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Rheological properties of silicone-surfactant-based wormlike micellar solution
Colloids and Surfaces A 581 (2019) 123841
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Rheological properties of silicone-surfactant-based wormlike micellar solution
T
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Kenji Aramakia, , Misaki Fujiia, Yuichi Sakanishib a b
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan Organic Chemical Products Company, DAICEL Corporation, 2-18-1,Konan, Minato-ku, Tokyo, 108-8230, Japan
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Wormlike micelle Silicone surfactant Anionic surfactant Viscoelasticity Low-temperature stability
One of the major drawbacks of ionic-surfactant-based wormlike micelles (WLMs) is their poor low-temperature stability, which severely limits their application. Generally, an ionic surfactant with a linear C12–C16 alkyl chain has a Krafft point around or above room temperature. Therefore, there is a demand for ionic-surfactant-based WLMs with greater lowtemperature stability. Silicone surfactants have flexible hydrophobic chain structures. Hence, they are expected to be suitable candidates for WLMs that can be stable at low temperatures. Sodium trimethylsilyl tetra(dimethylsiloxane) decyl sulfate (Si5C10SO4Na) and benzyltrimethylammonium chloride (BTAC) were used as surfactant and electrolyte, respectively, to obtain the WLM system with desired properties. No precipitation of surfactant crystals was observed in the WLM system obtained in this study when temperature was decreased to as low as 0 °C, indicating excellent low-temperature stability. Steady rheological measurements on the viscous solutions show shear thinning corresponding nearly to a power law relation (viscosity ∝ [shear rate]−1). Zero shear viscosity (η0) increased with increase in R, reaching a maximum at around R = 0.3 and decreasing thereafter. Oscillatory shear measurements for the viscoelastic samples, formed around the maximum-viscosity composition, show that the storage modulus (G′) and the loss modulus (G′′), with respect to the oscillation frequency (ω), cross each other and fit the Maxwell model very well in the low-ω region. The normalized Cole-Cole plot of G′′ / G′′max against G′ / G′′max was obtained as a semicircle centered at G′ / G′′max = 1, as is typical for WLM systems.
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Corresponding author. E-mail address: [email protected] (K. Aramaki).
https://doi.org/10.1016/j.colsurfa.2019.123841 Received 19 June 2019; Received in revised form 19 August 2019; Accepted 19 August 2019 Available online 20 August 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 581 (2019) 123841
K. Aramaki, et al.
1. Introduction
(PTHC, 98%) and benzyltrimethylammonium chloride (BTAC, 99.0%,) were purchased from Tokyo Chemical Industry Co., Japan. All the chemicals were used as received. Deionized (Millipore filtered, conductivity 15–25 μS mol−1) water was used as solvent.
Viscoelastic solutions or gels can be obtained in surfactant self-assemblies, such as lyotropic liquid crystals, α-type hydrated crystals, and wormlike micelles (WLMs). WLMs are flexible tubular micelles with lengths in the range from several hundreds nanometers to several microns. They are formed due to the sphere-to-rod transition followed by one-dimensional micellar growth under particular temperature and concentration conditions. Entangled WLMs form a transient network to thicken the aqueous solution. Viscosity of WLM solutions with a relatively high surfactant concentration was found to be 5–6 orders of magnitude higher than that of water. WLMs can be formed by mixing appropriate amounts of ionic [1–3] and nonionic surfactants [4–6] in an aqueous system. Such mixed surfactant systems are typically found in formulations of detergents and personal care products, where the WLMs improve foam stability and detergency. Ionic surfactants in the presence of electrolytes are also typically used to obtain WLMs. Aqueous cationic surfactant solutions in presence of electrolytes such as alkyl quaternary ammonium salts or alkyl pyridinium salts with sodium salicylate have been extensively studied both from a basic [7–11] as well as from an application point of view. These studies focused especially on the drag reduction (DR) effect [12–16], which reduces energy consumption in liquid transport or circulation systems. Anionic surfactants with electrolytes [17,18] and catanionic mixtures [19–26] can also form WLMs. One of the drawbacks of such ionic-surfactant-based WLMs is that they are highly unstable at low-temperatures. This consequently limits their application. The Krafft point of an ionic surfactant with a linear C12 - C16 alkyl chain is generally around or above the room temperature, e.g. 25–27 °C for hexadecyltrimethylammonium bromide (CTAB) [27], and 15–20 °C for sodium dodecyl sulfate (SDS) [27]. It is therefore necessary to develop ionic-surfactant-based WLMs that are stable at low-temperatures. Silicone surfactants have unique properties such as flexible chain structure and strong surface tension lowering ability or their surface activity in both aqueous and nonaqueous systems. Hence, they are regularly used for various applications, and novel functionalities have been proposed in the recent years, e.g. their role in methane hydrate formation [28], their antibacterial properties [29], and their use as organic templates of nanomaterial preparation [30–32]. In this work, we found that an anionic silicone surfactant, namely sodium trimethylsilyl tetra(dimethylsiloxane) decyl sulfate (Si5C10SO4Na), can form WLMs in the presence of an electrolyte. We also demonstrated that the WLM system shows increased low-temperature stability because of the high flexibility of the hydrophobic tail of the surfactant – a dimethylsiloxane chain. We also studied rheological properties of this highly viscous WLM system by static and dynamic rheological measurements. There are only a few reports on silicone-surfactant-based WLM formation [33–37] and, in addition, these are limited in dilute systems and not studied viscoelastic properties.
