Eur. Phys. J. E (2015) 38: 84 DOI 10.1140/epje/i2015-15084-5

THE EUROPEAN PHYSICAL JOURNAL E

Regular Article

Crystalline fibrillar gel formation in aqueous surfactantantioxidant system Linet Rose Joseph1 , B.V.R. Tata2 , and Lisa Sreejith1,a 1 2

Soft Materials Research Laboratory, Department of Chemistry, National Institute of Technology Calicut, India Condensed Matter Physics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamilnadu, India Received 13 December 2014 and Received in final form 14 April 2015 c EDP Sciences / Societ` Published online: 10 August 2015 –  a Italiana di Fisica / Springer-Verlag 2015 Abstract. Cetyltrimethylammonium bromide (CTAB) is a well-known cationic surfactant capable to micellize into diverse morphologies in aqueous medium. We observed the formation of an opaque gel state from aqueous CTAB solution in the presence of the aromatic additive, para-coumaric acid (PCA). Optical microscopic images revealed the presence of large fibrils in the system at room temperature. Gel nature of the fibrils was confirmed by rheological measurements. Presence of interstitial water in the fibrils was recognized with Raman spectroscopy. On heating the sample above 30 ◦ C, the fibrillar gel state changes to a transparent liquid state with Newtonian flow properties. Dynamic light scattering study hinted the presence of small micelles in the solution above 30 ◦ C. Thus the system showed a temperature-dependent structural transition from opaque water-swollen gel to transparent micellar liquid. The formation of waterswollen fibrillar network is attributed to surfactant-additive intermolecular interactions in aqueous medium. Transition to micelle phase above 30 ◦ C is related to Kraft transition which is observed at significantly lower temperature for CTAB in the absence of PCA. The structural features of PCA play a key role in promoting fibrillar network formation and elevating the Kraft transition in aqueous solution of CTAB.

1 Introduction Amphiphilic self-assembly in aqueous medium is controlled by a delicate balance between various non-covalent interactions such as hydrophobic and electrostatic interactions, hydrogen bonding and van der Waals forces [1]. Water gelation by amphiphiles is an active area of research owing to the possibility of incorporating both hydrophilic and lipophilic moieties of practical importance into the gel and their subsequent release in a controlled fashion [2]. Coagels, resulting from the aqueous self-assembly of amphiphiles are widely used as ointment base for topical administration of pharmaceuticals, since they can solubilize and stabilize drugs and foster their permeation through skin. Coagels are actually hydrated liquid crystalline phases which can exhibit sharp XRD pattern and optical birefringence [3]. Plate-like hydrated crystals are reported in concentrated solution of the cationic surfactant cetyltrimethylammonium bromide (CTAB) below its Kraft point (≈ 24 ◦ C) [4]. Introduction of amphiphilic surfactants with long hydrocarbon chain to aqueous medium disrupts the  Supplementary material in the form of a .pdf file available from the Journal web page at http://dx.doi.org/10.1140/epje/i2015-15084-5 a e-mail: [email protected]

hydrogen-bonded network of water, leading to reorientation of H-bonding and structuring of water molecules around the non-polar moieties. Self-aggregation of surfactant molecules is entropically advantageous at this point as it breaks the water structuring [5,6]. In ionic surfactant solution, micelle formation starts only above a critical concentration (CMC) and a critical temperature (Kraft point). Below Kraft temperature, surfactant solution exists in a two-phase form, comprising of hydrated crystals and water. Hydrated solid formation is favoured by the van der Waals attraction among alkyl chain of surfactant molecules, leading to crystallization [7,8]. Additives can effectively modify the critical parameters and aggregation pattern of amphiphiles. Phenolic antioxidants can influence the physical and structural properties of aqueous micelles depending on the nature of substituents and substitution site [9,10]. Para-coumaric acid (PCA), an antioxidant of plant origin can be employed effectively to prevent oxidation in emulsions [11–14]. But the effectiveness can vary depending on the type of molecular interactions and solubilization site. In addition to antioxidant activity, PCA is known for its ability to polymerize to biodegradable forms and is considered as a mesogenic bio monomer whose homo polymer shows liquid crystalline properties. Kaneko and co-workers have synthesized highly oriented liquid crystalline homo polymer from PCA, which showed

