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Examination of Glass Transitions in CO2-Processed, Peracetylated Sugars Using Sum Frequency Generation Spectroscopy Michael L. Hurrey† and Scott L. Wallen*,‡ Department of Chemistry and the NSF Science and Technology Center for EnVironmentally Responsible SolVents and Processes, Kenan and Venable Laboratories, The UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 ReceiVed March 4, 2004. In Final Form: January 13, 2006 The present study utilizes vibrational sum frequency generation (SFG) spectroscopy to study changes in the surface crystallinity of various peracetylated sugars, a class of materials that have a high affinity for carbon dioxide (CO2). Studies of the solid-air interface of acetylated β-cyclodextrin (Ac-β-CD) and sucrose octaacetate (SOA) show that diffuse reflectance SFG spectroscopy is sensitive to changes in crystallinity from processing with either heat or solvation in CO2, due to the loss of signal after glassification occurs. β-D-Glucose pentaacetate (Ac-β-GLC) was used as a control for this experiment due to the fact that it does not undergo a crystalline phase transition, regardless of processing conditions. The crystalline to amorpohous transitions of these bulk materials were verified using differential scanning calorimetry (DSC) as a function of thermal and CO2 processing. In addition, preliminary results suggest that the SFG technique is sensitive in detecting the degree of crystallinity at the interface as a result of incomplete processing and presents new opportunities for the examination and detection of surface crystallinity changes.
Introduction Environmental remediation has been an important concern over the past decade. The cost and difficulty of removing pollutants after they have been introduced to the environment has motivated industry to focus on methods to prevent pollution. In recent years, it has become evident that the replacement of organic solvents and the reduction of water usage are of primary concern in pollution prevention and in the development of green chemical processing methods.1 The search for alternatives was largely a result of changes in law and public opinion which both support an increased awareness of the environmental impacts of chemical processing. Another consideration is the economy of a particular process. From each of these points of view, CO2 has become an attractive alternative in the replacement of water and organic solvents for many industrial applications.2 This is due to the fact that CO2 is nontoxic, recyclable, nonflammable, inexpensive, and possesses tunable solvent properties with densities and viscosities readily adjusted between that of a gas and a liquid. Although CO2 is a green house gas, sources of CO2 for industrial uses would come from manufacturing where CO2 is a byproduct.2 Recent efforts have taken advantage of CO2 as a green solvent in nanoparticle production,3-5 performing organic,6 inorganic,7 and catalytic reactions,8-12 producing * Corresponding author. E-mail:
[email protected]. Phone: (919) 5933899. † Present address: Vertex Pharmaceuticals, 130 Waverly St., Cambridge, MA 02139. ‡ Present address: International Technology Center, 8100 Brownleigh Dr., Raleigh, NC 27617. (1) Poliakoff, M.; Anastas, P. Nature 2001, 413, 257. (2) DeSimone, J. M. Science 2002, 297, 799. (3) Cooper, A. I. AdV. Mater. 2001, 13, 1111. (4) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langmuir 1999, 15, 6613. (5) Ji, M.; Chen, X.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631. (6) Oakes, R. S.; Clifford, A. A.; Rayner, C. M. J. Chem. Soc. Perkins Trans. 1 2001, 917. (7) Yonker, C. R.; Wallen, S. L.; Linehan, J. C. J. Microcolumn Sep. 1998, 10, 153. (8) Hyde, J.; Leitner, W.; Poliakoff, M. High-Pressure Chem. 2002, 371.
