Pluronic-P105 PEO-PPO-PEO Block Copolymer in Aqueous Urea

Apr 10, 1995 - (cmc) andtemperature (cmt) values for the amphiphilic block copolymer dissolved in ...... lization. The 13C chemical shift measurements...
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Langmuir 1995,11, 2442-2450

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Pluronic-P1O5 PEO-PPO-PEOBlock Copolymer in Aqueous Urea Solutions: Micelle Formation, Structure, and Microenvironment Paschalis Alexandridis,*lt Vassiliki Athanassiou, and T. Alan Hatton" Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 66-307, Cambridge, Massachusetts 02139 Received September 15, 1994. I n Final Form: April 10, 1995@ The effects of urea on the micellization properties of a poly(ethy1ene oxide)-block-poly(propy1eneoxide)block-poly(ethy1ene oxide) (PEO-PPO-PEO)copolymer (commercially available as Pluronic P105) and on the structure and microenvironment of the micelles are reported. Critical micellization concentration (cmc)and temperature (cmt)values for the amphiphilic block copolymer dissolved in uredwater mixtures (urea concentration 0, 1, 2, and 4 M) were obtained using a dye solubilization method and corroborated with data from surface tension, density, and fluorescence spectroscopy experiments. Urea increased the cmc and cmt of the PEO-PPO-PEO copolymer; the effect of urea on the cmt was more pronounced at low copolymer concentrations and diminished at concentrations of -2.5%. The thermodynamic parameters of micelle formation in the presence of urea were estimated using a closed association model; the enthalpy of micellization was positive (endothermic) and decreased upon increasing the urea concentration. The surface activity and the partial specific volume of the block copolymer decreased with an increase in the urea concentration, whereas the hydrodynamic radii of the copolymer micelles, determined using dynamic light scattering, remained unaffected by the presence of 4 M urea in the solution. The micropolarity in copolymer solutions in uredwater was probed as a function of temperature using the 11/13intensity ratio of the pyrene vibrational fine structure recorded in fluorescence emission spectra; a small decrease in the micropolarity of the micelle core was observed in the presence of urea. The microviscosityin the micelle interior, estimated from the intramolecular excimer fluorescence of the hydrophobic probe bis(1pyrenylmethyl) ether (dipyme),also exhibited a small decrease with an increase in the urea concentration. The findings presented here are discussed in the context of the molecular mechanism underlying the effects of urea.

Introduction

heating has been shown to induce micelle formation in aqueous solutions of a number of these amphiphilic block Water-soluble poly(ethy1ene oxidel-block-poly(propy1ene copolymers, with the micelle core dominated by the oxidel-block-poly(ethy1ene oxide) (PEO-PPO-PEO) cohydrophobic PPO and the corona composed of hydrated polymers are nonionic macromolecular surface active PEO segments. agents that are commercially available as Pluronic or The addition of electrolytes, such as simple salts, or Synperonic polyols. Variation of the copolymer molecular nonelectrolytes, such as urea, is a common method for weight and composition during the synthesis allows the altering the solvent properties of water. The aggregational production of molecules with hydrophobidhydrophilic and surface properties of surfactants in solutions are very properties that meet the specific requirements of various sensitive to the presence of such c o ~ o l u t e s . ~The ~ J strong ~ applications, such as detergency, dispersion stabilization, effect of added salts on the aggregation behavior ofPluronic foaming, emulsification, lubrication, etc.ls2 The hydroF68, P85, and L64 PEO-PPO-PEO copolymers in aqueous phobichydrophilic character of the PEO-PPO-PEO cosolution, for instance, has been described by Bahadur and polymers in aqueous solutions can also be altered by c o - w o r k e r ~ , ~ who ~ - ~ ' showed that KCNS increased the varying the solution temperature or modifying the propcloud point in both L64 and P85 solutions, whereas erties of the aqueous solvent. The effects of temperature addition of KBr, KC1, and KJ? decreased the cloud point. on the properties and structure of PEO-PPO-PEO copolymer solutions have been studied e ~ t e n s i v e l y ; ~ - l ~ The onset of micelle formation was shifted in the same direction as the cloud point by the salts, but to a lesser degree.l6 The cloud point of Pluronic F68 was reduced by * Authors to whom correspondence should be addressed. 50 "C when 1.0M KF was added to the aqueous s01ution.l~ ' Present address: Physical Chemistry 1, Chemical Center, University of Lund, P.O. Box 124, Lund S 22100, Sweden. Urea [(NH2)2COI, a well-known protein denaturant,has Abstract published in Advance ACS Abstracts, J u n e 15,1995. been found to be a n efficient modifier of the porperties of (1) PZuronic and Tetronic Surfactants. Technical Brochure, BASF aqueous solutions and, in particular, of aqueous micellar Corp., Parsippany, NJ, 1989. (2)Schmolka, I. R.J.Am. Chem. Oil SOC.1977,54,110. solutions (see, e.g., Kresheck13 and references cited @

(3) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988,126,171. (4)Wanka, G.; Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 266,101. (5) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27,4145. (6) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995,273, 2. (7) Malmsten, M.; Lindman, B. Macromolecules 1992,25,5440. (8) Mortensen, K.; Pedersen, J . S. Macromolecules 1993,26, 805. (9) Alexandridis, P.; Hatton, T. A. InDynamicProperties ofznterfaces and Association Structures; Shah, D. O., Ed.; AOCS Press: Champaign, IL, 1995. (10)Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994,27,2414. (11)Alexandridis, P.; Hatton, T. A. Colloids Surf. A 1995,96, 1.

0743-746319512411-2442$09.00/0

(12)Alexandridis, P.; Nivaggioli, T.; Hatton, T. A. Langmuir 1995, 11, 1468. (13)Kresheck, G.C. In Water-A Comprehensive Treatise, Vol 4: Aqueous Solutions of Amphiphiles and Macromolecules; Franks, F., Ed.; Plenum Press: New York, 1975. (14)Schick, M. J., Ed. Non-ionic Surfactants, Physical Chemistry; Marcel Dekker: New York, 1987. (15) Bahadur, P.; Li, P.; Almgren, M.; Brown, W. Langmuir 1992,8, 1903.

