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Unusual viscoelasticity behaviour in aqueous solutions containing a photoresponsive amphiphile
Journal of Colloid and Interface Science 407 (2013) 370–374
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Unusual viscoelasticity behaviour in aqueous solutions containing a photoresponsive amphiphile Yutaka Takahashi ⇑, Yuki Yamamoto, Shinichi Hata, Yukishige Kondo ⇑ Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
a r t i c l e
i n f o
Article history: Received 11 May 2013 Accepted 6 June 2013 Available online 21 June 2013 Keywords: Cationic surfactant Azobenzene Wormlike micelle Self-assembly Viscosity Photorheological fluid
a b s t r a c t Here, we report unusual behaviour of the viscoelasticity of surfactant aqueous solutions consisting of cationic cetyltrimethylammonium bromide (CTAB) and an anionic photoresponsive amphiphile, sodium [4(4-butylphenylazo)phenoxy]acetate (C4AzoNa). When C4AzoNa molecules are trans-isomers, spheroidal micelles are formed in the surfactant solution, the viscosity of which is low. Irradiation of this solution by ultraviolet (UV) light yields an aqueous solution of CTAB/cis-C4AzoNa (cis-isomers of C4AzoNa), which is a highly viscous gel consisting of wormlike micelles. The drastic change in the surfactant solution viscosity is attributed to the formation and disruption of wormlike micelles. The geometrical structural transformation of the azobenzene groups in the C4AzoNa molecules of the CTAB/C4AzoNa mixture would lead to a change in the critical packing parameter of the mixture, thereby inducing the morphological transformation of the aggregates (spheroidal micelles to wormlike micelles). To our knowledge, this is the first report of a drastic increase in surfactant solution viscosity by UV light irradiation. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Surfactants and amphiphilic molecules can self-assemble into a variety of nanostructures or microstructures in aqueous solutions above a critical aggregate concentration. The morphologies of aggregates depend primarily on the molecular structure of a surfactant and aqueous solution conditions (e.g., pH, temperature, and the existence of additives). An aqueous solution of wormlike micelles has viscoelastic characteristics that are not exhibited by solutions of other aggregates because the wormlike micelles are three-dimensionally intertwined. Recently, a number of researchers have worked on controlling the morphology of aggregates formed from surfactants by external stimuli [1–4]. Such an ability is expected to permit temporal and spatial control over DNA delivery [5,6] and the release of substances such as drugs [7]. In particular, the ability to control the formation and disruption of wormlike micelles by external stimuli such as temperature [8,9], pH variation [3,10], and redox reaction [11] has attracted much attention because the viscosity of the solutions can be drastically changed. Light is more attractive than other stimuli as an external stimulus because it is relatively easy to use and its use generates no pollution. There have been many studies to control the formation of wormlike micelles by ultraviolet (UV) light irradiation of mixed ⇑ Corresponding authors. Fax: +81 3 5228 8313. E-mail addresses: [email protected] (Y. Takahashi), ykondo@rs. tus.ac.jp (Y. Kondo). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.06.010
systems consisting of an ionic surfactant and an ionic photoresponsive derivative such as p-coumaric acid [12], o-methoxycinnamic acid (OMCA) [13,14], sodium cinnamate [15,16], or azobenzene [17,18]. In these studies, the viscosity of the mixed systems greatly decreases by UV light irradiation. Photoresponsive compounds containing an arylethenyl group such as coumaric acid, OMCA, or sodium cinnamate show poor reversibility. In contrast, the lightreversibility of azobenzene is superior to that of the others, implying that azobenzene is a good candidate as an additive to control the viscoelasticity of surfactant solutions in application fields. Wormlike micelles containing trans-azobenzene are disrupted by UV light irradiation and reformed by visible (Vis) light irradiation [17]. However, there are very few studies on the opposite lightresponsive behaviour, in which UV light irradiation increases and Vis light decreases the surfactant viscosity. Although Raghavan et al. have shown that the viscosity of mixed solutions consisting of a zwitterionic surfactant and OMCA increases upon exposure to UV light, the mixture exhibited no reversible change in viscosity upon repetitive exposure to UV and Vis lights [19]. Sakai et al. have succeeded in increasing the viscosity of mixed aqueous solutions consisting of cetyltrimethylammonium bromide (CTAB)/sodium salicylate (NaSal)/sodium 3,30 -azobenzene dicarboxylate (Azo2Na) upon UV light exposure and decreasing the viscosity of the CTAB/ NaSal/Azo2Na mixed solution under Vis light irradiation [20]. However, the change in viscosity with repetitive exposure of the solution to UV and Vis light was small; UV light irradiation increased the mixed solution viscosity by only an order of magnitude from 1.5 Pa s from 0.23 Pa s.
Y. Takahashi et al. / Journal of Colloid and Interface Science 407 (2013) 370–374
Here, we report a new photorheological fluid based on wormlike micelles formed from a cationic surfactant, CTAB, and an anionic photosensitive amphiphile, sodium [4-(4-butylphenylazo)phenoxy]acetate (C4AzoNa; Scheme 1). We have succeeded in forming wormlike micelles (thus increasing the viscosity of the surfactant solution) by UV light irradiation and disrupting them (thus decreasing the viscosity of the solution) by Vis light irradiation for the aqueous mixture. We thereby demonstrate a drastic change in the surfactant solution viscosity (of three orders of magnitude) by UV and Vis light irradiation.
