Substantia. An International Journal of the History of Chemistry 6(2): 15-26, 2022 Firenze University Press www.fupress.com/substantia ISSN 2532-3997 (online) | DOI: 10.36253/Substantia-1736 Citation: Tatini D., Raudino M., Sarri F. (2022) Light-Modulated Rheological Properties in Green Innovative For- mulations. Substantia 6(2): 15-26. doi: 10.36253/Substantia-1736 Received: Mar 07, 2022 Revised: Jul 11, 2022 Just Accepted Online: Jul 12, 2022 Published: September 1, 2022 Copyright: © 2022 Tatini D., Raudino M., Sarri F. This is an open access, peer- reviewed article published by Firenze University Press (http://www.fupress. com/substantia) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All rel- evant data are within the paper and its Supporting Information files. Competing Interests: The Author(s) declare(s) no conflict of interest. Light-Modulated Rheological Properties in Green Innovative Formulations Duccio Tatini*, Martina Raudino, Filippo Sarri Department of Chemistry “Ugo Schiff ” and CSGI, University of Florence, Via della Last- ruccia 3, 50019 Sesto Fiorentino (Firenze), Italy *E-mail: duccio.tatini@unifi.it Abstract. The addition of azorubine to a viscoelastic aqueous dispersion of sodium oleate (NaOL, 0.43 M, 13% w/w) and KCl (up to 4% w/w) leads to a green gel-like sys- tem whose rheological behavior can be efficiently and reversibly triggered from remote by using UV light. Rheology, Differential Scanning Calorimetry (DSC) measurements and phase behavior studies indicate that the original texture of the NaOL dispersion is significantly hardened upon UV irradiation for 8 hours in the presence of azorubine, showing a seven hundred-fold increase in viscosity. The UV treatment brings about the trans to cis isomerization of azorubine, which modifies the structure of the NaOL wormlike micellar system, leading to a more entangled, close-textured network. The cooperative effect of KCl on the fluid viscosity is found to be concentration-dependent. The system slowly reverts to its original rheological behaviour after standing for about 1 day. These results are relevant for the development of stimuli-responsive innovative systems based on biocompatible, non expensive and commercially available materials that can be used in a wide range of applications, such as in drug delivery or enhanced oil recovery, where a quick change in the physico-chemical features of the system is required but difficult to be performed. Keywords: green chemistry, sodium oleate, azorubine, viscosity, stimulus-responsive. INTRODUCTION Responsive or “smart” materials are functional materials whose proper- ties can undergo controlled and reversible changes in response to an external stimulus [1–4]. The applied stimulus or external field include thermal, elec- trical, magnetic, pH, UV-visible light, ionic or metallic interactions or com- binations thereof [5,6]. The formulation of gel-based stimuli-responsive sys- tems with specific performances is crucial for a great number of applications, and particularly when it is impossible or very difficult to switch on/off their properties, and a remotely controlled trigger is necessary [5,7]. Among these systems, viscoelastic surfactant (VES) solutions have attracted great attention due to their unique features and versatility that can be harnessed in a variety of high-tech and everyday applications [8–15]. The amphiphilic nature of VES molecules leads to the self-assembly in solution into small aggregates, which show a complex phase behavior: the 16 Duccio Tatini, Martina Raudino, Filippo Sarri simplest structure are spherical micelles, but also hex- agonal, lamellar, vesicular, cubic, reverse phases can be observed, depending on the surfactant packing param- eter and global packing constraints [2,16–19]. Under certain conditions (i.e. surfactant concentra- tion, salinity, temperature, presence of different coun- terions, change in the composition of the solvent, etc.) spherical micelles may undergo uniaxial growth and form elongated and flexible structures, usually referred to as ‘‘wormlike’’ micelles (WLM) [19–21]. Above a critical concentration these systems show vis- coelastic properties, like polymer solutions or bicontinuous three-component ionic microemulsions, as a result of the formation of a densely entangled network [19,22–26]. In our previous series of works we extensive- ly investigated the main physicochemical properties and the phase behavior of wormlike micellar systems based on sodium oleate (NaOL), a safe, eco-friendly and cost-effective surfactant [19,26]. In the first part we reported on the structure, thermal properties and rheological