512 scans were collected for each spectrum from 4000 to 650 cm-1 at a resolution of 4 cm-1. The spectra were ratioed against the background spectrum taken on the clean subphase surface immediately prior to the spreading. All of the resulting spectra display raw data.
Results and Discussion Neutron Reflectivity Studies. The acquired data and best-fit curve for H-N6 on D2O (system 1) are shown in Figure 1. The smooth nature of the curve, without the presence of obvious maxima, is consistent with a diffuse film layer. This would be expected for a film similar to that of nylon 6 6, composed of fibrils containing varying numbers of stacked hydrogen-bonded sheets, interdispersed by the aqueous medium. Smooth curves of this nature were found during all subsequent crystallization experiments. Tables 2 and 3 show the data extracted from model fits for H-N6 on D2O and for D-N6 on saturated H-ASP in D2O, respectively. Volume fraction values were calculated on the assumption that the nylon 6 (N6) is either 100% crystalline, or 100% amorphous, with the values for the latter case given in brackets in italic. It is our opinion that
the nylon 6 is ∼50-75% crystalline, and this is borne out by the negative volume fraction calculated for DL-aspartic acid in the saturated DL-aspartic acid system, obtained assuming the nylon 6 film is totally amorphous (see Table 3). The good agreement between the water and saturated DL-aspartic acid systems obtained when the parameters are calculated on the assumption that the nylon 6 film is crystalline also supports this hypothesis (compare Tables 2 and 3). The surface concentration of nylon 6 on the subphase was ∼1.8 mg m-2. Hence, the expected film thickness would be ∼24 Å, assuming a fibrilar structure similar to that of nylon 6 6, and a corresponding amount of interfibrilar water. For comparison, a film layer composed entirely of nylon 6 fibrils with no intervening water would have a thickness of 16 Å for H-N6 and 14 Å for D-N6 assuming 100% crystallinity of the spread film, or 17 Å for H-N6 and 15 Å for D-N6 assuming it is 100% amorphous. Consequently, the film thickness and roughness values presented in Tables 2 and 3, together with the high proportion of water present within the film layer, are consistent with a nylon 6 film structure similar to that reported for nylon 6 6.14,15 Crystallization Experiments. The nylon 6 film was found to promote the crystallization of DL-aspartic acid at the air-aqueous interface, producing crystals ranging from ∼0.01 mm to 0.5 mm in length, which occupied ∼1-5% of the film covered surface after 8 h. Related in-situ optical microscopy and morphology studies using an Olympus microscope and Nima trough fitted with a glass window revealed that the (110) faces often grew beneath the films together with significant quantities of the (1h 11) and (2h 02) faces, and a minor amount of the (200) face; all of these faces have low surface tensions and are generally seen in DL-aspartic acid crystals grown from aqueous solution. No DL-aspartic acid crystals were visible at the film-free air-aqueous interface outside the trough barriers. All neutron reflectivity crystallization data were modeled initially by assuming that two layers were present on the subphase. We were expecting the first layer to have an almost invariant SLD value consistent with that of nylon 6 fibrils with intervening water, while the second layer would have been essentially water initially (recall that DL-aspartic acid comprises only ∼2% of the total mass in a 150% supersaturated solution, which has a negligible
3596
Langmuir, Vol. 20, No. 9, 2004
Jamieson et al.
Table 4. SLD, Roughness, Uniform Layer Thickness, and Volume Fractions, φ, Derived for D-N6 on 150% Supersaturated H-ASP in D2O (System 3)a timepoint
F/10-6 roughness/ thickness/ Å Å Å-2
0-2 h
7.01
9
35
2-4 h
6.81
9
39
4-6 h
6.54
17
50
6-8 h
6.18
26
81
φN6 0.40 (0.43) 0.36 (0.38) 0.28 (0.30) 0.17 (0.19)
φWater
φAsp
0.60 0.00 (0.76) (-0.18) 0.41 0.23 (0.62) (0.00) 0.33 0.39 (0.49) (0.21) 0.30 0.53 (0.33) (0.48)
a The values in brackets in italics correspond to amorphous nylon 6 material.
