Hydrophobic Complexation of Poly(vinyl caprolactam) with Sodium

The interaction of poly(vinyl caprolactam) (PVCAP) with sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in aqueous solutions ...
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Hydrophobic Complexation of Poly(vinyl caprolactam) with Sodium Dodecyl Sulfate and Dodecyltrimethylammonium Bromide in Solution Q. Qiu,† P. Somasundaran, and B. A. Pethica* Langmuir Center for Colloids and Interfaces, Columbia University, 500 West 120th Street, Room 911, New York, New York 10027 Received November 26, 2001. In Final Form: February 22, 2002 The interaction of poly(vinyl caprolactam) (PVCAP) with sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in aqueous solutions has been studied systematically by measuring the phase separation temperature, hydrodynamic radius, pyrene solubility, and surfactant binding isotherms. Both surfactants were observed to elevate the solution cloud point of PVCAP and cause the polymer to undergo a coil to globule transition. This transition occurs at a concentration about 1/10 of the critical micelle concentration (cmc) for SDS in the absence of polymer but at the cmc with DTAB. The results indicate that PVCAP interacts with SDS monomers but only with micelles in the case of DTAB. The phase behavior of the PVCAP/SDS/DTAB ternary system shows that the binding of SDS to PVCAP is reversible on changing the concentration of the free surfactant monomer in solution. Potentiometric titration of PVCAP and measurements of pyrene solubility in its mixtures with surfactants suggest that complexation of PVCAP and SDS is due to a combination of ion-dipole and hydrophobic effects. PVCAP and DTAB micelles interact through hydrophobic inclusion of polymer segments into the DTAB micelles. Pyrene is not solubilized by PVCAP in solution alone. Addition of SDS to PVCAP solutions induces marked pyrene solubilization well below the cmc, characterized by a region indicating saturation adsorption of the pyrene to the PVCAP/SDS complex. Above the SDS cmc, solubilization of pyrene increases linearly with SDS concentration, corresponding to inclusion of the pyrene into the SDS micelles. In contrast to SDS, the addition of DTAB to a solution of PVCAP shows no pyrene solubilization until the cmc is reached.

Introduction Interactions between water-soluble polymers and surfactants have been the subject of extensive study due to their inherently interesting properties and important applications in detergency, mineral separation, pharmaceuticals, rheological control, enhanced oil recovery, paint and coatings, and so forth.1-5 Of the various polymer/ surfactant combinations, nonionic polymers and ionic surfactants have attracted much attention. Particular interest has been shown in interactions with poly(vinyl pyrrolidone) (PVP),6-10 poly(ethylene oxide) (PEO),11,12 ethyl hydroxyl ethyl cellulose (EHEC),13-15 poly(acrylamide) (PAM),16 and poly(N-isopropyl acrylamide) (PNIPAM).17-19 A common feature of these polymers is * Corresponding author. Department of Chemical Engineering, Princeton University, The Engineering Quadrangle (A220C), Princeton, NJ 08544. † Present address: Unilever Research Laboratory, 45 River Rd., Edgewater, NJ 07020. (1) Goddard, E. D. Colloids Surf. 1986, 19, 255. (2) Goddard, E. D. Colloids Surf. 1986, 19, 301. (3) Robb, I. D. Surfactant Sci. Ser. 1981, 11, 109. (4) Satio, S. Surfactant Sci. Ser. 1987, 23, 881. (5) Somasundaran, P.; Cleverdon, P. J. Colloids Surf. 1985, 13, 73. (6) Folmer, B. M.; Kronberg, B. Langmuir 2000, 16, 5987. (7) Thibaut, A.; Misselyn-Bauduin, A. M.; Broze, G.; Jerome, R. Langmuir 2000, 16, 9841. (8) Ma, C.; Li, C. Colloids Surf. 1990, 47, 117. (9) Chari, K.; Hossain, T. Z. J. Phys. Chem. 1991, 95, 3302. (10) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637. (11) Ghodbane, J.; Denoyel, R. Colloids Surf. 1997, 127, 97. (12) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (13) Nahringbauer, I. Langmuir 1997, 13, 2242. (14) Hoff, E.; Bystrom, B.; Lindman, B. Langmuir 2001, 17, 28. (15) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588. (16) Mougdil, B. M.; Somasundaran, P. Colloids Surf. 1985, 13, 87. (17) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (18) Lee, L. T.; Cabane, B. Macromolecules 1997, 30, 6559.