2.2. Sample preparation All the components were weighed in screw-capped test tubes and homogenized by using a vortex mixer. Then, the samples were kept at 25 °C for at least 24 h. The concentration of Si5C10SO4Na in water was kept fixed at 150 mM. The composition of the added electrolytes is denoted by the electrolyte/Si5C10SO4Na molar ratio (R). 2.3. Conductivity measurement Conductivity measurements were performed by using DS-72, Horiba, Japan. The conductivity cell used was of the model 3551-10D (cell constant: 0.1012 m−1). A magnetic stirrer was used to mix the system when changing the temperature. 2.4. Surface tension measurement Surface tension measurements were performed by using a Wilhelmy-type automatic surface balance model K100 (Krüss, Germany) at 25◦C. 2.5. Rheological measurement The rheological measurements were carried out by using an AR-G2 rheometer (TA Instruments, U. S. A.). A cone-and-plate geometry with a cone angle of 1° was used. We used two different upper (moving) plates of diameters 60 and 40 mm, depending on the viscosity of the samples. The sample plate temperature was controlled by a Peltier device at 25 °C. Oscillation-strain sweep tests were performed with a constant oscillation frequency at 1 Hz to determine linear viscoelastic region. Subsequently, oscillation-frequency sweep tests were performed (0.01–100 rad s−1). 3. Results and discussion 3.1. Micellization of Si5C10SO4Na in water The critical micelle concentration (CMC) of Si5C10SO4Na in water was measured by the conductivity and the surface tension method. Fig. 2 shows conductivity or surface tension plotted against the change in concentration. The CMC of Si5C10SO4Na was determined to be 0.027 mM at 25 °C from the inflection point of the conductivity plot, which is a reasonable value by considering the CMC shown in the surface tension plot. We also performed CMC measurements in the presence of electrolyte BTAC as shown in Fig. S1.The CMC in the presence of BTAC was obtained as 0.06 mM which is slightly higher than the BTAC-free system. The degree of dissociation of the ionic surfactant around CMC (α) is estimated by the ratio S2/S1, where S1 and S2 are the slope of the conductivity plot above and below CMC, respectively [38]. The value of α was calculated to be 0.71 for the BTAC-free system while 0.90 for the BTAC-added system. Higher CMC and α value in the BTACadded system indicates BTAC is a chaotropic agent.
2. Experimental 2.1. Materials The anionic surfactant, sodium trimethylsilyl tetra(dimethylsiloxane) decyl sulfate (Si5C10SO4Na) (chemical structure shown in Fig. 1) was obtained from Daicel. Co., Japan. p-Toluidine hydrochloride
3.2. Steady shear viscosity and low-temperature stability It is well known that WLMs are formed by adding an organic electrolyte such as PTHC to an aqueous solution of SDS, resulting in an abrupt increase in the viscosity of the solution [17]. We examined whether the same phenomenon can be seen for an aqueous solution of Si5C10SO4Na, which has the same hydrophilic group as SDS. When PTHC was added to 150 mM aqueous solution of Si5C10SO4Na, a highly
Fig. 1. Chemical structure of Si5C10SO4Na. 2
Colloids and Surfaces A 581 (2019) 123841
K. Aramaki, et al.
Fig. 2. Conductivity and surface tension as a function of surfactant concentration in the water/Si5C10SO4Na system at 25 °C.
