Langmuir 1995,11, 725-729
725
Solubilization of Some Synthetic Perfumes by Anionic-Nonionic Mixed Surfactant Systems. 1 Yoshikazu Tokuoka,tJ?*Hirotaka Uchiyama,? Masahiko Abe,tJ and Sherril D. Christian5 Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278, Japan, Institute of Colloid and Interface Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan, and Institute for Applied Surfactant Research, The University of Oklahoma, Norman, Oklahoma 73019 Received November 3, 1993. I n Final Form: October 20, 1994@ The solubilization of synthetic perfumes (eugenol, linalool, benzyl acetate, a-ionone, a-hexylcinnamaldehyde, and d-limonene)by sodium dodecyl sulfate (SDS)/hexadecylpoly(oxyethy1ene)ethers (C16PoE20) mixed surfactants has been studied by measurement of the maximum additive concentration (MAC) and the distribution coefficientbetween the micellar and bulk phase (10.The MAC of the perfume at any mole fraction of C16POE2o increases with the hydrophilicity of the perfume. The MAC of the more hydrophilic perfume in the mixed systems is smaller than the calculated MAC (assuming additivity) because of hydrophilic-hydrophilic interactions between the head groups of anionic and nonionic surfactants in the mixed micelle. On the other hand, the radius of the mixed micelle in these systems is large compared to the radii of pure component micelles, causing the MAC values of the less hydrophilic perfumes in the mixed systems to be larger than the calculated MAC. Moreover, the values of K for each perfume in the mixture ofthe surfactants are smallerthan the idealgvalues; this effectincreases with a decreasein the hydrophilicity of the perfume, except in the case of d-limonene. The magnitude of these changes in K values in the surfactant mixtures depends on the relative importanceof surfactant-surfactant and surfactant-perfume interactions.
Introduction Solution properties of surfactant mixtures are often superior in applications to the properties of the individual s ~ r f a c t a n t s . l -Surfactants ~ used in practical applications almost always consist of mixtures of surface active agent^.^-^ Mixed surfactant systems are also of great interest in scientific and industrial applications. One can postulate that the tendency to form a micellar structure in mixed surfactant solutions would be substantially different from that in pure surfactant solutions. Since solubilization is directly related to micelle formation, it is difficult to predict the solubilization properties of a mixed surfactant system based upon properties of single surfactant systems. Of the literally hundreds of solubilization studies of organic compounds by mixed surfactant m i ~ e l l e s , ~ -most l ~ involve measurements with normal alkanes, alcohols, and oil-soluble dyes, but few studies have been made with synthetic perfume compounds.
* To whom correspondence should be addressed.
t Faculty of Science and Technology, Science University of Tokyo. Institute of Colloid a n d Interface Science, Science University of Tokyo. Institute for Applied Surfactant Research, The University of -Oklahoma. II Current address: Research and Develoument Division. S.T. Chemical Co., Ltd, 4-10,l-chome, shimo-ochid, Shinjuku-ku,Tokyo 161, J a p a n . Abstract published in Advance A C S Abstracts, February 1, 1995. (1)Lange, H.; Beck, K. H. Kolloid 2.2.Polym. 1973,251,424. (2)Abe, M.; Ogino, K In Mixed Surfactant Systems; Dekker: New York, 1993. (3)Moroi, Y.;Nishikido, N.; Matsuura, R. J . Colloid InteTface Sci. 1977,50,344; Moroi, Y.; Akisada, H.; Sato, M.; Matsuura, R. J . Colloid Interface Sci. 1977,61, 233. (4)Gvering, P.;Nilsson, P. G.; Lindman, B. J . Colloid Interface Sci. 1986,105, 41. (5)Osbome-Lee, I. W.; Schechter, R. S.; Wade, W. H.; Barakat, Y. J . Colloid Interface Sci. 1986,108, 60. (6)Malliaris, A.; Binana-Limbele, W.: Zana, R. J . Colloid Interface Sci. 1986,110,114. (7)Nishikido, N. J . Colloid Interface Sci. 1977,60,242. (8) Uchiyama, H.; Abe, M.; Ogino, K. J . Jpn. Oil Chem. SOC.1986, 35, 1031.
