Wormlike Micelle Formation by Acylglutamic Acid with Alkylamines

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Wormlike Micelle Formation by Acylglutamic Acid with Alkylamines Kenichi Sakai,†,* Kazuyuki Nomura,† Rekha Goswami Shrestha,† Takeshi Endo,† Kazutami Sakamoto,‡ Hideki Sakai,† and Masahiko Abe†,* †

Department of Pure and Applied Chemistry in Faculty of Science and Technology and Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan ‡ Faculty of Pharmacy, Chiba Institute of Science, 15-8 Shiomi, Choshi, Chiba 288-0025, Japan S Supporting Information *

ABSTRACT: Rheological properties of alkyl dicarboxylic acid-alkylamine complex systems have been characterized. The complex materials employed in this study consist of an amino acid-based surfactant (dodecanoylglutamic acid, C12Glu) and a tertiary alkylamine (dodecyldimethylamine, C12DMA) or a secondary alkylamine (dodecylmethylamine, C12MA). 1H NMR and mass spectroscopic data have suggested that C12Glu forms a stoichiometric 1:1 complex with C12DMA and C12MA. Rheological measurements have suggested that the complex systems yield viscoelastic wormlike micellar solutions and the rheological behavior is strongly dependent on the aqueous solution pH. This pH-dependent behavior results from the structural transformation of the wormlike micelles to occur in the narrow pH range 5.5−6.2 (in the case of C12Glu-C12DMA system); i.e., positive curved aggregates such as spherical or rodlike micelles tend to be formed at high pH values. Our current study offers a unique way to obtain viscoelastic wormlike micellar solutions by means of alkyl dicarboxylic acid-alkylamine complex as gemini-like amphiphiles.

1. INTRODUCTION Wormlike micelles are elongated (locally cylindrical) micelles that can become extremely long, flexible, and entangled with each other forming a transient network structure. This highly entangled network structure results in an increased viscoelasticity of the solution phase, being similar to conventional polymer solutions. However, there is a remarkable advantage of surfactant wormlike micellar solutions over polymer solutions; i.e., wormlike micelles are formed as a result of self-assembly of surfactant molecules and the transient network structure is dynamically reformable against shear motion. Hence, the preparation of wormlike micellar solutions has attracted much attention not only from academic but also from industrial standpoints.1,2 In general, surfactant molecules are associated with each other at concentrations above a critical micelle concentration (cmc) and form spherical micelles in aqueous media. In order to obtain viscoelastic wormlike micellar solutions, it is necessarily required to control packing geometry of surfactant molecules under appropriate conditions. In most instances, this structural transition from spherical to wormlike micelles is induced by the addition of cosurfactants.3−6 In this work, we demonstrate the formation of wormlike micelles in aqueous media by means of an amino acid-based surfactant, dodecanoylglutamic acid (C12Glu). Such amino acid-based surfactants are generally accepted as environment and human-friendly materials because of their biodegradable and less toxic nature.7 However, their relatively large headgroups sometimes cause a difficulty in preparing wormlike micelles. Shrestha and Aramaki have been focusing on the formation of wormlike micellar solutions in several amino acidbased surfactant systems.8−14 For example, C12Glu yields © XXXX American Chemical Society

wormlike micellar solutions as a result of mixing with a cationic surfactant (hexadecyltrimethylammonium bromide, C16TAB).8 The key idea of this earlier study to prepare wormlike micellar solutions lies in the fact that the water-insoluble amino acidbased surfactant is partially neutralized by 2,2′,2″-nitrilotriethanol (or triethanolamine, TEA). Hence the viscoelastic properties of the wormlike micellar solutions are largely dependent on the degree of neutralization by TEA as well as the C16TAB concentration (at a given concentration of the amino acid-based surfactant). We have also demonstrated that amino acid-based geminilike surfactants (acylglutamyllysilacylglutamate sodium salts) are able to form wormlike micelles as a result of mixing with quaternary ammonium type monomeric surfactants.15 One of the most interesting properties we had seen in this earlier study is the fact that highly viscoelastic wormlike micellar solutions are obtained even at relatively low concentrations of the amino acid-based surfactants (0.3 wt %). We assume that this property is reflective of the gemini-like structure of the amino acid-based surfactants; i.e., the packing parameter of gemini surfactants is generally larger than that of monomeric surfactants, and they exhibit excellent association capability in aqueous media at relatively low concentrations. Recently, we have proposed a possible method for the easy preparation of new gemini-like surfactant systems by means of aqueous mixtures of an alkylamine with an alkyl dicarboxylic acid.16 The stoichiometric complex consisting of the alkylamine Received: September 18, 2012 Revised: November 16, 2012