perfluoro13 and other polymeric materials,14,15 as well as improving lithographic methods by taking advantage of the low surface tension of CO2.3,16 One of the main efforts in this research is in the development of water supporting reverse microemulsions for reactions and polymerizations that would normally lead to large amounts of waste.17-23 To aid in the production of these materials, it is important to understand the interactions between CO2 and functional groups that promote solvation in this environmentally benign solvent. Previously, ab initio calculations and Raman vibrational studies have shown that small carbohydrate molecules with a high degree of acetylation are extremely soluble in CO2 because of cooperative intermolecular interactions.24-27 The model compounds that lead to enhanced CO2 solubility are peracetylated sugars.25,27 (9) Licence, P.; Ke, J.; Sokolova, M.; Ross, S. K.; Poliakoff, M. Green Chem. 2003, 5, 99. (10) Carter, C. A. G.; Baker, R. T.; Tumas, W.; Nolan, S. P. Chem. Commun. 2000, 347. (11) Jacobson, G. B.; Lee, C. T., Jr.; Johnston, K. P.; Tumas, W. J. Am. Chem. Soc. 1999, 121, 11902. (12) Pesiri, D. R.; Morita, D. K.; Walker, T.; Tumas, W. Organometallics 1999, 18, 4916. (13) Romack, T. J.; Kipp, B. E.; DeSimone, J. M. Macromolecules 1995, 28, 8432. (14) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Chem. ReV. 1999, 99, 543. (15) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 119, 6399. (16) Kendall, J.; Desimone, J. M.; Carbonell, R. G.; McAdams, C. L. PCT Int. Appl. 2002, 49. (17) Yates, M. Z.; Apodaca, D. L.; Campbell, M. L.; Birnbaum, E. R.; McCleskey, T. M. Chem. Commun. 2001, 25. (18) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127. (19) Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1988, 92, 2903. (20) Consani, K. A.; Smith, R. D. J. Supercrit. Fluid. 1990, 3, 51. (21) McFann, G. J.; Johnston, K. P.; Howdle, S. M. AIChE J. 1994, 40, 543. (22) Jacobson, G. B.; Lee, C. T., Jr.; daRocha, S. R. P.; Johnston, K. P. J. Org. Chem. 1999, 64, 1207. (23) Johnston, K. P.; Randolph, T.; Bright, F.; Howdle, S. Science 1996, 272, 1726. (24) Raveendran, P.; Wallen, S. L. J. Am. Chem. Soc. 2002, 124, 12590. (25) Raveendran, P.; Wallen, S. L. J. Am. Chem. Soc. 2002, 124, 7274. (26) Blatchford, M. A.; Raveendran, P.; Wallen, S. L. J. Am. Chem. Soc. 2002, 124, 14818. (27) Raveendran, P.; Ikushima, Y.; Wallen, S. L. Chem. ReV. 2005, 38, 478485.
10.1021/la0494223 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/12/2006
CO2-Processed, Peracetylated Sugars
Peracetylated sugars have inherent advantages over other CO2 soluble compounds due to the fact that they are inexpensive, renewable, and nontoxic. In addition, the dissolution of these materials under a variety of CO2 conditions suggests that these materials are suitable precursors to surfactant building blocks for reverse microemulsion systems25 as well as potential pharmaceutical excipients.28 With regard to the latter, this particular industry is especially waste intensive with respect to the quantity of solvents utilized in processing. The use of CO2 as a solvent has been problematic due to the lack of CO2-soluble active pharmaceutical ingredients (API) and excipients.28 Peracetylated sugars offer the opportunity to replace existing excipients since it is possible to use a CO2-based melt process at room temperature to entrain API in the solid excipient.28 Before these materials can be used at even the formulation level, a full characterization of changes in crystal structure under varying thermodynamic conditions must be investigated. One of the most important aspects of drug formulation is the crystallinity of both the API and excipients used. The presence of different crystalline states and polymorphs of each material have large effects on the overall shelf stability as well as how the materials will interact in a real biological system. Determining the long-range order of a material in most cases is straightforward. DSC and X-ray diffraction are the most readily available techniques to investigate long-range order, although they only probe the bulk characteristics of the material studied. Infrared (IR) and Raman spectroscopic techniques have also been used to study crystallinity changes in polymorphs.29 The benefit of these techniques is that they can be performed in situ and depending on the particular experimental setup (small angle diffuse reflectance IR and polarized Raman microspectroscopy) can provide surface specific information about these materials; however, surface selectivity and sample handling can prove difficult for these techniques. Studies of the bulk interactions of the peracetylated sugars have been important in explaining the solvation of these materials in CO2.24-27 However, to better understand the intermolecular interactions present at the surface and effects due to different environmental and processing conditions, a surface selective technique must be employed. SFG spectroscopy is a nonlinear, second order, vibrational technique that selectively probes the interface between two bulk centrosymmetric media. These changes could be caused by hydration, pressure, temperature, or solvation and presumably the surface of a material changes before the bulk. SFG detection of these changes would allow the possible study of specific orientational information that would allow a clearer understanding of mechanisms of crystallinity changes in materials. In the present study, SFG is used to investigate changes in long-range order at the surface of several sugars as a function of CO2 solution processing and changes in thermodynamic conditions.