*
@
Synthetic perfumes are often used in industrial products such as cosmetics, foods, detergents, pesticides, and coating materials. The synthetic perfumes are used in various forms with other materials such as emulsifiers or solubilizers, depending on, for example, the properties of the medium, the solubility of the perfume in the solvent, and the stability of the perfume. In particular fields of cosmetic and food science,surfactants are used to solubilize oily synthetic perfumes. The aqueous solution solubilizing the perfume becomes transparent and the perfumes are stabilized. Hence, understanding how the perfume interacts with the surfactant in a n aqueous solution is essential for many industrial applications of perfumes. In previous papers,l7J8we reported the solubilization of some synthetic perfumes by anionic and/or nonionic single surfactants in aqueous solution. It was found that the hydrophilic property of the synthetic perfume affected its solubilization. In the present paper, we report the solubilization of some typical synthetic perfumes (eugenol, linalool,benzyl acetate, a-ionone, a-hexylcinnamaldehyde, and d-limonene) by sodium dodecyl sulfate and hexadecyl poly(oxyethy1ene)ethers mixed surfactant systems, obtained by the maximum additive concentration (MAC) method. Distribution coefficients between the micellar and bulk phases ( K ) are inferred for each compound as a (9) Nugara, N.; Prapaitrakul, W.; King, Jr.,A. D. J . Colloid Interface Sci. 1987,120,118. (10)Muto, Y.;Asada, M.; Takasawa, A.; Esumi, K.; Meguro, K J . Colloid Interface Sci. 1988,124,632. (ll)Treiner, C.; Nortz, M.; Vaution, C.; Puisieux, F. J . Colloid Interface Sci. 1989,125,261. (12)Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. J . Colloid Interface Sci. 1989,130, 254. (13)Uchiyama,H.;Tokuoka,Y.;Abe,M.; 0gin0,KJ . ColloidInterface Sci. 1989,132,88. (14)Abe, M.; Kubota, T.; Uchiyama, H.; Ogino, K. Colloid Polym. Sci. 1989,267,365. (15)Treiner, C.; Nortz, M.; Vaution, C. Langmuir 1990,6,1211. (16)Zhao, G.; Li, X. J . Colloid Interface Sci. 1991,144,185. (17)Abe,M.;Tokuoka,Y.;Uchiyama,H.;Ogino,K. J . Jpn.Oi1Chem. SOC.1990,39,565. (18)Tokuoka,Y.; Uchiyama, H.; Abe, M.; Ogino, K J . Colloid Interface Sci. 1992,152,402.
0743-746319512411-0725$09.00/0 0 1995 American Chemical Society
726 Langmuir, Vol. 11, No. 3, 1995
Tokuoka et al.
Table 1. Chemical Structure, Abbreviation, Molecular Weight, HLB, and Purity of Synthetic Perfumes Used in This Study chemical synthetic perfume abbreviation molecular weight HLB purity (%) structure ~
0 A
eugenol
EL
164.21
6.9
>98
linalool
LL
154.25
5.2
> 98
benzyl acetate
BA
150.17
4.2
>98
a-ionone
IN
192.30
3.0
>95
a-hexylcinnamaldehyde
HCA
216.33
2.1
> 95
d -1imon en e
LN
136.24
0.7
95
YH20COCH3
CH~HCHO
d
function of the relative concentrations of the two surfactants. Mixing effects of the two surfactants on the MAC and the K are discussed. Experimental Section Materials. Anionic surfactant, sodium dodecyl sulfate (SDS), wasthepurestgradeproduct(>99.7%purity)ofTokyoKaseiKogyo Co., Ltd., Tokyo, Japan. It was recrystallized from ethanol and extracted with ether. Nonionic surfactant, hexadecyl poly(oxyethy1ene)ethers (c16POEZO; C16H330(CHzCHzO)zoH),was supplied by Nihon Surfactant Industries Co., Ltd., Tokyo, Japan. They have a narrow molecular weight distribution. Their purities were ascertained by surface tension measurements and differential scanning calorimetry. Synthetic perfumes, eugenol (EL),linalool (LL),benzyl acetate (BA), a-ionone (IN), a-hexylcinnamaldehyde (HCA), and dlimonene (LN), were supplied by Hasegawa Kouryo Co., Ltd., Tokyo, Japan, and were used without further purification. Chemical structures, molecular weights, hydrophilic-lipophilic balance (HLB), and purities are shown in Table 1. The HLB values were calculated using the method of F ~ j i t a . ' ~As , ~can ~ be seen from the HLB values in Table 1,the hydrophilic properties of the perfumes follow the order EL > LL > BA > IN > HCA > LN. Water used in the experiments was twice distilled and deionized with an ion exchanger (NAN0 pure D-1791 of Barnstcad Co. Ltd.) and distilled again prior to use. The resistivity of the water was about 18.0 MQcm and the pH was 6.7; the surface tension was 72.1 mN/m a t 30 "C. Preparationof Surfactant Solutions and Aqueous Solutions Including Synthetic Perfume. Into several 100-mL glass-stoppered Erlenmeyer flasks, 20-mL portions of a given concentration of surfactant or water were placed, and varying amounts of a synthetic perfume were added. The mixtures were stirred by a shaker (Model SS-82D type of Tokyo Rikakikai Co., Tokyo, Japan) for 12 hand allowed to stand for 12 h to establish a solubilization equilibrium a t 30 "C. (19)Fujita, A.Kagaku No Ryoiki 1967,11, 719. (20) Fujimoto,T.InNew Introduction to SurfuceActiueAgents; Sanyo Chemical Industries Ltd.: Kyoto, Japan, 1985, p 197.