behavior of NaOL aqueous dispersions in the presence of a single salt (KCl) via cryo-TEM, rheol- ogy and  DSC  experiments [19]. In part 2 we illustrated the specific ion effect induced by the addition of dif- ferent salts on viscoelastic dispersions of sodium and potassium oleate, and we systematically discussed it in terms of the Hofmeister series [26]. In a previous work we imparted a voltage-dependent responsiveness to an NaOL aqueous dispersion through the addition of car- bon black particles [27]. Prompted by these studies on green formulations with suitable responsiveness to different physical stimuli, in the present contribution we developed a NaOL-based viscoelastic dispersions that, in the presence of minimal amounts of azorubine, a biocompatible dye, and a salt (KCl) undergo a remarkable change in their rheological properties upon irradiation at a specific wavelength. NaOL finds application in a wide number of indus- trial products and commercial formulations, like health- care products, cleansers, thickening agents, emulsifiers, lubricants and fluids for enhanced oil recovery [28–32]. Thanks to its negative charge, NaOL is more biodegrad- able and less harmful for the environment compared to cationic surfactants [33]. NaOL shows a very interesting phase behavior [34] and forms different nano- and micro-structures in solu- tion upon the addition of electrolytes or as a result of pH variation [19]. For an extensive discussion about the structural features and the rheological behavior of NaOL dispersions please see references [19,26,35–41] and refer- ences therein. In this work we report on a moderately concentrated (0.43 M, 13% w/w) dispersion of NaOL in water that gives rise to a wormlike micellar network with peculiar struc- tural properties and rheology. The choice of this surfactant concentration is related to the NaOL/water systems’ phase diagram: at this concentration and 25° C the dispersion converts from a simple fluid micellar L1  to a viscous L1* phase that shows shear-dependent birefringence [19]. Azorubine is a synthetic azo dye approved for food decorations and coatings and as drink additive [42]. Its chemical structure is shown in Figure 1. The presence of the azo moiety enables a trans-cis isomerization upon irradiation with light at an appropriate wavelength, usu- ally in the UV region [43–46]. The process may revert spontaneously upon heating since the trans isomer is thermodynamically more sta- ble, or can be induced through irradiation with a visible light [47]. Previous studies reported on the introduction of dif- ferent chromophores in VES systems in order to obtain light-responsive fluids with tunable rheological properties. These photo-active molecules include synthetically modi- fied azobenzenes, [48–54] p-coumaric acid [55,56] and cinnamic acid derivatives [57–59]. We selected azorubine because of its unique advantages in terms of availability, simple manipulation, low cost and, above all, safety. This work is a proof-of-concept that shows the effi- cacy of combining completely biocompatible and non- toxic materials to obtain a versatile formulation with a viscosity and rheological behaviour that can be remark- ably modified through the irradiation with UV light. Moreover, all these features make these systems very attractive for a wide range of applications, including agriculture, food industry, cosmetics, “smart” materials, enhanced oil recovery, shale gas extraction, drug deliv- ery and controlled release. MATERIALS AND METHODS Materials Sodium oleate (ACS reagent grade, Riedel-De Haёn, Seelze, Germany) and potassium chloride (> 99 %, Sig- ma-Aldrich, Milan, Italy) were used as received without any further purification. Azorubine (Carmoisine, Food Figure 1. Trans-cis isomerization of azorubine. 17Light-Modulated Rheological Properties in Green Innovative Formulations red 3 or E 122, disodium 4-hydroxy-3-((4- sulphonaton- aphthyl)azo) naphthalenesulfonate), food grade quality) was supplied by F.lli Rebecchi S.r.l. (Piacenza, Italy) and used without any further purification. All solutions and dispersions were prepared with Milli-Q water (resistivity > 18 MΩ cm at 25°C). Sample Preparation Sodium oleate viscoelastic formulations were pre- pared by the addition of a weighted amount of sur- factant to KCl aqueous solutions at different concentra- tions (0, 0.1, 0.5, 1, 2, 3, 3.5, 4 % w/w) under constant stirring at room temperature. The final concentration of NaOL was 0.43 M (13 % w/w) in all the samples. The dye-loaded samples were prepared following a similar procedure: to a 0.18 % w/w (3.6 10-3 M) azorubine aque- ous solution the