Page 2 of 8

excellent cell compatibility suitable for biomedical applications [15]. PCA can also be employed for modifying the material properties of existing polymers owing to its high structural rigidity and aromaticity [16]. Also, the ability of PCA to enter into hydrogen bonding and π-π stacking, might enable it to modify the aggregation behaviour of ionic surfactants, especially in the temperature range where intermolecular interaction energy dominates over thermal kinetic energy [17]. In our attempt to examine the effect of PCA on assembling properties of cationic micelles, we observed the formation of hydrated thread-like aggregates in aqueous solution of CTAB. Interpenetration of the water-swollen fiber bundles resulted in gelation of the system with a turbid appearance. Above 30 ◦ C, a transition to clear micellar liquid is observed. Microstructural properties of the fibrillar gel as well as the micellar liquid were studied using rheology and dynamic light scattering. Huang and co-workers have reported hydrated gel formation in aqueous solution of C-16 tailed imidazoliumtype surfactant in the presence of sodium salicylate, below 21 ◦ C [18]. The fibrillar gel formed in the present system comprising of CTAB and PCA showed stability up to 30 ◦ C and the ability to incorporate water insoluble molecules. Interestingly, we noticed the absence of such fibrillar network formation in CTAB–ortho-coumaric acid (OCA) system under identical conditions. OCA acts as a strongly binding counter ion and induces elongation of CTAB micelles to thread-like micelles (TLMs) resulting in a transparent, Maxwell-type viscoelastic system. The para positioning of polar functional groups in PCA prevents its strong binding to CTAB micelles. Many studies have revealed that position isomers differing only in the location of substituent on the aromatic ring can influence micellar growth in entirely different ways owing to the differences in their orientation and penetration depth in micellar system. Johnson et al. carried out heat capacity measurements on cetyltrimethylammonium solution with orthoor para-substituted hydroxy benzoate ions as counter ions. Their results revealed that ortho-substituted counter ions penetrate into the head group region and are less hydrated than para-substituted ions which prefer to remain outside the palisade layer. Para-hydroxy-substituted counter ions lack a clear cut division into hydrophobic and hydrophilic parts, which adversely influence their penetration ability [19]. On the other hand Simmons et al. reported that parasubstituted phenols are efficient to promote the formation of fibrous organogels through stacked arrangement, whereas ortho-substituted phenols are not [20]. Oda et al. studied water gelation by gemini surfactants, comprising of two identical hydrophobic chains linked by a spacer moiety at the two head groups. The spacer in gemini surfactant can be a flexible species like ethylene group or a rigid aromatic species like benzene or stilbene molecule [21,22]. Menger and Littau showed that gemini surfactants with rigid spacer prefer linear alignment in aqueous medium rather than conventional micelle formation [23]. Inhibiting effect of the rigid spacer on micellization was observed

Eur. Phys. J. E (2015) 38: 84

Fig. 1. Chemical structure of para-coumaric acid.

by Wang and co-workers [24]. Zhou et al. found that in gemini surfactants with aromatic spacer, π-π stacking interaction of benzene rings along with intermolecular interaction of alkyl chains plays an important role in stabilizing the crystalline aggregates [25]. Fibril formation observed in CTAB-PCA system at ambient conditions suggests the stabilization of CTAB bilayers by PCA molecules. PCA with para-positioned polar functional groups plausibly acts as a physical spacer between CTAB head groups thereby stabilizing the hydrated crystals. π-π stacking ability of PCA offers additional assistance to the fibrils.

