Structure and Rheology of Wormlike Micelles Formed by Fluorocarbon

May 28, 2017 - From the viscoelastic parameters, the steady state compliance, Je, and the terminal relaxation time, τw, which were independently obta...
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Structure and Rheology of Wormlike Micelles Formed by Fluorocarbon−Hydrocarbon-Type Hybrid Gemini Surfactant in Aqueous Solution Ken Morishima,† Seiya Sugawara,† Tomokazu Yoshimura,‡ and Mitsuhiro Shibayama*,† †

The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoyanishi-machi, Nara 630-8506, Japan

‡

S Supporting Information *

ABSTRACT: The structure and rheological properties of wormlike micelles formed by a fluorocarbon−hydrocarbontype hybrid gemini surfactant in an aqueous solution were investigated by means of small-angle X-ray scattering (SAXS) and viscoelastic measurements. The cross-sectional structure (the radius of the hydrophobic core and the thickness of the hydrophilic shell) and the aggregation number per unit axial length of wormlike micelles were evaluated by a model fitting analysis of SAXS profiles. Both parameters for the hybrid gemini surfactant were smaller than those of a corresponding hydrocarbon−hydrocarbon-type gemini surfactant. On the other hand, the viscosity of the hybrid gemini surfactant was higher than that of the hydrocarbon−hydrocarbon-type gemini surfactant. From the viscoelastic parameters, the steady state compliance, Je, and the terminal relaxation time, τw, which were independently obtained by dynamic viscoelastic measurement, we revealed that a larger number of entanglements and a longer contour length of the hybrid gemini surfactant led to the higher viscosity. These results obtained by the rheological measurements were consistent with those obtained by SAXS analysis.

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INTRODUCTION Gemini surfactants, consisting of two monomeric surfactants linked with a spacer, have been synthesized and studied as high performance surfactants because of their lower critical micelle concentration (cmc) and surface tension than those of monomeric surfactants.1−5 Since the packing parameter p = v/a 0 l c for gemini surfactants is higher than that for corresponding monomeric surfactants due to the presence of two hydrophobic chains in a molecule, they form wormlike micelles by themselves even in the absence of salt at low concentrations. Here, v, a0, and lc are the volume of the hydrophobic group, the apparent area of the hydrophilic group, and the length of the hydrophobic chain, respectively6 As a result, an aqueous gemini surfactant solution exhibits high viscosity even at low concentrations.7−21 Therefore, gemini surfactants are more effective surfactants than monomeric surfactants and are promising materials for reduction of total consumption in commercial products. Furthermore, recently, hybrid (or hetero) gemini surfactants, which have different hydrophobic and/or hydrophilic groups in the surfactant, have been synthesized.22−25 Since they show excellent properties derived from the two different functional groups in the surfactant, they have attracted increasing attention as a designer’s surfactant. Gemini surfactants containing fluorocarbon chains show superior physicochemical performance, such as lower cmc and © 2017 American Chemical Society

surface tension, to corresponding hydrocarbon−hydrocarbontype gemini surfactants.26−30 This is due to strong hydrophobicity of the fluorocarbon chain. To date, cationic28,29,31 and anionic26,32 type fluorocarbon gemini surfactants have been synthesized and their micellization behaviors investigated. They show various morphology (e.g., wormlike micelle, vesicle, and bilayer sheet) depending on the chemical structure of the surfactants. In addition to these properties, fluorocarbon-type gemini surfactants are expected to exhibit high viscosity, chemical stability, deforming property, and so on. However, the poor solubility of fluorocarbon chain to water is a bottleneck for an application to daily necessities. Based on these situations, fluorocarbon−hydrocarbon-type hybrid gemini surfactants, which have both advantages of fluorocarbon and high solubility, have recently attracted much attention although the use of fluorocarbon surfactants is limited because of their toxicity in the surroundings. Fluorocarbon−hydrocarbon hybrid surfactants (surfactant molecules consisting of fluorocarbon and hydrocarbon chains connected to one hydrophilic group) have been first synthesized by Guo et al.33 Afterward, Yoshino et al. developed the chemical stability of the hybrid surfactant.34−37 They Received: March 16, 2017 Revised: May 26, 2017 Published: May 28, 2017 6084