proper amount of potassium chloride and then of the surfactant were added. The final con- centrations of KCl were 0.1, 0.5, 1, 2, 3, 3.5, 4% w/w. For all the samples we used Milli-Q water, which was boiled for 4 h, filtrated and stocked under argon. As reported in our previous works the samples were freshly prepared and tested within 1 hour for the rheological and DSC experiments [19,26]. All these precautions are necessary in order to avoid the uptake of atmospheric CO2 by the samples [60]. UV Irradiation About 10 mL of sample were placed in a quartz con- tainer and irradiated using a Camag UV lamp (wave- length 254 nm, 8 W, Muttenz, Switzerland) for 8 hours. All the experiments were conducted at 25 °C, and the distance between the sample and light source was fixed at 5 cm. Rheology Measurements Rheology experiments were carried out on a Paar Physica UDS 200 rheometer using a plate-plate geom- etry with a diameter of 40 mm and a measurement gap of 300 μm. The temperature was fixed at 25.0 ± 0.1 °C using a Peltier control system for all the measurements. The samples were equilibrated for 15 min at the set tem- perature before being tested. Frequency sweep meas- urements were carried out within the linear viscoelas- tic range at a strain value of 1%, which was previously determined by means of an amplitude sweep test. The storage and loss moduli (G’ and G’’, respectively) were measured over the frequency range of 10−3  and 102 Hz. The flow curves were acquired in a torque range between 10−1  and 5·103  mN·m. The experimental viscosity (η) was fitted with the Cross model (see the Results and Discus- sion) to obtain the zero-shear viscosity  (η0),  the infinite- shear viscosity (η∞),  the shear relaxation exponent (m) and the consistency (C). The experiments were repeat- ed at least three times, and silicon oil was applied to the rim of the measurement geometry to prevent water evaporation from the sample. Differential Scanning Calorimetry Differential Scanning Calorimetry (DSC) was per- formed by means of a DSC-Q2000 by TA Instruments (Philadelphia, PA). The samples were sealed in alu- minum hermetic pans and the measurements were con- ducted under N2 atmosphere, with a flow rate of 50 mL/ min. The samples were first cooled from 20 °C to − 60 °C at 10 °C/min, then heated up to 50 °C at 5°C/min. The thermograms which show overlapping endothermic peaks were analyzed using a linear combination of expo- nentially modified Gaussian (EMG) functions, as report- ed elsewhere [19,26,61,62]. RESULTS AND DISCUSSION Flow curve experiments Figure 2 shows the flow curves obtained before (sol- id lines) and after (dashed lines) UV irradiation from the aqueous dispersions of sodium oleate (NaOL, 13% w/w) in the presence of KCl at different concentrations (0, 0.5, 2 and 3% w/w). The curves for the other KCl concentra- Figure 2. Flow Curves before (solid lines) and after (dashed lines) UV irradiation for 13% w/w NaOL (0.43 M, black) and in the pres- ence of KCl at 0.5% (green), 2% (blue) and 3% w/w (red). The flow curves for non-irradiated samples are reprinted from [19] with per- mission from Elsevier. Copyright 2021. 18 Duccio Tatini, Martina Raudino, Filippo Sarri tions are reported in Figure S1 in the Supporting Infor- mation. The experimental data on non-irradiated sam- ples are reproduced from [19] with permission. All the samples show the typical flow behavior of wormlike micellar networks, i.e., a Newtonian pla- teau followed by a steep decrease in the viscosity at high shear stress, due to the shear-induced alignment of cylindrical aggregates. Moreover, a concentration- dependent increase in the viscosity is observed upon the addition of KCl [19,63]. The fitting of the experimental viscosity (η) data was performed using the Cross model [64]: 𝜂𝜂 = 𝜂𝜂! + 𝜂𝜂! − 𝜂𝜂" 1 + (𝐶𝐶�̇�𝛾)# (1) Here η0 is the zero-shear viscosity, m the shear relaxation exponent, η∞ the infinite-shear viscosity, the shear rate and C the consistency. The extracted fitting parameters are reported in Table S1 in the Supporting Information. The results nicely agree with the literature data and confirm the shear thinning behavior of the dispersions, originating from the micelles’ entangle- ment and their progressive alignment at high shear rates [19,26,34,65,66]. The UV irradiation does not affect the flow proper- ties of the formulations: the profiles, as well as the values of η0 exhibit minor fluctuations within the experimental uncertainty before and after the UV treatment. The flow curves for the NaOL samples at different KCl concentration (0, 0.1%, 0.5% and 4%) in the pres- ence of 0.18% w/w (3.6 10-3 M) azorubine