Background SFG. Sum frequency generation is a three wave mixing, nonlinear, optical technique postulated theoretically by Bloembergen and Pershan in the 1960s30 and first demonstrated experimentally on surfaces by Shen in 1987.31 For the SFG experimental setup, visible (ωV) and tunable IR (ωIR) beams coherently impinge upon a sample, inducing a nonlinear (28) Blatchford, M. A.; Wallen, S. L. Mol. Pharm. submitted. (29) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi, N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147. (30) Bloembergen, N. Appl. Phys. B-Lasers O 1999, B68, 289. (31) Hunt, J. H.; Guyot-Sionnest, P.; Shen, Y. R. Chem. Phys. Lett. 1987, 133, 189.
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polarization that can be measured at the sum frequency (ωSF) (ωSF ) ωV + ωIR).32,33 This polarization is governed by
I ∝ P ) 0(χ(1)‚E + χ(2):EE + χ(3)lEEE + ‚‚‚)
(1)
where P is the polarization, 0 is the permittivity of free space, χ(n) is the electric susceptibility, and E is the electric field strength. Since SFG is a second-order process, the χ(2) term will dominate the above equation. The χ(2) is dependent on nonresonant enhancements in the bulk (χ(2) NR) and vibrational resonances 32,34 that are both Raman and IR active within a system (χ(2) ) R according to (2) (2) χ(2) ) χ(2) NR + χR (ωIR) ) χNR +
NAnνMlmν∆p
∑ωV - ωIR - iΓν
(2)
where, N is the surface number density, Anν is the IR transition moment, Mlmν is the Raman transition strength, ∆p is the population difference, ωV is the resonant frequency, ωIR is the tunable IR frequency, and iΓν is a damping constant for a particular transition, ν.33-35 For χ(2) R to have both a Raman and IR transition, the molecule cannot have a center of symmetry.33,36 An interface, by definition, is a noncentrosymmetric environment due to a lack of inversion, which allows resonant enhancements. Equation 2 is a simplification of χ(2), which is a 27-element tensor relating molecular orientation and the lab frame to the incident and detected electromagnetic wave vectors37
[ ][
ESF βxxx βxyx βxzx χ(2) ) EV βyxy βyyy βyzy EIR βzxz βzyz βzzz
]
(3)
In eq 3, EV, EIR, and ESF represent the visible, infrared, and detected sum frequency electric fields respectively, and βijk is the molecular polarizability. If azimuthal symmetry is assumed, there are four nonzero, nonredundant solutions to the above (2) (2) (2) (2) (2) (2) 32,33,38,39 equation (χ(2) xxz ≡ χyyz, χxzx ≡ χyzy, χzxx ≡ χzyy, and χzzz). These solutions relate the polarization of the emitted and detected radiation directly to the orientation of a molecule. These polarization methods can be employed to determine the absolute orientation of a surface adsorbed or associated species.32,40,41 However, in the case of bulk powders, the random nature of the particles at the surface scatters light isotropically making polarized SFG impossible.42 This fact will play an important role in the interpretation of the SFG signal of powdered peracetylated sugars in the present study. DSC. Differential scanning calorimetry monitors changes in heat flow into a material, which is related to the enthalpy as it undergoes a phase transition.43 For macromolecules, in addition (32) Gragson, D. E.; Richmond, G. L. J. Phys. Chem. B 1998, 102, 3847. (33) Williams, C. T.; Yang, Y.; Bain, C. D. Langmuir 2000, 16, 2343. (34) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. Langmuir 1998, 14, 6722. (35) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. J. Phys. Chem. 1996, 100, 7617. (36) Dreesen, L.; Humbert, C.; Hollander, P.; Mani, A. A.; Ataka, K.; Thiry, P. A.; Peremans, A. Chem. Phys. Lett. 2001, 333, 327. (37) van der Ham, E. W. M.; Vrehen, Q. H. F.; Eliel, E. R.; Yakovlev, V. A.; Alieva, E. V.; Kuzik, L. A.; Petrov, J. E.; Sychugov, V. A.; van der Meer, A. F. G. J. Opt. Soc. Am. B 1999, 16, 1146. (38) Yang, Y. J.; Pizzolatto, R. L.; Messmer, M. C. J. Opt. Soc. Am. B 2000, 17, 638. (39) Schaller, R. D.; Saykally, R. J. Langmuir 2001, 17, 2055. (40) Briggman, K. A.; Stephenson, J. C.; Wallace, W. E.; Richter, L. J. J. Phys. Chem. B 2001, 105, 2785. (41) Pizzolatto, R. L.; Yang, Y. J.; Wolf, L. K.; Messmer, M. C. Anal. Chim. Acta 1999, 397, 81. (42) Ma, G.; Allen, H. C. J. Am. Chem. Soc. 2002, 124, 9374.
