Reversible Photorheology in Solutions of Cetyltrimethylammonium

Oct 1, 2010 - We show photorheology in aqueous solutions of weakly entangled wormlike micelles prepared with cetyltrimethylammonium bromide (CTAB), ...
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Reversible Photorheology in Solutions of Cetyltrimethylammonium Bromide, Salicylic Acid, and trans-2,4,40 -Trihydroxychalcone M. Pereira  Instituto Superior de Engenharia de Lisboa, Area Cientı´fica de Fı´sica Rua Conselheiro Emı´dio Navarro, 1, 1949-014 Lisboa, Portugal

C. R. Leal  Instituto Superior de Engenharia de Lisboa, Area Cientı´fica de Fı´sica Rua Conselheiro Emı´dio Navarro, 1, 1949-014 Lisboa, Portugal, and CENIMAT/I3N, Faculdade de Ci^ encias e Tecnologia, Universidade Nova de Lisboa, Campus da Caparica, 2829-516 Caparica, Portugal

A. J. Parola and U. M. Scheven* REQUIMTE/CQFB, Departamento da Quı´mica, Faculdade de Ci^ encias e Tecnologia, Universidade Nova de Lisboa, Campus da Caparica, 2829-516 Caparica, Portugal Received June 4, 2010. Revised Manuscript Received August 30, 2010 We show photorheology in aqueous solutions of weakly entangled wormlike micelles prepared with cetyltrimethylammonium bromide (CTAB), salicylic acid (HSal), and dilute amounts of the photochromic multistate compound trans-2,4,40 -trihydroxychalcone (Ct). Different chemical species of Ct are associated with different colorations and propensities to reside within or outside CTAB micelles. A light-induced transfer between the intra- and intermicellar space is used to alter the mean length of wormlike micelles and hence the rheological properties of the fluid, studied in steady-state shear flow and in dynamic rheological measurements. Light-induced changes of fluid rheology are reversible by a thermal relaxation process, at relaxation rates which depend on pH and which are consistent with photochromic reversion rates measured by UV-vis absorption spectroscopy. Parameterizing viscoelastic rheological states by their effective relaxation time τc and corresponding response modulus Gc, we find the light and dark states of the system to fall onto a characteristic state curve defined by comparable experiments conducted without photosensitive components. These reference experiments were prepared with the same concentration of CTAB, but different concentrations of HSal or sodium salicylate (NaSal), and tested at different temperatures.

I. Introduction Cetyltrimethylammonium bromide (CTAB) is a canonical member of the family of cationic surfactants whose aqueous solutions can form wormlike micelles stabilized by noncovalent bonds, in the case of CTAB upon the addition of polar or charged additives. Such wormlike systems have been studied for several decades and are used in food, cosmetics, and engineering applications. A large body of literature and a good number of excellent reviews1-5 exist, and we shall therefore limit our introductory remarks to the recapitulation of a few salient facts relevant to the present work. Above the critical micelle concentration, CTAB in water forms charged spherical micelles which, upon the addition of sodium salicylate (NaSal) or salicylic acid (HSal), can grow into very long and reasonably stiff worms, whose persistence lengths are much larger than their diameter. By mean field theory, the lengths of these worms are distributed exponentially.6,7 When the mean length of the self-assembled worms vastly exceeds the *E-mail address: [email protected]. (1) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869–6892. (2) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933–973. (3) Richtering, W. Curr. Opin. Colloid Interface Sci. 2001, 6, 446–450. (4) Cates, M. E.; Fielding, S. M. Adv. Phys. 2006, 55, 799–879. (5) Dreiss, C. A. Soft Matter 2007, 3, 956–970. (6) Mukerjee, P. J. Phys. Chem. 1972, 76, 565–570. (7) Israelatchvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1975, 72, 1525–1568.