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2.5. Rheological measurements Measurements were performed on a stress-controlled rheometer (HAAKE MARS, Thermo Fisher Scientific K.K.) by using the geometry of a cone-type plate with a diameter of 35 mm and cone angle of 1° at 25.0 ± 0.1 °C. A sample cover was used to avoid the change in sample concentration by evaporation during measurements. Frequency sweep measurements were performed in the linear viscoelastic regime of the samples, as determined previously by dynamic strain sweep measurements. The zero-shear viscosity of the samples was obtained by extrapolating the curve of the viscosity and shear rate to zero-shear rate.
2. Experimental section 2.6. Dynamic Light Scattering (DLS) 2.1. Materials and sample preparation Cetyltrimethylammonium bromide (CTAB) was purchased from Tokyo Chemical Industry Co., Ltd. and used without purification. Sodium [4-(4-butylphenylazo)phenoxy]acetate (C4AzoNa) was synthesised by the Williamson synthesis of 4-(4-butylphenylazo)phenol with ethyl bromoacetate, followed by hydrolysis with sodium hydroxide solution (Scheme 2). Solutions of 100 mM CTAB were prepared in high-purity H2O (Milli-Q pure water; R = 18 MX cm, c = 72.8 mN/m at 25 °C) and were added to a desired amount of C4AzoNa. The aqueous CTAB/C4AzoNa (100 mM/35 mM) solutions were stored at 25.0 ± 0.1 °C for at least 48 h to ensure equilibration. Then, each measurement was carried out.
2.2. Light irradiation method An aqueous CTAB/C4AzoNa solution (1 ml) was placed in a vial and then irradiated by UV or visible light with stirring in a dark room. The cis isomerisation of trans-C4AzoNa was performed on a Handy UV Lamp SLUV-8 (AsOne, Japan), which emitted UV light with a wavelength of 365 ± 15 nm. CTAB/cis-C4AzoNa solutions were illuminated with visible light (LAX-Cute, Asahi Spectra Co., Ltd.; 100-W Xe lamp; wavelength, 400–700 nm).
2.3. UV–Vis measurements Solutions were diluted to final concentration of 0.35 mM C4AzoNa solutions before measurements because the absorbance at the original concentration was too high in UV–Vis measurements. The diluted solutions were placed in a quartz cuvette with an optical path length of 1 mm. These spectra were recorded using a V-570 UV–Vis spectrometer (JASCO, Japan) at 25.0 ± 0.1 °C.
2.4. NMR measurements NMR measurements were carried out at 30 °C on a Bruker Avance DPX-400 spectrometer equipped with a QNP probe operating at 400 MHz for 1H nuclei. All samples were prepared using D2O (Acros Organics; 99.8 atom%D) as the solvent.
Scheme 1. Reversible photoisomerisation of C4AzoNa by light irradiation.
Solutions were filtered through a 0.20 lm membrane filter of hydrophilic cellulose acetate before measurement. Subsequent DLS measurements (He–Ne, 633 nm) were performed with a Zetasizer Nano ZS (Malvern Instruments Ltd., the United Kingdom) by using a noninvasive back-scatter (NIBS) optical system as the detector. 3. Results and discussion 3.1. Photoisomerisation of aqueous CTAB/C4AzoNa solution C4AzoNa was synthesised according to Scheme 2 and added to an aqueous solution of 100 mM CTAB to attain a concentration of 35 mM. UV–Vis absorption spectra for the aqueous solution of CTAB/trans-C4AzoNa (100 mM/35 mM) showed an absorption band at 334 nm corresponding to the trans-isomer of the azobenzene group in C4AzoNa (Fig. 1). The ratio of trans-C4AzoNa in this photostationary state of the trans-isomer was 95%, the value of which was estimated from the 1H NMR signal of the aromatic protons in the azobenzene group of C4AzoNa. The absorbance at 334 nm decreased with UV light irradiation of the surfactant solution and an absorption band assigned to the cis-isomer appeared at 442 nm. UV–Vis measurements indicated that UV light irradiation of the aqueous CTAB/C4AzoNa solution for at least 180 min led to photostationary states of the cis-isomer for C4AzoNa, where the molar ratio of cis-isomer/trans-isomer was 67/33. Subsequent Vis light irradiation for 30 min caused conversion of cis-C4AzoNa into the trans-isomer, resulting in a cis-isomer/trans-isomer ratio of 10/ 90. Repetitive irradiation with UV and Vis light led to reversible trans–cis photoisomerisation of C4AzoNa. 3.2. Photoinduced change in viscosity of CTAB/C4AzoNa mixture As seen in Fig. 2a, the CTAB/trans-C4AzoNa mixture was a lowviscosity fluid, and its zero-shear viscosity (g0) was 0.01 Pa s. When the mixture was irradiated by UV light, it became a highly viscous gel (Fig. 2b). The value of the zero-shear viscosity for the CTAB/cisC4AzoNa mixture was ca. 10 Pa s, a value of which is 3 orders of magnitude higher than the viscosity of the CTAB/trans-C4AzoNa mixture. Subsequent Vis light irradiation of the viscous gel returned the mixture to a low-viscosity fluid (g0 = 0.02 Pa s). Fig. 3 shows the changes in the zero-shear viscosity of the aqueous CTAB/C4AzoNa mixture by reversible trans–cis photoisomerisation that occurred by repetitive irradiation with UV and Vis light. The value of the zero-shear viscosity for the mixture where Vis light was irradiated after UV light irradiation was not identical to that of as-prepared CTAB/trans-C4AzoNa solution. As described above, this is due to the coexistence of cis-C4AzoNa, which is not completely reconverted to the initial photostationary state of the trans-isomer. However, repetitive trans–cis photoisomerisation
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Scheme 2. Synthesis of photoresponsive amphiphile C4AzoNa.
