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salts (LGT) of N-lauroyl l-glutamic acid, at different degrees of neutralization. ... be indexed to the crystallographic space group Pm3n. 1H NMR ...
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Langmuir 2002, 18, 4699-4703

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Self-Assembly in Some N-Lauroyl-L-glutamate/Water Systems Daisuke Kaneko,*,†,‡ Ulf Olsson,† and Kazutami Sakamoto‡ Division of Physical Chemistry 1, Center for Chemical and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden and AminoScience Laboratories, Ajinomoto Company, Incorporated, Suzuki-cho 1-1, Kawasaki-ku, Kawasaki-shi, 210-8681, Japan Received December 6, 2001. In Final Form: March 5, 2002 Acylglutamate is a divalent anionic surfactant whose properties are affected by its degree of neutralization. In this study, we present phase maps for aqueous systems of the potassium (LGP) and triethanolamine salts (LGT) of N-lauroyl L-glutamic acid, at different degrees of neutralization. A cubic phase forming between the isotropic micellar and the hexagonal phase was detected in both LGP/water and LGT/water systems. The stability of this cubic phase was promoted by increasing the degree of neutralization. The lattice parameters of the phase were determined by small-angle X-ray scattering (SAXS). SAXS spectra from samples of this cubic phase could be indexed to the crystallographic space group Pm3n. 1H NMR self-diffusion measurements have also been performed in the isotropic micellar and cubic regions. The self-diffusion coefficient of the surfactant decreases with increasing its concentration. In the cubic phase, the self-diffusion is of the order of 10-13 m2 s-1, demonstrating that the cubic phase consists of discrete micelles.

Introduction Anionic surfactants are widely used for cosmetic and toiletry products such as shower gels and hair shampoos. Today, the properties demanded of these surfactants are not only detergency and foaming power but also mildness and biodegradability. From this point of view, surfactants derived from amino acids are well-known to be mild surfactants and hence are increasingly used in cleansing products. Long-chain N-acylglutamates (see Figure 1) are surface active and can be used as surfactants for cosmetics and toiletries.1,2 Transparent soap is one of the applications of acylglutamate. Used in high concentration (approximately 50 wt %), it shows a higher transparency compared to transparent soaps made from fatty acid salts. The transparent soap of acylglutamate is shown in Figure 2. Transparent soaps not only look attractive, but many of them also protect the skin well and are mild to use. Many points regarding the crystal structure of transparent soap remain to be clarified. McBain and Ross3 have demonstrated using X-ray diffraction that transparent soap made from fatty acid salts is made up of fine crystals. It consists of rather fine crystalline structures the size of which is too small to provide an optical discontinuity when compared with the wavelength of visible light.4 However, the structure of transparent soap made from acylglutamates seems to be different from that made from fatty acid salts. Its features, that is, extreme transparency, nonbirefringence, and high stiffness, are similar to the features of a micellar cubic phase.5 In many systems, one finds a cubic structure between the isotropic micellar * To whom correspondence should be addressed at Ajinomoto Co., Inc., Japan. E-mail: [email protected]. † Lund University. ‡ Ajinomoto Co. (1) Takehara, M.; Moriyuki, H.; Yoshimura, I.; Yoshida, R. J. Am. Oil Chem. Soc. 1972, 49, 143. (2) Takehara, M. Colloids Surf. 1989, 38, 149. (3) McBain, J. M.; Ross, S. Oil Soap 1944, 24, 97. (4) Ogino, K. J. Jpn. Oil Chem. Soc. 1964, 13, 804. (5) Fontell, K. Colloid Polym. Sci. 1990, 268, 264.

Figure 1. Chemical structure of N-acyl-L-glutamic acid. Acylglutamate is obtained by neutralizing with a base. RCOis the acyl radical.