2 Materials and methods 2.1 Materials Cetyltrimethylammonium bromide (CTAB) was purchased from BDH England (99% assay), para-coumaric acid (PCA) (99% assay) and ortho-coumaric acid (98+ %) from Alfa Aesar England. All the chemicals were used as received. Samples were prepared in deionized Milli-Q water and kept in a water bath at 60 ◦ C with stirring about one hour for homogeneity. The resulting samples were cooled and stored at 25 ◦ C. pH of 100 mM CTAB/40 mM PCA sample at 35 ◦ C was found to be 3.13. Figure 1 shows the chemical structure of PCA. 2.2 Optical microscopy Optical microscopy images were recorded on an Olympus BX 51 bright field microscope equipped with Olympus DP 72 digital camera. Birefringent images under crossed polarizer were obtained with Olympus BX 51 polarizing microscope. Fluorescent mode image of the sample was recorded using an Olympus IX 71 fluorescence microscope. 2.3 Rheological measurements Rheological measurements were performed on an MCR301 rheometer (Anton Paar, Germany) with a parallel plate measuring system (50 mm diameter). Sample temperature was maintained to the accuracy of ±0.01 ◦ C. The viscosities of samples were obtained from steady shear measurements with shear rate ranging from 0.01 to 100 s−1 . Dynamic frequency spectra were obtained in the linear viscoelastic regime of each sample as determined by strain sweep measurements. All the frequency sweep measurements were performed in the angular frequency range of 0.05–100 rad/s.

Eur. Phys. J. E (2015) 38: 84

Page 3 of 8

2.4 Dynamic Light Scattering (DLS) DLS measurements were performed using Malvern 4800 Autosizer employing 7132 digital correlator (Malvern INstruments UK) at scattering angle of 130◦ . A vertically polarized light of wavelength 532 nm from a diode-pumped solid-state laser was used as the incident beam. The intensity correlation function was analyzed by the method of cumulants where unimodal distribution of relaxation of time is considered. 2.5 Raman spectroscopy Raman spectrum of the sample was obtained using a micro Raman spectrometer (Renishaw, UK, model Invia) with 514 nm laser excitation. Sample was loaded on a stainless steel gasket and the spectra were recorded at a fixed temperature of 25 ◦ C. 2.6 Transition temperature monitoring The temperature at which samples changes from turbid fibril state to clear liquid state was visually monitored using He-Ne LASER (632.8 nm) and CCD camera. LASER was allowed to pass through the sample taken in a cuvette fitted with a sensitive thermometer. The images corresponding to optical brightness were captured using a CCD camera at different temperatures and converted into corresponding % transmittance values programmatically.

3 Result and discussion CTAB is known to form spherical micelles in aqueous medium above its CMC, 4 × 10−4 M [26]. Presence of certain aromatic additives can bring micellar growth and elongation at low surfactant concentrations [27,28]. In order to monitor the effect of the aromatic additive PCA on structural transitions of CTAB micelles, we prepared a series of samples at varying concentrations of CTAB and additive. The concentration of CTAB showed a marked effect on improving the aqueous solubility of PCA, which is otherwise showing negligibly low water solubility. But no dramatic increase in the viscosity of dilute aqueous CTAB solution (at 35 ◦ C) was observed on successive increase in the concentration of PCA. Influence of PCA concentration on the viscosity behaviour of CTAB solution is presented in fig. 2. Viscosity of 0.1 M CTAB solution remained almost unaltered on successive increase in the concentration of PCA. A marginal increase in viscosity was observed in 0.2 M CTAB solution at high PCA concentration. In 0.3 M CTAB solution, a perceivable increase in viscosity occurred at high PCA concentration. However the observed viscosity increase is small as compared to the reported effect of other polar aromatic additives like salicylic acid. Para positioning of functional groups in PCA adversely influence its penetration ability into surfactant micelles.

Fig. 2. Variation in the viscosity behaviour of aqueous CTAB solution as a function of PCA concentration at 35 ◦ C.