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persistence length, they can be thought of as Gaussian coils and entropic springs which, above some overlap concentration C*, form an entangled, shear thinning, and viscoelastic network reminiscent of polymer solutions. However, unlike dense polymers the self-assembled micellar networks evolve not only by lengthpreserving mechanisms of Rouse motion,8 reptation in a tube defined by the surrounding polymers,9,10 or breathing modes,11 but also significantly by the scission and reaction events described by Cates’ reptation-reaction model12 for the interaction of scission, recombination, and reptation in the stress relaxation process. In the limit of fast breakage τbreak , τrep, where τrep is the reptation time, dynamic rheological experiments show Maxwellian behavior for a great number of well-entangled wormlike micellar solution, as predicted by Cates and observed with the water/CTAB/NaSal system13-16 and the water/CTAB/HSal system.17 For a perfect Maxwell fluid, the Cole-Cole plot of (8) Rouse, P. E. J. Chem. Phys. 1953, 21, 1272–1280. (9) de Gennes, P. G. J. Chem. Phys. 1971, 55, 572–579. (10) (a) Doi M., Edwards S. F. J. Chem. Soc., Faraday Trans. 2, 1972, 74, 1789-1801, 1802-1817, 1818-1832;(b) J. Chem. Soc., Faraday Trans. 2, 1979, 75, 38-54. (11) de Gennes, P. G. J. Chem. Phys. 1980, 72, 4756–4763. (12) Cates, M. E. Macromolecules 1987, 20, 2289–2296. (13) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081–1086. (14) Shikata, T.; Hirata, H.; Kotaka, T Langmuir 1988, 4, 354–359. (15) Hartmann, V.; Cressely, R. Colloid Surf., A 1997, 121, 151–162. (16) Cressely, R.; Hartmann, V. Eur. Phys. J. B 1998, 6, 57–62. (17) Shikata, T.; Hirata, H.; Kotaka, T Langmuir 1989, 5, 398–405.

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the viscous modulus G00 (ω) against the elastic modulus G0 (ω) produces a characteristic semicircle. Systematic deviations from Maxwell behavior do occur in the weakly entangled regime reached at low concentrations of the growth-promoting additive. One observes shear thickening and the formation of shear-induced structures under steady-shear experiments.15,16 The rheology in the weakly entangled regime is more complex and considerably less well understood theoretically than that of the well-entangled regime. The experimental work discussed in this paper was performed on samples in this weakly to moderately entangled regime, designated regimes I and II by Shikata et al.13 A pertinent example for this regime near and above the overlap concentration is the water/CTAB/NaSal system at temperature T = 24 °C and [CTAB] = 100 mM, where the zero-shear viscosity rises 105-fold as the NaSal concentration is raised from 20 mM to 30 mM,16 which is evidence of the dramatic growth of mean micellar length and entanglements. The spectacular variation of steady-state viscosity with small changes of salicylate concentration implies the possibility of producing “smart” CTAB-based fluids using charged or polar additives whose propensity to enter a CTABmicelle and stabilize the wormlike morphology can be altered by external stimuli, for example, light.18 Since the early work by Wolf,19 photorheological effects have been investigated in CTABbased surfactant systems, for example, Sakai et al.20 reporting “photo-switchable” rheology in aqueous CTAB/NaSal/AZTMA solutions where AZTMA is an azobenzene derivative, or the irreversible photosensitivity demonstrated in aqueous CTAB and trans-ortho-methoxycinnamic acid mixtures, where UV illumination led to viscosity changes by 4 orders of magnitude.21 In the present work, we employ a photoactive species which thus far has been explored mainly for its photochromic properties, rather than its potential for photorheology, the multistate compound trans-2,4,40 -trihydroxychalcone, Ct. The experiments were in part motivated by prior photochemical results in aqueous Ct-solutions in the presence of CTAB,22 where a UV-induced ejection of the solvatochromic photoproduct-its UV-vis spectrum depends on the chemical environment-from the interior of CTAB micelles into the aqueous extramicellar space could be observed. Ct has the same basic structure as natural chalcones derived from anthocyanins, the ubiquitous colorants responsible for most of the red and blue colors of flowers and fruits. In aqueous solutions, Ct gives rise to a multiequilibrium system involving several chemical species interconvertible by light and pH. As in the examples of CTAB-based photorheological fluids given above, a UV-induced trans-cis isomerization indirectly causes changes in micellar morphology associated with changes of the viscoelastic response of the micellar fluid. In our case, the concentration of the photoactive compound is very low compared to that of CTAB ([Ct]/[CTAB] = 1%). We employ a mixture of Ct and salicylic acid to stabilize a wormlike micellar morphology. The observed photorheological phenomena are reversible by thermal relaxation, at a reversion rate that can be tuned by adjusting the acidity of the solution. We arrive at a dilute system of wormlike micelles quite similar to the canonical CTAB/NaSal system, but one in which the mean micellar length is tuned perturbatively by light, without having to change temperature (18) Eastoe, J.; Vesperinas, A. Soft Matter 2005, 1, 338–347. (19) Wolff, T.; Emming, C. S.; Suck, T. A.; Von Bunau, G. J. Phys. Chem. 1989, 93, 4894–4898. (20) Sakai, H.; Orihara, Y.; Kodashima, H.; Matsamura, A.; Ohkubo, T.; Tsuchiya, K.; Abe, M. J. Am. Chem. Soc. 2005, 127, 13454–13455. (21) Ketner, A. M.; Kumar, R.; Davies, T. S.; Elder, P. W.; Rhagavan, S. R. J. Am. Chem. Soc. 2007, 129, 1553–1559. (22) Gomes, R.; Parola, A. J.; Laia, C. A. T.; Pina, F. Photochem. Photobiol. Sci. 2007, 6, 1003–1009.

