Rheological Properties of Viscoelastic Solutions in ... - ACS Publications

Apr 25, 2016 - Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, College of Chemistry and Chemical. Engineering...
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Rheological Properties of Viscoelastic Solutions in a Cationic Surfactant−Organic Salts−Water System Xilian Wei,* Peipei Geng, Chuanhong Han, Yan Guo, Xiaoxiao Chen, Junhong Zhang,* Yingtian Zhang, Dezhi Sun, and Shiyan Zhou Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, Shandong 252059, People’s Republic of China S Supporting Information *

ABSTRACT: Viscoelastic solutions formed in the mixed aqueous solutions of 3tetradecyloxy-2-hydroxypropyltrimethylammonium bromide (R14HTAB) and aromatic salts such as sodium salicylate (NaSal), sodium 1-hydroxynaphthalene-2-carboxylate (1SHNC), and sodium 2-hydroxynaphthalene-3-carboxylate (2SHNC) were systematically studied by steady and dynamic shear rheology in terms of concentration and temperature. In the absence of a salt, R14HTAB only produced spherical or short cylindrical micelles within a range of concentrations of 100−400 mmol kg−1. The addition of aromatic salts induced one-dimensional growth of micelles generating wormlike micelles. Zero-shear viscosity of the solutions shows viscosity maxima behavior in the examined range of salt concentration, where the strongest and the most stable network structures were formed. The changes in the viscoelastic behavior are a result of variation of the structural relaxation time, indicating that the flow behavior is primarily controlled by micellar kinetics. The microstructure of wormlike micelles and the reason for the variation in the microstructure with an increase in the additive concentration has been analyzed by infrared and 1H NMR spectra measurements. The zero-shear viscosity reduces exponentially with increasing temperature.

1. INTRODUCTION Surfactants and their mixtures in aqueous solution self-assemble to form diverse microstructures such as micelles (spherical, cylindrical, and wormlike), vesicles, planar lamellae, tubes, fibers, etc.1−3Among these self-assembly structures, viscoelastic wormlike micelles have been one of the hot topics for a long time because of their various potential applications,4 such as fracturing fluids used in oil fields,5,6 heat-transfer fluids, drag reduction agents,7−9 personal and home care products,10 as a novel sieving matrix for DNA separation,11 as a soft template for synthesis of nanomaterials, etc.12 Among the numerous wormlike micelle systems, solutions of cationic alkyltrimethylammonium surfactants combined with aromatic counterions as model wormlike micelle systems have been widely studied.13−25 The 2-hydroxypropoxy group was inserted between the hydrophobic chain and the polar headgroup of RnTAB (n = 12, 14, 16, and 18) to obtain a new surfactant, 3-alkoxy-2-hydroxypropyltrimethylammonium bromide (simplified as RnHTAB, n = 12, 14, 16, and 18). This modified in molecular structure endows special properties to the system. For example, the sterilization abilities of 0.005 mol/ L aqueous solutions of RnHTAB on Escherichia, Staphylococcus aureus, and Pseudomonas aeruginosa are lower than those of analogous surfactants with no 2-hydroxypropoxy group in exactly the same situation; these features greatly extend its applicability. In addition, these cationic surfactants show not only various assembling properties26−28 in aqueous solution, © 2016 American Chemical Society

but also show the characteristic of thermotropic liquid crystals in the solid state.29 In the present paper, we investigate the rheological properties of mixed aqueous solutions of 3-tetradecyloxy-2hydroxypropyltrimethylammonium bromide (R14HTAB) and sodium salicylate (NaSal), sodium 1-hydroxynaphthalene-2carboxylate (1SHNC), and sodium 2-hydroxynaphthalene-3carboxylate (2SHNC) (Figure 1) using viscosity measurements.

