J. Phys. Chem. 1083, 87, 251-254
25 1
Comparative Studies between Monomeric and Polymeric Sodium 10-Undecenoate Micelles C. M. Paleos,” C. I. Stasslnopoulou, and Angelos Malllarls’ Nuclear Research Center “Demokrltos”,Athens, Qeece (Received:February 18, 1982; In Flnal Form: September 15, 1982)
When aqueous solutions of sodium 10-undecenoateat concentrations above the cmc of the surfactant (8.24 g/dm3; 0.04 M) are irradiated by y rays the low molecular weight polysoap poly(sodium 10-undecenoate)is formed which has a degree of polymerization equal to ten and exhibits micellar behavior by solubilizing hydrophobic molecules. The cmc of the polymeric micelle is equal to zero, each micelle being built of a single poly(sodium 10-undecenoate)molecule. These micelles, called “intramolecular”micelles, are known to have equal hydrated size with the corresponding monomeric micelles of sodium lhndecenoate. Fluorescence studies with the micropolarity probe pyrene show that the fluorophore penetrates the intramolecular polymeric micelle less than the monomeric one. This is understood in terms of the more compact structure of the polymeric micelle due to the presence of polymerization-induced covalent bonds among the terminal methylene groups of the undecenoate chains. When the concentration of poly(sodium 10-undecenoate)in water exceeds 2.06 g/dm3 hydrophobic aggregation of intramolecular micelles occurs which leads to the formation of large “intermolecular” micelles having an intrinsic viscosity of 0.079 100 mL/g compared to 0.048 100 mL/g for the intramolecular and monomeric micelles.
Introduction Micelles which are aggregates of surfactant ions formed in aqueous media above a certain concentration, the critical micellar concentration (cmc) characteristic for each surfactant, have been the subject of intensive research during the past 2 decades because of their immediate relation to some fundamental physicochemical (hydrophobic effect), biological (models for biomembranes), and technological (chemical catalysis, charge separation in photoredox reactions) problems. On the other hand, polysoaps, defined as polymers to whose backbone structure long aliphatic chains bearing terminal ionic groups have been chemically attached,’ can exhibit micellar properties. It has been shown for instance’ that a polyelectrolyte which has a certain percentage of long aliphatic side chains assumes a compact structure in aqueous solutions due to van der Waals interactions among its alkyl chains and also solubilizes hydrophobic molecules in a fashion resembling typical micellar behavior. It has also been demonstrated that the hydrophobic character of the side chain is an important factor for some polyelectrolytes to exhibit micelle-like properties.2 Thus, the sodium salts of polyethylene-maleic acid-N-alkyl amides behave like micelles only when the alkyl chain contains more than six methylene groups, the shorter homologues demonstrating simple polyelectrolyte behavior.2 Although a comparison of the properties of polysoaps and micelles is interesting from the general point of view of interfacial chemistry, we are not aware of a truly comparativestudy involving closely related systems. In this article we report on comparative studies between ordinary micelles of sodium 10-undecenoate, referred to as “monomeric micelles” and micelles of poly(sodium 10undecenoate) referred to as “polymeric micelles”. We prefer the term polymeric micelle for the poly(sodium 10-undecenoate) instead of polysoap because polysoaps have been usually associated with polymers of rather high molecular weights (105-106 d a l t o n ~whereas )~ poly(sodium 10-undecenoate) has the modest number-average molecular (1) U. P. Strauss and N. L. Gershfeld, J.Phys. Chem., 58,747(1954). (2)C. P. Kurzendoerfer and M. J. Schwuger, “Surface Active Agenta Symposium”, 1979,p 147. (3)S. K. Sinha and A. I. Medalia, J. Am. Chem. SOC. 79,281 (1957).
weight of approximately 2000 daltons4; it is in a fact a low molecular weight polysoap with a short hydrocarbon backbone and long aliphatic side chains with terminal ionic headgroups (I). This terminology also agrees with the i -CHz-C
H--ilo
I (CHz
18
I COO-N:
I
name adopted by most authors for vesicles which have been stabilized by polymerization and which have been called polymeric or polymerized vesicle^.^ Moreover, the study of polymeric micelles that have been stabilized by covalent bonding and therefore the monomers do not exchange between the micellar and the bulk aqueous phases is of interest in view of the potential use of these systems as charge-separating media in photoredox reactions and solar energy converters.6
Experimental Section 10-Undecenoic acid (Ferak) was purified by two consecutive vacuum distillations whereas pyrene (Aldrich, 99%+) was extensively zone refined. Sodium 10-undecenoate was prepared7 by interacting equimolar quantities of the acid with sodium ethoxide in absolute ethanol. The precipitate was filtered, washed with ethanol, dried, redesolved in water, and treated with ether to remove possible traces of unreacted acid. The solution was lyophilized and the solid obtained was recrystallized from absolute ethanol and dried under vacuum over phosphorus pentoxide. The purity of the salt was confirmed by elemental analysis as well as by its ‘HNMR and IR spectra. Triply (4)C.E.Larrabee and E. D. Sprague, J. Polym. Sci., Polymn. Lett. Ed., 17,749 (1979).
