J. Phys. Chem. B 2009, 113, 1615–1622
1615
Viscoelastic Wormlike Micelles in Mixed Nonionic Fluorocarbon Surfactants and Structural Transition Induced by Oils Suraj Chandra Sharma,† Rekha Goswami Shrestha,‡ Lok Kumar Shrestha,‡ and Kenji Aramaki*,‡ Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo UniVersity of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan, and Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan ReceiVed: September 22, 2008; ReVised Manuscript ReceiVed: NoVember 10, 2008
Formation and rheological behavior of viscoelastic wormlike micelles in an aqueous solution of a mixed system of nonionic fluorocarbon surfactants, perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(CH2CH2O)nH (abbreviated as C8F17EOn, n ) 20 and 3), was studied. A partial ternary phase diagram of water/C8F17EO20/C8F17EO3 was constructed at 25 °C by visual inspection through crossed polarizers. In the water/surfactant binary system, C8F17EO20 forms an isotropic micellar solution over a wide concentration range (∼80 wt %). The micellar solution of the C8F17EO20 can solubilize a significant amount of C8F17EO3, and the solubility increases with increasing C8F17EO20 concentration. With successive addition of C8F17EO3 to the aqueous C8F17EO20 solution, viscosity increases rapidly, and a viscoelastic solution is formed. The viscosity of the viscous sample was ∼5th order of magnitude of pure water. The viscoelastic solution follows the Maxwell model typical of wormlike micelles at low-frequency region. With further addition of C8F17EO3 the viscosity decreases, and phase separation occurs. Addition of perfluoropolyether oil, (C3F6O)nCOOH, to the viscoelastic solution decreases the viscosity monotonically until phase separation. On the other hand, when perfluorodecalin oil, C10F18, is added, viscosity first decreases and attains a limiting value before excess oil phase separates out. The viscosity decrease in water/surfactant/oil systems is possibly caused by the microstructural transition in the network structure. Small-angle X-ray scattering (SAXS) measurements were performed to complement the rheological data. It has been found that the C8F17EO3 induces one-dimensional growth to the C8F17EO20 micelles. On the other hand, when (C3F6O)nCOOH is added, wormlike-sphere type transition is more likely to occur. 1. Introduction Fluorocarbon surfactants are a particular class of surfactants that are more surface-active and reduce the surface tension of water to an extent, which is in general unachievable with hydrocarbon surfactants. The presence of fluorine atoms alters the properties of surfactants and particularly its hydrophobicity and critical micellar concentration (cmc).1 It has been reported that for nonionic surfactants 1 CF2 group is equivalent to 1.7 CH2 groups.2 Besides, the carbon-fluorine bond is among the most stable known covalent bond with the heat of formation of the C-F bond in CF4 being 486 kJ/mol.3 In addition, fluorine atoms are close to the optimum size to shield carbon atoms without steric stress. These factors combine to make fluorocarbon chains thermally and chemically stable relative to hydrocarbons. The good chemical and thermal stability of fluorocarbon surfactants is an important consideration to operate in harsh environments such as extremes of pH, high temperatures, or in combination with strong oxidizing or reducing agents. Because of unique properties of fluorocarbon surfactants, they are irreplaceable in many applications such as water and oil repellents for textiles, surface modifiers for plastic, paper, and metal, in coating, in fire fighting foams, etc.1,4-8 * Corresponding author: e-mail
[email protected]; Fax +81-45-339 4300. † Tokyo University of Science. ‡ Yokohama National University.
The fluorocarbon surfactants usually behave in a similar way as the hydrocarbon analogues do and possess similar phase sequences and aggregate structures. However, they also exhibit major important differences. Since the fluorocarbon chain is relatively stiffer than the hydrocarbon chain because of the bulky fluorine atoms, the fluorocarbon chain bears an unusual character of low cohesive energies, and hence, the surfactants of this class display a greater tendency to form aggregates with low curvatures, including cylindrical micelles and bilayer structures.9,10 The cylindrical micelles undergo one-dimensional growth and form flexible aggregates known as wormlike micelles. Wormlike micelles are long cylindrical micelles, which are believed to have their contour lengths in the several hundreds of nanometers or even micrometer range. At a certain concentration called the overlap concentration wormlike micelles start to form transient networks that exhibit viscoelastic properties (both viscous (viscous modulus, G′′, > elastic modulus, G′) and elastic properties (G′ > G′′)). It is not always necessarily true that wormlike micelles show viscoelastic character. Viscoelastic character is possible only if the wormlike micelles entangle with each other, forming a transient network structure, and it is called viscoelastic wormlike micelles. The viscoelastic properties of surfactant solutions are analogous to those observed in polymer solutions. Viscoelastic solutions of wormlike micelles have been studied mostly in cationic or anionic fluorocarbon surfactants aqueous systems.5,10,11 There are several reports on thermoresponsive
10.1021/jp808390c CCC: $40.75 2009 American Chemical Society Published on Web 01/20/2009
1616 J. Phys. Chem. B, Vol. 113, No. 6, 2009 SCHEME 1: Molecular Structure of C8F17EOn (in the Present Study Amphiphiles with n ) 20 and 3 Are Considered)