Para-substituted ions generally prefer to remain outside the palisade layer and hence fail to induce considerable viscoelasticity to dilute or semi-dilute surfactant solution. In highly concentrated surfactant solutions, intermicellar distance is comparatively less and additive may bring slight viscosification via inter micellar association. The structural feature of PCA may favour micellar association in concentrated surfactant solution through electrostatic interaction between para-positioned polar functional groups and positively charged CTAB micelles on either side. Immediately after heating and stirring, the solutions appeared clear and homogeneous. But on cooling to room temperature we observed the growth of fibrils and eventually the solutions turned to opaque mass comprising of fibrous bundles. Samples showed the ability to support its own weight in an inverted vial. Samples were prepared in different compositions by varying the CTAB concentration from 0.1 M to 0.3 M and additive concentration from 0.01 M to 0.1 M and fibril formation was observed in all the samples. We carried out an investigation of the nature of fibrils formed in a representative sample comprising of 100 mM CTAB and 40 mM PCA and the results are reported herein. Other compositions also showed almost similar kind of behaviour. Figure 3 shows the microscopic view of 100 mM CTAB/40 mM PCA sample, disclosing the presence of fibrillar network in the sample at room temperature. The polarized optical image shown in fig. 3b indicates the birefringent nature of the fibrous bundles. PCA, like other phenolic compounds, exhibits native fluorescence. The fluorescent mode optical image of fibrils formed in the CTAB/PCA sample is presented in Fig. 3c and d. The incorporation of the PCA molecules into the water-swollen fibrils can be inferred from the fluorescent image. On heating, the sample became transparent and liquid-like, and no structures were seen under optical microscope. In order to monitor the temperature limit above which the fibrils transform to clear liquid phase, LASER was passed through the sample and captured the % transmittance as optical brightness images using CCD camera over a selected temperature range. Optical brightness was

Page 4 of 8

Eur. Phys. J. E (2015) 38: 84

Fig. 3. (a) Optical microscope image (scale bar: 50 μm), (b) polarizing microscopy image (scale bar: 50 μm), (c) fluorescent mode microscopy image under blue light and (d) fluorescent mode image under UV light of 100 mM CTAB/40 mM PCA sample at 25 ◦ C.

Fig. 4. Transmittance behaviour of 100 mM CTAB/40 mM PCA sample at different temperatures.

Fig. 5. The steady shear rheological response of 100 mM CTAB/40 mM PCA sample at two different temperatures.

then converted into corresponding % transmittance values. Temperature-dependent transmittance behaviour of the system is presented in fig. 4. At lower temperatures the sample was turbid and possessed zero transmittance. A sharp rise in transmittance was observed at 30 ◦ C and the transparency persisted at higher temperatures. To examine the flow properties of the system, we carried out steady shear rheological measurements. The steady shear response of CTAB /PCA sample at two different temperatures is shown in fig. 5. A very high initial viscosity and strongly shear thinning behaviour (sharp viscosity drop with shear rate) is observed at 25 ◦ C, hinting the presence of network structure with yield stress [29–31]. In contrast, a Newtonian-type shear-independent flow pattern is exhibited by the same sample at 32 ◦ C. Also the

viscosity is found to be very low (≈ 0.001 Pa s), of the order of viscosity of water. Such low viscosity is expected for spherical micellar system [32]. In order to further elucidate the viscoelastic properties of the system, oscillatory shear measurements were performed in the linear viscoelastic regime. Frequency dependence of the elastic and viscous moduli (G and G , respectively) of the sample at different temperatures is depicted in fig. 6a. At 20 and 25 ◦ C, both the moduli are almost independent of frequency. The dominance of G over G is observable throughout the measured frequency range. The supremacy of elastic modulus over viscous modulus and the nearly frequency-independent behaviour of both the moduli are indicative of elastic gel nature of the system [29,33]. Conversely, a viscoelastic response with G dominating at the low frequency region and G dominating

