Rheological behavior and microstructure of an anionic surfactant micelle solution with pyroelectric nanoparticle

Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 267–275

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Rheological behavior and microstructure of an anionic surfactant micelle solution with pyroelectric nanoparticle Mingliang Luo a,∗ , Zilong Jia a , Houtai Sun a , Lejun Liao b , Qingzhi Wen a a b

College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266555, PR China Changqing Downhole Technology Co., Ltd., CCDC, Xi’an 710018, PR China

a r t i c l e

i n f o

Article history: Received 5 October 2011 Received in revised form 26 November 2011 Accepted 15 December 2011 Available online 24 December 2011 Keywords: Anionic surfactant Nanoparticle Viscoelastic micelle Rheology Microstructure Pyroelectric effect

a b s t r a c t The rheological behavior and microstructure of anionic fatty acid methyl ester sulfonate sodium (MES) surfactant micelle solution with pyroelectric nanoparticle were investigated by dynamic light scattering (DLS), freeze-fracture transmission electron microscopy (FF-TEM) and rheological measurement. The formation of wormlike micelle due to salt addition was observed and the nanoparticle can accelerate the micelle growth and entanglement with each other. The effect of concentration of surfactant, added salt and pyroelectric barium titanate (BaTiO3 ) nanoparticle were investigated, respectively. The viscosity of MES micelle solution increases with the temperature rise within a certain range because of the pyroelectric effect of nanoparticles. At low temperature (∼25 ◦ C), the sample with 0.6 wt.% nanoparticle behaves like an elastic gel with an infinite relaxation time and viscosity. The change of microstructure and rheological properties for MES viscoelastic micelle solutions may be attributed to the electrostatic interaction and pseudo-crosslinking between micelles and nanoparticles. MES viscoelastic micelle solution can retain high viscosity at elevated temperature due to nanoparticle addition and has very attractive for reservoir stimulation and enhanced-oil-recovery in higher temperature reservoirs. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Wormlike micelles are long, threadlike aggregates of surfactants or other amphiphiles due to the elongation of rod-like micelles under appropriate solution conditions [1], which can entangle with each other, forming transient networks that exhibit viscoelastic behavior. These viscoelastic surfactants (VES) can be used as thickening agents in many industrial applications, especially in hydraulic fracturing and acidizing [2–5]. Viscoelastic behavior of VES is similar to that of polymers in semidilute or concentrated solutions [6–8]. However, unlike conventional polymer, the wormlike micelles are in equilibrium with their monomers, and can incessantly break and recombine under the external conditions, e.g. temperature, hydrophobic additives, high shear rate, etc. [9,10]. It was observed that the viscosity of VES can drop by several orders of magnitude when it contacts with hydrocarbons, which easily cleans up from the formation pores or natural fissures [2]. However, the VES in comparison to the conventional polymers mainly has a significant deterioration of their rheological characteristics at elevated temperatures and the filter cake cannot be formed on the surface of fracture, which leads to high leak-off volume into the

∗ Corresponding author. Tel.: +86 532 86981936; fax: +86 532 86981936. E-mail address: yfsailing [email protected] (M. Luo). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.052

formation in the process of hydraulic fracturing, frac-packing and other applications [11,12]. To overcome the above drawbacks of VES micelle solutions, James and Huang proposed to use blends of VES with nanoparticles [12,13]. It is expected that the addition of nanoparticles into VES micelle solution can be resistant to viscosity reduction and leak-off velocity enhancement at elevated temperature. At the past several years, most of the investigations on the rheological properties of cationic VES micelle/nanoparticles composite solutions at constant temperature were concerned [14–18]. Some researchers have been proposed that the rheological modification of VES micelle solutions by addition of nanoparticles simply attributes to the effect of nanoparticles on the bulk electrostatic properties of the fluid [15]. Others have proposed that the nanoparticles materially are participated in the viscoelastic network through the formation of effective cross-linking [13] or double networks [19]. But the effect of temperature on the rheological properties of VES micelle/nanoparticle composite solutions is few concerned, which possibly results in a great application risk in high temperature reservoirs. In addition to the electrolyte, the VES micelle/nanoparticle composite solution described in the work mainly consists of (i) barium titanate (BaTiO3 ) nanoparticle with pyroelectric effect, which exhibits an increase in electrostatic charge on their crystal surface due to spontaneous polarization when heated below Curie temperature (120 ◦ C), and (ii) fatty acid methyl ester sulfonate sodium (MES, molecular formula, RCH (SO3 Na) COOCH3 , R: C16-18 alkyl),