effect on the SLD value of water), but would have an increasing thickness and changing SLD value over time, due to the increasing amount of DL-aspartic acid material present as the crystallization progressed. However, the model fitting always produced a constant SLD value for the second layer, corresponding to that of water; that is, a single layer on the subphase always resulted. The SLD value for this single layer (the “film” layer) decreased throughout the crystallization, while the layer thickness increased. Hence, these results indicated the incorporation of DL-aspartic acid material within, and not below, the nylon film, showing that the nylon and DL-aspartic acid were intimately associated with one another throughout the crystallization. Because all of the reflectivity data were best modeled with just a single layer on the subphase, the number of adjustable parameters was three, the SLD, thickness, and roughness of the “film” layer, and so we would not expect our data to include inaccuracies due to a large number of adjustable parameters. Table 4 shows the best-fit parameters obtained from the neutron reflectivity data gathered during the crystallization of DL-aspartic acid in the D-N6 film, H-ASP, and D2O system (system 3). The results reveal the substantial swelling by over 100% of the nylon film layer due to the incorporation of DL-aspartic acid, with the amount of water present, given by the product of the layer thickness and volume fraction values, remaining fairly constant. Indeed, after 8 h the “film” layer was composed mainly of DLaspartic acid, as shown by the SLD value reaching a value lower than that of both the D-N6 and the D2O. As reported for system 2, our supposition that the nylon 6 is principally crystalline is further supported by the negative value for the volume fraction of H-ASP obtained for the 0-2 h data by assuming an amorphous nylon 6 film. The resulting coverage of visible DL-aspartic acid crystals at the surface after 8 h was only ∼1-5%, far lower than the volume fraction of DL-aspartic acid incorporated into the film layer. This suggests that a substantial proportion of the incorporated DL-aspartic acid existed in the form of subvisible clusters, presumably ranging from subcritical nucleus sizes to larger nuclei, whose subsequent growth to visible dimensions may have been impeded by film adsorption. The relatively low crystal surface coverage also explains why the subphase has an SLD value typical of water, and not that of DL-aspartic acid crystalline material and water. The increase in the roughness value with time is indicative of the variation of the DL-aspartic acid incorporation within the film layer. Figure 2 shows the SLD depth profiles for system 3 at the different time intervals. Each curve depicts a gradual change in the SLD value with depth, until the SLD value of the subphase is obtained. The corresponding composition and layer thickness changes in the surface layer are shown schematically in Figure 3.
Figure 2. Change in SLD profile against depth from surface for D-N6 on 150% supersaturated H-ASP in D2O (system 3).
Figure 3. Composition of the “film” layer over four timepoints for D-N6 on 150% supersaturated H-ASP in D2O (system 3). Table 5. SLD, Roughness, Uniform Layer Thickness, and Volume Fractions, φ, Derived for D-N6 on 150 % Supersaturated H-ASP in NRW (System 4)a timepoint
F/10-6 roughness/ thickness/ Å Å Å-2
0-2 h
2.80
16
34
2-4 h
2.48
28
55
4-6 h
2.00
40
100
φN6
φWater
φAsp
0.41 0.59 0.00 (0.44) (0.66) (-0.10) 0.25 0.49 0.26 (0.27) (0.48) (0.25) 0.14 0.51 0.35 (0.15) (0.45) (0.40)
a The values in brackets in italics correspond to amorphous nylon 6 material.
