Effect of alkyl chain asymmetry on catanionic mixtures of hydrogenated and fluorinated surfactants

Journal of Colloid and Interface Science 341 (2010) 261–266

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Effect of alkyl chain asymmetry on catanionic mixtures of hydrogenated and fluorinated surfactants Elena Blanco a, Carlos Rodriguez-Abreu b, Pablo Schulz c, Juan M. Ruso a,* a

Soft Matter and Molecular Biophysics Group, Department of Applied Physics, University of Santiago de Compostela, E-15782, Spain Instituto de Química Avanzada de Cataluña, Consejo Superior de Investigaciones Científicas (IQAC/CSIC), Jordi Girona, 18-26, 08034 Barcelona, Spain c Departamento de Química e INQUISUR, Universidad Nacional del Sur, 8000 Bahía Blanca, Argentina b

a r t i c l e

i n f o

Article history: Received 1 July 2009 Accepted 5 September 2009 Available online 2 October 2009 Keywords: Fluorinated Hydrogenated Catanionics Rheology Mixed

a b s t r a c t In this work we studied and compared the physicochemical properties of the catanionic mixtures cetyltrimethyl-ammonium bromide–sodium dodecanoate, cetyltrimethyl-ammonium bromide–sodium perfluorodacanoate, octyltrimethylammonium bromide–sodium perfluorodacanoate and cetyltrimethyl-ammonium bromide–sodium octanoate by a combination of rheological, transmission electron microscopy (TEM) and polarized optical microscopy measurements. The binary mixtures of the surfactants have been analyzed at different mixed ratios and total concentration of the mixture. Mixtures containing a perfluorinated surfactant are able to form lamellar liquid crystals and stable spontaneous vesicles. Meanwhile, system containing just hydrogenated surfactants form hexagonal phases or they are arranged in elongated aggregates. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction The study of mixtures of surfactants is a subject of great interest both from practical and theoretical points of view. In the first case a fundamental understanding of these systems is required for different industrial and daily applications such as detergents, cosmetics or pharmaceutical and petroleum industry. The second case is focused on the variety of behaviors that the mixtures can show as compared to those showed by pure surfactant systems. In this sense, many scientists have devoted particular attention to mixtures of cationic and anionic surfactants. In 1987, P. Jokela, B. Jönsson and A. Khan introduced the term ‘‘catanionic” to designate an equimolar mixture of two oppositely charged surfactants (the parent surfactants) from which the inorganic counterions are completely removed. The catanionic surfactant thus has no net charge, and one long alkyl chain organic ion acts as a counterion of the other [1–3]. These systems have allowed cheap and goodquality samples to be manufactured for the development of nanotechnology due to the wide range of aggregate microstructures by adjusting composition and packing parameters. The range of different structures observed in these systems is largely a result of electrostatic interactions between charged heads. As a general tendency, spontaneous formation of thermodynamically stable vesicles appears to be a feature of these mixtures [4]. Kaler et al. proposed that highly nonideal mixing of the two kinds of surfactants groups was held responsible for the stability [5]. Zemb * Corresponding author. Fax: +34 981 520 676. E-mail address: [email protected] (J.M. Ruso). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.09.062

et al. have shown that can form hollow aggregates with a regular icosahedral shape, which are stabilized by the presence of pores located at the vertices of the icosahedra [6a,b]. Recently Rico-Lattes and co-workers have demonstrated the role of the cohesive energy parameter on catanionic systems [7a,b]. In the last years there was an increasing interest in mixtures of fluorocarbon- and hydrocarbon-based surfactants that have been reported to have a limited mutual solubility. Initially, the coexistence of two types of micelles, one fluorocarbon-rich and another hydrocarbon-rich was postulated on the basis of the mutual repulsion of surfactant alkyl moieties [8,9]. However, based on 19F and 1 H chemical shift data, Nordstierna et al. [10] have recently proposed a single type of micelles within which fluorinated surfactants are preferentially coordinated by fluorinated ones and hydrogenated surfactants by hydrogenated ones. It is not straightforward to find experimental studies dealing with the effect of the symmetry, or asymmetry, between both alkyl chains. In the first case the lattice of liquid crystal energy is lower and the system can precipitate or form packed vesicles in equimolar ratio. For the second case due to the different alkyl chain length, a dense packing is sterically unfavorable and instead of precipitates or vesicles, elongated micelles are formed due to favorable packing. Thus, precipitation can be avoided if surfactants contain one long and one short alkyl chain [11–13]. However, it is important to take into account that some irregular behavior could be related to the even alkyl chain carbon numbers of the tail [14,15]. In this work, we extend our earlier studies on diluted mixtures of hydrogenated and fluorinated mixtures [16–18] to obtain highly viscous fluids or liquid crystals at low total surfactant con-

