Langmuir 1996,11, 4241-4249
4241
Aggregation of Amphiphilic Molecules in Supercritical Carbon Dioxide: A Small Angle X-ray Scattering Study John L. Fulton” and David M. Pfund Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory, Richland, Washington 99352
J. B. McClain, T. J. Romack, E. E. Maury, J. R. Combes, E. T. Samulski, and J. M. DeSimone” University of North Carolina at Chapel Hill, CB#3290, Venable and Kenan Laboratories, Chapel Hill, North Carolina 27599-3290
Malcolm Capel$ National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973 Received April 3, 1995. I n Final Form: July 27, 1995@ Highly soluble amphiphilicmaterials are shown to form aggregates in supercritical Con. The strategy for synthesis of these amphiphilic molecules involves incorporating CO2-philic segments that, for this study, are perfluorinated alkyl chains. These CO2-philic regions function like the hydrocarbon tails of conventional surfactant molecules used in liquid organic solvents. Synthesis and characterization of three different GO2 amphiphiles are reported. Subsequent small angle X-ray scattering (SAXS)measurements were used to characterize the aggregation of these materials in supercritical C02. Each of the three amphiphiles studied showed a different type of aggregation behavior. A graft copolymer consisting of a COz-philic backbone and CO2-phobic grafts associated into a micellar structure in the presence of water to promote hydrogen bonding. These aggregates contain approximately 600 grafts in the core. The commerciallyavailable surfactant perfluoroalkylpoly(ethy1ene oxide) or F(CFZ)~-~OCHZCH~O(CHZCH~O)~-BH, forms classic reverse micelle structures having radii of about 84 A under the conditions of high pressure required to solubilize the material. A third amphiphile, the semifluorinated alkane diblock molecule F(CFZ)~~(CH~ may ) ~ ~form H , small aggregates of at most 4 unimers per aggregate. The improved understanding of amphiphile aggregation in GO2 will aid in the development of new routes for polymer and organic synthesis in this relatively benign solvent.
Introduction Supercriticalfluids are becoming increasingly important solvent systems for use in polymer science and engineering.1,2 Supercritical C02, in particular, is a widely used solvent due to its low cost, moderate critical conditions (T,= 31 “C, P, = 73.8bar), and environmentally benign nature. Recently it has been shown that only a few classes of polymeric materials are significantly soluble (tens of percent) in supercritical C02 a t relatively “mild” (T< 100 “C, P < 350 bar) conditions, these being amorphous fluoropolymers and silicone^.^-^ We categorize these materials as being “COZ-philic”,while conventional polymers, having either hydrophilic or lipophilic character, are relatively insoluble in COZ and are termed ‘‘CO2phobic”. Through use of CO2-philic fluorocarbon repeat units in copolymers, amphiphilic materials can be molecularly engineered for use as surfactants in supercritical COZ. We have reported the synthesis of block copolymers and
* Corresponding authors.
Operated by Battelle Memorial Institute.
* Department of Biology. +
Abstract published inAduanceACSAbstracts, October 1,1995. (1)McHugh, M.; Krukonis, V. Supercritical Fluid Extraction; Butterworths: Boston, MA, 1986. (2)DeSimone, J. M.; Guan, Z.; Elsbemd, C. S. Science 1992,257, 945-947. (3)Guan, 2.; Combes,J. R.; Elsbernd, C. S.; DeSimone,J. M. Polym. Prepr. (Am. Chem. SOC.,Diu.Polym. Chem.) 1993,34,447. (4)Hoefling. T.;Sofesky, D.; Reid, M.; Beckman, E.; Enick, R. M. J. Supercrit. Fluids 1992,5,237. (5)Consani, K.A,; Smith, R. D. J. Supercrit. Fluids 1990,3,51-65. @
graft copolymers designed specifically to be amphiphilic in C02.‘jJ These materials have been shown to phase separate in the bulk producing two distinct glass transition temperatures by DSC analysis. It was also found that a copolymer (PFOA-g-PEO) of poly(ethy1ene oxide) grafts (PEO) on a backbone of poly(1,l-dihydroperfluorooctylacrylate), or PFOA, could facilitate the uptake of both a COz insoluble dye, methyl orange, and water into the supercritical C02 continuous phase. Development of amphiphilic molecules for use as surfactants in supercritical C02 and other supercritical fluids has led to the discovery and characterization of reverse micelles and microemulsion phases in supercritical fluid media.8-11 Smith et al. have utilized the existence of these inverse microemulsions to polymerize water soluble monomers in supercritical alkanes.lZJ3 The application of scattering techniques to the study of these sub-micrometer sized aggregates, reverse micelles, and microemulsions has proved to be an invaluable technique in their characterization. Previous small angle X-ray (6)Guan, 2.;DeSimone, J. M. Macromolecules 1994,27,5527. (7)Maury, E. E.; Batten, H. J.; Killian, S. K.; Menceloglu, Y. Z.; Combes, J. R.; DeSimone, J. M. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1993,43,664. (8)Fulton. J. L.:Smith. R. D. US Patent 5.158.704.Oct 27. 1992. (9)Harri Harrison, K.;Goveas, J.;Johnston, K. P.;’O’RearE.A. Laigmuir 1994,10, 3536-3541. (1C (10)Hoefling, T. A,; Enick, R. M.; Beckman, E. J. J. Phys Chem. 1991,95, 7127-7129. i9S.l; 95,7127-7129. (11)Fulton, J. L.;Smith, R. D. J. Phys. Chem. 1988,92,2903-2907. (12)Beckman, E. J.;Fulton. J. L.; Smith, R. D. US Patent 4,933,404, Jun 12,1990. (13)Beckman, E. J.;Smith, R. D. J. Phys. Chem. l990,94,345.
