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tion) built up over long times and also increased strongly with increasing net normal pressure. It is suggested that these remarkable effects can be rationalized on the basis of the compressibility of an ultrathin film near a solid boundary and an eventual vitrified state imposed by low levels of external normal pressure. Much more study, both experimental and theoretical, will be needed to understand the associated trapped structures which appear to evolve with time, the nature of the defects which accommodate the ultimate sliding motion, and the structure of these layers during the times that they do accommodate sliding. This study of fluids in intimate contact with solid
boundaries emphasizes the profound influence that a boundary can have on the structure and dynamics of molecular fluids, leading to loss of fluidity when the fluid is sufficiently thin. Acknowledgment. We thank I. Bitsanis and J. N. Israelachvili for discussions and comments and S. J. Clarson and A. J. Semlyen for providing the PPMS sample. This work was supported by the National Science Foundation, Grants MSM-88-19796 and NSF-DMR-8612860, with generous assistance from the IBM, Eastmar Kodak, and 3M Corporations.
Fluorescence Studies of the Interaction of Sodium Dodecyl Sulfate with Hydrophobically Modified Poly(ethylene oxide) Yong-Zhong Hu, Cheng-Le Zhao, and Mitchell A. Winnik' Department of Chemistry and Erindale College, University of Toronto, Toronto, Canada, M5S 1 A1
Pudupadi R. Sundararajan Xerox Research Centre of Canada, 2660 Speakman Dr., Mississauga, Canada, L5K 2Ll Received July 3, 1989. In Final Form: February 5, 1990 A poly(ethy1ene oxide) (PEO, M = SOOO), 1, containing pyrene (Py) groups at both ends attached via ether linkages has been prepared (l),and the fluorescence properties of ita aqueous solutions have been examined. These solutions show a weak excimer emission which is at first strongly enhanced and then suppressed by adding increasing amounts of sodium dodecyl sulfate (SDS) to the solutions. From the sharp onset of enhanced excimer emission and a correspondingly sharp change in the vibrational structure of the locally excited Py emission, we infer the formation of mixed micelles with an apparent cmc of 8 X M. SDS-polymer interaction occurs at concentrations well below that of SDS with either Py itself or PEO itself, indicating that in the hydrophobically modified polymer 1 the chain and end groups act cooperatively to promote interaction with the surfactant. At low SDS concentrations, the chains tend to cyclize to put both Py groups in the same micelle.
Introduction There has been a long-standing interest in the interaction of water-soluble polymers with small amphiphilic mo1ecu1es.'-' Part of this interest derives from the impor(1) (a) Goddard, E. D. Colloids Surf. 1986,19,255. (b) Goddard, E. D.; Hannan, R. B. In Micellisation, Solubilization and Microemulsion; Mittal, K . L., Ed.; Plenum: New York, 1977; Vol. 2. (2) Robb, D. In Lucaesen Renders, E. J., Ed. Anionic Surfactants: Physical Chemistry of Surfactant Action: Marcel Dekker: New York, 1981; p 109. (3) (a) Moroi, Y.; Akisada, H.; Saito, M.; Matuura, R. J . Colloid Interface Sci. 1977, 61, 233. (b) Carlsson, A.; Kalstrem, G.;Lindman, B. Langmuir 1986,2,536. (c) Winnik, F. M.; Winnik, M. A.; Tazuke, S. J . Phys. Chem. 1987,20,38. (4) (a) Bloor, D. M.; Wyn-Jones, E. J. Chem. SOC.,Faraday Tram. 2 1982, 78, 657. (b) Francois. J.: Davantis, J.: Sabbadin, J. Eur. Polym. J. 1985,21, 165. (5)Ja) .&a, R.; Lang,.J.; Lianos, P. In Dublin, P., Ed. Microdomoms m Polymer Solutions; Plenum: New York, 1984; Polym. Prep. Am. Chem. SOC.1982.23 (1). 39. (b) Zana. R.: Lane. J. J. Phvs. Chem. 1985,89, 41. (c) Lis&, E. A:; Abuin; E. J.'Colloid h e r f a c e 3 c i . 1985, 105, 1. (d) Turro, N. J.; Baretz, B. H.; Kuo, P.-L. Macromolecules 1984, 17, 1321.