Determination of Maximum Additive Concentration and Solubility in Water. After equilibrium was established in the solutions, turbidities were determined by measuring the transmittance at 700 nm with a double-beam spectrophotometer (Model MPS-2000 of Shimadzu Co., Tokyo, Japan) with a quartz cell (10-mm light pass length), as described in the previous paper.l7 The solubility of each perfume in water was measured by using a total organic carbon analyzer (Model TOC-5000 of Shimadzu Co. Ltd.), which determined the amount of total organic carbon in the aqueous solution. Samples for the solubility measurements were prepared by placing excess perfume in aqueous solution and stirring for 12h. The undissolved organic phase was removed before the measurement by utilizing a glass filter (Glass Microfibre Filter, Whatman Ltd. England) when necessary to separate the small particles from the aqueous solution. Determination of Micellar Compositions. The micellar surfactant mole fraction ( X M )is not always equal to the stoichiometric surfactant mole fraction in the aqueous solution (ZO.l5 Turro et al. determined X M by using a pyrene probing method.21 The ratio between the intensities of the first and the third peaks of the pyrene fluorescence spectrum is proportional to the permittivity of the localized pyrene environment. The permittivity obtained from the fluorescence peaks of pyrene in the mixed micelle (EM) is related to the following relationship:
where €1 and €2 correspond to the permittivities in the pure surfactant micelles. The micellar mole fraction,XM,in the SDSC16POEzo system was determined by using eq 1and by measuring the fluorescence spectrum of pyrene solubilized in single and mixed surfactant micelles according to the method described in the previous paper.17
Results The solubility of each perfume in pure water (EL, 9.8 x mom; LL, 7.7 x mom; BA, 1.4x mom; IN, 5.6 x m o m ; and LN, 4.6 mom; HCA, 8.1 x (21) Turro, N.J.;Kuo, P. L.; Somasundaran,P.; Worg, K. J.Phys. Chem. 1986,90,288.
Solubilization of Some Synthetic Perfumes
Langmuir, Vol. 11, No. 3, 1995 727 0
;Conc. of CI6POEm(mom)
o.oO0 0.002 0.004 0.006 0.00s 0.010
,
I
0.010 0.00s 0.006 0.004 0.002 O.oO0
0 ;Conc. of SDS (mom) 0.010
0.008
E
v
0.006-
u
0.004 -
XC,SOE,
Figure 1. Concentration of perfume in micelle phase vs the
mixed system mole fraction O f C16POEzoin the SDS-C~~POEZO at 30 "C. ;Conc. of C#OEU, (mom) O.OO0 0.002 0.004 0.006 0.008 0.010 0.010 0.008 0.006 0.004 0.002 o.oO0
0 ;Conc. of SDS (mom)
0.04
v
0.02
0.01
O%U
XCI8OE2a
Figure 3. MAC of LN with mole fraction of C16POEzo in the S D S - C ~ ~ P O Emixed ~ O system (total concentration of 1.0 x mol/L) and concentration of pure surfactant at 30 "C. 0 is the
MAC in the mixed surfactant system; 0, MAC in the SDS system; and e, MAC in the C16POEzo system. systems (right-filled circle), the relation between the MAC and the mole fraction of C16POE2o is depicted. In the pure surfactant systems (open and closed circles), the concentrations of the single surfactants correspond to the concentrations in the mixed surfactant solution. In other words, the concentration ratio of each surfactant is equal to their molar ratio in the mixed surfactant solution. Moreover, the dashed line in the figures represents a n estimated MAC (Ccdfor the mixed system based on the additivity mixing rule (eq 2) using the MAC in pure surfactant solution,
0.03
"E
0
Oi2
014
0:s
