Premicellar Aggregation of Fatty Acid N-Methylethanolamides in

Langmuir 2006, 22, 2039-2044

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Premicellar Aggregation of Fatty Acid N-Methylethanolamides in Aqueous Solutions Takaya Sakai,*,† Youhei Kaneko,† and Kaoru Tsujii‡ Materials DeVelopment Research Laboratories, Kao Corporation, Minato 1334. Wakayama-shi, Wakayama 640-8580, Japan, and Nanotechnology Research Center, Research Institute for Electronic Science, Hokkaido UniVersity, N21, W10 Kita-ku, Sapporo 001-0021, Japan ReceiVed September 28, 2005. In Final Form: December 7, 2005 The aggregation behaviors of an excellent nonionic foam booster, namely, fatty acid N-methylethanolamide (NMEAX; X indicates the carbon number of the acyl group), in aqueous solutions have been studied by equilibrium surface tension (γ), solubilization of oil-soluble dye, and steady-state fluorescence techniques. NMEA, having a longer alkyl chain than NMEA-08, clearly had two break points on the γ versus log C (where C is concentration) curves. The solubilization of the oil-soluble dye for NMEA aqueous solutions began at the break point of higher concentration in the γ versus log C curves, so this concentration was confirmed to be the critical micellization concentration (cmc). Above the cmc, however, a separate oil phase of NMEA was observed instead of micelles of limited size. Another break point at lower concentration was also observed in plots of the fluorescence intensity ratio of pyrene, I1/I3, versus log C of NMEA. The gradual decrease of I1/I3 and the appearance of excimer emission of pyrene in the concentration region between the two break points suggest the existence and growth of premicellar aggregates and the solubilization ability of pyrene. Consequently, this break point at lower concentration was assumed to be the critical premicellization concentration (cac). The surface tension reduction in the premicellar region decreased with increasing alkyl chain length of NMEA.

Introduction Fatty acid alkanolamides, such as fatty acid monoethanolamide (MEA) and fatty acid diethanolamide (DEA), have been widely used in the formulations of detergents, cosmetics, and some other consumer products. In such commercial products, mixed surfactant systems of anionic surfactants and cosurfactants are generally employed to obtain better performance for the products. Fatty acid alkanolamides increase the foaming power as a cosurfactant. The fatty acid N-methylethanolamide (NMEA), a nonionic surfactant of alkanolamides, has a lower melting point and better solubility in mixed surfactant systems than other widely known fatty acid alkanolamides (e.g., MEA, DEA). However, NMEA has not been used in industrial and consumer products, and there are very few studies concerning its characteristics.1 Our results on the foam-boosting activities of NMEA indicate that this agent enhances the foamability of anionic surfactants systems rather than their foam stability.2 Furthermore, it was found that dilute solutions of various mixed surfactant systems with NMEA show the typical viscoelastic behavior of wormlike micellar solutions3-5 and interesting phase behaviors.6,7 NMEA has a very simple molecular structure, but its solution behaviors * To whom correspondence should be addressed. E-mail: sakai.takaya@ kao.co.jp. Phone: +81-73-426-7970. Fax: +81-73-426-7909. † Kao Corporation. ‡ Hokkaido University. (1) Weil, J. K.; Pariis, N.; Noble, W. R.; Smith, F. D.; Stirton, A. J. Am. Oil Chem. Soc. 1971, 48, 674. (2) Sakai, T.; Kaneko, Y. J. Surf. Deterg. 2004, 7, 291. (3) Rodriguez, C.; Acharya, D. P.; Hattori, K.; Sakai T.; Kunieda, H. Langmuir 2003, 19, 8692. (4) Acharya, D. P.; Hattori, K.; Sakai, T.; Kunieda, H. Langmuir 2003, 19, 9173. (5) Acharya, D. P.; Hossain, M. K.; Feng, J.; Sakai, T.; Kunieda, H. Phys. Chem. Chem. Phys. 2004, 6, 1627. (6) Rodriguez, C.; Sakai, T.; Fujiyama, R.; Kunieda, H. J. Colloid Interface Sci. 2004, 270, 483. (7) Hossain, M. K.; Acharya, D. P.; Kunieda, H.; Sakai, T. J. Colloid Interface Sci. 2004, 277, 235.