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to melting temperatures (Tm), DSC allows determination of glass transition temperatures (Tg) and crystal formation temperatures (Tc). Most materials exist in either a purely crystalline, amorphous, or semicrystalline state, which is directly related to the longrange order of the molecular system.44 Heating the sample at a particular rate and measuring the change in heat flow can monitor phase transitions specific to the type of material present. When a temperature dependent change in heat is performed at constant pressure, the change in enthalpy of the material can be determined.45
(δqδt ) ) δHδt P
(4)
The change in heat flow is normalized to a reference material (typically a known amount of alumina or an empty aluminum pan), which enables endothermic and exothermic processes to be plotted as a function of temperature in a DSC thermogram. A sharp change in heat flow (a Gaussian peak) is indicative of a true phase transition, whereas a slow change in heat flow (a negative sigmoidal curve) typically indicates a glass transition in a material.46 At this temperature, a change in the local degrees of freedom of the material allows an amorphous polymer, for example, to be converted to a brittle, glasslike form.45 The mechanical properties of the substance change as a result of a rapid change in the specific heat, coefficient of thermal expansion, free volume, and dielectric constant.44,45
Figure 1. Schematic of the molecular structure of the peracetylated sugars studied. (A) Ac-β-GLC; (B) SOA; (C) Ac-β-CD.
Experimental Section Chemicals and Methods. Supercritical fluid extraction (SFE) grade CO2 (Scott Specialty Gases Inc.) was used as received. SOA (97+%; Sigma Chemical Co.), Ac-β-GLC (99%; Fluka Chemical Co.), and Ac-β-CD (industrial grade; gift from Cargill Inc.) were dried just below their respective melting temperatures for 2 h and stored in a desiccator prior to analysis (structures shown in Figure 1). Thermally processed materials were heated in a furnace (Thermolyne 48000) for 2 h and cooled in a desiccator. CO2-processed materials were kept at constant pressure using a syringe pump (ISCO 100DX) while being heated in a tube furnace (Eppendorf TC-50) for 2 h so that the materials were completely solubilized in CO2 before venting. SFG samples were prepared by grinding the samples with a mortar and pestle and then compressing the solid samples between two 1 × 3 cm acid cleaned glass slides, to ensure that the samples were macroscopically flat, and then placed on the SFG sample stage. SFG spectra were collected using the ssp polarization scheme and were normalized by dividing the SF signal by the input radiant flux. Because the SF intensity is proportional to the product of the impingent electromagnetic fields (see eq 1), SFG data under different conditions can be directly compared. Rapid expansion of supercritical solutions (RESS) samples were prepared by placing dried material in a cell, pressurizing with CO2, and then rapidly depressurizing. During depressurization, the solution was sprayed onto cleaned slides through a 250 µm i.d. capillary restrictor until a noticeable layer was present. This is important to ensure that the film present is thick enough to be characterized by the sum frequency process. Results have shown47 that the signal is dependent on the thickness and properties of the film studied. Therefore, the RESS coatings were made thick enough to facilitate alignment and should not affect the SFG results because only the surface layer is SFG (43) Reading, M.; Luget, A.; Wilson, R. Thermochim. Acta 1994, 238, 295. (44) Ferry, J. D. Viscoelastic Properties of Polymers; John Wiley & Sons: New York, 1961. (45) Zallen, R. J. The Physics of Amorphous Solids; John Wiley & Sons: New York, 1983. (46) Tanaka, K. Solid State Commun. 1985, 54, 867. (47) SFG response as a function of film thickness has shown that alignment of samples is nontrivial. Monolayer sensitivity is possible only for films that are homogenious and SFG active. SFG spectra of varying thickness of Ac-β-GLC films are available in the Supporting Information.
