Wormlike Micelles Formed by Sodium Erucate in the Presence of a

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Wormlike Micelles Formed by Sodium Erucate in the Presence of a Tetraalkylammonium Hydrotrope Yixiu Han,†,§ Yujun Feng,*,†,‡ Huanquan Sun,‡ Zhenquan Li,‡ Yugui Han,‡ and Hongyan Wang‡ †

Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, People's Republic of China Center for Enhanced Oil Recovery of SINOPEC, Shengli Oilfield Company, Dongying 257015, People's Republic of China § Graduate School of the Chinese Academy of Sciences, Beijing 100049, People's Republic of China ‡

bS Supporting Information ABSTRACT: Anionic wormlike micelles, particularly those formed by longchain carboxylate surfactants, are relatively less documented though their cationic or zwitterionic counterparts are frequently reported. In this study, the wormlike micelles of sodium erucate (NaOEr), a C22-tailed anionic surfactant with a monounsaturated tail, in the presence of a tetraalkylammonium hydrotrope were investigated for the first time. The different effects of two hydrotropes, benzyl trimethyl ammonium bromide (BTAB) and tetramethyl ammonium bromide (TMAB), on the phase behavior and rheological behaviors were compared, and the influences of surfactant concentration and temperature on the rheological properties of NaOEr solutions were also examined. Both organic salts can lower the Krafft temperature of NaOEr solutions and thus improve its water solubility, but BTAB can make TK drop more sharply. At a fixed NaOEr concentration, less BTAB is demanded to induce the formation of viscoelastic solution and to obtain the maximum viscosity of NaOEr solution; at a constant salt concentration, with increasing NaOEr content, the NaOErBTAB system shows a larger zero-shear viscosity (η0), relaxation time, and plateau modulus but lower overlapping concentration than those of the NaOErTMAB system. The occurrence of maximum η0 with increasing salt content for the NaOErBTAB system results from the formation of vesicles and L3 phases, which were verified by cryo-TEM observations. η0 shows an exponential decrease with increasing temperature; nevertheless it still remains above 103 mPa 3 s even at 90 °C.

1. INTRODUCTION Ionic surfactants in aqueous solution can self-assemble into long flexible wormlike micelles in the presence of specific salts,1 which promote the micelle growth by screening the electrostatic repulsions between their charged headgroups. Entanglement of these wormlike micelles into a transient network imparts viscoelasticity to the surfactant solutions, reminiscent of high-molecular-weight polymers. Unlike polymers, however, wormlike micelles are always in reversible and dynamic equilibrium with surfactant monomers, and thus they are also called “equilibrium polymers” or “living polymers”.2 Such unique rheological properties make wormlike micelles candidates for polymers in some special applications, such as oil well stimulation,3 drag reduction,4 and tertiary oil recovery.5 The most popular wormlike micelles reported so far are formed by cationic surfactants bearing a C16 tail.3,6,7 Recently much attention has been shifted to the worms formed by long-chain surfactants that normally bear an unsaturated C22 hydrophobic tail, such as cationic surfactants erucyl trimethylammonium chloride (ETAC),8 erucyl bis(hydroxyethyl)methylammonium chloride (EHAC),8,9 and protonated N-erucamidopropyl-N,N-dimethylamine (UC22AMPM)10 and zwitterionic surfactants dimethyl amidopropyl betaine (EDAB)11 and 3-(N-erucamidopropyl-N, N-dimethyl ammonium) propane sulfonate (EDAS).12,13 Compared r 2011 American Chemical Society

with their short-chain counterparts, these C22-tailed amphiphiles show stronger viscoelasticity,8,11 better thermostability,8,11 and lower overlapping concentrations.12,14 Nevertheless, the wormlike micelles formed by anionic surfactants are relatively less documented, though they are believed to be more environmentally benign for less toxicity and better biodegradability than cationic surfactants.3,1517 To date, two classes of anionic surfactants, i.e., alkyl sulfate and alkyl carboxylate, are employed to form wormlike micelles. Sodium dodecyl sulfate (SDS) is well-known to aggregate into spherical micelles in the presence of a monovalent inorganic salt,18 but excessive NaCl could induce the sphere-to-rod micelle transition in concentrated SDS solutions.19,20 Light scattering and cryogenic transmission electron microscopic (cryo-TEM) studies both verified the existence of entangled wormlike micelles, but the SDSNaCl system did not exhibit visible viscoelastic properties even at high surfactant and salt concentrations.21 Upon continuously adding Al(NO3)3, SDS solutions experienced coagulation, redispersion, and an isotropic micellar L1 Received: January 16, 2011 Revised: April 3, 2011 Published: May 05, 2011 6893