[39]. The onset of non-Newtonian behavior shifts to a lower shear rate when R increased from 0.288 to 0.332 while to higher shear rates when R > 0.364. For highly viscous samples with R in the range 0.288–0.364, flow birefringence was observed through crossed Nicols, suggesting that micelles with high anisotropy were formed. Since this is frequently observed in WLM solutions, it strongly suggests the formation of WLMs in this system as well. A zero-shear viscosity (η0) at each BTAC/Si5C10SO4Na ratio (R) was obtained from Fig. 4 as an extrapolation of Newtonian plateau and η0 is plotted against R in Fig. 5. The viscosity increases with increase in R, reaching a maximum, and decreasing thereafter One can understand the increase in viscosity below the maximum viscosity composition as the contribution of one-dimensional micellar growth. Entangled WLMs should be formed at around the maximum viscosity composition. Above the maximum viscosity composition, the viscosity is decreased because of the formation of a multi-connected network of micelles or branched micelles, that suppresses the solution viscosity due to the mobility of the branched points along the contours of the WLMs [40]. The maximum η0 is reached to several hundred Pa s, which is almost similar value in the other ionic surfactant-based system (SDS with polyoxyethylene alkyl ether) at the same surfactant concentration [41]. In the SDS-PTHC system, the surfactant composition dependence of η0 was reported. The η0 is reached to a maximum at 75 mM surfactant but the value is less than 1 Pa s [17], which is much lower value of the present system.
Fig. 3. Photo of samples at R = 0.288 and 0.332 immediately after being kept at 5 °C.
viscous solution was obtained for R ≈ 0.35, which suggests WLM formation. However, PTHC has low chemical stability, and after 1 day or more, this sample turned brown in color and the viscosity also decreased. Therefore, the same study was carried out using BTAC, which is also an organic electrolyte but with higher chemical stability than PTHC. A highly viscous solution was now obtained at around R = 0.3, with no change in appearance and viscosity over a considerable period. The sample was placed in a constant temperature bath, and precipitation of the surfactant crystal was not observed at 5 °C as shown in Fig. 3. Hence, it was shown that a WLM solution with excellent chemical and low-temperature stability can be obtained by using the silicone surfactant Si5C10SO4Na. It is well known that a WLM aqueous solution with relatively high surfactant concentration acts as a non-Newtonian viscoelastic body, exhibiting high viscosity. We encountered instances of such high viscosity in the present system and carried out rheological measurements to ascertain its behavior. The results from steady-state viscosity measurements for the water/BTAC/Si5C10SO4Na system ([Si5C10SO4Na] = 0.15 M) at various BTAC/Si5C10SO4Na ratios (R) are shown in Fig. 4. The viscosity remains almost constant for the values of R between 0 and 0.155, indicating Newtonian behavior. For R = 0.288 and above, however, Shear thinning behavior, namely monotonical decrease in viscosity with increasing shear rate above a certain shear rate, is observed. The shear thinning region shows slightly less steep decay compared to a power low relation η ∝ [shear rate]−1, which is often seen in other wormlike micellar systems due to shear banding
3.3. Dynamic rheological behavior We performed frequency sweep (oscillatory-shear) measurements for the samples around the maximum viscosity composition in the water/BTAC/Si5C10SO4Na system. Fig. 6(a)–(c) are representative plots for the change in the elastic (storage) modulus (G′) and the viscous (loss) modulus (G′′) with respect to oscillation frequency (ω). In the low-frequency region, G′′ was superior than G′, which is a liquid-like behavior. On the other hand, solid-like behavior (G′ > G′′) was observed in the high-frequency region. Such a crossover of G′ and G′′ curves are typically observed in general WLM systems. The Maxwell model expressed by the Eqs. (1) and (2) [42] can generally be fitted to the G′ and G′′ curves in WLM systems with a single relaxation time (τR).
G′ =
ω2τR2 G0 1 + ω2τR2
(1)
G" =
ωτR G0 1 + ω2τR2
(2)
The plateau modulus (G0) is given by the fixed value G′ at high ω. The relaxation time (τR) can be estimated from the equation: τR = 1/ωc
(3)
where ωc is the frequency at the crossover of the G′ and G′′ curves. The 3
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Fig. 4. Plot of viscosity for the micellar solutions of the water/BTAC/Si5C10SO4Na system ([Si5C10SO4Na] =150 mM) at 25 °C vs. shear rate at various BTAC/ Si5C10SO4Na molar ratios (R). (left) R = 0–0.332, (right) R = 0.364–0.576.
