Solubilization and Emulsification of Perfume in Discontinuous Cubic

Solubilization of three typical perfume compounds, d-limonene (LN), β-ionone ... moieties of surfactant upon addition of perfume is evaluated by calc...
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Langmuir 2000, 16, 6891-6897

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Solubilization and Emulsification of Perfume in Discontinuous Cubic Phase Md. Hemayet Uddin,† Noriko Kanei,†,‡ and Hironobu Kunieda*,† Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan, and Fragrance Laboratory, Soda Aromatic Co. Ltd., Funakata 1573-4, Noda, Chiba 270-0233, Japan Received March 3, 2000. In Final Form: June 7, 2000 Solubilization of three typical perfume compounds, d-limonene (LN), β-ionone (IN), and geraniol (GL), in the discontinuous micellar cubic (I1) phase of the water-polyoxyethylene dodecyl ether (C12EO25) system was investigated by phase study and small-angle X-ray scattering (SAXS). The hydrocarbon perfume LN increases the maximum melting temperature of the I1 phase, whereas GL decreases it. This phenomenon is related to their solubilization location in the micelle. LN tends to be solubilized inside the aggregates and makes a perfume core (swelling). On the other hand, GL is solubilized in the surfactant palisade layer and expands the effective cross-sectional area per surfactant, aS (penetration). The penetration tendency is in the order GL, IN, and LN, judging from the SAXS analysis. The change in repulsion between hydrophilic moieties of surfactant upon addition of perfume is evaluated by calculating an optimum cross-sectional area, aS,opt, at which the EO chain is neither compressed nor expanded. It is found from the calculation of aS/aS,opt that the repulsion considerably increases in the LN system whereas it decreases in the GL system.

Introduction Perfume is one of the most versatile products used in our everyday life. It is often used in many industrial products such as cosmetics, perfumeries, foods, pharmaceuticals, detergents, pesticides, coating materials, and so forth and as odor-control or masking agents. Most of the perfume-containing products, especially foods and medicines, are water-based. Perfumes are, in general, strongly hydrophobic organic compounds and, hence, insoluble in water. They must be solubilized or emulsified in an aqueous medium by surfactants when they are applied to such formulations. Moreover, the use of alcohol as a solvent, with the exception of some attempts to use long-chain alcohols or glycols,1 has been restricted recently because of its undesired environmental effects. The solubilization of perfume compounds in aqueous solutions of anionic,2 nonionic,3 and nonionic-anionic4,5 surfactants has been investigated. There have also been a few studies to clarify the different phases and structures in water/surfactant/fragrance systems.6-13 The effect of ethanol on the solubilization of perfume compounds by * To whom correspondence should be addressed. Phone and Fax: +81-45-339-4190. E-mail: [email protected]. † Graduate School of Engineering, Yokohama National University. ‡ Fragrance Laboratory, Soda Aromatic Co., Ltd. (1) Ebrsama, T.; Teshima, Y. Eur. Pat. Appl. 1995, 687, 460. (2) Abe, M.; Tokuoka, Y.; Uchiyama, H.; Ogino, K. J. Jpn. Oil Chem. Soc. 1990, 39, 565. (3) Tokuoka, Y.; Uchiyama, H.; Abe, M.; Ogino, K. J. Colloid Interface Sci. 1992, 152, 402. (4) Tokuoka, Y.; Uchiyama, H.; Abe, M. J. Phys. Chem. 1994, 98, 6167. (5) Tokuoka, Y.; Uchiyama, H.; Abe, M.; Christian, S. D. Langmuir 1995, 11, 725. (6) Friberg, S. E. In Novel Cosmatic Delivery Systems; Magdassi, S., Touitou, E., Eds.; Marcel Dekker: New York, 1998. (7) Uchiyama, H.; Christian, S. D.; Scamehorn, J. F.; Abe, M.; Ogino, K. Langmuir 1991, 7, 95. (8) Herman, S. J. Cosmet. Toiletries 1994, 109, 71. (9) Friberg, S. E.; Vona, S. A. J. Soap, Cosmet., Chem. Spec. 1994, August, 32. (10) Suhaimi, H.; Rose, L. C.; Ahmad, F. B. H. Pertanika J. Sci. Technol. 1995, 3, 141.

