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Behavior of wormlike micellar solutions formed without any additives from semi-fl uorinated quaternary ammonium salts Gennifer Padoan,ab Elisabeth Taffi n de Givenchy,aAlessandro Zaggia,b Sonia Amigoni,aThierry Darmanin,aLino Conteband Fr ed eric Guittard*a In the present work a series of fl uorinated quaternary ammonium salts were synthesized in two steps. The physical properties of the obtained surfactants were characterized by DLS, zeta potential, viscosity, and Cryo-SEM. The surfactants with a long fl uorinated chain form long and highly fl exible aggregates that lead to highly viscous solutions and the formation of an entangled network made of fl exible worm-like micelles. The viscoelastic properties of the surfactants that form hydrogels without any additives were characterized along with zeta potential and Cryo-SEM observations. They present a maximum viscosity as a function of worm-like micelles formation. The increase of the fl uorinated and hydrocarbon chain lengths contributes to the enhancement of this viscoelastic behavior. Introduction Surfactant solutions represent a well-documented class of self- assembled structures providing a powerful tool for several applications in pharmaceutical and cosmetics industries.15 The addition of an inorganic,6a strongly binding organic salt,7 or an oppositely charged surfactant leads to the growth of aggregates. This growth generally leads to highly viscous solutions and the formation of an entangled network made of exible worm-like micelles, oen referred to as thread-like micelles.8The length of these aggregates can reach up to 1 mm, but the diameters are only a few times the length of the constituent molecules. Prudhomme and Warr investigated the behavior of solu- tions of tetradecyltrimethylammonium salicylate9and they observed a rheological behavior of wormlike micelles in the surfactant solution similar to that of exible polymers. Yet, above a critical concentration (c*), few weight percent, wormlike micelles entangle into a transient network similar to a solution of exible polymers; they behave in exactly the same way as the equivalent macromolecules and entangle with each other to form transient networks, which display remarkable viscoelastic properties in a manner analogous to semi-dilute polymer solutions. Wormlike micelles are dynamic systems and the aggregates are constantly breaking and recombining, making them living polymers and proving an increase in viscosity with shear rate.10 The polymer analogy has been the catalyst for recent theo- retical advances in understanding the properties of the aniso- metric micellar solutions. By incorporating the dynamic nature of micelles into polymer solution theories, Cates11has been able to predict scaling laws for both equilibrium and dynamic properties of these systems. Because the micelles are in ther- modynamic equilibrium with the surrounding solvent, they are not geometrically xed like polymer molecules. As a conse- quence, the connecting or branching points of the micelles are able to move along the primary axis of the micelles. Theselivingpolymershaveoneextrapropertythatthenormal polymer system lacks (and hence their name): if either polymer chains or surfactant worm-like micelles are subjected to a high extensional force, they can be snapped in half. While polymer fragments always remain thereaer at this lower (halved) molecular weight, the broken surfactant micelles can reform undermorequiescentconditions,sincetheyarethermodynamic objects held together by reversible bonds. If elastic liquids are needed to withstand continual damage, and yet survive, then these micellar systems are much more mechanically robust. Micellar microstructures have a strong inuence on the rheological properties of aqueous surfactant solutions.12The transition to a shear-thickened uid is commonly attributed to the formation of shear-induced structures, where phase sepa- ration takes place between a surfactant-rich and a surfactant- poor phase. Their unique viscoelastic behavior is exploited to tune the rheology in numerous applications with or without the use of polymers or additives. For this reason they have been suggested for drag reducing uids in central-heating systems.13 aUniv. Nice Sophia Antipolis, CNRS, Laboratoire Physique Mati ere Condens ee, UMR 7336, Parc Valrose, 06100 Nice, France. E-mail: guittardunice.fr; Fax: +33 492076156; Tel: +33 492076159 