24802
2006, 110, 24802-24805 Published on Web 11/17/2006
Anionic Hybrid Threadlike Micelle Formation in an Aqueous Solution Kenji Nakamura and Toshiyuki Shikata* Department of Macromolecular Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan ReceiVed: September 29, 2006; In Final Form: NoVember 2, 2006
Novel anionic hybrid threadlike micelle formation was found in an aqueous solution of an anionic surfactant, sodium tetradecylsulfate (NaC14S), and partially quarternized polyelectrolyte at ca. 0.55, poly(N,N-diallylN-methylamine-ran-N,N-diallyl-N-ethyl-N-methylammonium bromide) (P(DAM/DAEMBr)). The system precipitated insoluble complexes at a composition of iso-electric points, forming long and stable hybrid threadlike micelles at a composition close to an iso-molar point between NaC14S and P(DAM/DAEMBr) in monomer units. Then, the system turned into transparent liquids and showed remarkable viscoelasticity due to entanglements between the formed anionic hybrid threadlike micelles.
Threadlike micelles are an example of supramolecular materials formed by surfactants. They are long enough to entangle with each other in aqueous solutions to show remarkable viscoelasticity.1,2 Ordinary threadlike micelles are constructed between surfactants and hydrophobic low-molecular-weight additives such as cetyltrimethylammonium bromide (CTAB) and sodium salycilate.3 On the other hand, the hybrid threadlike micelles consist of surfactants and polyelectrolytes instead of low-molecular-weight additives and are classified as polymerized ordinary threadlike (or wormlike) micelles4 and surfactantpolyelectrolyte complexes. It is well known that most polyelectrolytes form insoluble solid-state complexes with oppositely charged surfactant ions in aqueous solutions because of strong electrostatic interactions between them.5 We have recently found the formation of water-soluble hybrid threadlike micelles between CTAB as a surfactant and sodium salts of partially sulfonated polystyrenes, poly(styrene-ran-sodium styrenesulfonate) (P(St/NaSS)), as a polyelectrolyte.6 In the system, the strong electrostatic interaction between CTA+ and the polyelectrolyte was effectively reduced by controlling the degree of sulfonation. The system showed profound viscoelastic behavior,7 and the structure8 of the formed hybrid threadlike micelles in aqueous solution was fully investigated using a small-angle neutron scattering technique. In this paper, we report the formation of hybrid threadlike micelles consisting of an anionic surfactant and relatively common, commercially available cationic polyelectrolytes in an aqueous solution. Since most of the known threadlike micelles are formed by cationic surfactants such as CTAB, and the formation of (even ordinary) anionic threadlike micelles9 long enough to entangle each other is not common, the formation of anionic hybrid threadlike micelles is significantly novel. For the anionic hybrid threadlike micellar formation in this study, we employed sodium tetradecylsulfate (NaC14S) as an anionic surfactant and poly(N,N-diallyl-N-methylamine-ranN,N-diallyl-N-ethyl-N-methylammonium bromide) (P(DAM/ DAEMBr))10 as a polyelectrolyte.11 The aqueous micellar system, NaC14S:P(DAM/DAEMBr)/W, turned transparent and * Corresponding author. E-mail:
[email protected] 10.1021/jp066424i CCC: $33.50
showed marked viscoelasticity only under the conditions that the concentration of the polyelectrolyte in monomer units (cP) was higher than 200 mM, a ratio, cDcP-1, of surfactant concentration (cD) to cP was in a narrow restricted range from 0.8 to 1.2 (close to the iso-molar point),12 and that the used P(DAM/ DAEMBr) had the degree of quarternization (xq) from 0.45 to 0.55. These strongly indicated that the effective reduction of strong electrostatic interaction between the surfactant and polyelectrolytes and careful control of the power of hydrophobic interaction13 between them are essential factors for the formation of anionic hybrid threadlike micelles consisting of NaC14S and P(DAM/DAEMBr). It should be noted that aqueous solutions of P(DAM/ DAEMBr) and sodium dodecylsulfate (NaC12S) or sodium hexadecylsulfate (NaC16S) as an anionic surfactant, instead of NaC14S, showed less viscosity and/or became turbid even at carefully tuned conditions.14 Furthermore, when we employed a methyl quarternized polymer, poly(N,N-diallyl-N-methylamine-ran-N,N-diallyl-N,N-dimethylammoniumbromide)(P(DAM/ DADMBr)), as a polyelectrolyte in an aqueous NaC14S solution, the system also became turbid. To investigate the formation of hybrid threadlike micelles in the NaC14S:P(DAM/DAEMBr)/W system, dynamic viscoelastic measurements were performed over an angular frequency (ω) that ranged from 1.0 × 10-1 to 1.58 × 102 rads-1 using a stresscontrolled rheometer15 at a temperature range from 25 to 40 °C. The dependencies of storage and loss moduli (G′ and G′′) on ω for NaC14S:P(DAM/DAEMBr)/W with varying cD ()cP) ) 200, 300, and 400 mM at 25 °C are shown in Figure 1.16 The viscoelastic spectra, G′ and G′′ vs ω, are slightly broader than those of ordinary threadlike micellar systems, which are well described with the behavior of a simple Maxwell element,2,3,17 for the system at cD ()cP) ) 200 and 300 mM. The spectra were much broader for the system at cD ()cP) ) 400 mM. The value of zero-shear viscosity
(η0 ) lim |η*| ) lim xG′2 + G′′2/ω) ωf0
ωf0
of the NaC14S:P(DAM/DAEMBr)/W system at cD ()cP) ) 200 © 2006 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 110, No. 49, 2006 24803
Figure 1. Dependencies of storage and loss moduli, G′ and G′′, on angular frequency, ω, for NaC14S:P(DAM/DAEMBr)/W with varying cD ()cP) ) 200, 300, and 400 mM at 25 °C. Solid line represents the dependence of η* on ω for the system with cD ()cP) ) 200 mM.
