Peptide-Based Gemini Amphiphiles: Phase Behavior and Rheology of

Article pubs.acs.org/Langmuir

Peptide-Based Gemini Amphiphiles: Phase Behavior and Rheology of Wormlike Micelles Rekha Goswami Shrestha,*,† Kazuyuki Nomura,† Masashi Yamamoto,‡ Yukio Yamawaki,‡ Yoshinaga Tamura,‡ Kenichi Sakai,*,† Kazutami Sakamoto,†,§ Hideki Sakai,† and Masahiko Abe*,† †

Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan ‡ Functional Additives Division, Asahi Kasei Chemicals Co., 1-3-1 Yakoh, Kawasaki, Kanagawa 210-0863, Japan § Faculty of Pharmacy, Chiba Institute of Science, 15-8 Shiomi, Choshi, Chiba 288-0025, Japan S Supporting Information *

ABSTRACT: Aqueous binary phase behavior of a peptide-based gemini amphiphile with glutamic acid and lysine as spacer group, acylglutamyllysilacylglutamate (m-GLG-m where m = 12, 14, and 16), has been reported over a wide range of concentration and temperature. Lauroylglutamyllysillauroylglutamate, 12-GLG-12, selfassembles into spherical micelles above critical micelle concentration (CMC). The micellar region extends up to 32 wt %, and an ordering of spherical micelles into micellar cubic phase, I1, takes place at 33 wt % at 25 °C. The phase transition, I1 - hexagonal liquid crystal, (H1) lamellar liquid crystal, (Lα) has been observed with further increase in concentration; moreover, mixed phases are also observed between the pure liquid crystal domains. Similar phases were observed with 16-GLG-16 above 50 °C (Krafft temperature). The partial ternary phase behavior shows that the micellar solutions of m-GLG-m can solubilize a large amount of cationic amphiphile, alkyltrimethylammonium bromide, CnTAB, (where n = 14 (TTAB) and 16 (CTAB)) at 25 °C. An addition of CnTAB to the aqueous solutions of 16-GLG-16 in a dilute region forms a transparent solution of viscoelastic wormlike micelles at very low concentration (0.25 wt %) even at ambient condition. A mixture of oppositely charged amphiphiles, m-GLG-m and CnTAB, exhibits synergism as a result the amphiphile layer curvature, becomes less positive, and favors the transition from sphere to rod to transient networks (wormlike micelles). The gemini amphiphile, 16-GLG-16, forms wormlike micelles at relatively low concentrations compared to others reported so far. Viscosity increases by six orders of magnitude compared to that of pure solvent. The hydrophobic chain length of m-GLG-m and coamphiphile affects the rheology; the maximum viscosity achieved with 16-GLG-16/H2O/CTAB is higher than that of 14-GLG-14/H2O/CTAB, 12-GLG-12/ H2O/CTAB, and 16-GLG-16/H2O/TTAB systems. These temperature-sensitive systems exhibited viscoelastic behavior described by the Maxwell mechanical model with a single stress relaxation mode.

1. INTRODUCTION

difference that the networks of wormlike micelles break and reform dynamically. The transformation of spherical to elongated micelles is an extensively investigated aspect, from both experimental and theoretical viewpoints. The rheology of amphiphile solutions, as well as a number of other properties,10 is influenced by these transformations. They have largely been exploited for many practical applications such as fracturing fluids in oil fields, drag reducing agents, and also in the formulation of home and personal care products.11 Gemini amphiphiles consisting of two monomeric amphiphiles linked by a spacer at the level of their headgroups have a high potential for practical applications because of their

At concentrations above the critical micelle concentration (CMC), amphiphiles self-associate to spherical or spheroid micelles in aqueous or nonaqueous media.1 These micelles undergo shape transition when an appropriate parameter is modified. An increase in concentration,2,3 temperature,4 or addition of a compatible coamphiphile5,6 brings about micellar growth. In most instances, this process results in the formation of elongated (locally cylindrical) micelles that can become extremely long and entangle with each other forming a transient network structure showing viscoelastic properties. Such micelles have been referred to as “wormlike”,7 “threadlike”,8 or “polymer-like”;9 that represents some of the properties of these micelles. They are generally flexible, thereby justifying the expressions “wormlike” and “threadlike”. The network structure is similar to that of polymers with the © 2012 American Chemical Society