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an anionic surfactant, which was prepared by the addition of sulfur trioxide to the ␣-carbon of a methyl ester and subsequently neutralized with a base. Recently it was found that MES is able to form viscoelastic solutions in aqueous salt solution [20]. Compared to the cationic surfactant, MES surfactant is less expensive, biodegradable and lower adsorptive on the negative-charged rock surface, which can decrease the formation damage caused by wetting alternation of rock surface [21,22]. In the present work, the rheology and microstructure of MES micelle solution with and without nanoparticles were studied. The effect of temperature, added salt and surfactant concentration were investigated using rheological measurement, dynamic light scattering (DLS) and freeze-fracture transmission electron microscopy (FF-TEM). The effect of pyroelectric nanoparticle on the rheological behavior of VES solution was analyzed. A good understanding of the rheological behavior of the system will be obtained by the combination of these techniques. 2. Materials and methods 2.1. Materials The anionic surfactant fatty acid methyl ester sulfonate sodium (MES) was provided by Lion Daily Chemical (Qingdao) Co., Ltd., which was recrystallized three times from ethanol and characterized by the critical micelle concentration (6.52 × 10−4 M, close to the reported value in ref [23], 6.68 × 10−4 M). The surface tension measurements showed that no surface tension minimum was found. Sodium chloride (NaCl) from Aldrich (>99.8% purity) and barium titanate (BaTiO3 ) from Aldrich (20–40 nm, ≥99% purity) were used as received. Samples were prepared by weighting the appropriated amounts of surfactant, salt, nanoparticles and deionized water. The samples were stirred and then heated at 45 ◦ C for 30 min to remove any entrained air bubbles, finally placed for equilibration at room temperature for 1 day.

where  is the viscosity of solution, which can be approximated to that of water. All measurements were performed at 25 ◦ C if not specified. 2.4. Freeze-fracture transmission electron microscopy The freeze-fracture transmission electron microscopy (FF-TEM) was carried out using a freeze fracture apparatus (Hitachi HFZ-1) on a nitrogen-cooled support and a TEM (JEOL Model JEM-1200EX). The procedure has been described in several literatures [25–27]. A thin layer of the sample (20–30 ␮m) was placed on a thin copper holder and then rapidly quenched in liquid nitrogen. The frozen sample was fractured at −150 ◦ C, in a high vacuum better than 10−5 Pa with the liquid nitrogen cooled knife in the freeze etching unit. To increase the temperature to −100 ◦ C and keep the high vacuum, the fractured sample was placed 30–60 min in order to evaporate the water adsorbed on the surface of the sample. Then to decrease the temperature to −120 ◦ C again, the replication was done using unidirectional shadowing at an angle of 45◦ , with platinum–carbon (Pt–C) and 1–10 nm of mean metal deposit. The replicas were washed with double distilled water and were observed in transmission electron microscope. 3. Results and discussion 3.1. Rheology of MES viscoelastic micelle solutions 3.1.1. Effect of MES surfactant concentration Some cationic or anionic surfactants can self-assemble into long, flexible wormlike micelles in the presence of salt, and the entanglement of these micelles into a transient network gives viscoelasticity to the surfactant solutions. Fig. 1 shows the zero-shear viscosity