The results from the D-N6 film on H-ASP in NRW system (system 4) are presented in Table 5. Although data from this system showed higher background levels as compared to those of system 3 due to the greater extent of incoherent scattering from the hydrogen in NRW, the results clearly show a similar extensive incorporation of DL-aspartic acid material within the nylon film. There is good agreement between the two crystallization systems, although the crystallization in the NRW system has proceeded at a slightly faster rate to produce a thicker near-surface layer, and the amount of water in this layer has also increased. Once more, our supposition that the nylon 6 is principally crystalline is further supported by the negative value for the volume fraction of H-ASP obtained for the 0-2 h data by assuming an amorphous nylon 6 film. External Reflection FTIR Studies. To gain further insight into the film-induced crystallization process, the crystallization of hydrogenous DL-aspartic acid beneath hydrogenous nylon 6 was followed in situ using external
Studies of
DL-Aspartic
Acid Crystallization
Langmuir, Vol. 20, No. 9, 2004 3597
Table 6. Observed External Reflection FTIR Bands for Nylon 6 and
nylon 6 film bands/cm-1
DL-aspartic
DL-aspartic
acid bands seen prior to visible crystal formation/cm-1
acid bands seen on visible crystal formation/cm-1
986 vw, broad 1071 w, broad 1117 vw, broad 1141 w, broad
853 s 893 s 986 w 1071 s 1117 m or w 1141 m
1213 w, broad
1213 vs
1171 vw 1202 w 1265 w 1313 s 1349 sa 1543 sa 1639 sa 2851 vw, broad 2865 w, broad 2920 vw, broad 2930 w, v broad 3134 w, broad 3304 m
DL-Aspartic
Acid
assignment and ref CH2 rock33 CC stretch33 CC stretch33 CN stretch33 NH3+ rocking33 NH3+ rocking33 CH2 twist/wag31 amide III & CH2 wag/ CH2 twist/wag31 CH2 twist?33 amide III & CH2 wag31 CH2 wagging33 CH bend33 amide II31 amide I31 CH2 symmetric stretch33 CH2 symmetric stretch31 CH2 asymmetric stretch33 CH2 asymmetric stretch31 NH3+ asymmetric stretch33 NH stretch31
a These bands become increasingly obscured by the increasing intensity water vapor bands that arise as the background spectrum taken at the start of the experiment ages.
reflection FTIR. Table 6 lists the observed band positions attributable to the nylon 6 film and DL-aspartic acid crystals, together with their corresponding band assignments. The nylon 6 film bands occur at positions similar to those observed for nylon 6 6 spread films and are consistent with the film adopting a hydrogen-bonded sheet structure parallel to the film surface akin to the sheet structure found in its bulk R-phase crystal.31 The absence of so-called “amorphous” bands at 1124 and 983 cm-1 also supports the view that the film is not predominantly amorphous, although a very weak band at 1170 cm-1, controversially attributed to the amorphous phase, is just discernible.32 The DL-aspartic acid bands have been assigned with reference to published L-aspartic acid crystal transmission data,33 because data for DL-aspartic acid crystals could not be found in the literature; hence, these are only tentative assignments. Significantly, the first evidence for the incorporation of DL-aspartic acid material within the film layer occurs prior to any visible crystal formation and is shown by the appearance of very weak bands at approximately 1213, 1141, 1117, and 1071 cm-1 after 90-120 min, see Figure 4a. Of these bands, the 1213, 1141, and 1071 cm-1 are more intense, although the 1213 cm-1 band is partially obscured by the 1202 cm-1 amide III band of the nylon 6 film. No other DL-aspartic acid bands are discernible, although the 1700-1350 cm-1 region is obscured by water vapor bands, so we cannot rule out the occurrence of further bands in this region. These weak bands increase slightly in intensity over time, again without any observable crystal formation occurring in the beam-sampled region, see Figure 4b. The emergence of one further band at 986 cm-1 is also just discernible in the 150-min data (31) Rotter, G.; Ishida, H. J. Polym. Sci., Part B: Polym. Phys. 1992, 30, 489-495. (32) A recent study validated the use of the 1124 cm-1 band as an amorphous band; however, there is much debate over whether the 1170 and 983 cm-1 bands can also be used for this purpose, see: Vasanthan, N.; Salem, D. R. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 536547. (33) Lopez Navarrete, J. T.; Hernandez, V.; Ramirez, F. J. Biopolymers 1994, 34, 1065-1077.