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centration, focusing on the effect of the alkyl chain length, mixed ratio and the substitution of H by F atoms. The hydrogenated cationic surfactants chosen were octyltrimethylammonium and cetyltrimethyl-ammonium bromide. The anionic hydrogenated surfactants were: sodium octanoate and dodecanoate, and the fluorinated ones were sodium perfluorooctanoate and sodium perfluorodacanoate. Rheology, transmission electron microscopy and polarized optical microscopy were used to track the behavior of the different systems.

2. Experimental 2.1. Materials Octyltrimethylammonium bromide (C8TAB), cetyltrimethylammonium bromide (CTAB), sodium octanoate (C8HONa), sodium perfluorooctanoate (C8FONa) and sodium perfluorodecanoate (C10FONa) of at least 97% purity were obtained from Lancaster Synthesis Ltd. Sodium dodecanoate (C12HONa) with purity over 99%, were obtained from Sigma Chemical Co. All materials were of analytical grade, and solutions were made using doubly distilled and degassed water.

2.2. Rheology Rheological experiments were performed on a Bohlin CS-10 stress-controlled rheometer. A Couette geometry with a cup of 27.5 mm diameter and a bob type Mooney cell was used. The cell was heated by a reservoir of fluid circulating from a Julabo thermostated bath. The sample was equilibrated for at least 20 min at each temperature prior to each experiment. Both steady and dynamic rheological experiments were performed at each temperature. Dynamic frequency sweep measurements were performed in the linear viscoelastic regime of the samples, as determined previously by dynamic stress sweep measurements. For the steady-shear experiments, an equilibration time of 90 s was given at each shear stress.

2.3. Transmission electron microscopy (TEM) The morphological examination of the structures was performed by transmission electron microscopy (CM-12 Philips). Samples for TEM were prepared by the negative-staining technique with a 2% (w/v) phosphotungstic acid. A carbon Formvar-coated copper grid was put into the solution for 1 min and then into the sodium phosphotungstate for another 1 min. Then the grids are dried. In between and thereafter, excess liquid was sucked away with filter paper. For each system, at least two TEM samples were prepared and observed independently.

2.4. Polarized optical microscopy Samples were placed between two glass slides and observed between crossed polarizers in a Nikon Eclipse E20 polarizing microscope with a COPLIX 54 electronic camera. The samples were then left to evaporate to detect the phases formed as a function of decreasing peripherally water content. The peripheral evaporation technique consists of letting the samples to slowly evaporate trough the limits of the cover slide, thus creating a concentration gradient from dry crystals outside to the original solution at the center of the sample. Subsequently, a drop of water was added to produce peripheral dilution and observe the formation of hydrated phases.