Figure 1. Chemical structures of R14HTAB, NaSal, 1SHNC, and 2SHNC. Received: Revised: Accepted: Published: 5556

January 21, 2016 April 18, 2016 April 25, 2016 April 25, 2016 DOI: 10.1021/acs.iecr.6b00238 Ind. Eng. Chem. Res. 2016, 55, 5556−5564

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Industrial & Engineering Chemistry Research

mmol kg−1 at 25 °C; the concentration of NaSal was varied from 0 to 200 mmol kg−1. In the presence of NaSal, the viscosities of all solutions exhibit a Newtonian plateau and a non-Newtonian shearthinning behavior at low shear rates and high shear rates, respectively. By extrapolating the plateau values, the zero-shear viscosity (η0) data of four R14HTAB concentrations are obtained as a function of additive contents and are shown in Figure 2. For solutions with R14HTAB concentrations of 100

The aim of the study was to understand the influence of mixture ratio, temperature, and the type and molecular structures of the added salts on the viscoelastic behaviors of the mixed solution. This investigation should provide useful reference data for promoting the use of these mixed systems of cationic surfactants and organic salts in the fields of household and oil-field chemicals for the future.

2. MATERIALS AND METHODS 2.1. Materials. The surfactant 3-tetradecyloxy-2-hydroxypropyltrimethylammonium bromide (R14HTAB) was synthesized according to our work reported in the literature.29 The product was purified by recrystallization three times using ethyl acetate. The product was characterized by 1H nuclear magnetic resonance (NMR) spectroscopy and elemental analysis (shown in the Supporting Information). NaSal and SHNC were purchased from Aldrich Chemical Reagent Co. (purity >99%). The deionized water was redistilled from alkaline potassium permanganate with a surface tension of 72.7 mN m−1 at 20.0 °C. 2.2. Methods. Samples tested by weighing the required amount of the surfactant and additives were homogenized using a stirrer at a desired temperature. Then they were stored in a thermostat for at least 160 h to reach equilibrium. Rheological measurements were carried out by a stress controlled rheometer (AR2000ex, TA Instruments, USA). A sample cover was used to minimize water evaporation during the experiment. Dynamic frequency spectra were performed on a fixed stress σ (selected at the linear range). The frequency varied from 0.03 to 600 rad·s−1. All measurements were repeated two times to ensure reproducibility. The zero-shear viscosity (η0) of the system was obtained by extrapolating the shear rate to zero using the Carreau or Cross model. 2.3. Fourier Transform Infrared (FTIR) Spectroscopic Measurements. The FTIR spectra of sample were measured using a Nicolet-6700 infrared spectrometer (Thermo Co., USA) at room temperature. The solid samples of R14HTAB and NaSal dried by a vacuum desiccator under P2O5 were recorded by using KBr pellets. R14HTAB or R14HTAB/NaSal mixed samples were dissolved in CDCl3 to prepare the liquid sample. The infrared spectrum was recorded in the 4000−500 scanning range. 2.4. NMR Measurements. 1H NMR spectra on the R14HTAB/NaSal/D2O (containing an internal reference of TMSP) solution were carried out with a 400-MHz NMR spectrometer (Varian Co., USA).

Figure 2. Variation of η0 with CNaSal in R14HTAB solutions at 25 °C. The R14HTAB concentrations (CR14HTAB) are expressed in the figure.

and 200 mmol kg−1, the η0 increased continuously with increasing NaSal concentration (CNaSal) until a maximum value was reached, indicating the generating of entangled rodlike micelles. Beyond this, η0 reduce to a plateau region. This behavior is analogy to the conditions of many conventional aqueous ionic surfactants/additive systems.31−35 There is no doubt that the strong interaction between NaSal and R14HTAB is the reason for the augmentation in η0 with increasing Csalts. The aromatic anions neutralize the charges of the micelle headgroups, which weakens the electrostatic repulsions between them, while the insertion of the aromatic rings into the palisade layer increases the critical packing parameter and consequently reduces micellar curvature, favoring growth from spherical to cylindrical micelles. The decrease of η0 at high salt concentration may be owing to the micellar branching.4,36−38 The intermicellar junctions formed because of micelle branching can glide along the worm bodies and act as the stress-release points, which reduced the entanglement points between micelles and decreased the elasticity of the solutions. However, for systems with CR14HTAB = 300 and 400 mmol kg−1, the viscosity curves show a double-peak behavior with increasing CNaSal, similar to other cationic surfactant/additive mixed aqueous solutions.39−42 The physical appearance of the 300 mmol kg−1 NaSal/R14HTAB solution at the first maximum is shown in Figure 3. The solution was almost solid, showing a drawing behavior. It can be inferred that the complicated viscosity curves may have been a result of a combination of different existing mechanisms for stress relaxation, which will be discussed along with relaxation time. The viscosity curves are divided into four regions for each value of CR14HTAB. In region I, η0 increases gradually with increasing NaSal concentration and reaches the first maximum, where the solutions show strongly viscoelastic properties, indicating that the micelles have achieved their full length at this maximum. In region II, η0 decreases to a minimum as CNaSal