0743-7463/95/2411-4241$09.00/00 1995 American Chemical Society
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4242 Langmuir, Vol. 11, No. 11, 1995 Scheme 1. PEO Macromonomer Synthesis
CHI
ethane (3M) (Freon-113), and potassium tert-butoxide (1M in THF Aldrich) were used as received. Ethylene oxide (Kodak) CHZCH2 was vacuum distilled from calcium hydride. Vinylbenzyl iodide was synthesized fromp-vinylbenzyl chloride using the Finkelstein reaction. a,a'-Azobis(isobutylnitri1e) (Kodak) (AIBN) was recrystallized from methanol. 1,l-Dihydroperfluorooctylacrylate (3M) (FOA) was passed through columns of decolorizing carbon and alumina. The Zonyl FSO-100 fluorosurfactant was graciously donated by E. I. du Pont de Nemours and Co. and used as received. Synthesis of the PFOA-g-PEOGraft Copolymer. Synthesis of the graft copolymer was carried out using the macromonomer method established by Milkovich.l* The PEO macromonomer was prepared by anionic polymerization of ethylene oxide followed by termination withp-vinylbenzyl iodide as outlined in Scheme 1. The polymerizations were initiated by potassium tert-butoxide in THF in a low-pressure glass reactor at 40 "C under an argon atmosphere (40 psig). After 12 h the reactor was cooled to room temperature and a 2-fold excess of p-vinylbenzyl iodide was added. The reaction was stirred for 15 min before the macromonomer was precipitated into a large excess of hexanes. Copolymerization of macromonomer and FOA (Scheme 2) was carried out in a round bottom flask. Calculated amounts of FOA, macromonomer, and AIBN were added and deoxygenated by argon purge. A 2:3 mixture of Freon-113 and (SAXS)14J5and neutron (SANS)16J7 scattering studies of THF was added under argon. The flask was placed in a water supercritical fluids have demonstrated that these are bath a t 60 "C. After 24 h, the resulting copolymer was isolated powerful techniques to derive structural information not by rotary evaporation and purified by Soxhlet extraction in water only for aggregation of amphiphilic species but also for for 12 h. The copolymer was dried a t ambient temperature in the determination of the molecular structure of pure and vacuo. Synthesis of the Diblock F(CF2)lo(CH2)loH. The semimodified supercritical systems. fluorinated alkanes were synthesized according to methods The work presented here is focused on the small angle previously reported by Hopken and are outlined in Scheme 3.19 X-ray scattering study of three unique amphiphilic Polymer Characterization. A Waters 150-CV chromatomaterials in supercritical COP: (i) a poly(1,l-dihydroaph with Ultrastyragel columns of 100,500, lo3, lo4, and lo5 perfluorooctylacrylate-g-ethyleneoxide) polymer or PFOAporosities in THF was used with polystyrene standards to g-PEO similar to the one found previously to promote the determine the molar mass and molar mass distribution of all dispersion ofwater into supercritical COP;(ii)a commercial THF soluble polymers. Proton NMR analysis of PFOAg-PEO nonionic perfluoroalkylpoly(ethy1ene oxide) surfactant, was carried out in Freon-113KDCla using a Bruker AC-200 at F(CF2)6-10(CH2CH20)3-8H, with a trade name of Zonyl 200 MHz to determine the mole % PEO in the copolymer by taking the ratio of methylene protons in the PEO to the 1,lFSO-100 manufactured by Du Pont; (iii)a semifluorinated dihydro protons in the fluoroalkyl group. alkane diblock molecule consisting of ten perfluorinated SAXS Data Acquisition. Techniques used to collect SAXS carbons attached to ten hydrogenated carbons, F(CF2)loF H data for supercritical fluid solution have been previously (CH2)loH or CloClo. described.14J5 Briefly, the scattering cell is composed of a monoblock of 316 stainless steel containing windows for transExperimental Section mission of the X-ray beam and an optical view window. The Materials. C02 for the SAXS experiments (Scott Specialty sample volume of the scattering cell is about 10 mL, and the Gases, "SFC" grade) was used as received. COz for synthesis contents of the cell are stirred while the cell is mounted on the (Air Products, 99.5%) was passed through columns of molecular beamline goniometer using a magnetically-coupled stir bar. A sieves and reduced copper oxide catalyst (BASF R3-11). sapphire view-port incorporated in the cell provides a means of Distilled, deionized water was used through out. Tetrahydroviewing the sample to determine the number of phases. The furan (Fisher Certified Grade) (THF) was refluxed over sodium/ X-ray transmission windows consist of small diamond windows benzophenone and distilled under argon. Hexanes, methanol (3 mm diameter x 0.5 mm thick) that are brazed to the tip of a (Mallinekrodt, HPLC grade), 1,1,2-trifluoro-1,2,2-trichloro- standard Vd-in. high-pressure fitting (assembly