A Dually Effective Inorganic Salt at Inducing Obvious Viscoelastic

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A Dually Effective Inorganic Salt at Inducing Obvious Viscoelastic Behavior of both Cationic and Anionic Surfactant Solutions Ting Lu, Lian’gen Xia, Xiaodong Wang,* Aiqing Wang, and Tao Zhang* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People's Republic of China ABSTRACT: Hydrazine nitrate (HN), an inorganic salt, was first found to have dual effects on inducing obvious viscoelasticity of both cationic and anionic surfactant solutions. It was interesting that the surfactant solutions exhibited characteristic wormlike micelle features with strong viscoelastic properties upon the addition of this inorganic salt. The rheological properties of the surfactant solutions have been measured and discussed. The apparent viscosity of the solutions showed a volcano change with an increase of the HN concentration. Correspondingly, the microstructures of the micelles in the solutions changed with the apparent viscosity. First, wormlike micelles began to form and grew with an increase of the HN concentration. Subsequently, the systems exhibited linear viscoelasticity with characteristics of a Maxwell fluid in the intermediate mass fraction range, which originated from a 3D entangled network of wormlike micelles. Finally, a transition from linear micelles to branched ones probably took place at higher HN contents. In addition, the origin of the dual effects brought by HN addition on inducing viscoelasticity in both cationic and anionic surfactant solutions was investigated.

’ INTRODUCTION Viscoelasticity, induced by the entanglement of wormlike or threadlike micelles, has been observed in various surfactant systems and drawn considerable interest in both fundamental and applied science owing to the systems' special rheological properties.1
6 The rheology of these viscoelastic surfactant fluids is similar to that of the solutions of flexible polymers.7 However, unlike covalent chemical bonding polymers, self-assembled wormlike micelles are in dynamic equilibrium with their monomers, which are called “living polymers” owing to their ability to break and recombine rapidly.8,9 The viscoelastic wormlike micelles often show relaxation behavior that can be described by a Maxwell model with a single relaxation time, a living polymer model proposed by Cates et al.9
11 Viscoelastic wormlike micelles formed by ionic surfactants, especially cationic surfactants, upon addition of various additives have been studied extensively.12
18 The additives are either simple inorganic salts or structure-forming benzyl hydrotropes, with the molar ratio of salt to surfactant typically above 1 for the former while much lower for the latter. The most interesting features of wormlike micellar solutions are their fascinating microstructural evolution and their linear and nonlinear viscoelastic behavior, depending on the interaction between the surfactant and additive molecules. Inorganic counterions (e.g., F
, Cl
, Br
, NO3
) bind moderately to cationic micelles and thus lead to gradual micellar growth by screening the repulsion between the charged headgroups,19
22 but for the same charged surfactants, i.e., anionic surfactants, the inorganic salts only play a role in regulating the ionic strength of the solutions.23 Moreover, at high concentrations of surfactant and salt, phase separation r 2011 American Chemical Society

often occurs due to the formation of precipitate, which is generally called the salting-out effect.24
26 Compared with inorganic salts, organic counterions or hydrotropic salts that strongly bind to the micellar surface are highly efficient in promoting micellar growth16,22,27 and inducing wormlike micelles at a significantly low ratio of salt to surfactant. However, similar to inorganic salts, only the oppositely charged hydrotropic salts can induce such microstructure transition and viscoelastic behavior in surfactant systems. On the whole, these additives induce complicated viscoelastic responses which are strongly dependent on the electric properties of the additives, and only the oppositely charged additives can show strong interaction with the surfactants, regardless of whether they are inorganic salts or hydrotropic salts. Although simple additive effects for ionic surfactants have been studied extensively, there are few reports of the abovementioned effects induced only by one salt for both cationic and anionic surfactants. It is believed that the present study is the first example that inorganic salt induces micellar growth and viscoelastic behavior significantly in both cationic and anionic surfactant solutions. The inorganic salt discussed here is an ionic compound, hydrazine nitrate (HN), which is composed of positively charged [NH2NH3+] and negatively charged [NO3
]. We focus on the viscoelastic behavior of cationic surfactant cetyltrimethylammonium bromide (CTAB) and anionic surfactant Received: May 19, 2011 Revised: June 28, 2011 Published: July 01, 2011 9815

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sodium dodecyl sulfate (SDS) solutions as a function of added HN mass fraction.