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Langmuir Scheme 1. Chemical Structure of Surfactants: (a) 12-s-C3C6F, (b) 12-s-12, and (c) DTAB (s = 3, 6, and 12)

water purified with a Millipore Milli-Q system. All solutions were stirred overnight with a magnetic stirrer and were allowed to stand overnight before measurements. SAXS Measurements. Small-angle X-ray scattering (SAXS) measurements were performed using the BL03XU beamline at SPring-8 (Japan). PILATUS (Dectris) was used as an area detector. The X-ray wavelength, λ, the irradiation time, and the camera length (the distance between sample and detector) were chosen at 1.0 Å, 3 s, and 1.2 m, respectively. The magnitude of the scattering vector q is expressed as q = (4π/λ) sin(θ/2). (λ and θ are wavelength and scattering angle, respectively). The sample solutions were loaded in a quartz capillary cell (inner diameter = 2.0 mm) and measured at room temperature (around 25 °C). In the following are shown the scattering intensity functions corrected for the solvent scattering after circular averaging of the two-dimensional data. SAXS Analysis. In general, the scattering intensity from particulate systems is described by

reported thermoresponsive behavior for the hybrid surfactant, which has never been exhibited by monomeric surfactants or mixture systems. However, the solubility of the hybrid surfactants in aqueous solution was low. To avoid the poor solubility, Oda et al. have synthesized the two hydrophilic groups type “hybrid gemini” surfactant and investigated the micellization behavior and micellar structure in a wide concentration region by means of electrical conductivity, NMR, and cryo-TEM measurements.38 They showed the formation of branched wormlike micelles and vesicles in a cryoTEM picture. However, other micellar properties, such as detailed structure and viscoelastic properties, for fluorocarbon− hydrocarbon hybrid gemini surfactant systems have never been investigated after their report because of the difficulty of the synthesis. Recently, we have efficiently synthesized a fluorocarbon− hydrocarbon-type hybrid gemini surfactant. In this study, we investigated the micellar structure and rheological properties of the hybrid gemini surfactant solution. We demonstrate the effect of an introduction of fluorocarbon chain to a gemini surfactant on the rheological properties as well as micelle structure by comparing with corresponding hydrocarbon− hydrocarbon-type gemini surfactant.

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I(q) = NpP(q) S(q)

(1)

where Np, P(q), and S(q) are the number density of particles, the form factor, and the structure factor, respectively. P(q) for a core−shell cylinder is given by

P(q) =

EXPERIMENTAL SECTION

∫0

2π

|A(q , θ)|2 sin θ dθ

A(q) = 2(ρ1 − ρ2 )Vcore j0 (qH cos θ)

Samples. A hybrid-type gemini surfactant with a fluorocarbon and hydrocarbon chain (12-s-C3C6F, Scheme 1a) and hydrocarbon− hydrocarbon-type gemini surfactant with two hydrocarbon chains (12-s-12, Scheme 1b) were investigated in this study. 12-s-C3C6F was synthesized by quaternization of N,N,N′,N′-tetramethyldiaminoalkane, dodecyl bromide, and 3-(perfluorohexyl)propyl bromide. 12-s-12 was synthesized by quaternization of N,N,N′,N′-tetramethyldiaminoalkane and dodecyl bromide (Scheme 2). Here, we chose 12-s-12 as a

(2) J1(qR core sin θ) qR core sin θ

+ 2(ρ1 − ρ2 )Vshell j0 [q(H + dshell) cos θ ]

J1[q(R core + dshell) sin θ ] q(R core + dshell) sin θ

(3) j0 (x) =

sin x x

(4)

Vcore = πR core 2L

Scheme 2. Synthetic Scheme of 12-s-C3C6F

(5) 2

Vshell = π(R core + dshell) (L + 2dshell)

(6)

where ρ0, ρ1, and ρ2 are the scattering length densities of the solvent, the core, and the shell, respectively.40 Rcore and dshell are the core radius and the shell thickness. L is the length of the core−shell cylinder, and H = L/2. J1(x) is the first order Bessel function of x. We further assume that the Rcore has a log-normal distribution. The structure factor S(q) for charged rods is given by the reference interaction site mode (PRISM),40 which is given by S(q) =

c(q) = reference sample because the volume of hydrophobic groups is comparable with that of 12-s-C3C6F. The volume of the fluorocarbon is 1.5 times as large as that of a hydrocarbon with same carbon number.39 (i.e., the volumes of -C6F13 and -C9H19 are almost equal). As a result, the volumes of hydrophobic groups are comparable between -C3H6C6F13 and -C12H25 (= -C3H6-C9H19). A monomeric surfactant dodecyltrimethylammonium bromide (DTAB, Scheme 1c), which was purchased from Wako Pure Chemical Industries (Japan), was also used as a reference surfactant. These surfactants were dissolved in pure