are highly unique. Therefore, understanding the properties of aqueous NMEA solutions is quite important. Premicellar aggregation behavior is one of the most interesting subjects for understanding the self-assembly of surfactants in aqueous solutions. It is known that some surface-active agents form premicellar aggregates at concentrations below the critical micellization concentration (cmc). Imae et al. reported that the surface tension vs concentration curves of a nonionic surfaceactive dye in aqueous methanol solutions have two clear break points, which indicates two-step micellization of the molecules.8 Ozeki et al. showed that crown ether surfactants have several break points on surface tension vs concentration curves and proposed that the formation of premicellar aggregates occurs in the bulk solution.9,10 The most well-known surfactants that form premicellar aggregates are gemini surfactants. Premicellization seems to be a rather general effect in gemini surfactant solutions. Menger et al. provided the first evidence for this interesting property of gemini surfactants using anionic and cationic gemini surfactants.11 Since then, many studies on premicellar aggregation of cationic gemini surfactants have been performed. The sizes of premicellar aggregates of gemini surfactants have been investigated extensively, and the aggregation number is assumed to be 2-5.12-14 It is interesting to note that the effects of premicellization hardly appear in the surface tension vs concentration curves of gemini surfactant solutions, unlike the situation for the two surfactants mentioned above. Both of these surfactants have very bulky and complicated structures, which (8) Imae, T.; Mori, C.; Ikeda, S. J. Chem. Soc., Faraday Trans. 1 1982, 78, 1359. (9) Ozeki, S.; Ikegawa, T.; Takahashi, H.; Kuwamura, T. Langmuir 1988, 4, 1070. (10) Ozeki, S.; Ikegawa, T.; Inokuma, A.; Kuwamura, T. Langmuir 1989, 5, 222. (11) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (12) Rosen, M. J.; Liu, L. J. Am. Oil Chem. Soc. 1996, 73, 885. (13) Pinazo, A.; Wen, X.; Perez, L.; Infante, M. R.; Franses, E. I. Langmuir 1999, 15, 3134. (14) Mathias, J. H.; Rosen, M. J.; Davenport, L. Langmuir 2001, 17, 6148.

10.1021/la052640r CCC: $33.50 © 2006 American Chemical Society Published on Web 02/02/2006

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Figure 1. Chemical structure of fatty acid N-methylethanolamides, NMEA-X, where X ) 06, R ) C5H11; X ) 08, R ) C7H15; X ) 10, R ) C9H19; X ) 12, R ) C11H23; X ) 14, R ) C13H27.

affects their aggregation behaviors through steric interactions between molecules such as preferable conformations and the stacking of hydrophilic groups. In this article, we report a study of the aggregation behaviors of NMEA with various alkyl chain lengths in aqueous solutions using equilibrium surface tension, solubilization, and steadystate fluorescence measurements. Although NMEA has a very simple molecular structure, it turns out that NMEA tends to form premicellar aggregates over a wide range of concentrations below the cmc. It was also found, interestingly, that the oil phase of NMEA separates instead of forming micelles above the cmc. We use the term “cmc” in this article for the concentration at which the separated phase starts to appear, as the apparent phenomenon is quite similar to micelle formation unless the solution is centrifuged. Materials and Methods Materials. Fatty acid N-methylethanolamides, designated as NMEA-X (where X indicates the carbon number of the acyl group, i.e., X ) 08, octanamide; X ) 10, decanamide; X ) 12, dodecanamide; X ) 14, tetradecanamide) and shown in Figure 1, were prepared by the condensation reaction of fatty acid methyl esters and 2-(methylamino)ethanol in the presence of alkaline catalyst (sodium methylate). Alkaline water was added to the reaction mixtures. The reaction mixtures were aged at 50 °C for 3 h and then extracted with a chloroform/hexane mixture (10/0-3/7 by volume). The NMEAs used in this study were of excellent purity (> 99.5%). The oil-soluble dye o-toluene-azo-β-naphthol (Wako Pure Chemical Industries Ltd.) and pyrene (>98% pure; Wako Pure Chemical Industries Ltd.) were used without further purification. Distilled water, which was checked to give the surface tension of 72.0 mN/m at 25 °C, was used to prepare the sample solutions.