supplied by Omley Industries, Bend, OR). X-ray path lengths were 2.4 mm for all (14)Pfund, D. M.;Zemanian, T. S.; Linehan, J. C.; Fulton, J. L.; samples. Yonker, C. R. J. Phys Chem. 1994,98,11846-11857. The X-ray scattering (SAXS)data were acquired on the Time(15)Fulton, J.L., Pfund, D. M. Inproceedings ofThirdlnternationaZ Resolved Diffraction Facility (station X12B) at the National Symposium on Supercritical Fluids; Perrut, M., Ed.; Strasbourg,France, Synchrotron Light Source (Brookhaven National Laboratory), 1994. (16)Kaler, E. W.; Billman, J. F.; Fulton, J. L.; Smith, R. D. J.Phys. (18)Schultz, G. 0.;Milkovich,R. J. Appl. Polym. Sci. 1982,27,4773. Chem. 1991,95, 458-462. (19)Hopken, J. Ph D Dissertation, Univ. Twente Enschede: The (17)Londono, J. D.; Shah, V. M.; Wignall, G. D.; Cochran, H. D.; Bienkowski, P. R. J. Chem. Phys. 1993,99,466-470. Netherlands, 1991. I
r
Aggregation of Amphiphilic Molecules using a custom-builttwo-dimensionalgas delay-linedetector (10 cm x 10 cm, 512 x 512 pixels), interfaced to a real-time histogramming memory system. The optical system provides a doubly-focused(spotsize 0.5 mm x 0.5mm fwhm)monochromatic A,U,l) spanning 0.9-1.5 8 .For X-ray beam (bandpass -5 x these experiments the incident wavelength was 1.38 8. The measurement time was about 10 min per condition. Before comparison, the results from separate experiments made under different sample and instrument conditionswere scaled to remove differences due to detector nonuniformity, beam intensity, and sample transmission. The absolute intensities were not measured so a scaling parameter must be included in the fitting routines. However, all the scattering curves for the three amphiphiles reported in this study have been normalized to the same upstream photon flux. This allows for an independent check of the calculated sizes for the three different systems by comparisonofthe scattering intensities at low q. Thus,the sizes determined from the various fitting procedures were consistent with the scattering intensities measured for the three different systems. Sample preparation involved the introduction ofliquid or solid solute directly into the scattering cell. Scatteringmeasurements were performed by starting with the highest-density solution and then discharging small amounts of the solution to obtain progressively lower pressures. Through this technique, the overall mole fraction of the solute remained constant. Fluid pressure was monitored to *l bar with an electronic transducer (Precise Sensors, Inc., No. C451),which was calibrated against a deadweight tester (Ashcroft,No. 1305-D). The temperature of the SAXS cell was controlled using a three-mode controller with a platinum resistance probe (Omega,No. N2001) to an accuracy of f 0 . 2 "C.
Results and Discussion It is known that block and graft copolymers phase separate into micelle-like domains that are spherical with a narrow size distribution when dissolved in a solvent selective to one of the blocks.20-22These aggregates form by clustering of the insoluble portion of the molecule into a collapsed core structure surrounded by the solvated soluble portion of the polymer. A similar strategy is employed in creating an amphiphilic copolymer for supercritical COz. By selection of a COZ-philic portion comprised of perfluorocarbon segments, the insoluble copolymer portions can be shielded from interaction with the COz solvent and from interactions with the other insoluble copolymer segments in the solution. In COZ the exterior fluorocarbon segments will be solvated, thus shielding the interior COz-phobic regions. A key factor limiting the solubility of higher molecular weight (> 1 kDa) aggregates in COz appears to be the strong interaggregate attractive interaction^.^^,^^,^^ It is perhaps the highly repulsive nature and rigidityz5of the perfluorinated alkyl segments on the exterior surface of the aggregate that aid in the dissolution of macrostructures in a lowdielectric-constant solvent such as COz. In the sections that follow we discuss the aggregate structure of the three different semifluorinated amphiphiles including the large graft copolymer PFOAgPEO and the two smaller species, the nonionic surfactant, F(CFZ)~-~OCHZCHZO(CHZCHZO)~-~H and the diblock material, F(CFz)lo(CHz)loH. (20)Piirma, I. Surfactant Science Series; Marcel Dekker, Inc.: New York, 1992. (21)Vagberg, L. J. M.; Cogan, K. A.; Gast, A. P. Macromolecules 1991,24,1670. (22)Riess, G.;Hurtez, G.; Bahadur, P. Encyclopedia of Polymer Science & Engineering; Wiley: New York, 1985;p 324. (23)Tingey, J. M.;Fulton, J. L.; Smith, R. D. J.Phys. Chem. 1990, 94,1997-2004. (24)Peck D.G.;Johnston, K. P. J. Phys. Chem. 1993,97,56615667. (25)Yee, G. G.;Fulton, J. L.; Smith, R. D. J.Phys. Chem. 1992,96, 6172-6181.
