Controlled Release of Volatile Fragrance Molecules from PEO-b-PPO

pubs.acs.org/Langmuir © 2010 American Chemical Society

Controlled Release of Volatile Fragrance Molecules from PEO-b-PPO-b-PEO Block Copolymer Micelles in Ethanol-Water Mixtures Damien L. Berthier,*,† Isabelle Schmidt,†,‡,§ Wolfgang Fieber,† Christophe Schatz,‡,§ Anton Furrer,† Kenneth Wong,† and S
ebastien Lecommandoux‡,§ †

Firmenich SA, Division Recherche et D
eveloppement, B.P. 239, CH-1211 Gen eve 8, Switzerland, ‡ Universit
e de Bordeaux, ENSCPB, 16 avenue Pey Berland, 33607 Pessac Cedex, France, and § CNRS, Laboratoire de Chimie des Polym eres Organiques, UMR5629, Pessac, France Received December 22, 2009. Revised Manuscript Received January 29, 2010

Active materials that can solubilize in different compartments of a sample show release properties which might be of interest in some applications where a delayed release of solutes for instance is required. We studied perfume solutes in compartments of Pluronic block copolymers of different compositions and molecular weights over a range of ethanol-water mixtures. Phase diagrams were constructed to identify and map micellar phases, then dynamic light scattering was used to characterize the solute-swollen micelles; NMR provided with the partition of solutes between solvent and micelles, and equilibrium constants Kc were estimated using headspace analysis. Finally solute-evaporation rates were measured by thermogravimetry. We focused on two typical behaviors: when solubilization in a micellar compartment occurs, delayed release increased with Kc. When solubilization was limited or absent, either because no micelles form or, in the presence of micelles, because solubilization was minor or absent, delayed release was correspondingly absent.

1. Introduction The presence of volatile fragrant molecules is generally associated with a feeling of pleasantness or cleanness in consumer products. Stability and lasting perception of these molecules are decisive factors for the acceptance of these products and their related beneficial effects.1 Because of their high volatilities, the perception of fragrances is quite limited over time. Indeed, the perfume industry is interested in delivery systems which maintain a fragrant smell over time.2-5 Effective additives prolong the long-lastingness of volatile ingredients over a certain time-period by counteracting the effect of their high volatility and favoring their persistence. Different physical delivery systems, like microcapsules or microparticles, have been developed to control and extend the release of volatile ingredients by diffusion through the shell or out of the matrix, and also to protect active molecules against degradation. Release can be triggered by rubbing the capsules trapped on a surface to break them or upon their decomposition under certain conditions of temperature or pH.2,3 Other delivery systems have been developed for aqueous systems using liposomes, cyclodextrin derivatives, or surfactant micelles in aqueous systems.6,7 Among the different categories of physical delivery systems, amphiphilic block copolymers have been used to encapsulate *Corresponding author. E-mail: [email protected]. Fax: þ41 22 780 33 34.

(1) Milotic, D. J. Consumer Behav. 2003, 3, 179–191. (2) Ness, J.; Simonsen, O.; Symes, K. Microspheres, Microcapsules Liposomes 2003, 6, 199–234. (3) Park, S.-J.; Arshady, R. Microspheres, Microcapsules Liposomes 2003, 6, 157–198. (4) Barantsevitch, E.; Kantor, M.; Milstein, S. Proc. Int. Symp. Controlled Release Bioact. Mater. 1998, 25, 326–327. (5) Kreuzer, G.; Ternat, C.; Nguyen, T. Q.; Plummer, C. J. G.; Ma˚nson, J.-A. E.; Castelletto, V.; Hamley, I. W.; Sun, F.; Sheiko, S. S.; Herrmann, A.; Ouali, L.; Sommer, H.; Fieber, W.; Velazco, M. I.; Klok, H.-A. Macromolecules 2006, 39, 4507–4516. (6) Tokuoka, Y.; Uchiyama, H.; Abe, M. Colloid Polym. Sci. 1993, 272, 317– 323. (7) Friberg, S. E. Adv. Colloid Interface Sci. 1998, 75, 181–214. (8) Saito, Y.; Miura, K.; Tokuoka, Y.; Kondo, Y.; Abe, M.; Sato, T. J. Dispersion Sci. Technol. 1996, 17(6), 567–576.

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active ingredients like fragrances or drugs in water.8-15 The hydrophobic core of micelles is an efficient reservoir for hydrophobic molecules.12-15 Several factors influence the loading efficiency in the micelle core, such as the molecular volume of active ingredient and/or its interfacial tension with water,16 the size of the micelle core,17 or the micelle core-ingredient compatibility.12-14,17,18 In general, the ingredient which is more compatible with the core-forming polymer block interacts with it more favorably19 and is more easily dissolved in it than a less compatible molecule.20 An advantage of copolymer micelles regarding encapsulation is their stability, which is thermodynamically and kinetically driven.21-24 Block copolymer micelles have thus a better capacity (9) Kayali, I. Jordan J. Appl. Sci., Nat. Sci. 2003, 5(2), 42–49. (10) Zhang, Z.; Barber, J. L.; Friberg, S. E. J. Dispersion Sci. Technol. 2000, 21(2), 146–160. (11) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414–2425. (12) Kabanov, A. V.; Nazarova, I. R.; Astafieva, I. V.; Batrakova, E. V.; Alakhov, V. Y.; Yaroslavov, A. A.; Kabanov, V. A. Macromolecules 1995, 28, 2303–2314. (13) Zhao, J.; Allen, C.; Eisenberg, A. Macromolecules 1997, 30, 7143–7150. (14) Kataoka, K.; Kwon, G. S.; Yokoyama, M.; Okano, T.; Sakurai, Y. J. Controlled Release 1993, 24, 119–132. (15) Upadhyay, K. K.; Agrawal, H. G.; Upadhyay, C.; Schatz, C.; Le Meins, J.-F.; Misra, A.; Lecommandoux, S. Crit. Rev. Ther. Drug Carrier Syst. 2009, 26, 157–205. (16) Nagarajan, R.; Barry, M.; Ruckenstein, E. Langmuir 1986, 2, 210–215. (17) Liu, J.; Xiao, Y.; Allen, C. J. Pharm. Sci. 2004, 93, 132–143. (18) Hurter, P. N.; Hatton, T. A. Langmuir 1992, 8, 1291–1299. (19) Elsabahy, M.; Perron, M.-E.; Bertrand, N.; Yu, G.-E.; Leroux, J.-C. Biomacromolecules 2007, 8, 2250–2257. (20) Nakashima, K.; Bahadur, P. Adv. Colloid Interface Sci. 2006, 123-126, 75–96. (21) Loh, W. Encyclopedia of Surface and Colloid Science; Hubbard, A., Ed.; Marcel Dekker, Inc.: New York, 2002; pp 802-813. (22) Halperin, A. Supramolecular Polymers; Ciferri, A., Ed.; Dekker: New York, 2000; Chapter 3. (23) Dufresne, M. H.; Fournier, E.; Jones, M.-C.; Ranger, M.; Leroux, J. C. In Block Copolymer Micelles;Engineering Versatile Carriers for Drugs and Biomacromolecules; Gurny, R., Ed.; B. T. Gattefoss
e: Saint-Priest, France, 2003; pp 87-102. (24) Bugrin, V. S.; Kozlov, M. Y.; Baskin, I. I.; Melik-Nubarov, N. S. Polym. Sci., Ser. A. 2007, 49, 463–472.