dx.doi.org/10.1021/jp2004634 | J. Phys. Chem. B 2011, 115, 6893–6902

The Journal of Physical Chemistry B phase where threadlike micelles were observed.22 However, the weak viscoelasticity of SDSAl(NO3)3 micellar solution is closely dependent on the high concentrations of SDS and Al(NO3)3, as well as temperature.23 Introducing oxyethylene groups onto the SDS chain can enhance the interaction between headgroups and counterions, and thereby sodium dodecyl trioxyethylene sulfate (SDES) shows a stronger ability to aggregate than SDS and no precipitation occurs even in the presence of Al3þ or Ca2þ.19,24 However, these SDES systems still present moderate viscosity; the maximum viscosity of 80 mM SDES solution containing 800 mM Al3þ or Ca2þ is only slightly higher than 1.0 103 mPa 3 s.19,24 Compared with inorganic salts, organic hydrotropes are more efficient in promoting micellar growth with a lower ratio of salt to surfactant (CS/CD) because they offer both electrostatic screening and hydrophobic interaction.6,7 The Kaler team25 found that a small amount of p-toluidine hydrochloride could induce SDS micelle growth, but the maximum viscosity of micellar solutions was less than 1.0 103 mPa 3 s whatever the SDS concentration was. In the presence of pentylammonium bromides or p-toluidine halides, sodium hexadecyl sulfate (NaC16S) also forms anionic threadlike micelles.26 Due to the longer tail of NaC16S than that of SDS, the maximum viscosity increases to 105 mPa 3 s at CS/CD ∼ 1. However, an undesirable restriction of these two anionic systems is that the ratio of CS/CD must be precisely controlled in order to avoid the occurrence of precipitation. Probably due to its good solubility27 and relatively long chain, oleate salts are most frequently used to form wormlike micelles among alkyl carboxylates. Lin and Eads28 observed the viscoelastic response of wormlike micelles constructed by 2.65 wt % potassium oleate (KOA) in 0.65 wt % NaHCO3 aqueous solution, and the Philippova team29 reported the wormlike micelles formed by the “KOAKCl” system, of which rheological behaviors were very relevant to the high KOA concentration and temperature. Recently Zheng and his co-workers30 found that both Na3PO4 and Na2CO3 could make sodium oleate (NaOA) solution viscoelastic, but the least amounts of these salts to induce NaOA micelle growth were 500 and 200 mM, respectively. However, examples of hydrotropic binding counterions used for oleate salt samples are quite scarce. Raghavan et al.17 found that the NaOA solutions became viscous upon adding triethylammonium chloride owing to its strong binding ability to NaOA, but this system exhibited a cloud point phenomenon with increasing temperature. The same team31 also observed that NaOA solution in the presence of a fraction of octyl trimethylammonium bromide presents a large viscosity at a relatively high total surfactant concentration (3 wt %) at ambient temperature. In short, current anionic systems still behave as slightly viscous liquids despite sufficient salts and highly concentrated surfactants present, or even if at room temperature their viscosities can reach high values, but at higher temperatures the high viscosities cannot be maintained. It is also worth noting that the reported anionic wormlike micelles to date are mainly formed by surfactants with a C18 tail and below. Referred from cationic systems that C22-tailed systems show predominant viscoelasticities and better stabilities at high temperatures than their C16-tailed counterparts,610 it is possible to achieve much higher viscosities and stabilities by using a C22-tailed anionic surfactant. Furthermore, the available studies on C22-chain anionic surfactants have been just focused on phase behavior32 and aggregating properties.33 To the best of our knowledge, there are still no reports concerning wormlike micelles and related rheological behaviors of this type of long-chain anionic

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Scheme 1. Chemical Structures of the Cationic Hydrotropes, TMAB and BTAB, and the Anionic Surfactant, NaOEr, Used in This Work

surfactants. As surveyed above, additives are usually indispensible for ionic surfactants to form long wormlike micelles due to repulsions between headgroups. Although organic counterions are more effective to promote the micelle growth than inorganic ones, the anionic systems containing hydrotropes usually present precipitation or cloud point behaviors.17,25,26 On the other hand, in tetraalkylammonium (TAAþ)alkyl carboxylate samples, no such clouding or precipitation occurs,17,33,34 but the steric hindrance of TAAþ restricts the NaOA micelle growth, so no viscosity increase is observed in NaOATAAþ solutions.17 In this work, the wormlike micelles formed by the long-chain anionic surfactant sodium erucate (NaOEr) (Scheme 1), which bears a C22 tail possessing a cis unsaturation bond at the 13carbon position, were investigated for the first time. The effects of two hydrotropic salts, benzyl trimethyl ammonium bromide (BTAB) and tetramethyl ammonium bromide (TMAB) (Scheme 1), on both solubility and rheological behaviors were compared. The stronger hydrophobic BTAþ was found to be more favorable to facilitate micelle growth than TMAþ. The transition of micellar morphology upon increasing salt concentration predicted from rheology was ascertained by cryo-TEM observations, and the influences of surfactant concentration and temperature on rheological behaviors were also examined.

2. EXPERIMENTAL SECTION 2.1. Materials. TMAB and BTAB (Sinopharm Chemical Reagent Co.), both of analytical grade, were used as received. NaOEr was obtained by neutralizing erucic acid (95%, Sipo Chemical Co. Ltd., China) using NaOH at stoichiometric ratio. In a mixture of water and ethanol (with a ratio of 1/4), NaOH and erucic acid reacted for 8 h at 80 °C continuously. The product was recrystallized twice using a mixture of cold acetone and alcohol (with a ratio of 2/1) to remove the unreacted fatty acid and the excessive salt. Then the filtered product was dried under vacuum to remove the residual solvents. Finally, white NaOEr powder was obtained. Homogeneous surfactant solutions were obtained by dissolving a designed amount of NaOEr and an appropriate hydrotropic salt in triply distilled water with gentle agitation at 50 °C, and then left at least 24 h to reach equilibrium prior to measurements. The pH of solution was set at ∼10 to prevent soap hydrolysis.35 2.2. Phase Behavior Observation. Phase behavior was recorded by visual observation following the previously reported procedure.36 The phase boundary as a function of temperature was determined by noting a transparent sample upon heating. The transition temperatures (clearing points) reported here were reproducible within (0.5 °C. 2.3. Rheology. Rheological experiments were performed on a Physica MCR301 (Anton Paar, Austria) rotational rheometer 6894

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Figure 1. Krafft point plotted as a function of molar ratio of hydrotropic salt to NaOEr, CS/CD, where CD is fixed at 1 wt %. Photographs A and B, corresponding to the same sample at CS/CD ∼ 3 of the NaOErBTAB system, represent the typical clear micellar solution above TK and crystals below TK, respectively.

equipped with concentric cylinder geometry CC27 (ISO3219) with a measuring bob radius of 13.33 mm and a measuring cup radius of 14.46 mm. All the dynamic rheological measurements were conducted in the linear viscoelastic regimes, and the stresscontrolled mode was applied to all the experiments. For the steady-shear experiments, sufficient time was allowed before data collection at each shear rate to ensure the viscosities reached their steady values. All the measurements were carried out at 50 °C unless the temperature effect was examined. 2.4. Cryo-TEM Observation. The cryo-TEM observation was conducted in a controlled-environment vitrification system, as previously described.12 The relative humidity was kept close to saturation to prevent drying of the sample during preparation, and the specimens were prepared by dissolving NaOEr and BTAB at 50 °C to ensure complete solubilization. The acceleration voltage was 200 kV, and the working temperature was kept below 170 °C. The images were recorded digitally with a charge-coupled-device camera (Gatan 832) under low-dose conditions with an underfocus of approximately 3 μm.