G′ and G′′ could be fitted well to the Maxwell model in the low ω region; however, in the high ω region, deviation from the model for the G′′ curve by showing a rise with increase in ω, even after reaching a minimum. This deviation from the Maxwell model in the high ω region is considered to be due to the transition of the relaxation mode from the slower to the faster mode such as the Rouse modes [43]. It can also be seen in the normalized Cole-Cole plot of G′′ / G′′ max against G′ / G′′ max [44] for the samples at R = 0.288, 0.332 and 0.364 as shown in Fig. 6(d). In this plot, the Maxwell model is described as a semicircle, centered at G′ / G′′ max = 1. Among three compositions, one can clearly see the sample at R = 0.332 follows the most to the Maxwell model. One can obtain G0 and τR by fitting the oscillatory-shear measurements data to the Maxwell model. The values for G0 and τR are shown in Table 1. As can be seen, G0 was lower and τR was longer at R = 0.332 than at R = 0.288. The plateau modulus, G0, can be related to the hydrodynamic correlation length or the mesh size of the entangled WLM network, ξ, by the following expression [45].
Fig. 5. Variation of zero-shear viscosity (η0) as a function of the BTAC/ Si5C10SO4Na ratio (R), for the water/BTAC/Si5C10SO4Na system ([Si5C10SO4Na] =150 mM) at 25 °C.
G0 ≈ kB Tξ −3
(4)
The mesh size, ξ, shown in Table 1, was almost the same for R = Fig. 6. Variation of storage modulus, G′ (filled symbols) and loss modulus, G′′ (open symbols) as a function of oscillatory-shear frequency (ω) in the water/ BTAC/Si5C10SO4Na system ([Si5C10SO4Na] =150 mM) at 25 °C for (a) R = 0.288, (b) R = 0.332, and (c) R = 0.364. Solid and dotted lines represent the Maxwell model. (d) Normalized Cole-Cole plot at R = 0.288, 0.332 and 0.364.
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Table 1 Plateau modulus (G0) and relaxation time (τR) for the water/BTAC/ Si5C10SO4Na system ([Si5C10SO4Na] =150 mM) at 25 °C. R
G0 / Pa
τR / s
ξ / nm
η0 / Pa⋅s
0.288 0.332
130 129
4.0 6.3
31.6 31.7
460 675
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0.288 and 0.332. As described by the following equation, the relaxation time τR is determined by the two characteristic times τB and τrep, which correspond to the scission/recombination time of WLMs and the chain repetition time. [45] τR ∼ (τBτrep)1/2
(5)
Maxwell model behavior is generally observed under τB ≪ τrep. Therefore the τR is almost the reflection of the τrep, which is also the reflection of the length and volume fraction of the micelles, according to the following relation [46].
τrep ∼ L¯ 3Φ3/2
(6)
where L is the length of the WLMs and φ is the volume fraction of surfactant. Hence, an increase in τR can imply an increase in τrep which can be caused by an increase in L , as can be seen from Eqs. (5) and (6), with φ being almost constant. The τR value at R = 0.332 was longer than at R = 0.288, indicating longer WLMs, and thus, higher viscosity at R = 0.288, considering that mesh size or network density was similar in both cases. 4. Conclusions We report the formulation of ionic-surfactant-based WLMs with enhanced low-temperature stability using a silicone surfactant, sodium trimethylsilyl tetra(dimethylsiloxane) decyl sulfate (Si5C10SO4Na). In the presence of benzyltrimethylammonium chloride (BTAC) as the electrolyte, a highly viscous solution was obtained at an appropriate molar ratio of electrolyte to surfactant. As expected, no precipitation of the surfactant crystal was observed above 0 °C, establishing its lowtemperatures stability. Steady and dynamic rheological measurements on the viscous solutions confirm typical rheological behavior of an entangled WLM system such as shear thinning, which almost follows a power law relation ( η ∝ [shear rate]−1); and they result in a crossover of the storage modulus (G′) and the loss modulus (G′′) curves w.r.t. oscillation frequency (ω), which follows the Maxwell model. This study clearly demonstrates the formulation of an anionic WLM system which is stable at low-temperatures, and can be highly beneficial for various industrial applications. Additionally, to the best of our knowledge, this paper is the first to report on rheological properties of a viscoelastic WLM system with a silicone surfactant. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.123841. References [1] V. Croce, T. Cosgrove, C.A. Dreiss, G. Maitland, T. Hughes, G. Karlsson, Impacting the length of wormlike micelles using mixed surfactant systems, Langmuir 20
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Rheological properties of silicone-surfactant-based wormlike micellar solution
T
⁎
Kenji Aramakia, , Misaki Fujiia, Yuichi Sakanishib a b
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan Organic Chemical Products Company, DAICEL Corporation, 2-18-1,Konan, Minato-ku, Tokyo, 108-8230, Japan
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Wormlike micelle Silicone surfactant Anionic surfactant Viscoelasticity Low-temperature stability