viscoelasticity in some hybrid anionic surfactants containing both fluorocarbon and hydrocarbon chain in their molecules.12-15 However, the formation and properties of viscoelastic wormlike micellar solutions with nonionic fluorocarbon surfactants system are receiving much attention these days due to absence of complicated interaction between the counterions and head groups, which is not possible in the ionic systems.16-19 The oils are known to modify the spontaneous curvature and, hence, the geometry of the aggregate structures changes, and as a result, the solution properties differ significantly. For example, the viscosity of the wormlike micellar solution changes greatly when a small amount of oil is solubilized. Several reports on the oilinduced rod-sphere transition with hydrocarbon surfactants are available,20-23 but relatively little is known regarding the effect of fluorinated oils on the properties and structure of viscoelastic wormlike micelles with fluorocarbon surfactants.16,18,24 In the present contribution, we report the formation of viscoelastic wormlike micelles in mixed nonionic fluorocarbon surfactants in an aqueous system and the onset of wormlikesphere type transition induced by the fluorinated oils. The study is mainly based on rheomerty and small-angle X-ray scattering (SAXS). The rheological data are well supported by the SAXS measurements. 2. Experimental Section 2.1. Materials. Nonionic fluorocarbon surfactants, perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(C2H4O)nH, designated as C8F17EOn (n ) 20 and 3), were obtained from Mitsubishi Materials (Japan). The schematic molecular structure of the surfactant is shown in Scheme 1. The surfactants were used as received without further purification. Perfluoropolyether with structure F-(CF2CF2CF2O)n-CF2CF2COOH (designated as (C3F6O)nCOOH) was kindly provided by Daikin Industries Ltd., Japan. The average molecular weight of (C3F6O)nCOOH is 3600, which gives n ≈ 21. The polydispersity is 1.14. Perfluorodecalin (C10F18) of purity 95% (mixture of cis and trans) was the product of Aldrich. Millipore filter water was used throughout the experiments. 2.2. Phase Diagram. Samples for the construction of phase diagram were prepared in 5 wt % surfactant concentration interval by weighing the required amounts of reagents into test tubes fitted with screw cap. The samples were mixed using a dry thermobath, a vortex mixer, and repeated centrifugation. After mixing, the samples were kept in a temperature-controlled water bath at 25 °C with an accuracy of (0.5 °C for a minimum 24 h for equilibration. Finally, the equilibrium phases were identified by visual observation (through crossed polarizes). 2.3. Rheological Measurements. Samples for rheological measurements were homogenized and kept in water bath at 25 °C for at least 24 h to ensure equilibration before performing the measurements. All the rheological measurements were performed in a stress-controlled rheometer, AR-G2 (TA Instruments), using cone-plate geometries (diameters 60 mm for lowviscosity sample and 40 mm for high-viscosity sample, each with a cone angle of 1°) with the plate temperature controlled by a Peltier unit. A sample cover provided with the instrument was used to minimize the change in sample composition by
Sharma et al. evaporation during the measurement. Frequency sweep measurements were performed in the linear viscoelastic regime of the samples, as determined previously by dynamic strain sweep measurements. The zero-shear viscosity (η0) of the samples was determined from a steady-shear measurement by extrapolating the viscosity to zero-shear rate for less viscous samples or from the values of G0 and τR as obtained from oscillatory measurements for viscous samples by using the following relation:
η0 ) G0τR
(1)
where G0 is the plateau modulus which is related with the number density of entanglements in the transient network and τR is the relaxation time which is related to the average length of the wormlike micelles. 2.4. Small-Angle X-ray Scattering (SAXS). For the structural investigation of the micellar aggregates, SAXS measurements were carried out on the C8F17EO20/C8F17EO3/water system with or without oils. The measurements were performed using a SAXSess camera (Anton Paar, PANalytical) attached to a PW3830 laboratory X-ray generator with a long fine focus sealed glass X-ray tube (KR wavelength of 0.1542 nm) (PANalytical). The apparatus was operated at 40 kV voltages and 50 mA current. The SAXSess camera is equipped with a focusing multilayer optics and a block collimator for an intense and monochromatic primary beam with low background, and a translucent beam stop for the measurement of an attenuated primary beam at q ) 0. Samples were enclosed into vacuum tight thin quartz capillary with an outer diameter of 1 mm and thickness of 10 µm, and the same capillary was used for all measurements to attain the exactly the same scattering volume and background contribution. The sample temperature was controlled with a thermostated sample holder unit (TCS 120, Anton Paar). The scattered intensity was first measured on an image plate (IP) detection system Cyclone (Perkin-Elmer), and the two-dimensional intensity data were finally transformed into one-dimensional scattering curves as a function of the magnitude of the scattering vector by using SAXSQuant software (Anton Paar). All data were normalized to the same incident primary beam intensity for the transmission calibration and were corrected for the background scattering from the capillary and the solvent. The SAXS data of the micellar solutions are analyzed by the generalized indirect Fourier transformation’s (GIFT) method,25-27 which gives pair-distance distribution functions (PDDFs), p(r), and structures of particles in real space. The details on the theory of SAXS and data treatment methods are described elsewhere.28-32 3. Results and Discussion 3.1. Phase Behavior. A partial ternary phase diagram of the water/C8F17EO20/C8F17EO3 system at 25 °C is shown in Figure 1. In the water/C8F17EO20 binary system, a micellar solution (Wm) is formed over a wide range of surfactant concentration (∼80 wt % of C8F17EO20) at a 25 °C. The detailed aqueous binary phase behavior and the micellar structure of the C8F17EO20 in water are described elsewhere.33 This micellar solution of C8F17EO20 can solubilize a significant amount of C8F17EO3, which is evident from the height of the Wm domain in the ternary phase diagram. Because of bulky and stiff hydrophobic tail and small hydrophilic group, C8F17EO3 itself cannot form discrete aggregates in water. Because of the same reason, incorporation of C8F17EO3 in the aggregates of C8F17EO20 reduces the average
Viscoelastic Wormlike Micelles
Figure 1. Partial phase diagram of water/C8F17EO20/C8F17EO3 ternary system. In the phase diagram, the thick solid line (curve) represents the phase boundary between Wm and (Wm + LR) phases, the symbols are the experimental data points, and the shaded area enclosed by a thin solid line separates the high viscous Wm region from low viscous Wm region.