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Figure 1. Schematic of the states (chemical species) originated by trans-2,4,4 0 -trihydroxychalcone (Ct) in aqueous solutions. Transitions between the intramicellar Ct and extramicellar AHþ occur via short-lived intermediate states Cc and B2.

or composition. Additionally, photochromism provides a visual indication of the micellar state the fluid is in. In the next sections, we briefly review the multistate nature of Ct, followed by a description of the optical and rheological experiments, results, and a concluding discussion.

II. Samples and Methods Multiple States of trans-2,4,40 -Trihydroxychalcone. This compound has been studied in the presence of aqueous solutions containing spherical CTAB micelles,22 where two distinct forms of the compound were shown to be associated with photochromism and associated different propensities to reside either within a CTAB micelle, the trans-chalcone Ct, or outside of the micelles, the flavylium cation AHþ, both shown surrounded by boxes in Figure 1. Irradiation of the neutral trans-chalcone Ct produces the cis-isomer Cc, which in turn tautomerizes to the closed-ring form B2. This form, depending on pH, evolves either to the positively charged flavylium cation, AHþ, or to the neutral quinoidal base A with pKa(AHþ) = 3.1 in aqueous solution. The intermediate states Cc and B2 are short-lived.23 In the present work, we stabilized wormlike-rather than spherical-CTAB-salicylate micelles by adding HSal to produce a wormlike micellar solution with acidic pH of ∼2. To this, we added Ct, with the expectation that we would be able to observe photoreactions producing AHþ, followed by its ejection from the micelles to the bulk, similar to what was observed in the prior work with spherical CTAB micelles in acidic solutions.22 We surmise that the different shapes of transchalcone Ct and A and the positive charge of AHþ are drivers for the ejection from the positively charged micelles. Synthesis of trans-2,4,40 -Trihydroxychalcone (Ct). All reagents and solvents used were of analytical grade. The NMR spectra at 298.0 K were obtained on a Bruker AMX400 operating at 400.13 MHz (1H) and at 100.62 MHz (13C). Elemental analysis was obtained on a Thermofinnigan Flash EA 1112 2eries instrument. trans-2,4,40 -Trihydroxychalcone (Ct) was isolated from the corresponding 7,40 -Dihydroxyflavylium hydrogen sulfate. The flavylium was synthesized by modification of a method originally described by Robinson and Pratt 24 and later extended by Michaelidis and Wizinger,25 using sulfuric acid instead of gaseous hydrochloric acid. 2,4-dihydroxybenzaldehyde (20 mmol, 2.76 g) and 4-hydroxyacetophenone (20 mmol, 2.72 g) were dissolved in 20 mL of acetic acid. Five milliliters of concentrated sulfuric acid (23) (a) Figueiredo, P.; Lima, J. C.; Santos, H.; Wigand, M. C.; Brouillard, R.; Maanita, A. L.; Pina, F. J. Am. Chem. Soc. 1994, 116, 1249–1254. (b) Pina, F.; Melo, M. J.; Ballardini, R.; Flamigni, L.; Maestri, M. New J. Chem. 1997, 21, 969–976. (c) Pina, F.; Lima, J. C.; Parola, A. J.; Afonso, C. A. M. Angew. Chem., Int. Ed. 2004, 43, 1525–1527. (d) Galindo, F.; Lima, J. C.; Luis, S. V.; Parola, A. J.; Pina, F. Adv. Funct. Mater. 2005, 15, 541–545. (24) Robinson, R.; Pratt, D. D. J. Chem. Soc. Trans. 1922, 1577–1585. (25) Michaelidis, C.; Wizinger, R. Helv. Chim. Acta 1951, 34, 1761–1770.

