Investigation of Novel Triple-Responsive Wormlike Micelles

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Investigation of Novel Triple-Responsive Wormlike Micelles Mingwei Zhao, Mingwei Gao, Caili Dai, Chenwei Zou, Zhe Yang, Xuepeng Wu, Yifei Liu, Yining Wu, Sisi Fang, and Wenjiao Lv Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01011 • Publication Date (Web): 08 Apr 2017 Downloaded from http://pubs.acs.org on April 9, 2017

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Investigation of Novel Triple-Responsive Wormlike Micelles

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Mingwei Zhao,∗ Mingwei Gao, Caili Dai,* Chenwei Zou, Zhe Yang,

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Xuepeng Wu, Yifei Liu, Yining Wu, Sisi Fang, Wenjiao Lv

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School of Petroleum Engineering, State Key Laboratory of Heavy Oil Processing,

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China University of Petroleum (East China), Qingdao, Shandong, 266580, P. R.

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China.

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∗

Mingwei Zhao Caili Dai

Email: [email protected] Tel: +86-532-86981183 Fax: +86-532-86981161 Email: [email protected] Tel: +86-532-86981183 Fax: +86-532-86981161 1

ACS Paragon Plus Environment

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Abstract

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Smart wormlike micelles with stimuli-tunable rheological properties may be useful in

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a variety of applications, such as in molecular device and sensors. The formation of

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triple-stimuli responsive systems so far has been a challenging and important issue. In

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this work, a novel triple-stimuli (photo-, pH- and thermo-responsive) wormlike

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micelle is constructed with N-cetyl-N-methylmorpholinium bromide (CMMB) and

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trans-cinnamic acid (CA). The corresponding multi-responsive behaviors of wormlike

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micellar system were revealed by cryogenic transmission electron microscopy

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(cryo-TEM), rheometer and 1H NMR. Rheological properties of wormlike micellar

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system under different temperatures, pH conditions and UV irradiation time are

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measured. As confirmed by 1H NMR, chemical structure of CA molecule can be

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altered by the multiple stimulation from exotic environment. We expect it is a good

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model for triple-responsive wormlike micelles, which is helpful to understand the

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mechanism of triple-responsiveness and widen their applications.

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2


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Introduction

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In the recent years, the self-assembly behaviors of surfactants have been of particular

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interest in numerous industrial processes. The surfactants can self-assemble to form

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aggregates with different microstructures and shapes in aqueous solution, 1, 2, 3, 4, 5 such

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as spherical micelles, rodlike micelles, wormlike micelles, vesicles, lamellar phases

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and liquid crystals. Among these aggregates with various morphologies and structures,

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considerable attention from both theoretical researches and industrial applications

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have been focused on wormlike micelles, due to their unique viscoelastic behaviors

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and wide potential applications. 6, 7 Wormlike micelles show remarkable viscoelastic

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properties analogous to polymer solutions. However, the difference between polymer

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solutions and wormlike micelles is that the latter can constantly break and reform

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within an equilibrium process. 8, 9, 10 Thus, wormlike micelles are also called ‘‘living’’

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or ‘‘equilibrium’’ polymers.

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Recently, much attention has been paid to construct smart wormlike micelles,

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which are responsive to external stimuli, such as light, electric, magnetism, CO2, pH,

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temperature, and redox reactions.

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stimuli-responsive wormlike micellar systems reported heretofore generally focus on

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single external stimuli sensitivity. 16, 17, 18 Multi-stimuli responsive systems are rarely

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studied, which are sensitive to two or more stimuli conditions. This kind of

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multi-stimuli responsive wormlike micelles can provide unique opportunities to

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coordinate their responses to each stimulus, as well as accurately regulate the release

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profile under the combined action of multi-stimuli conditions.

4, 9, 11, 12, 13, 14, 15

As is known, most of the

3


19, 20

These

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multiple-responsive systems can enhance the versatility of materials and would

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provide many intriguing possibilities. 21 There have been abundant literatures reported

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on multi-stimuli responsive polymers. Only a few studies have been reported

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concerning multi-stimuli responsive micelles based on small organic molecules. Yu et

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al. used a surface active ionic liquid/azobenzene derivative mixed solution to form

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wormlike micelles with photoresponsive viscoelastic behavior. Besides, the average

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contour length of wormlike micelles can be controlled by temperature. 7 Huang et al.