Published on Web 02/10/2010

DOI: 10.1021/la904832d

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Berthier et al. Table 1. Composition and Physico-Chemical Properties of Pluronic Copolymers

copolymer L-43 L-64 L-81 L-101 L-121 P-103 P-123

Mw (g/mol)

PEO (wt %)

total PEO units

PPO (wt %)

total PPO units

cmc (wt %)

HLB

1850 2900 2750 3800 4400 4950 5750

30 40 10 10 10 30 30

12 26 6 8 10 34 40

70 60 90 90 90 70 70

22 30 43 59 68 60 69

0.4 0.14 0.0063 0.0008 0.0004 0.003 0.04

12 15 2 1 1 9 8

to load actives and to stay aggregated upon high dilution compared to monomeric surfactants.21,25-29 Furthermore, polymer micelles afford a long dissociation time (hours, days) in comparison to monomeric systems (few seconds).25 Aqueous systems with block copolymers of poly(ethylene glycol) (PEO) and poly(propylene glycol) (PPO) are widely used, for example in the cosmetic industry under the trade name of Pluronic.28 The phase diagrams in water of Pluronic are also well-known.11,12,30 Their self-assembly is strongly influenced by the hydrophobic interaction of the PPO block.11 Their cmcs also depend on the molecular weight of the PPO block, and influence the free energy of micellization.11,21,30 Pluronic copolymer micelles have already been used as fragrance delivery systems in water to encapsulate benzyl formate,8 limonene,9 or linalool.31 The presence of aggregates (micelles or vesicles) slowed the evaporation of the volatile molecules by decreasing their vapor pressures when favorable intermolecular interactions were present.31-37 Aqueous systems have charted the basic behavioral tendencies of Pluronic systems; however, other solvent systems have nonetheless elicited interest and research. Ethanolic systems with Pluronic have been studied, though less than aqueous ones. The influence of ethanol, for example, on the phase diagrams of different Pluronic polymers, has been reported.38-40 Studies in water-ethanol mixtures indicated that addition of ethanol changes the physical-chemical parameters of the solvent. Ethanol is a good solvent for both PEO and PPO blocks and thereby prevents self-hydration of the copolymer and increases its solubility: ethanol is distributed between the PPO core and the solvent.41,42 It has also been shown that addition of ethanol decreased the aggregation number and size of micelles, whereas it increased the cloud point, the critical micelle (25) Lavasanifar, A.; Samuel, J.; Kwon, G. S. Adv. Drug Delivery Rev. 2002, 54, 169–190. (26) Hagan, S. A.; Combes, A. G.; Garnett, M. C.; Davies, M. C.; Ilum, L.; Davis, S. S. Langmuir 1996, 12, 2153–2161. (27) Tancrede, P.; Barwicz, J.; Jutras, S.; Gruda, I. Biochim. Biophys. Acta, Biomembr. 1990, 1030, 289–295. (28) Schmolka, I. R. J. Am. Oil Chem. Soc. 1977, 54, 110–116. (29) Rodriguez-Hernandez, J.; Ch
ecot, F.; Gnanou, Y.; Lecommandoux, S. Prog. Polym. Sci. 2005, 30, 712–724. (30) Lopes, J. R.; Loh, W. Langmuir 1998, 14, 750–756. (31) Suzuki, K.; Saito, Y.; Tokuoka, Y.; Abe, M.; Sato, T. J. Am. Oil Chem. Soc. 1997, 74(1), 55–59. (32) Friberg, S. E.; Fei, L.; Aikens, P. A. Recent Res. Dev. Phys. Chem. 1997, 1, 193–208. (33) Friberg, S. E. Int. J. Cosmet. Sci. 1997, 19, 75–86. (34) Friberg, S. E.; Szymula, M.; Fei, L.; Barber, J.; Al-Bawab, A.; Aikens, P. Int. J. Cosmet. Sci. 1997, 19, 259–270. (35) Friberg, S. E.; Yin, Q.; Aikens, P. A. Int. J. Cosmet. Sci. 1998, 20, 355–367. (36) Fei, L.; Szymula, M.; Friberg, S. E.; Aikens, P. A. Prog. Colloid Polym. Sci. 1998, 111, 85–91. (37) Friberg, S. E.; Yin, Q.; Aikens, P. A. Colloids Surf., A 1999, 159, 17–30. (38) Alexandridis, P.; Yang, L. Macromolecules 2000, 33, 5574–5587. (39) Ivanova, R.; Alexandridis, P.; Lindman, B. Colloids Surf., A 2001, 183-185, 41–53. (40) Kwon, K.-W.; Park, M. J.; Hwang, J.; Char, K. Polymer J. (Tokyo, Jpn.) 2001, 33(5), 404–410. (41) Ivanova, R.; Lindman, B.; Alexandridis, P. Langmuir 2000, 16, 3660–3675. (42) Alexandridis, P.; Ivanova, R.; Lindman, B. Langmuir 2000, 16, 3676–3689. (43) Ganguly, R.; Aswal, V. K.; Hassan, P. A.; Gopalakrishnan, I. K.; Yakhmi, J. V. J. Phys. Chem. B 2005, 109, 5653–5658.