Fig. 1. UV–Vis spectra of the CTAB and C4AzoNa (100 mM/35 mM) mixture in water. Solid and broken lines correspond to the CTAB/trans-C4AzoNa and CTAB/cisC4AzoNa, respectively.
caused drastic reversible changes in the solution viscosity. Fig. 4 shows the dependence of the storage modulus, G0 , and the viscous modulus, G00 , of the aqueous CTAB/C4AzoNa mixtures on the frequency of the stress-controlled rheometer. Both G0 and G00 showed frequency dependence of the dynamic viscoelasticity. According to the Maxwell model, the typical viscoelastic property for the formation of wormlike micelles shows liquid-like behaviour (G0 < G00 ) at a
Fig. 3. Change in the zero-shear viscosity for aqueous CTAB/C4AzoNa (100 mM/ 35 mM) mixture with reversible trans–cis photoisomerisation. Open circles: CTAB/ trans-C4AzoNa; filled circles: CTAB/cis-C4AzoNa.
frequency region lower than the point at which G0 intersects with G00 and solid-like behaviour (G’ > G00 ) at a frequency region higher than the crossover point of G0 and G00 [21]. Therefore, the plot of G0 and G00 fits with the Maxwell model, and an intersection point of G0 and G00 appears when wormlike micelles are formed. The Maxwell model is described by the following equations:
G0 ðxÞ ¼ G00 ðxÞ ¼
x2 s 2 G0 1 þ x2 s2 xs 1 þ x2 s 2
G0
where s is the relaxation time and G0 is the plateau modulus. In the case of the CTAB/trans-C4AzoNa mixture, the viscoelastic behaviour did not indicate the formation of wormlike micelles. On the other hand, in the case of the CTAB/cis-C4AzoNa mixture, the plots of G0 and G00 were well fitted to the Maxwell equation, suggesting that the frequency dependence of the dynamic viscoelasticity for the CTAB/cis-C4AzoNa mixture revealed the formation of wormlike micelles. The wormlike micelles were disrupted by Vis light irradiation and then reformed by subsequent UV light irradiation. 3.3. Possible mechanism for photoinduced change in viscosity
Fig. 2. Photographs showing physical appearance of the aqueous CTAB/C4AzoNa (100 mM/35 mM) mixture (a) before and (b) after UV light irradiation.
Here, we discuss the drastic changes in the surfactant solution viscosity when the trans-isomer was photoisomerised to the cisisomer in the aggregates formed from the CTAB/C4AzoNa mixture. The geometrical structure transformation of amphiphilic molecules that have an azobenzene group causes a transition of aggregates formed from surfactants and changes their interfacial properties by light irradiation. In previous studies, researchers have reported that changes in the solubility of an azobenzene compound in water by UV light irradiation leads to the disruption of
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Fig. 4. Variation in the storage modulus, G0 (open circles), and viscous modulus, G00 (filled circles), as a function of frequency for aqueous (a) CTAB/trans-C4AzoNa, and (b) CTAB/cis-C4AzoNa mixtures. Maxwell fits to the experimental data are drawn as solid lines.