’ EXPERIMENTAL SECTION Materials. Inorganic salt HN was synthesized and purified according to a literature method.28 Cationic surfactant CTAB was a product of A.R. grade of Tianjin Kermel Co. and was recrystallized five times from ethanol and acetone before use. Anionic surfactant SDS and nonionic surfactant Triton X-100 were purchased from Acros Organics Co. and used as received. The purity of the surfactants was examined, and no surface tension minimum was found in the surface tension curve. 1,6Diphenyl-1,3,5-hexatriene (DPH; 98%) was purchased from Sigma Co. and used as a fluorescence probe. Sodium bromide (NaBr) and urea were purchased from Tianjin Kermel Co., and sorbitol was from Alfa Organics Co. Both of them were of A.R. grade. The water used was MilliQ with 18.2 MΩ 3 cm electrical resistivity. NaBr was calcined at 500 °C for 6 h before use. Sample Preparation. The solutions of the mixed systems were prepared by simply dissolving HN with surfactant stock solutions. Samples were vortex mixed and equilibrated in a thermostatic bath before investigation. The surfactant concentrations [CTAB] and [SDS] were fixed at 200 mM. The concentration of HN (%) was varied with the mass fraction of the whole sample. Methods. Rheology. The rheological properties of the samples were measured with an Anton Paar MCR301 rheometer. A cone
plate sensor was used with a diameter of 25 mm and a cone angle of 1°. All the measurements related to CTAB and SDS were performed at 35.00 ( 0.05 and 25.00 ( 0.05 °C, respectively. A sample cover provided with the instrument was used to minimize water evaporation. Frequencysweep experiments were performed in the linear viscoelastic regime determined previously by dynamic strain sweep measurements. Fluorescence Anisotropy. Steady-state fluorescence anisotropy (r) was carried out on an FLS920 fluorescence spectrometer (Edinburgh Instruments, U.K.) equipped with a semiconductor thermostat. DPH was used as the fluorescence probe, and its stock solution (1.0  10
3 M) was prepared in tetrahydrofuran. A certain amount of stock solution was added to a tube and vacuumized to remove the solvent. Then the concentration of DPH was adjusted to 1.0 μM by adding an appropriate amount of the sample solution. The excitation wavelength was 350 nm, and the emission was monitored at 427 nm. The r value was calculated employing the equation r ¼ ðIVV
GIVH Þ=ðIVV þ 2GIVH Þ

ð1Þ

where IVV and IVH are the fluorescence intensities polarized parallel and perpendicular to the excitation light and G is the instrumental correction factor (G = IHV/IHH). Freeze-Fracture Transmission Electron Microscopy (FF-TEM). Fracturing and replication were carried out in a freeze-fracture apparatus (BAF060, BAL-TEC Co.) at
140 °C. Pt/C was deposited at an angle of 45° to shadow the replicas, and C was deposited at an angle of 90° to consolidate the replicas. The resulting replicas were examined in a JEM100CX II transmission electron microscope. Fourier Transform Infrared (FT-IR) Spectroscopy. The IR measurements were collected with an infrared spectrometer (Bruker EQUINOX 55) in the attenuation total reflection (ATR) mode. Theoretical Considerations. In the case of a Maxwell fluid, the storage modulus (elastic modulus) G0 and the lost modulus (viscous modulus) G00 are given by the following equations: G0 ðωÞ ¼

G0 ðωτR Þ2 1 þ ðωτR Þ2

ð2Þ

Figure 1. Variation of shear viscosity as a function of the CTAB concentration at a shear rate of 0.1 s
1. The mass fraction of HN was fixed at 5%. The inset is the variation of the relaxation time as a function of the CTAB concentration.

G00 ðωÞ ¼

G0 ωτR 1 þ ðωτR Þ2

ð3Þ

Here, τR is the relaxation time, ω the frequency, and G0 the plateau modulus. The value of τR is estimated as 1/ωc, where ωc is the crossover frequency at which G0 and G00 intersect. G0 is measured at high frequency where G0 reaches a plateau. Also of interest is a linear plot of G00 versus G0 which reveals the semicircle characteristic of a Maxwell fluid and is known as the “Cole
Cole” plot, which is expressed as  2 1 1 ¼ G0 2 ð4Þ G002 þ G0
G0 2 4