3. RESULTS AND DISCUSSION 3.1. Formation of Salt Induced Viscoelastic Wormlike Micelles. In the salt-free system, R14HTAB forms spherical micelles with very low viscosity at surfactant concentration (CR14HTAB) < 300 mmol kg−1, and shows the characteristics of a Newtonian fluid, meaning the presence of spherical micelles in the system. At CR14HTAB > 300 mmol kg−1 the shear thinning phenomenon has been observed above the critical shear rate (γ̇c), implying micellar growth above this concentration. The overlap concentration of R14HTAB is 500 mmol kg−1 at 25.0 °C.30 When NaSal is added to the R14HTAB aqueous solution, the salt induced R14HTAB yields viscoelastic wormlike micelles at low concentration, for instance 100 mmol kg−1. Here, the R14HTAB concentrations used were 100, 200, 300, and 400 5557

DOI: 10.1021/acs.iecr.6b00238 Ind. Eng. Chem. Res. 2016, 55, 5556−5564

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Industrial & Engineering Chemistry Research log η0 = − 2.92 + 2.59 log(C R14HTAB)

Thus, these relationships can be used to design viscoelastic micellar surfactant solutions with desired rheological properties. Equations 1 and 2 indicate that the values of CNaSal and η0 increase with increasing R14HTAB concentration to the first viscosity maximum. However, the values of CNaSal/CR14HTAB decrease after the first viscosity maximum as R14HTAB concentration is increased, which is analogous to results previously reported in the literature.40 This is perhaps related to the aggregation morphology of pure R14HTAB solutions at different concentrations, in which spherical micelles were observed at CR14HTAB = 100 or 200 mmol kg−1 and cylindrical micelles were seen at 300 or 400 mmol kg−1.30 It is possible that the concentration of NaSal required to form an entangled network at the viscosity maximum was higher for the spherical micelles than for rodlike ones. Significantly, each CNaSal/CR14HTAB value is less than 1 irrespective of the surfactant concentration or maximums; i.e. the micellar surface does not reflect an optimal charge neutralization and possesses excess positive charge, indicating that electrostatic interactions do not primarily occur between the R14HTAB and NaSal molecules. These results are very different from the ratios reported in the literature,39−42 where Csalt/Csurfactants ≤ 1 at the first maximum, and the second maximum occurs when the Csalt/Csurfactants > 1. Such results are uncommon in the literature, and may be associated with the hydroxyl groups on the R14HTAB molecule or the surface charge of the micelles. In our previous work,29 a hydrogen bond has been confirmed to form between the oxygen and hydrogen and the counterion, bromine, and thus it can be deduced that the hydroxyl group of R14HTAB can form intermolecular hydrogen bonds in aqueous solutions, resulting in hydrogen-bonded dimers. This hydrogen bonding increases aquation of the micellar surface and stimulates the dissolution of counterions. Thus, the surface charge of these micelles should be higher than those without analogous hydrogen bonding. For the charged micelles, the rate of increase in micellar length with concentration is greater than that of neutral micelles when the oppositely charged Sal− ions are added to R14HTAB solutions.40 Perhaps a measurement of the zeta potential with increasing NaSal concentration will be able to explain this; however, because of limitations in the experimental setup, such a measurement cannot be performed in our laboratory. We conducted infrared (IR) spectroscopy to confirm this hypothesis. Figure 5 shows the IR spectra of R14HTAB and NaSal/R14HTAB mixed solution. In Figure 5a, only a weak broad peak was observed at about 3417 cm−1 for the dry solid form of NaSal, which is lower than the general values 3650− 3580 cm−1 of free hydroxyl group.45 This implies intramolecular hydrogen bonding in NaSal because the IR peak corresponding to the associated state of hydroxyl groups is in the 3400−3200 cm−1 range. R14HTAB in the dry solid form or 300 mmol kg−1 solutions in CDCl3 show similar IR spectra, with two weak characteristic peaks at 3417 and 3263 cm−1 belonging to the hydroxyl group in the connection state, which is induced by intermolecular hydrogen bonding (Figure 5b,c). When 80 and 120 mmol·kg−1 NaSal were added to 300 mmol·kg−1 R14HTAB in CDCl3 (Figure 5d,e), the two weak absorption peaks merge into a broad peak at 3417−3263 cm−1. This result implies that, as well as intramolecular and