Received: June 1, 2012 Revised: October 16, 2012 Published: October 17, 2012 15472

dx.doi.org/10.1021/la3022358 | Langmuir 2012, 28, 15472−15481

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mide, CnTAB (where n = 12, 14, and 16), were purchased from Tokyo Chemical Industries Co. Inc., Japan. Monomeric surfactant, disodiumlauroylglutamate, 12-G was received from Asahi Kasei Chemicals Co., Japan. 12-G has single hydrophobic tails with two carboxylates as a hydrophilic group. All chemicals were used as received without further purification. The water used in this study was deionized with a Barnstead NANO Pure Diamond UV system and filtered with a Millipore membrane filter (pore size 0.22 μm). 2.2. Phase Diagram. Samples were prepared by weighing the required amounts of reagents into test tubes fitted with screw cap and mixed using a vortex mixer. They were stored in a dry thermobath at 25 °C for equilibration. Phases were characterized by visual observation (through a crossed polarizer), cross-polarized microscopy, and peaks were identified by small-angle X-ray scattering (SAXS). No precipitation was visually observed when the micellar solution was left for a week at room temperature. 2.3. Rheological Measurements. Samples with required amounts of reagents in vials were homogenized and kept in a dry bath at 25 °C for at least 24 h to ensure equilibration before performing rheological measurements. Measurements were performed in a stress-controlled rheometer, AR-G2 (TA Instruments) using cone−plate geometries (diameter of 60 mm with cone angle of 2° 1′ 9″ for low-viscosity samples and diameter 40 mm with cone angle of 2° 0′ 4″ for high-viscosity samples). In the temperature effect study on the rheological behavior, we maintained the samples at each temperature for 30 min (for samples of low viscosity) to 1 h (for samples of high viscosity) before the measurements (sufficient to attain a stability in the rheological parameters (viscosity, η, and plateau modulus, Go), as suggested by a separate rheological measurement as a function of time. Frequency sweep measurements were performed in the linear viscoelastic regime of the samples determined previously by dynamic strain sweep measurements. The zero-shear viscosity of the samples was determined either from a steady shear rate measurement of less viscous samples by extrapolating the viscosity in shear-rate curve to zero-shear rate or from the values of Go and τR as obtained from oscillatory measurements (eq 3, in the Results and Discussion section). 2.4. Microscopic Analysis. Micrographs of liquid crystals were taken using an Olympus IMT-2. A small amount of samples was placed in the hollow (0.5 mm depth) of a slide glass and covered with a cover glass.

excellent ability to lower surface tension of water. In addition, it is expected that their biocompatibility would be good compared with that of a conventional amphiphile. The physicochemical property of gemini amphiphiles having a spacer chain between two hydrophilic groups in the molecule is influenced by the spacer chain.12−14 It has been found that most of the reports deal with the physicochemical behavior of the gemini amphiphile solutions in dilute regions15−17 but only a few in a wide range of compositions.18,19 Gemini amphiphiles have been found to show viscoelastic properties in solution.19−21 The biodegradability and biocompatibility of amphiphiles have become as important as their functional performance from the environmental perspective. Therefore, amphiphiles from renewable sources that are natural, less toxic, and biodegradable are the preferred choices for food, pharmaceutical, and cosmetic formulations. Lipopeptides are one of such kinds; initially, they were used as preservatives for medical and cosmetic applications. The peptide-based gemini surfactant satisfies the environmental concerns and low-toxicity requirements providing new incentives and opportunities for the formulation and development of new products. In this context, we have investigated the aqueous binary phase behavior of peptide-based gemini amphiphiles with glutamic acid and lysine as spacer groups, acylglutamyllysilacylglutamate, m-GLG-m (where m = 12, 14, and 16) in a wide range of concentration and temperature. The formation and rheological behavior of viscoelastic wormlike micelles in mixed aqueous systems of m-GLG-m and cationic coamphiphile, CnTAB (where n = 12, 14, and 16) has been studied in detail. Rheological behavior of highly viscous solutions in both steady and dynamic modes has confirmed the formation of viscoelastic wormlike micelles: coamphiphile induced micellar elongation and their entanglement. The rheology of wormlike micelles depends on the temperature and alkyl chain length of the amphiphile and coamphiphile. This m-GLG-m promotes recovery of smooth and moisturized hair and skin conditions; therefore, we believe this study holds importance in the academic field as well as for application purposes.

3. RESULTS AND DISCUSSION 3.1. Aqueous Binary Phase Behavior of m-GLG-m. The aqueous binary phase diagrams of monomeric amphiphile, disodiumlauroylglutamate, 12-G, and gemini amphiphiles, 12GLG-12 and 16-GLG-16, as a function of temperature have been shown in Figure 1a,b,c, respectively. The structures have been confirmed by cross-polarized optical microscopic images and SAXS (Figure SI 1a−f). Monomeric amphiphile disodiumlauroylglutamate, 12-G, forms micellar solution, Wm, over a wide range of concentration (Figure 1a) (up to ∼30 wt %); above it, the Wm transforms to highly viscous micellar cubic phase, I1, that extends up to ∼60 wt % at 25 °C. The large headgroup formed after the neutralization of carboxylic acid groups on the amphiphile with Na-ion favors the formation of micellar cubic phase.22 With increase in concentration, a narrow region of highly viscous anisotropic phase of hexagonal liquid crystal, H1, is formed. Nevertheless, there is a mixed phase of I1 + H1 in between the two pure I1 and H1 domains. It has been observed that I1 domain gets narrower and H1 domain shifts toward the lower concentration as the temperature of the system is raised. The effective cross-sectional area, aS, decreases with increasing temperature. The water molecule bound to the hydrophilic head groups tends to depart at higher temperature, and since the concentration of counterion is high at these concentrations,