that they interact strongly with anionic surfactants but weakly or not at all with nonionic or cationic surfactants. A model of “necklaces” of polymer-surfactant complexes, with individual polymer molecules carrying separate micellelike surfactant aggregates along the polymer chains, has been supported by surface tension, NMR, viscometry, IR spectroscopy, small-angle neutron scattering (SANS), and other techniques.20-21 Recently, thermoprecipitating nonionic polymers have become of interest due to their potential use in biotechnology and as separation aids.22 A less studied member of this class is poly(N-vinyl caprolactam) (PVCAP), which exhibits a reversible phase separation from aqueous solutions with a lower consolute temperature (LCT) in the range of physiological temperatures. PVCAP also forms complexes with dyes, proteins, enzymes, surfactants, and polymers.23 Makhaeva et al.24 studied the coil to globule transition of PVCAP on addition of sodium dodecyl sulfate (SDS) and cetylpyridinium chloride (CPC). At low surfactant concentrations, the polymer is observed to shrink until a minimum is reached after which further addition of surfactant causes increase of the polymer dimension, and with CPC the final size is even 20 nm larger than in the surfactant-free condition. Many issues remain unresolved regarding the nature of polymer/surfactant complexation. For example, it is unclear whether the positive component of the large dipole of the cyclic amide groups (19) Shi, X.; Li, J.; Sun, C.; Wu, S. Colloids Surf. 2000, 175, 41. (20) Cabane, B.; Duplessix, R. J. Phys. 1982, 43, 1529. (21) Sukul, D.; Pal, S. K.; Mandal, D.; Sen, S.; Bhattacharyya, K. J. Phys. Chem. B 2000, 104, 6128. (22) Galaev, I. Y.; Mattiasson, B. Enzyme Microb. Technol. 1993, 5, 354. (23) Kirsh, Yu. E. Prog. Polym. Sci. 1993, 18, 519. (24) Makhaeva, E. E.; Tenhu, H.; Khokhlov, A. R. Macromolecules 1998, 31, 6112.

10.1021/la011702k CCC: $22.00 © 2002 American Chemical Society Published on Web 04/05/2002