Figure 3. Appearance of NaSal/R14HTAB (300 mmol·kg−1) mixed solution at first maximum viscosity.

is increased. Thereafter, the curve enters region III, where η0 again increases and achieves a second maximum at a CNaSal/ CR14HTAB of ∼0.45 for two of the curves. Finally, when CNaSal is further increased, η0 drops again in area IV and reaches low values at higher salt contents. The zero-shear viscosity and NaSal concentration give power law relationships, i.e., η0 ∝ C3.88 and η0 ∝ C2.28 for the first maxima, and η0 ∝ C1.77and η0 ∝ C1.67 for the second maxima, respectively. All these results fall into the wide range of exponents (1.5−8.5) reported for viscoelastic micellar surfactant systems.43 The dependence of CNaSal as a function of R14HTAB concentration at the first maximum is shown in a bilogarithmic graph (Figure 4), where a linear relationship is observed

Figure 4. Log−log plot of NaSal concentrations at viscosity maxima as a function of surfactant concentration.

between both the CNaSal and CR14HTAB. Similar phenomena have also been found for cetylpyridinium chloride (CPyCl)/NaSal, cetyltrimethylammonium bromide (CTAB)/NaSal, and 1tetradecyl-3-methylimidazolium bromide (C14mimBr)/NaSal mixed solutions.39,40,44 The relationship obtained is log(C NaSal) = 0.51 + 0.59 log(C R14HTAB)

(2)

(1)

Moreover, there is a similar linear relationship between η0 and CR14HTAB: 5558

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the proton is larger, such as 4H, 3H, 5H, 1′H, 5′H, and 7′H. The chemical shift change (Δδ) of each group against CNaSal is shown in Figure 6. All curves show an obvious turning point

Figure 6. Variance of chemical shift for different groups as a function of NaSal concentration in 300 mmol·kg−1 R14HTAB solution. Different protons are marked in the figure.

Figure 5. FTIR spectra of different samples: (a) NaSal and (b) R14HTAB in solid form. Then from the bottom, (c) 0, (d) 80, and (e) 120 mmol·kg−1 NaSal added into 300 mmol·kg−1 R14HTAB in CDCl3.

corresponding to the first maximum viscosity as shown in Figure 2, implying that changes in interaction between R14HTAB and NaSal molecules are most prominent at this point. Δδ of 4H is the maximum among these groups. The range ability of 7′H is obvious before and after the maximum viscosity; second is that of 5′H, which shifts upfield and is different from other protons in the hydrophobic R14HTAB chain. It is known that 1H NMR signals for protons located on the benzene ring should be shifted upfield via the ring current, and the signal for protons on the edge of the ring is moved downfield. If intermolecular hydrogen bonds are destroyed, the 1 H NMR signals will shift upfield.47,48 Thus, by combining these results with the IR spectra, it is concluded when the NaSal is added to the R14HTAB solution before the first viscosity maximum, the negatively charged Sal− ions can strongly bind to the R14HTAB micellar surface, bringing about effective screening of charges between the headgroups. The salicylate counterion even can be embedded into the micellar palisade layer near the polar heads of the R14HTAB molecules. This solubilization partly disrupts the intermolecular hydrogen bond between R14HTAB molecules, leading to a dramatic increase of Δδ for the 7′H signal and the