0 1995 American Chemical Society

Langmuir, Vol. 11, No. 7, 1995 2443

PEO-PPO-PEO Copolymer in Aqueous Urea Solutions

Monte Carlo study by Hernandez-Coboset al.42suggested therein). Addition of urea increased the critical micellization concentration (cmc)of nonionic18-21 and i o n i ~ ~ ~ p that ~ ~ ,urea ~ ~ dimerization does occur at concentrations surfactants, decreased the hydrodynamic radius of nongreater than -5%, a result corroborated by molecular ionic20 and i o n i ~micelles, ~ ~ , ~ increased ~ the aggregation dynamics simulations of 2 M aqueous urea solutions number of fluorine-labelednonionic surfactant micelles,lg presented by Astrand et a1.43 In contrast, the Ramanand raised the cloud-point temperatures of nonionic spectroscopicresults of Hoccart and TurelP do not indicate surfactant solutions.20,21r26 The increase in the cmc was the presence of a significant concentration of urea dimers, generally explained in terms of the enhanced solubility of but rather the hydration of urea via the formation of the surfactant hydrophobic moiety in the presence of hydrogen bonds with water molecules. Dempsey and urea.20r22,23,27 Often, changes in the thermodynamic M01yneux~~ found the experimentally observed increases parameters of micellization have been attributed to the in aqueous solubility of 4-hydroxybenzoicacid and its alkyl stabilization of the monomers in aqueous urea solutions, esters in the presence of urea to be consistent with 1:l but the interactions of the micelles with urea in the association with urea. The FT-IR spectra of a valeric acidpostmicellar solutionsare also significant.28Besides these, urea-water system measured by Fukushima and He46 other possibilities such as specific urea-surfactant insupported the interpretation that the increase in the teractions and binding have also been suggested.29 A solubility of valeric acid in water by the addition of urea fundamental understanding of these effects remains a is due to the existence of an inclusion structure. Electronchallenging unsolved problem. This is illustrated in the spin echo,36resonance spectro~copy,~~ and fluorescence following quote by Calvaruso et “In the light of the s p e c t r o ~ c o p ystudies ~ ~ ~ ~intimated ~ that urea replaces some above findings and considerations we can conclude that water molecules in the hydration shell around the solute. a [kinetic] study such as that performed in the present These findings appear to support the direct mechanism, work can be considered as an indirect means of demonbut it is still not clear which type of interaction is strating the significant action of urea in modifying the responsiblefor the behavior of surfactants in aqueous urea properties of [the SDS] micellar solutions, even though solutions. the specific mechanism cannot be assessed.’’ In the context of a detailed t h e o r e t i ~ a and l ~ ~experi~~~ Two different mechanisms have been proposed to mental study on the m i c e l l i z a t i ~ n ~ and ~ ~ solubili~ ~~~-~~ explain the action of urea in aqueous solutions: (i) an z a t i ~ n in ~ ~ aqueous , ~ ~ poly(ethy1ene oxidel-block-polyindirect mechanism, according to which urea acts as a (propylene oxide)-block-poly(ethy1ene oxide) copolymer “structure-breaker’’ and thus facilitates the hydration of solutions, we investigated the effects of urea on the nonpolar solute^,^^,^^ and (ii)a direct mechanism, whereby micellization properties of a representative PEO-PPOurea has almost no effect on the water structure, replaces PEO copolymer (Pluronic P105) and the structure and some of the water molecules in the hydration shell of the microenvironment of the micelles formed. To the best of solute, and self- associate^.^^-^^ The indirect mechanism our knowledge, no other information on PEO-PPO-PEO has received the most attention and is widely accepted;13 copolymersin uredwater solutionshas yet been published. many experimental results seem to support the idea that Cmt (critical micellization temperature) and cmc data the addition of urea to water does destroy the solvent structure (e.g., see references cited by Baglioni et a1.36,37). obtainedfrom a dye solubilizationmethod are corroborated with surface tension, density, and fluorescence spectrosNevertheless, the experimental techniques used in most copy results. A closed association model is used to obtain studies do not provide information at a molecular level. the standard free energies (AGO), enthalpies (AH’), and Early Monte Carlo and molecular dynamic studies on entropies (AS’’) of micellization. The hydrodynamic radii aqueous solutions of urea found no evidence of water of the copolymer micelles and their size distribution are structure breaking but self-association of urea; this was, determined as a function of temperature using dynamic however, contested by other works that predicted no light scattering, while the micropolarityand microviscosity association of urea in aqueous solution^.^^-^^ A recent ~~

(18)Shick, M. J. J . Phys. Chem. 1964,68, 3585. (19) Muller, N.; Platko, F. E. J . Phys. Chem. 1971,75,547. (20) Brieanti, G.: Puwada.. S.:. Blankschtein, D. J . Phys. Chem. 1991, 95,8989. (21) Ruiz, C. C.; Sanchez, F. G. J . Colloid Interface Sci. 1994,165, 110. (22) Abu-Hamdiyyah, M.; Al-Mansour, L. J . Phys. Chem. 1979,83, 2236. (23) Das Gupta, P. K.; Moulik, S. P. Colloid Polym. Sci. 1989,267, 246. (24) Mazer, N. A.; Carey, M. C.; Kwasnick, R. F.; Benedek, G. B. Biochemistry 1979,18,3064. (25) Caponetti, E.; Causi, S.; De Lisi, R.; Floriano, M. A,; Milioto, S.; Triolo, R. J . Phys. Chem. 1992,96,4950. (26) Han, S. K.; Lee, S. M.; Kim, M.; Schott, H. J . Colloid Interface Sci. 1989,132,444. (27) Wetlaufer, D. B.; Malik, S. K.; Stoller, L.; Coffin, R. L. J . Am. Chem. SOC.1964,86,508. (28) Jha, R.; Ahluwalia, J.C. J . Chem. Soc., Faraday Trans. 1993, 89,3465. (29) Singh, P. K.; Ahluwalia, J. C. J.Surf. Sci. Technol. 1986,2,51. (30) Calvaruso, G.; Cavasino, F. P.; Sbriziolo, C.; Turco Liveri, M. L. J . Chem. SOC.,Faraday Trans. 1993,89,1373. (31)Frank, H. S.; Franks, F. J . Chem. Phys. 1968,48, 4746. (32) Nozaki, Y.; Tanford, C. J . Biol. Chem. 1964,238,4074. (33) Stokes, R. H. Aust. J . Chem. 1967,20, 2087. (34) Roseman, M.; Jencks, W. P. J . A m . Chem. SOC.1976,97,631. (35) Muller, N. J . Phys. Chem. 1990,94,3856. (36) Baglioni, P.; Ferroni, E.; Kevan, L. J . Phys. Chem. 1990,94, 4296. (37) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J . Phys. Chem. 1990,94,8218. I

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(38)Kuharski, R. A.; Rossky, P. J. J. Am. Chem. SOC.1984,106, 5786. (39)Kuharski, R. A,; Rossky, P. J. J . A m . Chem. SOC.1984,106, 5794. (40) Tanaka, H.; Nakanishi, K.; Touhara, H. J . Chem. Phys. 1986, 82,5184. (41) Boek, E. S.; Briels, W. J. J . Chem. Phys. 1993,98,1422. (42) Hernandez-Cobs, J.;Ortega-Blake, I.;Bonila-Marin, M.; MorenoBello, M. J . Chem. Phys. 1993,99,9122. (43) k t r a n d , P.-0.;Wallqvist, A,;Karlstrom, G. J . Phys. Chem. 1994, 98,8224. (44) Hoccart, X.;Turell, G. J . Chem. Phys. 1993,99,8498. (45) Dempsey, G.; Molyneux, P. J . Chem. Soc., Faraday Trans. 1992, 88,971. (46) Fukushima, K.; He, H. C. Bull. Chem. SOC.Jpn. 1993,66,1820. (47) Sarkar, N.; Bhattacharyya, K. Chem. Phys. Lett. 1991,180,283. (48) Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. Macromolecules 1993,26,5030. (49) Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. Macromolecules 1993,26,5592. (50) Alexandridis, P.; Athanassiou, V.; Fukuda, S.; Hatton, T. A. Langmuir 1994,10,2604. (51)Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. J.Am. Oil Chem. SOC.1996,72,in press. (52) Nivaggioli, T.; Alexandridis, P.; Hatton, T. A.; Yekta, A.; Winnik, M. A. Langmuir 1996,11,730. (53) Nivaggioli, T.;Tsao, B.; Alexandridis, P.; Hatton,T. A. Langmuir 1996,11, 119. (54) Hurter, P. N.; Hatton, T. A. Langmuir 1992,8, 1291. ( 5 5 ) Hurter, P. N.; Alexandridis, P.; Hatton, T. A. In Solubilization in Surfactant Aggregates; Christian, S. D., Scamehorn, J. F., Eds.; Marcel1 Dekker: New York, 1995; Chapter 6.