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reported dual-responsive (pH and temperature) vesicles by simply adding a responser,

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SC (sodium cholate), to a stimuli-inert DTAB/SDS (dodecyl triethyl ammonium 22

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bromide/sodium dodecyl sulfate) vesicular aqueous solution.

The use of

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temperature and salt as stimuli for controlling the structure of wormlike micelles was

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reported by Croce et al. 23 The majority of multi-stimuli responsive wormlike micelles

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reported focus on dual-responsive. Few studies are concentrated on triple-stimuli

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responsive wormlike micelles. How to construct a triple-stimuli responsive system is

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a great challenge.

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In this work, we describe the design and formation of a triple-stimuli responsive

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wormlike micellar system, which can respond to UV light-, pH- and thermo- stimuli

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conditions.

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We envisaged the probability of designing a triple stimuli sensitive wormlike

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micellar system by introducing multi-stimuli responsive solubilizer into aqueous

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surfactant solution. The typical photo-responsive molecule trans-cinnamic acid

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(trans-CA) and cationic surfactant N-cetyl-N-methylmorpholinium bromide (CMMB) 4


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are chosen to construct the triple-stimuli responsive wormlike micelles. Under

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different temperatures, pH conditions and UV irradiation time, the transition of

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self-assembled aggregates was observed. As confirmed by cryogenic transmission

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electron microscopy (cryo-TEM) and 1H NMR, the mechanism of multi-responsive

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wormlike micelles was discussed in detail. We expect it is a good model for

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self-assembly of surfactants with multi-stimuli, which is helpful to understand the

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nature of smart systems and widen their applications.

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Materials and methods

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Materials

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The cetyl bromide, N-methyl morpholine and trans-CA were all obtained from

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Aladdin Reagent Co., Ltd. (Shanghai, China) and used as received. All solvents

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(methylbenzene, diethyl ether, ethyl acetate and methyl alcohol) used were of

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analytical grade. All solutions were prepared in deionized water and then kept in a

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thermostatic bath at 25 °C to reach equilibrium.

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Synthesis

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CMMB was prepared according to the references reported before. 24, 25 A solution of

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30.5 g (0.10 mol) of cetyl bromide in 50 mL of methylbenzene was added dropwise to

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a solution of 10.1 g (0.10 mol) of N-methyl morpholine in 50 mL of methylbenzene

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with stirring vigorously, under N2 atmosphere. The mixture was refluxed for 48 h at

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90 °C. After the solvent was removed by rotary evaporation, the raw product obtained

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was washed three times with the mixture of diethyl ether/ethyl acetate three times.

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The solid was collected and recrystallized three times in the mixture of methyl 5


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alcohol/diethyl ether. The white solid product was dried under vacuum at 35 °C for

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more than 48 h. CMMB was produced as a white crystal flake: (yield 79.3%). 1H NMR

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(400 MHz, Deuterium Oxide) δ 4.27 – 4.09 (dtd, J = 18.2, 14.1, 13.4, 4.5 Hz, 0H), 3.74 – 3.62 (p,

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J = 7.8, 7.1 Hz, 0H), 3.41 – 3.36 (s, 0H), 1.97 – 1.88 (t, J = 8.3 Hz, 0H), 1.54 – 1.49 (s, 0H), 1.43

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– 1.38 (s, 1H), 1.03 – 0.95 (t, J = 6.4 Hz, 0H).

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Sample preparation

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The typical sample solutions were prepared by mixing CMMB and trans-CA in the

8

desired proportions. Samples were mixed and equilibrated in a thermostatic bath

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(25 °C) for at least two weeks before investigation.

10 11

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H NMR measurements

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H NMR measurements were performed on a MP-400 nuclear magnetic resonance

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spectrometer (Varian Company, US) at a proton resonance frequency of 400.15 MHz.

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The solutions were prepared in D2O prior to the 1H NMR experiments.

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Rheological measurements

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The rheological properties of samples were measured with a Physica MCR301

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rheometer made by Anton Paar GmbH with a Rotor CC27 system. Keeping each

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sample for 15 min on the plate to reach equilibrium before testing. Dynamic stress

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sweeping measurement was performed at a frequency of 6.28 rad·s-1 (1 Hz). Dynamic

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frequency sweep measurement was obtained in the linear viscoelastic region of each

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sample.