One of the major drawbacks of ionic-surfactant-based wormlike micelles (WLMs) is their poor low-temperature stability, which severely limits their application. Generally, an ionic surfactant with a linear C12–C16 alkyl chain has a Krafft point around or above room temperature. Therefore, there is a demand for ionic-surfactant-based WLMs with greater lowtemperature stability. Silicone surfactants have flexible hydrophobic chain structures. Hence, they are expected to be suitable candidates for WLMs that can be stable at low temperatures. Sodium trimethylsilyl tetra(dimethylsiloxane) decyl sulfate (Si5C10SO4Na) and benzyltrimethylammonium chloride (BTAC) were used as surfactant and electrolyte, respectively, to obtain the WLM system with desired properties. No precipitation of surfactant crystals was observed in the WLM system obtained in this study when temperature was decreased to as low as 0 °C, indicating excellent low-temperature stability. Steady rheological measurements on the viscous solutions show shear thinning corresponding nearly to a power law relation (viscosity ∝ [shear rate]−1). Zero shear viscosity (η0) increased with increase in R, reaching a maximum at around R = 0.3 and decreasing thereafter. Oscillatory shear measurements for the viscoelastic samples, formed around the maximum-viscosity composition, show that the storage modulus (G′) and the loss modulus (G′′), with respect to the oscillation frequency (ω), cross each other and fit the Maxwell model very well in the low-ω region. The normalized Cole-Cole plot of G′′ / G′′max against G′ / G′′max was obtained as a semicircle centered at G′ / G′′max = 1, as is typical for WLM systems.
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Corresponding author. E-mail address: [email protected] (K. Aramaki).
https://doi.org/10.1016/j.colsurfa.2019.123841 Received 19 June 2019; Received in revised form 19 August 2019; Accepted 19 August 2019 Available online 20 August 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 581 (2019) 123841
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1. Introduction
(PTHC, 98%) and benzyltrimethylammonium chloride (BTAC, 99.0%,) were purchased from Tokyo Chemical Industry Co., Japan. All the chemicals were used as received. Deionized (Millipore filtered, conductivity 15–25 μS mol−1) water was used as solvent.
Viscoelastic solutions or gels can be obtained in surfactant self-assemblies, such as lyotropic liquid crystals, α-type hydrated crystals, and wormlike micelles (WLMs). WLMs are flexible tubular micelles with lengths in the range from several hundreds nanometers to several microns. They are formed due to the sphere-to-rod transition followed by one-dimensional micellar growth under particular temperature and concentration conditions. Entangled WLMs form a transient network to thicken the aqueous solution. Viscosity of WLM solutions with a relatively high surfactant concentration was found to be 5–6 orders of magnitude higher than that of water. WLMs can be formed by mixing appropriate amounts of ionic [1–3] and nonionic surfactants [4–6] in an aqueous system. Such mixed surfactant systems are typically found in formulations of detergents and personal care products, where the WLMs improve foam stability and detergency. Ionic surfactants in the presence of electrolytes are also typically used to obtain WLMs. Aqueous cationic surfactant solutions in presence of electrolytes such as alkyl quaternary ammonium salts or alkyl pyridinium salts with sodium salicylate have been extensively studied both from a basic [7–11] as well as from an application point of view. These studies focused especially on the drag reduction (DR) effect [12–16], which reduces energy consumption in liquid transport or circulation systems. Anionic surfactants with electrolytes [17,18] and catanionic mixtures [19–26] can also form WLMs. One of the drawbacks of such ionic-surfactant-based WLMs is that they are highly unstable at low-temperatures. This consequently limits their application. The Krafft point of an ionic surfactant with a linear C12 - C16 alkyl chain is generally around or above the room temperature, e.g. 25–27 °C for hexadecyltrimethylammonium bromide (CTAB) [27], and 15–20 °C for sodium dodecyl sulfate (SDS) [27]. It is therefore necessary to develop ionic-surfactant-based WLMs that are stable at low-temperatures. Silicone surfactants have unique properties such as flexible chain structure and strong surface tension lowering ability or their surface activity in both aqueous and nonaqueous systems. Hence, they are regularly used for various applications, and novel functionalities have been proposed in the recent years, e.g. their role in methane hydrate formation [28], their antibacterial properties [29], and their use as organic templates of nanomaterial preparation [30–32]. In this work, we found that an anionic silicone surfactant, namely sodium trimethylsilyl tetra(dimethylsiloxane) decyl sulfate (Si5C10SO4Na), can form WLMs in the presence of an electrolyte. We also demonstrated that the WLM system shows increased low-temperature stability because of the high flexibility of the hydrophobic tail of the surfactant – a dimethylsiloxane chain. We also studied rheological properties of this highly viscous WLM system by static and dynamic rheological measurements. There are only a few reports on silicone-surfactant-based WLM formation [33–37] and, in addition, these are limited in dilute systems and not studied viscoelastic properties.