’ RESULTS AND DISCUSSION Viscoelastic Behavior in Cationic Surfactant Solutions. Steady-State Rheological Results. From appearance, all the sam-

ples of HN/CTAB mixtures were transparent and homogeneous over the studied concentration range. Figure 1 shows the trend of steady shear viscosity as a function of the surfactant concentration with the HN mass fraction fixed at 5%. Viscosity was measured at a shear rate of 0.1 s
1. In Figure 1, the shear viscosity increased with increasing CTAB concentration and also the relaxation time, which will be discussed later in the section “Dynamic Rheological Behaviors”. The viscosity increased abruptly at a certain CTAB concentration, i.e., near [CTAB] = 50 mM. This is the threshold concentration separating the dilute and semidilute regimes. Usually, the surfactant solutions in the semidilute regime show typical non-Newtonian responses such as shear thinning/thickening viscosity and viscoelasticity.17 To observe a more obvious rheological response, the effect of HN addition was studied at a fixed CTAB concentration of 200 mM, which is far above the threshold value. Before addition of HN, the CTAB solution was a transparent spherical micelle solution and its apparent viscosity was quite close to that of water. Upon the addition of HN, the viscosity increased gradually and gel-like solutions were obtained finally. Figure 2a shows the steady-shear viscosity of the sample with 9816

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Figure 2. Steady rheological results for the mixed system of HN/CTAB at 35 °C: (a) shear viscosity versus shear rate for various mass fractions of HN; (b) variation of zero-shear viscosity as a function of the HN mass fraction.

Figure 3. Dynamic rheological response as a function of the HN mass fraction at 35 °C: (a) variation of the elastic modulus (G0 ) and viscous modulus (G00 ) as a function of the frequency (ω) [the data are shifted along the vertical axis by 10a to avoid overlapping (a = 2, 1, 0,
1,
2,
3, and
4 from top to bottom)]; (b) variation of the relaxation time.

variation of the HN mass fraction in a 200 mM CTAB solution. All the samples displayed a Newtonian plateau at low shear rates and shear-thinning behavior at high shear rates. This phenomenon was observed previously in many surfactant systems composed of wormlike micelles.29,30 Figure 2a also shows that the greater the amount of HN added, the higher the shear rate required for the occurrence of the shear-shinning phenomenon. This implies that the chain recombination process of the wormlike micelles proceeds faster than the scission process at a higher mass fraction of HN. The variation of zero-shear viscosity (η0) as a function of the HN mass fraction is shown in Figure 2b. η0 here was determined by extrapolating the viscosity
shear rate curve to zero shear rate. As the mass fraction of HN was increased, there was a rapid increase in η0, followed by a maximum at around 8.7% (i.e., the molar ratio of HN to CTAB is 5). Afterward, η0 decreased with a further increase of HN. It is known that η0 is related to the type and morphology of various aggregates, even providing useful information on the molecular arrangement in the aggregates. Here, the initial rapid increase in η0 reflects the growth of the wormlike micelles at low HN mass fraction. The micelles eventually develop into long, flexible entities and form a transient entangled network, which results in a maximum of η0. On the other hand, the decrease of η0 is generally explained as a decrease of the micellar contour length31 or as the formation of branched micelles.32
34

Dynamic Rheological Behaviors. Dynamic frequency sweep measurements were applied to further characterize the viscoelastic behavior of the mixtures of HN/CTAB. Figure 3a shows all the samples possess elastic modulus G0 and viscous modulus G00 , and both G0 and G00 have one crossover at a specific oscillation frequency (ω) depending on the amount of HN. Therefore, it is considered that the addition of HN into CTAB solutions could induce strong viscoelasticity. As shown in Figure 3a, the systems showed a liquidlike behavior (G0 < G00 ) in the low frequency region, but both G0 and G00 increased with ω. However, in the high frequency region, the elastic modulus G0 approached the plateau modulus G0 and a solidlike behavior (G0 > G00 ) was observed. This is a typical viscoelastic behavior shown by wormlike micellar solutions.6 It was also found that G0 and G00 were proportional to ω2 and ω in the low frequency region, respectively. Thus, the data points of G0 and G00 could be well fitted to the Maxwell equations, indicating that the systems possess Maxwell behaviors and belong to a Maxwell liquid. Deviation from the Maxwell behavior occurred in the high frequency region, which was in agreement with the theory of Cates et al.9 It is shown in Figure 3b that the single relaxation time τR also exhibited a maximum against the HN mass fraction in the HN/ CTAB systems. The trend and position of the maximum point in Figure 3b agree well with those in steady rheological measurments (Figure 2b), indicating the consistency and correctness of our steady-state and dynamic rheological results. Meanwhile, the 9817