0:s
-.-
where S C S Dand ~ Sc1620 are the moles of the synthetic perfume solubilized per unit mole of SDS and CI6POEzo, CSDSand c1620 are the concentrations of SDS and c16POE20 in the mixed surfactant solution, and SLak,is the solubility of the perfume in water. If the dissimilar surfactants form nearly pure component micelles rather than mixed micelles, the MAC would be expected to be comparable to the calculated MAC (Ccd). The results plotted in Figure 2 show that the experimental MAC of EL in the S D S - C ~ ~ P O Esolution ~O is obviously smaller than the calculated MAC (dashed line). In contrast to the solubilization behavior of EL, the MAC of LN is larger than the calculated MAC, as shown in Figure 3. A mole fractional distribution coefficient ( K ) can be defined as follows: nmic Per
where the superscripts "mic))and "aq" denote the micellar is the mole and the aqueous bulk phase, respectively. Xper fraction of the perfume in each micellar or aqueous solution phase, riper and nsudare the number of moles of synthetic perfume and of surfactant, and nwateris the number of
Tokuoka et al.
728 Langmuir, Vol. 11, No. 3, 1995 Table 2. Apparent Mole Fractional Distribution Coefficients of the Perfume ( x 109 in the SDS-ClsPOE2o Mixed System with Stoichiometric Mole Fraction ( X ) and Micellar Mole Fraction (xm) of ClsPOEzo
X XM EL LL BA IN HCA LN
0.00 0.00
0.20 0.38
0.40 0.62
0.60 0.69
0.80 0.71
1.00 1.00
3.80 4.36 1.55 24.59 135.12 11.57
3.57 3.77 1.55 19.30 52.07 13.38
3.59 3.78 1.73 24.07 80.06 20.52
3.58 4.02 1.70 24.06 100.73 22.15
3.77 4.02 1.88 26.76 100.72 25.28
4.08 4.43 2.08 31.15 120.00 28.90
o BA
1.5
A
HCA LN
lzzSs-4 0.
...........................
.............. -- .............
I
moles of water in the bulk phase. Since nwater is always much greater than the summation of nr: and nit;f, the (nizr rz::J term in eq 3 can be neglected, and eq 3 can be rearranged to give
+
Where CMand Cw refer to the MAC and the solubility of the perfume in water, respectively. CS is the total surfactant concentration and C,,, is the total monomeric surfactant concentration in the solution with the perfume. Here, since the value of K given from eq 4 neglects the partial molar volume of the surfactants, the K becomes the apparent mole fractional distribution coefficient. However, using the same surfactant system, one can compare the mixing effect of the surfactants on the K of each perfume. The C,, under solubilization conditions is generally different from the monomeric surfactant concentration in the pure surfactant s o l ~ t i o n ,which ~~,~~ corresponds to the critical micelle concentration (cmc). The C ,,, of SDS solutions with each perfume were m o m ; LL, 1.7 x mol&; measured:I7 EL, 8.0 x BA, 1.8 x low3m o m ; IN, 4.8 x mom; HCA, 8.7 x mol&, respectively. These mol&, and LN, 4.9x C,, values are employed t o calculate the K values in the SDS system. In the cases of nonionic single and anionicnonionic mixed surfactant systems, the values of C,, of the solubilized perfume solutions are unknown. Nevertheless, the cmc values ofthe surfactants are much smaller mol&) under the given experimental than CS(1.0 x ~ o n d i t i o n s .Therefore, ~~ C,, in eq 4 can be neglected in calculating K for the nonionic and the mixed surfactant systems, and their values of K for each perfume are obtained according to eq 4. The Kvalues are listed, along with values of X and XM,in Table 2.