nonionic surfactants was reported.14 The vapor pressures of perfume compounds in sodium dodecyl sulfate aqueous solutions,15-18 the variation of fragrance vapor pressure with time in traditional alcohol-based formulations,19 and the correlation between the polarities of such perfume compounds and the HLB of nonionic surfactants20-22 were studied. Recently in our laboratory, the effect of added perfumes on the phase behavior of the HLB (hydrophile lipophile balanced) temperature and the correlation with the solubilization mechanism of perfume compounds in aqueous surfactant systems have been studied.23 All the systems concerning the solubilization of perfume compounds, as far as we know, were performed in the micellar solution or in the hexagonal and lamellar liquid crystalline phases. As far as we are aware, no work on solubilization of perfume compounds in the cubic phase in the water/surfactant system has been completed. Cubic phases are transparent isotropic liquid crystals, which are often highly viscous and thermally stable over a long period of time. As the hydrophilicity of nonionic surfactant increases with the increase in the number of oxyethylene units, the nonionic surfactant with a long oxyethylene (11) Hamdan, S.; Ahmad, F. B. H.; Laili, C. R.; Fauziah, H. Orient. J. Chem. 1995, 11, 220. (12) Yang, J.; Rong, G.; Friberg, S. E.; Aikens, P. A. Int. J. Cosmet. Sci. 1996, 18, 43. (13) Friberg, S. E.; Yang, J.; Huang, T. Ind. Eng. Chem. Res. 1996, 35, 2856. (14) Tagawa, M.; Tabata, Y.; Ohba, N. Nippon Keshohin Gijutsusha Kaishi 1979, 13, 47. (15) Akahoshi, R.; Horike, S.; Noda, S. Nippon Kagaku Kaishi 1984, 12, 1974. (16) Akahoshi, R.; Horike, S.; Noda, S. Nippon Kagaku Kaishi 1985, 2, 214. (17) Akahoshi, R.; Horike, S.; Noda, S. Nippon Kagaku Kaishi 1985, 5, 943. (18) Horike, S.; Akahoshi, R. Nippon Kagaku Kaishi 1996, 12, 1033. (19) Sorrentino, F. Ph.D. Thesis, University of Bourgogne, Dijon, 1984. (20) Moore, C. D.; Bell, M. Soap Perfum. Cosmet. 1957, 30, 69. (21) Strianse, S. J.; Lanzet, M. Proc. Sci. Sect. Toilet Goods Assoc. 1960, 34, 8. (22) Angla, B. Soap Perfum. Cosmet. 1967, 40, 169. (23) Kanei, N.; Tamura, Y.; Kunieda, H. J. Colloid Interface Sci. 1999, 218, 13.

10.1021/la000327d CCC: $19.00 © 2000 American Chemical Society Published on Web 07/29/2000

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Table 1. Chemical Structure, Molecular Weight, Molar Volume, and Purity of Perfume Compounds

polar end has a tendency to form cubic phases. There are four types of cubic phases:24 discontinuous micellar (I1), reversed micellar (I2), bicontinuous (V1), and reversed bicontinuous (V2) cubic phases with different lattice spaces. The solubilization of ordinary hydrocarbon in the I1 phase has been studied in some polyoxyethylene alkyl ether (CmEOn)-type hydrophilic nonionic surfactant/water systems.25,26 Although a small quantity of oil is solubilized in the I1 phase, a large amount of oil can be incorporated into the very stable so-called cubic-phase-based emulsion in the I1 + O (excess oil) region.25 The properties of cubic phases might soon be extensively used both in fundamental study and in practical applications. In the present study we report the solubilization of three typical perfume compounds, d-limonene, β-ionone, and geraniol, in the cubic phase of the water/polyoxyethylene dodecyl ether (C12EO25) system. The solubilization of perfumes in the cubic phase and their thermal stability largely depend upon the nature of added perfume compounds, and they have been discussed by the observed phase behavior and the measured SAXS results. Materials and Methods Materials. The polyoxyethylene dodecyl ether, having an average of 25 oxyethylene units per molecule (represented by C12EO25), was purchased from Tokyo Kasei Kogyo Co. Ltd., Japan. The synthetic perfume compounds d-limonene (LN), β-ionone (IN), and geraniol (GL) were procured from Yasuhara Chemical, Givaudan Roure, and Kuraray, respectively. The chemicals were used without further purification. The chemical structures, molecular weights, and purities of the perfume compounds are shown in Table 1. Their purities were determined by gas chromatography (Hewlett-Packard HP 5890). Sample Preparation and Phase Diagram Determination. Appropriate amounts of surfactant, water, and perfume were weighed, and the mixtures were immediately flame-sealed in ampules. The samples in the fluid-solution regions were mixed by a vortex mixer, and the liquid crystalline samples were centrifuged repeatedly in alternating directions over the course of several days (when viscous) until they attained equilibrium. The samples were then kept in a thermostat to stand at constant temperature (∼25 °C) for at least 1 week before the first measurement was carried out. Different phase states were identified by direct visual inspection of the samples and by crossed polarizers for birefringence. Liquid crystals were distinguished by optical microscopy and small-angle X-ray scattering. Molar Volumes of Surfactant and Perfumes. The molar volumes of perfume compounds, shown in Table 1, were calculated (24) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1998, 14, 2627. (25) Rodriguez, C.; Shigeta, K.; Kunieda, H. J. Colloid Interface Sci. 2000, 223, 197. (26) Kunieda, H.; Shigeta, K.; Suzuki, M. Langmuir 1999, 15, 3118.