Langmuir, Vol. 11, No. 11, 1995 4243
PFOA-g-PEO Graft Copolymer. The PFOA-g-PEO copolymer was characterized by several different techniques. By utilizing the macromonomer technique to synthesize the graft copolymer, we were able to characterize both the molecular weight and molecular weight distribution of the PEO grafts prior to their incorporation into the PFOA backbone. The macromonomer technique also allows for the synthesis of a copolymer with very well defined and uniform graft length. The number average molecular weight of the PEO macromonomer was determined to be 5 kg/mol with a molecular weight distribution of 1.10 by GPC analysis. In addition, the final graft copolymer was found, by proton NMR analysis, to contain 15% ethylene oxide based on the total weight of the polymer. While characterization of the PEO macromonomer prior to copolymerization is straightforward, the analysis of the graft copolymer proved to be very difficult because of the very low solubilities in conventional solvents; the polymer being soluble only in COP and Freon-113. Both GPC analysis and membrane osmometry have been attempted without success to characterize the molecular weight and molecular weight distribution of the copolymer. Common methods of polymer analysis fail when dealing with such amphiphilic materials due to extremely nonideal solution behavior brought on as a result of aggregation at very low concentrations.20~22 It is these nonidealities however, which give rise to the interesting solution properties examined by SAXS. Figure 3 shows the small angle X-ray scattering data for the PFOA-g-PEO in carbon dioxide at 60 "C and at three different pressures (470, 300, and 255 bar). The lower pressure data sets are very near to the phase boundary that occurs below 238 bar for the 0.6% solution and at slightly lower pressure, 234 bar, for the 1.9% solution. In both cases an appreciable amount of water has been added that is equivalent to a water-to-surfactant weight ratio of 0.32. At a pressure just below the phase boundary pressure the lower phase has the appearance of a semisolid floc that precipitates to the bottom of the cell. Generally we note that as the pressure of the system is reduced there is an increase in the scattering intensity across the entire q range. This is in part due to decreases in the density of the supercritical fluid solvent increasing the scattering contrast between the particle and the solvent (see Figure 2). There is also qualitative evidence of a small increase in the size of the particles as the pressure is reduced as shown by the shift in the scattering peaks to lower q . Due to the complete insolubility of 5000 g/mol PEO in COz, we propose a model where the PEO grafts exist in the center of a core-shell structure. This structure is depicted in Figure 1. With this model the PFOA backbone forms a shell limiting the C02-PEO interaction, thus facilitating the stabilization of a micellar structure. The PEO coil is mostly collapsed since the scattering shows a pattern characteristic of a solid sphere (oscillatory)rather than scattering behavior one would expect for a expanded, coiled polymer (monotonic decay). The added water may form intermolecular hydrogen bonds between different parts of the PEO chain aiding in the contraction of the polymer coil. There is also the possibility of incorporation of small amounts of COZ into the polymer matrix. All the scattering data for PFOA-g-PEO show a characteristic peak at about q = 0.041 A -l. There is evidence of a second scattering peak at approximately q = 0.06 A -l in many of the scattering cuTes. The oscillations in the region from 0.012 < q < 0.07 A -l are consistent with scatteringfrom spherically shaped particles having a small
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4244 Langmuir, Vol. 11, No. 11, 1995
PFOA backbone as shell
PFOA
-,
r
I
PEO
I
502 -I
6?
PEO grafts as water swollen core
470 bar
Figure 1. Proposed structureof a PFOA-g-PEOgrafi copolymer micelle in supercritical C02.
degree of polydispersity (seeeq 1). The q range fiom about 0.07 to 0.1A -l represents a crossover to a region at higher q that is dominated by scattering from smaller scale structures; most likely the individual segments of the PFOA backbone. The electron density of the PFOA chain (0.502 e-/A 3, is much higher than either the solvent (0.23 e-/A 3, or the PEO (0.37 e-/A 3, blocks. Hence, at high q, the scattering will be dominated by contributions from local structure of the PFOA chain. As stated, in the lower q region, scattering is dominated by the macromolecular aggregate. In this region, contributions to the total excess X-ray scattering, I(q),due to microemulsion solutions can arise from two sources: the droplet contributions,P(q),which depend solely upon the radius of the droplet, and an appropriate structure factor, S(q),which accounts for attractive or repulsive interactions between the droplets. The magnitude of the scattering vector, q, is given in terms of the scattering angle 8 by q = (4di)sin 8. In this approach the total scattering from a monodisperse system is simply I ( q )= nP(q)S(q),where n is the droplet number density. When the system is sufficiently dilute, interdroplet interactions are negligible and S(q) = 1. For a system consisting of spherical particles having a core with radius Rcore and scattering length density of pcore, surrounded by an outer concentric shell of radius Rshell of a second material with scattering length density Pshell the particle form factor can be written as
where the function fo(x) = 3(sin x - x cos x)/x3, the core volume is given by Vcore = 4dicore3/3,and similarly for the overall volume including the outer shell, Vtotal = 4d?&el13/3. To treat the polydispersity of the system we use a Gaussiandistribution of sizes having a mean outer radius, Rshell, and a distribution standard deviation of 0. The fitting procedure assumes that the polydispersity of the system o/Rshell, can be well described by a Gaussian distribution. A schematic of the scattering contrasts used in the fitting of eq 1 to the scattering data is given in Figure 2. The scattering contrast of the supercritical COa phase is variable and depends upon the density of the solution. The scattering contrasts of the PFOA and PEO segments were estimated from the bulk densities of the pure materials. A variable included in the fit was the length of the PFOA tail, &,ail = &hell - R,,, which was the same for every size particle in the distribution. The effect of small changes in the electron density or thickness of the shell only weakly affect the overall fitted radius since,
-30
-20
-10
0
Distance,
10
20
30
A
Figure 2. Electron density profile across the interfacial region for a PFOA-g-PEO particle in supercritical CO2. The electron density of the C02 phase is dependent upon the pressure of the system.