Discussion Mixing Effect of Anionic and Nonionic Surfactants on MAC. The MAC of a solubilizate is affected both by its hydrophilicity and by its molar volume. Here, since the molar volume of the perfumes used is nearly equal,17one can explain the solubilization characteristics of the perfumes based on the hydrophilicity of the perfumes. In practice, Figure 1 shows that the amount of synthetic perfume solubilized in the micelle increases with its hydrophilic property. The MAC and the solubilizing capacity of polar organic solutes such as alcohols and normal fatty acids, solubilized into the hydrophilic part (palisade layer) of the micelle, are generally larger than those of nonpolar solutes such as normal alkanes, (22) Shirahama, K.; Kashiwabara, T. J . Colloid Interface Sci. 1971, 36,65. (23) Manabe, M.; Kawamura, H.; Yamashita, A.; Tokunaga, S. J . Colloid Interface Sci. 1987,115,147. (24) Ofino, K.; Kakihara, T.; Uchiyama, H.; Abe, M. J . Am. Oil Chem. Soc. 1988,65,405.
where Cexpis the experimental value of the MAC of a synthetic perfume in a mixed surfactant solution and Ccal is the calculated MAC from eq 2. When R is equal to zero, this implies that there is no mixing effect of surfactants on the MAC. The R values as a function of the mole fraction of C16POEzo in the mixed solution are shown in Figure 4. The synthetic perfumes seem to be classified into two distinct groups, smaller than zero (EL, LL, and BA) or larger than zero (IN, HCA, and LN). We have reported that in anionic-nonionic mixed surfactant systems the mixed micelle is formed more easily by nonionic surfactants having longer alkyl chains (shorter polyoxyethylenechains)than by nonionic surfactants with shorter alkyl chains (longer POE chain^).^^-^^ We have also reported that interactions between the hydrophilic head groups of the surfactants play an important role in stabilizing anionic-nonionic mixed m i ~ e l l e s . ~More~-~~ (25) Kuroiwa, s. J.Jpn. Oil Chem. SOC.1985,34,479. (26) Mackay, R. A. In Nonionic Surfactants; Schick, M. J . , Ed.; Dekker: New York, 1987, p 333. (27)Abe, M.; Tsubaki, N.; Ogino, K. J . Jpn. Oil Chem. SOC.1983,32, 672. (28) Abe, M.; Tsubaki, N.; Ogino, K. Colloid Polym. Sci. 1984,262, 584. (29) Abe, M.;Tsubaki, N.; Ogino, K. J . Colloid Interface Sei. 1985, 107,503. (30) Ogino, K.;Tsubaki, N.; Abe, M. J . Colloid Interface Sci. 1985, -107. .. , 509). - - -. (31) Abe,M.; Kakihara,T.;Uchiyama,H.; Ogino,K.J.Jpn. Oil Chem. SOC.1987,36,135. (32) Abe, M.; Ogino, K. In Mixed Surfactant Systems; Dekker: New York, 1993; p 2. (33) Ogino, K.; Uchiyama, H.; Ohsato, M.;Abe, M. J . Colloid Interface Sci. 1987,116,81.
Solubilization of Some Synthetic Perfumes
Langmuir, Vol. 11, No.3, 1995 729
over, in a study of the solubilization of azobenzene, which presumably solubilizes in the hydrocarbon core of the micelle, it was found that the MAC in the anionic-nonionic mixed system was greater than the summation of the MACs for the individual surfactant solution^.'^ The solubilization domain in the mixed micelles becomes larger than that of the pure surfactant micelles as a result of an increase in the radius of the mixed micelle with a n electric double l a ~ e r . ~ ~ , ~ ~ Although the mixed micelle is easily formed in the SDSCl$OEzo mixed system,24 hydrophilic-hydrophilic interactions may cause the hydrophilic part of the mixed micelle to be more rigid than that of single surfactant micelles. The region in which EL, LL, and BA can solubilize (that is, the hydrophilic part near the micellar surface) may therefore become smaller (or less accessible) than in the single surfactant micelles; therefore, it is reasonable that the R values of EL, LL, and BA are negative. On the contrary, the solubilization of IN, HCA, and LN, which occurs in the less hydrophilic interiorregion of the mixed micelle, may be influenced less by hydrophilic-hydrophilic interactions at the micelle surface. As a general rule, the effect of a n increase in the radius of the mixed micelle should be greater for the solubilization of hydrophobic perfume compounds than for hydrophilic components. In such systems, it will be observed that R values are positive, as is shown for IN, HCA, and LN in Figure 4. Mixing Effect of Surfactants on Apparent Distribution Coefficient ( K ) . The mixing effect of the surfactant on the MAC of the perfume was discussed above by applying the additivity mixing rule. The synergistic solubilization effects occur as a result of the interaction of a solubilizate with the individual surfactants and the interaction between the surfactants in the mixed micelle. To gain a better understanding of the synergistic solubilization effects, therefore, it is preferable to discuss the synergistic solubilization effect based on the thermodynamic ground. However, it should be noted that the thermodynamic definition of the synergistic solubilization effect may be different from the practical definition such as the additivity mixing rule.35 Treiner et aZ.11J5have suggested that the distribution coefficient of a neutral organic solute in a mixed surfactant system follows the relationship shown in eq 6 , In K = XMIn Kl