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was added dropwise (with clear increase of a dark orange color after each drop) and the solution left under stirring overnight. The brown precipitate that formed was filtered and washed with ethyl acetate and ethyl ether. The solid was pump-dried yielding 2.34 g (35%), with the following NMR spectrum: 1H NMR (D2O, pD ≈ 1, 298 K) (ppm): 8.79 (1H, d, 3J = 8.7 Hz), 8.11 (2H, d, 3J = 8.7 Hz), 7.98 (1H, d, 3J = 8.7 Hz), 7.88 (1H, d, 3J = 8.8 Hz), 7.27 (1H, d, 4J = 2.1 Hz), 7.21 (1H, dd, 3J = 8.8 Hz, 4J = 2.1 Hz), 6.90 (2H, d, 3J = 8.7 Hz)). To isolate the trans-chalcone, the obtained flavylium was dissolved in 0.1 M sodium hydroxide in order to form unprotonated trans-chalcones Ct2-/Ct3-. The reaction was followed by UV-vis. After about 20 min, the solution was stable and was carefully neutralized with hydrochloric acid until pH of 6.5. Below pH of about 7.5, a brown solid starts to precipitate. After the precipitation was complete, the solid was filtered, then washed with cold water and ethyl ether. After drying, 1.08 g (61%) were obtained. 1H NMR (CD3OD, 298 K) (ppm): 8.02 (1H, d, 3J = 15.6 Hz), 7.94 (2H, d, 3J = 8.7 Hz), 7.64 (1H, d, 3J = 15.6 Hz), 7.49 (1H, d, 3J = 8.2 Hz), 6.87 (1H, d, 3J = 8.7 Hz), 6.35 (3H, m). 13C NMR (CD3OD, 298 K) (ppm): 191.91, 163.53, 162.71, 160.72, 142.32, 132.08, 131.55, 118.99, 116.33, 115.63, 109.04, 103.57. EA calcd. for C15H12O3 3 2H2O: C, 61.64; H, 5.52. Found: C, 61.83; H, 5.87. Fluid Preparation. CTAB and HSal of analytical grade were purchased from SIGMA-Aldrich and Acros Organics, respectively, and used as received. Deionized water was stirred with a magnetic stirrer, and CTAB, trans-chalcone Ct, and HSal were added successively to produce solutions with the desired concentrations. After preparation, samples were left to equilibrate for at least 24 h in the dark. The pH was confirmed to be about 2 using pH-indicator paper. All fluid samples were prepared with concentrations of [CTAB] = 100 mM, with acid concentrations [HSal] between 20 mM and 40 mM and trans-chalcone concentrations between 0 mM and 5 mM. The observed viscosity and the viscoelastic response confirm our expectation that the samples contained wormlike micelles indeed, as would be expected from published rheological results on [CTAB]:[HSal] in comparable concentration ranges.17 Optical Spectroscopy. The photochromic properties of several samples were determined quantitatively by UV-vis absorption spectroscopy with a Cary 100 spectrophotometer, producing similar results for all tested samples. The sample fluid was introduced into the gap between two quartz plates separated by about 100 μm. The sample was then illuminated using sunlight. During the exposure to light, the photochromic yellowish solution of Ct begins to darken after a few minutes, indicating the formation of the flavylium cation AHþ. The sample was placed in the spectrometer, and UV-vis absorption spectroscopy was performed over the next 90 min, permitting real-time monitoring of the reversion of optical spectra from those of characteristic of the illuminated state to those associated with the dark state. Optorheology. Rheological measurements were performed in a TA Instruments AR500 stress-controlled rheometer, using a standard cone and plate geometry with cone diameter 40 mm, cone angle 2°, and a gap of 57 μm. The cone was made of transparent acrylic. An external (solarium) lamp with sufficient spectral weight in the UV was installed to illuminate the sample and cone from the side, allowing us to drive the UV-light induced structural transition in situ. A water-filled aquarium fitted with an optical mask was placed between the lamp and the sample, thus shielding the rheometer and sample from the heat generated by the lamp. Additionally, the sample temperature was controlled using a Peltier element installed in the plate. Three types of rheological experiments were performed in the presence or absence of Langmuir 2010, 26(22), 16715–16721