intermolecular hydrogen bonding (for NaSal and R14HTAB, respectively), there may also be weak intermolecular hydrogen bonding between NaSal and R14HTAB. Nuclear magnetic resonance (NMR) spectroscopy is a very practical tool for studying the association between amphiphile and hydrotrope.46 To elucidate the interaction between R14HTAB and NaSal, 1H NMR spectroscopy is used to explore the location of salicylate anions in the micelle. The Supporting Information shows the 1H NMR signals for R14HTAB/NaSal mixed solutions at CR14HTAB = 300 mmol·kg−1 in D2O. The observed chemical shifts are shown in Table 1. We can see from Table 1 that the 3H, 5H, and 4H signals shift upfield relative to the location of the free Sal − with increasing C NaSal concentration, while the 6H proton signal hardly changed. However, the signals for −CH3 (1′), −(CH2)13 (2′), CH2−N (6′), and N−CH3 (7′) in the R14HTAB molecule shift downfield, but the protons in the CH− (5′, lively hydrogen) group shift upfield. The signals for 3′H and 4′H in the molecule are almost absent in the 1H NMR spectra. To facilitate the discussion, we chose several groups where the chemical shift of

Table 1. 1H NMR Chemical Shifts (δ, ppm) of R14HTAB (300 mmol·kg−1) in the Existence of Different Concentrations of NaSal at 25 °C

chemical shift (δ, ppm) NaSal 1 2 3 4 5 6 7

R14HTAB

NaSal (mmol·kg−1)

4H

3,5H

6H

1′H

2′H

5′H

6′H

7′H

0 60 80 100 120 140 150

7.456 7.154 7.160 7.199 7.181 7.158 7.167

6.947 6.730 6.750 6.785 6.769 6.745 6.753

7.827 7.828 7.825 7.823 7.835 7.828 7.826

0.827 0.880 0.905 0.965 0.939 0.879 0.911

1.245 1.284 1.308 1.371 1.346 1.270 1.336

4.391 4.341 4.305 4.205 4.269 4.321 4.299

2.835 2.879 2.895 2.922 2.924 2.876 2.907

3.225 3.266 3.267 3.541 3.300 3.508 3.516

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Figure 7. Variations of G′ (filled symbols) and G″ (open symbols) with the shear frequency ω for aqueous solutions of R14HTAB (300 mmol kg−1) (a) and the normalized Cole−Cole plots (b). The solid line represents the best fit to the Maxwell model at 25 °C. The NaSal concentrations are expressed in the figures.

Figure 8. Relaxation time τR (a) and plateau modulus G′∞ (b) for each of these systems are plotted with corresponding CNaSal at 25 °C.

upfield shift of 5′H in R14HTAB. Subsequently, with increasing concentrations of Sal− arranged from the palisade layer up to the micellar interface, the micelles grow rapidly until the viscosity reaches the first maximum. This evolution was accompanied by an obvious increase in viscosity and elasticity, in which the strongest and the most stable network structure is formed, i.e. with maximum viscosity and longest micelles. Therefore, the comprehensive forces of the system, such as electrostatic interactions, hydrophobic effect, and hydrogen bonding, were maximized at this point. Because of this synergy, the NaSal concentrations needed to reach the maximum viscosity should be less than in mixed systems of cationic surfactants without hydroxyl groups, such as CTAB/NaSal. This is also a main reason for the small molar ratio of NaSal to R14HTAB at the second viscosity maximum. After the maximum, the 7′H NMR signal is slightly lower than at the maximum, meaning that Sal− is bound to the micelle surface and not into the micellar palisade layer. The reduction in viscosity is probably due to damage of the network structure or micellar branching with further NaSal addition. The formation of the second maximum in viscosity is a remaining problem, although a possible reason for the formation of the second viscosity maximum value is that the new interlinkage between micelles is enhanced by increasing CNaSal, which is a relevant micellar kinetics process. 3.2. Dynamic Rheology. For a Maxwell fluid, the storage modulus (G′), the lost modulus (G″), and the complex viscosity (η*(ω)) can be described as33,34,49 G′ =