1 1 + βc(q) Prod(q)

(7)

3[sin{2q(R + lD)} − 2q(R + lD) cos{2q(R + lD)}] {2q(R + lD)}3 (8)

{

{1 + 2(B + C)}2 + 2D 1 + B + β=

4

(1 − B − C)

5 C 4

} −1

(9)

where B = πR Ln, C = (4/3)πR n, and D = (1/2)πRL n. lD is the Debye length, and Prod(q) is the form factor for the infinitely thin rod and is given by 2

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3

2

DOI: 10.1021/acs.langmuir.7b00902 Langmuir 2017, 33, 6084−6091

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Langmuir Prod(q) =

2∫

0

qL −1

t

4 sin 2

sin t dt

qL

−

qL 2 2

= 0.03−0.08 Å−1) depends on the concentration (first peak). On the other hand, the scattering peaks in the high q region (q ∼ 0.3 Å−1, second peak for 12-3-C3C6F; and q ∼ 0.2/0.4 Å−1, second/third peaks for 12−3−12, respectively) are independent of concentration. The concentration dependences of these peak positions q* are plotted in Figure 2. The second and third

( )

(qL)

(10)

The fitting analysis of the scattering functions was conducted with the software of Igor Pro. For calculation of P(q) of a core−shell cylinder, we utilized “SANS and USANS data reduction and analysis package” released from NIST (https://www.ncnr.nist.gov/programs/ sans/data/red_anal.html) which works on Igor Pro. S(q) of PRISM was calculated with the procedure written on Igor Pro by our group.41 Rheological Measurements. Rheological measurements were conducted by means of a stress control rheometer (MCR-501, Anton Paar). A cone plate with a diameter of 25 mm and a cone angle of 2.0° was used as a jig. In the steady flow viscosity measurement mode, the shear rate was varied in the range of 10−2 to 103 s−1. In the dynamic viscoelasticity measurement mode, the strain of 30% was applied, and the angular frequency, ω, was varied in the range of 10−1 to 102 s−1. All measurements were performed at 25 °C.

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RESULTS AND DISCUSSION Overview of SAXS Profile. Figure 1 shows the SAXS profiles of 12-3-C3C6F and 12-3-12 aqueous solutions at various

Figure 2. Weight fraction dependence of q*. Filled markers represent the first peaks. Open markers represent the second and third peaks.

peaks, which are independent of concentration, correspond to the micellar shape, and the profile at the high q region can be represented by the form factor P(q) (we demonstrate the fitting analysis later). On the other hand, the first peak is an increasing function with increasing concentration. By using Bragg’s diffraction equation (d* = 2π/q*), the interparticle distance d* is obtained as a decreasing function with increasing concentration. Therefore, the first peak, which is related to the structural factor S(q), gives the correlation distance between the micelles. d*s are well reproduced by a power law function of the weight fraction, ϕwt; q* ∼ ϕwt0.33 in the 123-C3C6F system and q* ∼ ϕwt0.5 in the 12-3-12 system. Generally, polyelectrolyte aqueous solutions show q* ∼ ϕwt0.5 in the semidilute regime, which reflects an ordered packing of rodlike scatterers.42 The results of 12-3-12 are similar to polyelectrolyte solutions with an ordered packing of wormlike micelles. On the other hand, the concentration dependence of the 12-3-C3C6F is more gradual than that of 12-3-12. Similar behavior has also been observed for hydrocarbon gemini surfactant (12-2-12) and trimeric surfactant (12-3-12-3-12).43 In et al. conjectured that wormlike micelles grow and branches are formed with increasing concentration, so that they take a different packing state from linear wormlike micelles. Although the theoretical verification has not been achieved, we suggest that the broad concentration dependence of q* for 12-3-C3C6F in this work also occurs from the same origin. Model Fitting Analysis of SAXS Results. In this study, we performed fitting analysis using the core−shell cylinder model as a form factor, P(q), and the polymer reference interaction site model (PRISM) as a structure factor, S(q), to represent the charged wormlike micelle. The fitting curves shown as the solid lines in Figure 1 well reproduce the experimental results in a wide q range except for the deviations

Figure 1. SAXS profiles of aqueous gemini surfactant solutions: 12-3C3C6F (upper) and 12-3-12 (lower). Reversed triangles represent the first peak (closed) and the second/third peaks (open). Solid lines are fitting curves. The data points and theoretical curves are vertically shifted for visibility of each set of data.