2. EXPERIMENTAL SECTION 2.1. Materials. Peptide-based gemini amphiphiles, sodium acylglutamyllysilacylglutamate, m-GLG-m (where m = 12, 14, and 16) were received from Asahi Kasei Chemicals Co., Japan. The INC name of m-GLG-m is sodium dilauramidoglutamide lysine. 12-GLG-12 and 16-GLG-16 were received as solid, while 14-GLG-14 was a 27 wt % aqueous solution. Scheme 1 shows the schematic molecular structure of m-GLG-m, which has two hydrophobic tails with three carboxyltates. Cationic coamphiphiles, alkyltrimethylammonium bro-

Scheme 1. Schematic Molecular Structures of m-GLG-m (as a Typical Example)a

a

Where m = 12, 14, and 16. This material is a mixture of isomers at the peptide linkages. 15473


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Figure 1. Aqueous binary phase diagrams of (a) 12-G, (b) 12-GLG-12, and (c) 16-GLG-16 over a wide range of compositions and temperatures. Wm, micellar phase; I1, discontinuous micellar cubic phase; H1, hexagonal liquid crystal; and Lα and Lα′, lamellar liquid crystalline phases.

are bridged by the spacer chain and their conformation is restricted, their aS is still smaller than double the value of aS for the monomeric molecule. The H1 domain is quite a bit wider than that of its monomer counterpart. The packing constraints of the lipophilic core tend to increase with increasing amphiphile concentration, inducing the H1 to Lα phase transition. Nevertheless, mixed phase I1 + H1 forms between H1 and I1 pure liquid crystal domains (57−59 wt %) at 25 °C. This mixed domain as well as H1 shifts toward lower concentrations as the temperature of the system is increased. The micellar domain remains unchanged; however, the highly viscous, gel-like discontinuous cubic phase, I1, domain gets narrower with temperature, similar to its monomer counterpart. The Lα phase at 80 wt % gets transformed into mixed phase of Lα + Lα′ above 50 °C, and gets wider at higher temperatures shifting toward lower concentrations. However, such phase transformation was not observed in the case of disodium 2,3didodecyl-1,2,3,4-butanetetracarboxylate.19 The replacement of dodecyl group with hexadecyl group (i.e., increasing the hydrophobicity of amphiphile) affects the phase behavior. Phase behavior of the 16-GLG-16/H2O system is very similar to that of 12-GLG-12/H2O, however, only above 50 °C (Krafft temperature) (Figure 1c). Solid-phase domain increases on increasing the hydrocarbon chain length of the amphiphile and the solid melts at higher temperatures. Above 50 °C, the micellar region extends up to ∼30 wt %. Above 30 wt %, it transforms into I1. Then, I1 + H1, H1, and Lα form in a sequence with increase in concentration. The Lα domain is wider than that of the 12-GLG-12 system. It does not form mixed Lα + Lα′ phase. The amphiphile is in a solid state above 80 wt % even at higher temperatures.

the dissociation of carboxylic groups of surfactant is also reduced, preventing an increase of aS; therefore, aggregates with a higher packing parameter are formed shifting the domain toward the lower concentration. Then, the lamellar liquid crystal, Lα, is observed that widens at higher temperature, and transformed into mixed phase of Lα + Lα′ above 60 °C. A solid phase is present above 70 wt % existing throughout the whole temperature range, i.e., the phase boundary is not temperaturesensitive. 12-GLG-12, which is a quasi-dimeric form of 12-G with two hydrophobic tails but three carboxylates instead of four, forms similar phases in similar sequence as a function of concentration and temperature. The range for H1 domain is also considerably different between the two systems, wider for the gemini counterpart. This difference can be attributed to the quasi-dimeric structure of 12-GLG-12 at which aS is smaller than the double of the aS for the monomeric molecule (12-G). The aqueous phase behavior of anionic amphiphile gemini (sodium 1,2-bis(N-dodecanoyl β-Alanate)-N-ethane) and the corresponding monomeric amphiphile (sodium N-dodecanoylN-methyl β-Alanate) also shows similar phase behavior.18 The Wm for 12-GLG-12 extends up to 32 wt %, while the phase boundary remains constant throughout the whole temperature range measured. A transformation of disordered micelles to ordered discontinuous micellar cubic phase, I1 occurs at 33 wt %. This behavior, however, is different from that observed for the anionic gemini amphiphile with no spacer group, disodium 2,3-didodecyl-1,2,3,4-butanetetracarboxylate,19,23 in that the W m transforms directly into H 1 with an increase in concentration. With a further increase in concentration, H1, and Lα liquid crystalline phases are formed, sequentially. Although the two hydrocarbon chains of gemini amphiphiles 15474