are reported in Figure 3. The flow curves at the other KCl concentra- tions are shown in Figure S2 (see the Supporting Infor- mation). In the presence of azorubine all the formulations exhibit a shear-thinning behaviour that is similar to that found for the NaOL-KCl systems. The results obtained from the fitting of the experimental data with the Cross model are reported in Table S1 in the Supporting Infor- mation. After UV irradiation the viscosity of the sam- ples with 0.1%, 0.5% and 4% KCl increases from 6.94 to 13.6, from 278.8 to 728.3 and from 1.852 to 730.1 Pa∙s, respectively. For the other KCl concentrations no signifi- cant variations in the formulation viscosity are observed after the UV treatment. These findings lead to two important conclusions: (i) azorubine has a remarkable effect as photo-active mol- ecule in modifying the viscosity of the fluids after UV irradiation. A similar light-modulated viscosity change is reported for binary mixtures of NaOL and a light- responsive cationic azobenzene dyes [50,54]. In the work of Lu et al. the UV irradiation and the resulting trans- to-cis isomerization of the azo dye (1-[2-(4-phenylazo- phenoxy)-ethyl]-3-methylimidazolium bromide) induces a decrease in the viscosity of the system, with a transi- tion from a gel-like structure to a Newtonian fluid [50]. In our case the opposite effect on the viscosity and the strengthening of the wormlike three-dimensional net- work is observed: a similar behavior is reported by Liu and coworkers on dilute NaOL dispersions in the pres- ence of three different imidazolium surfactants upon UV irradiation [54]. (ii) The presence of KCl plays a key role in modulating the effect of azorubine. The experi- mental data reported in Table S1 show that the trans- to-cis isomerization of azorubine induces an increase in the formulation viscosity when KCl concentration is lower than 1% w/w. Between 1% and 3.5% no remark- able changes are observed before and after the UV irra- diation, suggesting that the major contribution to the strength of the system is provided by the salt. The behavior of the sample containing azorubine and 4% KCl is peculiar and deserves a deeper analysis. The viscosity increases by two orders of magnitude, but the rheological profile shows some differences after the UV treatment (see Figure 2, pink curves). A first New- tonian plateau appears in the low stress regime, then the viscosity rapidly decreases at a critical shear stress which is considerably lower than the viscosity breakdown point before the irradiation. After this initial decrease the flow curve shows a second, less pronounced plateau, fol- lowed by the typical shear-thinning region at high stress values. A similar behavior was reported by Griffiths et al. in 2004 for carbon black particles dispersed in an acrylic polymeric matrix [67]. In this case the presence of an intermediate (secondary) Newtonian plateau was ascribed to the interaction between the polymer layer and the polymer matrix, in particular to the viscous drag between the polymer chains adsorbed on the parti- Figure 3. Flow curves before (solid lines) and after (dashed lines) UV irradiation for NaOL 0.43 M mixture in the presence of 0.18% w/w azorubine (3.6∙10-3 M) at different KCl concentration: 0 (black), 0.1% (orange), 0.5% (green) and 4% w/w (pink). 19Light-Modulated Rheological Properties in Green Innovative Formulations cles with the polymer in solution. In a more recent work Sochi depicts this intermediate region as a characteristic feature of viscoelastic fluids in porous media flow, that may be attributed to the time-dependent nature of the viscoelastic fluid when the relaxation time of the fluid and the characteristic time of the flow become compa- rable [68]. Polacco and coworkers observed two dis- tinct shear-thinning phenomena in polymer-modified asphalts: the first shear-thinning was ascribed to a rigid rearrangement of the aggregate structure, that involves a temporary detachment of polymer chains. As a result, the polymer is able to move between different domains, inducing a transitory strengthening of the network [69]. In our system the appearance of this second- ary Newtonian plateau may reflect the formation of ordered structures with a different degree of organiza- tion. To the best of our knowledge this is the first time that such behavior is found and reported for wormlike micellar dispersions. As a matter of fact the cited litera- ture sources deal with polymeric blends [67,69] or to a more general viscoelastic behavior in porous media flow [68]. Further work is necessary to deepen and clarify this phenomenon. The high