2.2. Sample preparation All the components were weighed in screw-capped test tubes and homogenized by using a vortex mixer. Then, the samples were kept at 25 °C for at least 24 h. The concentration of Si5C10SO4Na in water was kept fixed at 150 mM. The composition of the added electrolytes is denoted by the electrolyte/Si5C10SO4Na molar ratio (R). 2.3. Conductivity measurement Conductivity measurements were performed by using DS-72, Horiba, Japan. The conductivity cell used was of the model 3551-10D (cell constant: 0.1012 m−1). A magnetic stirrer was used to mix the system when changing the temperature. 2.4. Surface tension measurement Surface tension measurements were performed by using a Wilhelmy-type automatic surface balance model K100 (Krüss, Germany) at 25◦C. 2.5. Rheological measurement The rheological measurements were carried out by using an AR-G2 rheometer (TA Instruments, U. S. A.). A cone-and-plate geometry with a cone angle of 1° was used. We used two different upper (moving) plates of diameters 60 and 40 mm, depending on the viscosity of the samples. The sample plate temperature was controlled by a Peltier device at 25 °C. Oscillation-strain sweep tests were performed with a constant oscillation frequency at 1 Hz to determine linear viscoelastic region. Subsequently, oscillation-frequency sweep tests were performed (0.01–100 rad s−1). 3. Results and discussion 3.1. Micellization of Si5C10SO4Na in water The critical micelle concentration (CMC) of Si5C10SO4Na in water was measured by the conductivity and the surface tension method. Fig. 2 shows conductivity or surface tension plotted against the change in concentration. The CMC of Si5C10SO4Na was determined to be 0.027 mM at 25 °C from the inflection point of the conductivity plot, which is a reasonable value by considering the CMC shown in the surface tension plot. We also performed CMC measurements in the presence of electrolyte BTAC as shown in Fig. S1.The CMC in the presence of BTAC was obtained as 0.06 mM which is slightly higher than the BTAC-free system. The degree of dissociation of the ionic surfactant around CMC (α) is estimated by the ratio S2/S1, where S1 and S2 are the slope of the conductivity plot above and below CMC, respectively [38]. The value of α was calculated to be 0.71 for the BTAC-free system while 0.90 for the BTAC-added system. Higher CMC and α value in the BTACadded system indicates BTAC is a chaotropic agent.
2. Experimental 2.1. Materials The anionic surfactant, sodium trimethylsilyl tetra(dimethylsiloxane) decyl sulfate (Si5C10SO4Na) (chemical structure shown in Fig. 1) was obtained from Daicel. Co., Japan. p-Toluidine hydrochloride
3.2. Steady shear viscosity and low-temperature stability It is well known that WLMs are formed by adding an organic electrolyte such as PTHC to an aqueous solution of SDS, resulting in an abrupt increase in the viscosity of the solution [17]. We examined whether the same phenomenon can be seen for an aqueous solution of Si5C10SO4Na, which has the same hydrophilic group as SDS. When PTHC was added to 150 mM aqueous solution of Si5C10SO4Na, a highly
Fig. 1. Chemical structure of Si5C10SO4Na. 2
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Fig. 2. Conductivity and surface tension as a function of surfactant concentration in the water/Si5C10SO4Na system at 25 °C.