Alerandridis et al.

2444 Langmuir,~Vol. 11, No. 7, 1995 afforded by the micelles are probed using fluorescence spectroscopy. Materials and Methods Materials. The Pluronic P105 PEO-PPO-PEOblock copolymer was obtained as a gift from BASF Corp., Parsippany, NJ, and used as received (lot No. WPDI-522A). P105 has a nominal molecular weight of 6500 and 50 wt % PEOl and can be thus represented by the chemical formula EO~~POFAEO~~. Urea (CA 57-13-6)was purchased from Mallinckodt, Paris, KY, and used (DPH) as received(GenARgrade). 1,6-Diphenyl-1,3,5-hexatriene (CA1720-32-7)was obtained from MolecularProbesInc.,Eugene, OR, and used as received. Benzordeflphenanthrene(pyrene)(CA 129-00-0)was purchased from Sigma Chemical Co., St. Louis, MO, and was recrystallized three times from ethanol. Bis(1pyrenylmethyl) ether (dipyme) was ~ynthesized~~ following a procedure similar to the one reported by Georgescauld et al.56 Density. A calculating digial density meter DMA 45 (Anton Paar, Graz, Austria) was employed to measure the densities of aqueouscopolymer solutions;the density is determinedby placing the sample in a U-shaped tube and measuring the period of oscillations of this tube electronica1ly.l2 The instrument was calibrated by measuring the density of water and air (over the temperature range of interest) for which precise density values are available in the literature.12 The densities of aqueous urea solutions were in good agreement with the values reported by Guckeret al.57Temperature controlwas achieved using a Neslab RTE-110 refrigerated batldcirculator. SurfaceTension. The Wilhelmyplate method was employed for measuring the surface tension of the copolymer solutions. A Kriiss KlOTtensiometer (KriissUSA,Charlotte, NC)was used.50 The temperature was maintained at 25 “Cusing a Neslab RTE110 refrigerated batldcirculator. All glassware was washed in a 1 N NaOH-ethanol bath and then in a nitric acid bath, and thoroughly rinsed with Milli-Qwater before use. The platinum Wilhelmy plate was washed using acetone, rinsed in Milli-Q water, and flamed until red hot before each mea~urement.~~ Light Scattering. Dynamic light scattering measurements were performed at a scattering angle of 90”using a Brookhaven Model BI-2OOSM instrument (Brookhaven Instrument Corp., Holtsville, NY).The light source was a Model 95 Lexel8 W CW argon ion laser (Lexel Laser, Inc., Fremont, CA) operating at 1 = 514nm. The signal analysis was perfonnedusing a BI-9000AT Digital Correlator.12 The dynamic light scattering data were interpreted by using a cumulants analysis. The mean micellar hydrodynamicradius was calculated,in the context ofthe StokesEinstein relation, from the first cumulant by using available5* viscosity data for uredwater solutions. Spectroscopy. UV-visible absorption spectra of the copolymer/DPWwater samples were recorded in the 300-500 nm range using a Perkin-Elmer Lambda 3B UV-vis spectrophotometer (Perkin-Elmer Corp., Nonvalk, CT). The main absorption intensity peak, characteristic of DPH solubilizedin a hydrophobic environment, was at 356 nm.l0 Fluorescence spectra were recorded on a SPEX FluoroMax spectrofluorometer (SPEX Industries,Inc., Edison, NJ) using a 0.5 nm bandpass in the “s/r” mode (to correct for variations in lamp intensity). Wavelengths ofexcitation(Aex)were chosen accordingto the maximumintensity obtained in the excitation spectra; depending on the solution, Lex x 335 nm and A,, 372 nm. All samples were aerated, magnetically stirred, temperature controlled using a thennostated cuvette holder connected to a circulating water bath, and examined at right-angle g e ~ m e t r yThe . ~ pyrene ~ ~ ~ ~concentration was 3 x 10-7Min all samples;no excimer formationwas observed, even in the micellar solutions.

Results and Discussion 1. Micellization Phase Diagram. The solubilization of DPH, a hydrophobic molecule that is sparingly soluble (56) Georgescauld, D.; Desmasez, J. P.; Lapouyade, R.; Babeau, A.; Richard, H.; Winnik, M. Photochem. Photobiol. 1980,31, 539. (57) Gucker, F. T., Jr.; Gage, F. W.; Moser, C. E. J.Am. Chem. SOC. 1938,60,2582. (58) Venkatesan, V. K.; Suryanaryana, C. V. J.Phys. Chem. 1966, 60,775.

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Urea concentration (M) Figure 1. Effect of urea on the cmc of Pluronic P105 PEOPPO-PEO copolymer solutions. 45

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Concentration (“10) Figure 2. Micellization boundary for Pluronic P105 dissolved in uredwater solutions. At temperatures below the micellization boundary, the copolymers do not associate; micelles coexist in equilibrium with unimers above the micellization boundary. Copolymer

in water but partitions preferentially in micelles (where it has a characteristic UV-visible spectrum significantly different than that in water), was employed to detect the formation (i.e., the cmc and cmt values) of the copolymer micelles.1° The cmc values of the Pluronic P105 PEOPPO-PEO copolymer at a given temperature generally increased with increasing urea concentration as seen in Figure 1. The cmt values for P105 solutions in the presence of urea were higher than those for P105 dissolved in plain water (for a given copolymer concentration), intimating that the micelle formation becomes more difficult upon increasing urea concentration. The cmt data are plotted as a function of copolymer concentration in Figure 2 in the form of a micellization “phase diagram” for the Pluronic P105 solutions in uredwater. At temperatures below the micellization boundary the copolymers exist as individual molecules in solution (unimers), whereas micelles coexist in equilibrium with unimers above the micellization boundary. The effect of urea on the CMT was more pronounced at low copolymer concentrations and diminished at concentrations of -2.5%. In fact, it appears that the cmc vs temperature curves intersect at -20 “C; this behavior is similar to the intersection of the solubility curves of methane, ethane, and butane in water and uredwater solutions at a given temperature (different for each solute) reported by Wetlaufer et al.27and is related to the opposing contributions of enthalpy and entropy to the formation of micelles (see section 2 for discussion on the thermodynamics of micelle formation). As for the Pluronic P105 PEO-PPO-PEO copolymer, urea has been shown to increase the cmc of nonionic surfactants such as poly(oxyethylene)alkanols,ls8,8,8trifluorooctyl hexaoxyethylene glycol monoether,lg ndodecyl hexaethylene oxide (C12EOs),20and Triton-100.21