salt concentration may be responsible for the formation of complex, more entan- gled structures after UV exposure. These structures are responsible for the remarkable increase in the vis- cosity of the formulation and exhibit a low resistance to the applied stress, as confirmed by the low critical stress value. This hypothesis is also confirmed by the fact that during the sample preparation the azorubine solution becomes turbid at this salt concentration, sug- gesting the formation of aggregated structures. The same salt-induced behavior is observed in pure azoru- bine solutions, without sodium oleate, and presumably reflects the formation of piled up structures stabilized by π-stacking interactions [70,71]. For all the irradiated samples the system recovers its original rheological behavior after standing for about 1 day, indicating a complete reversibility of the process. Figure 4 reports the values of η0 as a function of KCl concentration before (solid lines) and after (dashed lines) the UV treatment. For the two sets of samples the viscosity steeply increases in the dilute regime, then it reaches a maxi- mum and progressively decreases at higher concen- trations of salt. This behavior has been widely report- ed for a large number of wormlike micellar systems [22,39,50,63,72–80]. The initial viscosity increase is ascribed to the formation, growth and entanglement of the cylindrical aggregates. The decrease in the fluid vis- cosity after the maximum upon salt addition is due to the lateral branching along the rod-like micelles, that provide an extra route for mechanical stress relief [72,81,82]. A detailed discussion about the thermodynamic justifica- tion and the driving forces behind branches formation can be found in our previous works [19,26]. The experimental results show that azorubine induc- es two distinct effects on the salt curves (Figure 3). The first effect is the shift of the peak maximum to lower KCl concentrations. Azorubine is an amphiphilic mol- ecule that can penetrate across the micellar interface at least partially. This results in a flattening of the micelle/ water interface with a significant lowering in the surface curvature. Moreover, the electrostatic repulsion between different tubular micelles is screened and the viscosity increases even at lower concentrations of salt. The second effect is the modification of the salt curve shape after the maximum. In the presence of azorubine the viscosity decreases more rapidly, as a con- sequence of the increased branching density. Rogers et al. reported that for several mixed surfactant/salt viscoe- lastic systems, the branching effect is more evident with sodium salicylate, a hydrotropic salt that can penetrate more efficiently underneath the water/micellar interface salt respect to a simple inorganic salt like KCl [83]. This effect due to branched structures is quite common and found also in nonionic or zwitterionic systems [41]. Oscillatory-shear measurements Oscillatory shear experiments were performed to explore the viscoelastic behavior of NaOL-KCl-azoru- bine systems upon UV irradiation. The storage (G’) and loss (G’’) moduli before and after UV irradiation for the dispersions of sodium oleate in the presence of KCl at different concentrations are shown in Figure 5. The fre- Figure 4. Zero-shear viscosity (η0) for the formulations containing 0.18% w/w azorubine (red) and the reference samples without the dye (black) as a function of the salt concentration (a) before (solid line) and after (dashed line) UV irradiation. The trend for non-irra- diated NaOL-KCl samples (black solid line) are reprinted from [19] with permission from Elsevier. Copyright 2021. 20 Duccio Tatini, Martina Raudino, Filippo Sarri quency sweep curves for other KCl concentrations are reported in the Supporting Information. For all the samples two distinct regimes are observed, i.e. a predominant viscous behavior at low frequencies (G’’ > G’) and mainly an elastic behavior at higher frequencies (G’’ < G’). The crossover point of the storage and loss moduli (and the corresponding crosso- ver frequency ωc) marks the transition between the two different regions in the viscoelastic spectrum [84]. The decrease in ωc upon salt addition reflects the formation of a more entangled WLM structure with slower relaxa- tion mechanisms [19]. No remarkable variations in G’ and G’’ are observed before and after the UV irradiation with the exception of NaOL alone, that exhibits higher values of the storage and loss moduli in the low frequen- cy region after the UV treatment. Figure 6 reports the storage and loss moduli obtained in the frequency sweep