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Conclusion Interestingly, k F E r is larger for (bpz)ReCH-DTF than for (dab)ReCH-DTF in both solvents. Since forward ET is only The experiments on the (b)ReCH-DTF complexes clearly moderately exothermic, the process is expected to be in the demonstrate that by using a metal complex chromophore and a "normal" free energy region. As a result, the larger kFET for rigid organic spacer rates of rapid intramolecular ET between (bpz)ReCH-DTF is expected due to the fact that AGFET is more redox sites that are held at a well-known separation distance can exothermic for this complex. be readily determined. Experiments in progress are focused on By contrast, kBET is larger for (bpz)ReCH-DTF than for using rigid organic spacers of varying lengths and on increasing (dab)ReCH-DTF, despite the fact that AGBET is more exothermic the range of driving forces for forward and back E T by varying for the dab complex. I n each complex the back-reaction involves the diimine ligands on the Re chromophore. These studies will ET from the diimine anion to the DTF cation; this is essentially allow examination of the solvent and distance dependence of the ET from an organic radical anion to an organic radical cation. reorganization energy for ET in these organic donor-metal comAs a result, there should be a relatively small inner-sphere replex chromophore-quencher systems. organization energy for back ET (e.g., 0.3-0.5eV).2 Taking into Acknowledgment. Acknowledgment is made to the donors of consideration the separation distance between the Re center and the Petroleum Research Fund, administered by the American the DTF moiety, the total reorganization energy for the back ET Chemical Society, for support of this research. Some of the reaction is probably in the range 1 .+I .4 eV.la.2-IoTherefore, back transient absorption experiments were performed at the Center ET is in the inverted free energy region in both solvents; this fact for Fast Kinetics Research, which is supported jointly by the very likely explains the observed inverted dependence of kBET on Research Technology Program of the Division of AGBET observed for the two (b)ReCH-DTF c ~ m p l e x e s . ~ ~ ~ - *Biomedical ~-~~ Research Resources of the National Institutes of Health (RR00886) and by The University of Texas at Austin. We thank (32) Based on the assumption that SoFET IS small or zero, the temperature Professor Russell Schmehl and Professor J.-M. Lehn for comments dependence data can be utilized to estimate the reorganization energy (A) and and advice and Mrs. Dorothy Freeto for travel support. the donor-acceptor electronic coupling (HAB) (refs 8 and IO). The assumption that A S ° F Eis~ small for the (b)ReCH-DTF complexes is probably valid (see ref IO) but cannot be tested due to the fact that the DTF oxidation is irreversible on the electrochemical time scale. Under this assumption the activation parameters for forward E T for ReCH-DTF in CH3CN suggest that X = 1 e V and H A 8 = 5 cm''
Supplementary Material Available: Preparation of and analytical data for the (b)ReCH-DTF and (b)ReCH complexes and CH-DTF (3 pages). Ordering information is given on any current masthead page.
Network Recrystatllzation in a Supercritical Fluid D. C. Steytler,* P.S. Moulson, S. A. Clark, and M. L. Parker AFRC Institute of Food Research, Colney Lane, Norwich NR4 7UA. U.K. (Receiued: September 25, 1990)
A new mechanism of network recrystallization has been demonstrated in supercritical C 0 2 in which a "saw tooth" pressure oscillation has been employed to induce growth of a filament microstructure in a crystalline material (aspartame). During the pressure cycles repeated deposition of material occurs onto the filaments which ultimately grow and interconnect to form small, discrete clusters of approximately 100 p n diameter.