Figure 1. Schematic representations of d, rI, rH, dL, and aS in the I1 (a), H1 (b), and LR (c) phases. elsewhere.23 The molar volume of C12EOn, VS, can be calculated by the following relation

VS ) VL + nVEO + VOH

(1)

where VL, VEO, and VOH represent the molar volumes of the lipophilic part of the surfactant (dodecyl group), the oxyethylene unit, and the hydroxyl group, respectively. n is the number of oxyethylene units. The values of VL, VEO, and VOH were taken as 215, 38.8, and 8.8 mL mol-1 at 25 °C, respectively.27 Thus, VS ) 1193 mL mol-1 is obtained from eq 1. The volume fraction of the lipophilic part of the surfactant in the system, φL, is calculated by the following equation

VL φL ) φS VS

(2)

where φS is the volume fraction of surfactant in the system. Small-Angle X-ray Scattering (SAXS). Interlayer spacings, d, of cubic, hexagonal, and lamellar liquid crystals were measured using SAXS, performed on a small-angle scattering goniometer with an 18 kW Rigaku Denki rotating anode goniometer (RINT2500) at ∼25 °C. The samples were placed in a metal slit and covered with plastic films (Mylar seal method) for the measurement. Different liquid crystals were identified by the SAXS peak ratios.26 It is assumed that spherical micelles are packed in a cubic array in the I1 phase; the hexagonal (H1) phase consists of infinitely long cylindrical micelles packed in a hexagonal array, and the lamellar (LR) phase consists of infinitely wide bimolecular layers stacked in a parallel way, as is schematically shown in Figure 1. According to the geometry of liquid crystals, the effective cross-sectional area per surfactant molecule, aS, is calculated by the following equations using the interlayer spacing, d, obtained from the SAXS measurement. For the I1 phase,

{

rI ) d

3 (φ + φO) 4πnc L

}

1/3

C

(3)

(27) Kunieda, H.; Ozawa, K.; Huang, K. L. J. Phys. Chem. B 1998, 102, 831.

Solubilization and Emulsification of Perfume

(

)

3νL φL + φO rI φL

as )

Langmuir, Vol. 16, No. 17, 2000 6893 (4)

where rI is the radius of spherical micelle in the I1 phase, nc is the number of micelles in a unit cell, and C is a constant (C ) (h2 + k2 + l2)1/2, where h, k, and l are Miller indices). The values of the constants (nc, C) are (1, 1) for simple-cubic, (2, x2) for body-centered cubic, and (4, x3) for face-centered cubic structures. νL is the volume of the hydrophobic part of one surfactant molecule, and φO is the volume fraction of perfume compound in the system. For the H1 phase,

{

rH ) d

2 (φL + φO) x3π

as )

(

}

1/2

)

2νL φL + φO rH φL

(5)

(6)

where rH is the radius of the hydrophobic part in the cylindrical micelle. For the LR phase,

d dL ) (φL + φO) 2 as )

(

)

νL φL + φO dL φL

(7)

(8)

where dL is half of the hydrophobic thickness in the LR phase.