Eur. Phys. J. E (2015) 38: 84

Page 5 of 8

Fig. 6. (a) Frequency spectrum of 100 mM CTAB/40 mM PCA sample at three different temperatures. Closed symbols represent G and open symbols represent G .  at 20 ◦ C,  at 25 ◦ C and • at 32 ◦ C. (b) Elastic and viscous moduli as a function of strain amplitude at two different temperatures.

at the high-frequency region was observed for the sample at 32 ◦ C. Oscillatory shear measurements suggested a gel to sol transition in the system upon increasing the temperature above 30 ◦ C. Another noticeable behaviour obtained was in the strain sweep measurement at two different temperatures. As can be seen from fig. 6b, at 25 ◦ C, a low strain limit is exhibited by the fibrillar gel sample. The moduli remain strain-independent only up to around 0.1% strain, beyond which gel disruption (monotonous decrease of G and G with strain) occurs. Such low strain limit values are generally shown by crystalline self-assembly. On the other hand, at 32 ◦ C, Both the moduli remain strain independent to a large extent and no cross over between G and G occurred within the observed limit. Also dominance of viscous modulus over elastic modulus can be observed in the entire range, which is consistent with the liquid nature of the sample at higher temperatures [34–36]. Rheological measurements indicated a transition from elastic gel to viscoelastic liquid upon increasing the temperature. In order to confirm the presence of micellar aggregates in the clear liquid obtained above 30 ◦ C, we conducted dynamic light scattering analysis. The intensityweighed distribution of the apparent hydrodynamic diameter, obtained for 100 mM CTAB/x mM PCA solution at 35 ◦ C is given in fig. 7a. No significant change in hydrodynamic diameter was observed with increase in PCA concentration. This supports the notion that PCA is incapable of bringing considerable micellar growth in dilute aqueous CTAB solution. However the intensity-weighed distribution suggests the presence of micellar aggregates with an average hydrodynamic diameter of 7 nm in 0.1 M CTAB-CPC solutions. Notice that, in ionic surfactant solutions in the absence of effective charge screening, inter particle repulsion will influence the accuracy of measurements [37,38]. Hence the apparent diameter obtained may not signify the exact size of the micelles. Figure 7b presents the variation in hydrodynamic diameter for x mM CTAB/40 mM PCA system. A successive

shift towards larger diameter region can be observed with increase in CTAB concentration. This suggests anisotropic growth of CTAB micelles to rod-like structures which is in agreement with the viscosity behaviour. Raman spectroscopy is an effective tool for detecting the presence of interstitial water in swollen aggregates. The region 3000–3800 cm−1 is highly informative of the O-H stretching. The presence of regular tetrahedrally bonded water is marked by the wave number component around 3200 cm−1 whereas the confinement of water between bilayered structures is identified by the component at around 3400 cm−1 . Attempts have been made to distinguish between the fraction of water molecules taking part in tetrahedral hydrogen bonding and the fraction present as interstitial water by comparing the relative intensities of the Raman peaks at 3200 cm−1 and 3400 cm−1 [39,40]. The Raman spectra obtained for swollen fibrillar sample and its pure components are depicted in fig. 8. The spectrum for 100 mM CTAB/40 mM PCA fibrils presents a broad peak centered at 3400 cm−1 which is absent in the spectra of anhydrous components, viz., CTAB and PCA. The low wave number component at 3200 cm−1 is relatively weak. Similar kind of spectra is previously reported for lamellar liquid crystalline structures formed in nonionic surfactant system [39]. The peak unambiguously evidences the presence of interstitial water in the fibrillar gel formed from the PCA aided self-assembly of CTAB. In order to identify the role of geometry of PCA on the observed aggregation behaviour, we carried out the rheological measurements of CTAB solution under identical conditions but using ortho-coumaric acid (OCA) as the additive instead of PCA. PCA and OCA are position isomers differing only in the location of functional groups on the aromatic ring. In the presence of OCA, the solution showed entirely different rheological properties. A significant increase in viscosity was observed in 100 mM CTAB/x mM OCA solution with progressive increase in OCA concentration (fig. S1 in Supplementary Material). 100 mM CTAB/50 mM OCA sample

Page 6 of 8

Eur. Phys. J. E (2015) 38: 84

Fig. 7. Intensity-weighed distribution of apparent hydrodynamic diameter of (a) 100 mM CTAB/x mM PCA and (b) x mM CTAB/40 mM PCA at 35 ◦ C.