wormlike micelles because the cis-type azobenzene compound preferentially dissolves in a bulk aqueous solution instead of penetrating into the surfactant aggregates [18]. The solubility of transC4AzoNa in water, as estimated from UV–vis measurements, was 1.7 mM. Dynamic light-scattering measurements for aqueous CTAB/trans-C4AzoNa mixtures indicated the formation of small micelles 5 nm in diameter. 1H NMR signals corresponding to the aromatic protons of the azobenzene group in the aggregates formed from the CTAB/trans-C4AzoNa mixture were broader than those corresponding to the aromatic protons of the azobenzene group in C4AzoNa dissolved in water (Fig. 5). In mixtures of a cationic surfactant with an anionic surfactant or organic salt, interionic interactions involving electrostatic attraction between oppositely charged ions are known to occur in aqueous solutions. The interaction and steric structures of catanionic surfactants (mixtures of a cationic surfactant and anionic surfactants) influence the morphology of the aggregates. CTAB and trans-C4AzoNa molecules will be closely packed in the aggregates by electrostatic attraction between hydrophilic groups with opposite charges, forming small micelles. On the other hand, UV light irradiation causes isomerisation of trans-C4AzoNa to the cis-isomer with a bulkier structure. The hydrophilicity of the cis-isomer is higher than that of the trans-isomer because the molecular dipole moment increases to 3.0 D as the trans-isomer is photoisomerised to the cis-isomer [22]. In fact, the solubility of cis-C4AzoNa in water estimated from UV–vis measurements was 6.7 mM—ca. 4 times higher than that of the trans-isomer. 1H NMR measurements indicated that signals corresponding to the aromatic protons of cisC4AzoNa in the aggregates were much sharper than those of the trans-isomer (Fig. 5). In addition, the chemical shift for these protons of cis-C4AzoNa moved upfield in the CTAB/cis-C4AzoNa mixture. Considering the NMR chemical shift, cis-C4AzoNa molecules will effectively penetrate into the surfactant aggregates despite their high solubility in water. Therefore, CTAB and cis-C4AzoNa molecules will be packed in the aggregates. However, the packing will be looser than that of trans-isomer because the cis-isomer is bulkier. In other words, trans-C4AzoNa molecules penetrate linearly into CTAB aggregates, whereas cis-C4AzoNa molecules orient within the aggregates as bulky structures (Fig. 6). Furthermore, we have investigated the morphological transition of mixtures of CTAB and sodium (4-phenylazophenoxy)acetate (AzoNa) having a structure in which a butyl chain (C4) in C4AzoNa is removed. Measurements of the frequency dependence of the dynamic viscoelasticity indicated the formation of wormlike micelles
Fig. 5. 1H NMR spectra for (a) saturated solution of C4AzoNa; (b) CTAB/transC4AzoNa mixture; (c) and CTAB/cis-C4AzoNa mixture in D2O. The eight assigned protons of the azobenzene group are shown by the letters a–d. The subscripts t and c denote trans- and cis-isomers, respectively. Signals of trans-C4AzoNa were considerably broadened with the formation of aggregates, thereby precluding interpretation of the chemical shift. In contrast, signals of cis-isomer shifted upfield by self-assembly of C4AzoNa.
in the CTAB/trans-AzoNa (100 mM/35 mM) mixture but not in the CTAB/cis-AzoNa mixture. The hydrophilicity of AzoNa is higher than that of C4AzoNa because AzoNa has no butyl chain (C4). The solubilities of trans- and cis-AzoNa in water were 25 mM and over 100 mM, respectively [18]. From these results, cis-AzoNa molecules cannot penetrate into CTAB aggregates formed in the CTAB/ AzoNa (100 mM/35 mM) mixture because the solubility of AzoNa in water increases with trans–cis photoisomerisation. Thus, the change in the solubility in water would lead to the morphological transition of aggregates formed from the CTAB/AzoNa mixture. On the other hand, variation in the solubility of the C4AzoNa molecule with photoisomerisation will have little influence on the formation and disruption of wormlike micelles in CTAB/C4AzoNa mixtures. Consequently, the trans–cis photoisomerisation of the azobenzene group in C4AzoNa occurs in the aggregates of the CTAB/ C4AzoNa mixture. The morphological transition of the aggregates
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the surfactant solution viscosity drastically increased with UV light irradiation and then decreased with Vis light irradiation. On the other hand, UV light irradiation led to increase in the viscosity in previous research [12–17]. The solution viscosity of the CTAB/ C4AzoNa mixture increases by about 3 orders of magnitude with photoisomerisation of the trans-isomer to the cis-isomer in this system. To the best of our knowledge, this article is first report concerning a drastic increase in surfactant solution viscosity by UV light irradiation. This drastic change in surfactant solution viscosity results from the transformation of small micelles to wormlike micelles by photoisomerisation. In the other words, the geometrical structure transformation of the azobenzene group in the CTAB/ C4AzoNa mixture at the molecular level leads to the macroscopic change in solution viscosity. Wormlike micelles can be applied drag reduction agents under turbulent flow [24]. The formation of wormlike micelles leads to a transition from turbulent flow to laminar flow. The turbulent flow is effective in transfer and diffusion of heat and materials. Therefore, a switchable photorheological fluid that we have demonstrated here will be useful for effective control over drag reduction and transfer of heat and materials. References Fig. 6. Possible mechanism for the formation and disruption of wormlike micelles by light irradiation. When trans-C4AzoNa is photoisomerised to cis-C4AzoNa in the aggregates, the curvature of the aggregates decreases, thereby inducing the transition from small micelles to wormlike micelles.