(ωτR )2 1 + (ωτR )2

G0

G″ =

ωτR 1 + (ωτR )2

|η*(ω)| =

G0

(G′2 + G″2 )1/2 ω

(4)

(5)

where G0 is the plateau modulus and ω is the frequency. The relaxation time, τR = η0/G0, is estimated from ωc−1, which is the crossover frequency of two modules. Accordingly, a Cole−Cole plot (G″ vs G′) for a Maxwell fluid describes a semicircle. ⎛ G ⎞ ⎛ G ⎞2 G″2 + ⎜G′2 − 0 ⎟ = ⎜ 0 ⎟ ⎝ 2 ⎠ ⎝ 2 ⎠

(6)

When the solution cannot be described using a simple Maxwell fluid, the deviation behaviors from the semicircular shape are observed at a critical frequency ω which is the inverse of the rupturing time of micelles.50 Eventually, an upturn in G″ at the high-frequency regime can be observed, which is associated with the Rouse modes. This leads to the minima at the G″ vs G′ plot and G′∞ is obtained by extrapolating G′ to the x-axis. Thus, to obtain comparable consequences, the data of G″ and G′ should be normalized by G0, which is the maximum value of G′. The result was shown in the form of G″(ω)/G0 versus G′(ω)/G0.51 Figure 7a shows the dynamic scanning curves of several samples for the solution at CR14HTAB = 300 mmol kg−1. The normalized Cole−Cole plots are shown in Figure 7b. The solid lines represent Maxwellian evolution. One sees that all dynamic curves show the standard characteristic of a Maxwell model, and the shapes of the curves follow perfect semicircles in the low and intermediate regimes, signifying that these systems obey Maxwell fluid behavior with a single stress relaxation time

(3) 5560

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Industrial & Engineering Chemistry Research (tR). However, an upturn can be clearly seen at high frequencies, which has been attributed to a conversion of the relaxation from reptation−scission to Rouse modes. This behavior has been found in many viscoelastic micellar solutions of surfactant.15,18,22,24,31,32,35,42 The deviation of G″ from the model in the high-ω range is also another feature of wormlike micelles. The variation of tR and G′∞ values are plotted against CNaSal (Figure 8); the exact values of the G′∞ can be got from the Cole−Cole (G″ vs G′) plot (not shown). The trend of tR vs CNaSal curves (Figure 8a) is very similar to the variation of η0 in Figure 2. The locations of the extreme values are in good consistency with the corresponding maxima and minima of the viscous resistance. It is related to the length of wormlike micelles experiencing stress relaxation, as said above for the Maxwellian model. A gradual increase in G′∞ took place with increasing CNaSal until the first viscosity maximum (Figure 8b), beyond which the change is very small regardless of increases or decreases in the zero shear viscosity of the systems, meaning that the micellar structure does not change after the first viscosity maximum. The relationships between G′∞ and R14HTAB concentration comply with the scaling law G′∞ ∝ Cx, where x = 4.5, 3.1, 0.72, and 0.68 for 100, 200, 300, and 400 mmol kg−1 R14HTAB solutions, respectively, which is inconsistent with the theoretical prediction of x = 2.3 for different viscoelastic systems.32 Such deviation is due to the effects of synergy in solution. These variations in both G′∞ and τR with increasing additives have been reported in other mixed systems of surfactant/salts.35,42,52 The plateau modulus G′∞ is relevant to the entangled density ρe by the relation G′∞ = ρekBT39 for entangled polymers, and hence shows the size of the mesh in the solution. The increase in the plateau modulus G′∞ indicates that the degree of entanglements increases with the CNaSal until the first viscosity maximum where ρe reaches a maximum, suggesting another microstructure transition with further increase of CNaSal. Therefore, the changes in the viscosity beyond the first viscosity maximum are a consequence of a variation in the relaxation times of the sytems rather than the modulus. These results are very similar to the NaSal/CTAB micellar system reported by Hoffmann et al.42 We may conclude, therefore, that the viscoelastic behavior of the solution is controlled by micellar kinetics processes. That is to say, the drastic variation in the shape of the curve is controlled by the braking/reforming mechanism. At the maxima, the relaxation time is controlled by the reptation mechanism, and it is regulated by the kinetic mechanism at the minima.42 The decrease or increase in the viscosity of the system is primarily a conclusion of the scission and recombination of the entangled network. We also examined the impact of temperature on the viscoelastic behaviors of mixed solutions, and the flow activation energy was estimated according to Arrhenius law.22,50 η0 = Ae−Ea / RT