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UV illumination. The first were standard flow curves measuring viscosity as a function of steady shear rate, in which the shear rate was stepped through a succession of steady shear rates for each of which the viscosity was measured and recorded. Flow curve experiments were conducted at shear rates between 0.1 s-1 and 200 s-1 and were preceded by a preshear period lasting 30 s, at a shear rate of 10 s-1. At each targeted shear rate, the rate was kept constant sufficiently long to produce a minimum of 100 units of deformation. Second were dynamic rheology experiments in which small-amplitude oscillatory shear stress is imposed and the frequency dependent complex shear modulus is measured, thus determining the viscous and elastic response of the fluid as a function of frequency. The shear amplitude for all dynamic results reported here was less than 50%. In the third type of experiment, shear stress measurements were made at fixed shear rates, as a function of illumination, thus measuring the temporal evolution of the viscous response to the optically triggered isomerization and to subsequent relaxation processes occurring after the light was turned off. Unless stated otherwise, the fluid was kept at 20 °C.

III. Results Absorption Spectra vs Time. The color of the water/CTAB/ HSal/Ct mixtures under investigation changed from a light yellowish hue in samples that had been kept in the dark to a strong dark-orange upon exposure to sunlight or UV irradiation. This characteristic signature of UV-induced transformation of a Ct to AHþ, via the reaction chain of Figure 1, provided an easy qualitative check of the state of the fluid. Optical absorption spectra were then obtained to characterize the thermal reversion of AHþ to Ct in two samples with concentrations [CTAB]:[HSal]:[Ct] = 100:20:5 mM and [CTAB]:[HSal]:[Ct]=100:40:5 mM, shown in Figures 2 and 3, respectively. The absorption spectra show a decreasing AHþ absorption peak centered around 450 nm, combined with a growing Ct peak centered around 366 nm. The absorption maximum of the flavylium cation AHþ is the same as that observed in the absence of micelles, proving its ejection.22 The AHþ concentration is determined, up to a constant of proportionality, by integrating the absolute value of the difference between the irradiated state and the dark state absorption over wavelengths between 300 and 500 nm, at various times during the relaxation process. The time dependence of the thermal reversion process is monoexponential. The time constants for the decay of AHþ concentrations are τr =25 min and τr = 46 min for the two samples, respectively. These time constants are consistent with the relaxation rates derived from photolysis experiments determining rate constants for the same process, in the presence of spherical CTAB micelles.22 Furthermore, the observation that the relaxation occurs more slowly for the more acidic solution is consistent with the prior work as well. Our optical data thus indicate that the kinetics governing the thermal conversion of AHþ to Ct and the incorporation of Ct within the micelles are not altered significantly by the fact that the micelles are wormlike and have somewhat reduced surface charge compared to spherical micelles. We now turn toward rheological measurements to observe photorheological behavior associated with the presence or absence of Ct within the micelles. Steady-State Rheology. In the first type of rheometric measurements, viscosity was measured as a function of shear rate and lighting conditions. Figure 4a shows data from the sample with [CTAB]:[HSal]:[Ct] = 100:25:1 mM. The flow curves associated with the illuminated state and the dark states exhibit a Newtonian plateau of the shear rate at low shear rates, followed DOI: 10.1021/la102267d

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Figure 2. Optical absorption spectra of a fluid with [CTAB]: [HSal]:[Ct] = 100:20:5 mM, acquired in the dark, after UVillumination. The absorption peak centered around 450 nm is associated with extramicellar AHþ; the peak at 366 nm is associated with intramicellar Ct. The inset shows the absolute value of the difference between the time evolving absorption and the final dark state absorption, integrated over the wavelengths from 300 nm to 500 nm, and normalized to the initial value. This is proportional to the concentration of AHþ. The relaxation time of the monoexponential process is 25 min.

Figure 3. Optical absorption spectra of a fluid with [CTAB]: [HSal]:[Ct] = 100:40:5 mM, acquired in the dark, after UV illumination. The relaxation time is 46 min.

by a shear thinning regime for shear rates above some critical rate, similar to what is commonly observed for such wormlike surfactant solutions.4 The Newtonian viscosity at low shear rates was lower in the illuminated state, and the characteristic shear rate where the viscosity turns over to become shear-thinning is higher 16718 DOI: 10.1021/la102267d