in aqueous solutions of the mixture can be understood on the basis of the critical packing parameter (CPP) theory [23]. The value of the CPP is defined by the equation CPP = v/al (v, volume of the hydrophobic chain; a, cross-sectional area of the hydrophobic–hydrophilic interface; l, fully stretched length of the hydrophobic chain). The CTAB/trans-C4AzoNa (100 mM/35 mM) mixture formed small micelles, therefore, CPP < 1/3 for the mixture. The structural transformation of the azobenzene group in the aggregates may increase the volume of the hydrophobic chain (v) and subsequently increase the CPP value of the CTAB/C4AzoNa mixture (Fig. 4). The increase in the CPP value causes the formation of large aggregates with lower curvature. Considering the isomerisation ratio upon exposure of the solution to UV light, two kinds of C4AzoNa molecules coexist in the wormlike micelle: cis-C4AzoNa (67 mol%) and trans-C4AzoNa (33 mol%). As a result, the apparent CPP value in the CTAB/cis-C4AzoNa mixture may approximate values capable of forming wormlike micelles, 1/3 < CPP < 1/2. The change in the CPP with the photoisomerisation of C4AzoNa will lead to the transformation of small micelles to wormlike micelles. 4. Conclusions We demonstrated a new photoresponsive change in the viscosity of a surfactant solution by UV light irradiation. In this article,
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Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Unusual viscoelasticity behaviour in aqueous solutions containing a photoresponsive amphiphile Yutaka Takahashi ⇑, Yuki Yamamoto, Shinichi Hata, Yukishige Kondo ⇑ Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
a r t i c l e
i n f o
Article history: Received 11 May 2013 Accepted 6 June 2013 Available online 21 June 2013 Keywords: Cationic surfactant Azobenzene Wormlike micelle Self-assembly Viscosity Photorheological fluid
a b s t r a c t Here, we report unusual behaviour of the viscoelasticity of surfactant aqueous solutions consisting of cationic cetyltrimethylammonium bromide (CTAB) and an anionic photoresponsive amphiphile, sodium [4(4-butylphenylazo)phenoxy]acetate (C4AzoNa). When C4AzoNa molecules are trans-isomers, spheroidal micelles are formed in the surfactant solution, the viscosity of which is low. Irradiation of this solution by ultraviolet (UV) light yields an aqueous solution of CTAB/cis-C4AzoNa (cis-isomers of C4AzoNa), which is a highly viscous gel consisting of wormlike micelles. The drastic change in the surfactant solution viscosity is attributed to the formation and disruption of wormlike micelles. The geometrical structural transformation of the azobenzene groups in the C4AzoNa molecules of the CTAB/C4AzoNa mixture would lead to a change in the critical packing parameter of the mixture, thereby inducing the morphological transformation of the aggregates (spheroidal micelles to wormlike micelles). To our knowledge, this is the first report of a drastic increase in surfactant solution viscosity by UV light irradiation. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Surfactants and amphiphilic molecules can self-assemble into a variety of nanostructures or microstructures in aqueous solutions above a critical aggregate concentration. The morphologies of aggregates depend primarily on the molecular structure of a surfactant and aqueous solution conditions (e.g., pH, temperature, and the existence of additives). An aqueous solution of wormlike micelles has viscoelastic characteristics that are not exhibited by solutions of other aggregates because the wormlike micelles are three-dimensionally intertwined. Recently, a number of researchers have worked on controlling the morphology of aggregates formed from surfactants by external stimuli [1–4]. Such an ability is expected to permit temporal and spatial control over DNA delivery [5,6] and the release of substances such as drugs [7]. In particular, the ability to control the formation and disruption of wormlike micelles by external stimuli such as temperature [8,9], pH variation [3,10], and redox reaction [11] has attracted much attention because the viscosity of the solutions can be drastically changed. Light is more attractive than other stimuli as an external stimulus because it is relatively easy to use and its use generates no pollution. There have been many studies to control the formation of wormlike micelles by ultraviolet (UV) light irradiation of mixed ⇑ Corresponding authors. Fax: +81 3 5228 8313. E-mail addresses: [email protected] (Y. Takahashi), ykondo@rs. tus.ac.jp (Y. Kondo). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.06.010
systems consisting of an ionic surfactant and an ionic photoresponsive derivative such as p-coumaric acid [12], o-methoxycinnamic acid (OMCA) [13,14], sodium cinnamate [15,16], or azobenzene [17,18]. In these studies, the viscosity of the mixed systems greatly decreases by UV light irradiation. Photoresponsive compounds containing an arylethenyl group such as coumaric acid, OMCA, or sodium cinnamate show poor reversibility. In contrast, the lightreversibility of azobenzene is superior to that of the others, implying that azobenzene is a good candidate as an additive to control the viscoelasticity of surfactant solutions in application fields. Wormlike micelles containing trans-azobenzene are disrupted by UV light irradiation and reformed by visible (Vis) light irradiation [17]. However, there are very few studies on the opposite lightresponsive behaviour, in which UV light irradiation increases and Vis light decreases the surfactant viscosity. Although Raghavan et al. have shown that the viscosity of mixed solutions consisting of a zwitterionic surfactant and OMCA increases upon exposure to UV light, the mixture exhibited no reversible change in viscosity upon repetitive exposure to UV and Vis lights [19]. Sakai et al. have succeeded in increasing the viscosity of mixed aqueous solutions consisting of cetyltrimethylammonium bromide (CTAB)/sodium salicylate (NaSal)/sodium 3,30 -azobenzene dicarboxylate (Azo2Na) upon UV light exposure and decreasing the viscosity of the CTAB/ NaSal/Azo2Na mixed solution under Vis light irradiation [20]. However, the change in viscosity with repetitive exposure of the solution to UV and Vis light was small; UV light irradiation increased the mixed solution viscosity by only an order of magnitude from 1.5 Pa s from 0.23 Pa s.
Y. Takahashi et al. / Journal of Colloid and Interface Science 407 (2013) 370–374
Here, we report a new photorheological fluid based on wormlike micelles formed from a cationic surfactant, CTAB, and an anionic photosensitive amphiphile, sodium [4-(4-butylphenylazo)phenoxy]acetate (C4AzoNa; Scheme 1). We have succeeded in forming wormlike micelles (thus increasing the viscosity of the surfactant solution) by UV light irradiation and disrupting them (thus decreasing the viscosity of the solution) by Vis light irradiation for the aqueous mixture. We thereby demonstrate a drastic change in the surfactant solution viscosity (of three orders of magnitude) by UV and Vis light irradiation.