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Langmuir Cole
Cole plots of G00 versus G0 from the data presented in Figure 3a are shown in Figure 4. They reveal the semicircle characteristic of a Maxwell fluid. In summary, all these results suggest the evolution from a spherical micelle to a wormlike micelle upon the addition of HN in the cationic surfactant CTAB solution, which leads to a viscoelastic and gel-like behavior of the system. Viscoelastic Behavior in Anionic Surfactant Solutions. Interestingly, gel-like solutions were also formed in the anionic surfactant SDS solution by the addition of HN. To further identify this phenomenon, rheological measurements including steady-state and dynamic rheologies were carried out. Phase Behavior Studies. Compared with HN/CTAB systems, the phase behavior of HN/SDS aqueous solutions is slightly complex. In a 200 mM SDS solution, the addition of HN led to a homogeneous phase in the mass fraction region of 1.9
19%, and the solution was transparent with obvious viscosity. As the mass fraction of HN approached 22%, an aqueous surfactant twophase (ASTP) system was formed, with an opalescent upper phase and a transparent lower phase. This ASTP system remained unchanged until the HN content approached 40%. Since we are more interested in the viscoelastic behavior, the following investigation is focused on the homogeneous phase region, i.e., HN/SDS systems with HN contents below 19%.

Figure 4. Cole
Cole plots of the HN/CTAB mixtures at various mass fractions of HN.

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Rheological Studies. In Figure 5a, the steady-shear viscosity is plotted as a function of the shear rate for various mass fractions of HN at 25 °C with the SDS concentration fixed at 200 mM. With an increase of the HN content, the system changed from a Newtonian fluid to a non-Newtonian one and the shear-thinning phenomenon was observed in the flow curves when [HN] reached 5.4% (Figure 5a), indicating the growth of micelles from spherical to wormlike morphology. Similar to the HN/CTAB system, the plot of η0 against the HN mass fraction in HN/SDS systems also exhibited a maximum (Figure 5b), at which point the wormlike micelles reached the maximal contour length and entangled into a network. With further addition of HN to 7.1%, a dynamic rheological response appeared, and the Cole
Cole plots obtained from dynamic rheologicl measurement are shown in Figure 6a. As noted, G0 versus G00 deviated from the semicircle as the HN content increased, implying that the deviation from simple Maxwell fluid behavior became pronounced with the increase of HN. Moreover, the trend of relaxation time varying with the HN mass fraction (Figure 6b) was in accordance with that of steady zero-shear viscosity, which verified that both the apparent viscosity and the characteristic relaxation time could be used to describe wormlike micelles. A similar tendency of η0 versus [HN] (%) was also observed at 35 °C in the HN/SDS system (Figure 5b), although the η0 value was smaller than that at 25 °C. Thus, the steady shear and dynamic oscillation response strongly suggest that HN addition induces viscoelastic behavior in anionic surfactant SDS solutions. Fluorescence Anisotropy Study. DPH, one of the most widely used fluorescence anisotropy probes,35
38 was adopted to express the microviscosity in the interior of surfactant assemblies by fluorescence anisotropy r. Generally, the anisotropy rises with increasing viscosity of the microenvironment where DPH lies, which may reflect the molecular arrangement in the organized microstructures. Figure 7a shows the variation of the steady-state florescence anisotropy of DPH as a function of the HN concentration in the system of 200 mM SDS. It can be noted that the fluorescence anisotropy of DPH monotonously rose as the HN amount increased, consistent with the rheological results. Considering the fact that the SDS concentration here is far more than its critical micelle concentration (cmc) and DPH is solubilized in the hydrophobic core, it can be confirmed that the SDS molecules in the micelle tend to a more compact packing at high HN concentrations, which resists the rotation of DPH molecules in the micelles. Combined with the critical packing

Figure 5. Steady rheological results for the mixed system of HN/SDS at 25 °C: (a) shear viscosity versus shear rate for various mass fractions of HN; (b) variation of zero-shear viscosity as a function of the HN mass fraction. 9818

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Figure 6. Dynamic rheological results for the mixed system of HN/SDS at various mass fractions of HN (25 °C): (a) Cole
Cole plots; (b) variation of the relaxation time as a function of the HN mass fraction.