concentrations. Since the electron density distributions are significantly different between 12-3-C3C6F and 12-3-12 molecules, the SAXS profiles are very different between the two systems. However, these profiles have a common feature, i.e., the presence of more than a couple of distinctive peaks irrespective of concentrations. The peak in the low q region (q 6086

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Langmuir in q > 0.3 Å−1 for both systems and q < 0.04 Å−1 for 12-3-12. The former deviation derives from nonideality of the local structure (i.e., the core−shell cylinder model cannot express the local structure of micelle.) The latter is caused by an upturn of scattering intensity in the low q region, which may be due to a network structure of the wormlike micelle. In most of the measured q regions except for the above q region, the structure of the micelles is successfully reproduced by the theories of charged wormlike micelle. The value of L (contour length) used for the curve fitting is an apparent value because the wormlike micelles form an entangled network in high concentration solution. The apparent L means the length of a partial cylinder of wormlike micelle. Figure 3 shows the cross-sectional shapes (the core radius, Rcore, and the shell thickness, dshell) and the scattering length

Here, Ne is the number of electrons of hydrophobic chains of a surfactant molecule, which are 274 and 194 for 12-3-C3C6F and 12-3-12, respectively. re is the classical electron radius (=2.81 × 10−13 cm), and V′ is the core volume (=πRcore2) per unit axial length of the wormlike micelle. From the above equation, the aggregation number per 1 nm axial length was estimated to be 7.0 and 9.1 molecules for 12-3-C3C6F and 12-3-12, respectively. Steady Flow Viscosity and Zero-Shear Viscosity. The shear rate, γ̇, dependence of the viscosity, η, obtained by the steady flow viscosity measurement is displayed in Figure 4a for 12-3-C3C6F and 12-3-12 solutions. A shear-thinning behavior was observed in a high shear rate region for high concentration solutions of 12-3-C3C6F and 12-3-12. This behavior was also

Figure 3. Illustration of the scattering length densities of core, shell, and solvent (ρ1(r), ρ2(r), and ρ0(r), respectively) and the cross-section (Rcore and dshell) of wormlike micelles: 12-3-C3C6F (left) and 12-3-12 (right).

density variations, ρ(r), of micelles of 12-3-C3C6F and 12-3-12 obtained by fitting analysis, where r is the real space coordinate. The core radius of the wormlike micelle was determined by the length and volume of the hydrophobic group of the surfactants. The core radius of the 12-3-C3C6F was found to be smaller than that of 12-3-12. Since the volume of the fluorocarbon chain is about 1.5 times larger than that of hydrocarbon chain having the same number of carbon atoms, as described above, the volumes of hydrophobic groups (-C3H6-C6F13 and -C12H25) of the two gemini surfactants used in this study are almost the same. Therefore, the smaller core radius of the 1233-C3C6F seems to be due to a short length of the hydrophobic group (i.e., the extended lengths of -C3H6-C6F13 and -C12H25 are 1.29 and 1.67 Å, respectively) and/or the stronger hydrophobicity. From the hydrophobic core radius and the scattering length density of the core obtained by the fitting analysis, the aggregation number, n, per unit axial length of a wormlike micelle can be obtained as follows.

n=

Figure 4. (a) Shear rate dependence of viscosity for aqueous 12-3C3C6F (upper) and 12-3-12 (lower) solutions. (b) Concentration dependence of zero-shear viscosity for 12-3-C3C6F, 12-3-12, and DTAB solutions. Solid lines are eye guides. Broken line represents Roscoe’s empirical equation.

ρcore V ′ Nere

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(Figure S2). Although such a clear rubbery plateau was not observed for 12-3-12, G′ and G″ intersect in the vicinity of ω = 40 s−1, which indicates that a rubbery plateau region probably exists in the higher frequency region than the measurable frequency range of the rheometer. From the viscoelasticity spectra, the steady state compliance, Je, and the terminal flow relaxation time, τw, are obtained independently (Figure S1). The concentration dependences of Je−1 and τw are shown in Figure 6. These viscoelastic parameters