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When lauroylglutamic acid is neutralized with ethanolamines (mono-, di-, and tri-), varying as effects the liquid crystal domain-range.22 It has been observed that the I1 domain is wider for the larger as (neutralized with triethanolamine), while the H1 domain is wider for the smaller as (neutralized with monoethanolamine). The interlayer spacing, d and aS, gradually decreases, whereas the radius of the cylinder gradually increases with surfactant concentration. Since the counterion concentration is high at high surfactant concentration, the dissociation of the carboxylic group of amphiphile is reduced. This may decrease the repulsion between ionic headgroups, and thus, aS decreases. Additionally, increment of concentration causes hydration on the surfactant hydrophilic moiety to reduce, resulting in the suppression of repulsive force between not only aggregates, but also head groups. As a consequence, the higher the concentration, the higher the packing constraint is. Thus, the concentration-dependent phase transition arises from the suppressed headgroup area. The aS for the quasi-gemini nature of m-GLG-m is considerably smaller than twice the value of aS for monomeric amphiphile. Hence, m-GLG-m molecules are tightly packed at the interface of the cylindrical micelle. The small aS means that the hydrophobic chain of quasi-gemini is close to that in its fully extended form compared with that of the monomeric amphiphile molecule; the quasi-gemini molecule is longer than the monomeric amphiphile molecule. The spacer chain may restrict the conformation of the hydrocarbon chain of the gemini amphiphile surfactant. This fact supports the wider I1 domain for 12-G and wider H1 domain for the 12-GLG-12. The phase boundary observed in the dimeric amphiphiles studied here are temperature sensitive unlike the reported ones before.19,23 The spacer in m-GLG-m, the peptide-based headgroups (lysine and glutamic acid), is likely to play an important role in packing constraint, the presence of hydrogen bonding, leading to the temperature-dependent association behavior of the amphiphilic molecules, thereby exhibiting unique phase behavior.24 3.2. CnTAB Incorporated Aqueous Phase Behavior of m-GLG-m. The effect of the addition of cationic coamphiphile, CnTAB, on the aqueous solution of m-GLG-m has been studied, and the results are displayed in Figure 2 showing partial ternary phase diagrams of m-GLG-m/H2O/CTAB(C16TAB) and 16-GLG-16/H2O/TTAB(C14TAB) systems in the water-rich region at 25 °C. The Wm extends over a wide concentration range for 12GLG-12/H2O, 14-GLG-14/H2O, and 16-GLG-16/H2O systems (∼ up to 30 wt % along the binary axis) (Figure 1a). Here, we need to note that the Wm region for rheological measurements for 16-GLG-16/H2O systems is prepared above 50 °C (Krafft temperature), because it exists as a twophase dispersion of 16-GLG-16 solid and water at 25 °C. The Wm is transformed to highly viscous micellar cubic phase; I1 is above 30 wt % in all the systems studied. Therefore, spheroid or globular micelles are expected in the Wm regions. It is found that the lipophilic coamphiphile, CnTAB, is soluble in the aqueous micelles of m-GLG-m. The oppositely charged amphiphiles exhibit synergism.25−27 The additive here, CnTAB, increases the viscosity of micellar solution of mGLG-m slowly at first, then abruptly with increase in its concentration, forming highly viscous solutions. The composition for viscous samples is indicated by shaded areas in the partial ternary phase diagrams. These samples are optically isotropic at rest while exhibiting shear-birefringence. It is

Figure 2. (a) Partial ternary phase diagrams of m-GLG-m/H2O/ CTAB (where m = 12, 14, and 16), and 16-GLG-16/H2O/TTAB systems and (b) SAXS patterns for 3 wt % 12-GLG-12/H2O + 29 wt % CTAB, 3 wt % 14-GLG-14/H2O + 29 wt % CTAB, and 3 wt % 16GLG-16/H2O + 28 wt % CTAB at 25 °C; Wm, micellar phase; I1, discrete micellar cubic phase; and Lα, lamellar liquid crystalline phases. The shaded areas in the micellar phase region highlight the viscous region of wormlike micelles. The dotted lines in the phase diagrams represent the compositions along which the rheological measurements were performed and the stars represent the composition corresponding to (b).