(5)T. Kunitake, N.Nakashima, K. Takarabe, A. Tsuge, and H. Yanagi, J. Am. Chem. SOC.,103,5945(1981);P. Tundo, D.J. Kippenberger, P. L. Klahn, N. E. Pietro, T. C. Jao, and J. H. Fendler, Ibid. 104,456 (1982);H.H. Hub, B. Hupfer, H. Kock, and H. Ringdorf, Angew. Chem., Int., Ed. En&, 19,938(1980);S. L. Regen, B. Czech, and A. Singh, J. Am. Chem. SOC.,103,6638(1980). (6)M. Gratzel, Ber. Bumenges. Phys. Chem., 84, 981 (1980);M. Gratzel, Isr. J. Chem., 18,364 (1979);B.E.Horsey and D. G. Whitten, J. Am. Chem. SOC.100,1293 (1978). (7)R. Thundathil, J. 0. Stoffer, and 5. E. Friberg, J. Polym. Sci., Polym. Chem. Ed., 18,2629 (1980).
0022-385418312087-025 1$01.5010 0 1983 American Chemical Society
252
The Journal of Physical Chemistry, Vol. 87,
No. 2, 1983
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Flgure 1. Monomer to polymer conversion vs. y-ray dose.
distilled water having an electrical conductivity 2-4 X lo4 S cm-' was used throughout this work. Electrical con0 10 20 ductivity measurements were performed on a Metrhom C ( g/dm' ) Herisau E512 conductometer operating with a thermostated conductivity cell capable of regulating sample temFlgure 2. Electrical conductivity vs. surfactant concentration. The conductivity is expressed in S cm-' = Q-' cm-'. perature at 25 f 0.05 "C. Viscosities were measured at 25 f 0.05 "C with an Ubbelohede type viscometer calibrated I according to British Standard 188.'O Optical densities were determined with a Cary 210 spectrophotometer. Fluorescence spectra of thoroughly deoxygenated solutions were recorded on a fluorometer built of a Hilger and Watts D330 monochromator with automatic wavelength drive, in conjunction with a Hamamatsu R282 photomultiplier tube (S-5 extended). The excitation wavelength was set at 3126 A by means of an interference filter. The concentration of the probe fluorophorepyrene was kept as low as possible (M) and equal in all solutions to avoid erroneous results due to the reabsorption of the fluorescence peak at 3770 A. All monomeric and polymeric surfactant concentrations are expressed in g/dm3 to allow comparison of the results except for viscosity measurements where concentration is expressed in g/100 mL. WAVELENGTH ! A ! Polymerization was accomplished by irradiation with Figure 3. Fluorescence spectra of pyrene in (a) homogeneous and 3.5-6.7 Mrd of thoroughly degassed 0.1 M (20.6 g/dm3) (b) micellar media. aqueous solutions of sodium 10-undecenoate in oaC @ ' adjacent to the carbonyl group (2.08 6) which was not y-ray source providing a dose rate of 0.143 Mrd/h. The affected by the polymerization. polymer formed was separated from unreacted monomer by precipitation in ethanol. Infrared spectra of the preResults and Discussion cipitated solid confirmed the removal of all unreacted The main objectives of the present study were (i) to monomer by the total absence of the characteristic videtermine if the low molecular weight polysoap poly(s0brations of the double bond.8 The reported value of 2000 dium 10-undecenoate),forms micelles and (ii) to compare daltons for the number-average molecular weight4 was the micellar properties (aggregational behavior, additive found (Knauer vapor pressure osmometer) to be indesolubilization) of the monomeric and polymeric species. pendent of dose rate in the range 3.5-6.7 Mrd for solutions In Figure 2 the dependence of the electrical conductivity having a concentration of 0.1 M. This result is consistent k (S cm-') on the surfactant concentration is shown for the with the fact that polymerization of sodium 10-undecenoate occurs only at concentrations above its c ~ c , ~ monomeric and the polymer salts. Both solutions exhibit a breaking point in their corresponding curve, charactertherefore only surfactants associated with micelles undergo istic of typical micellar behavior. However, because the polymerization. Since the aggregation number of the change in the slope of the conductivity vs. concentration monomeric micelle-determined by least-squares fit of curve below and above the inflection point is not very osmotic coefficient data on the mass action model of mipronounced, particularly in the case of the monomer, we celle formation-is only smaller than any previously have examined the fluorescence spectra of pyrene solureported aggregation number, the degree of polymerization bilized in various concentrations of monomeric and polyalso should not exceed 10. The progress of the polymermeric surfactants. It was found that the fluorescence of ization was followed (in DzO) by 'H NMR spectroscopy pyrene dissolved in sodium 10-undecenoate aqueous soin a Varian XL-100 spectrometer operating in the FT lutions below the inflection point (8.24 g/dm3, 0.04 M) was mode and it is shown in Figure 1 as a function of y-ray identical with the fluorescence obtained from pyrene dose. Conversion percent was determined by the decrease dissolved in pure water (Figure 3a). Above the inflection of the peak intensities of the vinyl protons (4.5-6.1 6) as point, however, pyrene displayed very different spectra, compared to the total intensity of the methylene protons indicating solubilization of the fluorophore in micellar environment (Figure 3b). On the other hand, spectra ob(8) L. J. Bellamy in 'The Infrared Spectra of Complex Molecules", tained from pyrene dissolved in solutions of the polymeric Wiley, New York, 1966. (9) C. E. Larrabee, Diss. Abstr. Int. B , 41, 975 (1980). surfactant below and above the inflection point (2.06 g/