headgroup area at the interface or, in other words, reduces the interfacial curvature, and beyond the solubilization limit of the Wm phase, the LR phase separates out from the isotropic solution. Upon successive addition of C8F17EO3 to the micellar solution of C8F17EO20, no significant change in viscosity occurs in the dilute solution of the hydrophilic surfactant, but at higher concentration (above 15 wt % of C8F17EO20), viscosity increases gradually at first and then steeply, and a viscous solution is observed. These solutions show flow birefringence, which is seen when the samples are viewed under crossed polarizers while being shaken. With further addition of C8F17EO3 viscosity decreases, and ultimately a phase separation occurs. The shaded area in the phase diagram shows the approximate region of viscous solution inside the Wm domain, and the symbols are the experimental data points. The viscous region extends toward higher surfactant concentration also, but its boundary was not determined at compositions above 60 wt % of C8F17EO20. It should be noted that the surfactant concentrations in which viscoelastic wormlike micellar solutions are formed is much higher than the range reported in the literature. 3.2. Rheological Behavior. 3.2.1. Effect of Surfactant Concentrations. Figure 2 shows the steady shear rate (γ˙ )-viscosity (η) curves for 25 wt % C8F17EO20 + C8F17EO3 system at different mixing fraction of C8F17EO3, expressed in weight fraction of C8F17EO3 in total surfactant (X) at 25 °C. At lower value of X, η is independent of γ˙ ; i.e., Newtonian flow behavior is observed up to γ˙ ∼1000 s-1. At X ) 0.113, behavior is still Newtonian over wide range of shear rate, but shear thinning occurs at large deformation (γ˙ G 100 s-1). With increasing X up to X ) 0.191, the critical γ˙ for shear thinning shifts gradually to lower value and also the viscosity in the plateau region (low γ˙ region) increases, which shows that the system is getting more structured. This rheological behavior is typical of systems consisting of network structure formed by wormlike micelles. When network structure is deformed by applying a shear, shear thinning occurs due to alignment of aggregates under flow if the deformation is faster than the time required to regain equilibrium network structure, and with increasing network density the relaxation becomes slower, i.e., shear thinning begins at lower shear rate. However, with further increase in C8F17EO3 concentration (at X ) 0.208 and 0.247) the viscosity decreases, and a higher deformation rate is required to induce shear thinning. This indicates that some structural transformation occurs at X > 0.191. One possibility is that the system becomes less structured; i.e., micellar length decreases, and gradually network structure is lost. However, such structural changes do not seem to be convincing because the interfacial curvature should be continuously decreasing with increasing C8F17EO3
J. Phys. Chem. B, Vol. 113, No. 6, 2009 1617 concentration, and aggregates with flat bilayer are formed finally at phase separation. A more credible explanation for the change in the rheological behavior is that with increasing X, spontaneous interfacial curvature of aggregate gradually decreases and, with this, energy cost for the formation of hemispherical end-caps of the cylindrical aggregates becomes higher. The end-cap energy is minimized if the free ends fuse with cylindrical part of its own or another micelles, thus forming micellar joints34 or branching.35 Such joints reduce the viscosity because when a stress is applied micellar joint can slide along the cylindrical body (contour), thereby allowing a fast stress relaxation process. In some surfactant systems, micellar connections or branching points have been detected by cryogenic transmission electron microscopy (cryo-TEM), especially in the region where the viscosity decreases after the maximum.36-39 Figure 2b shows the plots of zero-shear viscosity, η0, as a function of X at two different C8F17EO20 concentrations (25 and 35 wt %). It can be seen that above a certain value of X, the viscosity increases steeply and attains the maximum, followed by a decline. When C8F17EO20 concentration is increased from 25 to 35 wt %, a rapid viscosity growth occurs at lower concentration of C8F17EO3, or in other words, the η0-X curve shifts toward lower X values, which can be attributed to the decrease in the effective cross-sectional area per surfactant molecule (as) at the surfactant-water interface of C8F17EO20 in the aggregate with increasing concentration. Note that upon increasing the concentration of C8F17EO20 the maximum viscosity has slightly decreased. Oscillatory shear (frequency sweep) measurements were carried out on the viscous samples at 25 °C. Figure 3a shows the representative plots of elastic modulus (G′) and viscous modulus (G′′) as a function of oscillatory shear frequency (ω) for samples compositions corresponding to viscosity maximum (see Figure 2b). Figure 3b shows the plot of G′ and G′′ as a function of ω for a sample of the 35 wt % C8F17EO20 + C8F17EO3 system at X ) 0.092, 0.134, and 0.191. The result shows that samples are viscoelastic in the time scale of measurement, with G′ < G′′ in the low-ω region and G′ > G′′ in the high-ω region. The viscoelastic behavior of the solution is attributed to the entanglement of the wormlike micelles to form a transient network. In the low-ω region the data points of G′ and G′′ fit well to following Maxwell’s mechanical model of viscoelastic material described by following equations, which considers a single process for stress relaxation, characterized by a parameter τR called the relaxation time.