Results and Discussion Phase Behavior of Water/C12EO25/β-Ionone at Constant Temperature. The phase behavior of the water/ polyoxyethylene dodecyl ether (C12EO25)/β-ionone (IN) system was examined at constant temperature (25 °C) and pressure (atmospheric pressure). The results are shown in Figure 2. In the water/surfactant binary system, an aqueous micellar solution (Wm) phase and a cubic phase are formed in a relatively dilute region of surfactant. The cubic phase is expected to be built of discrete micellar aggregates as a discontinuous cubic (I1) phase, previously observed in polyoxyethylene derivatives with a long EO chain.28-34 This structure has been proposed before as a body-centered cubic structure for some polyoxyethylene alkyl ether systems.25,35,36 In a concentrated surfactant region, water dissolves in surfactant and forms a reversed micellar solution (Om) phase. The solubilization of IN in the Wm phase is low, while it is considerable in the I1 phase, as is shown in Figure 2. Upon further addition of IN, an excess oil (O) phase separates from the phase boundaries of the single Wm and I1 regions. However, it is found that a large amount of IN (up to ∼80%) can be incorporated into the I1 + O (28) Mitchell, D.; Tiddy, G.; Waring, L.; Bostock, T.; McDonald, M. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (29) Bouwstra, J. A.; Jousma, H.; van der Meulen, M. M.; Vijverberg, C. C.; Gooris, G. S.; Spies, F.; Junginger, H. E. Colloid Polym. Sci. 1989, 267, 531. (30) Huang, K.; Shigeta, K.; Kunieda, H. Prog. Colloid Polym. Sci. 1998, 110, 171. (31) So¨derlund, H.; Sjo¨blom, J.; Warnheim, T. J. Dispersion Sci. Technol. 1989, 10, 131. (32) Fontell, K. Colliod Polym. Sci. 1990, 268, 264. (33) Shigeta, K.; Suzuki, M.; Kunieda, H. Prog. Colloid Polym. Sci. 1997, 106, 49. (34) Kunieda, H.; Shigeta, K.; Ozawa, K. J. Phys. Chem. B 1997, 101, 7952. (35) Sakya, P.; Seddon, J. M.; Templer, R. H.; Mirkin, R. J.; Tiddy, G. J. T. Langmuir 1997, 13, 3706. (36) Hakanson, B.; Hansson, P.; Regev, O.; Soderman, O. Langmuir 1998, 14, 5730.

Figure 2. Phase diagram of the water/C12EO25/β-ionone system at 25 °C. Notations are as follows: I1, discontinuous cubic phase; H1, hexagonal phase; LR, lamellar phase; V1, bicontinuous cubic phase; D1, sponge phase; Wm, micellar solution phase; Om, reversed micellar solution phase; O, an excess oil phase; II, two-phase region.

region in cubic-phase-based emulsions (having a cubic phase as the continuous phase). In the previous paper25 we reported that highly concentrated translucent emulsions are formed in this region in a water/C12EO25/decane system; however, the emulsions are not translucent in the present system. This can be ascribed to the large difference between the refractive index of the pure water/ C12EO25 cubic phase (1.41 when the mass ratio is unity) and that of IN (1.52). At a moderately higher surfactant concentration (∼58% to 70%), the I1 phase changes to the lamellar (LR) phase through the hexagonal (H1) phase with increasing ionone content. The H1 and LR phases were identified by optical microscopy and SAXS measurements. Although a large amount of IN is solubilized in the lamellar phase, the phase transition from LR to other types of self-organized structures with a negative surfactant curvature such as the reversed hexagonal (H2) phase does not take place, because of the steric hindrance of the long EO chain.26 A narrow bicontinuous cubic (V1) phase and fluid-sponge (D1) phase are also observed. The V1 phase is determined by its high viscosity and optical inactivity and, finally, by SAXS measurements. This phase diagram is similar to that of the water/C18: 1EO50.8/m-xylene system except that, in the latter one, the V1 phase is formed between the H1 and LR phases.26 The phase behavior was explained by the solubilization mechanism of the oil. In this system, m-xylene is mostly solubilized in the surfactant palisade layer, as can be understood by the continuous increase of aS, which induces the decrease of curvature of the surfactant layer.27 However, as the EO chain length becomes larger, the surfactant molecular curvature remains positive in the dilute region because the EO chain length is fully hydrated and the repulsion between the hydrophilic groups is large. On the other hand, the I1-H1-V1-LR transitions take place in the concentrated region of surfactant, because the extent of hydration decreases. Phase Behavior as a Function of Temperature. The phase behavior of polyoxyethylene-type nonionic surfactant in water/oil systems is highly influenced by