Sakai et al. Surface Tension Measurements. Surface tensions of the aqueous NMEA solutions were measured with a CBVP-Z tensiometer (Kyowa Interface Science Co. Ltd.) employing the Wilhelmy Pt plate technique. Measurements were made at 25 °C in an equilibrium state; i.e., the surface tension was taken after the change in the observed value was less than 0.1 mN/m for 1 h. Solubilization Measurements. A 10-mL centrifuge tube containing 5 mL of test solution and the required amount of o-tolueneazo-β-naphthol was ultrasonicated for 15 min and shaken gently in a water bath at 25 °C for a few days. The test solution was then filtered. The absorbance was measured at 490 nm to determine the solubilized amount of the dye. Steady-State Fluorescence Measurements. Steady-state fluorescence measurements employing pyrene as the probe were carried out on a Hitachi F-4010 spectrofluorometer. Emission spectra were recorded at an excitation wavelength of 339 nm. Pyrene was used as a probe at 1.0 × 10-6 mol/dm3. Observation of the Phase Separation of NMEA Solutions. The NMEA phase separations after centrifuging were confirmed by visual observation. The optical nature of the separated liquid phases was checked with crossed polarizers.

Results Equilibrium Surface Tension. Equilibrium surface tension, γ, is plotted against log C in Figure 2 for the NMEA series in water at 25 °C. From the viewpoint of the relationship between surface activity and the alkyl chain length of NMEA, Figure 2 displays the quite normal behavior as expected, that is, the γ vs log C plots are shifted to the lower-concentration region of NMEA. γcmc, which is the surface tension at the cmc, gradually decreases with increasing alkyl chain length of the NMEA series. The attained surface tensions of all NMEA series except NMEA-06 at about 25 mN/m are quite low. These results might originate from phase separation of NMEA instead of micelle formation, as discussed later. NMEA-06 is dissolved in water throughout the whole concentration range tested. The NMEA-06 solution exhibits the same typical γ vs log C plots as sodium dodecyl sulfate (SDS) and many other ordinary surfactants. Phase separation takes place at a certain concentration in the flat region of the surface tension for all members of the NMEA

Figure 2. Equilibrium surface tension (γ) versus log C of (b) NMEA-06, (0) NMEA-08, ([) NMEA-10, (4) NMEA-12, and (9) NMEA-14 aqueous solutions at 25 °C. C1 indicates the concentration of the break point at lower concentration. C2 indicates the phase-separation concentration.

Premicellar Aggregation of an Alkanolamide

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Figure 3. Solubilization ability of o-toluene-azo-β-naphthol for aqueous solutions of the NMEA series at 25 °C: (b) NMEA-06, (0) NMEA-08, ([) NMEA-10, (4) NMEA-12, and (9) NMEA-14. C2 values are indicated on the γ plots of the respective NMEAs in Figure 2.