Fig. 8. Raman spectra of (A) pure CTAB (B) pure PCA and (C) 100 mM CTAB/40 mM PCA fibrils.

showed non-Newtonian viscoelastic behaviour similar to a Maxwell type fluid (fig. S2) indicating the presence of long and entangled worm-like micelles in the sample [41]. Interestingly no macroscopic fibril formation was observed in the system even at very low temperatures; instead, the sample became more viscous and transparent jellylike in appearance, retaining the Maxwellian viscoelastic behaviour, on cooling below room temperature. This behaviour is typical of worm-like micellar solution where the micellar length increases with lowering of temperature [42]. OCA, with an aromatic ring bearing two nearby polar substituents can effectively bind to the palisade layer of surfactant micelles, thereby transforming the spherical micelle to worm-like micelles [43,44]. On the other hand, in PCA the presence of two polar groups para to each other makes the penetration of aromatic ring into the micelle palisade layer difficult and hence the molecules prefer to condense around the charged micelles, similar to an inorganic salt.

Typically, the existence of coagel or hydrated crystalline phase with water confined between bilayer of amphiphilic molecules is observed for many surfactant systems at very low temperature and at high surfactant concentration. The bilayer structure formation is attributed to crystallization of alkyl chain of surfactant molecules driven by van der Waals attraction, followed by counter ion binding [45,46]. Application of very high pressure may cause the formation of coagel phase even at ambient temperature [47]. In the present system hydrated fibril formation is observed under ambient pressure and temperature, in moderately concentrated surfactant solution. Our experimental results hint that PCA is capable of modifying the lateral stacking of CTAB alkyl chains and enhancing the thermal stability of aggregates by positioning itself in an appropriate location in between the crystallized chains. The special features of PCA viz., the presence of aromatic ring and two para-positioned hydrophilic moieties could be the factors triggering the unusual self-assembly in CTAB solution. The plausible location of PCA molecules is between the crystalized CTAB layers. Para-positioned polar functional groups in PCA may interact electrostatically with the CTAB head groups on either side. Hence PCA can be viewed as a rigid physical spacer positioned between CTAB layers. The ability of PCA to modify the ordering and mechanical strength of synthetic polymers through its rigidity and aromaticity is already mentioned. π-π stacking of PCA molecules between CTAB layers favours fibril formation in a way somewhat similar to the crystallization reported in gemini surfactants with rigid aromatic spacers. Formation of swollen gel by encapsulating the entire water medium supports the plausible bilayered lamellar-type arrangement. The steady shear and dynamic rheological results affirm the expected gel type behaviour of the swollen fibrils. The transition to normal liquid state at higher temperature can be viewed as fibrillar gel to micellar transition. The low viscosity of 100 mM CTAB/40 mM PCA solution

Eur. Phys. J. E (2015) 38: 84

above 30 ◦ C hints the presence of small spherical aggregates and the inability of PCA to bring elongation of micelles by offering effective charge screening. The rheological behaviour at high temperature also supports the presence of smaller aggregates. The confirmative evidence for the presence of small aggregates in the water-like solution at higher temperature is provided by dynamic light scattering study. According to conventional view, the bilayers lamellar structures can offer different microenvironments for the encapsulation of both hydrophilic and hydrophobic moieties [48,49]. Considering the excellent pharmacological activities of PCA including antioxidant, antiallergic and antimicrobial activates, the wise replacement of the present surfactant with similar non-cytotoxic choices will enable the extension of the system for drug solubilization and delivery purposes [50]. The antioxidant properties of PCA can be effectively utilized to offer protection to the solubilized drugs against oxidative degradation.