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concentration (cmc), and the gel point.43 Addition of ethanol decreased the amphiphilic character of the copolymer and the repellency of both hydrophobic and hydrophilic parts because ethanol acted as a cosolvent. The swelling of either block also modified the interfacial curvature, which is initially determined by the molecular architecture of the copolymer. More recently, Pluronic P-123 has been formulated in water and ethanol.44-46 Spherical micelles have been observed up to 30 wt % of ethanol at 30 °C, but not above.46 Ethanolic systems of Pluronic with solutes however have not been studied, and the behavior of these systems is not necessarily obvious. For example, on the basis of results from aqueous systems, all of the Pluronic polymers studied here are soluble and form micelles; also, all of the perfume solutes studied here are hydrophobic and insoluble, and furthermore, all should be found principally in micelles. In ethanolic solvents, these behaviors are found to be untrue. Indeed the main goal of the present work is to determine the influence of block copolymers on the evaporation of volatile molecules in ethanolic solutions. We construct the phase diagrams of seven Pluronic copolymers over a wide range of ethanol-water mixtures. The capacity to load volatile molecules in aggregates of copolymers is determined by their swelling as measured by dynamic light scattering. Then, we measure the partitioning of the volatile molecules between the copolymer phase and solvent with diffusion NMR spectroscopy and we estimate the equilibrium constant for micelle-volatile solubilization, Kc, with static headspace analysis. Finally, to determine the effect of copolymer micelles on the evaporation rate of volatile ingredients, we measure the mass losses of the volatile molecules as a function of time with thermogravimetry.

2. Experimental Section General Data. Commercially available reagents and solvents were used without further purification if not stated otherwise. Demineralized water was obtained with a Millipore-Synergy-185 water purifier. Ethanol is absolute ethanol from Carlo Erba (purity 99.8%). All percents are percents in weight (wt %) relative to the total weight of the formulation. Copolymers. Seven PEO-b-PPO-b-PEO triblock copolymers were used for this study, available under the trade name of Pluronic by BASF (L-43, L-64, L-81, L-101, P-103, L-121, and P-123, Table 1). They were used as received. Pluronic copolymers have a specific nomenclature which codes their main characteristics. L and P correspond to the physical state of the polymer, liquid or paste. The first number in the numerical designation (or first two numbers for a three number designation), multiplied by 300, indicates the approximate molecular weight of the hydrophobic propylene oxide block. The last number, multiplied by 10, (44) Soni, S. S.; Brotons, G.; Bellour, M.; Narayanan, T.; Gibaud, A. J. Phys. Chem. B 2006, 110, 15157–15165. (45) Bharatiya, B.; Guo, C.; Ma, J. H.; Hassan, P. A.; Bahadur, P. Eur. Polym. J. 2007, 43, 1883–1891. (46) Chaibundit, C.; Ricardo, N. M. P. S.; de Costa, F. M. L. L.; Wong, M. G. P.; Hermida-Merino, D.; Rodriguez-Perez, J.; Hamley, I. W.; Yeates, S. G.; Booth, C. Langmuir 2008, 24, 12260–12266.

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Berthier et al. Table 2. Chemical Structures and Characteristics of Volatile Active Ingredients Present in Model Perfume

indicates the ethylene oxide wt %. Regarding Pluronic solubility, all of them used in this study are soluble in ethanol, and L-81, L-101, and L-121 are insoluble in water. Perfume. Volatile molecules are provided by Firmenich SA and known as ethyl butanoate or ethyl butyrate, ethyl 2-methylpentanoate or applinate, 2,4-dimethyl-3-cyclohexene1-carbaldehyde or Triplal, methyl 2,2-dimethyl-6-methylene1-cyclohexanecarboxylate or Romascone, and (Z/E)-3,7-dimethyl2,6-octadienal or Citral. Their main characteristics are summarized in Table 2. Water/Ethanol Mixtures as Solvent. Water/ethanol mixtures with different ratios were used. The water-ethanol ratios are expressed as the final water/ethanol content (%) in the sample: water/ethanol =90/10-75/25-70/30-60/40-50/50-25/75-10/ 90 (%). Viscosity and refractive index of the different water/ ethanol mixtures were measured (see below) or obtained from the literature.47 Preparation of Samples. Samples were prepared by direct addition of the copolymer to the solvent mixture (ethanol þ water) at room temperature (T = 20 °C) and mixing with a magnetic stir bar overnight. The initial water-ethanol content ratios were adapted to obtain the targeted water/ethanol ratios in the final formulation. Samples without perfume were prepared to establish phase diagrams of Pluronic in water-ethanol mixtures. The final copolymer concentration was set at 5%. Samples contained 95% of the water/ethanol solvent. For samples containing fragrance, volatile ingredients were added to the polymer/water/ethanol mixture and then stirred one night as above to attain thermodynamic equilibrium. These samples contained 10% of volatile molecules, 85% of the corresponding water/ethanol mixture, and finally 5% of polymer. The 55.5/44.5 water/ethanol ratio was used to fix the final concentration of ethanol at 40% in the reference. Dynamic Light Scattering. DLS measurements were performed with a Zetasizer, Nanoseries, Nano-S apparatus (Malvern (47) Motin, M. A.; Kabir, M. H.; Huque, M. E. Phys. Chem. Liq. 2005, 43(2), 123–137.

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Article Instruments, U.K.) equipped with a 4 mW He-Ne laser at a wavelength of 633 nm. Scattered intensity was measured at 173° and at 20 and 32 °C. Data were analyzed using two analysis modes: (a) the CUMULANTS analysis mode was used to determine the hydrodynamic diameter, expressed as the mean size of particles (Zave), the polydispersity index (PDI), and (b) the non-negative-least-square (NNLS) analysis mode that provides a full size distribution resolving multiple modes as long as their ratio of diameters are above 1:4. Viscosity and Refractive Index. Viscosity measurements were carried out with a viscosimeter Viscolite 700 from Hydramotion. Solutions were introduced in a water bath at 20 and 32 °C. Density was measured at 20 and 32 °C, with a DMA 4500 densimeter from Anton Paar. Refractive index was measured at 20 and 32 °C, with a A. Kr€ uss Optronic GmbH DR6200T refractometer. NMR Spectroscopy. NMR diffusion measurements were carried out on a Bruker 400 MHz Avance II spectrometer equipped with a 5 mm diffusion probe (Bruker Diff 30). The gradient amplitude was calibrated to 1187 G cm-1 at maximum intensity by means of a sample of doped water and diffusion coefficients from literature. Diffusion coefficients were obtained from a stimulated echo pulse sequence.48 In a typical experiment 16 spectra were recorded with increasing gradient strength using a gradient pulse duration δ of 1 ms and a diffusion delay Δ of 20 ms. The maximal gradient strength was adjusted for each molecule in the mixture in order to obtain a complete decay of the diffusion curve. All experiments were analyzed using Bruker TopSpin 2.1 software. The Stejskal-Tanner relation (eq 1) was fitted to the echo decay I     I δ ln ¼ -γ2 g2 δ2 Δ - D I0 3

ð1Þ

where I(0) is the initial amplitude of an echo signal, γ is the proton gyromagnetic ratio, and D is the diffusion coefficient.48-50 In order to take into account obstruction effects due to the presence of the polymer molecules correction factors have been applied to all measured diffusion coefficients.51 The results can be interpreted by assuming two sites for the fragrance volatile molecule in the sample: free molecules diffusing with Df, the diffusion coefficient of the fragrance molecules in the solvent, and molecules incorporated in the polymer micelles, diffusing with Dp, the diffusion coefficient of the polymer in the mixture. The exchange between the two sites is fast on the NMR time scale52 and the observed diffusion coefficient D of the fragrance molecule is an average of the two diffusion coefficients, which is weighted by the fraction of molecules in either site. The fraction of volatile molecules incorporated into the polymer micelles, fp, can then be calculated (eq 2),53 thus: fp ¼

Df -D Df -Dp

ð2Þ

D and Dp are measured in the polymer solution whereas Df is obtained from a solution of the respective fragrance molecule in the solvent only. Signal overlap occurred for the polymer peaks with those of ethanol (no deuterated solvents have been used) and Dp was determined by fitting of the attenuation curve to a biexponential function.