Figure 7. Fluorescence anisotropy studies in 200 mM SDS solutions with variation of the HN concentration at 25 °C: (a) stead-state anisotropy r of DPH corresponding to viscosity variation; (b) emission spectra of DPH at a 350 nm excitation wavelength.

parameter theory,39 the compact packing favors the formation of big aggregates, e.g., long cylindrical micelles or wormlike micelles. In other words, HN addition causes the SDS molecules in micelles to pack compactly, resulting in the growth of the micelles from spherical to wormlike, and consequently, viscoelastic behavior is observed. Moreover, it was interesting to find that the fluorescence intensity of DPH showed an obvious change with the variation of the HN concentration (Figure 7b) at the same probe concentration (1  10
6 M). With an increase of the HN content, the fluorescence intensity of the DPH probe in SDS solutions decreased dramatically, along with the disappearance of characteristic ternary peaks at 8.7 wt % HN. The remarkable decrease in the fluorescence intensity of DPH suggests that HN acts as a quencher. This quenching phenomenon, induced by HN addition, was also found in the mixed systems of CTAB and HN, and the quenching effect was more obvious in HN/CTAB systems than in HN/SDS systems. The addition of only a small amount of HN (1.9 wt %) caused the fluorescence intensity of DPH to decrease to the extent of the disappearance of characteristic peaks. The reason why the quenching effect of HN is more obvious in the HN/CTAB system than in the HN/SDS system may be attributed to the Br
counterions of CTAB, which also show a quenching effect to some extent. Microstructure Characterization. As mentioned above, the spherical micelles undergo a transition from small aggregates to

wormlike micelles in the cationic surfactant CTAB or anionic SDS solutions with the addition of HN, along with the variation of viscosity passing a maximum. FF-TEM was employed to determine the micellar structure in viscoelastic solutions. FFTEM can resolve length scales from nanometers to micrometers, so it provides visual imaging of microstructured systems, especially for aggregates in surfactant systems such as lamellar structures and wormlike micelles.18,40
43 Figure 8 shows that three-dimensional network micelles were formed in the systems of HN/CTAB (8.7% HN, 200 mM CTAB) and HN/SDS (10.2% HN, 200 mM SDS), which have maximum zero-shear viscosity (Figures 2b and 5b, respectively). These network micelles were composed of various wormlike micelles induced by the addition of HN into CTAB or SDS solutions. The microstructures were consistent with the rheological results; i.e., the formation of network micelles is responsible for the appearance of maximum zero-shear viscosity. Origin of the Viscoelastic Behavior. According to the above experiments, it can be concluded that inorganic salt HN is indeed a dual-effect additive, by which the viscoelastic behavior of both cationic and anionic surfactant solutions can be induced and regulated. From the viewpoint of microstructures, wormlike micellar formation, elongation, and entanglement could account for the viscoelastic behavior in surfactant solutions. So, what caused micellar growth? To solve this problem, we investigated the factors that may govern micellar growth and evolution. 9819

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Figure 8. FF-TEM micrographs: (a) aqueous solution of 200 mM CTAB with 8.7% HN addition at 35 °C; (b) 200 mM SDS with 10.2% HN addition at 25 °C.