interesting to note that the CnTAB incorporated micellar solutions of 16-GLG-16 are isotropic and highly viscous even at room temperature. A phase transition of this transparent viscous micellar solution to turbid solution of vesicular dispersion, or birefringent liquid crystals (Lα or H1) depending on m-GLG-m concentration, has been observed with increasing conccentrations of CnTAB. This Wm to vesicular dispersion/or lamellar phase (Lα) transition suggests that the positively charged CTAB exhibits synergy with oppositely charged mGLG-m and reduces the curvature leading to transformation sphere to cylinder to network of long cylinder micelles. The Wm and viscous regions are wide for shorter acyl chain for mGLG-m. Similar phase behavior has been observed when CTAB is replaced with TTAB in 16-GLG-16/H2O system with a difference that the Wm is wider in the latter case. 3.3. Rheological Behavior. 3.3.1. Effect of Coamphiphile. Steady-shear rheological measurements (viscosity (η) vs shearrate (γ̇)) were performed in the ternary mixtures of m-GLG-m/ H2O/CnTAB, as a function of CnTAB at 25 °C, and the results are shown in Figure 3. It has been observed that the mixture of oppositely charged amphiphiles exhibit synergism, in turn forming highly viscous 15475


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Figure 3. (a) Viscosity (η) vs steady shear rate (γ̇) curves for 1 wt % 16-GLG-16/H2O with varying concentration of CTAB, (b) zero-shear viscosity (η0) vs CTAB concentration at 1 and 3 wt % of 16-GLG-16/H2O, and η0 curves for 3 wt % 16-GLG-16/H2O with TTAB, and (c) η0 for 3 wt % of 12-G/H2O, 12-GLG-12/H2O, 14-GLG-14/H2O, and 16-GLG-16/H2O as a function of CTAB at 25 °C.

1 and 3 wt % of 16-GLG-16/H2O and η0 as a function of TTAB in 3 wt % 16-GLG-16/H2O systems. Here, first we discuss the 16-GLG-16/H2O/CTAB system. It has been observed that 0.3 wt % 16-GLG-16/H2O forms wormlike micelles with CTAB (Figure SI 2; the lowest concentration of gemini amphiphile that forms wormlike micelles, so far reported). At constant 16GLG-16 concentration (1 wt %) and varying CTAB concentration, the viscosity shows a rather complicated behavior, with two maxima and one minimum. In the highly dilute concentration of CTAB ( η0max of 3 wt % 16-GLG-16/H2O/ TTAB system and the second maximum is not that prominent (see Figure 3b). Furthermore, DTAB (C12TAB) too promotes axial elongation of the cylinders but could not form a network structure (data not shown here). The acyl side-chain length of the amphiphile also affects the rheology. The η0max lowers as the chain is decreased from 16 to 12 via 14 (Figure 3c). The η0max of the 16-GLG-16 system is the highest among all. In the 14-GLG-14/H2O/CTAB system, the first maximum occurs at 14-GLG-14/CTAB = 1:2.4 indicating that the 14-GLG-14 also needs ∼2 mol of CTAB, and then the minimum occurs at 14-GLG-14/CTAB = 1:2.8, where complete neutralization occurs, and the second maximum (but not that prominent) at 14-GLG-14/CTAB = 1:4.6, the concentration of CTAB here is higher than that of the 16-GLG-16/CTAB system. Wormlike micelles could also be achieved with a much more hydrophilic system, 12-GLG-12; however, no η0max was observed, viscosity increases continuously till the phase separation, and a similar trend in η0 has been reported when the hydrophilicity of the system forming wormlike micelles is increased.28 It can be anticipated that micelles grow weakly and phase transformation occurs before the maximum is reached. The unreached maxima could be that of cation-rich micelles. It has been noted that, when 12-GLG-12 is added to the aqueous micellar solutions of CTAB, a single maximum is observed in the zero-shear viscosity plot (see the Supporting Information, Figure SI4). Thus, the lipophilic tail architecture of amphiphile and coamphiphile plays a crucial role in the rheological behavior. Similar rheological behavior has been observed for the CTAB incorporated aqueous solutions of monomer, 12-G; however, the viscosity is slightly lower than that of its dimeric counterpart and at the cost of higher concentration of CTAB. Thus, gemini amphiphile is more effective in enhancing the viscosity (at lower concentration compared to its monomeric counterpart). It is well-known that the bulk properties of amphiphile in solution largely depend on the self-assembled nanostructures. In this contribution, we have found that the rheology depends on the molecular structure of the amphiphile as well as coamphiphiles. The rheological behavior of the present system

Table 1. (a) Values of Zero Shear Viscosity (η0) for the 1 and 3 wt % 16-GLG-16/H2O Systems with CTAB and η0 for 3 wt % 16-GLG-16/H2O with TTAB (corresponding to Figure 3a,b) and (b) η0 for 3 wt % of 12-G/H2O,12-GLG-12/H2O, 14-GLG-14/H2O, and 16-GLG-16/H2O as a Function of CTAB Concentration (corresponding to Figure 3c) at 25 °C a

CTAB (wt %)

1 wt % 16GLG16/ H2O/CTAB (η0)

0 0.7 0.76 1.825 0.856 0.889 0.91 0.933 0.955 0.998

0.003 0.01 0.145 115 290 200 115 195 130

CTAB (wt %)


0 0.99 1.99 2.44 2.91 3.39 4.32 5.06 5.67 6.55 6.97 7.42 8.28

0.00125 17.5 1850 1600 1400 1550 1500 1400 850 775 630 550

TTAB (wt %)