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Cryo-transmission electron microscopy (cryo-TEM)

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Cryo-TEM samples were prepared in a controlled environment chamber at 25 °C. 6


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Five microlitre solution was placed onto a TEM copper grid using a micropipette, and

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a thin film was produced by blotting off the redundant liquid with two pieces of filter

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paper. After 10 seconds, the samples were immediately plunged into a reservoir of

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liquid ethane(cooled by the nitrogen) at -165 °C. The vitrified samples were stored in

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liquid nitrogen till they were transferred to a cryogenic sample holder (Gatan 626) and

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examined with a JEOL JEM-1400 TEM (120KV) at -174 °C. The phase contrast was

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enhanced by underfocus. The images were recorded on a cooled Gatan multiscan

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CCD and processed with Digital Micrograph software.

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UV-vis spectroscopy

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The UV-vis absorbance measurements of solution were carried out using a

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UNIC-2802 UV-vis spectrophotometer at room temperature. In this section, deionized

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water was used as the blank.

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Results and discussion

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The construction of wormlike micelles

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In order to investigate the construction of wormlike micelles in the CMMB/trans-CA

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system at 25 °C, samples with different trans-CA concentrations from 20 mM to 120

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mM at a fixed CMMB solution (60 mM) were studied (Fig. S1). The zero-shearing

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viscosity (η0) values for the mixed system with different trans-CA concentrations are

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presented in Fig. 1. The value of η0 fits the steady-state curves to the Carreau model. 4

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As shown in Fig. 1, the η0 value shows a slow increase at low concentrations, then

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there appears a sharp increase at 40-50 mM. It is up to the maximum of 11400 mPa·s

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at about 80 mM. With the further addition of trans-CA, η0 decreases. The 7


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phenomenon in η0 was typical to the conventional ionic surfactant/salt aqueous

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wormlike micellar systems. 8 After the addition of trans-CA molecules, the interfacial

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curvature of surfactant aggregates decreases with increasing salt concentration. 26 The

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counterions of trans-CA can bind to the headgroup of CMMB, which favors micellar

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growth. 27 When micelles grow further, they become flexible and can entangle with

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each other, which results in the rapid increase of viscosity. The microstructure of

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wormlike micelles is presented in Fig. S2 as a typical example. While at higher

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trans-CA concentrations, wormlike micelles transform from linear to branched

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network or break into several pieces, and hence, the viscosity decreases. 2, 3

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In order to further confirm the viscoelastic properties of samples, plots of the

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storage modulus G’ and the loss modulus G” versus the frequencies are shown in Fig.

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2a. The sample shows a more viscous behavior at low frequency regions (G” > G’).

13

With the increase of frequencies, both the elastic modulus G’ and the viscous modulus

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G” increase and cross at a frequency ωco (ωco = 1/τR). At higher frequencies, the

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sample shows a more elastic behavior (G” < G’). G’ continues to increase and reaches

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an apparent plateau value G0, while G” decreases to a minimum ( ) and then

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increases again. This phenomenon is quite accordant with typical Maxwell model,

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which is reported by Cates and Granek. 28

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The model can be expressed as follows: = /1 +

(1)

" = /1 +

(2)

= 1/

(3) 8


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= 2 ∗

(4)

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Here, ω, G0, and τR are angular frequency, plateau modulus and relaxation time,

2

respectively. The parameters ωco and G* represent the critical angular frequency and

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modulus where the curve of G’ and G” levels out. In some viscoelastic systems, G’

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do not have a constant value at high frequencies, and the estimation of G0 can be

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obtained from the modulus value of G*, calculated from equation (4).

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To clearly visualize whether the data fits the Maxwell model well or not, the Cole–

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Cole plot is commonly used. Cole–Cole plot (a curve of G” as a function of G’) is

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studied from the following equation: " + − /2 = /2

(5)

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As shown in Fig. 2b, the low-frequency data closely follow the Maxwell behavior,

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whereas the high-frequency data slightly deviate from the semicircular Cole–Cole

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plots. This phenomenon can be attributed to the Rouse relaxation modes. 29 With the

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further study, we find that indicating that the wormlike micellar solution shows

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pseudoplastic fluid features and obeys the Cox-Merz rule (the study is shown in Fig.