[39]. The onset of non-Newtonian behavior shifts to a lower shear rate when R increased from 0.288 to 0.332 while to higher shear rates when R > 0.364. For highly viscous samples with R in the range 0.288–0.364, flow birefringence was observed through crossed Nicols, suggesting that micelles with high anisotropy were formed. Since this is frequently observed in WLM solutions, it strongly suggests the formation of WLMs in this system as well. A zero-shear viscosity (η0) at each BTAC/Si5C10SO4Na ratio (R) was obtained from Fig. 4 as an extrapolation of Newtonian plateau and η0 is plotted against R in Fig. 5. The viscosity increases with increase in R, reaching a maximum, and decreasing thereafter One can understand the increase in viscosity below the maximum viscosity composition as the contribution of one-dimensional micellar growth. Entangled WLMs should be formed at around the maximum viscosity composition. Above the maximum viscosity composition, the viscosity is decreased because of the formation of a multi-connected network of micelles or branched micelles, that suppresses the solution viscosity due to the mobility of the branched points along the contours of the WLMs [40]. The maximum η0 is reached to several hundred Pa s, which is almost similar value in the other ionic surfactant-based system (SDS with polyoxyethylene alkyl ether) at the same surfactant concentration [41]. In the SDS-PTHC system, the surfactant composition dependence of η0 was reported. The η0 is reached to a maximum at 75 mM surfactant but the value is less than 1 Pa s [17], which is much lower value of the present system.
Fig. 3. Photo of samples at R = 0.288 and 0.332 immediately after being kept at 5 °C.
viscous solution was obtained for R ≈ 0.35, which suggests WLM formation. However, PTHC has low chemical stability, and after 1 day or more, this sample turned brown in color and the viscosity also decreased. Therefore, the same study was carried out using BTAC, which is also an organic electrolyte but with higher chemical stability than PTHC. A highly viscous solution was now obtained at around R = 0.3, with no change in appearance and viscosity over a considerable period. The sample was placed in a constant temperature bath, and precipitation of the surfactant crystal was not observed at 5 °C as shown in Fig. 3. Hence, it was shown that a WLM solution with excellent chemical and low-temperature stability can be obtained by using the silicone surfactant Si5C10SO4Na. It is well known that a WLM aqueous solution with relatively high surfactant concentration acts as a non-Newtonian viscoelastic body, exhibiting high viscosity. We encountered instances of such high viscosity in the present system and carried out rheological measurements to ascertain its behavior. The results from steady-state viscosity measurements for the water/BTAC/Si5C10SO4Na system ([Si5C10SO4Na] = 0.15 M) at various BTAC/Si5C10SO4Na ratios (R) are shown in Fig. 4. The viscosity remains almost constant for the values of R between 0 and 0.155, indicating Newtonian behavior. For R = 0.288 and above, however, Shear thinning behavior, namely monotonical decrease in viscosity with increasing shear rate above a certain shear rate, is observed. The shear thinning region shows slightly less steep decay compared to a power low relation η ∝ [shear rate]−1, which is often seen in other wormlike micellar systems due to shear banding
3.3. Dynamic rheological behavior We performed frequency sweep (oscillatory-shear) measurements for the samples around the maximum viscosity composition in the water/BTAC/Si5C10SO4Na system. Fig. 6(a)–(c) are representative plots for the change in the elastic (storage) modulus (G′) and the viscous (loss) modulus (G′′) with respect to oscillation frequency (ω). In the low-frequency region, G′′ was superior than G′, which is a liquid-like behavior. On the other hand, solid-like behavior (G′ > G′′) was observed in the high-frequency region. Such a crossover of G′ and G′′ curves are typically observed in general WLM systems. The Maxwell model expressed by the Eqs. (1) and (2) [42] can generally be fitted to the G′ and G′′ curves in WLM systems with a single relaxation time (τR).
G′ =
ω2τR2 G0 1 + ω2τR2
(1)
G" =
ωτR G0 1 + ω2τR2
(2)
The plateau modulus (G0) is given by the fixed value G′ at high ω. The relaxation time (τR) can be estimated from the equation: τR = 1/ωc
(3)
where ωc is the frequency at the crossover of the G′ and G′′ curves. The 3
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Fig. 4. Plot of viscosity for the micellar solutions of the water/BTAC/Si5C10SO4Na system ([Si5C10SO4Na] =150 mM) at 25 °C vs. shear rate at various BTAC/ Si5C10SO4Na molar ratios (R). (left) R = 0–0.332, (right) R = 0.364–0.576.