3 wt % 16GLG16/ H2O/TTAB (η0)

0 0.99 1.90 2.29 2.44 2.6 2.74 2.91 3.39 4.32 5.06 5.67 6.55 6.97

0.0012 0.057 9.5 18 20 17 12 9.1 6 3.5 1.2 0.35 0.16

b

CTAB (wt %) 0 0.99 1.99 2.44 2.91 3.39 4.32 5.06 5.67 6.55 6.97 7.42 8.28 10 12 15 17 20 22 25 27




3 wt % 12G/ H2O/CTAB (η0)

0.00125 17.5 1850 1600 1400 1550 1500 1400 850 775 630 550

0.001 0.001 0.002 3 200 139 163 175 220 220 220 210 200

0 001 0.00115

0.001 0.0011

0.00147 0.012 0.53 6.3 10 15 21 21 26 30 40 44.9 89.1 95.44 147 196.8 310

0.00115

0.00123 0.1 2 9.5 35 62 96

255

rather low values, and finally, transformation to two-phase turbid solution takes place at 16-GLG-16/CTAB ∼ 1:2.7 mol ratio. As is clear in Figure 3b, the η0 of the micellar solution of 16GLG-16 increases with CTAB. Thus, micellar growth appears to occur via the newly added amphiphile at the expense of the spheroid micelles of aqueous 16-GLG-16. The η0 curves shift toward the right side (toward higher CTAB concentration) upon increase in 16-GLG-16 concentration from 1 to 3 wt %, and the maximum viscosity that a system can reach, η0max, increases from ∼300 to 1750 Pa.s. In 3 wt % 16-GLG-16/H2O/ 15477


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Figure 4. (a) Variation of elastic modulus, G′ (closed circles), viscous modulus, G″ (open circles), and complex viscosity, |η*| (closed triangles) as a function of oscillatory shear frequency, ω, for 1 wt % 16-GLG-16/H2O + 0.89 wt % CTAB system, and the rheological parameters G0 (diamonds) and τR (circles) obtained from the Maxwell model fittings for (b) 1 wt % 16-GLG-16/H2O/CTAB. In panel a, the solid lines represent the Maxwell model fits, while they are guides to our eyes in panel b.

properties; elastic behavior dominates (G′ > G″) at high ω, while at low ω, the viscous behavior dominates (G″ > G′). Such rheological behavior indicates the presence of viscoelastic wormlike micelles in the system. The rheogram further demonstrates that, at high ω, G′ develops into a well-defined plateau value, G0 (mesh size or density of entangled network). These data fit reasonably well to the Maxwell model in the low ω region with a notable deviation in the high ω regions, which arises from the transition of “slower” reptation to “faster” relaxation mode. Rouse modes well explain such phenomena.32−34 Figure 4b shows the rheological parameters, G0 and τR, estimated from the Maxwell model fits. The G0 and τR have two maxima with CTAB, the same trend as in η0 and at similar CTAB compositions (see Figure 3b). G0 and τR increase monotonously with CTAB, then show a maximum value at a composition corresponding to the η0max, indicating an increase in the micellar length and network density of the wormlike micelles. On the other hand, beyond the η0max a sharp decreasing pattern reaching G0min and τRmin as η0min can be anticipated as an occurrence of complete charge neutralization of anionic 16-GLG-16 and cationic CTAB. Then, there is a turnover in the nature of wormlike micelles formed with excess CTAB composition. The plot witnesses a rise in the G0 and τR like η0 reaching a second maxima. A similar explanation can be assigned for the CTAB-rich region with increasing concentration of 16-GLG-16. A similar trend in G0 and τR has been observed for other systems mentioned in Figure 3 and has been depicted in Figure SI 5a-d (see Supporting Information). The increase of τR with coamphiphile in the first place could be understood as a micellar growth and an increase in the network density of entangled flexible wormlike micelles; the system is more viscoelastic. Under this condition, the system undergoes a stress relaxation process slowly.20 With further increase in CTAB, G0 and τR decreases indicating a structural change in the network that allows stress relaxation by an additional faster mechanism. This is possible because of either interconnected or smaller wormlike micelles; the results obtained here support the breaking of micelles for the decreased length in micelles. It should be noted that the values of G0 and τR both increase when the concentration of the 16-GLG-16 is increased from 1 to 3 wt %; and τR increases when the acyl side chain of m-GLGm is increased from 12 to 16 (see Supporting Information, Figure SI5); suggesting an increase in the length and degree of entanglement of long micelles to be the reason for the increased viscosity (see Figure 3b) and concluding that the 16-