for the q range of this study, the real space resolution (AR L Jdlqma,) is not high enough to uniquely define the shell structure. Figure 4 shows the core-and-shell fit (eq 1) to the low q region of the scattering curves. We apply eq 1 by assuming that the solutions are at high enough dilution so that any interparticle interactions contribute negligibly to the scattering in this q region, i.e., S(q) = 1. Table 1 gives the parameters derived from the fitting of eq 1to the scattering data. The outer radii of the aggregates are ab_out 125 with relatively low polpdispersities of about dRshell= 0.16. The radii of the particles clearly increase either as the concentration of the surfactant is increased or as the pressure of the system is reduced. The effects of both pressure and concentration on the size are also strongly indicated by the shift in the cusp and scattering peaks shown in Figure 3 to lower q. It is important to note that the fitted shell thicknesses given in Table 1 are probably not uniquely determined since the spatial resolution at this q range is not high enough to resolve this fine detail. The radius of the PEO core is approximately 105 A . This is much larger than the diameter of a single, collapsed 5K PEO chain, and hence all PEO chains attached to a single PFOA backbone as well as those from other graft copolymers aggregate in the core region. The number of 5K PEO segments in the core is approximately 600 based upon the measured volume of the micelle core (4/3~rR:~,)and the bulk density of PEO given as 1.13 g/cm3. Then, from NMR measurements we know EO comprises 15 wt % of the copolymer and assuming a total molecular weight of approximately 100 000 g/mol we estimate a degree of aggregation of approximately 120 unim'ers. Further if we assume that the PFOA portion of the polymer has a density of about 1.8 g/cm3, then an overall micelle size of 174 A results from the association of 120 unimers per aggregate. These assumptions prove valid in that the overall size and shell thickness are in reasonable agreement with the parameters measured by SAXS. The micelle-likestructure of the graft copolymer solution greatly enhances the solubility of water into the COZ solvent. The weight ratio of water-to-surfactant for both concentrations studied is 0.32 (w/w). Since the water only
Langmuir, Vol. 11, No. 21, 1995 4245
Aggregation of Amphiphilic Molecules IWOOOOO
0.69%(w/w) PFOA-g-PEO T = 60°C [H20Y[PFOA.g-PEOl = 0.32 (w/w)
a)
I
Table 1. Size and Polydispersity Parameters from Core with Shell Model Fit of the Scattering Data for PF'OA-g-PEOin Supercritical C02
I
-.
0.6 0.6 0.6 1.9 1.9 1.9
8
.. aI
....
001
255
4.A'
"I
1.9% (w/w) PFOA-g-PEO T = 60OC [H,O]/[PFOA-~PEO]= 0.32 (w/w
'1:. D..
.*. * .aI .I
I L " .
T = 60°C P=3Wbar
1
pressure bar
Fsolv nt,
470 300 255 470 300 255
0.275 0.244 0.230 0.275 0.244 0.230
polydiEpersity,
e-/A3
SIRshell
117 122 124 119 129 148
15.2 19.8 35.8 14.6 16.5 22.4
102 102 88.2 104 113 125
0.162 0.155 0.159 0.180 0.165 0.153
hydrogen-bond to-or "hydrate"-each of the EO units, and hence the excess water probably promotes the formation of interchain hydrogen bonds leading to larger aggregate size. The water is likely dispersed throughout the PEO chains rather than forming a water pool at the center of the micelle-like structure although this interpretation cannot be derived from the current SAXS results. The low polydispersity is consistent with previous measurements of micelle-likestructures formed with block and grafl copolymers in liquid solvents.20-22The slight increase in the size of the particles as the concentration is increased is likely due to a mass action-type of effect where aggregate size increases with increasing concentration. The effect of pressure on the micelle size may be a response to changes in the interfacial surface tension of the solutions. As the density of the supercritical fluid continuous phase is reduced, there are substantial increases in the interfacial tension between the fluid and the micelle aggregate. An increase in the micelle size would minimize the free energy by reducing the total interfacial area. Probable mechanisms for pressuredependent size in supercritical solutions have been discussed by F ~ l t o n and ~ ~ 2J o~ h~n s t ~ n . ~ ~ , ~ ~ The higher than expected scattering intensity in the region from 0.08 < q < 0.3 ikldeviates from the theoretical fit for a sphere (seeFigure 4). In this q range the scattering from the smaller spatial regions of the high-electrondensity PFOA polymer chain dominates. The PFOA backbone has a comblike structure with a cross-sectional dimension that is approximately equal to twice the length of a single perflourooctylacrylate segment. Since the PFOA backbone can be approximately as a series of connected rodlike segments, we fit the scattering in this region using an expression for a rod-shaped particle of radius, Reyl,and length L. A similar approach has been previously used for polymers30and for rodlike micelles.31 The scattering from N p noninteracting rodlike particles having volumes V, is32
where Prod and psolventare the scattering length densities of the polymer chain and the solvent, respectively. 001
q,
A'
0.1
Figure 4. Nonlinear, least-squares fit of spherical core-withshell model to the low q region ofthe PFOAg-PEO SAXSspectra in supercritical C02 at 60 "C and 300 bar for two different concentrations, 0.6 and 1.9% (w/w).In both cases, the waterto-surfactant weight ratio is 0.32.
associates with the polar PEO groups of the polymer, it is more instructive to consider the molar ratio of water to each EO unit on the PEO backbone. Thus the [H20]/[EOl is 5.2 for both concentrations studied. This water concentration is in excess of the amount required to
(26) Beckman, E. J.,Fulton, J. L., Smith, R. D. InSupercriticalFluid Technology: Reviews in Modern Theory and Applications; Bruno, T. J., Ely, J. F., Eds.; CRC Press: Boston, MA, 1991; pp 405-449. (27) Yee, G. G.; Fulton, J. L.; Blitz, J. P.; Smith, R. D. J. Phys. Chem. 1991,95, 1403-1409.