A

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Figure 1. Chemical structures of C12Glu (acid-type), C12DMA, C12MA, C12A, and C12TAB. were used as the C12Glu-alkylamine complex. In typical experiments, the C12Glu-alkylamine complex was added into pure water at a desired concentration and the solution pH was adjusted by the addition of a small amount of an NaOH aqueous solution. The final products (C12Glu-alkylamine complex) were characterized with analytical techniques which included 1H NMR (JEOL-ECP 500 MHz) and ESI-MS (FT-ICR MS Varian 910-MS) spectroscopies. 2.3. Physicochemical Analysis. Samples with required amounts of reagents in glass vials were homogenized and kept in a dry bath at 25 °C at least for 4 days to ensure equilibration before performing rheological measurements. Measurements were performed in a stresscontrolled rheometer, AR-G2 (TA Instruments) using cone−plate geometries (diameter of 60 mm with cone angle of 2° 1′ 9″ for lowviscosity samples and diameter 40 mm with cone angle of 2° 0′ 4″ for high-viscosity samples). Frequency sweep measurements were performed in the linear viscoelastic region of the samples determined previously by dynamic strain sweep measurements. The zero-shear viscosity (η0) of the samples was determined from steady shear rate measurements by extrapolating the viscosity in shear rate curve to zero-shear rate. 13 C NMR measurements (JEOL-ECP 500 MHz) were performed in D2O in the presence of sodium (3-trimethylsilyl)-1-propanesulfonate as an NMR interior reference and the solution pH was adjusted by NaOD. Unless otherwise stated, all measurements reported herein were performed at 25 °C (or at a room temperature of ca. 25 °C).

and alkyl dicarboxylic acid compounds results from a proton transfer from the acid to the amine in aqueous media. The physicochemical properties of aqueous mixtures of alkylamine with alkyl monocarboxylic acid have also been studied from the standpoint of their phase behaviors.17−19 On the basis of these earlier studies, we expect that wormlike micelles are formed in the aqueous amino acid-based surfactant (C12Glu) system as a result of complex formation with alkylamine compounds. Here, the alkylamine compounds play a role of an organic counterion like TEA to neutralize the carboxylic acid headgroups of C12Glu as well as an agent to control the packing parameter being suitable for wormlike micelles (1/3−1/2). It has been reported that gemini surfactants yield wormlike micelles in their aqueous solutions under appropriate conditions.20−26 Hereafter, we present (i) molecular characterization of the C12Glualkylamine complex on the basis of 1H NMR and mass spectroscopic data; (ii) physicochemical characterization of wormlike micellar solutions through rheology measurements; and (iii) effects of cationic species on the formation of wormlike micelles.

2. EXPERIMENTAL SECTION 2.1. Materials. C12Glu (disodium salts) was kindly supplied from Asahi Kasei Chemicals Co. Alkylamine compounds including dodecyldimethylamine (C12DMA) and dodecylamine (C12A) were purchased from Tokyo Chemical Industry (TCI). A secondary alkylamine, dodecylmethylamine (C12MA), was purchased from Alfa-Aesar. A quaternary ammonium type cationic surfactant, dodecyltrimethylammonium bromide (C12TAB), was purchased from Sigma-Aldrich and used as received without purification. The chemical structures of these materials are shown in Figure 1. The other chemicals including NaBr, HCl, NaOH, ethanol, acetone, and ethyl acetate were of analytical grade and used without further purification. CDCl3 including tetramethylsilane as an interior reference (Wako), D2O (Wako), NaOD (Cambridge Isotope Laboratories, Inc.), and sodium (3-trimethylsilyl)-1-propanesulfonate (Aldrich) were used for 1 H- and 13C NMR measurements. The water used in this study was deionized with a Barnstead NANO Pure Diamond UV system and filtered with a Millipore membrane filter (0.22 μm pore size). 2.2. Preparation and Characterization of C12Glu-Alkylamine Complex. An aqueous solution of C12Glu (disodium salts) was mixed with HCl in a separatory funnel and then ethyl acetate was added to the aqueous mixture. After vigorous mixing, the organic phase was evaporated. The residue obtained here was recrystallized from acetone and finally dried under a reduced pressure. We confirmed the exchange from sodium salt-type to acid-type through IR and 1H NMR measurements. Unless otherwise stated, the acid-type C12Glu sample was used in the following experiments. C12Glu (acid-type) was mixed with an excess amount of the alkylamine compounds in ethanol and the mixture was stirred for 48 h at room temperature. Then, the mixture was evaporated and the residue obtained here was recrystallized from acetone. After filtration, the residue was dried under a reduced pressure. The products obtained