3. RESULTS AND DISCUSSION 3.1. Phase Behavior. It is easily understandable that water solubility of a surfactant will decrease when lengthening its hydrophobic tail. Possessing an ultralong hydrophobic tail, NaOEr is poorly soluble in pure water at room temperature, which impeded its practical use in some industrial processes. Therefore, it is necessary to decrease its Krafft point (TK), i.e., the temperature at which 1 wt % surfactant is solubilized,37 to satisfy the demands of research and potential applications. TAAþ ions are well-known to be capable of improving solubilities of anionic surfactants,33,34 and thus the influence of two TAAþ hydrotropic salts, TMAB and BTAB, on the TK of NaOEr solution (1 wt %, ca. 27.8 mM) was examined first. As shown in Figure 1, different TK values were obtained with varied molar ratios of CS/CD. These TK data were combined to form a clearing temperature line, which represents the solubility boundary between the opaque and clear regions. In the NaOErBTAB system, TK decreases smoothly with increasing BTAþ concentration, while in the NaOErTMAB system an abrupt decrease of TK occurs after an initial slight drop. Both systems show “salting-in” character, but BTAþ exhibits a

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stronger ability to enhance NaOEr solubility. The NaOEr BTAB sample at CS/CD ∼ 3 was used as an example to illustrate the phase behavior of the micellar solution with varied temperature. Above TK, a homogeneous and transparent micellar solution is observed (photograph A in Figure 1) and no clouding occurs, while below TK a white waxlike gel phase gradually appears (photograph B in Figure 1), and this hard gel was thought to be a hydrated surfactant phase formed as a result of a usual Krafft phenomenon.32,33 Such micellar phase behavior shows an obvious difference from that of tetrabutylammonium (TBAþ) alkyl sulfate,38,39 which exhibits clouding behavior, but is consistent with other alkyl carboxylate surfactant systems, such as TBAþdocosanoate32 and TBAþpalmitate36 systems, that show Krafft behaviors. Moreover, no precipitation came into existence in all these TBAþ systems, which is superior to the existing anionic worm systems.26,27 According to Figure 1, to make sure the samples within the studied CS/CD range are in a fully solubilized state, all the rheological measurements in the following work were performed at 50 °C unless stated otherwise. Obviously, both the nature and the concentration of the counterions play crucial roles in the phase behavior of NaOEr solutions. When adding TMAþ to NaOEr solutions, a competition of adsorption and desorption will occur between Naþ and TMAþ.36 As TMAþ is amphiphilic and less hydrated, it exhibits a more effective binding ability to the headgroup than Naþ. As a result, Naþ will be easily replaced by TMAþ from headgroups. The adsorption of TMAþ onto the micelle surface has an effect on the micellar microstructure. The inserted alkyl of the TMAþ can disrupt the regular packing of the NaOEr tails in the crystal state, and the headgroups will be separated due to the steric hindrance. The existing form of a surfactant in water is based on a competition between the crystal state and the solubilized state;36 therefore, the dissolved state of NaOEr is more favorable in the presence of TMAþ. Possessing a big benzene ring, BTAþ has stronger hydrophobicity and binding ability than TMAþ; thus it can displace Naþ more readily and tends to disassemble the packing surfactant tails more easily for extra volume than TMAþ. Consequently, it shows a stronger ability to enhance NaOEr solubility. At low concentrations, the sparse TMAþ has little effect on NaOEr aggregating behavior, and therefore TK drops slightly. Only at high hydrotrope concentration can TK of the TMAþ system reduce significantly as that of the BTAþ system does. In short, both hydrotropes can enhance the NaOEr solubility, but BTAþ shows a stronger ability than TMAþ due to an extra benzene group in its molecule. 3.2. Effect of Hydrotrope Concentration on Rheological Behaviors. The micellar morphology and size as well as the bulk nature are closely related to the solution composition; thus two aspects, i.e., the effects of both hydrophobicity and concentration, of the organic salts on the rheological behaviors were examined at 70 mM NaOEr in this subsection. Steady rheograms of the NaOErBTAB system with different salt concentrations are shown in Figure 2A, and the corresponding data of the NaOErTMAB system are shown in Figure S2 of the Supporting Information. All the samples present Newtonian behavior and shear-thinning response at low- and high-shear-rate regions, respectively. Zero-shear viscosity (η0) was obtained by extrapolating the viscosity curve along the plateau in the Newtonian region to zero shear rate. The high η0 manifests the existence of an entangled network. In this low-shear-rate region, the applied deformation is small and the entangled micelles may be almost unaffected. In the shear-thinning regime, the viscosity decreases monotonically above a critical shear rate (γ_ c). Shear thinning is a typical behavior of 6895



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Figure 2. (A) Steady rheology and (B) dynamic rheology for NaOErBTAB system at different BTAB concentrations. The lines in the dynamic spectrum are fitted with the Maxwell model. For clarity, only the data of two sample solutions containing 100 and 500 mM BTAB are given, and the data for the left samples are presented in Figure S1 of the Supporting Information. Inset: normalized ColeCole plots for these two samples. The solid line represents the osculating semicircle at the origin. The NaOEr concentration is held constant at 70 mM.