Introduction
The benefits of supercritical fluids (SCF's) have been widely reviewed' and reports of their application in a variety of extraction processes are continually appearing2v3 Moreover, in recent years growing consumer awareness of residual solvent levels has led to increasing interest in applications of S C F C 0 2 extraction in the food indus t ry.435 Apart from the natural benefits of a nontoxic, nonflammable medium, C 0 2 , with a critical temperature of 31.05 OC, has many advantages over conventional petrochemical solvents. In particular, the high degree of compressibility in the S C F state enables large changes in density which in turn affect solvation. Under the control of pressure (and temperature) this facilitates selective separation of components. Also the high vapor pressure of C 0 2 ensures its complete removal at low temperatures so that volatile ( I ) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction. Principles and Practice; Butterworths: Boston, MA, 1986. ( 2 ) Proceedings of International Symposium on Supercritical Fluids, Nice, Or/. 17-19. 1988; Societe Francaise de Chimie: Paris, 1988; Vols. I , 2 . (3) Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J . M . L.,Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989. (4) Coenen, H.; Kriegel, E. Ger. Chem. Eng. 1984, 7, 335. ( 5 ) O'Toole. C.: Richmond. P.; Reynolds, J . Chem. Eng. 1986. June, 74.
0022-3654/90/2094-8748$02.50/0
and thermolabile substances are completely retained. This is particularly important in the recovery of more "natural" flavors and essential o i k 6 Chemical degradation is also minimized in C 0 2 extraction by the provision of a chemically inert, nonoxidative environment. Recently, interest in SCF's has broadened to include applications as a reaction and as a solvent for controlled recrystallization. The unique control of density through pressure (and temperature) in SCF's offers a new route to nucleation of solutes that is not provided by liquid solvents. One technique that has been commonly adopted, and is inherent in the separation stage of many extraction processes, is rapid expansion/decompression (6) Moyler, D. A.; Heath, H. B. "Flavours and Fragrances: A World Perspective". Proc. 10th Int. Congr. Essential Oils, Flavours and Fragrances, Washington DC, 16-20 Noo. 1984 1984, 41. ( 7 ) Nakamura. K.; Min Chi, Y.; Yano, T . Agric. Biol. Chem. 1988, 52, 1541. (8) Poliakoff, M.; Turner, J.; Upmacis, R. K. J . A m . Chem. Soc. 1986, 108, 3645. (9) Johnston, K. P.: Flarsheim. W.; Hrnjez, A. M.; Metha, A,; Fox, M.; Bard, A. Proceedings of Internationol Symposium on Supercritical Fluids, Nice, Oct 17-19 1988; Societe Francaise de Chimie: Paris, 1988; Vol. 2, p 907. (IO) Suppes, G.J.; Occhiogrosso, M. A,; McHugh, M. A. Reference 9, p 911.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Val. 94. No. 25. 1990 8749
Letters
,--
- -~
I comiyurrmR Y P
Figure 1. Schematic representation of a SCF recrystallization process.
I
i
P I
rFigure 2. Schematic diagram of thc equipment used to induce network recrystallization in SCFC02.
Figure 3. Typical saw-tooth pressure oscillation produccd by the air. drivcn pump.
COOCH, H, NCHCONHCHCH, I
I
,
CH COOH
Figure 4. The molecular structure of aspartame of the S C F solution. The method. which represents a recirculation
process. is illustrated schematically in Figure I . The crystalline solid is dissolved in an extraction vessel and the solution passed to a decompression (or nucleation) vessel where the material is precipitated as the supercritical fluid is rapidly expanded, often to atmospheric pressure. The earliest report of structural modifications to a crystalline material using a S C F was made by Hannay and Hogarth in 1879 who described the "snow"-like appearance of the precipitate formed." More recent developments in this S C F nucleation process have been pioneered by Krukonis et aI.l2 who have been concerned primarily with the production of smaller, more uniform particles of materials (e.& progesterone) that are difficult to comminute by other techniques (e.&. milling). Mohamed et a1.I' have undertaken a systematic study of the effects of the control variables used in the expansion process on the morphology of nucleated naphthalene. Significantly, the preexpansion temperature of dilute S C F solutions was found to have a strong influence upon particle size. In contrast to the above studies, Tavana et a1.I4 have examined S C F nucleation in a batch crystallizer which offers greater control over decompression rates. In a study of the nucleation of benzoic acid from S C F CO,, the effects of different cooling and decompression profiles on crystal size were determined. It was established Hogarth, J. Pme. R. Soe. (London) 1879, 29, 324. (12) Krukonir. V. J. Paper presented at Annual AlChE Meeting. San
(11) Hannay. J. B.:
Franrirco. No". 1984. (13) Mahamed. R. S.; Dcbenedetti. P. G.: Prud'hamme. R. K. AlChE J . IPR9.. 35. 325. (14) Tavana. A,: Randolph. A. D. AlChE J . 1989,35. 1625. ~
~
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Figure 5. SEM micrugiaphr d ( a ) untreated arpartame. and (b)-(d) aapartamc exposed to continuowpressure cycles.