3. RESULTS AND DISCUSSION 3.1. Molecular Characterization. As mentioned in Section 2.2, the C12Glu-alkylamine complex was prepared in ethanol, where the complex precipitated from the solution phase. We note that both the alkylamine compounds and C12Glu (acid-type) are molecularly soluble in ethanol, and hence the precipitates obtained in this reaction suggest the complex formation to occur. The molecular characterization data of the C12Glu-alkylamine compounds are shown in Supporting Information (Figures S1, S2, and Table S1), where one can see 1H NMR and mass spectroscopic data of each system. Briefly, we found that (i) the integral intensity ratio of 1 H NMR signals detected for each complex sample is good consistent with the corresponding value expected for the 1:1 (C12Glu-C12DMA and C12Glu-C12MA) or 1:2 (C12GluC12A) complex material; and (ii) the mass spectra support the formation of such 1:1 or 1:2 stoichiometric complex. We can suggest, therefore, that C12Glu forms a 1:1 complex with the tertiary and secondary alkylamine compounds (i.e., C12DMA and C12MA) whereas it forms a 1:2 complex with the primary alkylamine compound (i.e., C12A). This difference may result from a relatively large headgroup (in other words, steric hindrance) of the tertiary/secondary alkylamines. It is interesting to note that the 1:2 complex formed by C12Glu with the primary alkylamine is insoluble in aqueous media over a whole range of pH investigated, whereas the 1:1 complex B

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formed by C12Glu with the tertiary/secondary alkylamines exhibits pH-dependent aqueous solution properties as mentioned below. 3.2. Rheological Properties of C12Glu-C12DMA Complex System. Figure 2a shows the steady-shear viscosity

G″(ω) =

ωτR 1 + ω 2τR 2

G0

(2)

where G0 is the plateau modulus, G′ is the elastic modulus, G″ is the viscous modulus, ω is the angular frequency, and τR is the single stress relaxation time. It is possible to estimate experimentally (i) the G0 value from a constant value of G′ measured in the high ω region and (ii) the τR value from the G′ − G″ crossover frequency (ωc), i.e., τR = 1/ωc. The variations of G′ and G″ are shown in Figure 3 as a function of ω. In this

Figure 3. Elastic modulus (G′) and viscous modulus (G″) data obtained for the C12Glu-C12DMA complex system as a function of angular frequency (ω). The complex concentration is fixed at 3 wt %. The data obtained at pH 5.7 are only shown as a typical example. Fitting curves based on the Maxwell model are shown as solid curves.

study, oscillatory-shear rheological measurements were performed at a fixed complex concentration of 3 wt % under various pH conditions, and the data shown in Figure 3 are obtained at pH 5.7 (as a typical example). One can see in this figure that elastic behavior dominates at high frequencies (G′ > G″), whereas viscous behavior dominates at low frequencies (G′ < G″). This is a typical behavior of wormlike micellar solutions, and hence we can suggest that the C12Glu-C12DMA complex forms a transient network structure of wormlike micelles under these experimental conditions. In addition, the Maxwell model fitting curves reasonably agree with the experimental G′ and G″ data at low frequencies, although a significant deviation is seen for the G″ data at high frequencies. This deviation results from the transition of “slower” reptation to “faster” relaxation mode.27 We note that such a rheological behavior suggesting the formation of wormlike micelles is similarly observed in the C12Glu-C12DMA complex system in the range of pH 5.5−6.0. The rheological parameters (G0 and τR) estimated for the C12Glu-C12DMA complex system at a fixed complex concentration of 3 wt % are shown in Figure 4 as a function of solution pH. Here G0 measures the number of entanglements between wormlike micelles or the mesh size of the network structure, whereas τR is relevant to the contour length of the wormlike micelles.8 One can see in Figure 4 that, in the range of pH 5.6−5.8, the G0 value increases with decreasing pH, whereas the τR value shows a maximum at pH 5.7. This pH is consistent with the pH where the maximum η0 value is observed (see Figure 2b). The combination of the rheological data shown in Figures 2, 3, and 4 suggests the formation of a pH-dependent network structure of wormlike micelles. The non-Newtonian shear thinning behavior that is seen in the pH range of 5.5−6.0 results from the deformation of the transient network structure