Figure 3. Effect of organic salt concentration on (A) η0 and (B) G0 and τR at 50 °C for NaOErBTAB and NaOErTMAB systems. The NaOEr concentration is fixed at 70 mM.

worms, and was generally explained by the deformation of the entangled networks and shear alignment of the micelles.40 When the BTAB concentration is increased from 15 to 70 mM, η0 increases accordingly and γ_ c shifts gradually to lower values. After the BTAB concentration is increased above 70 mM, an opposite tendency occurs, which may signify the variation of micelle morphology with increasing salt concentration. As exhibited in the dynamic rheology spectrum (Figure 2B), at high frequencies, the elastic modulus G0 shows a plateau and prevails over viscous modulus G00 , indicative of a typical elastic response, whereas at low frequencies G00 exceeds G0 , showing an obvious viscous behavior. The data at low frequencies collapse onto a single pair of G0 and G00 curves with slopes of 2 and 1, respectively, a typical behavior characterized by the Maxwell model:7 G0 ðωÞ ¼

G0 ω2 τ R 2 1 þ ω2 τR 2

G00 ðωÞ ¼

G0 ωτR 1 þ ω2 τ R 2

ð1Þ

where G0 is the plateau modulus, i.e., the value of G0 at high frequencies, and τR refers to relaxation time and is given by η0 ¼ τ R G 0

ð2Þ

Within low and intermediate frequency regions, the spectrum exhibits good fitting to the Maxwell model, confirming that a

single relaxation time dominates the response; at high frequencies, G00 displays an upturn, which was usually interpreted as a transition in the relaxation mode from reptation at longer time scales to “breathing” or Rouse modes at short time scales.41 The accordance of these data with Maxwellian behavior can also be better illustrated by a semicircular shape of the Cole Cole plot: 2 G0 2 G0 00 2 0 ¼ ð3Þ G þ G 2 2 in which the imaginary part G00 is plotted against the real part G0 . The ColeCole plots of the two NaOEr solutions containing 100 and 500 mM BTAB are shown in the inset of Figure 2B, where G0 and G00 have been normalized by the plateau modulus G0. At low frequencies, they fit the semicircle well over a majority of frequencies and deviation occurs at high frequencies for nonreptative effects.41,42 This suggests that they can be described by the single-mode Maxwell constitutive equation over a considerable frequency range.19,42 To gain insight into the wormlike micelles formed by NaOEr, the rheological parameters were further investigated as a function of salt concentration (Figure 3). As proposed by Cates et al.,7,41 the magnitudes of η0 and τR are related to the average length of 6896



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Figure 4. Cryo-TEM images of NaOErBTAB system at different BTAB contents. (A) CS/CD = 1, wormlike micelles; (B) CS/CD = 14.3, coexistence of vesicles and sponge phases.

the wormlike micelles, whereas G0 is related to the density number of entanglement or mesh size in the transient network. G0 shows independence of salt category and salt concentrations, because it is just related to surfactant concentration rather than to salt content.42 However, η0 and τR both show maxima with increasing salt concentration for the two systems, and the maximum values of these two parameters for the same system occur at a similar salt concentration. For the 70 mM NaOEr solution, 15 mM BTAB is sufficient to produce viscous micellar solution, which can be thought as the threshold salt concentration, CS*.43 Such a value is significantly lower than that of the SDESAl3þ system with CS* equal to 500 mM in 80 mM SDES solution.19 When increasing BTAB concentration from 15 to 70 mM, η0 increases by 34 orders of magnitude. The maximum viscosity (ηmax) and corresponding relaxation time (τmax) appear at 70 mM BTAB, where the molar ratio of salt to surfactant, CS/CDηmax, equals unity, indicating that the BTAþ binding to NaOEr forms 1:1 ion pairs under such a situation. With further increase of salt concentration to 1 M, η0 decreases by 45 orders of magnitude. Similar viscosity peaks are ubiquitous in the wormlike micelle systems and are usually believed to signify either micelle reduction31,44 or a transition from linear to branched wormlike micelles at high salt concentration.2,9 Unlike the cetylpyridinium chloridesodium salicylate system, which exhibits two viscosity maxima with increasing salt content,45 no second peak was found in the NaOErBTAB system with successive addition of BTAB. For the NaOErTMAB system, the remarkable viscous solution comes into being at 250 mM TMAB, and ηmax and τRmax turn up at 1500 mM TMAB, where CS/ CDηmax corresponds to 21/1. Obviously, the CS* and the salt concentrations to reach ηmax and τRmax in the BTAB system are both smaller than those in the TMAB counterpart. The analogous behavior that the hydrotrope with an extra benzene ring causes a smaller CS* was also found in a cationic surfactant system.43 However, the ηmax and τRmax values of the two systems show no obvious differences, suggesting that the micellar length mainly rests on surfactant concentration when sufficient salt is added and the net charges on the micellar surface are almost thoroughly screened. When the TMAB concentration is beyond 3 M, the sample separates into two phases, and the rheological performance was not further examined under such a situation.

To elucidate the mechanism of the occurrences of viscosity maximum and subsequent viscosity reduction for the NaOEr BTAB system, cryo-TEM was used to observe the micellar shape and size. As shown in Figure 4A, for the sample with CS/CD ∼ 1 located at the viscosity peak in Figure 3A, one can clearly find elongated and flexible micelles, some of which are more than several hundred nanometers in contour length, and it is difficult to identify where they begin and end. These micelles overlap and entangle themselves with one another, accounting for the strong viscoelasitcity of this sample. When the BTAB concentration is further increased up to CS/CD = 14.3 (Figure 3A), coexistent sponge phases (L3 phases) and scattered vesicles (white arrows in Figure 4B) are observed throughout the solution. Similar sponge phases were also found by cryo-TEM in metallosurfactant solutions,46 a highly concentrated SDSpentanolwater system,47 and poly(ethylene oxide)-co-poly(butylene oxide) copolymer solutions.48 The diameters of the vesicles in Figure 4B range from several tens of nanometers to more than 100 nm. However, no obvious branched micelles or shortened threadlike micelles were found in the micrographs. The presence of vesicles and sponge phases should be the cause of reduced viscosity at high salt concentration because samples consisting of vesicles and sponge phases usually present low viscosity.4952 Moreover, due to the formation of vesicles, the entangled micelles will diminish and the viscosity will decrease subsequently, which is in good agreement with the findings that the solutions containing vesicles often present less viscous.49 According to Figure 4B, the gradual decrease in viscosity of the sample (Figure 3A) denotes that the switch from long micelles to vesicles may be continuous with increasing salt concentration. The different influences of the two organic counterions on the characteristic rheological parameters (η0, G0, and τR in Figure 3) result from the different chemical structures of the two hydrotropes. The micellar self-assembly behavior conventionally resorts to a prediction of packing parameter, P,53 a geometric quantity defined as V/al, where V is the volume of the lipophilic chain possessing maximum effective length l and a is the effective headgroup area. When P < 1/3, spherical micelles are formed; when 1/3 < P < 1/2, wormlike micelles are formed; when 1/2 < P < 1, vesicles or bilayers are formed. P can be tuned by changing 6897