active. For both the CO2-processed materials and RESS coatings, Ac-β-GLC, Ac-β-CD, and SOA were pressurized to 137.9, 275.9, and 69.0 bar and heated to 25, 50, and 25 °C, respectively. SFG. The optical layout for the EKSPLA SFG spectrometer is shown in Figure 2. The technique utilizes the output of a 25-ps mode-locked Nd:YAG laser operating at 10 Hz with three outputs, two at the fundamental (1064 nm) and one at the second harmonic wavelength (532 nm). Both fundamental beams are relay-imaged and directed into a harmonics unit. This ensures proper beam shape and reduced absorption from the air for seeding the optical parametric oscillator (OPO). The second harmonic is sent through a two-pass crystal to create an optical delay, ensuring that the three-wave mixing is in phase. The harmonics unit pumps the OPO with two frequency tripled fundamental beams that are used to seed a lithium borate (LBO) crystal, while a third beam of fundamental energy is used to seed either a silver gallium sulfide crystal (AgGaS2) or a gallium selenide crystal (GaSe) in the difference frequency stage to produce tunable IR light. The IR light generated can be tuned between 2.3 and 18.0 µm (4345-555 cm-1), supplying the second beam used in the experiment. The IR beam has pulse energies ranging between 300 and 18 µJ over the range of wavelengths specified with a beam diameter of approximately 0.5 mm. The visible pulse has a bandwidth of 2 cm-1 and an energy range of 100-200 µJ, which is controlled with a quarter wave plate. This system has a better resolution than previous setups48 from EKSPLA due to the decreased bandwidth of the Nd:YAG, which also controls the bandwidth of the OPO output based on a BBO-MOPA developed at UC-Irvine.49 The system is also equipped with an xyz-translational stage and cell holder that allows for spatial alignment of the three beams of light used in the experiment, with the visible and infrared input beams at 60 and 54° with respect to the surface normal, respectively. DSC. Differential scanning calorimetry was carried out using a Seiko Instruments DSC 220C with a liquid N2 SII cooled controller. All of the measurements and experimental conditions were controlled with a personal computer. Each DSC sample was weighed in a standard DSC sample pan with a Mettler AE163, crimped shut, and (48) Wang, J.; Chen, C.; Buck, S. M.; Chen, Z. J. Phys. Chem. B 2001, 105, 12118. (49) van der Veer, W. E. Proc. Soc. Photo-Opt. Inst. 2001, 4268, 14.
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Figure 2. Schematic representation of the EKSPLA SFG spectrometer. (A) 25 ps Nd:YAG 10 Hz laser; (B) OPO/DFG unit; (C) sample stage; (VT) vacuum tube; (PD) photodetectors; (LD) laser diode; (GP) Glan-Thompson Prism.
Figure 3. Normalized SFG response of the air-powder interface for several peracetylated sugars; solid line (Ac-β-GLC); dotted line (Ac-β-CD); dashed line (SOA).
Figure 4. SFG spectra of RESS coatings for several peracetylated sugars. (A) Ac-β-GLC; (B) Ac-β-CD (dotted line); (C) SOA (dashed line).
placed directly into the instrument. Approximately 10-15 mg of unprocessed or CO2-processed samples were loaded into sample containers and run under specific experimental parameters for each sugar. During each run, a steady stream of nitrogen was passed into the sample compartment to ensure that the samples were in an inert atmosphere and to provide a constant pressure for the analysis.