(48) Tanner, J. E. J. Chem. Phys. 1970, 52, 2523–2526. (49) Callaghan, P. T. In Principles of Nuclear Magnetic Resonance Microscopy; Clarendon: Oxford, U.K., 1991. (50) Stejskal, E. O. J. Chem. Phys. 1965, 43, 3597–3608. (51) S€oderman, O.; Stilbs, P.; Price, W. S. Concepts Magn. Reson., Part A 2004, 23A, 121–135. (52) Nilsson, M.; Ha˚kansson, B.; S€oderman, O.; Topgaard, D. Macromolecules 2007, 40, 8250–8258. (53) Stilbs, P. J. Colloid Interface Sci. 1981, 80, 608–610.

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The hydrodynamic radius RH of the copolymer can be calculated assuming a spherical shape and employing the Stokes-Einstein equation (the same way as for DLS experiments, eq 3) RH ¼

kB T 6πη0 Dp

ð3Þ

where kB is the Boltzmann constant, T is the temperature, and η0 is the viscosity of the solution, which we measure. Static Headspace (HS). Headspace or gas concentration was measured by GC-MS. According to Raoult’s law (eq 4), in an ideal solution, e.g. a dilute solute in water, the vapor pressure of the solute increases as a function of its solution concentration, P ¼ xP0

ð4Þ

where P is the vapor pressure of the solute in solution, Po the vapor pressure of the pure solute and x is the solute’s mole fraction in solution. In terms of its solution concentration, V, in wt %, then P ¼V P0 where V ¼

xMU ð1 -xÞMV þ xMU

for MU and MV the molecular weights of solute and solvent, respectively. Similar computations may be made to derive other units of concentration. We take the gas concentration to be directly proportional to vapor pressure and express headspace as the ratio P/Po or P/Po0 where the reference is measured at 32 °C instead of room temperature. We wish to measure the apparent equilibrium constant KC between volatile V and a Pluronic macromolecule M to form the complex V∼M where KC

V þM TV ∼ M and KC ¼

V ∼M VM

ð5Þ

where the italics denote solution concentrations. It can be shown using the expression above for Kc and the mass balances for volatile and polymer, VT ¼ V þ V ∼ M MT ¼ M þ V ∼ M that KC V 2 þ ðKC MT -KC VT þ 1ÞV -VT ¼ 0

ð6Þ

Here V is measured by headspace, and MT and VT are known; thus, KC may be determined since the equation is a quadratic in V. We determine the best fit KC for a series of VT by minimizing the error (Vexptl - Vcalc)2 between V measured experimentally and calculated, respectively, with a particular KC. Sample preparation: Typically 5 or 10 g samples were manually shaken or magnetically stirred for dissolution or emulsification. Aliquots of 1 mL were pipetted into 20 mL standard headspace vials for analysis. In cases where no homogeneous mixture could be prepared, we weighed very small amounts directly in the headspace vials using a Mettler AT20 analytical balance. 7956 DOI: 10.1021/la904832d

Liquid-gas equilibration was attained after conditioning at 32 °C for 3 h in the PAL agitator of a HP 6890 GCMS. Headspace concentrations were sampled by the Gerstel MPS 2 robot, a slightly modified CTC Analytics PAL system, equipped with a 2.5 mL headspace syringe. Gas, 1 mL, was withdrawn by the PAL then injected onto a Supelco 33837-05A GC column followed by MS detection: for the different raw materials the following ions are measured: ethyl butyrate, 71.2, 88.1 and 116.1; Romascone, 107.1, 122.1 and 182.1; background, the ions 74.1 and 202.0. The GC temperature profile upon injection was 60 °C for 5 min, ramped to 220 °C at 10°/min. Helium was the mobile gas phase; the split ratio was 20:1. Thermogravimetric Analysis (TGA). Taking into account the high volatility of the compounds in the sample (ethanol, water, and perfume molecule), 20 μL of volatile molecule solution was injected with a precision syringe (Hamilton-Bonaduz, Schweiz, 25 μL) into an aluminum crucible (Mettler-Toledo ME-26763 40 μL) previously weighed on a precision balance (Mettler-Toledo XP204S), giving the starting weight (between 17.93 and 18.33 mg). We measured that densities of the solution were slightly modified by the addition of copolymers. The corresponding masses weighted on the balance, before the TG analysis, were not impacted by this modification of the density. The crucible was capped during the transfer from the balance to the TGA autosampler (Mettler-Toledo TSO801RO). A chronometer was started at the same time to measure the elapsed time between the injection and the first point recorded by the TGA. This time was necessary to stabilize the temperature and the balance in the oven (32 °C). It cannot be controlled accurately and was between 2 and 3 min. This is the reason that the time difference at the beginning of the measurements is quite large between the first and the second point. However with this method, we know exactly the starting weight of the sample and the total duration of the evaporation. Isothermal measurements were carried out at 32 °C for 10 min under a constant nitrogen flow of 20 mL/min with a thermogravimetric analyzer (Mettler-Toledo TGA/SDTA851e) equipped with a microbalance (accuracy: 1 μg) and an accurate oven having an internal volume of 35 mL. The chronometer was stopped at the beginning of the TGA measurement after the stabilization of the balance in the oven. Measurements have been done in triplicate with a good repeatability. Heterogeneous samples were vigorously stirred before injection to be as homogeneous as possible. Extraction and GC Analysis. Samples were prepared as described in the previous section. TG analysis of a given sample was stopped at 2, 4, 6, 8, 10, and 15 min. The TG crucible was transferred to a GC vial (capacity of 1.8 mL, from VWR) and extracted with 1 mL of a solution of isooctane (2,2,4-Trimethylpentane, 99.5%, for analysis, from Acros Organics) and diethyl ether (for analysis, from Carlo Erba Reactifs SDS) at 9/1 by volume; this solution was spiked with 1,4-dibromobenzene at 150 mg/L (Sigma-Aldrich) as the internal standard. The extract was analyzed by gas chromatography using a system GC 7890A equipped with an auto sampler 7683 series from Agilent Technologies (1 μL injected) and a HP-5 column of 30 m, an inner diameter of 0.32 mm and a film thickness of 0.25 μm. The GC temperature profile was: 100 °C for 1 min, ramped to 170 °C at 10°/min for a total run time of 8 min. Helium was the mobile gas phase; the split ratio was 50:1. The retention times of ethyl butyrate and Romascone are 1.43 and 3.70 min, respectively. A calibration curve was measured and used to quantify each molecule. Samples were measured in triplicate.