Figure 5. Schematic illustration of different contributions of BTAþ and TMAþ to the evolution of NaOEr micelle morphology.

the nature or content of additives in the micellar systems. The variation of micellar morphology in this work is schematically illustrated in Figure 5. The two organic counterions here can both screen the charges on the micellar surface and promote the growth of micelles. An increase in the lipophilicity of the counterion leads to an increase in the degree of counterion binding and hence decreases the electrical charges on the micelles.43 Moreover, the aromatic ring can be intercalated between the hydrocarbon chains in the interior of the micelle, which was previously verified by NMR elsewhere.25 Hydrophobic volume is increased accordingly, and thereby the packing parameter is increased resulting from two contributions, i.e., the decrease of a and the increase of V. As for TMAþ, with only one CH3 group on each side chain, its hydrophobicity is relatively weak and even negligible. Owing to its weak binding ability, TMAþ cannot approach the headgroup closely enough, and therefore its screening ability is restricted. Comparatively speaking, BTAþ is more effective for promoting micellar growth, and less BTAþ is required to achieve a similar viscosity than TMAþ. This result is consistent with reported cationic surfactant systems containing strong hydrophobic counterions.8,26,43 Prior to the presence of maximum viscosity, i.e., CS/CD < 1, adding a small amount of BTAþ could promote the transformation of spherical micelles into worms, but when CS/CD is beyond unity, the spontaneous curvature is further decreased and the aggregating shape will be transformed to another accordingly. The bulky hydrophobic volume of BTAþ also plays an important role in the evolution of aggregate morphology, i.e., the progressive transition from wormlike micelles to vesicles and L3 phases. Possessing both a hydrophilic headgroup and a hydrophobic moiety, BTAB approximately acts as a short-tail cationic surfactant. It can penetrate into the micellar interior and reduce the spontaneous curvature to induce micellar growth. When the concentration of BTAB is significantly higher than that of NaOEr, for example, with CS/CD equal to 14.3/1 (corresponding to the sample in Figure 4B), the main component in the solution is the counterion rather than the surfactant, and the surfactant molecules are surrounded by BTAþ ions and do not play crucial roles any more in the micellar shape. Under such a condition, the packing parameter will be close to unity tuned by the crowded BTAþ ions. Consequently, the vesicles and even L3 phases are formed in the solution. The sponge phases have an intriguing surfactant bilayer arrangement, forming surfaces of zero or low mean curvature, and correspond to a disordered version of a bicontinuous cubic

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phase.54 The presence of hydrophobic salt and its high concentration should be responsible for the morphology transition in the sample. With the variation of CS/CD, cationic surfactant systems also present similar phase behaviors, such as micelle phases (L1 phases), lamellar phases (LR phases) including vesicle phases and bilayer phases, and L3 phases.49,50 3.3. Effect of Surfactant Concentration on Rheological Behaviors. Besides the type and content of counterions, the surfactant concentration itself undoubtedly plays another important role in determining the rheological behaviors of NaOEr aqueous solutions. Exhibited in Figure 6 are the steady and dynamic rheological results of surfactant solutions with different NaOEr concentration but at fixed 50 mM BTAB. The counterpart data for the TMAB system are shown in the Supporting Information. With increasing NaOEr concentration, the η0 increases monotonically and the critical shear rate, γ_ c, shifts to lower values accordingly (Figure 6A). Within the investigated frequencies, G0 and G00 have no intersections (Figure 6B), implying that these NaOEr solutions possess very long relaxation times.8 In fact, the calculated maximum relaxation time here is 65 s, nearly 3 orders of magnitude times higher than that of C16 cationic surfactant systems,6 but comparable to that of other C22-tail gel-like micelles under similar conditions.8 The rheological parameters plotted as a function of NaOEr concentration at constant 50 mM hydrotrope are shown in Figure 7. In the dilute regime, the average micellar length usually increases with surfactant concentration following a simple power-law model with an exponent of ∼1/2, and η0 varies linearly with CD conforming to the Einstein equation η0 ¼ ηwater ð1 þ KCD Þ

ð4Þ

7

where K is on the order of unity. Above a critical CD, i.e., overlapping concentration, C*, the wormlike micelles begin to entangle themselves to form large aggregates, which imparts huge viscoelasticity to the solution,12 and η0 will increase exponentially by several orders of magnitude, ∼Cn.55 Clearly, C* values for BTAB and TMAB systems are only 5 and 50 mM, respectively (Figure 7A), lower than 55 mM of the hexadecylpyridinium chloride system and 70 mM of the SDS system despite more concentrated salts present.2,23 The power-law exponents of η0 against NaOEr concentration are 4.5 and 10.6, respectively, for BTAB and TMAB systems. These values are significantly higher than 3.5 predicted in the theoretical model.56 In the absence of salt or for incomplete screening, a power-law dependence of η0 steeper than theoretical value has also often been observed in other systems,23,25,57,58 where power-law indices lie between 8 and 16. Insufficient electrostatic screening should be responsible for vast deviation from the theoretical model.57 Shown in Figure 7B are the τR and G0 of both systems versus surfactant concentration. The decrease of the terminal relaxation time upon increasing surfactant concentration is attributed to the formation of intermicellar connections.23,59 The plateau modulus G0 rises steadily with surfactant concentration and exhibits power-law behavior; i.e., G0 ∼ CDR. The fitted results show that the power-law indices for NaOErBTAB and NaOErTMAB are 1.5 and 2.1, respectively, close to those found in the EDAS system12 and the EHAC system.8 Clausen et al.60 suggested that an increase in these rheological parameters is due to the growth in the lengths of wormlike micelles. When the surfactant concentration increases, the micelle length and flexibility rise concomitantly, and the elongated micelles promote the increase of 6898