G′ and G′′ could be fitted well to the Maxwell model in the low ω region; however, in the high ω region, deviation from the model for the G′′ curve by showing a rise with increase in ω, even after reaching a minimum. This deviation from the Maxwell model in the high ω region is considered to be due to the transition of the relaxation mode from the slower to the faster mode such as the Rouse modes [43]. It can also be seen in the normalized Cole-Cole plot of G′′ / G′′ max against G′ / G′′ max [44] for the samples at R = 0.288, 0.332 and 0.364 as shown in Fig. 6(d). In this plot, the Maxwell model is described as a semicircle, centered at G′ / G′′ max = 1. Among three compositions, one can clearly see the sample at R = 0.332 follows the most to the Maxwell model. One can obtain G0 and τR by fitting the oscillatory-shear measurements data to the Maxwell model. The values for G0 and τR are shown in Table 1. As can be seen, G0 was lower and τR was longer at R = 0.332 than at R = 0.288. The plateau modulus, G0, can be related to the hydrodynamic correlation length or the mesh size of the entangled WLM network, ξ, by the following expression [45].
Fig. 5. Variation of zero-shear viscosity (η0) as a function of the BTAC/ Si5C10SO4Na ratio (R), for the water/BTAC/Si5C10SO4Na system ([Si5C10SO4Na] =150 mM) at 25 °C.
G0 ≈ kB Tξ −3
(4)
The mesh size, ξ, shown in Table 1, was almost the same for R = Fig. 6. Variation of storage modulus, G′ (filled symbols) and loss modulus, G′′ (open symbols) as a function of oscillatory-shear frequency (ω) in the water/ BTAC/Si5C10SO4Na system ([Si5C10SO4Na] =150 mM) at 25 °C for (a) R = 0.288, (b) R = 0.332, and (c) R = 0.364. Solid and dotted lines represent the Maxwell model. (d) Normalized Cole-Cole plot at R = 0.288, 0.332 and 0.364.
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Table 1 Plateau modulus (G0) and relaxation time (τR) for the water/BTAC/ Si5C10SO4Na system ([Si5C10SO4Na] =150 mM) at 25 °C. R
G0 / Pa
τR / s
ξ / nm
η0 / Pa⋅s
0.288 0.332
130 129
4.0 6.3
31.6 31.7
460 675
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0.288 and 0.332. As described by the following equation, the relaxation time τR is determined by the two characteristic times τB and τrep, which correspond to the scission/recombination time of WLMs and the chain repetition time. [45] τR ∼ (τBτrep)1/2
(5)
Maxwell model behavior is generally observed under τB ≪ τrep. Therefore the τR is almost the reflection of the τrep, which is also the reflection of the length and volume fraction of the micelles, according to the following relation [46].
τrep ∼ L¯ 3Φ3/2
(6)
where L is the length of the WLMs and φ is the volume fraction of surfactant. Hence, an increase in τR can imply an increase in τrep which can be caused by an increase in L , as can be seen from Eqs. (5) and (6), with φ being almost constant. The τR value at R = 0.332 was longer than at R = 0.288, indicating longer WLMs, and thus, higher viscosity at R = 0.288, considering that mesh size or network density was similar in both cases. 4. Conclusions We report the formulation of ionic-surfactant-based WLMs with enhanced low-temperature stability using a silicone surfactant, sodium trimethylsilyl tetra(dimethylsiloxane) decyl sulfate (Si5C10SO4Na). In the presence of benzyltrimethylammonium chloride (BTAC) as the electrolyte, a highly viscous solution was obtained at an appropriate molar ratio of electrolyte to surfactant. As expected, no precipitation of the surfactant crystal was observed above 0 °C, establishing its lowtemperatures stability. Steady and dynamic rheological measurements on the viscous solutions confirm typical rheological behavior of an entangled WLM system such as shear thinning, which almost follows a power law relation ( η ∝ [shear rate]−1); and they result in a crossover of the storage modulus (G′) and the loss modulus (G′′) curves w.r.t. oscillation frequency (ω), which follows the Maxwell model. This study clearly demonstrates the formulation of an anionic WLM system which is stable at low-temperatures, and can be highly beneficial for various industrial applications. Additionally, to the best of our knowledge, this paper is the first to report on rheological properties of a viscoelastic WLM system with a silicone surfactant. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.123841. References [1] V. Croce, T. Cosgrove, C.A. Dreiss, G. Maitland, T. Hughes, G. Karlsson, Impacting the length of wormlike micelles using mixed surfactant systems, Langmuir 20
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