10

3

2.2. Rheological measurements

2.3. Dynamic light scattering Dynamic light scattering (DLS) measurements were performed on a DynaPro Nanostar equipped with a Static Light Scattering detector ( = 658 nm) at the scattering angle 90◦ . The samples were prepared by mixing proper ratios of stock solutions of each surfactant component (pre-filtered through a 0.2 ␮m syringe filter) and equilibrated at least 48 h before measurements. The measurement was replicated at least three times for each sample, and an average value was calculated. The hydrodynamic radii, Rh , can be estimated by applying the Stokes–Einstein relation (for stick-boundary conditions [24]): D=

kT 6Rh

10

2

lll slope 2.9

1

0

/Pa.s

10

Zero-shear viscosity

The rheological measurements were performed with a ThermoHaake RS6000 rheometer using the cone plate system (diameter, 35 mm; angle, 2◦ ). To avoid the effect of solvent evaporation, a specially constructed vapor lock filled with the solvent was used. The sample was equilibrated for at least 20 min at certain temperature prior to conducting measurements. Dynamic measurements were carried out in the linear viscoelastic regime in a 0.01–100 rad s−1 frequency range. The zero-shear viscosity 0 of the sample was determined from controlled stress measurement by extrapolating the viscosity-shear stress curve to zero shear-rate. All measurements were performed at 25 ◦ C if not specified.

10

10

10

0

ll slope 6.7

-1

-2

l 10

10

slope 3.3

1 2

-3

-4

0.1

1 MES concentration/wt.%

10

Fig. 1. Zero-shear viscosity as a function of the concentration of surfactant MES at two different temperatures: 25 ◦ C (open symbols) and 60 ◦ C (closed symbols). Solvent: 4.0 wt.% NaCl in water.

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10

2.5wt.% 3.0wt.% 4.0wt.% 5.0wt.%

/Pa.s

1

s

0.1

0.01

1

10

/Pa Fig. 2. The apparent viscosity (s ) as a function of shear-stress () for the micelle solution with different MES concentrations at 25 ◦ C. Solvent: 4.0 wt.% NaCl in water.

breakage and recombination both occur often before the chain reptates out of the tube segment. But in region II (CMES = 3.0 wt.%), it should be noted that the part of experimental points at the high frequency region in Cole–Cole curves are deviated from the theoretical Maxwell model. This phenomenon has been found in many viscoelastic surfactant solutions, which probably is resulted from the appearance of Rouse modes or “breathe mode” [31–33]. 3.1.2. Effect of temperature The dependencies of viscosity on surfactant concentration at two different temperatures were shown in Fig. 1. It is seen that the viscosity decreases by up to 1–2 orders of magnitude at heating from 25 ◦ C to 60 ◦ C. There are some differences in MES concentration dependencies of zero-shear viscosity at different temperatures. Fig. 1 shows that above C* the viscosity curve at 60 ◦ C has only one slope (3.3) in contrast to the curve at 25 ◦ C, which has two different slopes. It can be due to the acceleration of the dynamic process of breaking and recombination of micelles at elevated temperatures, as a result of which the region of the curve corresponding to the “unbreakable” chains (with slope 6.7) disappears. The storage moduli G and loss moduli G as a function of frequency for the systems at different temperatures were investigated and plotted in Fig. 4. It is seen that the heating does not affect the value of plateau modulus G0 , which means that the mesh size 40 3.0 wt.% 3.5 wt.% 4.0 wt.% 5.0 wt.%