Figure 9. η0 as a function of temperature at the first viscosity maximum for both NaSal/R14HTAB mixed systems.

300 mmol kg−1, respectively, fall into the broad region of Ea values (70−300 kJ mol−1) for surfactant solutions.22,50,51,53,54 3.3. Evaluation of the End-Cap Energy. In order to confirm that the viscosity of systems is derived mainly from the entanglement of wormlike micelles or bridging between rodlike micelles, we calculated the end-cap energy (needed to form two end caps by destroying a micelle), Ec, for micellar systems. The mean-field theory describes the relation between the contour length of wormlike micelles, L̅, and Ec:55 L̅ ∼ ϕ1/2 exp[Ec /2kT ]

(8)

where k is the Boltzmann constant. Ec is also twice the scission energy. The contour length of micelle, L̅, can be estimated with the description of Granek and Cates49 at the minimum of G″:

I G″min ≈ e G∞ L̅

(9)

Here Ie is the mean contour length of between two entangled points and G″min is the minimum of G″ at the high-frequency range. Thus, Ec can be calculated from a semilogarithmic plot of G∞ L ≡ I ̅ versus 1/T (accurate value of L̅ is not required). G″ min

e

Figure 10 shows this change obtained from the frequency spectra at different temperatures for NaSal/R14HTAB = 70/ 200. The Arrhenius relationship for L̅ was verified, and from the slope we obtained Ec = 23.3 kJ mol−1 (23.8 kJ mol−1 for NaSal/ R14HTAB = 100/300). Compared with those reported for C16TAB surfactant micelles (∼49 kJ mol−1) and for micelles of a dimeric surfactant (98−172 kJ mol−1) and an unsaturated

(7)

where Ea is the flow activation energy and A is a constant. Both maximum values for CNaSal/CR14HTAB = 70/200 and 100/300 (mmol kg−1) in the 20−40 °C temperature range were selected. Figure 9 presents the plot of log η0 vs the reciprocal of the thermodynamic temperature for both systems. The plot is linear and indicates that η0 follows an Arrhenius characteristic. The Ea values of 136.3 and 155.1 kJ mol−1, obtained from the slope and corresponding to CNaSal/CR14HTAB = 70/200 and 100/

Figure 10. Ratio of overall contour length L̅ to entanglement length Ie for micelles in the 70/200 mmol kg−1 R14HTAB sample plotted vs 1/T in an Arrhenius plot. 5561

DOI: 10.1021/acs.iecr.6b00238 Ind. Eng. Chem. Res. 2016, 55, 5556−5564

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Industrial & Engineering Chemistry Research surfactant (160 kJ mol−1),55 this is rather low, and is a conclusion of the high surface charge of micelles. Since the endcap energy is a thermodynamic driving force of micellar growth, such a small Ec value is unfavorable for micellar growth. In other words, the wormlike micelles are not too long and are not easily broken. This result can account for the formation of network structure mainly by bridging between rodlike micelles rather than by entanglement of long wormlike micelles in the systems. 3.4. Effect of Counterion. Finally, as a comparison, we examine the impact of different counterions on the rheological property of the R14HTAB solution for further applications. Additive NaSal was replaced by sodium 1-hydroxynaphthalene2-carboxylate (1SHNC) and sodium 2-hydroxynaphthalene-3carboxylate (2SHNC). The R14HTAB concentration was fixed at 100 mmol kg−1. Figure 11 shows the η0 values of R14HTAB/