Figure 2. Transparent soap made from acylglutamate.

solution phase and the anisotropic hexagonal liquid crystalline phase. The first observation of the micellar cubic phase was the alkyltrimethylammonium chloride/ water system.6 Other binary aqueous systems were those containing egg lysolectin,7 lysolecithins with chain lengths C12-C16,8,9 zwitterionic compounds, and nonionic poly(ethylene oxide) derivatives.10-14 In addition, in ternary (6) Balmbra, R. R.; Clunie, J. S.; Goodman, J. F. Nature 1969, 222, 1159. (7) Reiss-Husson, F. J. Mol. Biol. 1967, 25, 363. (8) Eriksson, P.; Lindblom, G.; Arvidson, G. J. Phys. Chem. 1987, 91, 846. (9) Eriksson, P.; Lindblom, G.; Arvidson, G. J. Phys. Chem. 1985, 89, 1050. (10) Mitchell, J. D.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (11) Johans, E.; Finkelmann, H. Colloid Polym. Sci. 1987, 265, 304.

10.1021/la0117653 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/15/2002

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systems a micellar cubic phase was observed when a hydrocarbon was added to an aqueous surfactant system.15-18 However, the application of transparent soap using the micellar cubic phase has not been reported. In general, micellar cubic phase regions in pure anionic surfactant/water systems are often very narrow and are observed at relatively high temperature. For successful industrial application, the stability of micellar cubic phases is an important factor. Although many different monovalent surfactant/water systems have been studied, studies of divalent surfactants are rare. According to these studies involving the dodecylpentamethyl-1,3-propylenebis-(ammonium chloride)/water and dipotassium dodecylmalonate/water systems, the micellar cubic phase extends over a wide region.19,20 The increased stability of the small micelles with increasing surfactant charge is mainly at the expense of the cylindrical structure and the hexagonal phase. This effect of surfactant charge has been modeled by Poisson-Boltzmann calculations.21-23 Acylglutamate is a divalent anionic surfactant the properties of which are strongly affected by its degree of neutralization.24 According to the calculation, its micellar cubic phase would be expected to be wide and stable. In this study, we present phase maps for aqueous systems of the potassium and triethanolamine salts of N-lauroyl-L-glutamic acid, at different degrees of neutralization. The structure of transparent soap made from acylglutamate was also investigated. Results of smallangle X-ray scattering and NMR self-diffusion experiments have been combined to determine the structure of the various phases. Experimental Section Materials. N-Lauroyl-L-glutamic acid (LGA) was obtained from Ajinomoto Co., Inc., Tokyo. The purity of LGA was above 99%, as checked by high-performance liquid chromatography (HPLC). Potassium hydroxide, triethanolamine, diethanolamine, and monoethanolamine were obtained from Junsei Chemical Co., Ltd., Tokyo. Potassium salts (LGP), triethanolamine salts (LGT), diethanolamine salts (LGD), and monoethanolamine salts (LGM) were prepared by neutralizing LGA with each base, respectively. The degree of neutralization, varied from 1.0 to 2.0, was denoted as Z. Deionized water was used for the preparation of samples. Phase Maps. Samples were prepared by adding the components directly to glass tubes. They were then homogenized by repeated heating and centrifugation and subsequently left for at least 1 week at 25 ( 0.5 °C. The phase change was detected by direct visual inspection of the samples and with crossed polarizers for birefringence. The type of liquid crystal was determined by small-angle X-ray scattering (SAXS). (12) Lutton, E. S. J. Am. Oil Chem. Soc. 1966, 43, 26. (13) So¨derman, O.; Carlsto¨m, G.; Monduzzi, M.; Olsson, U. Langmuir 1988, 4, 1039. (14) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1997, 13, 23. (15) Ekwall, P.; Mandell, L.; Fontell, K. Mol. Cryst. Liq. Cryst. 1969, 8, 157. (16) Gradzielski, M.; Hoffmann, H.; Oetter, G. Colloid Polym. Sci. 1990, 268, 167. (17) Jousma, H.; Joosten, J. G. H.; Gooris, G. S.; Junginger, H. E. Colloid Polym. Sci. 1989, 267, 353. (18) So¨derman, O.; Johansson, L. B.-Å. J. Colloid Interface Sci. 1996, 179, 570. (19) Hagsla¨tt, H.; So¨derman, O.; Jo¨nsson, B.; Johansson, L. B.-Å. J. Phys. Chem. 1991, 95, 1703. (20) Hagsla¨tt, H.; So¨derman, O.; Jo¨nsson, B. Langmuir 1994, 10, 2177. (21) Gunnarsson, G.; Jo¨nsson, B.; Wennerstro¨m, H. J. Phys. Chem. 1980, 84, 3114. (22) Jo¨nsson, B.; Wennerstro¨m, H. J. Colloid Interface Sci. 1981, 80, 482. (23) Jo¨nsson, B.; Wennerstro¨m, H. J. Phys. Chem. 1987, 91, 338. (24) Kaneko, D.; Hattori, T.; Sakamoto, K. J. Jpn. Oil Chem. Soc. 1999, 48, 713.