30

G"/Pa

as a function of MES surfactant concentration that far exceed the critical micelle concentration (cmc = 2.61 × 10−2 wt.%) at 25 ◦ C. It was observed that the zero-shear viscosity remains rather low until MES surfactant concentration of ca. 2.35 wt.%, and then increases dramatically, which indicates that MES solution transits from a dilute to a semidilute regime and the wormlike micelles start to entangle with each other. Here, the concentration of 2.35 wt.% can be considered as a crossover concentration C* for MES surfactant micelles. At a fixed salt concentration, two different slope regions for MES concentration dependence of viscosity at 25 ◦ C were observed (Fig. 1, curve 1). The dependence of zero-shear viscosity 0 on the MES concentration in the more slope region (II) is characterized by the power law 0 ∼ C6.7 , while the second slope region (III) is characterized by the power law 0 ∼ C2.9 . The results can be explained from the viewpoint of micelle growth. At lower MES concentration, increasing MES only leads to an increase of the size and number of rod-like micelles, and the shorter micelles in MES micelle solution do not break during the characteristic reptation time (i.e. the reptation time  rep is smaller than the breaking time  b :  rep   b ) and this MES micelle solution was described as the case of “dead polymer” [28]. Therefore, in region I, the zero-shear viscosity shows little change. And further increasing MES concentration, a great number of longer micelles appear and become more flexible and can curve freely, the wormlike micelles easily form and finally entangle with each other. The entangled network structure occurs at a certain surfactant concentration, which can rapidly increase the viscosity of MES micelle solution with the increasing of MES concentration (seen in region (II)), similar to that of the polymer in this region. The transient network can reversibly break and recombine many times and be described as the case of “living polymer” [29]. However, at a higher MES concentration, the zero-shear viscosity shows less dependent on surfactant concentration at a fixed salt concentration (seen in region (III)), which reflects the approximatively equilibrium between breakage and recombination of the aggregates [30]. Meanwhile the wormlike micelle entanglement density of the system approaches a Maximum, which may suggest another microstructure transition upon further increase of MES concentration. The observed result also indicates that the viscosity of MES micelle solution is jointly affected by MES and salt concentration at a constant temperature and so shows different flow characteristics compared with the polymer solution at higher concentration region. The zero-shear viscosity is very low at 2.5 wt.%(17.8 mPa s). The apparent viscosity of the micelle solutions (s ) as a function of shear stress () were measured and plotted in Fig. 2. As MES concentration increased from 2.5 wt.% to 5.0 wt.%, a dramatic increase in 0 (from 17.8 mPa s to 12.79 Pa s). The steady rheology for the micelle solutions at MES concentration from 3.0 wt.% to 5.0 wt.% exhibits a Newtonian fluid at lower shear stress whereas non-Newtonian behaviors at higher shear stress. The high viscosity observed in the Newtonian plateau can be interpreted as the result of the presence of an entangled network of wormlike micelles. The shear-thinning behavior at higher shear stress can be explained by virtue of the viewpoint of the entanglement between polymer chains. As the shear stress increases, the breaking velocity of entangled point of MES micelle network is higher than the recombining velocity and the wormlike micelles are aligned, so the surfactant solution starts to shear thinning. Dynamic rheological measurements showed quite different behavior of the system in the II and III regions, which were illustrated in Fig. 3. In region III (CMES = 3.5, 4.0, 5.0 wt.%), the stress relaxation is monoexponential (well fitting the Maxwell model) and the Cole–Cole curves have semicircular shapes. Such simple rheological behaviors can be attributed to the fact that the micelle breaking time is shorter than the reptation time. As a result, the

269

20

10

0

0

15

30

45

60

75

G'/Pa Fig. 3. Cole–Cole plots for different concentration solutions of surfactant MES at 25 ◦ C. Solvent: 4.0 wt.% NaCl in water.

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10

2

10

1

10

0

2

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1

10 "

G min

(G',G")/(Pa)

G',G"/Pa

270

0

10 10

60 C

0

25 C

-1

0

10

-1

10

p =0 p =0.3 wt.%

-2

10

-2

-1

10

0

10

1

10

10

p =0.6 wt.%

-2

10

2

/(rad/s) -3

Fig. 4. Dynamic moduli as a function of frequency for the sample of 4.0 wt.% MES solutions at different temperatures. Solvent: 4.0 wt.% NaCl in water. Storage moduli G (closed symbols) and loss moduli G (open symbols).

10

2

10

1

10

-2

10

-1

10

0

10

1

10

/(rad/s) Fig. 7. Dynamic moduli as a function of frequency for the samples of 4.0 wt.% MES solutions with different nanoparticle concentrations at 25 ◦ C. Solvent: 4.0 wt.% NaCl in water. Storage moduli G (closed symbols) and loss moduli G (open symbols).

without nanoparticle 0.3wt.%

with increasing of the temperature. The reduction in the elastic modulus indicates a predominantly viscous response or more liquid-like behavior. The variations of G and G versus temperature confirmed the microstructure changes of micelle network. Moreover, the crossover of G and G was shifted to a higher frequency, which suggests that the relaxation time is reduced at elevated temperature.