3. Results and discussion 3.1. CTAB-C12HONa system At the beginning of this study different samples at different total concentration (CT) and mixed ratios, aF (where aF is the mole fraction of the anionic surfactants in the mixture, without considering the solvent) were prepared and analyzed by visual inspection. Only samples which exhibit viscoelastic properties were studied in detail. For the system CTAB-C12CHONa this was just possible for a total concentration (CT) of 90 mM and mixed ratio of aF = 0.6. Polarized microscopy has been used to characterize the phase behavior of this system at higher concentrations. It has been observed that the system present a strong tendency to form a hexagonal liquid crystalline phase formed by non isotropic elements. The striated and fanlike texture from Fig. 1a and b, respectively, of hexagonal phase are based on arrangements of the cylindrical molecular aggregates. The fanlike texture is the type of focal conic variation designated ‘‘fan structure”. The fan is a composite of focal domains whose hyperbolas are so elongated that they appear as a group of straight lines converging from their respective focal regions to a common point; the focal conic geometry, if visible at all, would thus be sough it at the outer edge of each sector of the fan [19a,b]. The rheological study performed for this system is illustrated in Fig. 2. At intermediate shear rates, the sample show non-newtonian shear thinning, whereas at higher shear rates a shear thickening behavior can be observed with a critical shear rate of about 3  103 s1. Shear thickening has been reported for other surfactant systems usually containing diluted rod-like micelles a few tens nanometers long [20]. These micelles undergo free Brownian motions at equilibrium state or under slow shear rates. However, they tend to align in the shear flow when the applied shear rate is above a critical value. In the end, larger aggregates are formed and flow birefringence is increased [18]. 3.2. CTAB-C8FONa system Following the same protocol, we have centered our measurements in the total concentration range from 200 to 300 mM and mixed ratio of aF = 0.1. Polarized microscopy of this sample show a typical Maltese cross texture which is typical of lamellar structures. Other surfactant solutions in the lamellar region had smooth sandlike or marble-like textures that are also consistent with lamellar phases [21] (Fig. 3). The rheological behavior is presented in Fig. 4. For both concentrations and at low shear rates, the samples show Newtonian behavior; however, above a certain critical shear rate the viscosity decreases as in a shear thinning fluid, also known as pseudoplastic system. The onset of thinning behavior takes place at a lower critical shear rate as the surfactant concentration increases. This non-Newtonian flow response and the corresponding increase in viscosity with concentration are attributable to structure-forming interactions and the anisometric nature of vesicles that orient themselves at high shear rates [22]. In fact, based in the phase diagram reported for this system by Hentze et al. [23] our samples are placed in the vesicle + lamellar liquid crystal region. The results of oscillatory shear measurements are shown in Fig. 4b. In these systems, liquid-like behavior (G0 < G00 ) is observed in the low-frequency region, but both G0 and G00 increase with frequency and viscoelastic behavior (G0 > G00 ) is observed in the high frequency region for the most concentrated sample. The system is thus viscoelastic in a wide range of frequencies. 3.3. C8TAB-C10FONa system For this system we have focused on a concentration range from 10 to 20 mM (with aF = 0.6 and 0.8). Polarized microscopy (not

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Fig. 1. (a) Polarized optical microphotograph of a CTAB-C12HONa sample (aF = 0.6) showing the textures observed in a thin film in water with concentration increasing from left to right (magnification 100). The concentration gradient was established by allowing water to penetrate into an array of crystals placed on a glass slide and covered with a cover slip. Nomenclature: I, isotropic micellar solution phase; H hexagonal phase. (b) Detail of a fully developed hexagonal phase corresponding to the sample in (a).

-3

7x10

(a)

-3

6x10

1 -3

viscosity (Pa s)

viscosity (Pa s)

5x10

-3

4x10

-3

3x10

0.1

0.01 -3

2x10

2

10

3

10

4

-4

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10

-3

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-1

-1

0

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3

4

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10

5

10

-1

Shear rate (s )

shear rate (s )

Fig. 2. Shear viscosity as a function of shear rate for a CTAB-C12CHONa sample at CT = 90 mM and aF = 0.6.

(b)

G`. G`` (Pa)

10

1

10 0

10 -1

10 -2 -2 10

10

-1

10

0

10

1

10

2

-1

ω (rad s ) Fig. 4. (a) Shear viscosity as a function of shear rate for the system CTAB-C8FONa at CT = 200 mM (squares) and CT = 300 mM (circles). aF is fixed at 0.1. (b) Storage modulus G0 (solid symbols) and loss modulus G00 (open symbols) as a function of angular frequency for the sample in (a) at CT = 300 mM and aF = 0.1.

Fig. 3. Positive and negative units (spherulites) typical for lamellar liquid crystals in a CTAB-C8FONa sample at CT = 300 mM and aF = 0.1. The sample has been observed between crossed polarizers (magnification 100).

shown) revealed some low-birefringence areas showing oily streaks and spherulites. TEM micrographs of these systems show

E. Blanco et al. / Journal of Colloid and Interface Science 341 (2010) 261–266

vesicles which have a rather polydisperse distribution (Fig. 5). The vesicles diameters range from about 60 nm to 100 nm. The spontaneous formation of vesicles in catanionic systems have been previously reported in numerous studies [24a,b,c]. The viscoelastic properties of samples at different concentrations and aF values were studied by oscillatory experiments. Results are given in Figs. 6 and 7. For the mixed ratio aF = 0.6, the loss modulus (G00 ) is higher than the storage modulus (G0 ) which reveals the mainly viscous behavior of the solution. Similar patterns were obtained for aF = 0.8 at total concentrations of 15 and 20 mM. This behavior has been found previously in systems containing a high concentration of vesicles [25,26]. However, for the lowest concentration (10 mM, aF = 0.8), a typical result for worm-like micellar solutions has been obtained [27] with a G0 G00 crossover at low frequency (high relaxation time). Other investigations have shown that a vesicle to worm-like micelle transition can be induced in three different ways, i.e. by increase of temperature, by addition of a surfactant, and by shearing [28a,b]. This has been explained as being the result of a process of dissociation of the oppositely charged amphiphiles that leads to a surface melting [29,30].