3. Results and Discussion 3.1. Phase behavior of Pluronics in Water-Ethanol Mixtures. Pluronic copolymers were used in this study with different compositions, molecular weights, cmc, and hydrophilic lipophilic balance (HLB) values (listed in Table 1). To determine the conditions for Pluronic self-assembly, we established isothermal Langmuir 2010, 26(11), 7953–7961

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Figure 1. Phase behavior as a function of HLB of Pluronic copolymers at 5 wt % in water-ethanol mixtures at 20 °C (a) and 32 °C (b): free chains (empty dots); micelles (gray dots); aggregates (black dots). Table 3. Solubility of 10 wt % Volatile Active Ingredients at 40 wt % of Ethanol at 5, 20, and 32 °C, in the Presence of L-43 and P-103a 40 wt % EtOH þ L-43

40 wt % EtOH ingredients

5 °C

20 °C

ethyl butyrate S S applinate I I Triplal I I Romascone I I Citral I I a Key: S = soluble; I = insoluble.

32 °C S I I I I

5 °C S I I I I

(T = 20 and 32 °C) phase diagrams with DLS measurements (Figure 1, parts a and b). We define micelles as thermodynamically well-defined and stable, self-assembled structures which scatter strongly and show a narrow polydispersity, whereas aggregates are unstable, selfassembled structures or clusters of micelles with a high polydispersity. Using these criteria, phase diagrams were established for 5% of copolymer at different water/ethanol ratios. A large part of the phase diagram consists of free chains or unimers (empty circles in Figure 1, parts a and b). Such solutions resulted for all Pluronic with ethanol content greater than 35%; below this amount of ethanol, we distinguish three areas of the phase diagram corresponding to free chains/unimers, micelles, and aggregates depending on the structure of the Pluronic used (Figure 1a). At T = 20 °C, at high HLB, unimers were present for Pluronic L-43 and L-64. On the other hand, at intermediate HLB, micelles were found for P-103 and P-123 between 10 and 25% of ethanol (gray dots in Figure 1a), whereas unstable aggregates were observed for low HLB polymers L-81, L-101 and L-121 over the same range of ethanol concentrations (black dots in Figure 1a). Indeed, it is known that addition of short chain alcohols, such as ethanol, increases the cmc and decreases the surface tension of the solvent mixture.44,45 Inspection of the phase diagram confirms that the Pluronic polymers, which formed structures only at low ethanol content (less than 35%), became more soluble with the addition of ethanol. Thus, Pluronic polymers with intermediate HLB values at 8 and 9, copolymers P-103 and P-123, form micelles at 20 °C. Phase diagrams of Pluronics L-43, P-103 and P-123 were then established at 32 °C, the temperature of the skin (Figure 1b). As observed at 20 °C, copolymers P-103 and P-123 form micelles in the same range of ethanol concentrations and seem to be good candidates to encapsulate volatile fragrance molecules in the presence of ethanol. These results are also coherent with polymer size: the phase diagram shows that self-assembled structures were formed for (54) Zhang, C.; Zhang, J.; Li, W.; Feng, X.; Hou, M.; Buxing, H. J. Colloid Interface Sci. 2008, 327, 157–161.

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40 wt % EtOH þ P-103

20 °C

32 °C

S I S I S

S S S I S

5 °C S S S I I

20 °C

32 °C

S S S I S

S S S S S

polymers with high molecular weights.11,54 P-103 and P-123, have the highest molecular weights (Table 1) and form micelles, whereas L-43 and L-64 are completely soluble (molecular weights at 1850 and 2900 g/mol respectively). 3.2. Phase Behavior of Pluronic at Low Ethanol Content (40%) in the Presence of Active Volatile Molecules. 3.2.1. Macroscopic Observations. Addition of hydrophobic molecules to a surfactant solution can be an additional driving force for the formation of micelles as described recently by Foster et al.55 For example, addition of hydrophobic ibuprofen to P-103 aqueous solution increased the aggregation number and the fraction of polymer micellized. Perfume compositions are complex mixtures of volatile molecules, having different volatilities, hydrophobicities and chemical functions.56-58 To locate zones of micelle formation, we construct the phase diagrams of Pluronic in solvent alone: Figure 1 identifies which copolymers form micelles in ethanolic solutions, e.g., P-103, where on the phase diagram they form, and which remain as unimers, e.g., L-43. The contrasting behavior of these two Pluronic copolymers having been thus established, they were selected to study with perfumes. Pluronic are formulated at 5% in the presence of 40% of ethanol and 10% of five hydrophobic volatile molecules: ethyl butyrate (EB), applinate, Triplal, Romascone (RO), and Citral (Table 2). The solubility of each volatile molecule was visually evaluated at 10% in the presence of 40% of ethanol, water being the balance of the solvent, and at 5, 20, and 32 °C (Table 3). Only EB was soluble under these conditions, certainly because it has the lowest log P value at 1.85, all the other molecules were incompletely soluble at 40% ethanol and they phase separated. Solubilities were then measured under similar conditions in the presence of 5% of copolymers L-43 and P-103 (Table 3). With the exception of EB, all molecules were insoluble at 5 °C with L-43. (55) Foster, B.; Cosgrove, T.; Hammouda, B. Langmuir 2009, 25, 6760–6766. (56) Arctander, S. Perfume and Flavor Chemicals; published by the author: Montclair, NJ, 1969. (57) Surburg, H.; Panten, J. Common Fragrance and Flavor Materials, 5th ed.; Wiley-VCH: Weinheim, Germany, 2006. (58) Kraft, P.; Bajgrowicz, J. A.; Denis, C.; Fr
ater, G. Angew. Chem., Int. Ed. 2000, 39, 2981–3010.