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S3). We calculated various rheological parameters for aqueous solutions with different

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trans-CA concentrations (the results are shown in Table S1).

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Thermal sensitivity of wormlike micelles

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In order to investigate the thermal sensitivity of wormlike micellar systems,

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steady-state and oscillatory measurements were studied in the range of 15-40 °C and

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the corresponding calculated parameters of wormlike micelles (60 mM CMMB/80

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mM trans-CA) are listed in Table 1. All the rheological parameters of wormlike 9


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micelles, including η0, G0, τR and L, are reduced with the increase of temperature. G0

2

is correlated to the entanglement degree of network. The decrease of G0 decays the

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network or entanglement of micelles. In general, with the increase of G0, the micellar

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joints formation in the network structure increases as well. 4, 30 The decrease of G0

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value with increasing temperature suggests that the entanglement degree of micellar

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network decreases. The decrease in η0 and τR can be attributed to the microstructural

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transition induced by temperature, i.e., the branching or breaking of micelles. 31 For

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the CMMB/trans-CA system, L is exponential decaying with increase of temperature

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according to equation (6), which induces the wormlike micelles breaking into short,

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rod-like micelles upon heating. 32

≈ ∅ [

2!" #

]

(6)

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where ϕ is the volume fraction of wormlike micelles, Ec is the end-cap energy, and kB

12

is Boltzmann’s constant. The decrease in L affects the dynamics of micellar stress

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relaxation. The τR is determined by competition between the micellar breaking and

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chain recombination. The reduction of L causes a reduction in the τR. 33 Thus, the

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decrease of viscosity is almost independent of the micellar branches in the network

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structure, which is due to the reduction of average contour length (L) of wormlike

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micelles.

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In order to further understand the effect of temperature on the characteristics of

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wormlike micelles, the energy changes, characterized as activation energy (Ea), was

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studied. The plot of ln τR versus the reciprocal of the absolute temperature (1/T) for

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the wormlike micelles is shown in Fig. 3a. The rheological data accord well with a 10


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linear relationship, implying that the relaxation time follows the Arrhenius equation: 34 = %

&

'#

(7)

2

where Ea is the flow activation energy, R is the universal gas constant, and A is a

3

constant. From the slope of ln τR versus 1/T, the calculated Ea is 174.2 kJ/mol, which

4

falls into the Ea range from 70 to 300 kJ/mol reported for other wormlike micelles. 35

5

It is worth noting that the thermal-switch imparts full reversibility to the process, as

6

shown in Fig. 3b. When temperature is 15 °C, viscosity of fluid is 188,000 mPa·s, but

7

it decreases to 500 mPa·s upon increasing temperature to 40 °C. The viscosity of fluid

8

can be switched between ∼105 and ∼102 mPa·s by varying the temperature for at least

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three cycles without any deterioration.

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pH sensitivity of wormlike micelles

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To investigate the pH sensitivity of wormlike micelles, the solution containing 60 mM

12

CMMB/80 mM trans-CA was chosen as a typical wormlike micellar system at 25°C.

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Steady and dynamic rheological measurements were employed to investigate the

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flowing behavior of the system as pH value varies. The steady shear rheological

15

curves of 60 mM CMMB/80 mM trans-CA solution at different pH values are

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presented in Fig. 4a. At pH 5.7, the solution shows a shear thinning non-Newtonian

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flow pattern, indicating the formation of wormlike micelles. 36

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As shown in Fig. 4b, the viscoelastic property of solution is indicated by dynamic

19

rheological measurement. The variation of elastic modulus (G’) and the viscous

20

modulus (G”) as a function of frequency (ω) exhibits a good fit to Maxwell model,

21

given by the equation (1) and equation (2). 14, 15 As shown in Fig. 4c, with the increase 11


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of pH, the value of τR (τR = 1/ωco) increases until reaching the maximum at pH 5.9,

2

and then decreases. When the pH value is higher than 7.45, the value of η0 is almost

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invariant. The value of τR is correlated to average contour length (L) of wormlike

4

micelles. The increase of τR can be due to the increase of micellar length. 13

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Fig. 4c also shows the η0 values under different pH conditions. The zero-shearing

6

viscosity η0 rapidly rises with the increase of pH, reaches the maximum at pH 5.9 and

7

then decreases. When the value of pH is larger than 7.45, η0 keeps constant.