G'(ω) )
G″(ω) )
ω2τR2 1 + ω2τR2
G0
(2)
G0
(3)
ωτR 1 + ω2τR2
As is evident from the Maxwell equations, in the lowfrequency region ω , ωc, G′ and G′′ scale with ω according to G′ ∼ ω2 and G′′ ∼ ω. In the high-frequency region or, more specifically, in the region of ω . ωc, however, G′ attains a plateau value equal to G0 whereas G′′ shows a monotonic decrease. The shear frequency corresponding to the G′-G′′ crossover, ωc, is equal to the inverse of τR. Considering reptation or diffusion of wormlike micelles along its own contour as the mechanism of stress relaxation in the entangled network, as
1618 J. Phys. Chem. B, Vol. 113, No. 6, 2009
Sharma et al.
Figure 2. (a) Steady shear rate (γ˙ )-viscosity (η) curves for 25 wt % C8F17EO20 + C8F17EO3 system at various mixing fraction of C8F17EO3 in total surfactant, X, and (b) variation of zero-shear viscosity (η0) as a function of X for 25 wt % C8F17EO20 + C8F17EO3 and 35 wt % C8F17EO20 + C8F17EO3 systems at 25 °C.
Figure 3. Variation of elastic modulus, G′ (open symbols), and viscous modulus, G′′ (closed symbols), as a function of oscillatory shear frequency (ω) as obtained by frequency sweep measurement at 25 °C for (a) 25 wt % C8F17EO20 + C8F17EO3, X ) 0.191 (squares) and 35 wt % C8F17EO20 + C8F17EO3, X ) 0.134 (circles), which correspond to the compositions at the viscosity maximum for each system, and (b) 35 wt % C8F17EO20 + C8F17EO3 system at different X. The solid lines in panel “a” and “b” represent fitting to the Maxwellian equation.
proposed by Cates et al.,40 the magnitude of τR is related to the average length of the wormlike micelles whereas G0 is related to the number density of entanglement in the transient network. In Figure 3, solid lines represent the Maxwellian fit to the data points. The Maxwell model describes the rheological behavior in the low-ω region, but in the high-ω region, experimental data show significant deviation, which is generally considered to be due to faster relaxation processes such as Rouse modes.41 As can be seen from Figure 3b, with increasing X, G0 increases monotonically whereas G′-G′′ crossover frequency shifts to the lower value and attains the lowest value at a composition corresponding to viscosity maximum (X ) 0.134) and finally shifts to higher value again. This change in crossover frequency corresponds to faster relaxation processes, which may be due to the formation of micellar joints. Similar changes in rheological behavior with increasing X is observed in the 25 wt % C8F17EO20 + C8F17EO3 system also. Figure 3a allows one to compare the dynamic rheological behavior of 25 wt % C8F17EO20 + C8F17EO3 and 35 wt % C8F17EO20 + C8F17EO3 systems at compositions corresponding to viscosity maximum. The Cole-Cole plots (plot of G′′ vs G′) for 25 wt % C8F17EO20 + C8F17EO3 and 35 wt % C8F17EO20 + C8F17EO3 systems as a function of X are supplied as Supporting Information. Variation of G0 and τR as a function of X for the 25 wt % C8F17EO20 + C8F17EO3 and 35 wt % C8F17EO20 + C8F17EO3 systems are shown in Figure 4. These parameters were obtained by Maxwell model fitting to the experimental data from the frequency sweep measurements. As in the case of the systems described in Figure 3, G′ in the high-ω region (say G′∞) is often higher than the perfect plateau value (G0) as predicted by Maxwell equations. Therefore, the values of G0 estimated from Maxwell equations should be considered as the lower limit for the shear modulus.
Figure 4. Variation of plateau modulus (G0) and relaxation time (τR) as a function of the mixing fraction of C8F17EO3 in total surfactant, X, for 25 wt % C8F17EO20 + C8F17EO3 (triangles) and 35 wt % C8F17EO20 + C8F17EO3 (diamonds) systems at 25 °C.