(28)Peck, D. G.; Johnston, K. P. J. Phys. Chem. 1991, 95, 95499556. (29) McFann, G. J.;Johnston, K. P. J.Phys. Chem. 1991,95,48894896. (30) Kirste, V. R. G. In Small Angle X-ray Scattering; Glatter, O., Kratky, O., Eds.; Academic Press: New York, 1982. (31) Shih, L. B.; Sheu, E. Y.; Chen, S.H. Macromolecules 1988,21, 1387-1391. (32) Livsey, I. J. Chem. SOC., Faraday Trans. 2 1987,83,1445-1452.
4246 Langmuir, Vol. 11, No. 11, 1995
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T I 65°C 300 bar H,O/PFOA-PEO I0.32 (wlv
6.3%(w/w) F'TFE-PEO
'..
T I 65-C
IDOW0
R,= 11.1 A
# 0.6%
Y 3
a
a
i
r:
z
iz
Ima,
IW
1wo 10
9,
Figure 5. Nonlinear, least-squaresfit of cylindrical rod model to mid- to high-q region of the PFOA-g-PEO SAXS spectra in supercritical COz at 60 "C and 300 bar for two different concentrations, 0.6 and 1.9%(w/w). In both cases, the waterto-surfactant weight ratio is 0.32.
Additionally, Y is defined by u = qRcyl(l- COS^(^))^, w is given by w = l/2qLcos(y),andJl(u)is the first-order Bessel function. The cross-sectional radius of the rod is defined by Rcyland y is the angle between the q vector and the symmetric axis of the micelle. Equation 2 is used as an approximation to decribed the cross sectional dimension of the polymer and is only valid when the persistence length, L,, of the polymer is much larger than its diameter and for q > lIL,. Figure 5 shows the cylindrical rod fits (Equation 2) to the higher q region of the scattering data. The measured cross sectional radius for the PFOA polymer is about 11 A at 300 bar and 65 "C. For comparison, the extended length of a single perflourooctylacrylate segment is about 13.5 A where adjacent perflourooctylacrylate segments are probably rotated to angles giving the least steric interference. Since there will be some coiling of the side chain about the backbone, the measured radius is in reasonable agreement with the calculated size. Diblock F(CFa)s-loCHaCH20(CHaCH20)s-~(Zonyl FSO-100). The Zonyl FSO-100 surfactant is a small diblock molecule consisting of a perfluoroalkane chain (number average molecular weight = 384 g/mol)) covalently linked to a poly(ethy1eneoxide)segment (number average molecular weight 340 g/mol). The surfactant composition has a significant amount of polydispersity given by the approximate formula
Figure 6 shows the scattering data for the 6.3%(wlw)
FSO-100in carbon dioxide at 65 "C and at three different pressures (230,350,and 530 bar). Below about 400 bar a small amount of solid precipitate formed and settled to the bottom of the cell. Since the surfactant is a polydisperse mixture of different oligomers, the precipitated material is most likely the fraction containing high molecular weight PEO. The scattering at lower pressure shows a dramatic transition in the shape and intensity of the scattering curves. These lower pressure spectra were taken of the upper clear phase in a system under twophase conditions. In the midq region at about 0.06 A-l the scattering intensity increases by about a factor of 4 as the pressure is reduced from 530 down to 230 bar. This effect may be due to large increases in the critical
A"
Figure 6. Small angle X-ray scatteringspectra for 6.3% (w/w) of the nonionic surfactant, F(CFZ)~-*~(CHZCHZO)~-~H (Zonyl FSO-100)in supercritical COz at 65 "C and three different pressures, 230,350,and 530 bar.
scattering behavior of the small oligomers in solution at these conditions. This critical scattering represents the formation of longer range, weakly associating surfactant molecules that are not highly ordered as for a micelle aggregate. The F'TFE-b-PEO sample used for these studies contained a range of molecdar weights. It is well known that the aggregate size increases as the length of the PEO chain is increased. We know that for conventional nonionic surfactants that the aggregation number increases quite dramatically with only a small increase in the number ofPEO units (e.g., five to eight). For example in COZ,the solubility of the hydrocarbon surfactant, CIZEO8 (e.g., octaethylene glycol dodecyl ether), is dramatically lower than that of either C12E05 or C 1 ~ E 0 3 .For ~~,~~ C1zEO5or C&O3 the aggregate sizes are small, containing approximately four molecules per aggregate. Presumably the aggregation numbers are much higher for the ClZEOB and these larger structures have limited solubility in COz because of intermicellar attractive interactions. Through fluorination of the alkyl tails the solubilities of these larger micelles can be greatly improved. For our experiment at 530 bar we see that the higher molecular weight material, now soluble, forms larger aggregates. These aggregates probably incorporate lower molecular weight material as well. The SAXS spectrum at 530 bar is single phase and at a high enough density to be mostly removed from critical scatteringeffects. The behavior represented by this SAXS spectra of the FSO-100 surfactant is characteristic of a system having a broad distribution of sizes. To extract size and particle distribution information from the 530 bar single phase system,we utilize an inversion technique of R e F i ~ r e n t i n In . ~order ~ ~ ~ to ~ get a reasonable measure ofthe size distribution, it is necessary to reduce the number of adjustable parameters by assuming that the spheres are homogeneous. Further, we assume that the interparticle interference is negligible (S(q)= 1)at high fluid densities and that the micelles represent a polydisperse system of homogeneous spheres. The scattering from such a system is given by: (33)Yee, G.G.;Fulton, J. L.; Smith, R. D. Langmuir 1992,8,377384. (34)Fulton, J. L.;Yee, G. G.; Smith, R. D. J.Supercrit. Fluids 1990, 3, 169-174. (35) ReFiorentin, S.J.Appl. Phys. 1982,53,245-249. (36)Darab, J. G.; Pfund, D. M.; Fulton, J. L.; Linehan, J. C. Langmuir 1994,10, 135-141.