Figure 2. (a) Steady-shear viscosity data obtained for the C12GluC12DMA complex system at various pH values and (b) the corresponding zero-shear viscosity (η0) data as a function of pH. The complex concentration is fixed at 3 wt %. We note that the C12Glu-C12DMA complex precipitates below the solution pH of 5.4, so the rheological data obtained above the solution pH of 5.5 are only shown.

data obtained for the C12Glu-C12DMA complex system under various pH conditions. The concentration of the complex is fixed at 3 wt % in aqueous solutions. Interestingly, the flow behavior is highly affected by the solution pH: a Newtonian fluid is obtained at pH 6.2, whereas shear-thinning behavior is observed in the pH range from 5.5 to 6.0. In the nonNewtonian pH range, the viscosity shows a maximum at pH 5.7. This behavior is clearly seen when we plot the resulting η0 data as a function of pH (see Figure 2b). It should be noted here that (i) the highly viscous samples are optically isotropic clear solutions; (ii) neither precipitation nor phase separation is seen for these viscous samples; and (iii) these highly viscous samples show shear-birefringence. These behaviors are typically seen for wormlike micellar solutions,8,10 and hence we assume that wormlike micelles are formed in such highly viscous sample solutions. It is known that dynamic viscoelastic data obtained for wormlike micellar solutions follow the Maxwell model:8 G′(ω) =

ω 2τR 2 1 + ω 2τR 2

G0

(1) C

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place with decreasing pH (from 5.7 to 5.5), accompanying with an increased number of entanglements between wormlike micelles. In typical wormlike micelle systems, a structural transition from spherical to wormlike micelles occurs in the following order as a function of cosurfactant concentration; spherical micelles−rodlike micelles−wormlike micelles with a highly transient network structure.5,6,8−14 This transition accompanies with a change in the curvature of micellar aggregates to lower positive values. In other words, the increased concentration of cosurfactant results in lesser positive curved micellar aggregates. From this point of view, it is possible to understand that the structural change of the C12Glu-C12DMA wormlike micelles occurs toward lesser positive curved aggregates with decreasing pH. This situation looks similar to the typical wormlike micelle systems. However, a clear difference between the typical systems and our current system lies in the driving factor for determining the micelle curvature: i.e., the driving factor is the concentration of cosurfactant in the typical wormlike micelle systems, whereas in our case the charge density (or the acidity) around the carboxylic acid headgroups as a function of pH. In order to study the pH-dependent property in more detail, we have performed 13C NMR measurements. Figure 6 shows

Figure 4. Plateau modulus (G0) and single stress relaxation time (τR) data obtained for the C12Glu-C12DMA complex system in the pH range 5.6−5.8. The complex concentration is fixed at 3 wt %.

at high shear rates. Within the pH range investigated in this study, the most viscoelastic fluid corresponding to the rigid structure of the wormlike micellar network is obtained at pH 5.7, where we can see the highest η0 and the highest τR values. We assume that the pH-dependent wormlike micelle formation occurs according to the following processes (see also Figure 5).

Figure 5. Wormlike micelle structures as a function of pH.

Newtonian fluids are obtained above the solution pH of 6.2, suggesting the formation of spherical or rodlike micelles. A decreased pH from 6.0 to 5.7 induces a micellar growth of the spherical or rodlike micelles into wormlike micelles and hence viscoelastic solutions with a transient network structure are obtained in this pH range. This structural transition is supported by the experimental results that the decreased pH results in the increased η0, G0, and τR values. A further decrease in the solution pH from 5.7 to 5.5 results in an increased G0, but the η0 and τR values are significantly decreased with decreasing pH. The increased G0 can be taken as an evidence of one-dimensional micellar growth and hence an increased number of entanglements of wormlike micelles, however, the observed maximum of τR implies some structural changes occur simultaneously that allow a faster stress relaxation.29 Similar results have been reported in the earlier papers,8,14 where it is suggested that a monotonous increase in G0 with a τR maximum results from a micellar branching or interconnection allowing a faster stress relaxation and a decreased viscosity. These papers focus on the addition of cosurfactants (cationic surfactants) on a micellar growth of C12Glu neutralized by organic counterions (like TEA), and hence the total amount of surfactants forming the wormlike micelles is not constant. This point is different from our current system. However, when taking the pH-dependent charge density around the carboxylic acid headgroups and a resulting change in micelle curvature into consideration (see also discussion shown below), one possible interpretation for the observed behavior is that branching or interconnection of the wormlike micelles takes

Figure 6. Chemical shifts of characteristic 13C NMR signals measured at various pH values. The C12Glu-C12DMA complex concentration is fixed at 3 wt %.