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Figure 3. Composition phase map of the LGP (Z ) 1.0)/LGP (Z ) 2.0)/water system at 25 °C. L1, I1, and H1 denote a micellar solution, a micellar cubic phase, and a hexagonal phase. SAXS. SAXS measurements were performed on a Kratky compact small-angle system equipped with a position sensitive detector containing 1024 channels of width 51.3 µm. Cu KR radiation of wavelength 1.542 Å was provided by a Seifert ID300 X-ray generator, operating at 50 kV and 40 mA. A 10 µm thick nickel filter was used to remove the Kβ radiation, and a 1.5 mm tungsten filter was used to protect the detector from the primary beam. The sample-to-detector distance was 277 mm. The samples were prepared with thin mica windows, and the spectra were recorded in a vacuum. NMR. The isotropic solution was characterized with the pulsed gradient spin-echo proton NMR (PGSE 1H NMR) method to determine the self-diffusion coefficients of the surfactant and water.25,26 The self-diffusion measurements were performed on a Bruker DMX-200 spectrometer operating at 200 MHz (for 1H nuclei). A standard 90°-180° pulse sequence and stimulated echo pulse sequence were used for the measurement. The selfdiffusion coefficient D was obtained by fitting eq 1 to the obtained NMR data.

I ) I0e{-(γGδ) D(∆-δ/3)} 2

(1)

In eq 1, I denotes the observed echo intensity, I0 is the echo intensity in the absence of field gradient pulses, g is the magnetogyric ratio, G is the field gradient strength, d is the duration of the gradient pulse, and D is the time between the leading edges of the gradient pulses.

Results and Discussion Phase Maps. Ternary phase maps for LGP (Z ) 1.0)/ LGP (Z ) 2.0)/H2O and LGT (Z ) 1.0)/LGT (Z ) 2.0)/H2O systems at 25 °C are presented in Figures 3 and 4. The two systems show essentially the same phase behavior. The micellar solution (L1) is succeeded by a micellar cubic phase (I1), which in turn is followed by a hexagonal phase (H1) when the concentration increases. The micellar cubic phases in LGP/H2O and LGT/H2O were stable over 6 months. The intermediate regions, consisting of multiphase between phases, were quite narrow and were not investigated in detail. In previous studies on a divalent surfactant, a “ribbon phase” with noncircular cylindrical aggregates has been reported,27 but this was not detected in the present systems. The stability of the micellar cubic phase was strongly enhanced by increasing the degree of (25) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1. (26) So¨derman, O.; Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26, 445. (27) Hagsla¨tt, H.; So¨derman, O.; Jo¨nsson, B. Liq. Cryst. 1992, 12, 667.

N-Lauroyl-L-glutamate/Water Systems

Figure 4. Composition phase map of the LGT (Z ) 1.0)/LGT (Z ) 2.0)/water system at 25 °C. L1, I1, and H1 denote a micellar solution, a micellar cubic phase, and a hexagonal phase.