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It is worth noting that the viscosity of the 60 mM CMMB/80 mM trans-CA

9

solution can be almost reversibly controlled by the addition of NaOH or HCl. Fig. 4d

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shows the variation of zero-shearing viscosity by varying the pH between 5.9 and 7.5

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for at least three cycles. The high pH sensitivity and invertible control of rheological

12

natures may greatly promote practical applications of such viscoelastic system.

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Photo sensitivity of wormlike micelles

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Due to UV irradiation, the molecular structure of CA can undergo an isomerization

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from trans to cis state. 37 The trans/cis transition can be detected by UV-vis spectra

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because of the difference in wavelength absorptions. 38 UV-vis spectra for trans-CA

17

before and after UV irradiation are shown in Fig. 5a. The red and black lines in Fig.

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5a represents the UV spectrum of trans-CA before and after UV irradiation,

19

respectively. The original absorbance peak of trans-CA is located at around 221 nm

20

and 267 nm. When irradiating with UV light for 4 h, a blue shift in the UV spectrum

21

of trans-CA, with the peaks shifting to 200 and 253 nm, is observed. A drop in peak

22

intensity is also presented. The change of the absorption band in the UV spectra 12


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demonstrates the isomerization of trans-CA to cis-CA. Both CA isomers have

2

inappreciable absorbance in the visible ranges of UV spectra. It means that

3

UV-irradiated samples cannot be altered by visible light. 39

4

The

1

H NMR technique is conducted to further confirm the structural

5

photoisomerization. The 1H NMR spectrum of trans-CA is shown in Fig. 5b. When

6

the sample was exposed to UV-light for 4 h, the aromatic protons of cis-CA shift

7

further upfield compared with those of trans-CA, due to the magnetically anisotropic

8

effect (Fig. 5b-ΙΙ).

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aromatic ring and carboxylate radical of cis-CA is smaller than that of trans-CA,

10

which results in the upfield shift of aromatic protons. As calculated from the 1H NMR

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spectrum of vinyl protons in this CA molecule, there are about 66% trans-CA and 34%

12

cis-CA after UV irradiation.

40

After isomerization of CA molecules, the distance between

13

The effects of UV irradiation on CMMB (60 mM)/trans-CA (80 mM) system were

14

studied. The sample before UV irradiation is shown in Fig. 6a-Ι. This sample initially

15

holds its weight for several minutes in the inverted vial, the magnetic stirrer bar

16

remains trapped in the system. Fig. 6a-ΙΙ shows the visual evidence of UV irradiation

17

effects on the sample. The photographs show that the sample has been transformed

18

into a flowing fluid - it can easily flow in the tilted vial. Fig. 6a proves a UV-induced

19

viscosity change in the CMMB/trans-CA sample. On account of the spectra in Fig. 5,

20

the macroscopic change is clearly due to the transition of CA from trans to cis. 20

21

Fig. 6b shows the dynamic rheological curves of CMMB/trans-CA system after

22

different UV irradiation time. With the increase of irradiation time, the junctions of G’ 13


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and G” are removed to higher frequencies, indicating the decrease of relaxation time

2

(τR = 1/ωco). The plot of τR versus UV-irradiation time is shown in Fig. 6c. The

3

decrease in τR means the reduction in the average length of wormlike micelles. 34 In

4

the CMMB/trans-CA system, the values of τR show a reduction with the increase of

5

UV irradiation time, hence decreasing the average length of wormlike micelles with

6

the extension of UV irradiation time. The reason of this phenomenon may be due to

7

the transition from wormlike micelles to spherical or rodlike micelles.

8

shows the steady shear rheological curves of the CMMB/trans-CA system after

9

different irradiation time. Before irradiation, the sample is highly viscous and shows a

10

non-Newtonian and shear-thinning response. After 240 min of UV irradiation, η0 is

11

lowered by 4 order of magnitude, achieving the value of 5.8 mPa·s. With the

12

irradiation time increases, η0 further deceases. Thus, the viscosity drop can be

13

controlled by the UV irradiation time. The rheological properties and macroscopic

14

changes of CMMB/trans-CA system suggest that the micelles become much shorter

15

or nonentangled.

