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Change in Viscoelastic Behaviors Due to Phase Transition of the Assembly Comprising Cetyltrimethylammonium Chloride/Cetyl Alcohol/Water Yoshifumi Yamagata*,† and Mamoru Senna‡ Beauty-Care Research Laboratories, Lion Corporation, 7-13-12 Hirai, Edogawa-ku, Tokyo 132-0035, Japan, and Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received October 26, 1998. In Final Form: March 30, 1999 The phase transition process of molecular assembly comprising cetyltrimethylammonium chloride (C16CA), cetyl alcohol (C16OH), and water was analyzed on the basis of dynamic viscoelasticity, mainly by using Cole-Cole plots. The assembly prepared at 75 °C showed onionlike multilamellar vesicles (0.2-5 µm in diameter) immediately after preparation but transformed on aging into a rosary network of vesicles and finally fused into lamellae via polyhedra. The dynamic modulus (G′) of the assembly monotonically increased with frequency, while the loss modulus (G′′) showed 2 peaks. The change in the angle parameters (RL, RS) calculated from Cole-Cole plots indicate the long and short time relaxation mechanisms, corresponding to the network of coagulated vesicles and lamellae, respectively.
Introduction Surfactants form various assemblies such as lamellar, hexagonal, and cubic liquid crystals. Their structures with and without salts or cosurfactants in aqueous solution have been extensively studied.1-3 Physical properties of the assembly of a surfactant with an aliphatic alcohol as a cosurfactant have been analyzed at the macroscopic level by rheology,4-6 microscopy,7,8 and thermoanalysis.9 Molecular microscopic level studies were also made by the small-angle neutron scattering10,11 or NMR.12,13 Using assemblies such as liquid crystals comprising a surfactant and aliphatic alcohol, the stability of bubbles and O/W emulsion can be improved14-17 and the viscosity of the system can be adjusted.18 Assemblies are therefore useful for practical purposes. Assemblies are not always stable, however, and external factors such as shear stress or heat can cause destruction or transition of their † Lion Corporation. Tel: (03) 3616-3396. Fax: (03) 3616-5376. E-mail:
[email protected]. ‡ Keio University.
(1) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081. (2) Horiuchi, T.; Tajima, K. Yukagaku 1992, 41, 1191. (3) Nastishin, Y. A. Langmuir 1996, 12, 5011. (4) Bohlin, L.; Fontell, K. J. Colloid Int. Sci. 1978, 67, 272. (5) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, 3972. (6) Montalvo, G.; Valiente, M.; Rodenas, E. Langmuir 1996, 12, 5202. (7) Ho, C. C.; Goetz, R. J.; El-Aasser, M. S.; Vanderhoff, J. W.; Fowkes, F. M. Langmuir 1991, 7, 56. (8) Yamashita, M.; Kameyama, K.; Kobayashi, R.; Asahina, A.; Aita, S.; Ogura, K. J. Electron Microsc. 1996, 45, 461. (9) Goetz, R. J.; El-Aasser, M. S. Langmuir 1990, 6, 132. (10) Porte, G.; Marignan, J.; Bassereau, P.; May, R. J. Phys. Fr. 1988, 49, 511. (11) Gradzielski, M.; Bergmeier, M.; Mu¨ller, M.; Hoffmann, H. J. Phys. Chem. B 1997, 101, 1719. (12) Griffiths, L.; Horton, R.; Parker, I.; Rowe, R. C. J. Colloid Int. Sci. 1992, 154, 238. (13) Auguste, F.; Douliez, J-P.; Bellocq, A-M.; Dufourc, E. J.; GulikKrzywicki, T. Langmuir 1997, 13, 666. (14) Shah, D. O. J. Colloid Int. Sci. 1971, 37, 744. (15) Friberg, S.; Jansson, P. O.; Cederberg, E. J. Colloid Int. Sci. 1976, 55, 614. (16) Fukushima, S.; Takahashi, M. Yamaguchi, M. J. Colloid Int. Sci. 1976, 57, 201. (17) Fukushima, S.; Yamaguchi, M.; Harusawa, F. J. Colloid Int. Sci. 1976, 59, 159. (18) Barry, B. W.; Eccleston, G. M. J. Texture Stud. 1973, 4, 53.
Figure 1. Electromicrograph for the assembly after storage for (a) 1 day and (b) 60 days.
structure.19,20 As a result, physical properties such as apparent viscosity often become out of control.19 We previously confirmed changes in the flow and the creep behaviors on aging a ternary assembly comprising cetyltrimethylammonium chloride (C16CA), cetyl alcohol (C16OH), and water prepared by stirring and mixing at high temperature.21,22 To the best of our knowledge, however, the relationship between the internal structure (19) Hassan, P. A.; Valauliker, B. S.; Manohar, C.; Kern, F.; Bourdieu, L.; Candau, S. J. Langmuir 1996, 12, 4350. (20) Gulik-Krzywicki, T.; Dedieu, J. C.; Roux, D.; Degert, C.; Laversanne, R. Langmuir 1996, 12, 4668.