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Figure 6. Effect of surfactant concentration on (A) steady rheology and (B) dynamic rheology of the BTAB/NaOEr system at 50 °C. For clarity, only 75 and 200 mM NaOEr samples are presented in the dynamic rheological spectrum. The data for the left samples of different NaOEr concentrations are exhibited in Figure S3 of the Supporting Information. The concentration of BTAB is fixed at 50 mM.

Figure 7. Effect of surfactant concentration on (A) η0 and (B) τR and G0 at 50 °C. The salt concentration is held constant at 50 mM.

η0 and τR. Furthermore, the growth of the micelles promotes the increase of mesh size, and thus the G0 increases upon increasing surfactant concentration. At the same surfactant concentration, the NaOErBTAB system shows bigger values of η0, τR, and G0 than those of the TMAB system, which signifies that the former samples possess stronger viscoelasticity. For the BTAB system, the sample viscosity at 75 mM NaOEr can achieve above 105 mPa 3 s, but in the TMAB system more than 100 mM NaOEr is needed to acquire such a high viscosity. However, for the shortchain surfactant systems, for example, more than 200 mM SDS in the presence of 1 M Al3þ is demanded to obtain a comparable viscosity at room temperature.23 Thus both the increase of additive hydrophobicity and lengthening of surfactant tails contribute to the enhancement in viscoelasticity. Moreover, comparing with the TMAB system, the BTAB system shows a smaller power-law-index value, indicating a more adequate screening. All the rheological parameters are listed in Table 1. Obviously, the parameters of the BTAB system predominate over those of the TMAB system. The different influences of BTAþ and TMAþ on rheological parameters can be ascribed to their geometry discrepancies again. It is reported that C* decreases with increasing inorganic salt concentration since the effective charge on the micelles is reduced consequently.2,59 In contrast to TMAþ, BTAþ shows stronger binding ability to surfactant and thus

Table 1. Rheological Parameters for NaOErBTAB and NaOErTMAB Systems NaOErBTAB

NaOErTMAB

Salt Concentration Effect (CD = 70 mM) CS* (mM)

15

250

CS/CDηmax ηmax (mPa 3 s)

1:1 2.0 106

21:1 2.03 106

120

113

14 058

15 147

τmax (s) _ G 0 (mPa)

Surfactant Concentration Effect (CS = 50 mM) C* (mM)

5

50

Rη0

4.5

10.6

RG0

1.5

2.1

τmax (s)

86

42

possesses more effective screening to negative charges at the same concentration. This is the first reason for the lower C* of the NaOErBTAB system. Second, the benzene ring in BTAþ acts as an effective bridge among surfactant tails and fills up the gap in the micelle interior caused by the big headgroups (Figure 5). Therefore, the NaOErBTAB system behaves as a catanionic pair and packs closely. In the presence of the same 6899



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Figure 8. (A) Steady and (B) dynamic rheological results for the “70 mM NaOEr þ 70 mM BTAB” system at different temperatures. The inset shows Arrhenius plots of η0 and τR vs 1/T. The slopes of the straight line yield the activation energy Ea.

counterions and surfactant concentrations, the effective tail amounts of the two systems are different; i.e., the NaOEr BTAB system carries more contributing surfactants to form micelles than the NaOErTMAB system. By comparison, the bulky TMAþ only offers interspaces among the surfactant tails, and the surfactant tails arrange not as compactly as those in the NaOErBTAB system. Therefore the hydrophobic interaction between surfactants will be impaired. In addition, the ultralong C22 hydrophobic chain in NaOEr also contributes to the low C*. Stronger hydrophobicity of NaOEr facilitates the surfactant molecules to aggregate at low concentration. We recently found that the critical micellar concentration (cmc) of EDAS could be proportionally correlated with its C*.12 The long hydrophobic tail results in a low cmc, and then the low cmc is responsible for the low C*. G0 and τR are both concerned with surfactant concentration, as there is a bigger effective surfactant concentration in NaOErBTAB samples, and thus bigger values of these two parameters are yielded in this system. 3.4. Effect of Temperature on Rheological Behaviors. In order to examine the micelle stability at high temperatures, the effect of temperature ranging from 50 to 90 °C on the rheological response of the sample “70 mM NaOEr þ 70 mM BTAB”, namely, the solution with maximum viscosity in Figure 3, was studied. As shown in Figure 8A, within the studied temperature scope, η0 decreases monotonically upon increasing temperature. It is worth noting that the viscosity still remains more than 103 mPa 3 s at 90 °C, which shows the good stability of the entangled micelles at high temperature. On the contrary, those short-tail systems are very sensitive to high temperature and even become negligibly viscous with slightly increasing temperature.17,23 The tolerance to high temperature for the NaOErBTAB system can be attributed to the long breaking time of the micelles because the surfactant possesses a long hydrophobic tail.11 Another reason for the good thermostability is that the BTAþ becomes less polar with temperature increase and adopts a more compact binding to the micellar surface.33 Such an effect will stand out within an ultralong-chain surfactant system, because counterions will be more crowded by those long tails. The frequency spectra (Figure 8B) show crossovers of G0 and G00 within the range of accessible frequencies, fitting to the Maxwell model well within the measured temperature range.