Figure 5. Binary phase maps for the LGT (Z ) 2.0), LGD (Z ) 2.0), and LGM (Z ) 2.0)/water systems at 25 °C. L1, I1, and H1 denote a micellar solution, a micellar cubic phase, and a hexagonal phase.

neutralization, and the cubic phase region at low Z was narrow. However, the cubic phase narrows slightly in the range of Z > 2.0 with increasing pH. Excess alkalinity would reduce the electrostatic repulsion of the headgroups in surfactant aggregates in the same manner as salts. Those of micellar cubic phases at high Z extended over large regions of concentration compared with monovalent surfactant/H2O systems. We have also investigated LGD and LGM systems in order to establish the effect of counterion size. The binary phase maps of LGT, LGD, and LGM/water systems at Z ) 2.0 are presented in Figure 5. Clearly, the cubic phase of each is also relatively large and the extent of the cubic phase increases with the size of the counterion. Hagsla¨tt et al. have reported the theoretically calculated phase diagram for a divalent surfactant/monovalent surfactant/2H2O system by use of the Poisson-Boltzmann cell model.20 According to this model, the spherical aggregate region is extended when Z increases, and the phase boundaries are shifted to a higher surfactant concentration. This model does not distinguish a micellar solution from a cubic phase, however, and therefore a spherical aggregate region of the theoretically determined phase diagram would correspond to the whole isotropic region, that is, both micellar solution and cubic phases. The trends of the phase behavior of LGP and LGT/water systems were in accordance with this theoretical prediction. In the LGP system, however, the liquid and liquid crystalline phases do not extend to the binary axis with the monovalent surfactant because

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Figure 6. SAX diffraction pattern obtained from a hexagonal phase, liquid crystalline sample of 66.4/33.6 LGP (Z ) 1.8)/ water wt % composition at 25 °C. The scattering curve is also shown in expanded intensity to expose the higher order Bragg peaks (marked with the arrows).

of its high Krafft temperature. The formation of large micellar cubic phases is a general property of divalent surfactant/water systems. Because of the high electric charge density of the micellar surface23 and bulky headgroups, micellar growth is suppressed when the surfactant concentration increases and thus a cubic phase is formed extensively. The experimental SAXS data and NMR data underlying phase maps of Figures 3 and 4 are presented in the following sections. SAXS. The characterization of liquid crystalline phases by X-ray diffraction is based on the long-range order in the liquid crystalline state which gives rise to Bragg reflections, the positions of which are characteristic of the different liquid crystalline phases. Two different liquid crystalline phases have been detected in LGP/water and LGT/water systems. The relative positions of the four peaks resolved in the diffraction pattern of Figure 6 (LGP (Z ) 1.8); concentration, 66.4 wt %) correspond to the 1:x3:2:x7 relationship, a confirmation of the hexagonal structure. An optically isotropic phase is formed between the L1 and H1 regions, as seen in Figures 3 and 4. The samples in this region are transparent, nonbirefringent, and quite stiff, features which are characteristic of a cubic phase. A representative SAXS spectrum obtained from this region is shown in Figure 7, which is obtained with LGP (Z ) 1.6; 41.5 wt %). The scattering curve is also shown on an expanded intensity scale (right-hand side) to expose the higher order peaks. There are three main families of cubic structures, primitive, body-centered, and face-centered, and within each family there are many crystallographic space groups with different symmetries. As for liquid crystalline phases, in general the Bragg reflections tend to be very weak for high hkl indices due to the inherent disorder of the structure and the decay of the micellar form factor at higher scattering angles. But as can be seen in Figure 7, the obtained Bragg diffraction peaks are relatively sharp in which case the correct peak position can be evaluated. SAXS spectra from samples of this cubic phase were consistent with the crystallographic space group Pm3n.28 The indexing of the diffraction data to the Pm3n space group was assessed by plotting the reciprocal d spacings (1/dhkl) versus m ) (h2 + k2 + l2)1/2, as shown in Figure 8. As can be seen, there is good (28) International Tables for Crystallography; Kluwer Academic Publishers: Dortrecht, The Netherlands, 1989.