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2.5. Rheological measurements Measurements were performed on a stress-controlled rheometer (HAAKE MARS, Thermo Fisher Scientific K.K.) by using the geometry of a cone-type plate with a diameter of 35 mm and cone angle of 1° at 25.0 ± 0.1 °C. A sample cover was used to avoid the change in sample concentration by evaporation during measurements. Frequency sweep measurements were performed in the linear viscoelastic regime of the samples, as determined previously by dynamic strain sweep measurements. The zero-shear viscosity of the samples was obtained by extrapolating the curve of the viscosity and shear rate to zero-shear rate.
2. Experimental section 2.6. Dynamic Light Scattering (DLS) 2.1. Materials and sample preparation Cetyltrimethylammonium bromide (CTAB) was purchased from Tokyo Chemical Industry Co., Ltd. and used without purification. Sodium [4-(4-butylphenylazo)phenoxy]acetate (C4AzoNa) was synthesised by the Williamson synthesis of 4-(4-butylphenylazo)phenol with ethyl bromoacetate, followed by hydrolysis with sodium hydroxide solution (Scheme 2). Solutions of 100 mM CTAB were prepared in high-purity H2O (Milli-Q pure water; R = 18 MX cm, c = 72.8 mN/m at 25 °C) and were added to a desired amount of C4AzoNa. The aqueous CTAB/C4AzoNa (100 mM/35 mM) solutions were stored at 25.0 ± 0.1 °C for at least 48 h to ensure equilibration. Then, each measurement was carried out.
2.2. Light irradiation method An aqueous CTAB/C4AzoNa solution (1 ml) was placed in a vial and then irradiated by UV or visible light with stirring in a dark room. The cis isomerisation of trans-C4AzoNa was performed on a Handy UV Lamp SLUV-8 (AsOne, Japan), which emitted UV light with a wavelength of 365 ± 15 nm. CTAB/cis-C4AzoNa solutions were illuminated with visible light (LAX-Cute, Asahi Spectra Co., Ltd.; 100-W Xe lamp; wavelength, 400–700 nm).
2.3. UV–Vis measurements Solutions were diluted to final concentration of 0.35 mM C4AzoNa solutions before measurements because the absorbance at the original concentration was too high in UV–Vis measurements. The diluted solutions were placed in a quartz cuvette with an optical path length of 1 mm. These spectra were recorded using a V-570 UV–Vis spectrometer (JASCO, Japan) at 25.0 ± 0.1 °C.
2.4. NMR measurements NMR measurements were carried out at 30 °C on a Bruker Avance DPX-400 spectrometer equipped with a QNP probe operating at 400 MHz for 1H nuclei. All samples were prepared using D2O (Acros Organics; 99.8 atom%D) as the solvent.
Scheme 1. Reversible photoisomerisation of C4AzoNa by light irradiation.
Solutions were filtered through a 0.20 lm membrane filter of hydrophilic cellulose acetate before measurement. Subsequent DLS measurements (He–Ne, 633 nm) were performed with a Zetasizer Nano ZS (Malvern Instruments Ltd., the United Kingdom) by using a noninvasive back-scatter (NIBS) optical system as the detector. 3. Results and discussion 3.1. Photoisomerisation of aqueous CTAB/C4AzoNa solution C4AzoNa was synthesised according to Scheme 2 and added to an aqueous solution of 100 mM CTAB to attain a concentration of 35 mM. UV–Vis absorption spectra for the aqueous solution of CTAB/trans-C4AzoNa (100 mM/35 mM) showed an absorption band at 334 nm corresponding to the trans-isomer of the azobenzene group in C4AzoNa (Fig. 1). The ratio of trans-C4AzoNa in this photostationary state of the trans-isomer was 95%, the value of which was estimated from the 1H NMR signal of the aromatic protons in the azobenzene group of C4AzoNa. The absorbance at 334 nm decreased with UV light irradiation of the surfactant solution and an absorption band assigned to the cis-isomer appeared at 442 nm. UV–Vis measurements indicated that UV light irradiation of the aqueous CTAB/C4AzoNa solution for at least 180 min led to photostationary states of the cis-isomer for C4AzoNa, where the molar ratio of cis-isomer/trans-isomer was 67/33. Subsequent Vis light irradiation for 30 min caused conversion of cis-C4AzoNa into the trans-isomer, resulting in a cis-isomer/trans-isomer ratio of 10/ 90. Repetitive irradiation with UV and Vis light led to reversible trans–cis photoisomerisation of C4AzoNa. 3.2. Photoinduced change in viscosity of CTAB/C4AzoNa mixture As seen in Fig. 2a, the CTAB/trans-C4AzoNa mixture was a lowviscosity fluid, and its zero-shear viscosity (g0) was 0.01 Pa s. When the mixture was irradiated by UV light, it became a highly viscous gel (Fig. 2b). The value of the zero-shear viscosity for the CTAB/cisC4AzoNa mixture was ca. 10 Pa s, a value of which is 3 orders of magnitude higher than the viscosity of the CTAB/trans-C4AzoNa mixture. Subsequent Vis light irradiation of the viscous gel returned the mixture to a low-viscosity fluid (g0 = 0.02 Pa s). Fig. 3 shows the changes in the zero-shear viscosity of the aqueous CTAB/C4AzoNa mixture by reversible trans–cis photoisomerisation that occurred by repetitive irradiation with UV and Vis light. The value of the zero-shear viscosity for the mixture where Vis light was irradiated after UV light irradiation was not identical to that of as-prepared CTAB/trans-C4AzoNa solution. As described above, this is due to the coexistence of cis-C4AzoNa, which is not completely reconverted to the initial photostationary state of the trans-isomer. However, repetitive trans–cis photoisomerisation
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Scheme 2. Synthesis of photoresponsive amphiphile C4AzoNa.