shown in Figure 4b. No associated changes in the nylon 6 film bands are apparent during this period. Random sampling of 10 apparently crystal-free film-covered surface regions also revealed these weak bands associated with DL-aspartic acid in 6 of the 10 cases; in the other 4 cases, the weak signals, if present, could not be distinguished above the spectral noise. Furthermore, similar weak intensity bands were also observed prior to visible DLaspartic acid crystallization in nylon 6 6 film-induced crystallization experiments. The widespread occurrence of these weak DL-aspartic acid bands may be correlated with the neutron reflectivity data, which showed a significant incorporation of DL-aspartic acid material into the film layer, despite the relatively low visible crystal surface coverage obtained after 8 h. Thus, this supports our claim that the majority of the adsorbed DL-aspartic acid material remains as subvisible clusters. Similar conclusions have been reached by Ahn et al.16 to explain the large-scale intensity changes observed in film IR bands on calcite crystallization, despite only ∼5-15% of the surface becoming covered with calcite crystals. The visible formation of DL-aspartic acid crystals on the beam-sampled surface is accompanied by a sharpening and large increase in the intensity of the weak signals at 1213, 1141, 1117, and 1071 cm-1, see Figure 4c, together with the appearance of many more crystalline bands, as listed in Table 6. Figure 5a and 5b shows the 3500-2800 and 1400-950 cm-1 spectral data obtained during the visible crystallization of DL-aspartic acid; not only are the DL-aspartic acid bands readily seen, but it is also evident from Figure 5a that the nylon 6 NH stretch at 3304 cm-1 increases dramatically in intensity as the crystallization proceeds; the weak nylon 6 methylene stretches also increase slightly in intensity. An intensity increase is also apparent for the nylon 6 amide I at 1637 cm-1, although the extent of the increase is harder to discern due to the overlapping water vapor bands; indeed, the latter obscure the amide II band at 1541 cm-1 to such a degree that the observation of any intensity increases for this band is precluded. The weakness and close proximity to stronger DL-aspartic acid crystalline bands also make it difficult to
3598
Langmuir, Vol. 20, No. 9, 2004
Figure 4. FTIR spectra from 1250 to 950 cm-1 showing the occurrence of weak DL-aspartic acid bands prior to any visible crystal formation. (a) Spectra taken after 120 min. The weak DL-aspartic acid bands are asterisked. (b) Spectra from 0 to 150 min showing the gradual appearance of the weak DL-aspartic acid bands. (c) Spectra comparing the sharper, more intense DL-aspartic acid bands found in regions of visible crystal formation with the weak DL-aspartic acid bands often seen in regions devoid of visible crystal formation.
discern the extent of intensity changes for the nylon 6 amide III bands at 1265 and 1202 cm-1. Nevertheless, the intensity of these amide III bands does appear to increase also. Similar film band intensity increases were also sometimes seen during DL-aspartic acid crystallization beneath the analogous nylon 6 6 spread films. It was difficult to observe these synergistic changes regularly, though, because this required the crystals to grow within the small ∼1 mm2 region sampled by the IR beam. More typically, crystals grew outside the beam-sampled area, necessitating careful movement of the trough so that IR
Jamieson et al.
data from these regions could be compared with regions devoid of crystals. Comparison of these regions often, but certainly not always, displayed similar film band increases in the regions with visible surface crystals. To obtain more information about the nylon 6 film band intensity increases, the dissolution process was also investigated at the end of the crystallization via the following procedure. FTIR data on a DL-aspartic acid crystal grown beneath the film were obtained by carefully sliding the trough along its platform, until a crystal was located in the IR beam. Approximately one-half of the saturated solution was then carefully replaced with water at 60 °C, and the resulting dissolution of the crystal beneath the film was followed by collecting spectra every 10 min. The data obtained, see Figure 5c and 5d, clearly show the decrease in the nylon 6 NH stretch band as the crystal dissolves. This confirms the reversibility of the synergistic process that occurs during the film-induced DL-aspartic acid crystallization. In external reflection FTIR studies with an incidence angle of 30°, bands due to groups vibrating parallel to the interface contribute more to an external reflection FTIR spectrum at the air-water interface, because the magnitude of the electric vector perpendicular to the interface is very low.34,35 Consequently, band intensity increases/ decreases can arise due to molecular reorientations that cause the particular vibration to become more/less parallel to the interface. Such an interpretation has previously been given for