can be explained in terms of the modulation in the selfassembled nanostructures. The m-GLG-m is expected to form spherical micelles with a high interfacial curvature above the CMC due to its bulky hydrophilic moiety. The interfacial curvature of aggregates gradually decreases with coamphiphile favoring a transition to rod-like structure. Besides, the prominent energy required for the formation of hemispherical end-caps of the cylindrical micelles also increases, and this energy is compensated by either fusing free ends of micelles with cylindrical part of its own or other micelles, thus forming micellar joints, or branching in the network structure. Such joints can slip along the cylindrical body, thereby allowing a faster and easier way of stress relaxation.29,30 Branching points also restrict the alignment of micelles under shear causing an increase in γ̇c.31 A transient network of long and flexible micelles exhibits viscoelastic properties. They undergo two stress relaxation processes: reptation or reptile-like motion along a micellar tube and reversible scission of micelles. They take place at two time scales, reptation time, τrep, and scission time, τb, as described by Cates et al.10,32 The viscoelasticity of the present systems could be explained by the Maxwell model. Maxwell’s equations for the viscoelastic wormlike micelles consider a single stress relaxation time, τR, given by (τb τrep)1/2 and is governed by the following equations.10,33 G′(ω) =

G″(ω) =

ω 2τR 2 1 + ω 2τR 2 ωτR 1 + ω 2τR 2

G0 G0

(1)

(2)

where G0 is called the plateau modulus, that is a constant value of elastic modulus, G′, at higher angular frequency, ω. The relaxation time, τR, can be estimated from the G′− G″ crossover frequency, ωc, i.e., τR = 1/ωc, when G′ = G″. Once G0 and τR are available, η0 can be calculated by following relation: |η*| ≈ η0 = G0τR

(3)

The dynamic properties of these solutions have been investigated by performing oscillatory-shear measurements on the viscous samples close to η0max in the η0 curves (see Figure 3). Figure 4 shows the rheogram for the 1 wt % 16-GLG-16/ H2O + 0.89 wt % CTAB as a typical example. They are expressed in terms of G′ and G″. These moduli depend on ω. The single composition possesses both elastic as well as viscous 15478


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Figure 5. (a) Zero shear viscosity, η0, and (b) relaxation time, τR, versus 1/T, (c) variation of elastic modulus, G′, and viscous modulus, G″, as a function of angular frequency, ω, and (d) variation of plateau modulus, G0 (closed diamonds), and relaxation time, τR (open circles), as a function of temperature as obtained by frequency sweep measurement for the 1 wt % 16-GLG-16/H2O + 0.89 wt % CTAB, where T is the absolute temperature. The lines in panels (a) and (b) are the fits to the Arrhenius equation (c), the fits to the Maxwell equation, while in (d) they are guides to the eyes.

ωc gives τR; higher ωc (lower τR) at higher temperature corresponds to the rapidly relaxing system. A decay in τR here can be attributed to the temperature-induced microstructural change. The evolution of associated G0 and τR as a function of temperature for the same set of samples described in Figure 5a is presented in Figure 5d. The G0, which is a measure of network density of the wormlike micelles, is almost constant although there is a wide variation in temperature (25 to 80 °C). However, the η0 and τR decay exponentially with temperature (see Figure 5a,b,d). The decrease in η0 and τR can be attributed to either the micellar branching or breaking, which has also been observed in other oppositely charged aqueous cationic− anionic amphiphile wormlike micelles.37,38 The breaking of micelles with temperature can be because, as the value of τR decreases with increasing temperature, ionic wormlike micelles are prone to break into short, rod-like micelles upon heating.37 Besides the wormlike micelles being in thermal equilibrium with their monomers, the average micellar length of the wormlike micelles/micellar contour length, L̅ , is a thermodynamic quantity, and it responds to changes in solution composition and temperature. It is clear that, when a wormlike micellar solution is heated, L̅ decays exponentially with temperature according to the following equation.37

GLG-16 seems to be the best viscosity enhancer in the series used here. 3.3.2. Effect of Temperature. The structure and dynamics of the wormlike micelles are highly influenced by temperature.35 It has been observed that an increase in temperature dehydrates the EO chain in polyoxyethylene-based amphiphile increasing the cpp value, transforming aggregates with less positive curvature, promoting 1-D micellar growth.36 It also can trigger branching, breaking the network structure.37 In order to observe the effect of temperature on the present system, rheological measurements were performed on 1 wt % 16-GLG16/H2O + 0.89 wt % CTAB varying temperature. The variation of η0 and τR with temperature is shown in Figure 5a,b, respectively. The η0 and τR decay exponentially with temperature as proven by the Arrhenius type of plot and follows the well-known equation for wormlike micelles.10,38 η0 = A e Ea / RT

(4)

τR = A e Ea / RT

(5)

where Ea is the flow activation energy, R is the universal gas constant, and A is a constant. The flow activation energies calculated from the slopes of the straight line fits are found to be 126 and 139 kJ/mol from log η0 and log τR versus 1/T, respectively; comparable to the values reported for other wormlike micelles.10,38 The decay in the η0 and τR with temperature can be due to the structural transformation in the network. The rheogram for the 1 wt % 16-GLG-16/H2O + 0.89 wt % CTAB as a function of temperature (Figure 5c) reveals that with increasing temperature the crossover frequency ωc shifts toward higher values, and then, there is no crossover beyond 80 °C in the measured ω range (0.001−100 s−1). The reciprocal of