16

1

41

Fig. 6d

H NMR studies were performed in order to investigate the effect of UV irradiation

17

time for CMMB (60mM)/trans-CA (80mM). The proton resonances of CMMB

18

(60mM)/ trans-CA (80mM) system with UV irradiation time of 0 min, 30 min, 60 min

19

and 120 min are shown in Fig. 7. For a comparison of spectra at different UV

20

irradiation time, a significant broadening of surfactant peaks was observed. With the

21

increase of UV irradiation time, the broadening of proton signals on surfactant

22

increases. The broadening increase of proton signals suggests a decreasingly binding 14


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of CA on the surface of micelle, indicating the breaking of wormlike micelles.

2

Therefore, the aggregate structure of system transforms to long rod-like micelles from

3

wormlike micelles due to the weaker binding with CA. 42

4

Triple-stimuli responsive mechanism

5

The theory of molecular packing parameter (P), 43 which is usually used to predict the

6

aggregate geometry, can explain the mechanism of aggregate transition. P=v/la

(8)

7

Here, a, v, and l are the effective area of surfactant headgroup at the surfactant-water

8

interface, the volume and length of hydrophobic chain, respectively.

9

It is acknowledged that without trans-CA, CMMB cannot pack closely with each

10

other due to the electrostatic repulsion between headgroups of surfactants. The

11

effective area of surfactant headgroup is relatively large and limits micellar growth.

12

With the addition of trans-CA, trans-CA molecules adsorb to the positively charged

13

headgroup of CMMB driven by electrostatic attraction and hydrophobic effect. 44, 45

14

The styryl group of trans-CA is embedded vertically in the hydrophobic interior while

15

the carboxylate radical of trans-CA is located next to the cationic headgroups at the

16

interface. The adsorption of trans-CA not only decreases the electrostatic repulsion,

17

but also enhances electrostatic screening. 46 This adsorption results in the decrease of

18

the effective area of surfactant headgroup (a). On the basis of equation (8), the

19

packing parameter (P) increases with the decrease of a. So the wormlike micelles are

20

formed in the system. The interactions between surfactant and CA molecules were

21

further examined by 1H NMR (the results are shown in Fig. S4). 15


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

The dominant driving force for forming wormlike micelles are the hydrophobic

2

effect and electrostatic interactions. 31, 47 Normal wormlike micelles have a constant

3

end-cap energy Ec as a function of temperature. 48 The dominant driving force for

4

micellization in general is expected to be strongly temperature-independent. (This

5

conclusion has been recognized by both experimental 31 and theoretical 48 studies.)

6

The predominant effect of temperature is the accelerated dynamics of surfactant

7

exchange. 48 At a higher temperature, surfactant molecules spend less time on their

8

end-caps due to the rapid exchange. Consequently, more end-caps can be formed,

9

which means that micelles become shorter. This phenomenon finally leads to a rapid

10

decrease in η0 upon heating.

11

The micellar structure is also significantly affected by pH values. The phenomenon

12

may be due to the different existing state of CA molecules. 13 At pH 5.7, the carboxyl

13

groups in trans-CA molecule are only weakly ionized. The electrostatic forces are

14

very weak between trans-CA and CMMB molecules. In this case, spherical micelles

15

are mostly built, because of electrostatic repulsion between the head groups of

16

CMMB molecules.

17

molecules takes place, and trans-CA becomes negatively charged. More carboxylate

18

radical can offer better charge screening to the headgroup of CMMB, thereby

19

decreasing the effective repulsion between headgroups. 50 The complex formation of

20

CMMB and trans-CA favors elongation of micelles due to the increase of the

21

surfactant packing parameter. With further increase of pH beyond 5.9, more ionization

22

of trans-CA molecules leads to the rise in ionic strength and strong mutual interaction

49

With the increase of pH, further ionization of trans-CA

16


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Langmuir

1

between trans-CA and CMMB molecules. The rise in ionic strength at higher pH may

2

lead to more flexible micelles, which would relax faster. The strong mutual interaction

3

between trans-CA and CMMB molecules may promote the transition from linear

4

micelles to the branched micellar network. As a result, viscosity of micellar system is

5

reversibly switched by regulating the pH between 5.9 and 7.45. 51 When pH is higher

6

than 7.45, all of the trans-CA exist in ionized state, and the further increase in pH

7

cannot influence the existential state of trans-CA. Hence the microstructure of

8

CMMB/ trans-CA would remain stable, and the value of η0 attains a plateau region.