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temperature. Moreover, even in the same surfactant system, the HLB temperature is changed depending on the nature of oil or perfume compounds. The effect of added d-limonene (LN), β-ionone (IN), and geraniol (GL) on the phase behavior of the water/C12EO25 system as a function of temperature was determined at the water/C12EO25 mass ratio of unity, as indicated by the arrow marked A in Figure 2, and the results are shown in Figure 3a and c. In the water/C12EO25 binary system (on the left-hand axis), the surfactant forms a discontinuous cubic (I1) phase which is stabilized up to ∼58 °C and melts to an aqueous micellar solution (Wm) phase above this temperature. In the LN system, a considerable amount of LN is solubilized in the I1 phase. With the further addition of LN, excess perfume (O) separated out, and a two-phase region (I1 + O) is formed. At the boundary, the interlayer spacing measured by SAXS becomes constant. With an increase of temperature, the solubilization of LN in the I1 phase increases and, finally, the I1 phase melts to form an oil-swollen aqueous micellar solution (Wm) phase or I1-H1-LR phase changes take place in a relatively LN-rich region. At a high temperature, the surfactant dissolves in the perfume, forms a water-swollen reversed micellar solution (Om) phase, and coexists with excess water (W). The melting point of the I1 phase (i.e., the phasetransition temperature from the cubic phase to Wm or other phases) increases with the incorporation of LN into the micelles, forming the cubic structure. That is, the thermal stability of the cubic phase increases with an increase in the solubilization of nonpolar perfume in the cubic phase. The maximum melting temperature of the cubic phase is designated as Tmax ) 100 °C. This phase behavior is similar to that of water/polyoxyethylene dodecyl ether/hydrocarbon systems.25 In these systems, the thermal stability of the cubic phase increases sharply upon addition of small amounts of oil (decane, hexadecane, or squalane). The phase behavior of the water/C12EO25/IN system is similar to that of the LN system. The exception is that the melting temperature of the I1 phase of this system first decreases and, after reaching a minimum, it increases with the increase of IN concentration. The maximum melting temperature of the I1 phase corresponds to the thermal stability (73 °C). The phase transitions I1-H1LR take place at relatively lower temperatures compared with those of the LN system. As is shown in Figure 2, the phase changes (I1-H1-LR) with an increase of perfume content in relatively surfactant-rich regions at constant temperature. It behaves similarly with an increase in temperature, as shown in Figure 3b. It can be attributed to the fact that hydration of the EO-chain of the surfactant molecule decreases with the decrease of water content or increase of temperature. The surfactant curvature tends to be less positive or zero. The phase behavior of the water/C12EO25/GL system is rather different from that of the two previously discussed systems. The solubilization of GL in the I1 phase is small. The melting temperature of the I1 phase decreases sharply and monotonically from that of the pure water/surfactant I1 phase, and the I1-H1-LR phase transitions occur even at room temperature. As is shown in Figure 3a and b, the surfactant layer curvature is changed from positive to negative with increasing temperature because the dehydration of the EO chain of C12EO25 is progressed. When the penetration tendency of the perfume or oil in the surfactant layer is large, the surfactant layer curvature is changed to be less positive or negative, whereas it changes to be more positive if oil is solubilized inside the aggregate, making an oil

Uddin et al.

Figure 3. (a) Phase diagram of the water/C12EO25/d-limonene system as a function of temperature. The water/C12EO25 mass ratio is unity. Phase notation as in Figure 2. W, an excess water phase; Tmax, the maximum melting temperature of the I 1 phase. (b) Phase diagram of the water/C12EO25/β-ionone system as a function of temperature. The water/C12EO25 mass ratio is unity. Phase notation as in Figure 2. (c) Phase diagram of the water/C12EO25/geraniol system as a function of temperature. The water/C12EO25 mass ratio is unity. Phase notation as in Figure 2.