PEO-PPO-PEO Copolymer in Aqueous Urea Solutions

Langmuir, Vol. 11, No. 7, 1995 2445

The ratio of cmc in the presence of urea (cmcu) to cmc in AGO = RT ln(Xcmc) (1) plain water was on the average 2 for the poly(oxyethy1ene)alkanols (EO = 7, 30; C = 12, 16) studied by Schick;ls where R is the gas law constant, T is the absolute cmcvlcmc decreased by 10-20% upon increase of temtemperature, and X,,, is the critical micellization conperature from 10 to 45 "C. Note, that the opposite trend centration, expressed in mole fraction units, at temperis observed in the experiments presented here (see Figure ature T. AGO is expressed in terms of the standard 1); cmcukmc for the Pluronic P105 solutions was apenthalpy of micellization, AHO, and the standard entropy proximately 2.5 at 25 "C and increased to -8 at 35 "C (4 of micellization per mole of surfactant, AS", as61 M urea). Briganti et a1.20found the cmc of C12EO6 to increase from (mole fraction units) at zero urea AGO = AlP - TAS" (2) concentration to -3.5 x at 4 M urea concentration (25 "C). The cmc of Triton-100 at 25 "C increased from The micellization free energy values, AG", are negative, 0.35 mM at zero urea concentration to -1 mM at 4 M urea since thermodynamically stable micelles are formed concentration.21 spontaneously; furthermore, AG"s become more negative The increase of cmc in nonionic surfactants caused by at higher temperatures, indicating a larger driving force urea was attributed by Schickls to increased hydration of for micellization.1° The standard enthalpy of micellization, the PEO segments caused by a reduction of the cooperative AH", can be calculated from the intercept of a linear fit structure of water. Han et aLZ6interpreted changes in (assuming that AH" is independent of temperature) of the cloud point to originate predominantly from the the AGO vs T data, in accordance with eq 2. AH" is positive, interaction of urea with the hydrated PEO corona of the indicating that the transfer of copolymer molecules from nonionic micelles, and suggested that the hydrocarbon solution to the micelle is an enthalpically-disfavored core was not involved. On the other hand, Briganti et endothermic process. Thus, a positive entropy contribua1.20concluded (in the context of a molecular model of tion must be the driving force for micellization of the PEOmicellization) that the dominant factor in determining PPO-PEO block copolymer (i.e., the aggregation of cothe cmc in CLEO,nonionic surfactants was the enhanced polymer molecules will lead to an increase of the overall solubility of the hydrophobic tail in urea-water solutions. solution entropy). Addition of urea to the P105 PEOThe significance of the enhanced solubility of the hydroPPO-PEO copolymersolution alters the magnitude of AH" phobic moiety in waterlurea is also supported by the findings of Wetlaufer et al.27and Das Gupta and M o ~ l i k . ~ ~but does not affect the fundamental mechanism (entropic driving force) responsible for the micellization. The differences between poly(propy1ene oxide), the hyThe enthalpy of micellization was lower in the presence drophobic part ofthe PEO-PPO-PEO copolymers, and the of urea and decreased further upon increase of the urea methyl groups should be kept in mind when comparing concentration, suggestingthat urea reduced the enthalpic these copolymers to CiEOj surfactants. The contribution barrier to micellization. AH" values for Pluronic P105 of one propylene oxide segment to the free energy of copolymer in uredwater solutions are plotted in Figure micellization (see section 2 for definitions) is 0.2-0.3 3 as a function ofurea concentration; AH"was 330 kJ/mol kT,495959 4-6 times smaller than the contribution of one in water and dropped to 230 kJ/mol in the presence of 4 methylene group (1.2 kr),4 reflecting the higher solubility M urea. The micellization entropy values for Pluronic (and specific interactions) of PPO in water. PPO can be P105 copolymer in uredwater solutions also decreased affected by urea through both the indirect (breaking the from 1.21kJmol-l-K-l a t zero urea concentration to 0.85 water "structure") and the direct (replacement of water kJ-mol-l-K-l at 4 M urea. The dependence of the micelmolecules and hydrogen bonding to urea molecules) lization enthalpy on the polymer molecular weight and mechanism, whereas the methylene groups involved in composition was examined for a number of PEO-PPOthe study of Briganti et a1.20 cannot interact through PEO copolymersin ref 10. Two main groups of copolymers hydrogen bonds. The temperature dependence of cmcu/ were identified:1° the relatively hydrophobic Pluronics cmc observed for the PEO-PPO-PEO copolymer, different P103, P104, P105, and P123 with AH" in the 300-350 from that of the poly(oxyethylene)alkanols,may indeed kJ/mol range and the relatively hydrophilic L64, P65, P84, indicate the additional mode of PPO interactions with and P85 with AH" in the 180-230 kJ/mol range. Urea urea. Note that the PPO block has been found responsible apparently shifts the hydrophobichydrophilic nature of for the micellization of the PEO-PPO-PEOcopolymers. Pluronic P105 and places the copolymer in the same group Indeed the micellization could be looked upon as a with e.g., P85, a PEO-PPO-PEO copolymer that has the microphase separation for PPO; the cloud point of a PPO same PPOPEO compositionas P105 but a lower molecular homopolymer in water has been shown to correspond weight (4600). roughly to the micellization onset of a PEO-PPO-PEO The decrease in the micellization enthalpy values in copolymer (PluronicP94) with PPO block size comparable aqueous urea solutions observed for Pluronic P105 is in to that of the PPO homopolymer.60 agreement with the calorimetric data for Pluronic L64 2. Thermodynamics of Micellization. The tem(E013P030E013)copolymer,@decylamine oxide,63CloEOj perature dependence of the cmc in surfactant solutions surfactants,28and other compounds cited in the review by can be analyzed to provide information on the thermoK r e ~ h e c k .Such ~ ~ a decrease in AH"can also be inferred dynamic parameters of micellization.61 The standard free from the temperature dependence of cmc data reported energy change for the transfet of 1 mol of amphiphile by Schickls for C10E07, CIOEO~O, and C16E030in 3 and 6 from solution to the micellar phase (free energy of M aqueous urea solutions. The results of J h a and micellization), AG", assuming an equilibrium between Ahluwalia2son the solution enthalpies in uredwater for unimer and micelles, is given by1OS6l CloEO, surfactants in both unimeric and micellar states indicated a decrease in hydration or structure around (59) Alexandridis,P. Thermodynamics and Dynamics of Micellization unimers in the pre-cmc region and a relative increase in and Micelle-Solute Interactions in Block-Copolymer and Reverse Micellar Systems. Ph.D. Thesis, Massachusetts Institute of Technolhydration of micelles due to enhanced hydrogen bonding ogy: Cambridge, MA, 1994. between ethylene oxide (EO)segments and urea molecules (60)Hvidt, S.; Jgrgensen, E. B.; Brown, W.; Schillen, K. J. Phys. Chem. 1994,98, 12320. (61) Hunter, R.J.Foundations of Colloid Science; Oxford University Press: New York, 1987; Vol. 1.