tance of these interactions in various applications as diverse as paints and enhanced oil recovery. Perhaps the best studied system of this genre is that of poly(ethy1ene oxide) (PEO) in water interacting with the anionic surfactant sodium dodecyl sulfate (SDS)."' SDS itself in water 8880ciates into micelles containing approximately 60 molecules at concentrations above 8 X M (its critical micelle concentration (cmc)). In the presence of PEO, this association takes place at somewhat lower amphiphile concentration. The critical concentration for this aggregation is essentially independent of PEO concentration and its molar mass. Chain length effects only appear for chains much shorter than M = lo4. The SDS-PEO aggregate has all the characteristics of a mixed micelle: a localized ensemble of SDS molecules incorporating the polymer chain into its structure. For long polymers, many of these aggregates adhere to a given (6) (a) Cabane, B.; Dupleesix, R.; Colloids Surf. 1986,13,12; J. Phys. (Les Uljs, Fr.) 1987, 48,651. (b) Cabane, B.; Dupleask, R. J. Phys. (Les Ulrs, Fr.) 1982, 43, 1528. (7) Cabane, B. J. Phys. Chem. 1977,81, 1639.
0 1990 American Chemical Society
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Letters
chain.'^' From neutron-scattering studies: the size of each individual polymer-bound micelle is comparable to that of an isolated SDS micelle, and the spacing of these micelles along the backbone seems to be dictated by Coulombic repulsions between the micelles. Fluorescence quenching studies indicate that the aggregation number of the bound micelles is somewhat smaller than that of free SDS micelles and that salt effects on the aggregation number of bound micelles are also smaller than on SDS micelles themsel~es.~ NMR experiments suggest that the PEO backbone units become incorporated into the micelle in the head-group region, near the hydrophobicaqueous interface. These structures assemble in the way they do because of an interplay between three competing types of interactions: (i) strong hydrophobic interactions between the surfactant molecules, (ii) Coulombic repulsion between micelles, and (iii) a much weaker interaction between the polymer and the micelles. I t is this weaker interaction which provides the driving force for mixed micelle formation at surfactant concentrations below ita normal cmc. One way of enhancing the strength of the polymersurfactant interaction is to introduce hydrophobic substituents into the PEO. This is the process used, for example, by the paint industry to produce "associative thickeners", which provide rather remarkable rheological properties in aqueous solutions. Such thickeners are typically a series of PEO segments of M = 8000, linked via diisocyanates,and end-capped by hydrophobic groups (e.g., C12H25 or Cl,H,, alkyl or alkylphenolic end groups). In order to model this type of system, we prepared the bis1-pyrenyl methyl ether of PEO (1). Here the pyrenes serve both as a fluorescent probe and as a hydrophobic substituent on both ends of the polymer chain.
Here we report the results of experiments on the fluorescence properties of 1 in water in the presence and absence of SDS. Our attention is focused on a single sample of 1of M,,= 8000,of very narrow molecular weight distribution, and containing 2.0 pyrene groups per chain. The synthesis of 1is nontrivial. The correspondingdiester prepared from pyrenebutyric acid and PEO is much more readily available' but tends to hydrolyze in aqueous s o l ~ t i o n .Unless ~ one is exceedingly careful, one's measurements can contain signals from the fluorescent byproducts of this hydrolysis.
Experimental Section The synthesis of 1 and its characterization will be reported in detail elsewhere.l0 All experiments were carried out in MilliQ water subsequently distilled in glass. Fluorescence spectra were obtained on a SPEX Fluorolog 112 spectrometer. Some samples (Figure 2) were purged of oxygen. Oxygen affects the (8) Cuniberti, C.; Perico, A. Eur. Polym.J. 1977,13,369. (9) Cheung, S. T.; Redpath, A. E. C.; Winnik, M. A. Makromol. Chem. 1982, 183, 1815. Subsequent experiments with aqueous solu-
tions were frustrated by hydrolysis of the end groups. Cheung, S. T., . unpublished. (10) Hu, Y.-Z., Winnik, M. A., in preparation. The key to the characterization of 1 is that analysis by HPLC of 1 in water (TSK3000 PW column) permits one to resolve the unsubstituted, monosubstituted, and disubstituted PEO derivatives.