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Table 4. Particle Size Diameter (and PDI) of P-103 and L-43 at 5 wt %, in the Presence of Ethyl Butyrate, Applinate, Triplal, Romascone, or Citral at 10 wt %, and 40 wt % of ethanol, measured by dynamic light scattering

Table 5. Hydrodynamic Diameter and Loading of Active Ingredients of P-103 and L-43 Particles at 5 wt %, in the Presence of Ethyl Butyrate, Applinate, Triplal, Romascone, or Citral at 10 wt %, and 40 wt % of Ethanol, Measured by NMR

hydrodynamic diameter (nm) and PDI at 32 °C ingredient

L-43

P-103

none ethyl butyrate applinate Triplal Citral Romascone

2.1 (0.20) 1.9 (0.60) 2.9 (0.21) 4.6 (0.08) 3.2 (0.04) two phases

2.8 (0.15) 2.9 (0.10) 6.6 (0.10) 8.4 (0.23) 4.7 (0.20) 12.0 (0.13)

Nevertheless, Triplal and Citral became soluble at 20 °C and Applinate at 32 °C, but not RO, which has the highest log P at 3.92 (Table 2). Moreover copolymer P-103 appears to be a better solubilizer. At 5 °C, Applinate and Triplal were already soluble, besides EB. Citral became soluble at 20 °C and RO at 32 °C, resulting in a bluish dispersion, typically indicating the presence of self-assembled micellar structures. From these results, we chose the volatiles which showed extremes of behavior, most soluble and most insoluble, namely EB and RO, respectively, to study further their evaporation with Pluronic. 3.2.2. Dynamic Light Scattering. Size measurements were carried out to determine if the solubilization effects obtained in the presence of copolymers L-43 and P-103 were linked to the formation of micellar structures. Solutions of 40% of ethanol, with L-43 or P-103 at 5% and volatile molecules at 10% were analyzed with dynamic light scattering (DLS) at 32 °C (Table 4). The phase diagram of copolymer L-43 at 40% of ethanol shows no self-assembly by DLS at 32 °C (Figure 1b). We observe a low scattered intensity and a high polydispersity, with a diameter of 2.1 nm, characteristic of unimers (Table 4). No change was observed after addition of EB, which is soluble under these conditions, indicating the continued presence of free chains (Table 4). When adding RO, a phase separation occurred. On the other hand, addition of applinate, Triplal, and Citral showed a significant diameter increase to 2.9, 4.6, and 3.2 nm, respectively. These larger particle diameters might indicate the formation of small micelles, swollen by the volatile molecules in the core, mainly with the Triplal, suggesting a better affinity of L-43 for this molecule. A similar behavior has been recently reported by Penfold et al. with micelles of dodecaethylene monodedecyl ether (C12EO12).59 Addition of a polar ingredient like phenylethylol led only to a small variation of the micelle hydrodynamic diameter. On the other hand, the addition of hydrophobic molecules, like dihydromyrcenol, resulted in substantial micellar growth. As mentioned above, copolymer P-103 at 40% of ethanol in water did not self-assemble at 20 °C. DLS measurements were conducted under similar conditions at 32 °C and gave a hydrodynamic diameter of 2.8 nm, characteristic of unimers (Table 4). Addition of EB did not provoke the formation of micelles, the measured diameter remained 2.9 nm (Table 4). Applinate, Triplal, or Citral were added to P-103 solution at 32 °C. Their presence induced a large increase of the scattered intensity, resulting in hydrodynamic diameters characteristic of micelles of 6.6, 8.3, and 4.6 nm respectively (Table 4). The addition of these three molecules thus had a strong impact on the formation and the swelling of the micelles. The trend for high log P molecules to provoke micelle formation was confirmed with RO: this molecule (59) Penfold, J.; Tucker, I.; Green, A.; Grainger, D.; Jones, C.; Ford, G.; Roberts, C.; Hubbard, J.; Petkov, J.; Thomas, R. K.; Grillo, I. Langmuir 2008, 24, 12209–12220.

7958 DOI: 10.1021/la904832d

ingredient

L-43 diameter (nm) at 32 °C

none 1.8 ethyl butyrate 1.7 applinate 1.8 Triplal 1.8 Citral 1.8 Romascone two phases

% of ingredient in L-43 particle at 32 °C

P-103 diameter (nm) at 32 °C

% of ingredient in P-103 particle at 32 °C

0 -2.8 9.5 18.3 6.7 0

2.8 2.8 3.4 3.7 3.0 5.84

0 0.4 15.9 20.7 12.3 56.4

became soluble at 32 °C in the presence of P-103 (Table 3) and resulted in micelles with a diameter of 12.0 nm (Table 4). From these results, we link the solubilization of hydrophobic molecules to the formation of micelles, and micellar growth can be attributed to the solubilization of hydrophobic fragrances in the micelle core thus increasing the core volume. This schema has been described by others, e.g. Pileni et al.60 On the other hand, the fragrance must have some affinity for the micelle to solubilize in it. For example, ethyl butyrate appears to have no or little effect on the size of either Pluronic polymers. 3.2.3. Partitioning Measurements by Diffusion NMR Spectroscopy. Diffusion NMR measurements were carried out for L-43 and P-103 and apparent hydrodynamic diameters of the polymers were obtained from the Stokes-Einstein equation (Table 5). The values for the copolymer P-103 agree with those obtained from DLS measurements. The presence of fragrances led to a swelling of P-103 indicating the formation of micelles. As with DLS the largest micellar sizes are obtained with RO, however, the diffusion NMR derived hydrodynamic diameters are generally smaller. Similar observations have been reported in literature for the triblock copolymer Pluronic P-105 in aqueous solution. The differences in micellar size between DLS and NMR spectroscopy were attributed to the polydisperse nature of the polymer.61 Assuming two sites of fragrance molecules in the sample, bulk liquid and micelles, respectively, the fraction in the copolymer micelles can be calculated. Apart from EB which is solvophilic and remains entirely in the water/ethanol phase, all fragrance molecules are partially dissolved in P-103, and for RO the fraction exceeds 50% (Table 5). A linear correlation between loading in the micelles and micellar size can be observed (Figure 2a). According to these data a cooperative micelle formation in the presence of fragrance molecules takes place, and high log P substances preferentially go to the hydrophobic core of the micelles.62 With the exception of Citral, fragrance loading increases linearly as a function of log P of the molecule similar to what has been described by Hurter et al., who demonstrated a strong correlation between the micelle-water partition coefficient and the log P for polycyclic aromatic compounds (Figure 2b).9 These observations are in agreement with previous NMR diffusion studies of fragrance molecules in the presence of star block copolymers63 and of surfactants, respectively.62 (60) Pileni, M.-P.; Zemb, T.; Petit, C. Chem. Phys. Lett. 1985, 118, 414–420. (61) Ma, J.; Guo, C.; Tang, Y.; Xiang, J.; Chen, S.; Wang, J.; Liu, H. J. Colloid Interface Sci. 2007, 312, 390–396. (62) Fischer, E.; Fieber, W.; Navarro, C.; Sommer, H.; Bencz
edi, D.; Velazco, M. I.; Sch€onhoff, M. J. Surfactants Deterg. 2009, 12, 73–84. (63) Fieber, W.; Herrmann, A.; Ouali, L.; Velazco, M. I.; Kreutzer, G.; Klok, H.-A.; Ternat, C.; Plummer, C. J. G.; Ma˚nson, J.-A. E.; Sommer, H. Macromolecules 2007, 40, 5372–5378.