10.1021/la981507e CCC: $18.00 © 1999 American Chemical Society Published on Web 05/26/1999
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Figure 2. Electromicrograph for lamellar phase comprising the ternary system. Figure 4. Plots of G* in the linear viscoelastic region and critical stress σc against storage time.
Figure 3. Plots of G* against stress σ for the assembly.
of assembly and the dynamic viscoelastic behaviors on aging has not yet been studied. In this paper, change in the dynamic viscoelastic behaviors of the above assembly with the phase transition is elucidated, where the ColeCole plot is used as a main analytical tool. Experimental Section Materials. Cetyltrimethylammonium chloride (C16CA, Tokyo Kasei Co., extra pure grade) and cetyl alcohol (C16OH, Tokyo Kasei Co., guaranteed reagent grade) were used without further purification. A 2 wt % C16CA and C16OH mixture at a fixed molar ratio, 1:2, was added to deionized water to give 150 g of a ternary mixture, which was heated to 75 °C for 2 h in a water bath. After being mixed and stirred by a rod with four paddles for 20 min, the mixture was cooled to 35 °C at 2 K‚min-1 while stirring. The product was defoamed in a bell jar by an aspirator and aged at 25 °C up to 60 days. Cryo-SEM Observation. For cryo-SEM studies, we used a JEOL JSM-6300F scanning electron microscope with a JEOL SM-31210 cryo system. The sample frozen in liquid nitrogen was sliced with an attached knife or a commercial razor blade to prepare a flat cross section. After evacuation, frost on the specimen surface was sublimed by etching for 20 s three times. The surface was observed without sputtering. Rheological Measurements. Rheometry was carried out at 25 ( 0.1 °C by a stress-controlled rheometer (Haake Rheo-Stress RS-100) equipped with a 4° cone and plate of 35 mm diameter. When a viscoelastic material is subjected to a sinusoidal stress
Figure 5. Plots of storage modulus G′ and loss modulus G′′ against frequency. σ, it deforms with the phase difference δ (0° < δ < 90°). The complex modulus G*, storage modulus G′, and loss modulus G′′ are given by
G* ) σ/γ
(1)
G′ ) G*/cos δ
(2)
G′′ ) G*/sin δ
(3)
where γ is the shear strain. We measured G* as a function of the stresses, G′ and G′′, and the frequency.
Results and Discussions (21) Yamagata, Y.; Senna, M. Colloids Surf. A: Physicochem. Eng. Aspects 1998, 132, 251. (22) Yamagata, Y.; Senna, M. Colloids Surf. A: Physicochem. Eng. Aspects 1998, 133, 245.
Structure of Dispersions. Figure 1a,b shows SEM micrographs of the assembly. After aging for 2 h, multilamellar vesicles were monodispersed, followed by
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Yamagata and Senna
Figure 6. Cole-Cole plots of the assembly.
coagulation to form a network in a rosary pattern after 1 day (Figure 1a). The dispersed phase forms a multilamellar vesicle structure with 0.2-5 µm in diameter. The majority of particles less than 1 µm are spherical. Bilayer structure inside particles are not exposed because of cutting at the interface between the dispersed particles and the continuous phase; the latter exhibits as the black gaps around dispersed particles. On further aging (60 days), vesicles changed into polyhedra as shown in Figure 1b, followed by the formation of a lamellae structure in part (Figure 2). The latter change was also observed in our previous study after addition of a cationic surfactant to C16OH dispersed solution.23 These SEM observations demonstrate the phase transition of the ternary assembly from the network of coagulated vesicles to the lamellae on aging. Changes in G′ and G′′ as a Function of Frequency and Aging. To evaluate the viscoelastic behaviors of the assembly, the frequency (ω) was fixed at 6.28 rad/s, and the stress (σ) dependency of the complex modulus (G*) was measured. The results are shown in Figure 3. At the lower stress region, G* remains nearly constant. When the shear stress exceeds a critical value, σc, G* decreases sharply. The critical stress, σc, increases with aging time and levels off after 3 days (Figure 4). At the same time, G* in the linear region increases and approaches a constant value after 3 days. On the basis of these results, the frequency dependence in the linear region was measured at the fixed stress, 1 Pa. In Figure 5a,b, the dynamic modulus (G′) and the loss modulus (G′′) at 1 Pa are plotted against frequency (ω). Both G′ and G′′ increases on aging in the entire frequency region. This suggests progressive formation of the assembly on aging. While G′ as well as ω increases monotonically, G′′ exhibits 2 peaks, showing two different frequency dependence. G′ represents elastic property while G′′ represents viscous property. Since G′ was 2-10 times higher than G′′, the assembly seems highly elastic. (23) Yamagata, Y.; Senna, M. Submitted for publication in Langmuir.