series except NMEA-06. The phase-separation concentrations of NMEA having various alkyl chain lengths are shown as C2 in Figure 2. It has already been reported that liquid-liquid phase separation takes place in NMEA-0815 and NMEA-10 aqueous solutions, that NMEA-12 forms a lamellar phase dispersed in water,15 and that NMEA-14 precipitates as a solid. One can clearly see the two break points on the γ vs log C plots of NMEA-08, -10, and -12. The break point at lower concentration on each surface tension curve is shown as C1. The higher break points agree with the corresponding phase-separation concentrations, C2. NMEA-14 also shows the normal γ vs log C curves having one break point. However, for NMEA-14, phase separation does not occur at break point C1. The phase-separation concentration, C2, for NMEA-14 is about 8 times higher than C1. All NMEAs except NMEA-06 are assumed to form the premicelles at the critical concentration C1. Furthermore, at C2, they probably aggregate to form micelles of infinitely large size, i.e., phase separate. We will take up this matter in detail in the Discussion section. Solubilization of an Oil-Soluble Dye. The solubilization behavior of an oil-soluble dye, o-toluene-azo-β-naphthol, in aqueous solutions of NMEAs was studied to determine the cmc’s of the NMEA series. The absorbance of the solubilized dye at 490 nm observed at 25 °C is plotted in Figure 3. The break point on the NMEA-06 curve appears at the same concentration as in the equilibrium surface tension curve shown in Figure 2. The C2 values in Figure 3 agree with the phase-separation concentrations or break-point concentrations observed in Figure 2. For all NMEAs except NMEA-06, the absorbance increases suddenly at C2. In the concentration region just above C2, NMEA-08 and -10 appear to form orange micellar solutions. However, when the solutions are centrifuged, an orange oil phase separates, and the bottom solution has the capability of solubilizing the dye. It is likely that micelles coexist in this concentration range. Thus, C2 is the beginning concentration of solubilization and might correspond to the cmc, i.e., the phase-separation concentration. Steady-State Fluorescence Measurements. The emission spectra of pyrene at various concentrations of NMEA-08 are (15) Feng, J.; Rodriguez, C.; Izawa, T.; Kunieda, H.; Sakai, T. J. Dispersion Sci. Technol. 2004, 25, 163.

Figure 4. Emission spectra of pyrene (1 × 10-6 mol/dm3) in NMEA08 aqueous solutions: [NMEA-08] ) (1) 3.0 × 10-3, (2) 4.0 × 10-3, and (3) 1.5 × 10-2 mol/dm3. In these results, the fourth and fifth peaks of the characteristic five maxima of pyrene monomer at 370-400 nm cannot be distinguished.

shown in Figure 4. The characteristic emission spectrum of pyrene monomer in the visible region shows five maxima at 370-400 nm. (In Figure 4, the fourth and fifth peaks come in succession, so two separate peaks cannot be distinguished.) The intensity ratio of the first and third peaks of the pyrene emission spectrum, I1/I3, is well-known to indicate the polarity of the probe environment,16 and a decrease of I1/I3 denotes a decrease of the polarity. The abrupt change of I1/I3 as a function of surfactant concentration has been commonly used to determine the cmc of surfactant solutions. Plots of I1/I3 vs log C for the NMEAs in water at 25 °C are shown in Figures 5-9. The critical concentrations of C1 and C2 in Figures 2 and 3 again appear as break points in Figures 6-9. In Figure 5, for the NMEA-06 system, the I1/I3 curve has a sharp drop at a particular concentration that corresponds to the break point for the γ and solubilization vs log C curves. It is evident that this concentration is the cmc of NMEA-06 and that the agent shows aggregation behavior typical of surfactants such as SDS, (16) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.

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Figure 5. I1/I3 vs log C of NMEA-06 in aqueous solution using pyrene at 25 °C. The excimer emission was not observed in the NMEA-06 system. The cmc was determined by equilibrium surface tension measurements.

Figure 6. (b) I1/I3 and (O) Iex/I1 [ratio of excimer (480 nm) to monomer (372 nm) fluorescence] vs log C for NMEA-08 in aqueous solution using pyrene at 25 °C. C1 and C2 are indicated in Figures 2 and 3.