the chemical shift data of characteristic 13C NMR signals as a function of pH. These signals are assigned as the carbons located at the α and γ positions of the C12Glu carboxylic acid headgroups.28 Clearly, a decreased pH results in a decreased chemical shift (i.e., upfield shift) for both of the signals. This indicates that the charge density around the C12Glu headgroups is decreased with decreasing pH. In other words, the neutralization by NaOH gradually proceeds from pH 5.5 to 6.0 for both of the α and γ carboxylic acid headgroups. This situation is similar to the results reported for the acid-type C12Glu aqueous solution system partially neutralized by TEA.28 It is reasonably understandable that greater positive curved aggregates such as spherical or rodlike micelles tend to be formed at high pH values, where an increased head-to-head repulsion (or an increased apparent cross-sectional area of headgroups) gives a decreased packing parameter of the C12Glu-C12DMA complex. We conclude, therefore, that the relatively large packing parameter of the C12Glu-C12DMA complex results in the wormlike micelle formation and the pHdependent rheological properties result from the change in the acidity around the C12Glu headgroups. D

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3.3. Effects of Cationic Species on Rheological Properties. In this section, we discuss effects of cationic species on rheological properties. The cationic species used here were C12TAB (quaternary ammonium type cationic surfactant) and C12MA (secondary alkylamine). As shown in Table S1 of the SI, 1:1 complex formation with C12Glu (acidtype) is suggested to occur for C12MA. In the case of C12TAB, aqueous mixtures of C12Glu (acid-type) and C12TAB were directly prepared at a mole ratio = 1:1 under various pH conditions. Figure 7 shows the resulting η0 values as a function

aqueous solutions of conventional surfactants in the presence of the third materials such as inorganic/organic salts or cosurfactants. Our current study offers a unique way to obtain viscoelastic wormlike micellar solutions by means of alkyl dicarboxylic acid-alkylamine complex as gemini-like amphiphiles.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1: Molecular characterization on the basis of 1H NMR (CDCl3) and mass spectroscopic data. Figure S1: 1H NMR (CDCl3) spectra measured for (a) C12Glu-C12DMA, (b) C12Glu-C12MA, and (c) C12Glu-C12A complex systems. Figure S2: ESI-mass spectra measured for (a) C12GluC12DMA, (b) C12Glu-C12MA, and (c) C12Glu-C12A complex systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.S.); [email protected]. tus.ac.jp (M.A.). Notes

Figure 7. Zero-shear viscosity (η0) data obtained for the C12GluC12DMA, C12Glu-C12MA, and C12Glu-C12TAB complex systems as a function of pH. The complex concentrations are fixed at 3 wt %. The C12Glu-C12MA complex precipitates below the solution pH of 6.1, so the rheological data obtained above the solution pH of 6.2 are only shown. Similarly, precipitation takes place for the C12GluC12TAB system below the solution pH of 5.4.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The amino acid-based surfactant was kindly supplied from Asahi Kasei Chemicals Co. This work was financially supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) 23550217.

of pH. The concentration of the complex is fixed at 3 wt %. In the case of C12Glu-C12MA complex system, we can see a similar shaped η0 curve with that for the C12Glu-C12DMA complex system, although the maximum η0 value is shifted from pH 5.7 (C12Glu-C12DMA) to pH 6.4 (C12Glu-C12MA). This pH shift is reflective of a change in basicity of the alkylamine compounds: i.e., the basicity is lower for C12MA than for C12DMA and hence a larger amount of NaOH to induce wormlike micelle formation is required for the C12GluC12MA complex system. In the case of C12Glu-C12TAB complex system, Newtonian (or quasi-Newtonian) fluids were obtained in a whole range of pH investigated (pH = 5.4 − 6.2) and the η0 values are much lower than those for the C12GluC12DMA complex system at a given pH value. This suggests that a micellar growth into wormlike micelles is not evidenced for the C12Glu-C12TAB complex system on the basis of the rheological data.

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REFERENCES

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4. CONCLUSIONS The amino acid-based surfactant (C12Glu) yields aqueous solutions of wormlike micelles as a result of complex formation with the tertiary and secondary alkylamine compounds (C12DMA and C12MA). This complex formation occurs at a mole ratio of 1:1. The alkylamine compounds play a role of an organic counterion to neutralize the carboxylic acid headgroups of C12Glu as well as an agent to control the packing parameter being suitable for wormlike micelles. The rheological behavior of the complex system is strongly dependent on the solution pH, being reflective of the pH-dependent structural transformation of the wormlike micelles. This is caused by the change in the acidity around the carboxylic acid headgroups as a function of pH. Wormlike micelles are generally formed in E

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