The key rheological parameters such as η0 and τR can be empirically described by Arrhenius relations:7 Ea τR ¼ A exp ð5Þ RT

Ea η0 ¼ G0 A exp RT

ð6Þ

where Ea is the activation energy, R is the gas constant, T is the absolute temperature, and A is the preexponential factor. According to eq 6, G0 is independent of temperature, which is also confirmed by experimental results (Figure 8B); however, the plots of τR and η0 against 1000/T both fall on straight lines with almost the same slope (the inset of Figure 8A). In other words, Ea could be obtained from the temperature dependence of either η0 or τR. For the present system, the Ea is ca. 164 kJ/mol, which is in the range found for other wormlike micelles.8,23 Typically, when a wormlike micellar solution is heated, the micellar contour length decays exponentially with increasing temperature.8,61 The end cap is energetically more unfavorable to form than the cylindrical body by a factor of the end-cap energy.8 However, at high temperatures, surfactant molecules can move more rapidly between the body and the hemispherical end cap of the worms, and hence the end-cap limitation is less severe. The end caps will be readily formed and the micelle growth will be restricted. The diminishment in micellar length, in turn, leads to an exponential decrease in rheological properties such as τR and η0. 3.5. Comparison with Other Wormlike Micelles. Compared to other wormlike systems, the NaOEr systems show some superiority. In contrast with those worms formed by other longchain surfactants, NaOEr could be simply prepared by neutralizing erucic acid, which can be extracted from rapeseed plants.62 It possesses stronger biodegradability and less toxicity than cationic systems.3,1517 From the points of view of simplicity and environmental concerns, it is more desirable than other long-tail surfactants. Compared with short-tail anionic worm systems, lesser amounts of hydrotropes and NaOEr are needed to form comparably viscoelastic solutions. In another words, like other C22-tail cationic and zwitterionic systems, the NaOErTAAþ systems show stronger viscoelasticity than those shorter-tail anionic systems at high temperatures (i.e., higher than 50 °C). Moreover, the NaOErTAAþ systems also avoid the occurrence of precipitation 6900


The Journal of Physical Chemistry B or cloud point behaviors which are common obstacles in the existing anionic worms induced by hydrotropes. In the presence of bulky TAAþ, NaOA is inclined to form globular micelles for steric hindrance;17 while NaOEr compensates this undesirable effect from TAAþ by its strong hydrophobicity and takes advantage of the big size of TAAþ to enhance its solubility instead.

4. CONCLUSIONS We investigate the wormlike micelles formed by the C22-tailed anionic surfactant NaOEr for the first time. The contributions of two hydrotropes BTAB and TMAB are 2-fold, i.e., enhancing NaOEr solubility and inducing micelle growth. The bulky volume of TMAþ occupied in headgroups impedes the tight packing of surfactants to form crystals and therefore promotes NaOEr to solubilize. Possessing an extra benzene ring than TMAB, BTAB shows a stronger ability to enhance NaOEr solubility. In the presence of BTAB or TMAB, the NaOEr micellar solutions show remarkable viscoelasticity like other C22-tailed systems. Besides the electrostatic screening, the counterion hydrophobicity also favors micelle growth. The decrease of a and increase of V doubly increase P, which promotes the shift of micellar morphology. Therefore BTAB presents a stronger inducement to micelle growth than TMAB. With increasing BTAB concentration, a successive micelle morphology transformation from wormlike micelles to vesicles and L3 phases occurs, which is verified by a combination of rheology and cryo-TEM observation. The excessive BTAþ is responsible for the formation of vesicles. Containing a strong hydrophobic counterion and a long-tail surfactant, the NaOErBTAB system possesses an extremely low overlapping concentration and exhibits good thermostability at high temperatures. Such a system may be an excellent candidate for use in rheology-control applications, such as displacing fluid to enhance oil recovery from high-temperature oil reservoirs, and a promising ingredient in cosmetic and personal care formulations. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional figures for effects of hydrotrope and surfactants concentrations on rheological behaviors, as well as additional cryo-TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: þ 86 (28) 8523 6874.

’ ACKNOWLEDGMENT Financial support from the Shandong Provincial government through the “Taishan Scholar Foundation”, the Sichuan Provincial Bureau of Science and Technology through its “Distinuished Youth Foundation” (2010JQ0029), and the Key Laboratory for Colloid and Interface Chemistry of the State Education Ministry at Shandong University through its open research fund program (200601) is greatly acknowledged. ’ REFERENCES (1) Cates, M. E. J. Phys. Chem. 1990, 94, 371. (2) Khatory, A.; Kern, F.; Lequeux, F.; Appell, J.; Porte, G.; Morie, N.; Ott, A.; Urbach, W. Langmuir 1993, 9, 933. (3) Maitland, G. C. Curr. Opin. Colloid Interface Sci. 2000, 5, 301.