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Figure 4. [CTAB]:[HSal]:[Ct] = 100:25:1 mM: (a) steady-state shear viscosity in the presence or absence of light, showing reduced plateau viscosity for the illuminated sample and a crossover at higher shear rates. Arrow marks shear rate γ_ = 0.5/s. (b) Steady-state shear experiments at a shear rate of γ_ = 0.5/s, as a function of illumination. UV light drives the fluid to a lower viscosity state; the dark-state viscosity is recovered after the light is turned off, on time scales consistent with the observed spectral recovery rates shown in Figure 2.

than the equivalent characteristic shear rate in the dark state. We also note a crossover of viscosities, where the illuminated sample exhibits higher viscosities at the highest shear rates, in contrast to the Newtonian flow region. We observed broadly similar behavior, including a crossover, in other light-sensitive samples with somewhat different concentrations of HSal and Ct. Figure 4b shows the result from an experiment probing the transition dynamics between the light and dark states: viscosity was measured at a fixed shear rate of γ_ = 0.5 s-1 while changing the lighting conditions. In the initial absence of light, a steady shear viscosity of about 7 Pa 3 s was measured for 10 min, at which point the light driving the Ct to Cc transition was turned on. In response, the viscosity dropped by a factor of 2 over the course of a few minutes, after which it stabilized. The rate of change and the magnitude for this light induced viscosity reduction depends on total photoproduction of AHþ, which in turn depends on the quantum yield of the transformation, the intensity of the applied light, and how well we managed to couple light into the cone and plate geometry. After steady state was established, we turned the light off again, to observe the recovery of the dark state viscosity, a nonexponential process occurring on a time scale, estimated using the 1/e time of recovery of about 15 min. This recovery time scale is reasonably consistent with the relaxation times measured in the spectroscopic experiments on samples with comparable pH discussed above, supporting the notion that differences in steady-state viscosities of illuminated and dark states are attributable to movement of Ct and AHþ between the intramicellar and extramicellar spaces, which also cause the photoinduced changes of coloration. The observed nonexponential nature of viscous recovery, as opposed to that of spectroscopic recovery, is presumably due to the fact that viscosity is not a linear function of the concentration of Ct within the wormlike micelles or due to the presence of viscoelastic relaxation time scales comparable to the inverse shear rate. Langmuir 2010, 26(22), 16715–16721

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Figure 5. Without UV illumination: storage G0 (ω) and loss G00 (ω) moduli as a function of angular frequency ω for [CTAB] = 100 mM and [Ct] = 1 mM, for three concentrations of salicylic acid. Arrows mark the crossover angular frequencies and moduli. The ColeCole plot of the insert shows the departure from Maxwell behavior. The dotted line shows the result of a [CTAB] = 100 mM, [NaSal] = 30 mM comparison experiment conducted at the same temperature of 20 °C.

We conducted control experiments with samples where HSal was replaced with sodium salicylate (NaSal), giving us wormlike micelles but no acidity. These samples showed neither photochromism nor viscosity changes upon UV illumination; thus, we rule out heating as a cause for the observed viscosity changes of Figure 4. Dynamic Rheology. Small amplitude measurements of frequency- and illumination-dependent rheology were then performed on three samples prepared with [CTAB] = 100 mM and [Ct] = 1 mM and concentrations of salicylic acid [HSal] =20, 25, and 30 mM, respectively. Frequency-dependent elastic moduli G0 (ω) and viscous moduli G00 (ω) measured in the dark are shown in Figure 5. The crossover where G0 (ω) = G00 (ω) = Gc defines an effective “characteristic” frequency somewhat similar to that of a perfect Maxwellian fluid where the crossover frequency defines the intrinsic relaxation time or frequency of the fluid. The effective frequency of each sample is marked by an arrow. It decreases by nearly 2 orders of magnitude with moderate increases of the [HSal] concentration from 20 mM to 30 mM, while the crossover modulus grows somewhat as well. The observed growth of the characteristic crossover time τc = 1/ωc with increasing HSal concentration is most certainly due to growth in the length of the wormlike micelles. We also note that a comparison experiment with the canonical water/CTAB/NaSal fluid in which [CTAB] = 100 mM and [NaSal] = 30 mM produced rather similar data, shown by the dotted line of Figure 5. Whether or not these are Maxwell fluids can be seen with Cole-Cole plots of the same data, shown in the inset. They are not, as the plots do not appear as Maxwellian semicircles, but sections of the data do appear to fall onto distorted arcs of a semicircle; for the most acidic solution ([HSal] =30 mM), with its presumably longer worms, a somewhat Maxwellian turnover is observed. The deviations from Maxwell behavior are not unexpected, because by design, the fluids are not in the well-entangled regime associated with Maxwellian Langmuir 2010, 26(22), 16715–16721