that large crystals could be produced by first cooling the solution to produce a high initial supersaturation and then slowly decompressing to establish a constant supersaturation during the growth stage. Conversely, small crystals were formed by maintaining a high supersaturation throughout the crystallization process.
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The Journal of Physical Chemistry, Vol. 94, No. 25, 1990
An interesting variation has been reported by Chang et al.” who decompressed a solution of &carotene in S C F ethylene into an aqueous solution of gelatin. The nucleation produced very small (100 nm) particles of almost perfect spherical symmetry. More recently, Krukonis et a1.I6 have introduced a new nucleation technique whereby S C F gases are used as “antisolvents” to precipitate crystalline solutes from organic solvents. This gas antisolvent recrystallization (GAS) method removes the restriction imposed by low solubilities in SCF’s and thereby extends the technique to a wider range of materials. In this paper we describe a new approach to S C F recrystallization whereby an interconnecting network is grown between the crystals to form them into larger aggregates. The method is distinct from previous decompression techniques since (i) the material is never completely dissolved in the S C F and (ii) the process operates through continuous pressure cycles. Application of this process to aspartame has produced a novel microstructure showing large clusters of crystals held together by a random three-dimensional array of crystalline filaments which grow during the application of pressure cycles.
Experimental Section 1 . SCF Recrystallization. The S C F nucleation tests were carried out in a small high-pressure optical cell in the apparatus shown schematically in Figure 2. The cell has an internal volume of 12 mL and is designed with a maximum operating pressure of 1000 bar. A Haake thermostated bath was used to control the temperature of the cell by circulating fluid through four channels in the cell body. The temperature was monitored (to f0.2 “C) by a Pt resistance thermometer housed within a Dynisco TPT 432A composite pressure/temperature transducer. High-pressure sapphire windows set at 180” provided a convenient means of visually monitoring bulk structural changes in the crystalline powder during the tests. Two pumps were used: one, a small handpump (Haskell MCP), acted as the pressurizing pump to achieve an initial system pressure and the second, an air-driven reciprocating pump (Haskell DSF 150), was employed between the pressurizing pump and the cell to induce controlled pressure cycles. This pressure-oscillation pump had the outlet check valve removed so that changes in volume during reciprocation of the piston established an oscillating pressure in the cell. Thc overall amplitude of the pressure oscillation is determined by the volume and compressibility of the fluid acted upon and the pump displacement per stroke. The form of the pressure oscillation induced by the reciprocating pump depends on its design and mode of operation. The rate of compression on the reciprocating pump employed can be controlled by throttling the air supply but the decompression step, triggered by a spool valve, is always extremely rapid. Pressure oscillations induced by this pump therefore resemble a ”saw-tooth” waveform with a slow rise in pressure followed by a rapid decompression. Introduction of a timer to control the air supply allowed the system to be held at a constant high pressure for a controlled period of time before decompression. The form of the pressure oscillation employed is shown in Figure 3. I n the experiment, aspartame (0.5 g) was placed in the cell which was brought to thermal equilibrium at 40 “C. The cell was then purged with C 0 2 gas to remove air. Liquid C 0 2was pumped into the cell, using the pressurizing pump, until the lower pressure level of 200 bar was attained. The reciprocating pump was then started to induce the required pressure cycles. During the experiment the ccll contents were well stirred by a small magnetic (15) Chang. C . J.; Randolph, A. D. AlChE J . 1989, 35, 1876. (16) Gallagher. P. M.: Coffey. M . P.; Krukonis, V . J.; Klasutis, N. Supercritical Fluid Science and Technology: Johnston, K. p.. Penninger, J . M . L.. Eds.: ACS Symposium Series 406; American Chemical Society: Washington. DC, 1989: p 334.