1

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264

10

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-2

-1

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ω (rad s ) Fig. 6. Storage modulus G0 (closed symbols) and loss modulus G00 (open symbols) as a function of angular frequency for the system C8TAB-C10FONa, at a total concentration of 10 mM and mixed ratio of a = 0.6 (squares) and a = 0.8 (circles).

3.4. CTAB-C8HONa system G' (10mM) G'' (10mM) G' (15mM) G'' (15mM)

1

10

0

10

10

1

10

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-1

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G', G'' (Pa s)

G', G'' (Pa)

For this system the rheological study was carried out for mixed ratios of aF = 0.6 and aF = 0.4 and different total concentrations. In all cases, shear thinning behavior is observed. We have obtained the zero shear viscosity (g0) of the samples; the results are shown in Fig. 8. The g0 values were calculated by extrapolating the viscosity data at low shear rates to a zero shear rate. In the range of total concentration studied here, two different behaviors have been found: an increase of viscosity with concentration for aF = 0.4, meanwhile the viscosity g0 shows a pronounced maximum as a function of total concentration for aF = 0.6. It is well-known that aqueous ionic solutions of surfactants can undergo uniaxial growth upon the addition of salt. As a result, the viscosity of the solutions increases. It is generally admitted that increasing the salt concentration amounts to an increase in the curvature energy of surfactant molecules in the end-cap relative to the one cylindrical body of the micelle. This leads to an increase in micellar length. The decrease of the viscosity is more difficult to explain and several possible explanations have been proposed, the simplest of which is a decrease of the micellar length. Previous studies have demonstrated that there is not a general rule for plots of zero shear viscosity as a function of the total concentration, there are systems with a more complex behavior that can exhibit even two maxima [31].

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G' (20mM) G'' (20mM)

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ω (rad s ) Fig. 7. Storage modulus G0 (closed symbols) and loss modulus G00 (open symbols) as a function of angular frequency for the system C8TAB-C10FONa at a fixed mixed ratio of a = 0.8 and total concentrations of 10 mM (squares) and 15 mM (circles). The inset shows the same features for a total concentration of 20 mM.

50

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Zero shear viscosity (Pa s)

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total concentration (mM)

Fig. 5. TEM images of C8TAB-C10FONa (CT = 10 mM, aF = 0.8).

Fig. 8. Variation of the zero shear viscosity for the system CTAB-C8HONa at two different mixed ratios a = 0.6 (squares) and a = 0.4 (circles) as a function of the total concentration.

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relaxation time (sR), G0 and G00 obey the following relations as a function of x:

260

210 mM 285 mM 360 mM 450 mM 570 mM

240 220 200 180

140 120 100 80 60 40 20 0

100

200

300

400

G' (Pa) Fig. 9. Cole–Cole plot for different concentrations of CTAB-C8HONa (aF = 0.4).

Oscillatory-shear (frequency sweep) measurements were performed on viscoelastic samples. All systems under study have shown a liquid-like behavior (G0 < G00 ) at the low-frequency region, but both G0 and G00 increase with x, and solid-like behavior (G0 > G00 ) is observed at the high frequency region. This is the typical viscoelastic behavior shown by worm-like micellar solutions. In the low x region the data points of G0 and G00 could be fitted to Maxwell equations, but in the high x region, experimental data deviates from the model, which is generally considered to be due to other relaxation processes such as Rouse modes [32]. A Cole–Cole plot provides a good picture of how well the data correspond to the Maxwell model with a single relaxation time; a semicircle is characteristic of a Maxwell fluid. Fig. 9 shows Cole–Cole plots of the system under study, indicating that these systems are well fitted by a simple Maxwell model [33]. Maxwellian type oscillatory rheological behavior of viscous micellar solutions can be related to the transient network formed by the entanglement of worm-like micelles [34]. In the Maxwell model of viscoelastic fluids with a single