bDipartimento di Ingegneria Industriale, University of Padua, Via Marzolo, 9-35131, Padua, Italy Electronic supplementary information (ESI) available: Characterizations, yields and cmc for the synthesized surfactants, complementary graphs of rheology, zeta potential and Cryo-SEM images. See DOI: 10.1039/c3sm51591e Cite this: Soft Matter, 2013, 9, 8992 Received 7th June 2013 Accepted 25th July 2013 DOI: 10.1039/c3sm51591e /softmatter 8992 | Soft Matter, 2013, 9, 89928999This journal is The Royal Society of Chemistry 2013 Soft Matter PAPER Published on 29 July 2013. Downloaded on 02/12/2017 01:48:19. View Article Online View Journal | View Issue Like all brous microstructures, the viscosity is high when the system is randomly dispersed in space and highly entangled, but once the ow has aligned the microstructures, the viscosity is quite low. This change from very high to very low viscosity takes place over a narrow range of stress or temperature. These self- assembled structures may reduce friction energy loss in turbulent ow by up to 90% at relatively low surfactant concentrationsunderappropriateowandtemperature conditions. This phenomenon is called drag reduction (DR) and it has signicant potential impacts on uid transport and on the environment.14The self-assembly of surfactant DR additives (DRAs) permits them to be used in recirculation applications such as in district heating/cooling systems.15,16 Cationic surfactants have been extensively investigated and it seems that they are the most eff ective surfactant drag reducers providing the proper conditions (such as the chemical struc- tures and concentrations of surfactant and counter-ion). Some cationic surfactants can spontaneously pack into cylindrical structures without any additives at room temperature. Most of the time, the addition of inorganic salts to ionic surfactants is needed to promote micellar growth and viscoelastic proper- ties.17The mechanism involved is primarily the screening of the electrostatic repulsion between the charged head-groups, which results in reduction of the optimal molecular area at the hydrocarbonwater interface, leading to an increase in the end- cap energy.18Rather, little is known about uorosurfactants, in particular cationic uorosurfactants, despite the technical interest of their potential usefulness due to their strong hydrophobicity and effi cient surface activity. Solutions of these surfactants have a number of peculiar properties because their micellar structures are formed due to weak non-covalent interactions and easily change in response to external actions. The introduction of uorinated chains in surfactants is very promising, thanks to their high hydrophobicity and stiff ness, for the formation of structures with relatively little curvature, such as cylindrical micelles.14,19 A schematic representation of the synthesized uoro- surfactants is given in Scheme 1. We report on the synthesis of these surfactants, zeta potential and rheological measurements. Moreover, viscoelastic properties of concentrated solutions of these hybrid surfactants as a function of temperature, stress and strain were studied along with Cryo-SEM observations. Materials and methods 3-Peruorobutyl-1,2-epoxypropane, 3-peruorohexyl-1,2-epoxy- propane and 3-peruorooctyl-1,2-epoxypropane were synthe- sized according to the previous procedure.20For the synthesis of hybrid surfactants, iodomethane and all chemicals were purchased from Aldrich and used without further purication. Unless specied, the solvents were used as received. Conr- mation of the structures of the intermediates and products was obtained by nuclear magnetic resonance (NMR) realized with a Bruker W-200 MHz instrument and mass spectrometry (MS) 80 using a Thermo TRACEGC instrument from Thermoscher 81Corp. tted with an Automass III Multi spectrometer (elec- tron ionization at 70 eV). Preparation of the semiuorinated tertiary amines (Anm) 3-Peruoroalkyl-1,2-epoxypropane(0.02mol)alkylbutyl(n4), hexyl (n 6), and octyl (n 8) was added under stirring to dialkylamine (0.022 mol) alkyl methyl or ethyl in methanol. The mixture was heated at 60 ?C for 6 to 15 h. Purication of the red liquid by column chromatography on silica gel (dichlor- omethane : methanol99 : 1)gaveaminesAnmasyellowliquids. The experiments were reproduced several times. In all cases, the yields (see details in the ESI) were reproducible (?3%). General procedure for the synthesis of the hybrid surfactants (FnHm) A mixture of a tertiary amine (Anm) and an excess of iodo- methane was reuxed in acetonitrile at 60 ?C for over 24 h. Scheme 1Synthetic route towards the studied surfactants FnHm. This journal is The Royal Society of Chemistry 2013Soft Matter, 2013, 9, 89928999 | 8993 PaperSoft Matter Published on 29 July 2013. Downloaded on 02/12/2017 01:48:19. View Article Online Iodomethane and acetonitrile were removed by evaporation, the residue was washed several times with anhydrous diethyl ether and recrystallized several times from a mixture of methanol and diethyl ether, and then the residue was washed several times with petroleum ether to give FnHmin the form of white solids (seeyields,chemicalcharacterizations,criticalmicellar concentration (cmc) and surface tensions (gS) in the ESI). Dynamic light scattering experiments TheZ-averagediameteristhemeanhydrodynamicdiameterand the polydispersity index is an estimate of the width of distribu- tion. Both of these parameters were calculated according to the International Standard on dynamic light scattering (ISO13321). All the measurements were done at a temperature of 25 ?C, whichwas controlled bymeans of a thermostatusing a Zetasizer Nano-ZSmodel3600(MalvernInstrumentsLtd)equippedwitha HeNe laser (l 633 nm, 4.0 mW). Measurements of the size of the surfactant aggregates wereperformed byDLS to examine the state of aggregation in aqueous solution. The aqueous solutions were prepared using deionizated water and ltered through a 0.45 mm lter. The time-dependent correlation function of the scattered light intensity was measured at a scattering angle of 90?relative to the laser source (red-scattering detection). Measurements were repeated several weeks aer prepara- tion, and no modication of the sizes was visualized. DLS diff usion coeffi cients (D) were evaluated by tting the equation of StokesEinstein: D (kBT )/6phr(1) where kBis Boltzmanns constant, T is the absolute tempera- ture, and h is the viscosity of the solvent. The radius (r) of the particles was estimated from their diff usion coeffi cient (D). Zeta potential measurements Zeta potential measurements of the surfactant complex were carried out using a Malvern Instruments Ltd NanoZs by taking the average of three measurements at the stationary level. The cell used was a 5 mm ? 2 mm rectangular quartz capillary. The temperature of the experiments was 298.15 ? 0.01 K controlled by a proportional temperature controller HETO. The zeta potential z were calculated from the electrophoretic mobilities (mE) using the Henry equation:21 z 3mEh 2303r 1 fka (2) where 30is the permittivity of vacuum, 3rand h are the relative permittivity and viscosity of the medium, respectively, a is the particle radius, and k is the Debye length. The function f (ka) depends on the particle shape and for our systems it was determined by fka 1:5 ? 9 2ka 75 2k2a2 ? 330 k3r3 (3) The Henry function is appraised to be 1 for small kr and 1.5 for large kr. Hence, for larger aggregates of relatively small potential, eqn (2) can be used to determine the zeta potential of a particle. Then, k is related to the ionic strength of electrolyte solution according to k (2000F2)/(303rRT)1/2I1/2(4) where F is the Faraday constant, 30the permittivity of a vacuum, 3rthe relative permittivity, R the gas constant, T the tempera- ture, and I the ionic strength of the electrolyte solution.22 Rheological measurements The rheological properties of the samples were measured using an Anton Parr Physica MCR-301 rheometer. Cone geometry was used in each case (50 mm diameter, 1 angle). The gap was 0.05mm.Asolventtrapwasusedtominimizewaterevaporation. Experiments were performed at 20, 30, 40, and 50 ? 1 ?C. The viscosityofthesamplewasobtainedfromsteady-shear measurements with the shear ranging from 0.3 to 500 s?1. Frequencysweepmeasurementswereperformedatagivenstress in the frequency region varying from 0.01 to 100 rad s?1. All the rheology/viscosity data given in this paper are average values of twomeasurements;thedeviationswereintherangeof?0.01Pas. Microscopic observations Cryogenic scanning electron microscopy (Cryo-SEM) images were obtained with an FESEM6700F JEOL microscope (Japan). One drop of the sample was rapidly frozen in nitrogen slush at ?220 ?C and transferred under vacuum into the cryofracture apparatus (Alto 2500 GATAN UK) chamber where it was frac- tured at ?100 ?C and maintained at this temperature for 7 min for sublimation. It was then