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core (swelling tendency). Judging from the effect of these perfume compounds on the HLB temperature in the previous paper,23 the penetration tendency is in the order of GL > IN > LN. Hence, it is considered that the maximum temperature of the I1 phase increases in the LN system due to the large swelling tendency of the perfume compound. In other words, LN is solubilized in the core of the aggregate, and the surfactant layer is changed to be more positive. In the IN system, first, the perfume compound is solubilized in the palisade layer (penetration), and then with increasing concentration, it makes the perfume core. This causes the minimum melting temperature of the I1 phase, as is shown in Figure 3b. On the other hand, GL is continuously solubilized in the palisade layer, and the surfactant layer is changed to be less positive or negative. These mechanisms of solubilization and its relation to the stability of the I1 phase are investigated at the molecular level by SAXS measurements in the following section. Effect of Added Perfume on the Water/C12EO25 Cubic Structure. The interlayer spacings, d, of liquid crystals were measured by SAXS for all three perfume systems. The results are shown in Figure 4a and c. Since the number of SAXS peaks is not enough to identify the detailed structure of the I1 phase, we calculated the aS for simple, body-centered (bcc) and face-centered (fcc) cubic structures. The differences in aS for different cubic structures are small except for the simple cubic structure, which is not common in the present nonionic surfactant systems. Because the I1 phase of the water/C12EO25 binary system was considered as a body-centered cubic structure,25 we report the aS values only for the body-centered cubic structure. When the radius of the micelle, rI, and aS are calculated, it is assumed that the shape of the micelle in the I1 phase is spherical. The calculated rI (1.83 nm) is slightly longer than the hydrophobic chain length in its extended form, which is consistent with slightly aspherical micelles. However, since the difference is small, the assumption of a spherical micelle approximately holds. The d increases upon the addition of LN or IN, as these are incorporated into the micelle core of the I1 phase. When excess oil separates from the I1 phase, the d becomes constant, whereas, in the GL system, d remains constant in the I1 or H1 phase but increases in the LR phase. On the other hand, aS increases in the I1 and H1 phases and becomes almost constant in the LR phase in this system. GL is mainly considered to be solubilized in the vicinity of the interface and, hence, to increase aS. As a result of continuous penetration of GL, the I1 phase changes to an H1 phase and, finally, to an LR phase. In the LR phase, however, GL is mainly solubilized in the hydrophobic part, and the thickness (dL) is increased but aS is almost unchanged, as is shown in Figure 4c. By examining Figure 4a-c, it is found that both rI and aS increase with increasing perfume content in all the three systems. However, the increase of aS follows the order LN < IN < GL. Penetration or Swelling of Perfume. The degree of penetration of fragrance compound into the surfactant palisade layer can be evaluated by the following method. Suppose that oil molecules are completely penetrated into the palisade layer and are not solubilized inside the aggregate cores. As a first approximation, the radius of the micelle forming the I1 phase, rI, is regarded as constant, and eq 3 is rewritten as follows

d)

{

rI,0 4πnc 1 C 3 (φL + φO)

}

1/3

(9)

where rI,0 is the rI value of a perfume-free I1 phase (water/ C12EO25 mass ratio is unity). In this case, the interlayer spacing should decrease upon the addition of perfume. On the other hand, for the alternative possibility, it is assumed that perfume compounds are solubilized only in the cores of aggregates and do not penetrate into the palisade layer. In other words, the hydrophobic part of the surfactant is in a pure state. In this case, aS would be constant, and the following equation is obtained by combining eqs 3 and 4:

( )

3νL 4πnc d) aS,0C 3

1/3(φ L

+ φO)2/3 φL

(10)