SF response for different RESS films coated onto glass slides. No SF signal was present for the RESS coatings with the exception of Ac-β-GLC. The difference between the RESS samples and the bulk powders was CO2 solvation and desolvation under rapid depressurization and cooling but otherwise there was no difference in the way the sugars were treated. After processing, visual examination of the sugars exposed to CO2 in the pressure vessel indicated that the samples were no longer a fine powder after CO2 was vented from the cell but had become brittle like a glass. This change in mechanical properties and the SFG results prompted a DSC study of the phase transitions of these materials as a function of processing conditions. DSC Results of Processed Sugars. To investigate any changes in bulk mechanical properties of the peracetylated sugars studied, DSC was used to characterize each sample as a function of thermodynamic and solution processing conditions. Figure 5 shows the DSC results for unprocessed as well as thermally and CO2-processed peracetylated sugars. Table 1 lists the phase transition temperatures and relative order of the bulk materials determined from the DSC results. As discussed earlier, a negative Gaussian peak suggests a melting transition of a crystalline material and an inverse sigmoidal curve indicates a glass transition in an amorphous material. It was found that drying the material before DSC analysis had little to no effect on the measured transition temperatures in the case of Ac-β-GLC and SOA (see Table 1). However, Ac-β-CD with insufficient drying gave rise to multiple endothermic peaks during the first heating of the material (see the Supporting Information). It was hypothesized that this was due to either a possible hydration layer inside the cyclodextrin cavity, due to the acetylation reaction or physisorbed H2O from the atmosphere. Heating the Ac-β-CD to just over 100 °C had little effect on the initial results. Only upon heating the sample in a furnace for 2 h at 200 °C were these hydration peaks removed and a single peak observed reflective of a crystalline melting transition (Figure 5B).
Results and Discussion Bulk Powder-Air Interface. Allen and co-workers42 have recently shown that a surface comprised of surfactant particles could be probed with broadband SFG. The underlying assumption in that study was that the diffuse reflectance of the surface particles nullifies any polarization information available because of the angularly diffused SF response. However, as long as there is some degree of order at the surface, the same resonance enhancements expressed in eq 2 can be obtained. In the case of the unprocessed acetylated sugars, the surfaces were not expected to have any surface specific ordering, which would result in no measured signal. However, these materials did give rise to an enhanced SF response. Figure 3 shows the nonpolarized SFG spectra for three peracetylated sugars (Ac-β-GLC, Ac-β-CD, and SOA) in the -CH stretching region. It was assumed that the SF response was angularly diffused as discussed above, and no orientational information can be quantified. However, to determine the origin of the surface ordering necessary for SF signal enhancements in these materials, a broad range of interfacial systems were studied. RESS Processed Sugar Films. Obtaining SFG signal from different sugar surfaces as a function of processing conditions was first attempted. Previous studies25 have shown these samples to be highly soluble in CO2, prompting the use of the RESS process. The coatings were prepared with the assumption that no preferential ordering would occur when spraying these materials from CO2 solutions, and the signal should be similar to the signal from the bulk powders. Figure 4 shows the normalized
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Figure 5. Examination of thermal and CO2-processed sugars by DSC. (A) Ac-β-GLC; (B) Ac-β-CD; (C) SOA. Solid, dotted, and dashed lines are for unprocessed, thermally processed, and CO2processed sugars, respectively. Insets in panels B and C of Figure 5 have the same x and y axes as the rest of the figure. Table 1. Phase Transition Data for Peracetylated Sugars under a Variety of Processing Conditions
Ac-β-GLC Unprocessed thermally processed CO2-processed Ac-β-CD Unprocessed thermally processed CO2-processed SOA Unprocessed thermally processed CO2-processed
Tm (wet)
Tm (dry)
135.0 134.8 b
135.0 135.1 136.8
221.6
224.3
89.3
Tg (wet)
Tg (dry)
ordera cr cr cr
127.7 134.2
129.9 126.4
cr am am
29.2 b
29.0 25.8
cr am am
88.6
a Long range order expressed as either crystalline (cr) or amorphous (am). b Wet CO2 processing was only performed on Ac-β-CD.