experiments before and after UV irradiation for the samples containing azorubine in the presence of KCl at different concentra- tions (0, 0.5 and 2%). The frequency sweep curves for other KCl concentrations are reported in the Support- ing Information. The formulations exhibit a viscoelastic behavior that looks similar to those reported in Figure 5, indicating that the two frequency-dependent regimes are present. The crossover frequency decreases when KCl concentration is increased from 0 to 0.5%, then it pro- gressively shifts to higher values upon further increase in the salt concentration. The UV treatment has a very minor effect on the viscoelastic properties of the formulations, with the exception of the sample containing KCl 4% (Figure S10 in the Supporting Information). In this case before the UV irradiation the storage and loss moduli show a crossover in the high-frequency region, followed by a remarkable drop at medium and low frequencies, which indicates a predominantly viscous behavior. After the UV treatment G’ and G’’ overlap for most part of the whole frequency range and after a slight decrease at high frequencies they level off to a relatively high con- stant value in the medium and low-frequency regime. This result is consistent with the remarkable viscosity increase observed after the irradiation (Figure 2) due to the presence of light-induced ordered structures. A Cole-Cole plot analysis was performed to describe the viscoelastic behavior of the NaOL dispersions in terms of a Maxwell model with a single relaxation time (results not shown). Unfortunately, this approach does not provide an accurate prediction for the experimen- tal values due to the presence of additional relaxation modes. For this reason, we calculated the continuous time-weighted relaxation spectrum using the values of the storage and loss moduli (Trios software, 5.2 version, TA instruments) [85,86]. The relaxation times τR estimat- ed from the spectra are reported in Figure 7. In the relaxation spectra an intense primary peak is observed for all the samples, and it was used to extrapo- late the relaxation times. Additional secondary peaks are clearly detectable, demonstrating the presence of concurrent relaxation modes (reptation, breaking and recombination, Rouse motion, etc.) [19]. The comparison between the relaxation times and the viscosity values (see Figure 4) shows a similar effect on both the flow and the viscoelastic properties of the dispersions induced by azorubine: the UV irradiation, thanks to the presence of the dye, increases the struc- turedness of the tridimensional micellar network, result- ing in similar trends of η0 and τR. Figure 5. Storage (triangles) and loss (circles) moduli before (filled markers) and after (empty markers) UV irradiation for NaOL 0.43 M in the presence of KCl at 0 (black), 0.5% (green) and 2% (blue). The frequency sweep curves for non-irradiated samples are reprint- ed from [19] with permission from Elsevier. Copyright 2021. Figure 6. Storage (triangles) and loss (circles) moduli before (filled markers) and after (hollow markers) UV irradiation for NaOL 0.43 M + azorubine 0.18 % mixture in the presence of KCl at 0 (black), 0.5 % (green) and 2 % (blue). 21Light-Modulated Rheological Properties in Green Innovative Formulations The only exception is represented by NaOL + KCl 4% in the presence of azorubine: in this case the relaxa- tion time after the UV treatment is very close to the val- ue of the non-irradiated sample. As we mentioned in the previous section, the remarkable increase in the viscosity observed at this salt concentration is probably due to the presence of aggregated structures. Differential Scanning Calorimetry Differential Scanning Calorimetry (DSC) experi- ments were performed to investigate the thermal prop- erties of the NaOL-KCL-azorubine systems. The thermo- grams of the NaOL dispersions in the presence of KCl at different concentrations (0, 3.5 and 4%) before and after UV irradiation are reported in Figure 8. For the DSC curves of the other investigated KCl concentrations see the Supporting Information. All the DSC thermograms show a free water melt- ing peak at around 0°C. For KCl concentrations above 0.5 % a secondary endothermic peak occurs between -15 and -11°C, and it is related to the melting of the “inter- facial” water molecules that are confined in the solvent- rich domains between the entangled cylindrical micelles [87–91]. A third endothermic peak appears when KCl con- centration reaches 4%, as observed in our previous work [19]. This thermal transition is attributed to the melting of freezable bound water, i.e. the