First, crystal structure analysis shows that HN is an ionic compound in which cationic charge is concentrated in [NH2NH3+] and anionic charg in [NO3
].44,45 The inorganic counterions of NO3
and NH2NH3+ bind moderately to cationic CTAB and anionic SDS micelles by electrostatic attraction and screen the repulsions between the charged headgroups, respectively. Thus, the area per headgroup of the surfactant molecule is decreased, resulting in an increase of the critical packing parameter p,39 which is beneficial to the formation of larger assemblies such as wormlike micelles. It has been proved that the efficiency of nonpenetrating counterions to promote micellar elongation increases following the sequence F
< Cl
< Br
< NO3
< ClO3
.46 Obviously, NO3
derived from HN plays an important role in inducing micellar growth and viscoelasticity of CTAB solutions. However, differing from a simple inorganic salt (e.g., NaBr, NaCl), a greater amount of HN is required to promote efficient micellar growth, a condition at which the salting-out phenomenon often occurs for a general inorganic salt. For example, viscoelastic behavior can be detected only above 7.1 wt % HN (the molar ratio of HN to SDS was 4) addition into 200 mM SDS solution, whereas 5.8 wt % NaBr addition would induce the salting-out phenomenon for the same concentration of SDS solution. So much HN addition, to some extent, plays an important role in changing the solvent nature. Hence, it can be speculated that there are other effects, besides the electrostatic screening effect, involved in micellar growth and viscoelasticity. On the other hand, to distinguish from the electrostatic screening effect, a nonionic surfactant was employed. Triton X-100, a typical nonionic surfactant, is usually used to break the molecular ordered self-assemblies in the system due to the branched hydrocarbon in the molecule, which is disadvantageous to close arrangement of molecules in aggregates.47
49 Even so, the addition of HN can still increase the viscosity of the Triton X-100 system (Figure 9), verifying that there are indeed other effects of HN operating on the above surfactant systems besides the electrostatic shielding effect. Second, we found that HN showed better solubility in water and a high concentration of HN solution (above 10 mol/L, equal to 48.7 wt %) exhibited a remarkably positive partial molar volume, which indicates there is strong interaction between HN and water molecules. From the view of the HN molecular

Figure 9. Shear viscosity versus mass fraction of HN at a shear rate of 0.1 s
1 in the 200 mM Triton X-100 system at 25 °C.

structure, a large number of hydrogen bonds can form, including HN
HN molecules and HN
water molecules. FT-IR results confirmed the above viewpoint. From Figure 10, it can be found that, with an increase of the HN concentration, the corresponding characteristic peak near 1348 cm
1, which is induced by bending vibration, shifted to a short wavenumber, confirming the formation and increase of hydrogen bonds. This hydrogenbonding interaction between HN and water resulted in the enhancement of hydrophobic interaction, just as with sorbitol additive, which is normally considered as a “water-structure maker”.50,51 Consequently, micellar growth is realized owing to the enhanced hydrophobic interactions. According to the above-mentioned explanation, the viscosity should decrease upon the addition of a “water-structure breaker”, such as urea, which can weaken hydrophobic interactions.52,53 This is indeed the case. Taking the HN/CTAB (8.7% HN, 200 mM CTAB) system as an example which has the maximum zero-shear viscosity, the viscosity of the sample and relaxation time (τR) in dynamic rheological measurement decreased gradually with an increase of the urea concentration (Figure 11). It should be noted that the generation of viscoelasticity is the result 9820

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Figure 10. Infrared spectra for HN/SDS mixtures with different HN concentrations at room temperature: (a) full spectra from 1000 to 4000 cm
1; (b) partly enlarged spectrum. The inset in (b) represents the variation of the peak value wavenumber versus the HN concentration.

have great potential in the understanding of various interactions in surfactant
inorganic salt systems.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (X.W.); [email protected] (T.Z.). Phone: 86-411-84379015. Fax: 86-411-84685940.

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grants 21003124 and 21076211) and Funds of the Chinese Academy of Sciences for Key Topics in Innovation Engineering (Grant YYYJ0703). Figure 11. Variation of the zero-shear viscosity as a function of the urea mass fraction in the mixed system of HN/CTAB (8.7% HN, 200 mM CTAB) at 35 °C. The inset is the variation of the relaxation time versus the urea mass fraction.

of both the electrostatic screening effect and hydrophobic interaction enhancement, by which HN addition can show dual effects on inducing viscoelastic behavior of both cationic CTAB and anionic SDS systems.

’ CONCLUSIONS Viscoelastic solutions with the characteristic Maxwell response have been formed with cationic CTAB and anionic SDS systems, which were induced by the same inorganic salt addition. The viscoelastic behavior in surfactant solutions is the result of the transition from spherical to wormlike micelles upon the addition of HN. It is proposed that the inorganic salt HN plays the role of water-structure maker, which can promote the hydrophobic interaction. The intermolecular interactions between the surfactants and salt and between salt and water, including the electrostatic screening effect and hydrophobic effect enhancement, account for this dual regulating performance of HN. To the best of our knowledge, the work described here is the first example of a single inorganic salt inducing obvious viscoelasticity of a cationic surfactant as well as an anionic surfactant solution. This interesting discovery along with the fundamental investigation on the origin of the dual effects could

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dx.doi.org/10.1021/la2018709 |Langmuir 2011, 27, 9815–9822