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in PVP is the site of the associative binding of SDS to the polymer. The same interaction may apply to explain the strong complexing capability of PVCAP for organic anions. In this work, we explored the interactions between PVCAP and SDS or dodecyltrimethylammonium bromide (DTAB) using a variety of techniques to understand the determining factors governing complexation. Materials and Methods Poly(vinyl caprolactam) was provided by International Specialty Products. It was synthesized by radical polymerization using L575 [(CH3)3C-O-O-CdO-CH2-(CH2)3CH3] as the initiator. The solvent used was 2-propanol. This polymer was precipitated twice by the addition of acetone to concentrated aqueous solutions of the polymer at room temperature and dried under vacuum. The weight-average molecular weight is 85K as determined by gel permeation chromatography using a multiangle light scattering detector. SDS was purchased from Acros (lot no. B0075657, formula weight (fw) 288.38) and is of analytical grade. DTAB was purchased from Aldrich (fw 308.35) and is of analytical grade. Both surfactants were used as received. Triply distilled water (TDW) was used in this study. The second distillation is from alkaline permanganate. Methods. Prior to each set of measurements, mixed solutions of polymer/surfactant at the desired concentrations were prepared and subjected to wrist-action shaking, using a gyratory shaker model G2 from New Brunswick Scientific Co., at 250 rpm for 2 h unless specified otherwise. (1) Dynamic Light Scattering. Dynamic light scattering was performed using a Brookhaven (model BI-DS) photon correlation spectrometer. To minimize dust effects, solutions were filtered through a 0.2µ Nalgene membrane (Nalge Nunc International Co.) prior to use. The filtration process produced no detectable effect on polymer concentration and, due to the relatively large pore size of the membrane, is unlikely to change the concentration or conformation of polymer or polymer/surfactant complexes. The sample, contained in a 20 mL glass vial, was then placed in the sample holder in a water bath thermostat. At least 30 min was allowed to reach the required temperature. All of the measurements were done in the dynamic mode at 90° detection. At least 10 measurements were repeated, and values were averaged. (2) Pyrene Solubility. Excess pyrene crystals were added to the solutions of interest contained in 20 mL glass vials. The samples were then wrapped with aluminum foil to avoid possible decomposition caused by direct contact with light and subjected to wrist-action shaking at 250 rpm for a week. No further solubilization was detected with longer time. The samples were centrifuged at 4000 rpm for half an hour, and the supernatant was filtered through a 0.2µ Nalgene membrane. The pyrene concentration was analyzed by UV absorbance at 335 nm using a Schimadzu UV-vis spectrophotometer. (3) Cloud Point. Solutions of polymer or polymer/surfactant mixtures were heated to increasing temperatures in a water bath until visibly cloudy and then cooled slowly. The temperature at which the solution clears was taken as the cloud point, measured to 0.1 °C. A Brinkmann PC/600 colorimeter was used to assist the determination of the clearing point. Measurements were repeated at least three times for each sample. (4) Binding Isotherms. An ultrafiltration technique was used to obtain the binding density of SDS to PVCAP. Solutions at desired polymer/surfactant concentrations were filtered through a 3000 molecular weight cutoff membrane (Fisher Scientific) by subjecting them to centrifugal force at 4000 rpm for 40 min. The SDS concentration in the filtrate was analyzed using two-phase titration25 with 1.1 mM cetyltrimethylammonium bromide (CTAB) as the cationic titrant. The binding density was expressed as the ratio of SDS molecules depleted from solution per PVCAP monomer. (5) Cloud Point and Total Organic Carbon (TOC) of PVCAP/ SDS/DTAB Ternary Mixtures. Around the 1:1 stoichiometric ratio of DTAB/SDS, precipitates were formed in solution. To remove these precipitates, all turbid mixtures were first cen(25) Li, Z.; Rosen, M. J. Anal. Chem. 1981, 53, 1516.

Figure 1. Effect of surfactant on the phase behavior of PVCAP. Polymer concentration, 1000 ppm; ionic strength, 0.01 M NaCl. Squares, SDS; circles, DTAB. trifuged at 4000 rpm for 30 min and the supernatant obtained was then filtered through a 0.2µ membrane. The filtrate was analyzed for TOC, using a TOC-5000 total organic carbon analyzer from Shimadzu, and its cloud point was determined.

Results and Discussion (1) Binary Systems: PVCAP/SDS and PVCAP/ DTAB. The phase behavior of PVCAP in the presence of SDS and DTAB is illustrated in Figure 1. In the absence of surfactants, the polymer solution at 1000 ppm and 0.01 M NaCl shows a phase transition temperature at 31 °C. This temperature is sharply elevated by the addition of SDS and DTAB above certain concentrations: 0.5 mM with anionic SDS and 12 mM with cationic DTAB. For DTAB in 0.01 M NaCl, 12 mM is the critical micelle concentration (cmc) indicating that PVCAP interacts with micelles of DTAB. With SDS, 0.5 mM is only about 1/10 of its cmc and the rising cloud point above this concentration shows that SDS monomers bind to the polymer. The observed difference in binding between SDS and DTAB is presumably caused by the weaker interaction of the shielded trimethylammonium group with the amide dipole as compared to SDS, the two alkyl chains being equivalently hydrophobic. Above the cmc, micelles of DTAB are formed and the hydrophobic PVCAP segments penetrate the hydrophobic cores of the micelles. The interaction of PVCAP with monomers of SDS involves hydrophobic forces between polymer and surfactant, with the possible formation of hydrophobic clusters of SDS molecules along the polymer chain. Figure 2 shows the pyrene solubility in SDS solutions with and without the presence of PVCAP. Pyrene is barely soluble in water or in aqueous PVCAP solutions, but when hydrophobic microdomains are present its solubility will increase in proportion to the volume of the microdomains. This is demonstrated in SDS solutions without polymer where a noticeable increase in pyrene solubility is detected only above the cmc and a linear slope is observed in that region. On addition of 500 ppm PVCAP to SDS solutions, the solubility of pyrene is enhanced over the whole range of SDS concentrations. Further addition of PVCAP to the solutions further enhances pyrene solubilization. The results show that for PVCAP in solution alone, the hydrophobic caprolactam groups along the PVCAP chains do not cluster into hydrophobic regions sufficient to solubilize pyrene. This is consistent with the probable architecture of the PVCAP chain in solution shown by