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(62) Alexandridis, P.; Holzwarth, J. F., manuscript in preparation. (63) Benjamin, L. J. Colloid Interface Sci. 1966,22,386.

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2446 Langmuir, Vol. 11, No. 7, 1995 350 330

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Figure 3. Enthalpy (AH") of micelle formation for Pluronic P105 in uredwater solutions, as a function of urea concentra-

tion.

in the post-cmc region. The enthalpy of transfer of CiEOj unimers from water to aqueous urea (AHtr,b)was positive and decreased with an increase in the number of EO segments; AHtr+,,i also decreased with an increase in urea concentration, reflecting a greater exothermic contribution from the interactions between the EO segments and ureaz8 (note that the enthalpies of transfer of hydrophobicgroups from water to aqueous urea solution are usually positive,27 while the enthalpies of transfer of ionic or hydrophilic groups are negativeG4zG5).The enthalpies of transfer of micelles from water to aqueous urea solution in the postcmc region, AH,,,,,, were negative and became more negative with increase in urea concentration. J h a and Ahluwalia28attributed this to the fact that, in the micellar state, all of the hydrophobic groups are buried inside the micellar core and hence the endothermic contribution of the interaction between the hydrophobic decyl group and urea is essentially the same as in the premicellar state; interactions between EO segments (hydrophilic block) and urea seemed to dominate over the hydrophobic group1 urea interactions.28 Although the hydrophobic part of the PEO-PPO-PEO copolymers is different than that of the C,EOj surfactants studied in ref 28,the decrease in the AHo values for P105 upon the addition of urea can be indicative of the exothermic interactions ofPEO with urea, interactions whose magnitude increases with an increase in the urea concentration. The PPO segments could also interact with urea as discussed in section 1. 3. Partial Specific Volume. Information on the conformation of the copolymer molecules in the micelles can be inferred from density measurements. The density of Pluronic P105 copolymer aqueous solutions (0.5 and 2.5% polymer concentration) of varying urea concentrations (0, 2, and 4 M) was investigated as a function of temperature for the 15-45 "C temperature range. The density of bulk P105 copolymer is higher than the water density;l urea solutions also have a density higher than that of water.57 The density of the Pluronic solutions decreased with increasing temperature. The rate of this decrease increased with temperature, a fact that can be related to the formation of micelles (cmt). To probe this effect, the partial specific volume values of the copolymer were calculated from the measured densities. The partial specific volume, Y , of a solute is a characteristic thermodynamic parameter which defines various intermolecular interactions.66 The experimental quantity at a given concentration, c, is the apparent partial specific , can be determined from density volume, ~ ( c ) which (64)Stern, J. H.; Kulluck, J. D. J . Phys. Chem. 1969,73,2795. (65)Hakin, A. W.;Beswick, C. L. Can.J . Chem. 1992,70,1666. (66)Armstrong, J. K.;Parsonage, J.; Chowdhry, B.; Lehame, S.; Mitchell, J.; Beezer, A,; Lohner,K.; Laggner, P. J.Phys. Chem. 1993, 97,3904.

10

15

20

25

30

35

40

45

50

Temperature ('C)

Figure 4. Partial specific volume values of Pluronic P105 in uredwater solutions, as a function of temperature. The cmt can be obtained from the intersection of the solid lines drawn in the graph.

measurements using the following relation:12@

where the values of Q and c are expressed in g/mL. Partial specific volume values of Pluronic P105 in uredwater solutions are plotted as a function oftemperature in Figure 4. The abrupt increase in Y(C) is indicative of a heatinginduced "phase transition".lZ This phase transition is related to the formation of micelles and can be observed for all solutions studied; the transition spans -10 K, in accord to the micellization transitions observed by differential scanning calorimetry in similar PEO-PPO-PEO copolymers.4,5~11~12,60~62~64 The cmt from such a set of data can be obtained12from the intersection of the solid lines shown in Figure 4. The cmt decreases with increasing polymer concentration (compare the 0.5% and 2.5% P105 solutions) as expected;lOJ1urea appears to increase the cmt of 2.5% P105 solutions, albeit to a small extend, in agreement with the trends discussed in section 1. The partial specific volume data indicate that the PEO-PPO-PEO copolymer chains become less dense (more expanded) during micellization. The 13C chemical shift measurements for the methyl carbons in the PPO block of a similar copolymer (Pluronic L64) indicated a change to an extended conformation in the micelles from a more coiled conformation in the unimer state and in the neat PPO h ~ m o p o l y m e r . ~ ~ On the other hand, the 13C chemical shifts for the methylene carbons of the PEO block where similar in the micelle state and in aqueous solutions of PEO homopolymer.67 The micelle size for PEO-PPO-PEO copolymers has been shown to depend on the length of the PPO block,8J1suggesting also that the PPO block occurs in an extended configuration in the micelle core. The partial specificvolume of the copolymer decreases (and the density increases) with increase in urea concentration,paralleling the effect of increasing the bulk copolymer concentration on Y. However, the magnitude of the change occurring in the value of Y over the micellization transition, as well as the temperature range over which this transition takes place, remains unaffected by the presence of urea. 4. Surface Activity. The surface activity of PEOPPO-PEO copolymers is a very important property in applications such as dispersion stabilization, foaming, emulsification, etc.11s50 The effect of urea on the surface activity of Pluronic P105 was investigated and the results are presented in Figure 5, where surface tension data for P105 solutions in uredwater (0, 2, and 4 M urea) at 25 (67)Almgren, M.; van Stam, 3.;Lindblad, C.; Li, P.; Stilbs, P.; Bahadur, P. J. Phys. Chem. 1991,95, 5677.

PEO-PPO-PEO Copolymer in Aqueous Urea Solutions 62

,

-

._....., . ......., . ......., . ....,.., . .......,

t

-A-

52

,

Langmuir, Vol. 11, No. 7, 1995 2447

....

(a) 4.5

P105 (no urea) P105 (2M urea) P105 (4M urea)

h

1

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Figure 5. Surface tension data for Pluronic P105 dissolved in uredwater, plotted semilogarithmically with respect to copolymer bulk concentration at various urea concentrations.