9

After UV irradiation, trans-CA is converted to cis-CA. It leads to remarkable

10

changes in their molecular structures, which would further influence the association

11

and packing of CMMB and CA. As the trans-CA, the carboxyl and aromatic ring are

12

distributed on different sides of carbon double bond, while as cis-CA, both of them

13

are distributed on the same side. Due to the molecular structural change, the steric

14

hindrance of cis-CA is much larger than that of trans-CA. The dipole moment of

15

cis-CA is lower than that of trans-CA, i.e. cis-CA is more hydrophilic. Thus, the

16

association of CA and CMMB is greatly weakened. Some CA molecules are free from

17

the micelles and the gap between headgroups of CMMB is widened, leading to the

18

increase in the effective area of surfactant headgroups. On the basis of equation (8),

19

the packing parameter (P) decreases with the increase of a. As a result, micelles

20

transform into much smaller structures and cannot entangle with each other, causing a

21

sharp drop of viscosity. A theoretical proposed model of the triple-stimuli (photo-, pH-

22

and thermo-responsive) changes in micellar structure is sketched in Fig. 8. 17


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1 2

Conclusion

3

In conclusion, we have designed a multi-responsive self-assembled system in which

4

viscosity can be controlled by UV-light, pH, and temperature. The system is based on

5

mixture of CMMB and trans-CA. The addition of trans-CA in to CMMB solution

6

brings microstructure transitions compared with the conventional surfactant system.

7

At low concentrations, most of micelles are spheroidal. With the increase of Ctrans-CA,

8

the sharply increase of zero-shearing viscosity indicates the appearance and growth of

9

elongated micelles. After UV irradiation, trans-CA is photoisomerized to cis-CA, and

10

the zero-shearing viscosity is reduced. Wormlike micelles can transform to the

11

spherical or rodlike micelles. The viscosity can also be reversibly switched by pH.

12

The pH-responsive viscosity property was ascribed to the microstructure transition

13

between wormlike micelles and spherical/rodlike micelles. The micellar contour

14

length decays exponentially with increasing temperature. The exponential decrease in

15

zero-shearing viscosity follows an Arrhenius law, which is ascribed to an exponential

16

reduction in micellar contour length. Thus, the multi-responsive self-assembled

17

system can be promising to enrich the application fields such as molecular device,

18

logic gates, and sensors.

19

Acknowledgements

20

The work was supported by the National Key Basic Research Program

21

(2015CB250904), the National Science Fund (U1663206, 51425406), the Chang

22

Jiang Scholars Program (T2014152), the Fundamental Research Funds for the Central 18


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Langmuir

1

Universities (14CX02184A, 16CX02056A).

19


Langmuir

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1

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molecular assemblies

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24. Choi, S.; Kim, K. S.; Cha, J. H.; Lee, H.; Oh, J. S.; Lee, B. B. Thermal and

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27. Sharma, S. C.; Shrestha, L. K.; Sakai, K.; Sakai, H.; Abe, M. Viscoelastic

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solution of long polyoxyethylene chain phytosterol/monoglyceride/water systems.

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behavior and rheology of wormlike micelles. Langmuir 2012, 28 (44), 15472.

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34. Kern, F.; Lemarechal, P.; Candau, S. J.; Cates, M. E. Rheological properties of

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semidilute and concentrated aqueous solutions of cetyltrimethylammonium

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35. Li, Q.; Wang, X.; Yue, X.; Chen, X. Wormlike micelles formed using Gemini 24


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surfactants with quaternary hydroxyethyl methylammonium headgroups. Soft

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37. Kan, T.; Hirota, N.; Terazima, M. Enthalpy changes and reaction volumes of

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39. Kumar, R.; Ketner, A. M.; Raghavan, S. R. Nonaqueous Photorheological Fluids

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Based on Light-Responsive Reverse Wormlike Micelles. Langmuir 2010, 26 (8),

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43. Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Theory of self-assembly of

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10 11 12 13 14 15

Benzoates

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46. Kumar, R.; Raghavan, S. R. Photogelling fluids based on light-activated growth of zwitterionic wormlike micelles. Soft Matter 2009, 5 (4), 797-803. 47. Hoffmann, H. Viscoelastic Surfactant Solutions. Acs Symposium 1981, 85 (85), 255-266. 48. Cates, M. E.; Candau, S. J. Statics and dynamics of worm-like surfactant micelles. Journal of Physics Condensed Matter 1999, 2 (12), 9790-9797.