Figure 7. (b) I1/I3 and (O) Iex/I1 vs log C for NMEA-10 in aqueous solution using pyrene at 25 °C. C1 and C2 are indicated in Figures 2 and 3.

which is the transition from monomers to micelles. In Figure 6, it can be seen that the value of I1/I3 remains constant at about 1.8 at low concentration of NMEA-08, which is the same behavior as in water. The onset of the decrease in I1/I3 occurs at a certain concentration of NMEA-08 that agrees well with C1. I1/I3 decreases gradually from 1.8 to 1.3 with increasing concentration of NMEA-08 over the wide range of concentration from C1 to C2. A plateau in the I1/I3 vs log C plots appears beyond C2. The same changes in I1/I3 with increasing NMEA concentration were

Sakai et al.

Figure 8. (b) I1/I3 and (O) Iex/I1 vs log C for NMEA-12 in aqueous solution using pyrene at 25 °C. C1 and C2 are indicated in Figures 2 and 3.

Figure 9. (b) I1/I3 and (O) Iex/I1 vs log C for NMEA-14 in aqueous solution using pyrene at 25 °C. C1 and C2 are indicated in Figures 2 and 3.

observed also for the NMEA-10, -12, and -14 solutions (Figures 7-9). The most interesting aspect is the gradual decrease in I1/I3 between concentrations C1 and C2, which is unlike the behavior of typical surfactants such as SDS and NMEA-06 mentioned above. In Figure 4, a broad emission band with a maximum at 480 nm can be seen at only a certain concentration range of NMEA08. The appearance of this band is attributed to the formation of an excimer, which occurs through the reaction of an excited pyrene molecule with another pyrene molecule in the ground state.17 The ratio in the emission intensity of the maximum of the excimer to the monomer spectrum of pyrene, Iex/I1, can be used to judge the efficiency of excimer formation. An Iex/I1 plot for aqueous NMEA-08 solutions is also given in Figure 6. One can see that the excimer emission of pyrene is interestingly observed at only the concentration range between C1 and C2, where the I1/I3 decreases gradually. Figures 7-9 also suggest that the fluorescence emission behaviors of all NMEAs except NMEA-06 are same as that of NMEA-08. In all cases except NMEA-06, Iex/I1 has a maximum at a particular concentration just below C2 () cmc, i.e., phase-separation concentration) and diminishes sharply with increasing concentration of NMEA. In addition, the excimer emission vanishes when the plateau is reached in each I1/I3 curve (>cmc). The excimer emission cannot generally be observed in dilute pyrene solutions of isotropic solvents. For example, it has been reported that a 2 × 10-6 mol/dm3 ethanol solution of pyrene does not exhibit the excimer (17) Sivakumar, A.; Somasundaran, P. Langmuir 1994, 10, 131.

Premicellar Aggregation of an Alkanolamide

emission.18 However, the results in this study show that, compared to ethanol solution, a lower pyrene concentration (1 × 10-6 mol/dm3) suffices in these systems for efficient excimer formation. Pyrene molecules can be assumed to be dissolved in hydrophobic microdomains, where the local concentration is much higher than the bulk concentration.