(b) 12

1

“C are plotted semilogarithmically with respect to copolymer bulk concentration. At low copolymer concentrations the surface tension decreased with increasing concentration, in accord with the Gibbs’ adsorption isotherm.68 A change in slope (break) was observed in the surface tension curve at a characteristic concentration (approximately after which the surface tension values continued to decrease until a plateau was reached (second break); the surface tension values remained approximately constant with fixther increase of copolymer concentration. The break a t the high-concentration part of the surface tension curve corresponds to the cmc, while the origin of the low-concentration break is less clear and has been attributed to formation of “monomolecular” micelle^,^,^^ the presence of i m p ~ r i t i e sand , ~ the rearrangement of the copolymer molecules adsorbed on the aidwater interface at complete surface coverage.50 The surface tension of the Pluronic P105 aqueous solution increased (and the surface activity decreased) in the presence of urea. The change in slope signifying the cmc appeared at higher copolymer concentrations, compared to the ones at zero urea concentration, in agreement with the dye solubilization results presented in section 1 (see also Figure 1). Addition of urea shifts the surface tension curve to higher copolymerconcentrations without affecting the features of the curve. Note that the “dip”, typical of the presence of i m p u r i t i e ~ ,discerned ~ ~ ! ~ ~ in the surface tension curve in the vicinity of the cmc also moves to higher copolymer concentrations when the urea concentration increases. The low-concentration break in the surface tension curve is also observed in the presence of urea at the same copolymer concentration (-2 x the urea data, however, are not adequate for distinguishing the mechanism responsible for the occurrenceof this break. The small effect of urea on the surface activity is also reflected in the values for surface area. The areas per Pluronic P105 molecule at the aidwater interface were calculated from surface tension data using the simple form of the Gibbs adsorption isotherm@that relates the surface (excess) concentration of the surfactant, r, to the surface tension and the surfactant chemical potential:

r = -(l/RT)(dy/d

In C)

(4) where the area per molecule,A, is equal to (TN)-l, y is the surface tension, N is Avogadro’s number, R is the molar gas constant, T i s the absolute temperature, and an ideal solution is assumed. In the absence of information on the exact composition of the Pluronic sample, the area per (68)Chattoraj, D. K.; Birdi, K. S. Adsorption and the Gibbs Surface Excess; Plenum Press, New York,1984. (69)Prasad, K. N.;Luong, T. T.; Florence, A. T.; Pans, J.; Vaution, C.;Seiller, M.; Puisieux, F. J. Colloid Interface Sci. 1979,69, 225. (70) Mysels, K. J. Langmuir 1986,2, 423.

45

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P105 2 5% (no urea) P105 2 5% (4M urea) F108 2 5% (no urea) F108 2 5% (4M urea)

-

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7

-

6

-

molecule estimated from eq 4 will be an average area. The bulk copolymer concentration range over which area per molecule values were extracted was 10-6-10-3 wt %. The values for surface area per Pluronic P105 molecule calculated in this way are approximately 100 k for all urea concentrations. Addition ofurea reduces the waterhydrocarbon interfacial tension, although the surface tension of aqueous urea solutions is slightly greater than that of pure water.34 The latter would indicate that urea desorbs from the airlwater interface; the indifference of the surface area to the urea bulk concentration could reflect this behavior. 5. Micelle Hydrodynamic Radius. The hydrodynamic radius ofthe micelles formed by Pluronic P105 PEOPPO-PEO copolymers in uredwater solutions was measured using dynamic light scattering. The exponential decay of the correlation of the intensity fluctuations, originating from the Brownian motion of the micelles, is measured during a dynamic light scattering experiment. The correlation decay is related to the diffusivity of the micelles according to the equation G = Dq2, where G is the inverse relaxation or correlation time, D is the effective translational diffusion coefficient, and q is the scattering vector.61 Figure 6a shows the temperature dependence of the translational diffusion coefficient of P105 micelles a t 0 and 4 M urea. D, ranging from 2.5 x lo-’ to 4.5 x lo-’ cm2/s, increased with increasing temperature and decreased with increasing urea concentration (at a given temperature). The “effective”hydrodynamic radius of the micelles, Rh, is obtained from D through the StokesEinstein equation, Rh = kTI(6nrD), where k is the Boltzmann constant, T i s the absolute temperature, and 17 is the solvent viscosity.61 When the values for temperature and viscosity are used in the Stokes-Einstein

Alexandridis et al.

2448 Langmuir, Vol. 11, No. 7, 1995 equation, the calculated hydrodynamic radii (Rh) of the copolymer micelles were found to be independent of temperature and urea concentration as seen in Figure 6b. Rh for P105 micelles was approximately 8 nm; the& value reported for 2.5% P105 at 25 “C is larger than the Rh at higher temperatures, probably due to the relatively high polydispersity observed at 25 “C (high polydispersity has been reported in PEO-PPO-PEO micellar solutions a t temperatures higher but close to the cmt12). Although there appears to be a trend of decreasing hydrodynamic radii with increasing temperature, the difference between &values at 35 and 45 “Cis within the experimental error. Also shown in the same plot for purposes of comparison are hydrodynamic radii of Pluronic F108 micelles in water with 0 and 4 M urea concentration. F108 is a PEO-PPOPEO copolymer with a PPO (middle) block of similar size to that of P105 but larger PEO blocks; the approximate chemical formula of F108 is E0132P050E0132(F108 molecular weight: 14 600). F108 forms micelles that have a hydrodynamic radius (-10.5 nm) larger than that of P105; the size difference reflects the larger PEO blocks of F108. Similarly to P105, the presence of 4 M urea has no effect on the hydrodynamic radius of the F108 micelles, depsite their extended PEO coronae. The constant values of the hydrodynamic radii with respect to temperature have been interpreted in terms of enhanced dehydration of EO segments with increasing t e m p e r a t ~ r e . ~ JThe ’ , ~ average ~ cross-sectional area of the surfactant headgroup (the PEO blocks), A, decreases as the temperature is i n c r e a ~ e d ,thus ~ ’ ~ reducing ~ the repulsive headgroup-headgroup interactions and allowing more PEO-PPO-PEOmolecules to associate in a micelle.52 According to the direct mechanism of urea action,3zurea perturbs the hydration shell of the headgroup by replacing some of the water molecules. Since a urea molecule is approximately 2.5 times larger than a water molecule, we would expect A to increase with the addition of urea at a fixed temperature, thus effectively increasing steric repulsions between the PEO blocks; this would then lead to a decrease in the micelle aggregation number. The absence of urea effects on the effective hydrodynamic radius of the P105 and F108 micelles (Figure 6b) would imply either that urea does not affect the aggregation size and the hydration of the PEO segments or that urea decreases the aggregation number and at the same time increases the “hydration layer” (by participating in it) that surrounds the micelle. Given the related experimental evidence from the surface area of PI05 in the presence of urea (see section 41, the former scenario seems more plausible. Note also that, while the larger size of the urea molecules should lead to a faster decrease rate forA with increasing temperature if urea were present in the hydration shell, and thus enable the micelles to grow as temperature is increased,20this has not been observed here. The absence of an effect of urea on the hydrodynamic radius of Pluronic P105 and F108 copolymer micelles reported here agrees with the findings of Bahadur et al.15 that the sizes of Pluronic F68 unimers and micelles in aqueous solution (estimated from dynamic light scattering, NMR self-diffusion measurements, and static light scattering) are similar in the presence and absence of salts, although the growth of the micelles started at a lower temperature and continued over a wider temperature range in the salt solution. Regarding the mechanism of interaction of urea with the PEO coronae of the PEOPPO-PEO copolymer micelles, it should be kept in mind that, in 4 M aqueous urea solution, the weight fraction of urea is approximately 0.25; under these conditions, the inclusion of urea in the solvation of the copolymer

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.