intensity changes observed during crystallization beneath nonpolymeric films, where it has been shown that the reorientation decreased the lattice mismatch between the film and underlying crystal face, and hence aided the crystallization process.16 However, for polymeric films, we are aware that the film accumulating around the growing crystal, without necessarily reorientating itself to a significant extent, could also lead to the same result. Considering that the hydrogen-bonded sheets are expected to lie parallel to the aqueous surface, this should already lead to a maximum in the intensity of the NH stretch for a crystalline, or mainly crystalline, nylon 6 film. The other nylon 6 bands listed in Table 6, except for the asymmetric methylene stretches, also have their transition dipole moments parallel to the surface for a mainly crystalline film, and so we would also expect these bands to initially have a maximum intensity. Because all of the nylon 6 bands appear to increase/decrease in intensity in unison (albeit that it is difficult to quantify the intensity increase for some of the bands), this does suggest that the film accumulation process is a more likely explanation for the observed intensity changes. Of course, some reorientation of the film will also occur during this accumulation process, but the reorientation is unlikely to be extensive because the increase in the NH stretch band would be far less evident if the NH stretching vibration was no longer approximately parallel to the aqueous surface. At this stage, we do not have definitive proof that the film accumulation process occurs due to the favorable interaction between the nylon 6 film and DL-aspartic acid crystal, or whether it merely occurs because the additional weight of the crystal as it grows causes the surface to be depressed, with additional polymeric material then filling the recess formed. However, if infilling was the main cause of the film accumulation in the visible crystal regions, we would have expected this effect to occur for all of the larger crystals growing beneath the film, which is not the case. (34) Mielczarski, J. A. J. Phys. Chem. 1993, 97, 2649-2663. (35) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373-1379.
Studies of
DL-Aspartic
Acid Crystallization
Langmuir, Vol. 20, No. 9, 2004 3599
Figure 5. Intensity changes observed in the nylon 6 NH stretch band during visible DL-aspartic acid crystal growth/dissolution. (a) and (b) show the 3500-2800 cm-1 and 1400-950 cm-1 spectra during the visible crystallization of DL-aspartic acid. The minute values indicated refer to the time elapsed after the spreading of the nylon 6 film. (c) and (d) show the 3500-2800 cm-1 and 1400-950 cm-1 spectra during the visible dissolution of a DL-aspartic acid crystal. The minute values indicated refer to the time elapsed after adding pure water to dissolve the crystal.
Also, the favorable adsorption of the nylon 6 film on the DL-aspartic acid crystals is consistent with the DL-aspartic acid material becoming incorporated within, and not below, the nylon 6 film. This agrees with the supposition that subsequent growth of many crystallites has been hindered by film adsorption caused by the mutual DLaspartic acid-nylon 6 attraction. Conclusions Neutron reflectivity was used to follow the crystallization of DL-aspartic acid under nylon 6 spread films with the aim of providing information about the composition and thickness of the interfacial region during the early stages of crystal growth. The data revealed the incorporation of significant quantities of DL-aspartic acid within the nylon 6 layer, rather than the existence of discrete film and crystal layers. After 8 h, the DL-aspartic acid comprised over 50% of the interfacial film layer. Ac-
cumulation of further DL-aspartic acid material to produce microscopic/macroscopic crystals occurred, but on a more limited scale, culminating in ∼1-5% crystal coverage of the surface over the same period. This suggests that much of the incorporated DL-aspartic acid material is of subcritical nucleus size and/or that subsequent film adsorption upon the nuclei impedes their further growth to visible dimensions. These data are supported by external reflection FTIR studies in which very weak bands attributable to DL-aspartic acid are observed in surface regions devoid of visible crystals. In regions with visible crystals, much larger and sharper DL-aspartic acid bands are seen. Changes in the intensity of the nylon 6 NH stretch and other nylon 6 bands are often observed during the visible crystallization and dissolution of DL-aspartic acid and are consistent with the reversible accumulation of the nylon 6 film around the growing crystals.
3600
Langmuir, Vol. 20, No. 9, 2004
Acknowledgment. We gratefully acknowledge EPSRC funding for M.J.J. and A.F.M. and the assistance of Prof. R. W. Richards (University of Durham) with the neutron data analysis. CCLRC is thanked for the provision of
Jamieson et al.
neutron beam facilities of ISIS at the Rutherford Appleton Laboratory. LA0361823