Discussion The γ vs log C plots for NMEA-08, -10, and -12 show a unique shape having two break points (Figure 2). It has been well-known that the formation of surfactant aggregates changes the slope of the γ vs log C curve. Some mixed surfactant systems including polymer components have been found to have such double break points in their γ vs log C plots.19,20 These reports have demonstrated that the binding of surfactants to polymer molecules starts at C1 and continues up to C2. Above C2, the surface tension of the solution is close to that of a solution containing regular surfactant micelles. C1 and C2 have been explained as the transition concentrations of aggregation. Moreover, it is known that only a few surfactants have multiple break points in their γ vs log C plots in single-component solutions.8-10 The change of slope at C1 in their γ vs log C plots is attributed to the formation of premicellar aggregates. The absorbance measurements of the solubilized dye with increasing concentrations of NMEA-08, -10, and -12 show that the solubilization starts at concentration C2 (Figure 3). It is widely known that the onset point of solubilization is the cmc, and this is widely used as a method to determine the cmc. Therefore, the C2 values for the NMEAs are thought to be the cmc’s, which corresponds to the phase-separation concentrations, as mentioned before. On the other hand, the solubilization of oil-soluble dye does not occur at concentrations below the cmc (i.e., C2). If NMEAs form premicellar aggregates at a certain concentration, C1, the aggregates might not have a size or structure sufficient to hold the dye molecules in their hydrophobic core portion. Thus, we should carefully determine whether C1 is indeed the critical premicellization concentration (cac). Figures 6-8 show the characteristic changes in the value of I1/I3 with increasing concentrations of NMEA-08, -10, and -12. The value of I1/I3 remains constant at about 1.8 at low concentration of NMEAs, which is the same behavior as in water. I1/I3 begins to decrease at C1 and diminishes gradually over the wide range of concentration between C1 and C2. This gradual decline implies a gentle increase in aggregation number of the premicellar aggregates.14 Iex/I1 plots for NMEA-08, -10, and -12 in Figures 6-8 can confirm the validity of the above model. The excimer emission of pyrene is observed only in the concentration region between C1 and C2. Every Iex/I1 plot has a maximum at a particular concentration just below C2 () cmc). The excimer emission diminishes sharply with increasing concentration of NMEA and vanishes above the cmc. The appearance of the excimer emission of pyrene in dilute solutions below the cmc shows that the premicellar aggregates provide a space where the local concentration of pyrene is much higher than the bulk concentration. The pyrene excimer is considered to form when pyrene molecules are close each other in the NMEA aggregates. When the aggregation number of the premicelles is small enough,12-14 the pyrene molecules are in intimate contact in a restricted hydrophobic environment. Near the cmc, the premicelles grow (18) The HandbooksA Guide to Fluorescent Probes and Labeling Technologies, 10th Web ed.; Invitorogen Corporation: Carlsbad, CA, 2005; Chapter 13; available at http://probes.invitrogen.com/handbook/figures/0917.html (accessed Jul 2005). (19) Arai, H.; Murata, M.; Shinoda, K. J. Colloid Interface Sci. 1971, 37, 223. (20) Cabane, B. J. Phys. Chem. 1977, 81, 1639.

Langmuir, Vol. 22, No. 5, 2006 2043 Table 1. Aggregation Parameters of NMEA Aqueous Solutions at 25 °Ca

NMEA-06 NMEA-08 NMEA-10 NMEA-12 NMEA-14

C1 () cac) (mol dm-3)

C2 () cmc) (mol dm-3)

1.00 × 10-3 1.22 × 10-4 8.15 × 10-5 1.28 × 10-5

3.15 × 10-1 4.04 × 10-2 c 3.29 × 10-3 c 2.77 × 10-4 c 7.48 × 10-5 c

γ1 γ2 ∆γb (mN m-1) (mN m-1) (mN m-1) 33.6 34.0 30.4 25.7

35.2 28.3 26.4 26.5 25.7

5.3 7.6 3.9 0.0

a All values were determined by equilibrium surface tension measurements with an exception (see footnote c). γ1 and γ2 indicate the equilibrium surface tension at C1 and C2. respectively. b ∆γ indicates the surface tension reductions () γ1 - γ2) for NMEA in the premicellar concentration region. c These C2 values were obtained from solubilization measurements using oil-soluble dye.