0.6wt.% 0

10

-1

10

-2

0.9wt.%

0

/Pa.s

10

10

-3

10

-4

3.2. Rheology of MES micelle solution with added nanoparticle

1

10

C MES /wt.% Fig. 5. Zero-shear viscosity (0 ) as a function of concentration of surfactant MES (CMES ) for different nanoparticle concentrations at 25 ◦ C. Solvent: 4.0 wt.% NaCl in water.

and hence the entanglement length le are unaffected by temperature. But simultaneously heating shifts the minimum value of the  loss modulus Gmin to higher frequencies (consistent with the presented results [34]), indicating a decrease of the average contour  /G ≈ l /L [8]. Thus, the drop length Lh of micelles according to Gmin e 0 of viscosity at heating seems to be due to the shortening of the micelles [35]. We also observed that the elastic moduli decrease

3.2.1. Effect of added nanoparticle concentration The zero-shear viscosities 0 of MES micelle solution with added 0.3 wt.%, 0.6 wt.%, 0.9 wt.% nanoparticles were measured at 25 ◦ C, respectively, and are plotted in Fig. 5. It is seen that the concentration dependence of 0 in MES micelle solution with added nanoparticles also show two different slope regions. At constant surfactant concentration, the zero-shear viscosity increases with increasing of nanoparticle concentration for all surfactant concentrations, which is illustratively demonstrated in Fig. 6. For surfactant concentrations above C*, the 0 increases by as much as a factor of ∼10 upon addition of only 0.6 wt.% nanoparticles. It is also observed that there are two effects induced by nanoparticle additive: (i) a significant shift of the entanglement concentration of the strong rise of viscosity to lower surfactant concentrations and (ii) a strikingly sharp increase of 0 with scaling law 0 ∼ C12.5 (instead of 0 ∼ C6.7 for MES micelle solution without nanoparticles). The observed decrease in the entanglement concentration with increasing of nanoparticle concentration can be reconciled by the effective lengthening of micelles due to the presence of pseudocrosslink of micelle–nanoparticle, similar to what was observed in

Fig. 6. Photographs of micellar solution of the samples of 4.0 wt.% surfactant MES with different nanoparticle concentrations: (a) 0;(b) 0.3 wt.% and (c) 0.6 wt.%. at 25 ◦ C. Solvent:4.0 wt.% NaCl in water.

10

1

10

0

25

20

/ (Pa.s)

2

271

without nanopartilce 0.3wt.% 0.6wt.%

15

0

10

s

/Pa.s

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10

10

-1

5

10

-2

1

10

/Pa

3.0

Fig. 8. The apparent viscosity (s ) as a function of shear-stress () for the samples of 4.0 wt.% MES solutions with different nanoparticle concentrations at 25 ◦ C. Solvent: 4.0 wt.% NaCl in water.

240 200

/(mPa.s)

160 120 80 40 0 20

30

40

50

60

70

T/(ºC) Fig. 9. Shear viscosity as a function of temperature for the samples of 4.0 wt.% MES solutions with different nanoparticle concentrations at shear rate 170 s−1 . Solvent: 4.0 wt.% NaCl in water (nanoparticle concentration: , 0; , 0.3 wt.%; , 0.6 wt.%).

3.5

4.0

4.5

5.0

CNaCl / (wt.%) Fig. 11. Zero-shear viscosity as a function of concentration of salt NaCl for the nanoparticle/surfactant micelle solution at 25 ◦ C. Surfactant: 4.0 wt.% MES in water (, 0; 䊉, 0.6 wt.%).