observed around equimolar ratios of SHNCs to R14HTAB. When the temperature increased, these pale-blue samples changed from cloudy to transparent viscoelastic solutions, and the aggregate state of the solution simultaneously varied from vesicles to entangled rodlike micelles. These results are somewhat similar to those reported by Hoffmann,56 and will be studied in a separate paper.

4. CONCLUSION In the R14HTAB solutions, adding NaSal and SHNCs can promote formation of rodlike micelles. The η0 of solutions shows through a maximum for systems with R14HTAB concentrations of 100 and 200 mmol kg−1, and two maxima for 300 and 400 mmol kg−1 ones. The NaSal concentrations and zero-shear viscosities at first maxima change linearly as a function of CR14HTAB on a logarithmic scale. All mixed systems showed a single Maxwell type of relaxation, and the rheological behavior was explained by the formation of three-dimensional networks built up from intermicellar junctions. The variations in τR with NaSal concentration also show a trend similar to that of η0, indicating that the viscosities are driven by a reptation mechanism at the maxima and a kinetic mechanism at the minima. The change of plateau modulus G′∞ is very small beyond the first viscosity maximum. The high viscosity of the solutions is mainly due to the linkages between the rodlike micelles rather than from entangled long micelles. Low flow activation energy also confirmed this hypothesis. 1H NMR and IR spectral analyses indicate that the location of solubilizates for salicylate in the micelles is from the micellar barrier layer to the micellar surface. A comparison experiment showed that the interaction between SHNCs and R14HTAB is stronger than that between NaSal and the same surfactant. We hope the present study may provide a possible route to adjust the selfassembly structure of mixed systems of cationic surfactant with additives, and to promote related practical applications.

Figure 11. Zero-shear viscosity of 100 mmol kg−1 R14HTAB solution as a function of additive concentration at 25 °C.

■

1SHNC and R14HTAB/2SHNC solutions as the function of SHNC concentration at 25 °C. Compared with the R14HTAB/ NaSal systems under the same conditions, although the trend of η0 with increasing CSHNC is the same, namely, there is only one viscosity maximum in each system, the positions of the extreme values move to the direction of lower SHNC concentration. The absolute value of η0 at the maximum is relatively large, indicating a stronger interaction between R14HTAB and SHNCs. Why are R14HTAB micelles sensitive to SHNCs? In comparing the molecules (see Figure 1), NaSal is identical to SHNCs; the key difference between the molecules is the presence of an extra benzene ring in the SHNCs. This implies that NaSal is less hydrophobic than the SHNCs; thus their counterions will adsorb on the R14HTAB micelles at lower concentrations, and show a stronger interaction compared to NaSal. This inference explains the results in Figure 11. Furthermore, the effect of 2SHNC on the viscosity of the R14HTAB solution is stronger than that of 1SHNC, indicating that the hydroxyl group on the side of the benzene ring is more advantageous for generating wormlike micelles than on the inside. In addition, we noticed that as CSHNC increased from 10 to 70 mmol kg−1, the external appearance of the mixed solution drastically changed. At low SHNC concentration, solutions were transparent, but they became gelatinous in the concentration region of 30−60 mmol kg−1. When CSHNC was increased further to 70−90 mmol kg−1, some precipitate formed, and light blue and slightly turbid dilute solutions were

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00238. 1 H NMR spectral data and elemental analysis dates of R14HTAB (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86-635-8230613 (X.L.W.). *E-mail: [email protected]. Tel.: +86-635-8230640 (J.H.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21473084, 21373106, 21073081), Project of Shandong Provincial Education Department (J12LD13) and Liaocheng University Scientific Research Fund (318011402), Teaching Research (311161518), and Experimental Technology Projects of Liaocheng University.

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