⎡ E ⎤ L̅ ≈ ϕ1/2 exp⎢ c ⎥ ⎣ 2kBT ⎦

(6)

where ϕ is the volume fraction of wormlike micelles, Ec is the end-cap energy (i.e., the excess energy associated with the hemispherical caps compared to the cylindrical body of the wormlike micelles), and kB is Boltzmann’s constant. A decrease in L̅ affects the dynamics of micellar stress relaxation. The τR is determined by competition between the micelle breaking and chain reptation, and Maxwell behavior is generally observed 15479


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when the breaking time τb is much lower than the reptation time τrep. In the fast breaking regime, the τR is equal to (τrepτb)1/2. Since τrep ∼ (L̅ )3, a decrease in L̅ will cause a drastic reduction in the reptation time, which has a dominant effect on the relaxation time. As with rising temperature, the fall of τR indicates that stronger temperature induces curvature reduction favoring formation of low local-curvature junctions, which reduces Lc more strongly. Reduction of τR indicating formation of shorter micelles with greater end-cap density and lower entanglement density cannot be completely discarded. The dynamic rheology results in Figure 5b anticipate the fall in viscosity of the systems. The Maxwell equations predict a monotonic decrease of G″ in the high-ω region, however, the wormlike micelles deviate from this behavior, showing an increase of G″ in the high-frequency region. The deviation is much more prominent at higher temperatures. This deviation is often associated with the stress relaxation by additional “faster” processes such as Rouse modes of cylindrical micelles, analogous to the polymer chain. The minimum value of G″ in the high-frequency region is related to L̅ according to the relation32 ″ l Gmin ≈ e G0 L̅

viscosity increases only slightly with the DTAB system, and it does not show any viscoelastic properties. These wormlike micelles exhibit viscoelastic behavior, could be described by the Maxwell mechanical model with a single stress relaxation mode, and are highly sensitive to temperature. Viscosity decreases by an order of magnitude upon small changes in temperature, mainly attributed to the breaking of micelles. It is possible to form wormlike micelles at very low concentrations of 16-GLG16 (0.25 wt %) through a wide range of concentration and temperature. The gemini amphiphile here forms wormlike micelles at lower molar volumes than their monomeric counterpart. We believe that this work finds an important place in the formulation of personal care products and also adds a positive dimension to the existing knowledge of rheology of wormlike micelles.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

(7)

*E-mail: [email protected]; [email protected]. jp; [email protected]. Phone: +81-47-124-1501 ext. 3662. Fax: +81-47-121-2439.

where le is the entanglement length, the contour length of the section of wormlike micelles between two entanglement points. It is seen that the heating does not affect the value of G0, which means that the mesh size and hence the entanglement length le are unaffected by temperature. Simultaneous heating shifts the minimum value of the loss modulus, G″min, to higher frequencies consistent with the presented results,39 indicating a decrease of L̅ of micelles according to the eq 7. Thus, a more convincing explanation of the drop in η0 and τR with increasing temperature here seems to be the shortening of the micelles.38

Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION Aqueous binary phase behavior of peptide-based gemini amphiphile, m-GLG-m, where m = 12 and 16, has been studied in detail over a wide range of concentration and temperature. The alternation in the acyl chain length of the amphiphile affects the Krafft temperature and the phase sequences. 12GLG-12 and 14-GLG-14 self-assemble into spherical micelles above the critical micelle concentration and transform to a micellar cubic phase with a space group at ∼33 wt % at 25 °C; 16-GLG-16 shows similar phase behavior but above 50 °C. It is found that CnTAB (where n = 12, 14, and 16) is solubilized in the aqueous micelles of m-GLG-m forming the viscoelastic micelles. The phase and rheological behavior of viscoelastic micelles in mixed aqueous systems of m-GLG-m and CnTAB is found be dependent upon the molecular structure and concentration of amphiphile and coamphiphile, as well as temperature. The CnTAB exhibits synergy with oppositely charged m-GLG-m micelles and decreases the curvature of the aggregates, and hence, favors sphere to rod transition, eventually forming a transient network of wormlike micelles. Viscosity of the micellar solution increases by six orders of magnitude. The zero-shear viscosity (η0) versus CnTAB concentration curve shows two peaks, and the position of the peak shifts to the right (at higher concentration of CnTAB) when the concentration of 16-GLG-16 is increased from 1 to 3 wt %. On the other hand, it shifts to the left on decrease of the hydrophobic chain length (TTAB) of the coamphiphile. The 15480


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Peptide-Based Gemini Amphiphiles: Phase Behavior and Rheology of

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