The shift of the η0 and τR curves toward the lower X values in the η0-X (Figure 2b) and τR-X (Figure 4) plots upon increasing the C8F17EO20 concentration in the mixed system corresponds to the higher extent of linear micellar growth. This is also evident from the increase in the G0 or network density upon increasing surfactant concentration. The lower value of τR at the maximum in the 35 wt % C8F17EO20 + C8F17EO3 system in comparison to the 25 wt % C8F17EO20 + C8F17EO3 system should not be considered as a lower extent of micellar growth in the former system. Instead, it might have arisen from the fact that with increasing surfactant concentration the spontaneous curvature decreases and the system favors micellar branching even at lower value of X so as to minimize energy cost of the formation of end-caps. Continuous growth of G0 in the given composition range where η0 and τR decrease shows that after branching the network density grows until the phase
Viscoelastic Wormlike Micelles
Figure 5. (a) Steady shear rate (γ˙ )-viscosity (η) curves as a function of the weight fraction of (C3F6O)nCOOH, W0, and (b) variation of zeroshear viscosity (η0) as a function of weight fraction of (C3F6O)nCOOH (circles) or C10F18 (squares), W0, for the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system. The solid lines are visual guide. All the measurements were carried out at 25 °C.
separation occurs. There are indications that the local structure at branching points evolve toward bilayer structure and ultimately separates out. 3.2.2. Effect of Fluorinated Oils. Effects of fluorinated oils (perfluoropolyether and perfluorodecalin) on the rheological behavior of viscoelastic wormlike micelles formed in the mixed system of water/C8F17EO20/C8F17EO3 have been studied at 25 °C. Figure 5a shows the steady shear rate (γ˙ )-viscosity (η) curves for the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system as a function of weight fraction of (C3F6O)nCOOH in total system, W0. As can be seen from the Figure 5a, with increasing concentration of oil, viscosity in the plateau region decreases and the critical shear rate (the shear rate at which shear thinning starts) shifts to higher shear rate region. Both these features indicate the formation of a less rigid network structure. One possible reason for the viscosity decay is wormlike-sphere type transition in the micellar structure induced by the oil. We will come to this point in the following section. The effects of fluorinated oils on the steady-shear rheological behavior of viscoelastic wormlike micellar solution can be seen more clearly in Figure 5b, where zero-shear viscosity (η0) is plotted as a function of weight fraction of (C3F6O)nCOOH or C10F18, W0, for the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system at 25 °C. The η0 is ∼275 Pa.s in the oil-free system. With successive addition of (C3F6O)nCOOH, the η0 decreases monotonously until phase separation occurs. In the case of C10F18, the η0 decreases initially for low concentration of oil and attains a plateau value. Phase separation occurs above W0 ) 0.008. In the case of (C3F6O)nCOOH, phase separation occurs above W0 ) 0.012. It has been reported that depending on the molecular structure, the oils are solubilized at different site of the aggregates. Recently, Rodriguez-Abreu et al. have studied the effects of oils (decane and p-xylene) on the rheology of viscoelastic wormlike micelles in mixed sucrose esters and nonionic cosurfactant.21 It has been found that decane reduces the maximum η0, whereas p-xylene does not. The p-xylene, however, shifts the viscosity maximum toward the lower cosurfactant concentrations. The oils used in the present study have entirely different structures and molecular weights and thus can have different effects to the interfacial curvature and rheology. The (C3F6O)nCOOH due to its amphiphilic nature penetrates the palisade layer of cylindrical aggregates, but a large section of the hydrophobic chain of the oil molecules forms a pool in the core of the aggregates and, hence, the curvature tends to be more positive, resulting into the spherical type of aggregates. On the other hand, C10F18 tends to be solubilized in the vicinity of the hydrophobic micellar core. It is possible that
J. Phys. Chem. B, Vol. 113, No. 6, 2009 1619
Figure 6. Variation of elastic modulus, G′ (open symbols), and viscous modulus, G′′ (closed symbols), as a function of oscillatory shear frequency (ω) as obtained by frequency sweep measurement at 25 °C for the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system at different weight fraction of (C3F6O)nCOOH, W0 in total system. W0 ) 0 (squares), 0.001 (circles), 0.003 (triangles), and 0.01 (diamonds). Thick solid lines are best fits to the Maxwell model.
Figure 7. Variation of plateau modulus, G0, and relaxation time, τR, as a function of the weight fraction of (C3F6O)nCOOH (circles) or C10F18 (squares) in total system, W0, for the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system at 25 °C.
the rigid network structure of wormlike or cylindrical micelles is retained in the case of C10F18 added systems. We will come to this point in section 3.3.2. Figure 6 shows plot of elastic modulus (G′) and viscous modulus (G′′) as a function of oscillatory shear frequency (ω) for the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system at different weight fraction of (C3F6O)nCOOH in total system, W0. With increasing (C3F6O)nCOOH concentration, the crossover frequency shifts toward higher-frequency region, suggesting a faster relaxation process with a less rigid network structure. The rheological parameters (G0 and τR) derived from the Maxwell model fittings to the experimental data are presented in Figure 7. The decreasing trend of the plateau modulus (G0) with increasing (C3F6O)nCOOH concentration indicates that the (C3F6O)nCOOH disrupts the network structure of viscoelastic wormlike micelles. Since the G0 is proportional to the number of entanglements N in the system,34 it seems that the increase in the aggregate curvature induced by added (C3F6O)nCOOH results reduction in the value of the N. The relaxation time, τR, follows a similar trend with increasing (C3F6O)nCOOH concentration and further clarifies the shortening of the average micellar length. As can be seen in Figure 7, the G0 and τR of the C10F18 added system is higher than that of the (C3F6O)nCOOH system, which suggests that the former oil has a weaker tendency to modify the network structure of the aggregates in comparison to the latter oil. Therefore, the viscosity of the C10F18 added system is
1620 J. Phys. Chem. B, Vol. 113, No. 6, 2009
Sharma et al.