It should be noted in Figure 5 that the DSC results are quite similar in the case of the larger sugars (Ac-β-CD and SOA), but the results for Ac-β-GLC were quite different. In fact the Ac-
β-GLC remains crystalline regardless of either thermal or solution processing. This is demonstrated by the persistence of a melting transition for all samples (Figure 5A). In contrast, Ac-β-CD and SOA are both crystalline after drying (Figure 5, panels B and C) but become amorphous after being processed (insets to panels B and C of Figure 5). In addition, the DSC data presented show an experimentally relevant change in Tg for the thermally and CO2-processed materials (insets to panels B and C of Figure 5). This is indicative of a different structure for thermally and CO2processed glasses. The DSC results reported here indicate that the materials are all crystalline prior to processing. After both thermal and solution processing, however, the larger sugars become amorphous and below a certain temperature (see Table 1) exist as glasses. SFG of Processed Sugars. After establishing the bulk mechanical properties of these materials using DSC, the SFG response of the same materials at their surfaces was investigated. It was mentioned earlier that, in order for there to be a resonant SF response from a solid surface, some degree of local order must be present. Recall that the diffuse reflectance of a powdered sample allows no orientational information to be extracted from SFG results. Figure 3 shows that the unprocessed materials all possess SFG active vibrational modes with fairly strong intensities. In addition, the RESS coatings show very little signal for Acβ-GLC and essentially no signal for the larger sugars. The data for the bulk sugars depicted in Figure 3 represent fully crystalline materials since no processing has taken place. The sugars used for the RESS coatings on the other hand (Figure 4) have become amorphous after solubilization in CO2 with the exception of Ac-β-GLC. A crystalline material has the unique property of having both short-range and long-range ordering, in which the latter can be described by specific geometric shapes. Due to the long-range ordering of the sugars, SFG signal was obtained for the unprocessed materials and processed Ac-β-GLC even in the absence of an oriented interface. These results were in good agreement with both DSC and prior SFG RESS film results, which verifies the sensitivity of SFG to crystallinity. Figure 6 shows SFG spectra for the air-solid interface of both unprocessed and CO2-processed sugars. Due to the argument that crystallinity has an effect on the SFG signal due to longrange order, the larger sugars, Ac-β-CD and SOA, would elicit no signal because processing these materials in CO2 makes them completely amorphous. This is in fact what is observed in Figure 6, panels B and C. In addition, because Ac-β-GLC remains crystalline after processing, it would be expected to have approximately the same signal as the untreated material. Although the signal for the treated Ac-β-GLC is present, it is about an order of magnitude less intense than the unprocessed sample. This is not surprising due to the slight change in the Tm of the material after processing (Figure 5A). It is important to note in Figure 6A that there is signal from the processed Ac-β-GLC, supporting the hypothesis that the SFG response is sensitive to the long-range order in the material being studied. The change in amplitude seems to suggest that the material has undergone some mechanical change after being processed, which is evident from the DSC results. The exact nature of the change is not known at this time, however, since the X-ray powder diffraction results indicate that the crystal structure of β-D-galactose pentaacetate (Ac-β-GAL) does not change as a function of CO2 solvation leads to the conclusion that the change in signal is not due to a change in long-range crystal structure for the bulk (results not shown) but rather to a surface associated change. One might suggest that the change could be due to a CO2-based densification since the material became brittle and more tightly packed after
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Figure 7. SFG response to changes in processing of Ac-β-CD at the air-solid interface; unprocessed (solid), partially CO2-processed (dotted), fully CO2-processed (dashed), thermally processed (dotdashed). The fully CO2-processed and thermally processed spectra have been offset for clarity.
Figure 6. SFG examination of surfaces after CO2-processing. (A) Ac-β-GLC; (b) Ac-β-CD; (c) SOA. Solid and dotted lines represent the unprocessed and CO2-processed materials, respectively.