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Figure 6. With UV illumination: Storage G00 (ω) and loss G00 (ω)

moduli as a function of angular frequency ω for [CTAB] = 100 mM and [Ct] =1 mM, for three concentrations of salicylic acid. The inset shows the same data in the Cole-Cole representation.

frequency response explained by Cates’ original reptation-reaction model.12 Nevertheless, the presence of Maxwellian arcs in the Cole-Cole plot indicates the presence of wormlike micelles, rather than some other micellar morphology, and the Cole-Cole plots do bear good resemblance to published data on water/CTAB/ NaSal systems at slightly higher temperatures.13,15 Figure 6 shows the result of the corresponding rheological experiments upon UV illumination. The curves for samples with [HSal] = 25 mM and [HSal] =30 mM shift to higher frequencies, and for the higher acid concentration, one observes a downward shift of the response curves, with a corresponding downward shift of the crossover modulus Gc = G0 (ωc) = G00 (ωc). For the least wormlike sample with [HSal] =20 mM, there is little change, and qualitatively, the overall appearance of the data and the Cole-Cole plots is maintained. The observed rheological changes are therefore not dramatic, particularly when compared to systems in which there is a triggered phase transition to a spherical or vesicular and thus unentangled micellar morphology.26 The absence of a structural phase transition, however, permits quantitative and unified comparison with other CTAB-based fluids exhibiting wormlike micelles in the poorly entangled regime, without invoking a specific rheological model. To make the comparison, we prepared and tested a range of samples, listed in Table 1, with the same concentration [CTAB] = 100 mM but different concentrations of HSal, NaSal, and Ct at higher concentrations; some were measured at different temperatures. We then represent their rheological state-taken to correspond to a well-defined state of entanglements and of mean micellar lengths-by two numbers, the characteristic crossover time τc=1/ωc and the corresponding crossover modulus where storage and loss moduli are equal Gc =G0 (ωc)=G00 (ωc). Our underlying assumption here is that all fluids in our comparison are essentially the same, except for the degree of entanglement governed by the unknown mean micellar length L µ φ1/2 exp(E/kbT) and the persistence length, where E is the end-cap energy whose value is (26) Davies, T. S.; Ketner, A. M.; Raghavan, S. R J. Am. Chem. Soc. 2006, 128, 6669–6675.

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Table 1. [CTAB] = 100 mM: Concentrations of Worm Stabilizing Additives, Temperature, and Lighting Conditions, for Labeled Data Represented in Figure 7a ID

[Ct] (mM)

[HSal] (mM)

N1 0 0 N2 0 0 N3 0 0 N4 0 0 N5 0 0 N6 0 0 NC1 1 0 NC2 1 0 S1 0 0 (S2) 0 0 S3 0 0 H1 0 30 H2 0 40 HN 0 30 HC1 5 25 HC2 5 27.5 HC3 5 30 a S1, (S2), S3 from Shikata et al.13

[NaSal] (mM)

T (°C)