Figure 8. (a) Normalized X-ray scattered intensities, I(q), of the 25 wt % C8F17EO20 + C8F17EO3 system at different weight fractions of C8F17EO3 in total surfactant, X, at 25 °C (symbols), and their GIFT fit functions (solid lines) and (b) the corresponding PDDF, p(r), functions. The arrows in (b) represent the maximum dimension of the micelles.
Figure 9. (a) Normalized X-ray scattered intensities, I(q), of the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system at different weight fraction of (C3F6O)nCOOH in total system, W0, at 25 °C (symbols) and their GIFT fit functions (solid lines) and (b) the corresponding PDDF, p(r), functions. For better visibility, the curves are multiplied by the factors 5 (W0 ) 0.001), 100 (W0 ) 0.003), 500 (W0 ) 0.006), and 5000 (W0 ) 0.010), and the arrows in (b) represent the maximum dimension of the particles, Dmax.
Figure 10. (a) Normalized X-ray scattered intensities, I(q), of the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system at different weight fraction of C10F18 in total system, W0, at 25 °C (symbols) and their GIFT fit functions (solid lines) and (b) the corresponding PDDF, p(r), functions. For better visibility, the curves are multiplied by the factors 5 (W0 ) 0.001), 100 (W0 ) 0.003), and 1000 (W0 ) 0.006), and the arrows in (b) represent the maximum dimension of the particles, Dmax.
higher compared to the viscosity of the (C3F6O)nCOOH added system (see Figure 5b). 3.3. SAXS Measurements. 3.3.1. Effects of C8F17EO3 Concentration on the Micellar Structure. Figure 8 shows scattering functions, I(q), and the corresponding pair-distance distribution functions (PDDF), p(r), deduced with the GIFT analysis of the SAXS data for the 25 wt % C8F17EO20 + C8F17EO3 systems at different X at 25 °C. The calculated form factor, P(q), and structure factor, S(q), for the 25 wt % C8F17EO20 + C8F17EO3 system as a function of X are supplied as Supporting Information. The suppressed forward scattering intensity with a peak in the intermediate q region (q ∼ 0.7 nm-1) for the 25 wt % C8F17EO20 system is a clear signature of repulsive interaction among the particle owing to the reduced osmotic compressibility of the system.42,43 Such feature in the scattering function is natural as the system is concentrated (surfactant concentration 25 wt %). In the present calculation, we have used the averaged
structure factor model for a hard-sphere (HS) interaction potential, S(q)av,44,45 which considers the Gaussian distribution of the interaction radius σ for individual monodisperse systems for polydispersity µ. Therefore, the micellar interpenetration is not possible. Upon successive addition of C8F17EO3 to the micellar solution of the C8F17EO20, the forward scattering intensity increases and the interaction peak shifts toward forward direction (low-q side). The microstructural changes induced by the C8F17EO3 is presented in the PDDF curves (Figure 8b) derived from the GIFT analysis of the SAXS data. In the absence of C8F17EO3, the PDDF curve is bell-shaped and symmetric, which indicates spherical type aggregates. The value of r at which p(r) becomes zero (Dmax) estimates the maximum length of the aggregates. As can be seen from the Figure 8b, when C8F17EO3 is added, the Dmax shifts to the higher-r side, and the PDDF curves become asymmetric, indicating elongated aggregates. At higher C8F17EO3 concentrations, a clear signature of cylindrical particles is observed as estimated from the
Viscoelastic Wormlike Micelles pronounced peak in the low-r side with an extended tail in the higher-r side of the PDDF curve.46,47 Thus, the viscosity increase in the ternary mixture of water/C8F17EO20/C8F17EO3 can be attributed to the one-dimensional micellar growth. Note that the Dmax depends on the maximum resolution (or qmin) of the SAXS equipment. Therefore, the length of the micelles extracted from the SAXS data mainly in the systems of very long cylindrical particles such as wormlike micellar solutions may not be the actual length of aggregates. The actual length of the wormlike micelles is believed to have their contour lengths several hundreds of nanometers or even in the micrometer range. However, within the resolution of the SAXS equipment and by careful analysis of the SAXS data, is it possible to see a general trend in microstructural transition of aggregates induced by system variables. 3.3.2. Effects of Fluorinated Oils on the Micellar Structure. Nonpolar organic solvents are practically immiscible in water, but it can be solubilized in the micellar solution. Generally, the oils tend to solubilize in the hydrophobic micellar core. As a result, the spontaneous curvature of the micelles changes and also the structure and aggregation number. In this paper, we have investigated the effects of two different types of fluorinated oils, namely perfluoropolyether, (C3F6O)nCOOH, and perfluorodecalin, C10F8, on the structure of viscoelastic wormlike micelles. Figure 9 shows the results of SAXS measurements at different concentration of (C3F6O)nCOOH. Figure 9a shows the normalized X-ray scattered intensities, I(q), of the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system at different weight fraction of (C3F6O)nCOOH in total system, W0, at 25 °C. For the sake of clarity, the curves are multiplied by certain factors (see caption of Figure 9). Minute observation of the scattering curves reveals that the forward scattering intensity decreases slightly with increasing weight fraction of the (C3F6O)nCOOH. In Figure 9b, the PDDF curves deduced from the GIFT analysis of the SAXS data are presented. From the PDDF curves, we can see a decreasing trend of Dmax with increasing (C3F6O)nCOOH concentration, and ultimately at W0 ) 0.010 globular type particle is observed. The Dmax changes from ∼24 to ∼10 nm. Therefore, the viscosity decreases with increasing (C3F6O)nCOOH concentration can be attributed to the shortening of wormlike micelles. In other words, we can say the viscosity decrease is due to wormlike-sphere type transition in the network structure. It is possible that the polar part of (C3F6O)nCOOH goes very close to the hydrophilic/ lipophilic interface of the surfactant molecule and nonpolar part makes an oil pool in the micellar core since the hydrophobic chain of the oil is longer than that of the host surfactant molecule. Kaneko et al. obtained similar results in their study on the effect of silicone copolymer oils.48 Next, we present the effect of C10F18 on the micellar structure. Figure 10 shows the scattering intensities and the PDDF curves of the 25 wt % C8F17EO20 + C8F17EO3 (X ) 0.191) system at different weight fraction of C10F18 in total system, W0. For the sake of clarity, the scattering intensity curves are multiplied by certain factors (see caption of Figure 10). As can be seen from the PDDF curves, the micellar length tends to decrease with C10F18 concentration, but a rodlike feature is still retained by the system. This might be the reason for higher viscosity with C10F18 systems compared to the (C3F6O)nCOOH system. 4. Summary Viscoelastic solution of wormlike micelle is formed in an aqueous solution of nonionic fluorinated surfactant, perfluoroalkyl sulfonamide ethoxylate (C8F17EO20), when a less hydro-
J. Phys. Chem. B, Vol. 113, No. 6, 2009 1621 philic cosurfactant, C8F17EO3, is added. Addition of C8F17EO3 reduces the interfacial curvature of the aggregates and induces one-dimensional micellar growth. With successive addition of the C8F17EO3, viscosity increases rapidly to form viscoelastic solutions and then decreases after the maximum, and ultimately phase separation occurs. The dynamic rheological behavior of the viscoelastic solutions can be fitted to Maxwell model at low shear frequency, which is characteristic of wormlike micelles. The turning point in the viscosity curve is associated with the formation of micellar joints. Increasing surfactant or cosurfactant concentration in the mixed nonionic system increases the extent of one-dimensional micellar growth, which is mainly attributed to the decrease in the spontaneous curvature of the aggregates and consequently a progressive increase in the energy cost for the formation of the hemispherical end-caps of the aggregates. Rheological study, consistent with the theoretical prediction, indicates that upon increasing any of the parameters that increases the energy cost of the formation of end-caps, free endcaps fuse to the cylindrical body to form branching or joints having relatively low free energy. Addition of fluorinated oils such as (C3F6O)nCOOH and C10F18 decreases the viscosity of the viscoelastic solution of the the water/C8F17EO20/C8F17EO3 system due to microstructural transition of the aggregates. Viscosity of the C10F18 added system is higher compared to the (C3F6O)nCOOH system due to retainable rodlike features of micelles. On the other hand, the lower viscosity of the (C3F6O)nCOOH added system is due to the formation of globular type of particles. Rheological results are qualitatively supported by SAXS measurements. Acknowledgment. S.C.S. and L.K.S. are thankful to the Japan Society for the Promotion of Science (JSPS) for financial support. Supporting Information Available: Cole-Cole plots for 25 wt % C8F17EO20 + C8F17EO3 and 35 wt % C8F17EO20 + C8F17EO3 systems as a function of X; calculated form factor, P(q), and structure factor, S(q), for the 25 wt % C8F17EO20 + C8F17EO3 system as a function of X. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909. (2) Ravey, J. C.; Gherbi, A.; Ste´be´, M. J. Prog. Colloid Polym. Sci. 1988, 76, 234. (3) Fletcher, P. D. I. In Specialist Surfactants; Robb, I. D., Ed.; Blackie A&P: Cambridge, UK, 1997; p 108. (4) Kissa, E., Ed. Fluorinated Surfactants and Repellant, 2nd ed.; Marcel Dekker: New York, 2001; Vol. 97, Chapters 1 and 8. (5) Hoffmann, H.; Wu¨rtz, J. J. Mol. Liq. 1997, 72, 191. (6) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976, 80, 2468. (7) Ravey, J. C.; Ste´be´, M. J. Colloids Surf., A 1994, 84, 909. (8) Wu¨rtz, J.; Mayer, J.; Hoffmann, H. Phys. Chem. Chem. Phys. 2001, 3, 3132. (9) Oelschlaeger, C.; Waton, G.; Buhler, E.; Candau, S. J.; Cates, M. E. Langmuir 2002, 18, 3076. (10) Wang, K.; Karlsson, G.; Almgren, M.; Asakawa, T. J. Phys. Chem. B 1999, 103, 9237. (11) Knoblich, A.; Matsumoto, M.; Murata, K.; Fujiyoshi, Y. Langmuir 1995, 11, 2361. (12) Abe, M.; Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Kasahara, Y.; Matsuzawa, H.; Iwahashi, M.; Momozawa, N.; Nishiyama, K. Langmuir 1997, 13, 2932. (13) Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Iwahashi, M.; Momozawa, N.; Abe, M. Langmuir 1997, 13, 5054. (14) Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Kamogawa, K.; Momozawa, N.; Abe, M. Langmuir 1998, 14, 4753. (15) Danino, D.; Weihs, D.; Zana, R.; Ora¨dd, G.; Lindblom, G.; Abe, M.; Talmon, Y. J. Colloid Interface Sci. 2003, 259, 382. (16) Sharma, S. C.; Kunieda, H.; Esquena, J.; Rodriguez-Abreu, C. J. Colloid Interface Sci. 2006, 299, 297.