water molecules that closely interact with the micellar surface and show a melting tem- perature remarkably different from bulk water [61,92–96]. The melting temperature, the relative enthalpy change and amount (%) of free water (∆Hmf, Tmf and Wf), interfacial water (∆Hmi, Tmi and Wi) and freezable bound water (∆Hmb, Tmb and Wb) for all the examined samples, before and after UV irradiation are listed in Table S2 in the Supporting Information. A detailed discussion about the theoretical background and the procedures for the calculation of the thermal parameters can be found in [19,26,61]. For the pristine non-irradiated samples Wf rapidly decreases, passing from 96% to 46% as the salt concen- tration increases. Similar values are obtained for the NaOL-KCl systems after the UV irradiation, demon- strating that the endothermic process (i.e. the melting of the free water) is not affected by the UV treatment, as expected. When KCl concentration increases, the melting peak temperatures Tmi shift to lower values; conversely ∆Hmi shows a progressive increase. As previously reported, the addition of KCl (and the consequent Na+/K+ exchange at the micellar surface) leads to the formation of elongated cylindrical micelles, and above a critical concentration (see Figure 4) to the branching of the WLM network [19]. For this reason the number of connections and junctions within the micellar network increase, as well as the number of water molecules confined within the intermicellar domains, as evidenced by the values of Wi. By comparing the values of the peak temperatures and of the enthalpy changes related to the melting of the interfacial and bound water we conclude that the UV treatment does not alter the hydration state of the NaOL/KCl systems. For the samples containing azorubine the values of ∆Hmf, Tmf, Wf, ∆Hmi, Tmi, Wi, ∆Hmb, Tmb and Wb are reported in Table S2 in the Supporting Information. The presence of the dye does not induce any remark- able variation in the amount of free and interfacial water, as well as in the values of peak temperatures and Figure 7. Relaxation time (τR) for the formulations containing azorubine (red) and the reference samples without the dye (black) as a function of the salt concentration before (solid line) and after (dashed line) the UV irradiation. The trend for non-irradiated NaOL-KCl samples (black solid line) are reprinted from [19] with permission from Elsevier. Copyright 2021. Figure 8. DSC heating curves before (solid lines) and after (dashed lines) UV irradiation for NaOL 0.43 M mixture in the presence of KCl at 0 (black), 3.5% (light blue) and 4% (pink). 22 Duccio Tatini, Martina Raudino, Filippo Sarri the relative enthalpy changes. The main difference is represented by the freezable bound water melting peak, that appears starting from KCl 3%. As indicated by the viscosity measurements, the negatively charged sulfonate groups in the azorubine molecule have a cooperative effect with KCl in screening the electrostatic repulsion between the micellar surfaces. This synergistic action reduces the amount of salt that is required for the for- mation of the micellar entangled network (and eventual- ly the branched structures) and affects the thermal tran- sition of the water molecules that closely interacts with the micellar surface, i.e. the freezable bound water. CONCLUSIONS In this work we illustrate a simple and inexpensive procedure for the formulation of green photo-responsive viscoelastic fluids starting from non-toxic, biocompat- ible, commercially available materials. The addition of a food azo dye, azorubine, to a dispersion of sodium oleate in the presence of KCl enables the modification of the formulation rheological properties through an external UV light stimulus. The UV treatment brings about the trans-cis isomerization of azorubine, which is partially intercalated between the surfactant polar heads, result- ing in a modification of the wormlike micellar structure of NaOL. This gives rise to a remarkable increase in the formulation viscosity and a variation of the rheological properties, as evidenced by viscosity and oscillatory- shear measurements. The effect of azorubine is medi- ated by the presence of the salt, since it occurs only in a specific range of KCl concentration. DSC measurements confirm the formation of wormlike micelles above a crit- ical salt concentration, which is lower in the presence of azorubine. The UV treatment does not affect the thermal transitions of the NaOL dispersions, since no significant variation is detected before and after the irradiation. 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