460
440
r-
500
X (nm)
-550
600
Figure 1. Fluorescence spectra of 1 (1 x 10* M) in the presence of various concentrations of SDS. The spectra shown correspond (front to rear) to [SDS] = 0, 1.0 X lo4, 5.1 X lo-', 1.0 x 10-4,2.0x 10+,4.0 x io+,6.i x 10-4,8.ix io+,i.o x 10-3, 2.8 x 3.5 X 1.0 X lo-*, and 2.1 x lo-* M. overall fluorescence intensity but not the shape of the spectrum. SDS (99.5%) was used as received.
Results and Discussion Fluorescence spectra of 1 in water and in aqueous SDS solutions are displayed in Figure 1. In water, the excimer emission, centered at 465 nm, is relatively weak. The excitation spectrum of the excimer is somewhat redshifted (1.0 nm) from that of the locally excited "monomer" emission and indicates that much of the excimer emission derives from preformed associated pyrene pairs. The excimer/monomer intensity ratio ZE/ZM is independent of the concentration of 1 over the range 1 X lo* to 2 X 10" M. As a consequence, we infer that the excimer formation in water is uniquely of intramolecular origin. SDS Effects. When SDS is added to these solutions, two different kinds of changes occur in the fluorescence spectrum, Figure 1. First, there is a pronounced change in the pattern associated with vibrational coupling in the monomer emission. Above a certain SDS concentration, there is a decrease in the ratio of the intensities of the first ( I M l ) to fourth ( I ) vibronic bands. This is remiMfl observed in pyrene itself, where niscent of the Ham effect solvent polarity acts to perturb the symmetry of the pyrene group and promote the oscillator strength of the (0,O) transition. Normally, this effect is very small in l-substituted pyrene derivatives, where the substituent itself serves to perturb the electronic symmetry. Here it appears that the &oxo substituent has a symmetrizing influence transition. Polar solvents enhance on the SI So (Lb) the intensity of the (0,O)band (ZMJ relative to that of the (0,3)band (ZM4). The magnitudes are very different than in the case of pyrene. For comparison, we provide in Table 1some values of ZMI/ZM4 for 1 obtained in various organic solvents. In Figure 2, we plot values of ZMl/ZM4vs log SDS concentration. No change is observed until a certain critical concentration is reached, and then there is a precipitous decrease in the value of this ratio. This kind of behavior is identical with that seen for pyrene itself when
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(11) (a) Ham, J. S. J. Chem. Phys. 1953, 21, 756. (b) Kalyanasundaran, K.;Thomas, J. K.J. Am. Chem. SOC.1977,99,2038; J. Phys. Chem. 1977,81,2176. (c) Nakajima, A. Bull. Chem. SOC.Jpn. 1977,44, 3272. (d) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984,62, 2560.
882 Langmuir, Vol. 6, No. 4, 1990
Letters
Table I. ZMl/ZMd Values for 1 in Various Solvents
I, 1,.
solvent toluene dichloromethane methanol tetrahydrofuran acetone water
I
0.93 1.02 1.04 1.05 1.09 1.14
III
1 I .o
- 5.0
-4.0
-3.0
-2.0
II
-u
m
sDs#
-1.0
Figure 3. Schematic model of the interaction of 1 with SDS in aqueous solution with increasing SDS concentration.
Log Csds
Figure 2. Plot of I M i / I M 4 (upper curve) and I /IM (lower curve) vs log [SDS]. These samples were degassezby purging with argon gas (20 min).