metallized with AuPd and intro- duced into the microscope chamber where it was maintained at ?100 ?C during the observation. To ensure reproducibility, the micrographs presented in this article were chosen from a large number of negatives. Results and discussion Dynamic light scattering discussion Fig. 1 shows the diameter (D) of the population of aggregates as a function of the solution concentrations for F4H1and F4H2. D increases as the concentration and the value of D was higher for F4H2suggesting the inuence of the head size (N+Me3for F4H1 against N+MeEt2for F4H2) on larger aggregates. Fig. 2 shows the diameter D of the aggregates in solution (at ve times the cmc) as a function of the uorinated chain length: it can be observed that higher values of D are reached with longer uorinated chains. It reects the ability of uorinated surfactants to form aggregates with lengths as long as several micrometers. These ndings are consistent with the earlier observations by Zana and co-workers10for hybrid surfactants having a very strong propensity for micellar growth and formation of micelles of very low curvature. Moreover, in our case, the presence of big counter-ions I? near to the polar head-groups of the surfactant molecules decreases the repulsion force between the head-groups. This 8994 | Soft Matter, 2013, 9, 89928999This journal is The Royal Society of Chemistry 2013 Soft MatterPaper Published on 29 July 2013. Downloaded on 02/12/2017 01:48:19. View Article Online reduction in the repulsion allows the surfactant molecules to approach each other more closely and form larger aggregates. However, the electrostatic eff ect of the I?counter-ion binding on ionic micelles and the change in the hydrocarbon- bonded structure of water cause hydrophobic interactions between surfactant molecules.23Also, due to Ostwald ripening,24 aggregates of larger curvature realized themselves into single species due to their higher chemical potential and the liberated single species adsorb to the aggregates of smaller curvature to make them further larger in size. DLS analyses clearly show the presence in the solution of diff erent sizes of aggregates and the transition from monomodal to bimodal structure with an increase in the uorinated chain length (see DLS curves in the ESI).25 The eff ect of a long uorinated chain on packing of surfactants is expected to aff ect strongly the curvature of a surfactant layer and the incorporation of a uorinated chain into the core provides a bimodal distribution for the long uorinated chains (F8Hmand F6Hm). In fact, it may be assumed that with an increasing number of carbon atoms in the uorinated chain, the hydrophobic interac- tions become more intensive and larger aggregates can be observed.Thesemeasurementssuggestspontaneous organizationinlargeaggregates(vesiclesorcylinders)forallofthe synthesized surfactants. Zeta potential discussion Zeta potential (z) measurements for F8H1and F8H2as a func- tion of concentration for 20 (C1), 30 (C2), and 40 (C3) times the cmc were carried out; these concentrations were chosen to observe the behavior until the maximal solubility of the synthesized surfactants was reached. The concentration over 40 times the cmc of F8H1was not realized because the surfactant is insoluble at this concentration. The main peak was found to be positive and it corresponds to the positive change of head groups in these quaternary ammonium surfactants.25 The main positive peak is probably attributed to large aggregates with a low degree of curvature and a weak adsorption of I?counter-ions at the micelle surface. Here, the positive zeta potential of F8H1and F8H2decreases with increasing surfactant concentration due to the formation of entangled networks of wormlike micelles caused by higher hydrophobic interactions. Also, zeta potential measurements were carried out at a concentration twice the cmc of FnHmwith n 4, 6, 8 and m 1, 2 in order to study the inuence of the uorinated chains. The measured values for the synthesized surfactants are summa- rized in Table 1. All values were positive, due to the head group, and the values increased on increasing the uorinated chain length. The uorinated chain enhances the hydrophobic interactions leading to an increase in the value of the zeta potential. Rheological discussion The stiff ness of the uorocarbon chain sometimes has an eff ect on the packing17leading uorinated surfactants, in aqueous solutions, to form cylindrical micelles wi