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Figure 7. SAX diffraction pattern obtained from a cubic phase, liquid crystalline sample of 41.5/58.5 LGP (Z ) 1.6)/water wt % composition at 25 °C. The scattering curve is also shown in expanded intensity to expose the higher order Bragg peaks (marked with the arrows). The arrows mark the positions of reflections afforded by the Pm3n crystallographic space group.

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Figure 9. Characteristic structural parameters of the cubic phase in the LGP/water system, the lattice parameter, a, the major axis of the micelle, l, and the effective area per surfactant molecule, AS, plotted versus the LGP concentration: (b) Z ) 1.6, (2) Z ) 1.8, and (9) Z ) 2.0.

On the basis of Fontell’s model, characteristic structural parameters of the cubic phase were calculated. The shape of micelles is assumed as hemisphere-capped cylinders,32 and a minor axis R is equal to the fully extended length of the lauroyl chain according to Tanford’s equation33 and the length of cylindrical part is bR. The major axis length, l, and the effective area per surfactant molecule, AS, at the hydrophobic/hydrophilic interface (which we take to be the hydrocarbon/glutamic acid interface) in the cubic phase are calculated by using the following equations:34

Vmic )

a3 ‚φ n L

4 3 πR + bR‚πR2 ) Vmic 3 l) Figure 8. Plot of the reciprocal d spacings (1/dhkl) of the reflections observed in the SAXS spectra of Figure 5 plotted versus m ) (h2 + k2 + l2)1/2.

agreement between the data and the Pm3n structure. For a cubic phase, such a plot should pass through the origin and be linear with a slope of 1/a, where a is the cubic unit cell lattice parameter. The cubic phase in the LGT/water system was also consistent with the Pm3n structure. The structure of the Pm3n micellar cubic phase, in particular, has been the subject of some debate. Luzzati et al. have previously proposed a structure for the cubic phase to be built up of a three-dimensional network of rods to form a cage in which a spherical micelle is enclosed.29 However, these workers have rejected this proposal30 and now favor a structure similar to the one proposed by Fontell.31 Fontell proposed that the Pm3n phase consisted of eight slightly elongated micelles per unit cell, which can be divided into two groups corresponding to two types of sites with different properties. Two of the eight micelles, occupying the center and the eight corners of the unit cell, are free to rotate around both minor axes. The remaining six, occupying the side of the unit cell, can rotate only laterally. (29) Tardieu, A.; Luzzati, V. Biochim. Biophys. Acta 1970, 219, 11. (30) Vargas, R.; Mariani, P.; Gulik, A.; Luzzati, V. J. Mol. Biol. 1992, 225, 137. (31) Fontell, K.; Fox, K. K.; Hansson, E. Mol. Cryst. Liq. Cryst. 1985, 1, 9.

As )

R(2 + b) 2

(4 + 2b) ‚V (4/3 + b)R L

(2) (3) (4) (5)

Here, Vmic is the volume of one micelle, a is the lattice parameter, n is the number of micelles per unit cell (n ) 8), φL is the volume fraction of the surfactant’s hydrophobic moiety, and VL is the volume of the surfactant’s hydrophobic moiety according to Tanford’s equation.33 In Figure 9, we present the characteristic structural parameters of the cubic phase. The values of a and AS were decreased with increasing the concentration of LGP, and l was increased. The distance between micelles diminishes, and the micelle approaches a more prolated shape as the LGP concentration increases; subsequently the cubic phase transforms into the hexagonal phase. The axis ratio (l/R) changed from 1.2 to 1.5. Similar values have been found in other systems.32,35-37 (32) Johansson, L. B.-Å.; So¨derman, O. J. Phys. Chem. 1987, 91, 5275. (33) Tanford, C. J. J. Phys. Chem. 1972, 76, 3020. (34) Badrı´guez, C.; Kunieda, H. Langmuir 2000, 16, 8263. (35) So¨derman, O.; Henriksson, U. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1515. (36) So¨derman, O.; Walderhaug, H.; Henriksson, U.; Stilbs, P. J. Phys. Chem. 1985, 89, 3693. (37) Medhage, B.; Almgren, M.; Alsins, J. J. Phys. Chem. 1993, 97, 7753.