light

25 30 30 30 30 40 25 25 27.5 32.5 40 0 0 5 0 0 0

20 20 23 25 29 20 20 20 25 25 25 20 30 20 20 20 20

n n n n n n n y n n n n n n n n n

tuned by adding intercalants. We make this assumption because the micellar volume fraction φ is about the same in all samples, as it is dominated by the equal CTAB concentration; by comparison, the volume of the intercalated salicylate is small and the somewhat larger intercalated Ct is present in small concentrations only, with [Ct]/[CTAB] = 0.01. We further assume the persistence length to be roughly the same, since all the wormlike micelles used here were mostly composed of CTAB. Stress relaxation is assumed to be dominated by similar disentanglement time scales, rather than scission, because the micelles are near the overlap concentration by design. With these conceptual simplifications, the same rheological state and mean micellar length characterized by (Gc, τc) should be attainable in experiments on fluids with the same concentrations of CTAB but different concentrations of growth-inducing additives or different temperatures. Our plot of Gc versus τc in Figure 7 shows that this is the case. The plot defines a state curve characteristic of the chosen concentration [CTAB] = 100 mM, where each position along the curve is be presumed to be associated with a mean micellar length L common to all samples. The dashed line in Figure 7 is a guide to the eye obtained by a simple fit to all the data except the photorheological data points themselves and except the data points marked HC and (S2) to which we will return below. The state curve encompasses experiments from the most viscoelastic solutions tested here, with [NaSal] = 40 mM at a temperature of 20 °C (N6), to the least viscoelastic at [NaSal] = 20 mM and temperature of 29 °C, with experiments conducted at other temperatures and with different intercalants lying in between. The data points marked NC on the state curve belong to the photorheological control experiment. Comparing with prior experiments with water:CTAB:NaSal, we find some of the data by Shikata et al.13 (S1 and S3) to agree with our data reasonably well. We cannot explain our failure to reproduce their data point (S2) in our experiment N4 of Table 1, but given the wealth of data at hand, we set this discrepancy aside. The photorheological data points extracted from Figures 5 and 6 fall onto or near the fitted state curve. Upon illumination, the measured crossover moduli of the two more acidic samples move toward the less viscoelastic part of the state curve on the left, as marked by the two arrows of Figure 7. The light-induced shifts indicate a reduction in overlap and mean micellar length L. The least viscous sample with the lowest concentration of [HSal] = 20 mM shows no photorheology, suggesting that here overlap is 16720 DOI: 10.1021/la102267d

Figure 7. Crossover modulus Gc = G0 (ωc) = G00 (ωc) as a function of effective of characteristic time τc = 1/ωc. The dashed line is an approximate state curve representing the rheological states of weakly entangled wormlike CTAB micelles with [CTAB]=100 mM, under conditions listed in Table 1. The light-induced rheological changes (arrows) connect points lying on the state curve.

poor and the degree of entanglement is too small for changes in mean micellar length to matter much. The observed photoinduced rheological changes are reversed by thermal relaxation once the light is turned off. The consistency of the state curve description and the fact that photoinduced shifts occur along the state curve, within the present range of parameters, supports our working assumption that the micellar persistence length is about the same for all experiments and that the scission rate is either the same for all experiments or not important in this dilute regime because disentanglement dominates stress relaxation. As part of our exploration of the experimental parameter space, we also conducted experiments, marked by the stars labeled HC in Figure 7, with a higher concentration of [Ct] = 5 mM. For these experiments, the mapping of rheological states onto the state curve breaks down, which may indicate that scission rates or the persistence length or both are now affected significantly by the intercalated photoactive compound. Possibly, the higher concentration of Ct induces branching, as has been observed in other systems with wormlike micelles.27 Some experiments with higher concentrations of HSal were tried as well, showing somewhat smaller relative photorheological changes; as a result, these were not pursued further.

IV. Conclusion We have shown that very small concentrations of Ct can give rise to a reversible photochromic and photorheological response of CTAB:HSal solutions, in the poorly entangled regime where viscoelastic properties depend strongly on the concentration of the intercalant. Light and thermal relaxation processes drive the transfer of the photosensitive compound Ct between wormlike CTAB micelles and the bulk, respectively. The rheological states observed in the presence or absence of light, characterized by the effective characteristic time τc = 1/ωc and the crossover modulus Gc, map onto those observed with experiments in which the (27) Couillet, I.; Hughes, T.; Maitland, G.; Candau, F.; Candau, S. J. Langmuir 2003, 19, 8536–8541.

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rheological state was altered by conventional means, i.e., changes in temperature or composition. This implies that at low concentrations of photoactive compound used here the local micellar dynamics, bending, persistence length, and scission are not affected much by the presence of the additional Ct. By contrast, the photosensitive intercalant does promote growth of the mean micellar length by modifying the end-cap energy. For higher concentrations of the photosensitive compound, the measured rheological properties of our photochromic and photorheological system no longer map onto those of our reference fluids without photoactive compounds, perhaps indicating changes in the breakage frequency or the emergence of branching points in the network of entangled micelles. The observed changes of rheology correlate with photochromic signatures (color changes) which

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Article

occur on the same time scale as the rheological changes. Absent acid, both photochromism and photorheological effects vanish, which is further evidence for their common origin. The rate at which ejected Ct returns into the micelle can be tuned by small changes of pH; for future experiments, this permits systematic and continuous exploration of the dilute entangled regime without requiring changes of temperature or composition, in experiments where samples are illuminated in situ and the resulting mean micellar length is set by the competition of light-induced ejection and thermal reversion. Acknowledgment. Supported by the Portuguese Foundation for Science and Technology FCT-MCTES under grant PTDC/ ENR/65170/2006.

DOI: 10.1021/la102267d

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