+ (1 - XM)In K2 + BX&
- XM) (6)
where K1 and Kz are the mole fraction distribution coefficients of a solute for the individual surfactants constituting the mixed micelle andXM corresponds to the micellar mole fraction of a surfactant having the value of K1. Moreover, B is an experimental parameter reflecting both the surfactant-surfactant interactions and the surfactant-solute interactions. When there is no synergistic effect on the partitioning of the solute, B is expected to be zero. Table 3 lists the analytical functions which represent the experimental data with the average B value a t any micelle molar fraction of C16POEz~, XM.As can be seen from Table 3, the B coefficients are negative for all the perfumes, and these coefficients become more negative as the hydrophilicity of the perfume decreases, except for LN. Such negative B values imply that K in the mixed surfactant system is in every case smaller than if the partitioning occurred ideally and that all of the perfumes are solubilized more readily in the single surfactant systems than in the mixed surfactant system. (34) Ogino, K.; Uchiyama, H.; Abe, M. Colloid Polym. Sei. 1987,265, 52. (35)Nishikido, N.Langmuzr 1991,7,2076.
Table 3. Characteristic Parameters of the Distribution Equation@la K = XMIn &620 + (1 - XM)In Ksns BXM(1 - XM) compd distribution equations
+
The B coefficient is closely related to the binary surfactant interaction parameter (a) estimated from regular solution theory. We obtained a n avalue of -6.18 in the SDS-C16POEZo system from surface tension measurements using the so-called regular solution theory.% In the case ofmixtures forming with large negative values of a,one will normally expect a greater reduction in the extent of solubilization of a polar solute in the mixed micellar system. However, values of the B coefficients must depend both on surfactant-surfactant interactions and on the surfactant-solute interactions in the mixed micelle. In systems where surfactant-solute interactions are not particularly strong, the B coefficient will be determined primarily by the a value. However, if there are strong surfactant-solute interactions in the mixed micelle, B may be less negative than expected, even if a is quite large and negative. Thus, a hydrophilic perfume may be attracted to the ether and sulfate head groups of the surfactants in the mixed micelle more strongly than a hydrophobic perfume. The progression of B values for EL, LL, BA, IN, and HCA in Table 3 indicates that hydrophilic groups of the perfume molecules tend to interact less and less strongly with the head groups as the hydrophilicity of these solutes decreases. On the other hand, the value of B for LN, the most hydrophobic perfume compound in this series, is nearly zero, indicating that solubilization of LN in the mixed systems is greatly enhanced compared to the solubilization of the polar solutes. This may result from a reduction in the Laplace pressure across the micelle-bulk interface, owing to a n increase in the aggregation number (and micellar radius) upon mixing of the surfactants.15 Since LN is practically nonpolar, its solubilization in the binary surfactant mixture may be influenced more by the increase in size ofthe surfactant than by the surfactant-surfactant interaction.
Conclusion The solubilization of typical synthetic perfumes in anionic-nonionic mixed surfactant solutions has been investigated using maximum additive concentration (MAC) measurements and determinations of the distribution coefficient between micellar and bulk phases (IO.In mixtures of SDS and Cl6POEzo, there is a negative synergistic effect on the MAC of the more hydrophilic perfumes (EL, LL, and BA), and a positive synergistic effect on the MAC of the less hydrophilic perfumes (IN, HCA, and LN). Examination of the values of the distribution coefficients, however, indicates a reduction in the tendency of the perfume components to be solubilized by the surfactant mixture, compared to the solubilization by the individual surfactants. This effect becomes greater as the solutes become less hydrophilic, although the negative effect practically disappears in the case of the most hydrophobic (least polar) perfume. LA9306342