N-Lauroyl-L-glutamate/Water Systems

Figure 10. Surfactant self-diffusion coefficients in the LGP (Z ) 1.6)/water system as a function of surfactant concentration at 25 °C.

We have discussed the cubic phase assuming a discontinuous cubic phase. This view is supported by the observed position of the phase in phase maps and the fact that the diffraction pattern was consistent with the Pm3n space group. However, there is nevertheless a possibility of a bicontinuous cubic phase. To confirm the type of cubic phase, NMR self-diffusion experiments have been conducted. NMR Self-Diffusion. The simplest experiment that will discriminate whether a cubic phase has a bicontinuous structure or whether it consists of discrete micellar units is the measurement of the self-diffusion coefficients for the surfactant and water in the system. In a bicontinuous surfactant, all components diffuse rapidly, while in the case of discrete micelles, the micellar components possess a low diffusion coefficient and the continuous phase components diffuse close to its bulk rate.38 1H NMR selfdiffusion measurements have been performed across the phase boundary separating the L1 and the I1 regions in LGP/water system, shown in Figure 10. The measured self-diffusion coefficient for water was of the order of 10-9 m2 s-1. On the other hand, the measured self-diffusion coefficient for the surfactant decreases rapidly in the cubic phase, of the order of 10-13 m2 s-1, as can be seen in Figure (38) Lindman, B.; Olsson, U. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 344. (39) Håkansson, B.; Hansson, P.; Regev, O.; So¨derman, O. Langmuir 1998, 14, 5730.

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10. The observation of slow self-diffusion for the surfactant demonstrates that the cubic phase consists of discrete micelles. We have also measured the self-diffusion of the LGT/water system and found the same tendency as the results of the LGP/water system. In addition, we could measure the self-diffusion coefficients for triethanolamine, which was the counterion, in the LGT/water system. The self-diffusion coefficients for triethanolamine in the cubic phase were much higher than the self-diffusion coefficients for surfactant, of the order of 10-10 m2 s-1. This result implies that a counterion in a micellar cubic phase diffuses as rapidly as a counterion in a micellar solution. Assuming that the diffusion process is a random walk from one aggregate to the other, from the value of self-diffusion coefficients and the size of the unit cell that was determined by SAXS, one can also estimate the lifetime (τR) of a surfactant in the micellar aggregate by using the following equation.18,38

τR )

f2 6D

(6)

Here, f is the (average) distance between two neighboring micelles. For the Pm3n lattice, we can approximate f ≈ a/2,18 where a is the dimension of the cubic lattice cell. Using data of LGP (Z ) 1.6) at 41.5 wt % (D ) 1.39 × 10-13 m2 s-1, a ) 89.4 Å), we obtain the lifetime: τR ) 24 µs. Conclusion The purpose of this study has been to investigate the phase behavior of acylglutamate, especially in the relatively high concentration range often utilized for producing transparent soap. To investigate this, phase maps were derived using SAXS and NMR techniques. The experiments reveal that the structure of the region utilized for the acylglutamate transparent soap is a cubic phase and relatively large compared with monovalent anionic surfactant systems. SAXS results show the crystallographic space group of the cubic phase to be Pm3n. The NMRdetermined self-diffusion coefficients for surfactant were low, indicating that the structure of the cubic phase was micellar. The formation of the cubic phase was promoted by increasing the degree of neutralization. The observed dependence on average surfactant valency was in good agreement with the predictions of Poisson-Boltzmann calculations.20,23 The micellar cubic phases obtained in acylglutamate/water systems were large and stable over a long period. Hence, other applications such as drug delivery and microemulsions can also be anticipated. LA0117653