to an infinitely large size (phase separation occurs) to solubilize the pyrene molecules separately. Consequently, Iex/I1 diminishes at this concentration. Several workers have also reported the appearance of excimer emission in dilute anionic surfactant solutions below the cmc in the presence of cationic pyrene derivatives.21,22 They have explained this appearance as resulting from the formation of premicellar aggregates between surfactants and probes through electrostatic attractions and premicellar aggregation being brought about by the presence of pyrene derivatives. However, this explanation cannot be applied to our results because the premicellization of NMEA is observed in the γ vs log C plots without any addition of pyrene probe. Moreover, NMEA and pyrene are nonionic compounds. Any special strong interactions, such as an electrostatic attraction, between nonionic NMEA and pyrene are not possible. Therefore, pyrene is reasonably assumed not to form any aggregates with NMEA but to be solubilized in the premicellar aggregates of NMEA, which are formed at concentrations between C1 and C2. NMEA-14 solution exhibits only one break point in the equilibrium surface tension (Figure 2) and solubilization (Figure 3) plots, and they are at different concentrations, C1 and C2. It is easily assumed that the break point at lower concentration, C1, is the cac and the other, C2, is the cmc by comparison with the other NMEA systems. In the I1/I3 plots for NMEA-14 (Figure 9), these two break points can be clearly observed. Furthermore, in the concentration region between C1 and C2, the strong excimer emission of pyrene was clearly observed (Figure 9). These fluorescence studies also show that the aggregation behaviors of NMEA-14 are the same as those of NMEA-08, -10, and -12. In all of the studies mentioned above, only NMEA-06 shows typical behaviors of surfactants such as SDS. That is, the sudden changes in the equilibrium surface tension, the solubilization ability of oil-soluble dye, and the I1/I3 value of the pyrene emission spectrum occur at a certain concentration (cmc). It is clear that NMEA-06 is directly converted from monomers to micelles at the cmc. This premicellization behavior of the NMEA series, which appears for surfactants having longer alkyl chains, is similar to that of gemini surfactants. Values of C1 (cac) and C2 (cmc) obtained from all of the above measurements are listed in Table 1. γ1 and γ2 indicate the equilibrium surface tensions at C1 and C2, respectively. ∆γ () γ1 - γ2) shows the surface tension reductions for the NMEA series in the premicellar formation region. ∆γ decreases with increasing NMEA alkyl chain length. For the NMEA-14 system, (21) Oliveira, M. E. C. D.; Ferreira, J. A.; Nascimento, S. M.; Burrows, H. D.; Miguel, M. G. J. Chem. Soc., Faraday Trans. 1995, 91, 3913. (22) Ghosh, S. K.; Pal, A.; Kundu, S.; Mandal, M.; Nath, S.; Pal, T. Langmuir 2004, 20, 5209.

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the surface tension is maintained at 25.8 mN/m in this concentration range. This phenomenon presumably suggests a change in the surface activity of the premicellar aggregates themselves with hydrocarbon chain length. Rosen et al. reported that premicellar solutions of some gemini surfactants having long alkyl chains showed smaller surface tension reductions and concluded that the premicellar aggregates had little or no surface activity. Thus, the surface tension reduction in the above solutions can be assumed to be due only to the monomeric surfactant molecules present.23

Conclusion Aqueous solutions of some fatty acid N-methylethanolamides (NMEAs), which are excellent nonionic foam boosters in anionic surfactant systems, have been studied in terms of equilibrium surface tension (γ), solubilization of an oil-soluble dye, and fluorescence measurements. The NMEAs having hydrophobic chains longer than the octanoyl group form premicellar aggregates at a certain concentration (cac) below the cmc. Furthermore, liquid or solid phase separation of the NMEAs takes place instead (23) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 1149.

Sakai et al.

of micelle formation above the cmc. The cac can be determined as a break point in the γ vs log C plots and/or as the onset of a decrease of the intensity ratio of the first and third peaks of the pyrene emission spectrum. The cac values observed by different techniques are in good agreement each other. Therefore, premicellar aggregates of NMEA are spontaneously formed regardless of the presence or absence of any substrates such as pyrene in the dilute NMEA solutions. This is quite a rare case, and it is interesting to note that premicelle formation is substantiated clearly in aqueous solutions of a single surfactant. In addition, the growth process of the premicellar aggregates was observed by fluorescence spectroscopy over a wide range of NMEA concentrations. The surface activity of these premicellar aggregates gradually decreases with increasing alkyl chain length, and the surface tension reduction in the premicellar concentration region for NMEA also diminishes. Acknowledgment. We are grateful to Dr. A. Kawamata of Kao Corporation for permission to publish this paper and also to M. Miyaki, H. Tajima, and M. Shimizu for enlightening discussions. LA052640R