used as a probe for micelle formation." Once aggregates form, pyrene partitions into the aggregate. Within the hydrophobic domains, it experiences a less polar environment, leading to a decrease in the (0,O)band intensity. The implication of the data in Figure 2 is that SDS associates with the hydrophobically substituted PEO to form a mixed micelle with an apparent cmc 1 order of magnitude lower than that of the cmc of SDS itself. The other feature of interest in Figure 1is the change in excimer-to-monomer intensity ratio as a function of SDS concentration. Above a certain concentration ZE/ ZM increases,and then at higher concentrations it decreases (Figure 2). The onset of the change of ZE/ZM corresponds to the sudden decrease in zMl/ZM4 seen in the upper curve in Figure 2. Mixed Micelles. One of the characteristics of the formation of mixed micelles between SDS and PEO itself is that the ap arent cmc is independent of the PEO con~entration.~?This same feature is observed here. Even at 20-fold higher concentrations of 1 we obtain an identical plot of IE/Zm vs SDS concentration. There is, nevertheless, one important distinction between this system and that involving PEO itself. Mixed micelles involving 1 and SDS form at concentrations about a factor of 3 smaller than those between SDS and PEO."' This is significant and unexpected. When pyrene itself is added to aqueous surfactant solutions, it is assumed and commonly found that the presence of one or even two pyrenes in a micelle has only a very small effect on This the cmc and the aggregation number of the mi~el1e.I~ concept extends to the more complex case where pyrene is used as a probe for mixed micelle formation between (12) Ananthapadmanabhan, K.P.; Goddard, E. D.; Turm, N. J.; Kuo, P. L. Langmuir 1985,1,352. (13) (a) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (b) Yekta, A,; Aikawa, M.; Turro, N. J. Chem. Phys. Lett. 1979,63,543. (c) Infelta, P. P. Chem. Phys. Lett. 1979,61,88. (d) Turro, N. J.; Gratzel, M.; Braun, A. M. Angew Chem. Eng. 1980, 19, 675. (0 Van der Auweraer, M.; Dederen, J. C.; Gelad(, E.; De Schryver, F. C. J. Chem.
Phys. 1981, 74, 1140.
ionic surfactants and nonionic polymers. The pyrene and the polymer act independently of one another, and thus the pyrene excimer method gives reliable values for the aggregation numbers for these mixed micelles.' Here the situation is quite different. With the pyrene groups bound to the polymer, they act cooperatively to promote an interaction between SDS and the polymer. Cyclized Chains. The increase in ZE accompanying mixed micelle formation suggests that initial micelle formation promotes end-to-end cyclization of the polymer chains. Excitation spectra indicate that prior to the peak in Figure 2 excimers derive from preassociated pyrene pairs. In other words, initial micelle formation occurs to produce polymer molecules with both pyrene chain ends in the same micelle. When the SDS concentration is increased, the ZE/ZM ratio reaches a maximum and then decreases. No corresponding changes occur in the ZM1/IM4 ratio. The local environment of each pyrene group remains essentially constant, but some other feature of the system changes. The most reasonable explanation is that above a certain concentration of SDS there are sufficient surfactant molecules to form micelles at each end of the polymer. I t is interesting to note that these changes are virtually complete at concentrations well below the normal cmc of SDS (8 X lo9 M)and that in this latter concentration range, where normal SDS micelles form, no changes are observed in ZE/ZM and only small changes in I M l / I M 4 are observed. We depict these phenomena as shown in Figure 3. At low SDS concentration, the polymer is essentially present as the swollen coil of a polymer in a good solvent. The fraction of cyclized chains (11) is somewhat enhanced because of hydrophobic interactions between the end groups. Once the apparent cmc is surpassed, mixed micelles form. These incorporate one (111) or two (IV) pyrenes plus some portion of the PEO chain. A t elevated SDS concentrations, micelles form at both chain ends (V), and excimer formation is suppressed. Conformational Effects. In order to get a feeling for the conformational contributions of the polymer to the phenomena we observe, we have carried out rotational isomeric state calculations on the PEO chain using parameters reported by Flory." These calculations ignore excluded volume effects, but this should have only a small effect on mean chain dimensionsfor the chain length examined here. We find for PEO of M = 8000 (DP= 182) a (14) Flory, P.J. Statistical Mechanics of Chain Molecules; WileyInterscience: New York, 1969.