4 Conclusions The addition of phenolic antioxidant, para-coumaric acid to aqueous CTAB solution is found to be capable of bringing unique aggregation properties to the system. An unusual crystalline fibrillar gel formation is observed in the system under ambient conditions. Optical birefringence and the ability to encapsulate water suggest the possibility of bilayered lamellar arrangement which is supposed to provide micro environment for the encapsulation of delicate hydrophobic species in a protected manner. Also the antioxidant properties of para-coumaric acid offer protection from oxidative stress to the encapsulated moieties and hence bio applicability to the system. The fibrillar structures are susceptible to transformation into micellar assemblies at higher temperature. This work was performed under the collaborative research scheme (CRS-K-0/5/20) of the UGC-DAE Consortium for Scientific Research, India. The financial support in the form of Research Assistant Fellowship and Contingency Grant is gratefully acknowledged. The authors are thankful to Dr. N. Sandhyarani and her research group (SNST, NIT Calicut) and Dr. Sujith A. (Dept. of Chemistry, NIT Calicut) for providing the optical microscope facility. The fluorescence optical microscope images are taken from the Spectroscopy and Analytical Test Facility Centre, IISC Bangalore. Thanks are due to Dr. T. R. Ravindran and K. Kamali (IGCAR, Kalpakkam) for help with Raman analysis and Dr. P. A. Hassan (BARC, Mumbai) for help with DLS studies.

References 1. J.N. Israelachvili, Intermolecular and surface forces, second ed. (Academic Press, New York, 1992). 2. L.A. Estroff, A.D. Hamilton, Chem. Rev. 104, 1201 (2004). 3. S. Palma, R. Manzo, P.L. Nostro, D. Allemandi, Int. J. Pharm. 345, 26 (2007).