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Figure 2. Comparison between the loading of volatile molecule and the diameter of micelle of copolymers (a), L-43 (empty markers), and P-103 (full markers). Representation of the loading as a function of the log P of the volatile molecules (b) in the presence of copolymers L-43 (empty markers) and P-103 (full markers), at 40 wt % of ethanol and 32 °C, derived from the results of NMR diffusion measurements.

Figure 3. Static headspace analysis of Romascone (a) and ethyl butyrate (b) in the presence of Pluronic L-43 and P-103 (5 wt %) at 40 wt % of ethanol and 32 °C.

Figure 4. Headspace concentrations P/Po0 of Romascone (a) and ethyl butyrate (b) with P-103 as a function of the total molar concentration of volatile molecules in solution PT; these data are used to calculate equilibrium constant, Kc. Best fits are indicated by the lines. Table 6. Approximate Equilibrium Constants Kc (L/mmol) for Romascone and Ethyl Butyrate and Copolymers P-103 and L-43 Kc (L/mmol)

Romascone

ethyl butyrate

P-103 0.061 0.01 L-43 0.019 naa a The results with polymer almost superpose on those without polymer, and the calculation gives Kc = 8.6  10-7 L/mmol.

On the other hand, for copolymer L-43 diffusion NMR showed no swelling in the presence of fragrance molecules. However, the polymer fractions of some of these fragrance molecules in L-43 assume nonzero values, though less pronounced than for P-103. Probably there is a certain affinity between the fragrance molecules and the copolymer, but in contrast to a cooperative micelle formation, as it is the case for P-103, these interactions seem to be weaker with L-43 and do not lead to pronounced swelling. 3.3. Influence of Block Copolymer Micelles on Evaporation of Ethyl Butyrate and Romascone. In order to understand better the influence of micelles on the evaporation of volatile Langmuir 2010, 26(11), 7953–7961

molecules, we used ethyl butyrate and Romascone as two model volatiles with different log P values. 3.3.1. Static Headspace Analysis of Active Volatile Molecules at 40% of Ethanol in Water and in the Presence of L-43 and P-103 at 5%. Headspace may be used to measure the concentration of a solute in solution. Here a complex solution of polymer aggregates solubilizes the volatile, the headspace P/Po0 measures the concentration of volatile which is “free” in the solvent while the balance is, we consider, in or associated with the aggregate. Figure 3 shows headspace vs concentration for different solutions of two volatiles. For RO, Figure 3a, in the simple ethanolic solution (]) P/Po0 , increased linearly with solution concentration until the solubility limit was attained at ∼1% RO. With the copolymer P-103 (), the headspace was below that of the simple solution: considerable perfume is solubilized in the P-103 micelles. The plateau of saturation was attained at 5-6%, which is consistent with the loading measured by NMR. The presence of micelles of P-103 increased the limit of solubilization of RO from DOI: 10.1021/la904832d

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Figure 5. Mass losses of solutions of ethyl butyrate measured by TGA (a) and extracted ethyl butyrate measured by GC analysis (b) as a function of time in the presence of Pluronic L-43 and P-103. Solvent is 40 wt % of ethanol, T = 32 °C. Values for the best-fit evaporation rates (slopes) are the lines of the plots in part a, values are given in Table 7.

less than 1% to more than 5%. The copolymer L-43 (Δ) had a similar effect on RO, but was less pronounced and thus indicates it is a less efficient reservoir. Alternatively, in the absence of L-43 aggregates, RO may not be solubilized but may simply associate with the polyethylene block and thus show up as a weak interaction. For EB (see Figure 3b) only minor differences between the simple and the complex solutions with copolymer were observed: no significant solubilization nor association with P-103 and L-43 copolymers can be determined. In fact, all profiles in Figure 3 are curved: this is due the high concentrations of RO and EB in the samples, a range which is comparable to that used in other measurements of this study, but which is no longer dilute in the sense of Raoult’s Law. The result is that the nature of the solvent changes as the solute increases. This effect precludes using these data to extract accurate equilibrium constants KC for the complexation between volatile and macromolecule M, to form V∼M. Nonetheless, Kc may be estimated using the initial points of the profiles, where the curvature is least pronounced. Figure 4 shows plots for the two volatiles and the polymer P-103: it presents the headspace data and the simulation of these data using the best fit KC, computed as described in section 2.9. The same computations are made for the volatiles and the copolymer L-43, the results are listed in Table 6. KC shows the strength of solute-micelle interaction: it is largest for RomasconeP103, compared to Romascone-L43. EB interacts very slightly with P103 and not at all with L43. 3.3.2. Thermogravimetric Analysis of Active Volatile Molecules at 40% of Ethanol in Water and in the Presence of L-43 and P-103 at 5%. Addition of copolymers reduces the chemical potential and influences the vapor pressure of volatile molecules, as described in the Flory mean-field approximation.64 Thermogravimetry (TG) is a convenient method to measure mass loss of a sample as a function of time in a dynamic process.65 However, in a mixture, TG does not indicate which component is lost by evaporation. Hence, we analyze the residual EB, or RO, by gas chromatography (GC) to measure its evaporation and to distinguish it from that of the solvent. We can determine if the different trends observed at equilibrium in solutions of volatiles and copolymers have an impact on the evaporation of the specifically the volatiles. Figure 5 shows the influence of copolymers L-43 and P-103 on mass losses (measured by TG) of solutions of EB as a function of time. (64) Ouali, L.; Leon, G.; Normand, V.; Johnsen, H.; Dyrli, A.; Schmid, R.; Bencz
edi, D. Polym. Adv. Technol. 2006, 17, 45–52. (65) Ternat, C.; Ouali, L.; Sommer, H.; Fieber, W.; Velazco, M. I.; Plummer, C. J. G.; Kreuzer, G.; Klok, H.-A.; Ma˚nson, J.-A. E.; Herrmann, A. Macromolecules 2008, 41, 7079–7089.