The Journal of Physical Chemisfry, Vol. 87, No. 2, 1983 253
Sodium 10-Undecenoate Micelles
dm3) exhibited micropolarity of the site of solubilization of the probe molecules intermediate between water and hexane (Figure 3), indicating micellar solubilization of pyrene below and above the inflection point. Similar results were found when the dye Sudan red was used as the solubilizate. It is concluded therefore that sodium 10undecenoate has indeed a cmc a t 8.24 g/dm3 whereas the polymeric surfactant forms micelles at all concentrations, each polymeric surfactant ion being a micelle by itself. Such behavior of polymeric surfactant has been observed with other systems,2J1and these micelles have been considered as intramolecular micelles2 with a cmc equal to zero.I1 At higher concentrations, however, intramolecular micelles can aggregate by hydrophobic forces to form large intermolecular micelles built of polymeric surfactant units. Apparently the formation of the intermolecular micelles is reflected in the inflection point of the conductivity vs. concentration curve of the polymeric surfactant occurring at 2.06 g/dm3 (Figure 2). In order to substantiate the conjecture concerning the hydrophobic nature of the intermolecular interactions leading to the formation of intermolecular micelles, we have studied the effect of urea on the inflection point of the conductivity vs. concentration curve of the polymeric surfactant. Should these interactions be of a hydrophobic nature the addition of a water structure breaker such as urea would shift the intermolecular cmc to higher surfactant concentration. Indeed, in the presence of 3 M purified urea the conductivity of the polymeric solution does not show a breaking point at 2.06 g/dm3, meaning that either the cmc has been shifted to much higher concentration, beyond the range of our measurements (20.6 g/dm3), or that the slope of the conductivity vs. concentration curve has changed so that the breaking point is not perceptible anymore. The latter explanation seems more reasonable since usually the addition of urea does not increase the cmd2 very much. In either case the aggregational behavior of the polymeric surfactant has been influenced by the presence of a water structure breaker indicating that the aggregating interactions are of a hydrophobic nature. To further c o n f m the formation of large intermolecular micelles by the aggregation of poly(sodium 10-undecenoate) surfactants at concentrations above 2.06 g/dm3 we have determined the intrinsic viscosity ( v ) of the solution of these aggregates. The reduced viscosities shown in Figure 4 were calculated by assuming as solvents solutions
having concentrations of 8.24 and 2.06 g/dm3 for the monomeric and the polymeric surfactants, respectively, instead of pure water. These results shown in Figure 4 clearly indicate that the monomeric and intermolecular polymeric micelles have very different hydrated sizes, the latter being considerably larger than the former. Independent viscosity studies,13however, have shown that the monomeric and the intramolecular polymeric micelles have indistinguishable intrinsic viscosities equal to 0.048( 100 mL/g). It is concluded therefore that, when sodium 10undecenoate undergoes micellar polymerization (surfactant concentration, 20.6 g/dm3; y-ray dose, 3.5-6.7 Mrd), poly(sodium 10-undecenoate) is produced which forms intramolecular micelles having zero cmc and a hydrated size equal to the size of the monomeric micelles whereas above the polymeric surfactant concentration of 2.06 g/dm3 large intermolecular micelles are formed. To compare the fashion in which monomeric and polymeric sodium 10-undecenoate micelles solubilize hydrophobic molecules we have examined the vibrational structure of the fluorescence of pyrene solubilized in the two micelles. Pyrene is a well-known probe for the micropolarity of the interior of micelles14 where it is solubilized due to its substantial hydrophobic character. Although there exists some ambiguity concerning the precise solubilization site of pyrene in a micellar system,14 the fluorescence spectrum of pyrene, and in particular the intensity ratio II/I3of the first to the third emission peak, has been widely employed as a measure of the polarity of the microenvironmentof the fluorophore.'