where aS,0 ) 0.583 nm2 is the aS value of the perfume-free I1 phase. In this case, d increases upon addition of perfume compound. In accordance with eqs 9 and 10, we calculated the change in d as a function of φO, and the results are shown in Figure 5. The solid lines correspond to the d values calculated by eqs 9 and 10, considering the complete penetration or swelling concepts, and the points with different symbols correspond to the experimental d values of the three perfume systems. From Figure 5 it is clear that the properties of perfume compounds change from swelling to penetration in the order LN < IN < GL. It is expected that, since LN (unsaturated hydrocarbon) is less amphiphilic (in other words, less hydrophilic), it tends to be solubilized inside the hydrocarbon part of the surfactant, forming a perfume core. Hence, it does not expand aS largely, and its ability to make the surfactant curvature negative is relatively low. As a result, the I1 phase of this perfume compound system is highly thermally stable. IN is at first solubilized in the palisade layer and then in the micelle core, as can be understood by the downward tendency of d values in the low perfume region, shown in the Figure 5. This could well be an explanation of the broad minimum in the melting temperature curve in Figure 3b. On the other hand, for the GL system Figure 5 reveals that GL largely penetrates into the palisade layer and, hence, the phase I1-H1-LR changes occur at lower temperatures compared to those of the other perfume compound systems. Mechanism of Changing Curvature. The effective cross-sectional area per surfactant, aS, is determined by the balance between the repulsion of hydrophilic moieties and the attraction of lipophilic moieties, due to the interfacial tension of water-lipophilic moieties.37,38 The repulsion due to hydration, polarity, steric hindrance, and chain configuration tends to widen the aS, whereas the interfacial tension tends to shrink it. However, it has not been known what is the major contribution to the repulsion of the EO chains. In the present systems, the hydration of the EO chains can be regarded as being approximately constant, since the water/C12EO25 mass ratio is fixed at unity and only perfume content is changed. Since the perfumes LN or GL are hydrocarbons or long-chain alcohols and their carbon number is comparable with the hydrophobic part of surfactant, the water-hydrophobic region interfacial tension is considered not largely changed by solubilization. In fact, water-hydrocarbon interfacial tension reveals the same value, ∼50 mN/m, even if the molecular weight of oil is largely changed.39 Therefore, (37) Hyde, S.; Anderson, S.; Larsson, K.; Blum, Z.; Laudh, T.; Lindin, S.; Ninham, B. W. The Language of Shape; Elsevier: Amsterdam, 1997; Chapters 3 and 4. (38) Kunieda, H.; Umizu, G.; Yamaguchi, Y.; Suzuki, M. Nihon Yukagakkaishi 1998, 47, 879. (39) Fowkes, F. M. J. Phys. Chem. 1980, 84, 510.

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Figure 5. Correlation between the interlayer spacing, d, and the swelling or penetration effects of perfume in the I1 phase: 4, LN; 0, IN; O, GL.

Figure 6. Schematic representation of the section of the spherical micelle in the I1 phase. The small solid circle is the section of the micelle in the absence of oil, whereas the large one is that of the oil-swollen micelle. (a) If the length of the hydrophilic part, rEO, is constant and equal to rEO,0, the EO chain is neither compressed nor expanded. (b) If the aS is constant while oil is swollen in the aggregate, the hydrophilic moiety is laterally compressed. (c) If the aS increases very much, rEO should be decreased and the EO chain laterally expanded. Figure 4. (a) Variation of d, rI, and aS, in the I1 phase of the water/C12EO25/d-limonene system as a function of volume fraction of d-limonene (φO) at 25 °C. d (0), interlayer spacing; rI (4), radius of the lipophilic core of the micelles; aS (O), effective cross-sectional area per surfactant molecule. (b) Variation of d, rI, and aS, in the I1 phase of the water/C12EO25/β-ionone system as a function of volume fraction of β-ionone (φO) at 25 °C. Notations as in part a. (c) Variation of d, rI, rH, dL, and aS, in the I1, H1, and LR phases of the water/C12EO25/geraniol system as a function of volume fraction of geraniol (φO) at 25 °C. Notations as in part 4a. rH, radius of a lipophilic core of cylindrical micelles; dL, half of the hydrophobic thickness in the LR phase.

the phase transition can mainly be attributed to the change in repulsion of the EO chains. When oil is solubilized in the micelles forming the I1 phase, the radius of the lipophilic part is extended from rI,0 to rI, as is shown in Figure 6a. If the aS remains constant, the hydrophilic moieties or EO chains are compressed because the crosssectional area occupied by one EO chain has to be shrunk, as is schematically shown in Figure 6b. In other words, the EO chain would be elongated if the sphere radius increases because the distance between the adjacent EO