observed in other wormlike micellar systems and is explained by a decrease of the number of entanglements by high speed flow in analogy to entangled polymer solutions.44−46 The zero-shear viscosity, η0, is determined from the viscosity at the low shear limit. The concentration dependence of η0 is plotted in Figure 4b for 12-3-C3C6F, 12-3-12, and DTAB solutions. Since DTAB forms only spherical micelles even at high concentrations in salt-free solutions, it has a very low viscosity as reproduced by Roscoe’s equation (η = η0(1 − 1.35ϕ)−2.5: η0 and ϕ are the solvent viscosity and the volume fraction, respectively) which is an empirical formula for a hard sphere dispersion.47 On the other hand, the gemini surfactants 12-3-C3C6F and 12-3-12 solutions showed a drastic increase in viscosity at low concentrations because of formation of the wormlike micelle. It is known that for a polymer solution without special interaction (good solvent system), η0 increases in proportion to c4.5.48 The concentration dependence for 12-3C3C6F and 12-3-12 solutions in 70−150 mmol/L is stronger than that for polymer solutions. The strong concentration dependence indicates a growth of average micelle length of the wormlike micelle. Such an increase in average micelle length with such concentrations is predicted by a theory6 and is consistent with experimental results with nonionic surfactant.49,50 Although the concentration dependence of the steep upturn in η0 is similar for both 12-3-C3C6F and 12-3-12 solutions, η0 of 12-3-C3C6F solution is higher than that of 12-312. The reason for the higher zero-shear viscosity will be discussed later using the results of dynamic viscoelasticity measurements. Furthermore, η0 reaches a maximum and then decreases with concentration for both gemini surfactant solutions. This behavior has already been observed in the previous studies of hydrocarbon-based gemini surfactants such as 12-3-12 series. According to In et al.,13,43 the decrease in η0 is due to branching. However, we propose another mechanism based on the change of the relaxation mechanism from the result of the dynamic viscoelasticity measurement described later. Viscoelastic Property. Figure 5 shows the angular frequency (ω) dependence of the storage modulus, G′, and

Figure 6. Concentration dependence of the reciprocal of the steady state compliance Je−1 (upper) and the terminal flow relaxation time τw (lower).

have the following relationship with the zero-shear viscosity, η0.48 η0 = Je−1τw

(12)

According to the viscoelastic theory for polymer solutions above the entanglement concentration, Je−1 is proportional to the rubbery plateau modulus, GN.48 Je−1 ∼ G N ∼ νkBT

(13)

Here, ν is the number density of entanglements and kB and T are the Boltzmann constant and absolute temperature, respectively. Je−1 increases with concentration in both 12-3C3C6F and 12-3-12. The increase of Je−1 means an increase of the density of entanglements, ν, with concentration. For polymer solutions, the concentration dependence of Je−1 is represented by a power law of Je−1 ∼ M0c2, where M is the molecular weight. The concentration dependence of Je−1 for 123-C3C6F and 12-3-12 is somewhat stronger than that of polymer solutions. The value of Je−1 for 12-3-C3C6F is higher than that of 12-3-12 at all concentrations. This means that the wormlike micelles of 12-3-C3C6F form more entanglements. This is consistent with the result of the aggregation number per unit

Figure 5. Angular frequency dependence of G′ (filled marker) and G″ (open marker): 12-3-C3C6F (left) and 12-3-12 (right).

the loss modulus, G″, obtained by dynamic viscoelasticity measurement. At any concentration, the terminal flow region as represented by the power law of G′ ∼ ω2 and G″ ∼ ω was observed at the low frequency limit. In the higher frequency region, a rubbery plateau region was observed for the high concentration of 12-3-C3C6F. At this concentration, the viscoelastic spectrum is expressed by the Maxwell model 6088

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Langmuir axial length obtained from SAXS measurement; i.e., the longer total axial length results in more ν (Figure 7).

Figure 8. Illustration of reptation relaxation and micelle scission relaxation.

Spacer Length Dependence. The spacer length dependence of zero-shear viscosity for 12-s-C3C6F and 12-s-12 (s = 3, 6, 12) solutions is shown in Figure 9. Both solutions exhibit a

Figure 7. Illustration of difference of ν and L between 12-3-C3C6F and 12-3-12.