* In view of our previous results, however, concerning the formation of large intermolecular micelles at polymer concentrations above 2.06 g/dm3 it is apparent that comparative studies are meaningful only between monomeric and intramolecular polymeric micelles which have identical size and shape, the only difference between the two systems being the polymerization-induced covalent bonds in the case of the polymeric micelles. In Figure 3b the fluorescence spectra of 10" M pyrene solubilized in 12.36 g/dm3 monomer and 1.65 g/dm3 polymer aqueous surfactant solutions are shown. The ratio of the 11/13 peak intensities is 1.19 f 0.03 for the monomeric and 1.32 f 0.03 for the intermolecular polymeric micelle. This definite difference in the micropolarity sensed by the probe molecule in the two micelles could be attributed either to differences in water penetration, more penetration in the case of the polymeric micelle, or to the fact that pyrene solubilizes at different sites in the two systems. If pyrene resides closer to the microinterface in the polymeric micelle than in the monomeric it will encounter more water and therefore its fluorescence spectrum will demostrate higher polarity. The first explanation, however, about different water penetration in the two micellar cores, is not supported by viscosity studied3 which have concluded that the monomeric and intramolecular polymeric micelles have indistinguishable hydrated sizes and therefore an equal number of water molecules are associated with each micelle (approximately 15 H 2 0 molecules per monomer subunitg). The alternative assumption that the probe molecules penetrate the core of the polymeric micelle less than that of the monomeric micelle seems reasonable also from the point of view of the expected more compact packing of the aliphatic chains in the polymerized system than in the nonpolymerized. The covalent bonding among the un-
(10)British Standards B.S. 188 (1977). (11)J. C.Salamone, S. C. Israel, P. Taylor, and B. Snider, J. Polym. Sci., 45, 65 (1974). (12)W.Bruning and A. Holtzer,J.Am. Chem. SOC.,83,4865 (1961).
(13)E.D.Sprague, D. C. Duecker, and C. E. Larrabee, J . Am. Chem. SOC.,103, 6797 (1981). (14)K. Kalyanasundaram and J. K. Thomas, J . Am. Chem. SOC.,99, 2037 (1977).
.........o
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0
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0)
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0
0.5
1
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Figure 4. Reduced viscosity vs. surfactant concentration. The concentration is expressed in C- ,C = actual concentration of the salt - critical micelle concentration of the monomeric OT polymeric micelle.
J. Phys. Chem. 1983, 87, 254-260
254
decenoate chains in the polymeric micelle forces their end methylene groups close to each other a t a distance equal to the length of the C-C bond (1.54 A) compared to a distance of approximately 4 A (twice the effective van der Waals radius of CH,) which separates the corresponding =CH2 groups in the monomeric micelle. The same polar microenvironment was also sensed by pyrene solubilized in poly(sodium 10-undecenoate) solutions above the intermolecular cmc (2.06 g/dm3) of the polymeric surfactant indicating that essentially the site of solubilization is not affected by the aggregation of the intermolecular micelles. Similar conclusions were drawn from measurements of the nitrogen hyperfine splitting constants of two nitroxide spin labels solubilized in these micelles showing that both probes penetrate deeper inside the monomeric than the polymeric mi~e1le.l~
Conclusions When aqueous solutions of sodium 10-undecenoate are
irradiated by y rays a t concentrations above the cmc of the surfactant, poly(sodium 10-undecenoate) is formed having a degree of polymerization equal to the aggregation number of the monomeric micelles. Poly(sodium 10-undecenoate) is an intramolecular micelle having a cmc equal to zero and a hydrated size equal to the size of the monomeric micelle. At polymeric surfactant concentrations above 2.06 g/dm3, however, poly(sodium 10-undecenoate) ions aggregate under the influence of hydrophobic interactions to form large intermolecular micelles. Although hydrophobic molecules are solubilized by polymeric micelles (intra and intermolecular) they cannot penetrate the micellar core as much as they penetrate the core of the corresponding monomeric (nonpolymerized) micelles, apparently because of the more compact packing of the undecenoate chains of poly(sodium 10-undecenoate) due to polymeric covalent bonding. Registry No. Sodium 10-undecenoate,3398-33-2.