previous studies [12,13]. Such a sharp increase of viscosity may be due to the formation of a strong network, in which some subchains are composed of elongated surfactant micelles, while other subchains are formed by physically crosslink micelle aggregates in solution. In micelle aggregates, each nanoparticle will behave as an object with much larger effective hydrodynamic radius and as the core of micelle crosslinking, which will produce a corresponding increase in viscosity. It was observed from Fig. 6 that small amounts of nanoparticles impart high viscosities to aqueous solutions. This aspect is further explored through dynamic rheological measurements, as shown in Fig. 7. Addition of ϕp = 0. 3 wt.% leads to an increase in G and G over the entire frequency range. In this case, the solution exhibits the features typically associated with fully entangled wormlike micelles, namely, Maxwell-like behavior of G and G at low frequencies, followed by crossover of G and G and eventually a rubbery plateau in G0 accompanied by a local minimum in G at sufficiently high frequencies. These results demonstrate that the addition of nanoparticles to an unentangled micellar solution with liquid-like behavior gives rise to significant viscoelasticity and entanglement at sufficient nanoparticle concentration, which is interpreted through two energetic scales, the micellar end-cap energy and micelle–nanoparticle adsorption energy [19]. At low

Fig. 10. Illustration of the proposed mechanisms of a strong network built by nanoparticles and MES micelles, (a) nanoparticle with electric neutrality before heated, (b) charged nanoparticle after heated, (c) micelle network, (d) micelle–nanoparticle network formation by pseudo-crosslinking and (e) micelle–nanoparticle junctions.

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1.0

(a)

0.8

0.8

0.6

0.6

f( )

f( )

1.0

0.4

0.4

0.2

0.2

0.0 0.01

0.1

(b)

0.0 0.01

1

0.1

1.0

(c)

0.8

0.8

0.6

0.6

f( )

f( )

1.0

0.4

0.4

0.2

0.2

0.0 0.01

0.1

1

/ms

/ms

1

/ms

0.0 0.01

(d)

0.1

1

/ms

Fig. 12. Plots of normalized intensity distribution functions of relaxation time f() at 90◦ for the samples: (a) 0.5 wt.% MES in water; (b) 2.5 wt.% MES in 4.0 wt.% NaCl solution; (c) 4.0 wt.% MES in 4.0 wt.% NaCl solution and (d) 4.0 wt.% MES in 4.0 wt.% NaCl solution containing 0.6 wt.% nanoparticles.

nanoparticle concentrations, micellar end-caps are in vast excess, such that each nanoparticle will result in the effective lengthening of micelles; another is the micelle–nanoparticle pseudo-crosslink themselves, which effectively join two or more micelles, creating additional viscoelasticity. These will result in a greater number of entanglements per effective micelle, which will serve as physical crosslinks of a micellar network, much like in a cross-linked polymer gel [36]. Moreover, the crossover of G and G shifts to a lower frequency with increasing of the nanoparticle concentration, which indicates that the relaxation time become longer at higher nanoparticle concentration. But MES micelle solution with 0.6 wt.% nanoparticle show a very different dynamic rheological response, with G’ exceeding G and both moduli being less dependent of frequency, which implies that there is no relaxation of stress even at long time scales; that is, the sample respond like elastic gels with infinite relaxation times. Meanwhile the gel-like character of MES micelle solution with 0.6 wt.% nanoparticle is also reflected in steady-shear rheology as an apparent yield stress for the sample. Fig. 8 shows the apparent viscosity of the samples of 4.0 wt.% MES micelle solutions with different nanoparticle concentration as a function of the shear stress at 25 ◦ C. The yield stress behavior is especially apparent for MES micelle solution with 0.6 wt.% nanoparticle and the viscosity is essentially infinite at shear stresses below about 3.6 Pa (which is, thereby, the value of the yield stress for this sample). Above this shear stress, the viscosity rapidly decreases by several orders of magnitude, that is, the sample “yields” [37]. Note that there is no plateau in the viscosity at low stresses, in contrast to the behavior observed typically for a viscoelastic wormlike micellar sample.