1622 J. Phys. Chem. B, Vol. 113, No. 6, 2009 (17) Acharya, D. P.; Sharma, S. C.; Rodriguez-Abreu, C.; Aramaki, K. J. Phys. Chem. B 2006, 110, 20224. (18) Sharma, S. C.; Acharya, D. P.; Aramaki, K. Langmuir 2007, 23, 5324. (19) Sharma, S. C.; Rodriguez-Abreu, C.; Shrestha, L. K.; Aramaki, K. J. Colloid Interface Sci. 2007, 314, 223. (20) Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388. (21) Rodrı´guez-Abreu, C.; Aramaki, K.; Tanaka, Y.; Lo´pez-Quintela, M. A.; Ishitobi, M.; Kunieda, H. J. Colloid Interface Sci. 2005, 291, 560. (22) Pokhriyal, N. K.; Joshi, J. V.; Goyal, P. S. Colloids Surf., A 2003, 218, 201. (23) Sato, T.; Acharya, D. P.; Kaneko, M.; Aramaki, K.; Singh, Y.; Ishitobi, M.; Kunieda, H. J. Dispersion Sci. Technol. 2006, 27, 611. (24) Sharma, S. C.; Rodriguez-Abreu, C.; Shrestha, L. K.; Aramaki, K. J. Phys. Chem. B 2007, 111, 12146. (25) Glatter, O.; Fritz, G.; Lindner, H.; Brunner, P. J.; Mittelbach, R.; Strey, R.; Egelhaaf, S. U. Langmuir 2000, 16, 8692. (26) Brunner-Popela, J.; Mittelbach, R.; Strey, R.; Shubert, K. V.; Kaler, E. W.; Glatter, O. J. Chem. Phys. 1999, 110, 10623. (27) Weyerich, B.; Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1999, 32, 197. (28) Glatter, O. Acta Phys. Austriaca 1977, 47, 83. (29) Glatter, O. J. Appl. Crystallogr. 1977, 10, 415. (30) Glatter, O. J. Appl. Crystallogr. 1980, 13, 577. (31) Glatter, O. J. Appl. Crystallogr. 1980, 13, 7. (32) Shrestha, L. K.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Aramaki, K.; Kunieda, H. J. Phys. Chem. B 2006, 110, 12266.
Sharma et al. (33) Shrestha, R. G.; Shrestha, L. K.; Sharma, S. C.; Aramaki, K. J. Phys. Chem. B 2008, 112, 10520. (34) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (35) Candau, S. J.; Oda, R. Colloids Surf., A 2001, 183-185, 5. (36) Lin, Z. Langmuir 1996, 12, 1729. (37) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420. (38) In, M.; Aguerre-Chariol, O.; Zana, R. J. Phys. Chem. B 1999, 103, 7747. (39) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205. (40) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869. (41) Granek, R.; Cates, M. E. J. Chem. Phys. 1992, 96, 4758. (42) Stradner, A.; Sedgwick, H.; Cardinaux, F.; Poon, W. C. K.; Egelhaaf, S. U.; Schurtenberger, P. Nature (London) 2004, 432, 492. (43) Stradner, A.; Glatter, O.; Schurtenberger, P. Langmuir 2000, 16, 5354. (44) Pusey, P. N.; Fijnaut, H. M.; Vrijm, A. J. Chem. Phys. 1982, 77, 4270. (45) Salgi, P.; Rajagopolan, R. AdV. Colloid Interface Sci. 1993, 43, 169. (46) Shrestha, L. K.; Sato, T.; Aramaki, K. Langmuir 2007, 23, 6606. (47) Shrestha, L. K.; Sato, T.; Aramaki, K. J. Phys. Chem. B 2007, 111, 1664. (48) Kaneko, M.; Matsuzawa, K.; Uddin, M. H.; Lo´pez-Quintela, M. A.; Kunieda, H. J. Phys. Chem. B 2004, 108, 12736.
JP808390C