Cole-Cole Analysis. On the basis of the data shown in Figure 5, G′ was plotted against G′′ to obtain ColeCole plots. The change in the plot on aging is shown in Figure 6. All the curves are expressed by the combination of 2 arcs with their center being a negative G′′ value. Shikata et al.1 and Kern et al.24 found that aqueous solutions of C16CA or cetyltrimethylammonium bromide (CTAB) containing specific salts such as sodium salicylate (NaSal) show the Cole-Cole plot of a semicircle with its center on the G′′ ) 0 line and exhibit Maxwell elementlike behavior. They attribute this to the threadlike micelles, formed by the cationic surfactant and salt. Their micelles are entangled, resulting in relaxation. Since the Cole-Cole plots in this study are expressed by 2 arcs, 2 different relaxation mechanisms may coexist. SEM observation reveals the network of vesicles connected in a rosary pattern and local lamellae. Two relaxation mechanisms, therefore, appear to be associated with these 2 distinct types of structures. Change in the Relaxation Time on Aging. On the basis of 2 arcs of the Cole-Cole plots, the apparent mean relaxation time τ was calculated from the frequency ωmax at the maximum G′′, using eq 4.
τ ) 1/ωmax
(4)
The change in τ on aging is shown in Figure 7. The mean relaxation time, τL, on the longer time (low frequency) side was 260 s after aging for 2 h, followed by a sluggish increase, reaching about 600 s after 30 days. On the other hand, the mean relaxation time, τS, on the shorter time (high frequency) side was about 2 s after aging for 2 h, being smaller than τL by about 2 orders of magnitude. Unlike τL, τS gradually decreases to 0.8 s after 30 days. From the creep tests of the same system, we previously proposed a 6-element model comprising 2 Voigt units in series with a Maxwell unit.22 The 2 retardation times obtained from Voigt models (τ1, τ2) were 14-17 s on the (24) Kern, F.; Zana, R.; Candau, S. J. Langmuir 1991, 7, 1344.
Phase Transition of a Molecular Assembly
Figure 7. Change in relaxation time with storage time.
Figure 8. Complex dielectric constant locus.
longer time side and 1-4 s on the shorter time side. On the shorter time side, the order of the retardation time obtained by the creep tests was the same as that of the relaxation time obtained by the Cole-Cole plots. However, the relaxation time on the longer time side, τL, was higher by 1 order of magnitude. This marked difference between τ1 and τL might be attributed to the short observation time, i.e., only to 120 s in the creep tests, and the retardation time was simulated using the data in this time scale. r-Parameter. Generally, there is similarity between viscoelastic and dielectric properties.25 Cole-Cole plot studies in the field of dielectrics,26-28 in which the dielectric constant ′ is plotted against the dielectric loss ′′, show arcs similar to those in Figure 6. When the center of the arc is located on the ′ axis (′′ ) 0), the substance shows single relaxation. When the center of the arc is located at ′′ < 0, however, distribution of the relaxation time is to (25) Ferry, J. D.; Williams, M. L.; Fitzgerald, E. R. J. Phys. Chem. 1955, 59, 403. (26) Barchini, R.; Saville, D. A. J. Colloid Int. Sci. 1995, 173, 86. (27) El-Shabasy, M.; Riad, A. S. Physica B 1996, 222, 153. (28) Sauer, B. B.; Stock, R. S.; Lim, K-H.; Ray, W. H. J. Appl. Polym. Sci. 1990, 39, 2419.
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Figure 9. Change in RL and RS with storage time.
be expected. The distribution width is expressed by the angle Rπ/2 (radian), defined in Figure 8. The parameter R is a constant between 0 and 1. A higher R-value indicates broader distribution of the relaxation time. In a colloidal dispersion, R increases with the concentration of the dispersed phase.26 We calculated the constant R from the Cole-Cole plot shown in Figure 6. The changes in the RL and RS obtained from the arcs on the longer time (low frequency) side and the shorter time (high frequency) sides, respectively, are shown in Figure 9. After the sample was aged for 2 h, RL was about 0.32 and RS about 0.14. While RL slightly decreased on aging, RS slightly increased. SEM observation reveals coexistence of a network of vesicles connected in a rosary pattern and local lamellae in this system. We observed the phase transition from the network structure to the lamellae occurs by coalescence or fusion of vesicles. These findings suggest that the decrease of RL on aging is attributed to the formation of the network of vesicles, whereas the increase in RS to the formation of lamellae. Conclusions The molecular assembly comprising of C16CA, C16OH, and water takes the form of multilamellar vesicles immediately after preparation, followed by a network formation of vesicles connected in a rosary pattern. On further aging, lamellae are formed. The Cole-Cole plots obtained from dynamic measurements are expressed by 2 arcs, indicating the coexistence of 2 different relaxation mechanisms. The viscoelastic behavior on the longer time side corresponds to form the network of multilamellar vesicles and that on the shorter time side to the lamellae, in accordance with our previous rheological observation.22 LA981507E