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Figure 2. Pyrene solubility in SDS solutions in the absence (curve 1, squares) and presence of 500 ppm (curve 2, triangles), 1000 ppm (curve 3, circles), and 2000 ppm (curve 4, diamonds) PVCAP. T ) 25 °C, 0.01 M NaCl.

models of the polymer, which indicate that the nonplanar caprolactam groups with their large dipole moments are inhibited from clustering by rotational hindrance around the N-C bond coupling them to the polymer backbone. Addition of SDS to PVCAP solutions leads to complexation featuring the formation of hydrophobic domains and the consequent solubilization of pyrene. Similar behavior has been also observed with PVP/SDS in solubilizing an organic dye, orange OT.26 In the case of DTAB/PVCAP, the coupling of the polymer to DTAB micelles appears to involve entry of a number of hydrophobic groups into the micelles, but the extent of folding of the polymer chain to allow more than one caprolactam group to enter the micelles is not known. The number ratio of DTAB micelles to PVCAP molecules is about 1:1 at a DTAB concentration of 1.1 times cmc and rises to 10:1 at twice the cmc, taking the aggregation number as 80.27 Unless the polymer association with the DTAB micelles is cooperative, the association is probably of one polymer molecule per micelle in the concentration range studied. The small and constant radius of gyration of the PVCAP/DTAB complexes above the DTAB cmc, as will be shown later, also argues for one PVCAP molecule per DTAB micelle, but larger ratios may be found with higher polymer concentrations. The pyrene solubility in the presence of PVCAP with added SDS shows two distinct regions: an initial nonlinear region resembling an adsorption isotherm concave to the SDS concentration axis, followed by a linear region with a slope similar to that for SDS solutions in the absence of polymer. This interpretation is confirmed by directly comparing the binding of SDS to the PVCAP, determined by the ultrafiltration method, with the solubilization of pyrene in the complex as shown in Figure 3. The saturation of SDS binding correlates with the onset of the linear region of increasing pyrene solubilization. The first (nonlinear) SDS concentration region in the pyrene binding is associated with the formation of micellelike hydrophobic clusters of SDS with the polymer, while the linear second region relates to the formation of free surfactant micelles in solution. The end of the nonlinear region in the pyrene solubility curve corresponds to saturation of the PVCAP/SDS complex. The binding isotherm in Figure 3 also indicates that at saturation there (26) Lange, H. Kolloid Z. Z. Polym. 1971, 243, 101. (27) Caponetti, E.; Causi, S.; De Lisi, R.; Floiano, M. A.; Milioto, S.; Triolo, R. J. Phys. Chem. 1992, 96, 4950.

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Figure 3. Correlation of pyrene solubility (circles) in binary mixtures of PVCAP/SDS with the binding density (squares) of SDS to PVCAP. PVCAP concentration ) 1000 ppm, T ) 25 °C, 0.01 M NaCl.

Figure 4. Potentiometric titration of 1000 ppm aqueous PVCAP solution (stars) compared with that of pure water (circles). Negative values are for H+. T ) 25 °C.