Solubilization and Emulsification of Perfume

Langmuir, Vol. 16, No. 17, 2000 6897

chains becomes close due to the decrease in angle for aS from the center of the micelle. It causes the increase in repulsion. On the other hand, if the aS is largely increased due to the large penetration of oil, the cross-sectional area per EO chain is laterally expanded, as is shown in Figure 6c. Since the EO chains are separated, the repulsion would decrease. We consider the optimum cross-sectional area per surfactant at the interface of the lipophilic part, aS,opt, at which the average cross-sectional area, a j EO, of the EO chain is neither compressed nor expanded while oil is solubilized. In the absence of oil, the radius of the lipophilic core rI,0 and the effective cross-sectional area aS,0 are calculated by eqs 3 and 4 using the condition φO ) 0. According to the packing model, we calculate the EO chain length, rEO, in the absence of oil by

(x ) 3

rEO ) RrI,0

φS -1 φL

(11)

where R > 1 is a correction factor to take into account the hydration of the EO chain. Since the EO chain is hydrated by water, the real EO chain length would be longer than the present rEO, because the volume of the hydrophilic part is the sum of the EO chain and hydrated water. In the present calculation for rEO, we consider R ) 1, as its real value is unknown. Since eq 4 holds for both actual aS and aS,opt, the following relation is valid

aS aS,opt

)

rI,opt rI

(12)

where rI,opt is the radius of the lipophilic core of the spherical micelle in which the effective cross-sectional area is aS,opt. Since rI,opt3 : (rI,opt + rEO)3 ) (φL + φO) : (φS + φO), we obtain

(x

rEO ) aS,opt rI aS

3

φS + φO -1 φL + φO

)

-1

(13)

Combining eqs 3, 11, and 13, we obtain

x

rI,0 ) aS,opt dC aS

3

4πnc 3

3

3

xφS - xφL 3 3 3 xφL(xφS + φO - xφL + φO)

(14)

Equation 14 shows how the EO chain is compressed or expanded when oil is solubilized. When aS/aS,opt is unity, the EO chain is relaxed (i.e., neither compressed nor expanded) while the solubilization proceeds. This means that the repulsion is unchanged. The aS/aS,opt values were calculated for the three systems, and the results are shown graphically in Figure 7. In the LN system, aS/aS,opt decreases with an increase of LN content. The repulsion of the EO chain is increased due to the decrease in distance between the EO chains. This facilitates the stability of the cubic phase in this system. The increase in repulsion may also influence the amount of solubilization of perfume in the I1 phase. In the LN system, the repulsion rapidly increases with increasing LN content and it resists reducing the surfactant layer curvature (increase in the

Figure 7. aS/aS,opt in the I1 phase as a function of φO. aS/aS,opt < 1 indicates that the repulsion of the surfactant EO chains increases with increasing oil content, whereas aS/aS,opt > 1 means that the attraction of lipophilic moieties increases. aS is the effective cross-sectional area at the interface in the I1 phase in each oil system. aS,opt is the optimum aS at which the hydrophilic moieties or EO chains are neither compressed nor expanded upon the solubilization of oil.

radius of the micelle). Therefore, the solubilization of LN is lower than that of IN. On the other hand, in the GL system, aS/aS,opt increases more stiffly upon the addition of oil. It means that the repulsion is decreased when the oil is swollen into the micelle. Then, the attraction force becomes larger than the repulsion. If spherical micelles change into cylinders and then into a bilayer phase, the EO chains become closer and the repulsion would increase, even at the same aS. Conclusion The “solubilization capacity” of the hydrophobic perfume compound LN and the less hydrophobic IN in the I1 phase of the aqueous C12EO25 system is large, whereas it is small for the hydrophilic perfume compound GL. In the latter case, the I1 phase is shrunk as the I1-H1-LR phase transitions take place because of the continuous penetration of GL into the surfactant palisade layer at constant temperature. The effective cross-sectional area per surfactant molecule, aS, at the water-hydrocarbon chain interface increases in the order LN < IN < GL as the hydrophilicity of these compounds increases in the same order (depending on the chemical structures of the typical perfume compounds). The simple geometrical relation aS/ aS,opt shows that the repulsion between the EO chains increases with increasing LN solubilization. Here, aS,opt means the cross-sectional area at which the EO chain is neither compressed nor expanded during the solubilization. This repulsion stabilizes the I1 phase of the LN system up to a high temperature. On the other hand, the repulsion decreases upon solubilization of GL because the aS becomes larger than aS,opt because of the penetration of the perfume compound in the surfactant palisade layer and, hence, phase changes take place. LA000327D