3.2.2. Effect of temperature Fig. 9 shows the shear viscosity as a function of temperature for the samples of 4.0 wt.% MES micelle solutions with different nanoparticle concentration (0, 0.3, 0.6 wt.%) at shear rate 170 s−1 . It was seen that the viscosity decreases with temperature for MES micelle solution without nanoparticles. Interestingly, it was observed that the viscosity increases with increasing of temperature at lower temperature range for the samples of 4.0 wt.% MES viscoelastic micelle solutions with nanoparticles. The critical temperature that the viscosity starts to decrease shift to higher temperature with the increase of nanoparticle concentration, which may be related with the pyroelectric effect of nanoparticle. The proposed mechanism on the phenomenon is illustrated in Fig. 10. With the increase in temperature, the original bound charges will be released and distributed on the surface of nanoparticles (seen in Fig. 10-(a) and (b)). These surface-charged nanoparticles are absorbed on the wormlike micelle surface more easily and lead to electrostatic screening of charged micelles, which can promote micelles entangle with each other and form the more stable cross-linking network (seen in Fig. 10-(d)). On the other hand, the nanoparticle absorbed on the micelle end-cap and micelle can form a micelle–nanoparticle stalk (seen in Fig. 10-(e)). Because the surface area of charged nanoparticle is much larger than the projected cross-section of the micelle, a single nanoparticle can accommodate many stalks in liquid–solid interface and acts as a junction of micelle, which results in an effectively longer micelle. These nanoparticles with several junctions will incorporate into the micelle network and act as a skeleton-like, which can enhance the network stability greatly. But the temperature exceeds the critical value, the viscosity decreases with the increase of temperature,

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Fig. 13. Representative FF-TEM micrographs for the samples: (a) 2.5 wt.% MES in water; (b) 4.0 wt.% MES in 4.0 wt.% NaCl solution; (c) 4.0 wt.% MES in 4.0 wt.% NaCl solution with 0.6 wt.% nanoparticles at 25 ◦ C.

which is attributed to the breaking of micelle network because the outer energy is greater than the association energy between micelle and nanoparticle, similar to that of the sample without nanoparticles. 3.2.3. Effect of salt concentration For many surfactant systems [28,30,38], it has been observed that the zero-shear viscosity increases with salt concentration, reaching a broad maximum and then decreasing at high ionic strength. This behavior is also observed in MES viscoelastic solution at different NaCl concentrations, as shown in Fig. 11. The increase in the zero-shear viscosity with increasing salt concentration has been explained in terms of micelle growth due to the screening of electrostatic repulsion between similarly charged head groups. Increasing the amount of salt leads to an increase in the curvature energy of the surfactant molecules in the end caps and is relative to that of the molecules in the cylindrical body of the micelles. As a consequence, an increase in micelle length is promoted, the viscosity is raised, and the micelles start to overlap, forming an entangled network. Several authors [37,39] have suggested that the decrease in viscosity at high salt content may be due to the formation of branched wormlike micelles. The crosslinks in the resulting multi-connected micelle network can slide along the micelles and hence serve as stress release points. Such a branched micelle network will therefore show a reduced viscosity compared to that of the entangled linear micelles. Thus, the maximum in viscosity with increasing salt concentration is expected to correspond to a shift from linear to branched micelles. Additionally, it can be seen from Fig. 11 that the salt concentration corresponding to maximum viscosity of the sample of MES solution with nanoparticles is somewhat lower than that of the MES solution without nanoparticles, which indicates that the

nanoparticle at these conditions slightly promotes the branching process. 3.3. Microstructure of MES micelle solutions with and without nanoparticles The dynamic light scattering (DLS) was employed to track the hydrodynamic properties of the microstructures. Plots of normalized intensity distribution functions of relaxation time f() at 90◦ for the samples of 0.5 wt.% MES in water, 2.5 wt.% MES in 4.0 wt.% NaCl solution, 4.0 wt.% MES in 4.0 wt.% NaCl solution and 4.0 wt.% MES in 4.0 wt.% NaCl solution containing 0.6 wt.% nanoparticles are shown in Fig. 12(a)–(d), respectively. A single relaxation mode was observed in the samples of 0.5 wt.% MES in water with the apparent hydrodynamic radius (Rh,app ) estimated to be 4.2 nm. Considering the fact spherical micelles formed in the MES system should have a radius roughly equal to the molecular length of the surfactants (ca. 2 nm), a larger micelle size is therefore consistent with a cylindrical morphology. With increase in surfactant concentration, addition of NaCl and nanoparticles, another mode appears, which has an equivalent Rh,app value of 24 nm, 43 nm and 58 nm for the samples of 2.5 wt.% MES in 4.0 wt.% NaCl solution, 4.0 wt.% MES in 4.0 wt.% NaCl solution and 4.0 wt.% MES in 4.0 wt.% NaCl solution containing 0.6 wt.% nanoparticles, respectively. Additionally, it is also observed that in the first mode, the equivalent Rh,app value of 4.2 nm, 8.8 nm and 16.5 nm for the samples of 0.5 wt.% MES in water, 2.5 wt.% MES in 4.0 wt.% NaCl solution and 4.0 wt.% MES in 4.0 wt.% NaCl solution increases with increase of surfactant concentration and addition of salt. According to DLS results, the spherical micelles are dominant in MES dilute solutions without inorganic salts. In the presence of inorganic salt, there exist strong electrostatic interactions between the cation of salt and the surfactant molecules and micelle