are approximately 0.6 bound SDS molecules per PVCAP monomer. This ratio is twice that of SDS binding to PVP in the presence of 0.1 M NaCl,28 probably due to the greater hydrophobicity of PVCAP. The binding of SDS to PVCAP was also studied over a pH range of 3-10. The binding, within experimental error, is not a function of pH, which is as expected from the titration data shown in Figure 4. In contrast to SDS, the complexation of PVCAP with micelles of DTAB does not produce hydrophobic sites additional to those of the DTAB micelles. Figure 5 shows the pyrene solubility in DTAB solutions in the absence and presence of 1000 ppm PVCAP. The effect of the polymer is minor, with a small apparent reduction in the DTAB cmc, indicating a modest stabilizing effect of the polymer on micelle formation. Fluorescent emission spectra observed for saturated pyrene in the SDS and DTAB/polymer systems are mostly dominated by the broad excimer band characteristic of concentrated pyrene solutions in hydrophobic domains. Further measurements of fluorescence decay rates and of emission spectra at lower pyrene concentrations should give further detail on these hydrophobic domains. (2) Ternary System: PVCAP/SDS/DTAB. PVCAP/ surfactant complexation was further examined by study(28) Fishman, M. L.; Eirich, F. R. J. Phys. Chem. 1971, 75, 3135.

Hydrophobic Complexation of PVCAP

Figure 5. Pyrene solubility in DTAB solutions in the absence (circles) and presence (squares) of 1000 ppm PVCAP. T ) 25 °C, 0.01 M NaCl.

Figure 6. Effect of DTAB addition to the PVCAP/SDS binary mixture: change in solution cloud point (squares) and total organic carbon (circles) after centrifuging. SDS concentration ) 5 mM, T ) 25 °C. For comparison purposes, the TOC data have been normalized to the corresponding value for 1000 ppm PVCAP.

ing the PVCAP/SDS/DTAB ternary system. Figure 6 shows the change in solution cloud point and total organic carbon (after filtration through a 0.2µ membrane) as DTAB is added to a solution of 1000 ppm PVCAP and 5 mM SDS in 0.01 M NaCl, which has a cloud point of ∼90 °C. As DTAB is added, the cloud point is gradually reduced, and up to the 1:1 stoichiometric ratio of DTAB/SDS, the solution shows a cloud point of 31 °C, the characteristic value for 1000 ppm PVCAP solution alone. No further change in cloud point is detected at DTAB/SDS ratios up to 2, where due to the interaction with SDS the free DTAB is still below its cmc. Furthermore, the solution TOC shows a minimum at the 1:1 ratio of DTAB/SDS, with a value identical to that of 1000 ppm PVCAP alone showing that the DTAB/SDS complex is filtered off. Beyond that point, the TOC increases sharply to the value expected for complete passage of the total mass through the filter. The monotonic decay in cloud point as the ratio DTAB/ SDS goes from 0 to 1 suggests that the amount of bound SDS is decreased on DTAB addition. At the 1:1 ratio of DTAB/SDS, only free PVCAP molecules are left in the solution and all surfactants are precipitated out. In the concentration range studied, DTAB does not directly interact with PVCAP but forms a complex with free SDS

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Figure 7. Effect of PVCAP/surfactant complexation on polymer configuration studied by dynamic light scattering at a 90° detector angle. The surfactant concentrations have been normalized to the cmc of the respective surfactant, i.e., 5.8 mM for SDS and 12 mM for DTAB. PVCAP concentration ) 1000 ppm, T ) 25 °C, 0.01 M NaCl.