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(4) Zhang, H.; Wang, D.; Chen, H. Arch. Appl. Mech. 2009, 79, 773. (5) Li, X. P.; Yu, L.; Ji, Y. Q.; Wu, B.; Li, G. Z.; Zheng., L. Q. Chin. Chem. Lett. 2009, 20, 1251. (6) Hassan, P. A.; Candau, S. J.; Kern, F.; Manohar, C. Langmuir 1998, 14, 6025. (7) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869. (8) Raghavan, S. R.; Kaler, E. W. Langmuir 2001, 17, 300. (9) Croce, V.; Cosgrove, T.; Maitland, G.; Hughes, T.; Karlsson, G. Langmuir 2003, 19, 8536. (10) Chu, Z.; Feng, Y. Chem. Commun. 2010, 46, 9028. (11) Kumar, R.; Kalur, G. C.; Ziserman, L.; Danino, D.; Raghavan, S. R. Langmuir 2007, 23, 12849. (12) Chu, Z.; Feng, Y.; Su, X.; Han, Y. Langmuir 2010, 26, 7783. (13) Chu, Z.; Feng, Y. Soft Matter 2010, 6, 6065. (14) Couillet, I.; Hughes, T.; Maitland, G.; Candau, F.; Candau, S. J. Langmuir 2004, 20, 9541. (15) Yang, J. Curr. Opin. Colloid Interface Sci. 2002, 7, 276. (16) Vlachy, N.; Merle, C.; Touraud, D.; Schmidt, J.; Talmon, Y.; Heilmann, J.; Kunz, W. Langmuir 2008, 24, 9983. (17) Kalur, G. C.; Raghavan, S. R. J. Phys. Chem. B 2005, 109, 8599. (18) Kumar, S.; David, S. L.; Aswal, V. K.; Goyal, P. S.; Kabir-ud-Din Langmuir 1997, 13, 6461. (19) Mu, J. H.; Li, G. Z.; Jia, X. L.; Wang, H. X.; Zhang, G. Y. J. Phys. Chem. B 2002, 106, 11685. (20) Magid, L. J.; Li, Z.; Butler, P. D. Langmuir 2000, 16, 10028. (21) Magid, L. G. J. Phys. Chem. B 1998, 102, 4064. (22) Angelescu, D.; Caldararu, H.; Khan, A. Colloids Surf., A: Physicochem. Eng. Aspects 2004, 245, 49. (23) Angelescu, D.; Khan, A.; Caldararu, H. Langmuir 2003, 19, 9155. (24) Mu, J. H.; Li, G. Z. Chem. Phys. Lett. 2001, 345, 100. (25) Hassan, P. A.; Raghavan, S. R.; Kaler, E. W. Langmuir 2002, 18, 2543. (26) Nakamura, K.; Shikata, T. Langmuir 2006, 22, 9853. (27) Mcbain, J. W.; Sierichs, W. C. J. Am. Oil Chem. Soc. 1948, 6, 221. (28) Lin, Z.; Eads, C. D. Langmuir 1997, 13, 2647. (29) Molchanov, V. S.; Shashkina, Y. A.; Philippova, O. E.; Khokhlov, A. R. Colloid J. 2005, 67, 606. (30) Cao, Q.; Yu, L.; Zheng, L. Q.; Li, G. Z.; Ding, Y. H.; Xiao, J. H. Colloids Surf., A: Physicochem. Eng. Aspects 2008, 312, 32. (31) Ziserman, L.; Abezgauz, L.; Ramon, O.; Raghavan, S. R.; Danino, D. Langmuir 2009, 25, 10483. (32) Zana, R. Langmuir 2004, 20, 5666. (33) Zana, R.; Schmidt, J.; Talmon, Y. Langmuir 2005, 21, 11628. (34) Koshy, P.; Verma, G.; Aswal, V. K.; Venkatesh, M.; Hassan, P. A. J. Phys. Chem. B 2010, 114, 10462. (35) de Mul, M. N. G.; Davis, H. T.; Evans, D. F.; Bhave, A. V.; Wagner, J. R. Langmuir 2000, 16, 8276. (36) Lin, B.; McCormick, A. V.; Davis, H. T.; Strey, R. J. Colloid Interface Sci. 2005, 291, 543. (37) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: San Diego, CA, USA, 1994. (38) Benrraou, M.; Bales, B.; Zana, R. J. Phys. Chem. B 2003, 107, 13432. (39) Bales, B.; Tiguida, K.; Zana, R. J. Phys. Chem. B 2004, 108, 14948. (40) Bautista, F.; Soltero, J. F. A. J. Phys. Chem. B 2002, 106, 13018. (41) Granek, R.; Cates, M. E. J. Chem. Phys. 1992, 96, 4758. (42) Raghavan, S. R.; Fritz, G.; Kaler, E. W. Langmuir 2002, 18, 3797. (43) Hassan, P. A.; Yakhmi, J. V. Langmuir 2000, 16, 7187. (44) Cappelaere, E; Cressely, R. Rheol. Acta 2000, 39, 346. (45) Oelschlaeger, C.; Schopferer, M.; Scheffold, F.; Willenbacher, N. Langmuir 2009, 25, 716. (46) Koutsantonis, G. A.; Nealon, G. L.; Buckley, C. E.; Paskevicius, M.; Douce, L.; Harrowfield, J. M.; McDowall, A. W. Langmuir 2007, 23, 11986. (47) Ponsinet, V.; Talmon, Y. Langmuir 1997, 13, 7287. (48) Battaglia, G.; Ryan, A. J. Macromolecules 2006, 39, 798. 6901



ARTICLE

(49) Davies, T. S.; Ketner, A. M.; Raghavan, S. R. J. Am. Chem. Soc. 2006, 128, 6669. (50) Sun, W.; Shen, Y.; Hao, J. Langmuir 2011, 27, 1675. (51) Miller, C. A.; Gradzielski, M.; Hoffmann, H.; Kraemer, U.; Thunig, C. Prog. Colloid Polym. Sci. 1991, 84, 243. (52) Hoffmann, H. Adv. Mater. 1994, 6, 116. (53) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (54) Leal, C.; Bilalov, A.; Lindman, B. J. Phys. Chem. B 2006, 110, 17221. (55) Berret, J. F.; Appell, J.; Porte, G. Langmuir 1993, 9, 2851. (56) Turner, M. S.; Cates, M. E. J. Phys. II 1992, 2, 503. (57) Kern, F.; Lequeux, F.; Zana, R.; Candau, S. J. Langmuir 1994, 10, 1714. (58) Koehler, R. D.; Raghavan, S. R.; Kaler, E. W. J. Phys. Chem. B 2000, 104, 11035. (59) Narayanan, J.; Manohar, C.; Kern, F.; Lequeux, F.; Candau, S. J. Langmuir 1997, 13, 5235. (60) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474. (61) Fischer, P.; Rehage, H. Langmuir 1997, 13, 7012. (62) Dierker, M.; Sch€afer, H. J. Eur. J. Lipid Sci. Technol. 2010, 112, 122.

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Wormlike Micelles Formed by Sodium Erucate in the Presence of a

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