Figure 2. Dependence of reciprocal of the steady-state compliance, Je-1, average relaxation time, τA, and zero-shear viscosity, η0, on cD ()cP) for NaC14S:P(DAM/DAEMBr)/W at 25 °C.
mM shown in Figure 1 is more than 104 times as high as that of an aqueous solution of the used P(DAM/DAEMBr) at cP ) 200 mM. The value of relaxation strength or plateau modulus
(G∞ ) lim G′) ωf∞
looked sensitive to the cD ()cP) values. The relaxation times (τ) evaluated from ω at which G′′ curves showed the maximums lengthened with increasing cD. These strongly indicated that the NaC14S:P(DAM/DAEMBr)/W system involved hybrid threadlike micelles long enough to entangle each other resulting in 3-dimensional entanglement networks of them in aqueous solution. For a detailed discussion of the entanglement density of the formed hybrid threadlike micelles, the reciprocal of steady-state compliance
(Je-1 ) η02/lim G′ω-2) ωf0
and the average relaxation time (τA ) η0Je) have been used instead of the plateau modulus, G∞, and relaxation time, τ, for systems showing rather broad viscoelastic spectra such as the entangling polymer systems.18 Figure 2 shows the dependence of Je-1 on cD ()cP) for NaC14S:P(DAM/DAEMBr)/W at 25 °C. The value of Je-1 markedly increased with increasing cD, and the relationship Je-1 ∝ cD2∼2.3 was clearly recognized as that previously seen in entangling polymer systems19 and the ordinary threadlike micellar systems.3 This strongly suggests that the origin for the elasticity in this hybrid threadlike micellar system is entanglement between the micelles, as previously seen in the entangling polymer and the ordinary threadlike micellar systems. Figure 2 also contains the dependencies of τA and η0 on cD ()cP) with the relationship τA ∝ cD1.7 and η0 ∝ cD4.0, respectively. These relationships somewhat resemble those predicted by a bond-interchange model (in a reptative regime): τA ∝ cD1.7; η0 ∝ cD4.0, by an end-interchange model: τA ∝ cD1.2; η0 ∝ cD3.5, proposed by Turner et al. 20 as extensions of the Cates’ living polymer model,21 and by the original Cates’ living polymer model: τA ∝ cD1.4; η0 ∝ cD3.7. However, the fact that the average relaxation time, τA, (and also Je-1) slightly but definitely lengthened by the addition of sodium bromide (NaBr) (see Figure 3) possibly leads us to consider that the
Figure 3. Dependencies of G′ and G′′ on ω for NaC14S:P(DAM/ DAEMBr)/W with varying NaBr concentrations, cS, at cD ()cP) ) 200 mM and 25 °C.
reason for the increase in τA with increasing cD (and cP) is attributed to the increase in Na+ and Br- concentrations with increasing cD (and cP), owing to partial dissociation of the surfactant and polyelectrolyte. At present, we are not able to identify the essential mechanism for the entanglement release in the hybrid threadlike micellar system. As a typical example, the temperature dependence of viscoelastic spectra for the NaC14S:P(DAM/DAEMBr)/W system at cD ) 240 and cP ) 200 mM is shown in Figure 4. The value of the plateau modulus, G∞, looked insensitive to temperature, while the relaxation time, τ, remarkably decreased with increasing temperature. Similar temperature dependencies of viscoelastic spectra were observed for other NaC14S:P(DAM/ DAEMBr)/W systems at different cD ()cP). Activation energies, E*, for τ were evaluated to be ca. 90 kJ mol-1 from Arrhenius plots, τ-1 vs reciprocal absolute temperature, irrespective of the value of cP. A possible reason for the observed elasticity of the system is the rubber elasticity owing to the entropic origin related to the conformation change of long hybrid threadlike micelle as in concentrated polymer systems19 or long supramolecular polymeric system.22 This can explain how the relaxation strength, G∞, or the reciprocal of steady-state compliance, Je-1, should be proportional to the absolute temperature. The reason for the constant relaxation strength can be attributed to the narrow temperature range examined in this study and a change of solution density with temperature.
24804 J. Phys. Chem. B, Vol. 110, No. 49, 2006
Letters lecular materials bearing a radius of 2.0 nm.26 The chemical modification of P(SS) into P(St/NaSS) was able to convert the surfactant-polyelectrolyte complexes to cationic hybrid threadlike micelles stable enough in the CTAB:P(St/NaSS)/W system.6 Consequently, combinations of a surfactant and oppositely charged polyelectrolyte forming insoluble complexes in aqueous solution with microstructures bearing hexagonal arrays of cylindrical supramolecular materials possess the possibility to be converted into those showing hybrid threadlike micelle formation by a careful choice of surfactant species and chemical modification to increase solubility of the formed cylindrical supramolecular materials.
Figure 4. Temperature dependence of viscoelastic spectra, G′ and G′′ vs ω, for NaC14S:P(DAM/DAEMBr)/W at cD ) 240, cP ) 200 mM from 25 to 40 °C.
Acknowledgment. K.N. wishes to express special thanks for a Research Fellowship for Young Scientists from the Japan Society for the Promotion of Science. References and Notes
Remarkable viscoelastic behavior relatively similar to that of this NaC14S:P(DAM/DAEMBr)/W system was also found in aqueous systems of so-called associating polymers with or without surfactants.23,24 In the associating polymer systems, a small amount (