Aggregation of Amphiphilic Molecules
The size distribution function, p(r), is the number of particles with radii between r and r dr. This function is determined by the inversion to within a constant factor. The function P(q;r) is given by eq 1 for the case of a homogeneous particle. All particles were assumed to have equal X-ray contrast. Briefly, the ReFiorentin method represents p(r) by a sum of cubic B - ~ p l i n e sthe , ~ ~coefficients of which are determined by least squares regression of eq 3 to the measured intensities I(q). The fitted scattering curve and the resulting particle size distributions are given in Figures 7 and 8, respectively. The scattering curve shown in Figure 7 had two distinct portions. The slowly decreasing intensity at q greater than about 0.08 A-l was due to the presence of a large number of small particles. These constitute the large peak in the size distribution function plotted in Figure 8. The mean radius of these particles was approximately 9.6 A and their polydispersity was 0.39. These structures were larger than the perfluorinated portion of a single surfactant molecule and therefore they most likely represent premicellar surfactant aggregates consisting of fewer than ten surfactant molecules. A small number ofmuch larger aggregates were also present in the sample. These were respopible for the large scattering at q less than about 0.08 A-l shown in Figure 7. The size distribution function for these aggregates is plotted in the inset of Figure 8. Their mean radius was 83.7 A,with a polydispersity of 0.31. These aggregates were of large enough sizes that they could be classified as conventional reverse micelles. These larger structures may contain a higher proportion of the surfactant molecules with six to eight PEO units, whereas the small premicellar structures may be composed of mostly the lower molecular weight fractions. Semifluorinated Diblock F(CF2)lo(CH2)l,,H. The semifluorinated alkane F ( C F Z ) ~ C H ~ ) I or O H"C~oCy,", has been shown to be highly soluble in COZand to form gels from 20 wt % liquid COZ solutions at room tempera t ~ r e From . ~ ~ the pioneering studies of these materials by Moller et al. they have found surfactant-like behavior in solvents selective to the hydrocarbon It has been suggested by results from preliminary light scattering studies that the C$yo material forms micelles of approximately 130 unimers in octane at 35 "C and low concentrations (0.3-1.9 wt %). Moller also found that the degree of aggregation decreased sharply at higher temperatures (60 "C) and in less selective solvents such as toluene. He proposed an aggregation phenomenon based on association of the fluorocarbon segments near the solution crystallization temperature. When the system is at higher temperatures, the entropic contribution dominates and aggregates are destroyed. Related studies on systems where the solvent is selective to the fluorocarbon segment have also been carried out. Systems such as C!Cyz in perfluorotributylamine and CiCy6 in perfluorooctane, which were studied by dynamic light scattering and fluorescence probe solubilization studies, were reported to form premicellar aggregates on the order of four to six molecules per aggregate.39 SAXS experiments herein will focus on 5-6 wt % concentrations ofthe C ~ o Cmaterial ~o in supercritical COz a t 65 "C. Figure 9 displays the scattering curves for the 6 wt % solution ofthis surfactant at 65 "C over a range ofpressures
Langmuir, Vol. 11, No. 11, 1995 4247 6.3% (w/w) FSO-100 T=65T P = 530 bar
+
(37)Iezzi, A.;Bendale, P.; Enick, R. M.; Turburg, M.; Brady, J. FZuid Phase EquiZib. 1989,52,307.
(38)Hopken,J.;Pugh, C.; bchering, W.; Moller,M.Makromol. Chem. 1988,189, 911 and references therein. (39)Turberg, M. P.;Brady, J. E. J.Am. Chem. Soc 1988,110,7797.
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Figure 7. Results of the distributionfit (line)of the scattering (ZonylFSO-100)in curve (points)of F(CFZ)~-~~(CHZCH~O)~-~H supercritical C02 at 65 "C, 530 bar, and concentration, 6.3% (w/w). I .o
6.3% (w/w)FSO-100 T=65"C P=530bar
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Figure 8. Distribution of particle radii for F(CF2)6-10(CH~CHZO)~-~H (Zonyl FSO-100)in supercritical COZat 65 "C, 530 bar, and concentration, 6.3%(w/w). Inset: Distribution of larger particles. from 220 to 450 bar. Scattered intensities are very weak, characteristic of small structures. As before,the scattering intensity increases at lower pressure in part because of the change in the scattering contrast from the lower density supercritical solvent. Because of the lack of any scattering peaks, there is no evidence for existence ofliquid crystal phases or any type of crystal structure under these condition of temperature, density, and concentration as have been observed in liquid organic solvents at higher concentrations. In the limit of low q for a solid particle it can be shown that eq 1 is equal to the Guinier approximation for the
Fulton et al.