15

25

35

45

55

Temperature (‘C)

Figure 7. Temperature dependence of the ratio of vibronic band intensities, 1 1 / 1 3 , of pyrene present in 1%Pluronic P105 copolymer dissolved in uredwater mixtures (urea concentration: 0, 2, and 4 M).

molecules seems unavoidable. Note also that poly(ethylene oxide) and urea are known to form a molecular complex in the solid state containing 2 mol of urea per ethylene oxide ~ e g m e n t ;this ~ ~is, ~ an~indication of specific hydrogen bonding interactions between PEO and urea. Briganti et al.zOfound that addition of urea reduced the effective intermicellar attraction in ClzEOs micelles and inferred that urea has a larger effect on the steric repulsions (by contributing to thicker hydration layer) than on the van der Waals attractions between the hydrated micelles. 6. Micropolarity. We recently reported the use of pyrene in probing the effects of temperature on the micropolarity of PEO-PPO-PEO copolymer aqueous solut i o n ~ .Pyrene ~~ exhibits a characteristic fluorescence emission spectrum that consists mainly of five bands, referred to (from shorter to longer wavelengths) as, 11, 1 2 , ,..,and 1 5 . The ratio of the vibronic band intensities, 11/13, of pyrene, often referred to as the “Py ~ c a l e ” ,depends ~~!~~ strongly on the polarity of the medium: the higher the ratio, the more polar the medium. Electronically excited pyrene is a reporter of the average micropolarity of the environment it visits during its lifetime (ca. 300 ns); in this sense, pyrene is attractive for studying restricted microenvironments such as micellar system^.^^,^^ The 11/13 ratios of the pyrene fluorescence emission intensity for aqueous Pluronic P105 solutions (1%copolymer) are presented in Figure 7 as a function of temperature for 0,2, and 4 M urea. A sharp decrease in 11/13 is observed for the P105 copolymer solutions at characteristic temperatures, different for each urea concentration, followed by a less dramatic linear decrease as temperature increases further. This sharp decrease has been attributed to the formation of micelles composed of a hydrophobic core into which pyrene partitions preferentially.12~51~5z~73~75 The temperature at which the micellization transition occurs increases with increasing urea concentration in accordance with the cmt values presented in Figure 2. The linear decrease in the 11/13 ratio, observed at temperatures higher than the cmt, is another salient feature of the 11/13variations in PEO-PPO-PEOcopolymer solutions. This decrease signifies a less polar microenvironment in the micelles with increasing temperature (71) Bailey, F. E., Jr.; France, H. G. J . Polym. Sci. 1961,49, 397. (72) Bogdanov, B. G.; Michailov, M.; Uzof, C. V.; Gavrailova, G. G. J . Polym. Sei. B : Polym. Phys. 1994,32,387. (73) Thomas, J. K. The Chemistry ofExcitation at Interfaces; ACS Monograph 181; American Chemical Society: Washington, DC, 1984; Chapter 5. (74) Dong, D. C.; Winnik, M. A. Can. J . Chem. 1984,62,2560. (75) Ananthapadmanabhan, K. P.;Goddard,E. D.;Turro, N. J.;Kuo, P. L. Langmuir 1985,1, 352.

Langmuir, Vol. 21, NO. 7, 1995 2449

PEO-PPO-PEO Copolymer in Aqueous Urea Solutions

and can be attributed either to changes in the composition of the micelle core (i.e., less PEO or water), or to a change in the medium polarity at constant core c o m p o ~ i t i o nIn .~~ this temperature range, the IdI3 values obtained for aqueous P105 solutions in the presence of urea are lower than those obtained with the P105 aqueous solutions a t zero urea concentration, suggesting that urea results in a more hydrophobic microdomain (note that the effect of urea on the 11/13ratio is small but statistically significant). Urea is known to increase the dielectric constant of water (the dielectric constant of 6 M uredwater solution is 92, compared to 78 for pure water76)and thus result in a more polar environment. On the other hand, the I J I 3 ratio measured in a 4 M uredwater solution (no copolymer present) is lower than the Ill&ratio in water, implying a lower micropolarity. This may be due to specific interactions ofurea with pyrene (similar to the inclusion structure detected in the valeric acid-urea-water system46).Since the micelle size appears unaffected by urea (section 51, and since urea does not partition in the micelle core,77the small decrease in the micropolarity observed in the presence of urea a t temperatures higher than the cmt must be due to a decrease in the overall solution polarity as sensed by pyrene. The micropolarity data will be compared together with that of microviscosity in section 7 to other literature reports on micellar systems in the presence of urea. 7. Microviscosity. The effect of urea on the microviscosity in the interior of the P105 micelles was probed using the hydrophobic molecule bis(1-pyrenylmethyl) ether (dipyme) that exhibits intramolecular excimer fluorescence(intensityIE)in competition with fluorescence from the locally excited pyrene chromophore (“monomer” emission, intensity I,; 1 5 is used here to represent The extent of excimer emission in dipyme depends upon the rate ofconformational change, the motion of the pyrene segments of dipyme being restricted by the local friction imposed by the environment. As a consequence, the intensity ratio IM/IE provides a measure of the “microviscosity” of its environment (the larger the I M / l E ratio, the more viscous the environment where the probe is located) and is a particularly powerfulmeans of monitoring changes in microviscosity as the system is subjected to external s t i m ~ l i . ~ ~ , ~ ~ ~ ~ ~ , ~ ~ Monomedexcimerintensity ratio (IM/IE) values of dipyme in Pluronic P105 PEO-PPO-PEO copolymer micelles are shown in Figure 8. IM/IE data are reported in the 35-50 “C temperature range, so that micelles are present in which dipyme can be solubilized (the solubility of dipyme in water is very limited). The microviscosity decreased monotonically with temperature for all systems studied, in accordance with what has been observed in bulk PPO and in micelles formed by other PEO-PPO-PEO copolym e r ~ . The ~ ~ IM/IE , ~ ~ values for the P105 micelles are comparable to those measured for bulk PPO of molecular weight 2000 (reported in ref 531, suggesting that the environment where dipyme is located is similar to bulk PPO. Lower microviscosities were observed for the P105 micelles in the presence of urea, resembling the microviscosity decrease seen in the PEO-PPO-PEO micelles upon decrease of the copolymer molecular weight.53 Note, though, that the decrease in IM/IE effected by increasing urea concentration is of smaller magnitude (-50%) than the decrease observed between P105 and P85 micelles. (76) Wyman, P.J . Am. Chem. SOC.1933,55, 4116. (77) Treiner, C.J. Colloid Interface Sci. 1982, 90, 444. (78) Winnik, F. M.;Winnik, M. A,; Ringsdorf, H.; Venzmer, J. J . Phys. Chem. 1991,95, 2583. (79) Yekta, A.;Duhamel, J.;Brochard, P.;Adiwidjaja, H.; Winnik, M.A. Macromolecules 1993,26, 1829.