Fig. 1. UV–Vis spectra of the CTAB and C4AzoNa (100 mM/35 mM) mixture in water. Solid and broken lines correspond to the CTAB/trans-C4AzoNa and CTAB/cisC4AzoNa, respectively.
caused drastic reversible changes in the solution viscosity. Fig. 4 shows the dependence of the storage modulus, G0 , and the viscous modulus, G00 , of the aqueous CTAB/C4AzoNa mixtures on the frequency of the stress-controlled rheometer. Both G0 and G00 showed frequency dependence of the dynamic viscoelasticity. According to the Maxwell model, the typical viscoelastic property for the formation of wormlike micelles shows liquid-like behaviour (G0 < G00 ) at a
Fig. 3. Change in the zero-shear viscosity for aqueous CTAB/C4AzoNa (100 mM/ 35 mM) mixture with reversible trans–cis photoisomerisation. Open circles: CTAB/ trans-C4AzoNa; filled circles: CTAB/cis-C4AzoNa.
frequency region lower than the point at which G0 intersects with G00 and solid-like behaviour (G’ > G00 ) at a frequency region higher than the crossover point of G0 and G00 [21]. Therefore, the plot of G0 and G00 fits with the Maxwell model, and an intersection point of G0 and G00 appears when wormlike micelles are formed. The Maxwell model is described by the following equations:
G0 ðxÞ ¼ G00 ðxÞ ¼
x2 s 2 G0 1 þ x2 s2 xs 1 þ x2 s 2
G0
where s is the relaxation time and G0 is the plateau modulus. In the case of the CTAB/trans-C4AzoNa mixture, the viscoelastic behaviour did not indicate the formation of wormlike micelles. On the other hand, in the case of the CTAB/cis-C4AzoNa mixture, the plots of G0 and G00 were well fitted to the Maxwell equation, suggesting that the frequency dependence of the dynamic viscoelasticity for the CTAB/cis-C4AzoNa mixture revealed the formation of wormlike micelles. The wormlike micelles were disrupted by Vis light irradiation and then reformed by subsequent UV light irradiation. 3.3. Possible mechanism for photoinduced change in viscosity
Fig. 2. Photographs showing physical appearance of the aqueous CTAB/C4AzoNa (100 mM/35 mM) mixture (a) before and (b) after UV light irradiation.
Here, we discuss the drastic changes in the surfactant solution viscosity when the trans-isomer was photoisomerised to the cisisomer in the aggregates formed from the CTAB/C4AzoNa mixture. The geometrical structure transformation of amphiphilic molecules that have an azobenzene group causes a transition of aggregates formed from surfactants and changes their interfacial properties by light irradiation. In previous studies, researchers have reported that changes in the solubility of an azobenzene compound in water by UV light irradiation leads to the disruption of
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Fig. 4. Variation in the storage modulus, G0 (open circles), and viscous modulus, G00 (filled circles), as a function of frequency for aqueous (a) CTAB/trans-C4AzoNa, and (b) CTAB/cis-C4AzoNa mixtures. Maxwell fits to the experimental data are drawn as solid lines.
wormlike micelles because the cis-type azobenzene compound preferentially dissolves in a bulk aqueous solution instead of penetrating into the surfactant aggregates [18]. The solubility of transC4AzoNa in water, as estimated from UV–vis measurements, was 1.7 mM. Dynamic light-scattering measurements for aqueous CTAB/trans-C4AzoNa mixtures indicated the formation of small micelles 5 nm in diameter. 1H NMR signals corresponding to the aromatic protons of the azobenzene group in the aggregates formed from the CTAB/trans-C4AzoNa mixture were broader than those corresponding to the aromatic protons of the azobenzene group in C4AzoNa dissolved in water (Fig. 5). In mixtures of a cationic surfactant with an anionic surfactant or organic salt, interionic interactions involving electrostatic attraction between oppositely charged ions are known to occur in aqueous solutions. The interaction and steric structures of catanionic surfactants (mixtures of a cationic surfactant and anionic surfactants) influence the morphology of the aggregates. CTAB and trans-C4AzoNa molecules will be closely packed in the aggregates by electrostatic attraction between hydrophilic groups with opposite charges, forming small micelles. On the other hand, UV light irradiation causes isomerisation of trans-C4AzoNa to the cis-isomer with a bulkier structure. The hydrophilicity of the cis-isomer is higher than that of the trans-isomer because the molecular dipole moment increases to 3.0 D as the trans-isomer is photoisomerised to the cis-isomer [22]. In fact, the solubility of cis-C4AzoNa in water estimated from UV–vis measurements was 6.7 mM—ca. 4 times higher than that of the trans-isomer. 1H NMR measurements indicated that signals corresponding to the aromatic protons of cisC4AzoNa in the aggregates were much sharper than those of the trans-isomer (Fig. 5). In addition, the chemical shift for these protons of cis-C4AzoNa moved upfield in the CTAB/cis-C4AzoNa mixture. Considering the NMR chemical shift, cis-C4AzoNa molecules will effectively penetrate into the surfactant aggregates despite their high solubility in water. Therefore, CTAB and cis-C4AzoNa molecules will be packed in the aggregates. However, the packing will be looser than that of trans-isomer because the cis-isomer is bulkier. In other words, trans-C4AzoNa molecules penetrate linearly into CTAB aggregates, whereas cis-C4AzoNa molecules orient within the aggregates as bulky structures (Fig. 6). Furthermore, we have investigated the morphological transition of mixtures of CTAB and sodium (4-phenylazophenoxy)acetate (AzoNa) having a structure in which a butyl chain (C4) in C4AzoNa is removed. Measurements of the frequency dependence of the dynamic viscoelasticity indicated the formation of wormlike micelles
Fig. 5. 1H NMR spectra for (a) saturated solution of C4AzoNa; (b) CTAB/transC4AzoNa mixture; (c) and CTAB/cis-C4AzoNa mixture in D2O. The eight assigned protons of the azobenzene group are shown by the letters a–d. The subscripts t and c denote trans- and cis-isomers, respectively. Signals of trans-C4AzoNa were considerably broadened with the formation of aggregates, thereby precluding interpretation of the chemical shift. In contrast, signals of cis-isomer shifted upfield by self-assembly of C4AzoNa.