The concentration dependence of τw has a maximum unlike the monotonically increasing of Je−1. This behavior corresponds to the behavior of η0 shown in Figure 6. This maximum has been also observed for other gemini surfactant solutions.13 The previous report has attributed the maximum to branching. However, this explanation lacks rheological evidence; even if branches are formed, the viscoelastic relaxation time does not decrease in the entangled concentration regime. In this study, we explained this behavior with another hypothesis based on the rheological viewpoint. To understand this concentration dependence, two viscoelastic relaxation mechanisms must be taken into consideration. According to the previous studies,51−54 a viscoelastic relaxation of entangled wormlike micelles occurs by (a) a reptation similar to polymer systems (reptation mode) or (b) a scission of the wormlike micelles (scission mode) (Figure 8). The relaxation time of the reptation mode increases with the concentration c and the contour length L as τrep ∼ c1.4L3.4, while that of the scission mode is a decreasing function of c and L as τsci ∼ c−1L−1. The experimental result of this study is explained by a switching of these dominant relaxation mechanisms, i.e., a switching from reptation relaxation to scission relaxation at the concentration cmax where τw reaches a maximum (cmax = 100 mmol/L for 12-3C3C6F and cmax ≤ 200 mmol/L for 12-3-12); the reptation mode is dominant in c < cmax, and the scission mode, in c > cmax. This switching of the relaxation mechanisms can be observed in the linear viscoelastic behavior (Figure S2). In Figure 6b, τw of 12-3-C3C6F is longer than that of 12-3-12 in c < cmax, and the magnitude is reversed at higher concentrations (c > 200 mmol/ L). Considering the theoretical prediction of τrep and τsci, we conclude that the average contour length of a wormlike micelle of 12-3-C3C6F is longer than that of 12-3-12. As described above, the high viscosity of 12-3-C3C6F solution is caused by the high values of ν and L (Figure 7).

Figure 9. Zero-shear viscosity of 12-s-C3C6F (left) and 12-s-12 (right).

similar s-dependence; η0(s=3) > η0(s=12) > η0(s=6). This spacer length dependence is consistent with the previous studies of cmc, the free energy of micellization, and aggregation number for hydrocarbon−hydrocarbon-type gemini surfactants.1,4,14 These physical parameters have been reported to have a maximum or a minimum at the vicinity of s = 6. This tendency is caused by the packing parameter. For s ≤ 6, a longer spacer chain makes the packing parameter smaller because the headgroup of a surfactant becomes larger (i.e., a0 become larger). Meanwhile, for s > 6, a long spacer chain works as a hydrophobic group, which makes the packing parameters large. As a result, the s-dependence of the packing parameter exhibits a minimum, which is responsible for the maximum of viscosity. We demonstrate that the tendency found in hydrocarbon−hydrocarbon-type gemini surfactants is also applied to the 12-s-C3C6F system.

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CONCLUSION We investigated the structure and rheological properties of micelles formed by fluorocarbon−-hydrocarbon-type gemini surfactant (12-3-C3C6F) in an aqueous solution by means of SAXS and viscoelasticity measurements. As a result of the model fitting analysis of SAXS profiles, we characterized the wormlike micelles formed by the gemini surfactants 12-3-C3C6F and the corresponding hydrocarbon-type gemini surfactants (12-3-12). The obtained cross-sectional shape (core radius, 6089

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Rcore, and shell thickness, dshell) and the aggregation number per unit axial length of the wormlike micelles of 12-3-C3C6F are smaller than those of 12-3-12. This structure reflects the rheological properties. The zero-shear viscosity of 12-3-C3C6F solution is higher than that of 12-3-12. This reason was elucidated from the viscoelastic parameters Je and τw obtained from dynamic viscoelastic measurements. By comparing Je and τw between 123-C3C6F and 12-3-12 systems, we concluded that the micelles of 12-3-C3C6F have a larger number of entanglements and longer contour length than that of 12-3-12. The former is consistent with the SAXS result. The latter is related to the end-cap energy; a wormlike micelle of 12-3-C3C6F is harder to make an end-cap than that of 12-3-12. Strong hydrophobicity of fluorocarbon is responsible for a larger number of entanglements and longer contour length. In conclusion, we demonstrated that the introduction of fluorocarbon to gemini surfactant leads to superior viscoelastic properties, and its origin was explained by the micellar structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00902. Fitting parameters from SAXS measurements, illustration for determining Je and τw from dynamic viscoelastic spectrum, and experiment vs Maxwell model comparison for 12-3-C3C6F (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tomokazu Yoshimura: 0000-0001-8283-1032 Mitsuhiro Shibayama: 0000-0002-8683-5070 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sport, Science, and Technology (No. 16H02277 to M.S.) The SAXS experiment was performed at the Frontier Soft Matter Beamline (FSBL; BL03XU) in SPring-8 (Proposal No. 2016A7210) and BL10C in KEK-PF (Proposal No. 2016G538). We are grateful to Atsushi Izumi (Sumitomo Bakelite, Co., Ltd) for assistance with SAXS measurement. This work was supported by the Photon and Quantum Basic Research Coordinated Development Program by MEXT Grant No. 13004017.

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REFERENCES

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