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Table 7. Evaporation Rates of Solutions of Ethyl Butyrate, Romascone, and Extracted Romascone in the Presence of Copolymers L-43 or P-103 and 40 wt % of Ethanol EB in system

evaporation rate -dm/dt (mg/min)

intercept (mg)

solvent þ EB solvent þ EB þ L-43 solvent þ EB þ P-103

1.080 1.080 1.070

15.40 15.95 15.84

RO in system

evaporation rate -dm/dt (mg/min)

intercept (mg)

solvent þ RO solvent þ RO þ L-43 solvent þ RO þ P-103 RO RO þ L-43 RO þ P-103

0.950 0.960 0.830 0.072 0.070 0.047

17.16 17.49 17.33 1.73 1.67 1.71

Mass loss profiles of the test solutions (EB, solvent and copolymer) are very close to that measured for the reference solution (EB and solvent). The trend line of each measurement was also plotted as a function of time to compute evaporation rates: the average rate (-dm/dt) was estimated to be 1.08 and 1.07 mg/min for solutions with copolymers L-43 and P-103 instead of 1.08 mg/min for the reference (Table 7). Thus, it appears that there is no impact of these copolymers on the evaporation of the three different samples of EB. If we track the loss of EB specifically by GC, we also find that all samples have equivalent evaporation profiles. These results confirm that the association between EB and copolymers L-43 and P-103 is absent, as observed in DLS, NMR spectroscopy, and static headspace analysis (Figures 2, and 4 and Table 4). Figure 6a shows similar results for RO: in the presence of L-43, the mass loss profile (by TG) of the test solution (RO, solvent and copolymer) is similar to that of the reference (RO and solvent) with evaporation rates of 0.95 and 0.96 mg/min respectively (Table 7): thus L-43 does not affect the evaporation of RO samples. A large amount of solvent may screen the interactions between the RO and the copolymer L-43, as suggested by the parallel curves and the small difference of the evaporation rates. This result was confirmed by GC of RO which shows similar evaporation profiles of test and reference samples (Figure 6b): the corresponding evaporation rates of RO were calculated to be 0.070 and 0.072 mg/min, respectively. These measurements again confirm that interactions between RO and copolymer L-43 are limited, as measured by DLS, NMR and static headspace analyses (Figures 2-4, and Table 4). On the other hand, with P-103, mass loss of RO solutions (by TG) is slower compared to that with L-43 or loss in the reference Langmuir 2010, 26(11), 7953–7961

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Figure 6. Mass losses of solutions of Romascone measured by TGA (a) and extracted Romascone measured by GC analysis (b) as a function of time in the presence of Pluronic L-43 and P-103 (same scale for ordinates in Figures 5a and 6a, different ranges). Solvent is 40 wt % of ethanol, T = 32 °C. Values for the best-fit evaporation rates of solutions (slopes) are the lines of the plots, values are given in Table 7.

(Figure 6a), its rate of 0.83 vs 0.96 mg/min. (Table 7). This trend was confirmed by the lower evaporation rate of RO (by GC) which decreased from 0.072 alone to 0.047 mg/min with P-103 (Figure 6b). Evaporation rates for both the solution (RO, solvent and P-103) and RO only are significantly lower than in the references. Therefore, P-103 micelles decreased the RO evaporation rate, even in the presence of large amounts of solvent. This difference of evaporation rate can be explained by the presence of RO in the micelle core. The micelles observed with DLS influences the partitioning of RO between the solvent phase and the polymer phase, as measured by NMR. This loading decreased the concentration of RO in the solvent phase and therefore its chemical potential. This reduction of the chemical potential decreased its vapor pressure under these conditions and thus decreased its evaporation. This interpretation was confirmed by the static headspace analysis: the micelles of P-103, contained most of the RO, thus its concentration in the continuous solvent-phase was decreased and consequently its concentration in the gas phase was also decreases. Ultimately, therefore, its evaporation was slower, as observed by TGA.

4. Conclusions Our objective in this work was to understand how block copolymer micelles can influence the evaporation of volatiles in ethanolic solutions. By controlling and slowing down evaporation rate, we hope to smell these volatiles over a longer period of time. The initial studies in this work have served to identify systems in which the polymer can solubilize the perfume and thus present the opportunity to control its release. On the basis of the phase diagram of Pluronic polymers in ethanolic solutions of less than 40% ethanol, we have identified polymers which exist as free chains or which form micelles, for example L-43 and P-103 respectively. If a Pluronic polymer chains self-assembles, fragrances can provoke them to form micelles and swell them. We observe that the higher the log P of the fragrance, the more that is solubilized in the micelles and consequently the larger the micelles become on swelling. Thus, the fragrances swell significantly P-103, which forms micelles, but little or not at all L-43, which does not micellize. This general behavior is observed

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by both DLS and NMR. Two examples of volatile molecules were chosen to study evaporation: ethyl butyrate, EB, which is soluble in 40-60 wt % ethanol-water and Romascone, RO, which is insoluble. Equilibrium constants, KC, as evaluated by headspace measurements, show that RO associates strongly with micelle-forming P-103; but RO associates only slightly with L-43, which exists only as a unimers, and thus have no capacity to solubilize. EB associates little if at all with either polymer: KC are small, thus EB apparently is in a good solvent, too good for it to solubilize in micelles of P-103 or to associate with unimers or L-43. Evaporation was evaluated by TG of the solution (volatile, solvent, and copolymer) or gas chromatography to measure only the volatile. EB shows a single evaporation rate which is the same for all systems whether alone in solvent or with either polymer: this result is consistent with the above observations that EB does not associate with the polymers, i.e., the equilibrium constant is small or null, it evaporates in the same way, oblivious to whether the polymers are present or absent. RO shows the same evaporation rate alone in solvent or with Pluronic L-43: the RO/L-43 couple is noninteractive. However RO evaporates ∼35% more slowly with P-103 than it does alone in solvent or with L-43. Again, this result is consistent with the above measurements which show that P-103 micelles solubilize a large amount of RO, swell as a result, and have a large equilibrium constant. Thus, to delay release, to control evaporation of volatiles, it is necessary to design a system where polymer and volatile(s) interact significantly. The mechanism of interaction may be described as an association, a solubilization or a complexation, the goal is to remove volatile from the solvent and thus reduce its free concentration;then its gas concentration is reduced and it evaporates more slowly. In the case of solvent mixtures, the solvent must be sufficiently poor for the polymer such that micelles form; it must also be sufficiently poor for the volatile such that the volatile partitions into the micelle. Acknowledgment. We acknowledge T. Zemb for fruitful discussions. The work was supported by ENSCBP-IPB, CNRS, University of Bordeaux, Region Aquitaine, and Firmenich SA.

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