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49. Linetrose, J.; Tata, B. V.; Talmon, Y.; Aswal, V. K.; Hassan, P. A.; Sreejith, L.

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Micellar solution with pH responsive viscoelasticity and colour switching

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51. Koehler, R. D.; And, S. R. R.; Kaler, E. W. Microstructure and Dynamics of

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27


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1

Table Captions:

2

Table. 1 The rheological parameters of wormlike micelles with different effects of

3

temperature, CCMMB = 60 mM; Ctrans-CA = 80mM; pH = 6.90 ± 0.01.

4

28


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Langmuir

1

Figure Captions:

2

Fig.1 Variation of zero-shearing viscosity (η0) as a function of different trans-CA

3

concentrations for CMMB at 60 mM, at 25 °C.

4

Fig. 2 G’ and G” versus frequencies for 60 mM CMMB solutions with different

5

trans-CA concentrations (a); Cole–Cole plots for the 60 mM CMMB and different

6

concentrations of trans-CA mixed aqueous solutions at 25 °C (b).

7

Fig. 3 An Arrhenius plot of ln tR versus 1/ T for the 60 mM CMMB and 80 mM

8

trans-CA solutions (a); Switchable viscosity of 60 mM CMMB/80 mM trans-CA

9

systems at different temperatures (b).

10

Fig. 4 Steady shear viscosity plot for 60 mM CMMB/80 mM trans-CA solution at

11

different pH values (a); G’ and G” versus frequencies for 60 mM CMMB/80 mM

12

trans-CA system with different pH values (b); Variations of zero-shearing viscosity

13

(η0) and the relaxation time (τR) as a function of 60 mM CMMB/80 mM trans-CA

14

solutions at different pH (c); Switchable viscosity of 60 mM CMMB/80 mM

15

trans-CA systems at different pH (d).

16

Fig. 5 UV-vis spectra of trans-CA before and after the UV irradiation. The

17

concentration of solution is 0.1 mM (a); 1H NMR spectra of trans-CA (Ι) before and

18

(ΙΙ) after UV irradiation (b).

19

Fig. 6 Photographs of a 60 mM CMMB/80 mM trans-CA sample (Ι) before and (ΙΙ)

20

after UV irradiation (a); Dynamic rheology of the CMMB(60 mM)-CA(80 mM)

21

mixture: before and after UV irradiation for 30, 60, 120 and 240 min (b); The

22

relaxation time τR versus the UV irradiation for CMMB(60mM)/trans-CA(80mM) 29


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

system at 25 °C (c); Steady shear viscosity plot for 60 mM CMMB/80 mM trans-CA

2

solution at different UV irradiation time (d).

3

Fig. 7 1H NMR spectra of CMMB(60mM)/trans-CA(80mM) system with UV

4

irradiation time of 0 min (a), 30 min (b), 60 min (c) and 120 min (d) at 25 °C.

5

Fig. 8 Schematic illustration of the aggregates formed by the CMMB/CA system in

6

aqueous solution.

7

30


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Langmuir

Table 1

1

Temperature

η0

G0

G∞'

Gmin"

ωco

τR

L

°C

mPa·s

Pa

Pa

Pa

rad/s

s

µm

15 20 23 25 28 30 40

188000 43000 23000 11400 5350 3200 500

24.2 23.4 23.0 22.4 19.5 19.0 13.2

24 22.9 22.8 22.6 19.6 19.2 15

1.2 2.8 2.6 3.3 4.4 5.3 --

0.1 0.6 1.2 1.6 4.3 7.1 42.1

7.69 1.75 0.83 0.63 0.23 0.14 0.02

1669.6-3130.4 887.6-1664.2 701.5-1315.4 547.9-1027.3 356.4-668.2 289.8-543.4 --

2

31


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1 2

Figure 1

3

32


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Langmuir

1 2

Figure 2

3

33


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 3

3

34


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Langmuir

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Figure 4

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Langmuir

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1 2

Figure 5

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Langmuir

1 2

Figure 6

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Langmuir

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1 2

Figure 7

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Langmuir

1 2

Figure 8

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Langmuir

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251x181mm (149 x 149 DPI)


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Investigation of Novel Triple-Responsive Wormlike Micelles

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