Langmuir 1990,6, 883-885 root-mean-squared end-to-end distance RF = 68 A. Coulombic repulsions between micelles at both chain ends would tend to elongate the chain. The fully extended (helical) length of this polymer, 504 A, corresponds to a sequence of (TTGTTG) states. We can estimate the conformational entropy loss on the cyclization step I11 IV in the following way. Assume the micelle on the chain end represents a sphere of radius ca.16.7 A, Le., a normal SDS micelle radius." If we assume that the other chain end is located on average within a sphere of radius R,, the fraction of this volume occupied by the micelle is ca. 0.015. If any conformation of the chain placing both pyrenes within the same micelle is deemed to be cyclized, the entropy loss corresponding to cyclization, AS = -R In (0.015), is ca. 8.3 cal K-' mol-'. At SDS concentrations below the peak in Figure 2, the entropy loss for the reaction I11 IV is offset by the gain in free energy through the hydrophobic binding of the second pyrene to the micelle. At higher SDS concen-
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(15) Gruen, D.W. R. Prog. Colloid Polym. Sci. 1985, 70, 6 .
883
trations, the equilibrium is among 111, IV, and V. Here the entropy loss on cyclization should be much less severe. The polymer containing micelles a t both ends should have a significantly lower entropy than the random chains I because of the tendency in V toward chain elongation due to Coulombic effects.
Summary We have examined by fluorescence spectroscopy aqueous solutions of the polymer'Py-PEO-Py (1) in the presence and absence of SDS. The most striking result is that the hydrophobic end groups interact cooperatively with the polymer and the surfactant to promote surfactant aggregation with the polymer at much lower concentrations than the surfactant cmc even in the presence of the unsubstituted polymer. At low SDS concentration, the polymers tend to cyclize so that the two end groups share a single mixed micelle. Acknowledgment. We thank NSERC Canada for its support of this research.
Effect of the Spreading Solvent on Monolayers of Valinomycin Herman E. Ries, Jr. Department of Molecular Genetics and Cell Biology, The Uniuersity of Chicago, Chicago, Illinois 60637 Received October 24, 1989 Pressure-area isotherms for valinomycin, a cyclic dodecadepsipeptide,are significantly different when the film is spread from four different solvents: benzene, chloroform, n-hexane, and cyclohexane. This effect of the solvent is in sharp contrast to that found with films of long-chain lipid-type compounds, which show no significant solvent effect. The highly polar internal ring structure of valinomycin in contrast to the relatively small terminal polar group of a typical lipid may account for the difference.
Introduction Because few film-forming compounds spread spontaneously, the use of volatile solvents has become an integral part of monolayer spreading.'.* For many years, possible effects of the spreading solvent on monolayer properties have been of some concern. Early studies of the solvents were performed on long straight-chain compounds such as fatty acids and a l ~ o h o l s . ~With - ~ conventional solvents, benzene, chloroform, and n-hexane, no significant differences were observed in the important parts of the pressure-area isotherms, that is, in the long central linear portions from which the extrapolated areas are obtained. However, in current studies on monolayers of valinomycin, marked differences are observed (1) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley: New York, 1982; p 114. (2) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Intarscience: New York, 1966;pp 30-33. ( 3 ) Cook, H. D.; Ries, H. E., Jr. J. Phys. Chem. 1956,60, 1533. (4) Cook, H. D.; Ries, H. E., Jr. J . Am. Chem. SOC.1969,81, 501. (5) Walker, D. C.; Ries, H. E., Jr. Nature 1964,203,292.
0743-7463f 90f 2406-0883$02.50f 0
in the isotherms obtained with four spreading solvents: benzene, chloroform, n-hexane, and cyclohexane. Valinomycin is a cyclic dodecadepsipeptide, an antibiotic, and an ionophore. It carries ions through the cell membrane because of its cyclic polar center and its nonpolar exterior. A schematic drawing of the valinomycin molecule is shown in Figure 1. Its structure is clearly different from those of the long-chain lipid-like molecules.
Experimental Section High-purity valinomycin preparations from the Calbiochem Corp. and from the Aldrich Chemical Co. gave monolayer properties that are essentially identical. The four spreading solvents were high-purity materials from American Burdick and Jackson; extremely dilute solutions were prepared gravimetrically. The monolayer experimentswere performed on a horizontalfloat film balance of the Langmuir-Adam-Harkins type? Sur(6) Ries, H. E., Jr.; Swift, H. J. Colloid Interface Sci. 1982,89, 245.
0 1990 American Chemical Society