I

2.6 tP105 1% (no urea) -0-

-

P1OS 1% (2M urea)

. r - - - P 1 0 51%(4Muraa)

-

1.6

I

14 1.2

34

I 37

40

43

46

49

52

Temperature ( ‘ C )

Figure 8. Temperature dependence of the ratio of monomer/ excimer intensity ratio ( I M / Z E ) of dipyme (indicative of microviscosity) in Pluronic P105 micelles in uredwater mixtures (urea concentration: 0, 2, and 4 M). The lines in the I ~ I vs E temperature plot are fits t o an exponential relationship.

A number of recent studies have employed spin and fluorescence probe molecules to assess the effects of urea on the microenvironment in micellar solutions; as shown in the following discussion, though, the results are rather conflicting. Electron spin resonance spectroscopy of cationic and nonionic spin probes, studied by Baglioni et al.37in sodium dodecyl sulfate (SDS) and in dodecyltrimethylammonium bromide (DTAB) solutions, indicated that urea slightly decreases the polarity and strongly increases (from 20 to 100% depending on the surfactant and on the urea concentration) the microviscosity of the micelle interface. A small increase (-10%) in the correlation time was also observed for spin probes that lay deeper in the micelle core (below the surfactant polar headgroups), suggesting that urea penetrates below the headgroups. Sarkar and B h a t t a ~ h a r y y have a ~ ~ studied the emission properties of p-toluidinonaphthalenesulfonate(TNS) in nonionic Triton-Xand anionic SDS micelles in the presence of urea. The emission properties of TNS are markedly dependent on the polarity and viscosity of the media. The main nonradiative pathway in the excited state of TNS is twisted intramolecular charge transfer (TICT).47The TICT processes are quite fast in the highly polar aqueous medium and, hence, the quantum yield of fluorescence (4f)of TNS is extremely low in water; at the micellar surface, the reduced polarity and increased microviscosity decrease the rate of TICT, resulting in a substantially higher &. This has been utilized to follow the process of micellization and determine the cmc. Above the cmc, the #f of TNS solutions containing urea is considerably lower than that without urea. Sarkar and Bhatta~haryya~’ took for granted the report of Baglioni et a1.36,37 (see above) and, since the observed decrease in +fofTNS was opposite to the urea-induced changes in the local polarity and viscosity reported by Baglioni et a1.,36,37 they concluded that urea directly interacts with TNS, decreases the number of TNS molecules bound to the micellar interface and, as a result, reduces the overall &. However, a more “direct” interpretation of the data of Sarkar and Bhatt a ~ h a r y y awould ~ ~ be a decrease in microviscosity and increase in micropolarity of the environment sensed by TNS. Miyagashi et aLE0 reported that the ratio of the microviscosity (probed using the fluorescence emission intensity of (N,NJV’,”-tetramethy1diamino)diphenyl ketoimine) between the bulk phase and SDS micelles was smaller in the presence of urea and virtually did not depend (80) Miyagishi, S.;Asakawa, T.;Nishida, M. J.Colloid Interface Sci. 1987, 115, 199.

Alexandridis et al.

2450 Langmuir, Vol. 11, No. 7, 1995 on the concentration of urea, indicating that the probe sensed the perturbation to a similar extent in both phases. Note that the probe used was ionic and, therefore, its fluorescencewould reflect changes near the micelle surface rather than in the hydrocarbon core. Recent fluorescence polarization studies in Triton X-100 micelles present in watedurea solutions suggested that the micelle core remained unaffected by the urea, while the mobility of a probe molecule located a t the micelle corona increased; the latter observation was interpreted as a direct interaction of urea with the hydration layer of the headgroups that would lead to a looser packing of these groups.21The small effect of urea on the microviscosity in P105 micelles observed here is in agreement with the study of Ruiz and Sanchez.21

Conclusions We have investigated the effects of urea on the micellization behavior and structure of micelles for a poly(ethylene oxidel-block-poly(propy1eneoxidel-block-poly(ethyleneoxide) copolymer,Pluronic P105, using an array of techniques. The copolymer concentrations needed to form micelles (cmc) increased with increasing urea concentration. PPO can be affected by urea through both the indirect (breakingthe water (‘structure”)and the direct (replacement of water molecules and hydrogen bonding to urea molecules) mechanism. The temperature dependence of cmc&mc observed for the PEO-PPO-PEO copolymer, different from that for the poly(oxyethy1ene)alkanols, may indeed indicate the additional mode of PPO interactions with urea. An investigation of the solubility and phase behavior of PPO in uredwater solutions would be very useful in clarifying such interactions but has not been done here. A positive (endothermic) enthalpy of micellization was estimated using a copolymer association model. The enthalpy of micellization was lower in the presence of urea and decreased further upon increase of the urea concentration, in agreement with other nonionic surfactants. Favorable enthalpic interactions between urea and PEO, PPO are most likely causing the decrease in AH”. Structural information on the PEO-PPO-PEO copolymer was obtained through density, surface tension, and dynamic light scattering experiments. The temperature dependence of the partial specific volume of the copolymers exhibited a “phase-transition” behavior at the cmt, showing the molecules to become less dense as they enter the

micelles; this is probably due to the extended configuration of the PPO blocks in the micelle core. The partial specific volume (v) of the copolymer decreases (and the density increases) with increase in urea concentration; however, the change in the value of v over the micellization transition, as well as the temperature range over which this transition occurs (-10 K), remains unaffected by the presence of urea. The surface tension of the Pluronic P105 aqueous solution increased (and the surface activity decreased)in the presence of urea. The area-per-molecule values for copolymers adsorbed at the airlwater interface (estimated from surface tension measurements) remained independent of addition of urea. The hydrodynamic radii (Rh)of the P105 micelles (determined to be 8 nm using dynamic light scattering) were also found to be independent of urea concentration. The absence of urea effects on the effective hydrodynamic radius of the P105 micelles and the surface area of P105 suggests that urea does not affect the aggregation number and the hydration of the PEO segments. A small decrease in the micropolarity of the micelle core was observed in the presence of urea and attributed to a decrease in the overall solution polarity. The microviscosity in the micelle interior, estimated from the intramolecular excimer fluorescence of the hydrophobic probe bis(1-pyrenylmethyl) ether (dipyme), decreased to a small extent with an increase in the urea concentration, resembling the decrease in microviscosity observed between P105 micelles and micelles formed by a copolymer of smaller molecular weight. Overall, the presence of urea makes P105 more hydrophilic (shift in the micellization boundary) but does not affect the structure of the copolymer (reflected in the surface area per molecule and the micelle size).

Acknowledgment. This work was supported by the U.S. Department of Energy under Grant No. DE-FGO292ER14262 and the Emission Reduction Research Center (ERRC), Newark, NJ. V.A. participated in this research project under the Massachusetts Institute of Technology (MIT) Undergraduate Research Opportunities Program (UROP). We thank Professor Daniel Blankschtein (MIT) for allowing us to use his tensiomener, Dr. Belinda Tsao for synthesizing bis(1-pyrenylmethyl)ether (dipyme),and Dr. Thierry Nivaggioli for helpful discussions on the fluorescence properties of dipyme. LA940740J