4248 Langmuir, Vol. 11, No. 11, 1995 9.5
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Figure 9. Small angle X-ray scattering spectra for 6%(w/w) F(CF~)~O(CHZ)IOH (CyoCyn)in supercritical COz at 65 "C and three different pressures, 220, 300, and 450 bar. particle form factor given by
P(q) = P(O)exp(-~,2q~/3)
(4)
where R, is the radius of gyration of the particle. The slope (-R,2/3) of the line from a plot of In P(q)versus q2 yields the radius ofgyration for the particle. For spherical particles, R, = (0.6)1'2r.The radius of gyration measured by SAXS is related to the particle electron distribution rather than the mass distribution so that for the C ~ o Cmaterial ~o the results will be strongly weighted by the fluorinated portion of the molecule. Although the Guinier approximation is derived from the assumption that the system can be characterized as a mondisperse system of spheres, it is known to be fairly insensitive to small differences in the shape or polydispersity of the system^.^^,^^ The Guinier relation accurately describes the intensity data when rq is less than 1,r being the radius of the particles. Linear regression analyses of data in this region of the intensity curves were used to obtain R, values. The results for the three different pressures are fitted to a Guinier analysis and shown in Figure 10. At the highest pressure the linearity of the data is excellent and the calculated radius of gyration is 5.8 . At lower pressures, the measured R, values increase slightly. There is also clearly some deviation from linearity particularly for the 220 bar data. These phenomena indicate that there may be some aggregate formation or alternatively there may be an increase in critical-like scattering on approach to the phase boundary. We can compare the measured R, with the calculated sizes of the individual C ~ o Cmolecules. ~o The electron density of the perfluorinated portion of the C ~ o C is ~o approximately 3 times higher than that of the protonated portion; thus the fluorinated portion dominates the scattering in the q range corresponding to the size domain of the molecular length. Scattering from the semirigid fluorinated segment can be approximated as that from a rigid rod having a length equivalent to the 10 carbon perfluorinated segment. For a rigid rod (needle)oflength h the radius of gyration is R, = (h2/12)1/2 or 4.2A for the perfluorinated segment. Corrections for folding of the
A
(40)Glatter, O., Kratky, 0.Small AngleX-ray Scattering; Academic Press: New York, 1982. (41)Hilfiker, R.; Eicke, H.; Sager, W.; Steeb, C.; Hofmeier, U. Ber. Bunsenges. Phys. Chem. 1990,94, 677.
0.00
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Figure 10. Guinier analysis of the F(CF~)IO(CH~)IOH or CyoCyosurfactant in supercritical COZ at 65 "C and 450 bar. hydrocarbon backbone (decreases R,) or for the contributions from the protonated portion of the chain (increases R,) are probably not justified given the limited resolution of the measurements. The predicted size is just slightly smaller than the 5.8 measured at 450 bar. This may indicate that there is a slight amount of aggregation but certainly nothing larger than three to four unimers per aggregate at this higher temperaturelpressure condition. It is possible that the extent of aggregation would be enhanced at temperatures well below the 65 "C of this study.
A
Conclusions Amphiphilic materials, that are synthesized or selected based upon a strategy to tailor them specifically for use in COZ,clearly form molecular aggregates. In this study we used amphiphiles with perfluorinated segments to promote the solubility in supercritical COZ. Small angle X-ray scattering measurements provided direct measurement of the aggregate size and geometry. Three widely differing amphiphiles were examined including the high molecular weight (- 100 000) graft copolymer PFOA-gPEO, a fluorinated nonionic surfactant, F(CFz)a-ioCH2CHz0(CHzCHz0)3-~H7 and diblock material, F(CF2)lo(CH2)loH. Each of these materials exhibited a different type of aggregation behavior. The PFOA-g-PEO material aggregates in supercritical COz, forming a core domain rich in PEO that is able to stabilize small amounts of water. The overall radius of this micelle is about 125 A and shows a very low size polydispersity, suggesting very uniform aggregates. The micelles contain approximately 600 grafts in the core of each aggregate. The shell of PFOA exists as a layer around the PEO core shielding it from the COZ continuous phase solvent. As the pressure is decreased,
Langmuir, Vol. 11, No. 11, 1995 4249
Aggregation of Amphiphilic Molecules
there is a small increase in the aggregate size perhaps in response to an increase in the interfacial tension at lower pressure. The nonionic surfactant, F(CFz)s-loCHzCHzO(CHzCHz0)s-gH or FSO-100, only completely dissolves at relatively high pressures at 65 “C. By their nature, nonionic surfactants having a smaller number of PEO units form a polydisperse micelle system. Additionally, the molecular weight distribution of the surfactant material studied probably further broadens the micellar distribution. An inversion of the scattering curves yields a roughly bimodal distribution of larger micelles in equilibrium with small oligomers possibly including monomer, dimer, and trimer species. oThe mean radius of the larger micelle structures is 84 A. Finally, the semifluorinated diblock material F(CF2)lo(CH&H forms at most only a small aggregate no larger than about four unimers per aggregate at 65 “C. It is possible that the relatively higher temperature of this study reduced the amount aggregation for these weakly interacting “amphiphilic” species.
The results of these structural deteminations for various amphiphiles in supercritical COZ will be useful in helping develop applications of these systems for organic or polymeric synthesis.
Acknowledgment. This research was supported in part (J.L.F. and D.M.P.)by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical SciencesDivision of the U.S. Department of Energy, under Contract DE-AC06-76RLO1830. This research was also supported by the Consortium for Materials Synthesis and Processing in Carbon Dioxide at the University of North Carolina-Chapel Hill, which is funded by the National Science Foundation, the Environmental ProtectionAgency, Dupont, Air Products, B. F. Goodrich, Eastman Chemical, Hoechst-Celanese, Xerox, Bayer, and GE. Additional support was also provided by the National Science Foundation (J.M. DeSimone, Presidential Faculty Fellow, 1993-1997). LA950266X