Temperature Dependence of the Fluorescence Lifetime of Benzene in Cryogenic Solutionst F. LI, J. Lee, and E. R. Bernstein' Department of Chemlstry, Colorado State University, Fort Collins, Colorado 80523 (Received: June 4, 1982; In Final Form: September 7, 1982)
Fluorescence lifetimes for CJ& in various hydrocarbon solvents have been measured as a function of concentration and temperature. For the solvents ethylene, ethane, and propane it is found that at low concentrations (-0.3 ppm or -4 X lo4 mol/L) the fluorescence lifetime is roughly temperature independent (90 IT I220 K) and equal to the gas-phase value of -150 ns. As the concentration is increased (1 I c I100 ppm), the lifetime decreases as temperature is increased, reaching a minimum value at about 150 K of roughly 70-100 ns, depending on the concentration. As the temperature is increased from 150 to 220 K, the lifetime increases to well over 100 ns. These trends can be understood on the basis of a monomer/excimer kinetic model in which benzene excimers form at low temperature and break apart at high temperature to regenerate the excited-state and ground-state monomers. In propene and 1-butene solvents, such behavior is not observed most likely due to solvent triplet-state quenching of the excited lBzubenzene monomer.
probe system because of its high symmetry, well-known Introduction spectra and kinetics, and solubility. The study of molecular electronic spectra of solute In the present study, fluorescence lifetimes of the first molecules in cryogenic molecular liquids has begun to draw excited singlet state of benzene are reported in several increasing attention in recent years.' Fluorescent and cryogenic liquids. The solvents employed in these studies phosphorescent probe molecules can provide information are methane, ethane, ethylene, propane, propene, and on local structure and microdynamics of chemical systems 1-butene. The experimental results are explained on the in liquid solution.2 Moreover, even at low concentrations basis of a monomer/dimer (excimer)kinetic model. At low (1 ppm or -1.5 X mol/L) absorption and emission temperature the ''cagen structure of the liquid provides an spectra are intense enough to provide good signal-tenoise effective mechanism for the formation of an excimer once detection ratios. Cryogenic liquids reduce the spectral congestion associated with hot bands and quite often solvent effects induce forbidden transitions by lowering (1) (a) G. Ya. Zelikina and T. G. Meister, Opt. Spectrosc. (Engl. the effective solute symmetry. Benzene, as a solvent probe Transl.),43,46 (1977); (b) J. W.Eastman and S. J. Rehfeld, J. Phys. in several cryogenic liquids, has proved helpful in studying Chem., 74,1438(1970);(c) W.H.Beattie, W. B. Maier, 11, R. F. Holland, solvent-solute interactions and liquid-state ~ t r u c t u r e . ~ ? ~S. M. Freund, and B. Stewart, SPIE Laser Spectrosc., 158,113 (1978); (d) S.M. Freund, W. B. Maier, 11, R. F. Holland, and W. H. Beattie, Anal. Cryogenic small-molecule liquids are employed in this Chem., KO, 1260 (1978). research because they are simple but still molecular, they (2)G.W.Robinson, R. A. Auerbach, and J. A. Synowiec, Chem. Phys. Lett., 82, 219 (1981). are good solvents for organic molecules, they possess a wide (3) (a) E. R. Bemtein and J. Lee, J. Chem. Phys., 74,3159(1981);(b) liquid range, and their low temperature allows for sharp M. W.Schauer, J. Lee, and E. R. Bernstein, ibid., 76,2773 (1982); (c) spectroscopic probe feature^.^" Benzene is a reasonable J. Lee and E. R. Bernstein, ibid., submitted. 'Supported in part by the
ONR and NSF.
(4)F. Li, J. Lee, and E. R. Bernstein, unpublished results.
( 5 ) F. Li, J. Lee, and E. R. Bernstein, J.Phys. Chem., 86, 3606 (1982).
0 1983 American Chemical Society