rapid depressurization. However, this cannot explain the changes in the Tm or the fact that a higher density material should have a greater quantity of SFG active surface sites per unit volume leading to an increased signal. The only viable explanation for the SFG and DSC results is that the surface structure has been changed, presumably becoming more disordered relative to the unprocessed or thermally processed material. Any of the explanations for the changes in the SFG signal shown in Figure 6 could also be applied to the observations for the RESS coatings (Figure 4). The signal for the Ac-β-GLC RESS coating is rather weak in comparison to the bulk powders seen in Figure 3, which could also support a change in surface morphology that is dependent on the processing conditions used. Nevertheless, the surface of Ac-β-CD and the SOA are both completely amorphous after CO2 processing resulting in no SFG activity. To fully understand the SFG response to the change in crystallinity, several different processing conditions were examined using Ac-β-CD to see if the response was specific to CO2-processing or to changes in crystallinity in general. DSC results suggested that thermally processing materials should also have the same effect on the SF signal as solution processing with CO2. The ex situ study comparing the effect of thermal and CO2-processing methods on Ac-β-CD is shown in Figure 7. Four different treatment methods were used to study changes in the crystallinity of this material. The cyclodextrin was first dried just below its melting temperature for several hours to ensure that residual H2O was removed and that no subsequent thermal degradation occurred (solid line in Figure 7). The material was also CO2-processed under the same conditions as discussed earlier except the equilibration time was changed (dotted line). It was
found that this material had become semicrystalline after only 30 min of CO2 processing due to the appearance of both a melting and glass phase transition in the DSC results (see the Supporting Information). The SFG data collected from a fully CO2-processed sample of Ac-β-CD is also presented in Figure 7 (dashed line). Last, the material was heated in a furnace above its melting temperature and allowed to cool in a desiccator, and then SFG data was collected (dot-dashed line). The thermally processed material exhibited no signal when placed in the SFG spectrometer, indicating that SFG is sensitive to crystallinity changes and surface glassification. Second, it was noted that the equilibration time used when CO2-processing sugars had an effect on the signal. If the sample was not fully processed (i.e., the material was semicrystalline), residual SFG signal was detected (dotted line in Figure 7). This is consistent with the hypothesis that SF signal is dependent on the surface crystallinity. In this case, a large amount of amorphous material would be surrounding small pockets of crystalline material (lamellae) in the partially processed Ac-β-CD, which would result in a smaller signal than the unprocessed material. When comparing these data with that of the fully processed material (dashed line in Figure 7), it is noted that there is very little difference in intensity, which suggests that the SFG technique is sensitive to a system which is partially crystalline.
Conclusions In this study, the validation of SFG spectroscopy to study changes in surface crystal structure was presented. The assumption that the signal of a solid, powder sample is due to some local orientation stems from the fact that, in general, SFG signals have a resonant component to the second-order hyperpolarizability χ(2) that within the dipole approximation has to have some orientation of the vibrational modes at the interface. In the case of acetylated sugars, the SF signal arising from these surfaces is sensitive to the overall crystallinity, since no other interface specific ordering is present. To verify that the signal arises from changes in crystal structure, various thermal and CO2-processing methods were employed. The nature of each material was also characterized with DSC as a function of processing method, and all of the acetylated sugars except Ac-β-GLC became amorphous after processing. Once the presence or absence of long-range order was determined, CO2 and thermally processed, as well as partially processed, materials were examined with SFG. These studies indicate the SFG signal is dependent on the surface crystallinity of the sugar. In the case of Ac-β-GLC, there is a detectable decrease in the SFG intensity for the CO2-processed
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sample that appears to indicate a more disordered surface structure compared to the unprocessed or thermally processed material. This difference is also apparent in shifts in melting temperature from the DSC results for this material. Further studies are needed to understand the mechanism of the change in the surface structure. Specifically, elucidation of how the crystal structure in Ac-β-GLC changes before and after processing as well as studying the CO2-induced crystallinity changes in Ac-β-CD and SOA. The successful study of changes in surface crystallinity using SFG has broad applications, including the investigation of the effects of environmental and processing conditions on API and excipients in pharmaceutical formulations.
Hurrey and Wallen
Acknowledgment. This material is based upon work supported in part by the STC Program of the National Science Foundation under Agreement No. CHE-9876674, and a NSF GAANN Fellowship (M.L.H.). The authors thank Cargill for the gift of the peracetylated sugars, Altos-Inc. and EKSPLA for their service and support of the SFG instrument, and Dr. P. Raveendran for his calculations regarding the vibrational spectra of the acetylated sugars. Supporting Information Available: Effect of Film Thickness on SFG Spectra. This material is available free of charge via the Internet at http://pubs.acs.org. LA0494223