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particles, which compress the hydration layer of surfactant headgroups and the electric double layer of micelles, resulting in a decrease in the area at the micelle surface of surfactant headgroup(s) and an increase in the surfactant packing parameter. Consequently, the addition of salt enhances micelle growth. To compare the samples with and without nanoparticles (Fig. 12(c) and (d)), the effect of nanoparticles on the apparent hydrodynamic radius of the first mode is much less than that of the second mode. Thus DLS results suggest the system underwent a significant growth from small cylindrical micelles into elongated wormlike micelles. It is interesting to note the two populations of micelles coexisted in this case with a relatively large gap in the size distribution. The micelle size distribution for the sample without nanoparticles in Fig. 12(a)–(c) may be explained by the second CMC theory proposed by Israelachvili et al. [40]. It is suggested that above the conventional CMC, the number of micelles increases with an average constant size as the concentration increases until, above a second critical concentration, refer as the second CMC, the micelles start to increase significantly. The key finding behind this theory is the existence of an energy barrier to overcome for growth of small micelles to occur [41]. Recently, Dubin et al. [42] and Yin et al. [43] also suggested the coexistence of bimodal micelle species with a relatively large gap in the size distribution is caused by the energetic instability of intermediate structures. Above the second CMC, much larger wormlike micelles were formed probably due to the energetically unfavorable formation of intermediate aggregates between the small micelles and wormlike micelles. However, adding nanoparticles into MES viscoelastic solution leads to different effect on the smaller micelles and larger micelles, which indicates two separate mechanisms. The first is to micelle overlapping results in the entanglement of smaller micelles, which may be caused by lower the repulsion between micelles due to nanoparticle electrostatic screening [15]. The second is the formation of nanoparticle-larger micelle junction [19], which may accommodate multiple micelle stalk and result in the substantial growth of micelle. In addition to DLS, we also used FF-TEM to study the microstructure in MES micelle solution with and without nanoparticles. Note that FF-TEM is a technique where the structure in a fluid sample is preserved by rapid freezing under a controlled environment. Fig. 13 shows representative micrographs for the samples of 4.0 wt.% MES in water, 4.0 wt.% MES in 4.0 wt.% NaCl solution and 4.0 wt.% MES in 4.0 wt.% NaCl solution with 0.6 wt.% nanoparticles at 25 ◦ C. As shown in Fig. 13(a)–(c), the long, entangled wormlike micelles were observed in the samples with and without nanoparticles at 25 ◦ C, in accordance with the DLS data. Simultaneously, it was also seen clearly that with addition of salt and nanoparticles, the apparent hydrodynamic radius of the micelle increases and the micelles are entangled with each other. Moreover, a distinctive feature of the micrographs in Fig. 13-(c) is that some of the micelles appear to intersect with nanoparticles, which is marked by the arrows. The observed phenomenon further corroborate the hypothesis given by Nettesheim et al. [14] that micelle–nanoparticle association occurs in viscoelastic surfactant solution. The observed microstructure of the samples with and without nanoparticles in the FF-TEM images could give us a better understanding on special rheological properties of these samples at different conditions.

4. Summary The rheological behavior and microstructure of MES surfactant micelle solution with and without pyroelectric nanoparticles were investigated by rheological measurement, DLS and FF-TEM. The results demonstrate that nanoparticle can promote the micelle

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