in solution thereby reducing its chemical potential, causing the bound SDS to desorb. The complexes formed by DTAB/ SDS beyond the critical ratio of 1:1 might be in the form of vesicles or mixed micelles. Whatever form they are, however, they seem not to interact with the polymer as suggested by the unaltered cloud point above the equivalence point. This property of PVCAP might be of biological importance where liposomes and membranes are involved. (3) Conformation of PVCAP on Complexation with SDS or DTAB. In addition to the change in cloud point, the complexation of PVCAP with surfactants also alters its configuration in solution. Figure 7 shows the hydrodynamic diameter of PVCAP in aqueous solutions in the presence of SDS or DTAB. In both cases, the polymer is seen to shrink in overall dimension. With SDS, shrinking starts at a much lower concentration than its cmc, while with DTAB shrinking starts at the cmc. The coil to globule transition of PVCAP upon addition of SDS was also observed by Makhaeva et al.24 They reported a minimum size at 3 mM SDS for a 500 ppm solution of PVCAP of molecular weight 4 × 106, after which the hydrodynamic radius is observed to increase with higher SDS concentrations. The expansion was attributed to the increased osmotic pressure inside the large polymer coil induced by the surfactant counterions and possible weakening of hydrophobic interactions between surfactant tails due to the repulsion of the similarly charged bound surfactants. The argument is open to discussion. The existence of the minimum depends on one experimental point, and since no error bars are given in the paper, it is hard to tell whether the minimum is significant. Nonetheless, the scheme proposed for the PVCAP/SDS complexation, that more than one nonadjacent polymer segment is associated with each micelle-like surfactant structure on the polymer backbone, clearly shows why PVCAP shrinks. Norenburg et al.29 studied fluorescence correlation spectra for a mixture of a terminally fluorescer-labeled SDS at a fixed concentration (10 nM) with unlabeled SDS at varying concentrations in aqueous solution with and without addition of PVP (types K90 and K30, corresponding to molecular weights of ∼106 and 5 × 104). The (29) Norenburg, F.; Klingler, J.; Horn, D. Angew. Chem., Int. Ed. 1999, 38, 1626.

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fluorescer group chemistry was not specified. The estimated diffusion coefficient for the labeled SDS decreased above the SDS cmc to a value corresponding to the size of an SDS micelle. Addition of PVP lowers the apparent cmc and showed a large increase in the size of the diffusing unit, clearly resulting from the association of the labeled surfactant with the large PVP molecule. At high polymer concentrations (overall monomer subunit/SDS ratios above about 5), the diffusion coefficient corresponds with that expected for the polymer alone, suggesting that almost all the fluorescer is bound to the polymer with little conformation change at these low SDS binding ratios. At lower polymer concentrations (higher SDS/monomer subunit ratios), interpretation of the data is ambiguous and could show polymer coiling as a result of surfactant binding, rapid exchange of the labeled SDS between free micelles and the PVP/labeled SDS complex, or both. Interpretation is complicated by the absence of information on the relative binding of the labeled and unlabeled SDS to the polymer. Conclusions The complexing of PVCAP with anionic SDS and cationic DTAB has been studied by measuring pyrene solubility, phase transition temperature, hydrodynamic dimension, and surfactant binding. It was found that PVCAP complexes through hydrophobic forces with both anionic SDS and cationic DTAB, the latter taking place only above the cmc of DTAB. Upon complexation, PVCAP shows enhanced hydrophilic character in solution as indicated by an elevated cloud point and undergoes a coil to globule transition in conformation, as shown by the dynamic light scattering measurements. The binding of SDS to PVCAP is a reversible process. As the chemical potential of the free SDS is reduced by

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the formation of vesicles or mixed micelles with DTAB, the SDS binding to the polymer decreases and the cloud point shifts toward that for the polymer solution without surfactant. A transition point is observed at the stoichiometric 1:1 ratio of DTAB/SDS where all the surfactants are precipitated out of the solution and only bare polymer is left. Further raising of the ratio causes redissolution/ redispersion of the surfactant precipitates but produces no effect on the solution cloud point suggesting the absence of interactions between the polymer and the surfactant complexes. Pyrene solubility in water is low and barely increases on addition of PVCAP, indicating that caprolactam groups in the polymer do not cluster into hydrophobic domains. In the presence of PVCAP, pyrene solubility in SDS solutions is enhanced and the data show two distinct regions. The initial nonlinear section is related to the binding isotherm of SDS to PVCAP and the formation of hydrophobic clusters in the complex. This is followed by a linear region related to saturation of the SDS/PVCAP complex at 0.6 SDS molecules per monomer subunit and the formation of free SDS micelles in solution. In contrast, the addition of PVCAP to DTAB solutions shows little interaction below the DTAB cmc. PVCAP undergoes coiling in associating with DTAB micelles, but with little change in the total micellar hydrophobic domain volume. Acknowledgment. The authors acknowledge valuable technical discussions with Professor Thomas W. Healy and the support of the National Science Foundation (EEC 9804618 and CTS 0089530) and the industrial sponsors for the Industrial/University Cooperation Research Center at Columbia University for advanced studies in novel surfactants. LA011702K