Letters follower driven by a magnetic stirrer beneath the cell. After 2 h the cell was decompressed and the sample removed for inspection by scanning electron microscopy (SEM). 2. Scanning Electron Microscopy. Samples of aspartame crystals were mounted onto S E M stubs by using adhesive tabs. They were coated with a layer of gold, approximately 20 nm thick, and examined in a Philips 501 B scanning electron microscope using an accelerating voltage of I5 kV.
Results and Discussion Aspartame (L-a-aspartyl-L-phenylalanine methyl ester) was introduced as a “natural” artificial sweetener in 1981 and current production exceeds several thousand metric tons per annum. The aspartame molecule (Figure 4) has an amphiphilic structure comprising a hydrophilic, zwitterionic aspartyl group linked with a highly hydrophobic phenylalanine ester. X-ray scattering studies” have revealed a space group P4, in which the molecules in the crystal are aligned in columns with the hydrophilic portion forming a polar core surrounded by a hydrophobic surface. The formation of this structure is believed to be responsible, at least in part, for the tendency toward fiber formation on crystallization. The conventional production process produces fine needle crystals (diameter < I O pm). One technique for structural modification of aspartame has been provided by a new crystallization process’* producing “bundles” of aspartame crystals (diameter 50-100 pm). This technique uses rapid cooling of a concentrated solution of aspartame followed by slow recrystallization without stirring. Structural features of untreated aspartame are shown in the SEM micrograph in Figure 5a. The needlelike crystals are highly disperse in size with lengths between I O and 50 pm. The effect of rapid decompression cycles induced by the piston pump was dramatic. During the course of the experiment, bulk properties of the aspartame were observed to change progressively to a less dense, more granular powder. SEM micrographs (Figure 5, b-d) clearly revealed unique structural changes had occured. The pressure cycles induce repeated dissolution and nucleation of some of the aspartame in the form of a fine web of crystalline filaments (Figure 5d). The filaments appear to originate from the host crystals and establish appreciable connectivity and branching during the growth process. Ultimately the individual crystals in the original sample become connected forming large (approximately 100 pm) aggregates effectively “bound together” by the web of filaments (Figure 5c). The aggregates are robust and retain their structural integrity when shaken or poured. Absence of chemical breakdown during the process was checked by mass spectrometry and HPLC which gave identical results for the untreated and S C F nucleated samples. Conclusion A new mechanism of S C F recrystallization has been demonstrated using pressure oscillations to induce growth of a unique filament microstructure in a crystalline material. The process requires pressure cycles having a short equilibration time at elevated pressure followed by rapid decompression. The structure suggests a mechanism of repeated deposition of molecules onto the growing filaments during each pressure cycle. In contrast to previous S C F nucleation studies, this process involves structural modifications in a fixed population of partially dissolved crystals. The application of S C F network nucleation to modify structural properties of particulate crystalline substances has been demonstrated using aspartame. However, the unique submicrometer structure formed suggests other novel applications in materials science in the production of membranes, filters, etc. (17) Hatada, M.; Jancarik, J.: Graves, B.: Kim, S. J . Am. Chem. SOC. 1985, 107, 4279. ( 18) Kishimoto, S.; Naruse, M. J . Chem. Biotechnol. 1988, 43. 7 1 ,