in the CTAB/trans-AzoNa (100 mM/35 mM) mixture but not in the CTAB/cis-AzoNa mixture. The hydrophilicity of AzoNa is higher than that of C4AzoNa because AzoNa has no butyl chain (C4). The solubilities of trans- and cis-AzoNa in water were 25 mM and over 100 mM, respectively [18]. From these results, cis-AzoNa molecules cannot penetrate into CTAB aggregates formed in the CTAB/ AzoNa (100 mM/35 mM) mixture because the solubility of AzoNa in water increases with trans–cis photoisomerisation. Thus, the change in the solubility in water would lead to the morphological transition of aggregates formed from the CTAB/AzoNa mixture. On the other hand, variation in the solubility of the C4AzoNa molecule with photoisomerisation will have little influence on the formation and disruption of wormlike micelles in CTAB/C4AzoNa mixtures. Consequently, the trans–cis photoisomerisation of the azobenzene group in C4AzoNa occurs in the aggregates of the CTAB/ C4AzoNa mixture. The morphological transition of the aggregates
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the surfactant solution viscosity drastically increased with UV light irradiation and then decreased with Vis light irradiation. On the other hand, UV light irradiation led to increase in the viscosity in previous research [12–17]. The solution viscosity of the CTAB/ C4AzoNa mixture increases by about 3 orders of magnitude with photoisomerisation of the trans-isomer to the cis-isomer in this system. To the best of our knowledge, this article is first report concerning a drastic increase in surfactant solution viscosity by UV light irradiation. This drastic change in surfactant solution viscosity results from the transformation of small micelles to wormlike micelles by photoisomerisation. In the other words, the geometrical structure transformation of the azobenzene group in the CTAB/ C4AzoNa mixture at the molecular level leads to the macroscopic change in solution viscosity. Wormlike micelles can be applied drag reduction agents under turbulent flow [24]. The formation of wormlike micelles leads to a transition from turbulent flow to laminar flow. The turbulent flow is effective in transfer and diffusion of heat and materials. Therefore, a switchable photorheological fluid that we have demonstrated here will be useful for effective control over drag reduction and transfer of heat and materials. References Fig. 6. Possible mechanism for the formation and disruption of wormlike micelles by light irradiation. When trans-C4AzoNa is photoisomerised to cis-C4AzoNa in the aggregates, the curvature of the aggregates decreases, thereby inducing the transition from small micelles to wormlike micelles.
in aqueous solutions of the mixture can be understood on the basis of the critical packing parameter (CPP) theory [23]. The value of the CPP is defined by the equation CPP = v/al (v, volume of the hydrophobic chain; a, cross-sectional area of the hydrophobic–hydrophilic interface; l, fully stretched length of the hydrophobic chain). The CTAB/trans-C4AzoNa (100 mM/35 mM) mixture formed small micelles, therefore, CPP < 1/3 for the mixture. The structural transformation of the azobenzene group in the aggregates may increase the volume of the hydrophobic chain (v) and subsequently increase the CPP value of the CTAB/C4AzoNa mixture (Fig. 4). The increase in the CPP value causes the formation of large aggregates with lower curvature. Considering the isomerisation ratio upon exposure of the solution to UV light, two kinds of C4AzoNa molecules coexist in the wormlike micelle: cis-C4AzoNa (67 mol%) and trans-C4AzoNa (33 mol%). As a result, the apparent CPP value in the CTAB/cis-C4AzoNa mixture may approximate values capable of forming wormlike micelles, 1/3 < CPP < 1/2. The change in the CPP with the photoisomerisation of C4AzoNa will lead to the transformation of small micelles to wormlike micelles. 4. Conclusions We demonstrated a new photoresponsive change in the viscosity of a surfactant solution by UV light irradiation. In this article,
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