xsR 1 þ ðxsR Þ2

G0

ð1Þ

G0

ð2Þ

The plateau modulus, G0, is given by G0 (x) at high x. The relaxation time, sR, may be estimated from the relation sR = 1/xc where xc is the crossover frequency. Calculated values for G0 and sR obtained from the Maxwell fit to the experimental data are plotted in Fig. 10. Since G0 values are related to the number of entanglements between worm-like micelles, i.e., the network mesh size, the increase in G0 with total concentration corresponds to an increase in the network density of the worm-like micelles. The relaxation time sR is determined by a competition between micellar breaking and chain reptation, and Maxwellian behavior is generally observed when the breaking time sB is much lower than the reptation time srep. In this fast breaking regime, the relaxation time sR is equal to (sBsrep)1/2. Because srep is proportional to the average micellar length, a decrease in the length would results in a drastic reduction in srep. This is the dominant effect on sR. 4. Comparison between the different systems As previously mentioned, we have only carried out rheological measurements in systems with evident viscoelastic behavior (see Table 1). Thus, it is not possible to compare systems at the same concentrations or mixture ratios (mainly due to differences in Krafft temperatures, coalescence, or phase separation of the mixtures). However some trends can be discussed. For example, only mixed systems with fluorocarbon surfactants are able to form vesicles regardless of the degree of asymmetry between hydrogenated and fluorinated chains. For the mixed systems with the same cationic surfactant (CTAB) the presence of work-like micelles has been observed for the greatest asymmetry in alkyl chain length (i.e. when C8HONa is the anionic surfactant). On the other hand, as can be seen in Table 1, there is a synergism in a certain range of

0.028

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Fig. 10. Plateau modulus (G0) and relaxation time (sR) as a function of total concentration in CTAB-C8HONa systems (a) aF = 0.4; (b) aF = 0.6.

τR (s)

0.10

200

τR (s)

G0 (Pa)

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G (Pa)

G'' (Pa)

1 þ ðxsR Þ2

G00 ðxÞ ¼

160

0

ðxsR Þ2

G0 ðxÞ ¼

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E. Blanco et al. / Journal of Colloid and Interface Science 341 (2010) 261–266

Table 1 Occurrence of viscoelasticity in hydrocarbon–fluorocarbon mixed catanionic systems at 25 °C. Mixed system

Viscoelasticity

CTAB-C12HONa

aF < 0.6

aF = 0.6

Noa

Yes CT = 90 mM aF = 0.1 Yes 200 < CT < 300 mM 0.4 < aF < 0.6 Yes 200 < CT < 300 mM 0.6 < aF < 0.8 Yes 60 < CT < 100 mM

CTAB-C8FONa

aF < 0.1 Noa

CTAB-C8HONa

aF < 0.4 Noa

C8TAB-C10FONa

aF < 0.6 No

a

aF > 0.6 No aF > 0.1 No

aF > 0.6 No

aF > 0.8 Noa

Krafft point above 25 °C.

mixing ratios for the appearance of viscoelastic behavior. Such a behavior is attributed to the formation of long micelles that eventually get entangled. Micellar growth is favored by a decrease in the interfacial surface area per molecule [35] (increase in the molecular packing parameter) due to charge matching in the catanionic mixture that reduces the repulsion between surfactant head groups. Fluorocarbon surfactants also usually show molecular packing parameters higher than their hydrocarbon counterparts [36] despite in the former the cohesive forces between the hydrophobic chains are stronger [37]. 5. Summary In this work the microstructural features of phase behavior of systems formed by cationic (octyltrimethylammonium bromide and cetyltrimethyl-ammonium bromide) and different anionic (sodium octanoate, dodecanoate, perfluorooctanoate and perfluorodecanaote) surfactants have been investigated. Their rich morphological diversity may exhibit broadly different properties: hexagonal liquid phase with aligned rod-like micelles under stress (CTAB-C12HONa), lamellar and vesicular structures (CTABC8FONa), vesicles which form worm-like micelles under stress (C8TAB-C10FONa) and worm-like micelles with and without stress (CTAB-C8HONa). Spontaneous vesicles have been observed just for systems containing perfluorinated surfactants. Application of stress to these systems tends to result in the formation of worm-like micelles. On the whole, the ability of catanionic surfactants to selfassemble producing different structures results in interesting properties that depend on both the interaction between hydrocarbon/ fluorocarbon chains and the interaction between the head groups. This balance can be really useful for nanofabrication purposes. We hope that this work may advance the very scarce investigations on catanionic surfactant resulting from mixtures of hydrogenated and fluorinated surfactants. Acknowledgments Juan M. Ruso thanks Dirección Xeral de Promoción Científica e Tecnológica do Sistema Universitario de Galicia for financial

support. C. Rodríguez-Abreu is grateful to CSIC (2007AR0031) and Ministerio de Ciencia e Innovación, Spain (CTQ2008-01979/ BQU) for research funding.

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