Page 7 of 8 4. R. Ranjini, M.V. Matham, N.T. Nguyen, Opt. Mater. 33, 1338 (2011). 5. D. Chandler, Nature 37, 640 (2005). 6. C. Tanford, J. Mol. Biol. 67, 59 (1972). 7. D. Lavabre, V. Pradines, J.C. Micheau, V. Pimienta, J. Phys. Chem. B 109, 7582 (2005). 8. S. Sasaki, J. Phys. Chem. B 113, 8545 (2009). 9. A. Heins, V. Garamus, B. Steffen, H. St¨ ockmann, K. Schwarz, Food Biophys. 1, 189 (2006). 10. A. Ota, H. Abramovi˘c, V. Abram, N.P. Ulrih, Food Chem. 125, 1256 (2011). 11. T.H. Gamel, E.S. Abdel, Agric. Food Sci. 21, 118 (2012). 12. I. G¨ ul¸cin, E. Bursal, M.H. S ¸ ehito˘ glu, M. Bilsel, A.C. G¨ oren, Food Chem. Toxicol. 48, 2227 (2010). 13. A. Logan, U. Nienaber, X. Pan, Lipid Oxidation: Challenges in Food Systems Canola-based phenolic applications (AOCS Press, USA, 2013). 14. J. Wang, F. Shahidi, J. Agric. Food Chem. 62, 454 (2013). 15. T. Kaneko, M. Matsusaki, T.T. Hang, M. Akashi, Macromol. Rapid Commun. 25, 673 (2004). 16. T. Kaneko, H.T. Tran, M. Matsusaki, M. Akashi, Chem. Mater. 18, 6220 (2006). 17. R. Kumar, A.M. Ketner, S.R. Raghavan, Langmuir 26, 5405 (2010). 18. Y. Lin, Y. Qiao, Y. Yan, J. Huang, Soft Matter 5, 3047 (2009). 19. I. Johnson, G. Olofsson, J. Colloid Interface Sci. 106, 222 (1985). 20. B.A. Simmons, C.E. Taylor, F.A. Landis, V.T. John, G.L. McPherson, D.K. Schwartz, R. Moore, J. Am. Chem. Soc. 123, 2414 (2001). 21. R. Oda, I. Huc, S.J. Candau, Angew. Chem. Int. Ed. 37, 2689 (1998). 22. S.K. Hait, S.P. Moulik, Curr. Sci. 82, 1101 (2002). 23. F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 115, 10083 (1993). 24. X. Wang, J. Wang, Y. Wang, H. Yan, P. Li, R.K. Thomas, Langmuir 20, 53 (2004). 25. M. Zhou, H.L. Liu, H.F. Yang, X.L. Liu, Z.R. Zhang, Y. Hu, Langmuir 22, 10877 (2006). 26. T. Imae, R. Kamiya, S. Ikeda, J. Colloid Interface Sci. 108, 215 (1985). 27. R. Abdel-Rahem, Adv. Colloid Interface Sci. 141, 24 (2008). 28. J.J. Cardiel, Y. Zhao, P. De La Iglesia, L.D. Pozzo, A.Q. Shen, Soft Matter 10, 9300 (2014). 29. J.H. Lee, J.P. Gustin, T. Chen, G.F. Payne, S.R. Raghavan, Langmuir 21, 26 (2004). 30. C.W. Macosko, Rheology: Principles, Measurements and Application, (VCH Publishers, New York, 1994). 31. K. Ueno, K. Hata, T. Katakabe, M. Kondoh, M. Watanabe, J. Phys. Chem. B 112, 9013 (2008). 32. H.H. Kohler, J. Strnad, J. Phys. Chem. B 94, 7628 (1990). 33. F.M. Menger, K.L. Caran, J. Am. Chem. Soc. 122, 11679 (2000). 34. S.R. Raghavan, Langmuir 25, 8382 (2009). 35. K. Saravanakumar, B.V.R. Tata, V.K. Aswal, Colloids Surf. A 414, 359 (2012). 36. S.H. Tung, Y.E. Huang, S.R. Raghavan, Soft Matter 4, 1086 (2008). 37. M. Corti, V. Degiorgio, J. Phys. Chem. 85, 711 (1981). 38. J.R. Rodr´ıguez, J. Czapkiewicz, Colloids Surf. A 101, 107 (1995).

Page 8 of 8 39. V.S. Marinov, Z.S. Nickolov, H. Matsuura, J. Phys. Chem. B 105, 9953 (2001). 40. V.S. Marinov, H. Matsuura, J. Mol. Struct. 610, 105 (2002). 41. M.E. Cates, S.J. Candau, J. Phys.: Condens. Matter 2, 6869 (1990). 42. D.P. Acharya, P. Varade, K. Aramaki, J. Colloid Interface Sci. 315, 330 (2007). 43. H. Rehage, H. Hoffmann, J. Phys. Chem. 92, 4712 (1988). 44. B.K. Roy, S.P. Moulik, Curr. Sci. 85, 1148 (2003). 45. S. Sasaki, J. Phys. Chem. B 111, 2473 (2007).

Eur. Phys. J. E (2015) 38: 84 46. S. Sasaki, J. Phys. Chem. B 112, 8586 (2008). 47. A. Islamov, C.R. Haramagatti, H. Gibhardt, A. Kuklin, G. Eckold, Physica B 385-386, 791 (2006). 48. Ambrosi, P.L. Nostro, L. Fratoni, L. Dei, B.W. Ninham, S. Palma, R.H. Manzo, D. Allemandi, P. Baglioni, Phys. Chem. Chem. Phys. 6, 1401 (2004). 49. A. Pannala, R.B.R. Halliwell, S. Singh, C.A. Rice-Evans, Free Radical Biol. Med. 24, 594 (1998). 50. S. Palma, R.H. Manzo, D. Allemandi, L. Fratoni, P.L. Nostro, Colloids Surf. A 212, 163 (2003).