10044
Langmuir 2007, 23, 10044-10052
Effects of Organic Salt Additives on the Behavior of Dimeric (“Gemini”) Surfactants in Aqueous Solution Laurent Wattebled† and Andre´ Laschewsky*,†,‡ UniVersita¨t Potsdam, Institut fu¨r Chemie, Karl-Liebknecht-Str. 24-25, D-14476 Golm/Potsdam, Germany, and Fraunhofer Institut fu¨r Angewandte Polymerforschung IAP, P.O. Box 600651, D-14406 Potsdam, Germany ReceiVed May 26, 2007. In Final Form: July 6, 2007 The effects of a series of aromatic anions, so-called hydrotropes, on characteristic solution properties of a family of ammonium gemini surfactants with dodecyl chains were explored. The stoichiometric addition of the organic salts to the geminis can result in clear solutions or in phase separation/precipitation, depending on the detailed nature of the added counterions and on the spacer group of the gemini surfactant. Many organic anions induce synergistic effects, strongly reducing the critical micellization concentration (cmc) and the surface tension at the cmc. Furthermore, a number of combinations of organic anions and geminis exhibit thickening of their aqueous solutions. The effects of the added salts are strongly enhanced for the gemini surfactants compared to the monomeric analogue N-dodecylN,N,N-trimethylammonium chloride. Even anions such as benzoate may be effective for thickening, and viscoelastic solutions can be obtained with salicylate despite the relatively short alkyl chains.
Introduction Amphiphilic molecules self-assemble in aqueous solution into a variety of structures such as spherical or wormlike micelles, vesicles, and so forth, depending on the molecular design and on the conditions under which aggregates are formed.1-3 Single-tail surfactants usually form spherical micelles in aqueous solution above their critical micellization concentration (cmc), which eventually grow to other shapes with increasing surfactant concentration. Studies on single-chain surfactants have shown that the counterions exert a strong influence on the cmc, aggregation number, and size and shape of aggregates of ionic surfactant systems.2-15 Alkyltrimethylammonium and alkylpyridinium surfactants are the most studied surfactant systems in this context. Usually, spherical micelles are formed in combination with halide counterions, whereas aromatic counterions often induce the formation of wormlike micelles at relatively low surfactant and counterion concentrations. Salicylate is particularly efficient in this respect.12,14-16 The formation of wormlike micelles is * To whom correspondence should be addressed. E-mail: andre.
[email protected]. † Universita ¨ t Potsdam. ‡ Fraunhofer Institut fu ¨ r Angewandte Polymerforschung IAP. (1) Laughlin, R. G. The Aqueous Phase BehaVior of Surfactants; Academic Press: London, 1994; Chapter 1. (2) Tadros, T. F. Applied Surfactants: Principles and Applications; Wiley VCH: Weinheim, 2005. (3) Myers, D. Surfactants Science and Technology, 3rd ed.; Wiley-Interscience: Hoboken, NJ, 2006. (4) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869. (5) Gaillon, L.; Lelie`vre, J.; Gaboriaud, R. J. Colloid Interface Sci. 1999, 213, 287. (6) Knock, M. M.; Bain, C. D. Langmuir 2000, 16, 2857. (7) Wang, Y.; Han, B.; Yan, H.; Cooke, D. J.; Lu, J.; Thomas, R. K. Langmuir 1998, 14, 6054. (8) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447. (9) Hugerth, A.; Sundelo¨f, L.-O. Langmuir 2000, 16, 4940. (10) Lin, H.-P.; Kao, C.-P.; Mou, C.-Y.; Liu, S.-B. J. Phys. Chem. B 2000, 104, 7885. (11) Appell, J.; Porte, G.; Poggi, Y. J. Colloid Interface Sci. 1982, 87, 492. (12) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933. (13) Rao, U. R. K.; Manohar, C.; Valaulikar, B. S.; Iyer, R. M. J. Phys. Chem. 1987, 91, 3286. (14) Bijma, K.; Engberts, J. B. F. N. Langmuir 1997, 13, 4843. (15) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712.
attributed to the strong binding of organic counterions to surfactant micelles to minimize the contact of their bulky hydrophobic part with water. The bound counterions screen the electrostatic repulsion between the ionic hydrophilic groups and increase simultaneously the hydrophobic interactions in the palisade layer of the micelle. This results in tighter packing, modifying the spontaneous curvature of the surfactant assemblies and the solution properties.2,3,17 A possible consequence of such a modified micellar morphology is the appearance of viscoelastic solutions, which arise from an entangled network of wormlike micelles. The latter micelles are often referred to as “living (secondary valence) polymers” because the association of surfactants results in long polymerlike chains which break and recombine in a dynamic process.17 Therefore, surfactant-counterion mixed systems have attracted considerable interest from both academic and industrial research. For instance, wormlike micelle containing systems are discussed intensely as drag reducing agents in recirculation systems and in fracturing fluids in oil production.18 The dimeric so-called “gemini” surfactants have attracted much attention by virtue of their appealing properties in comparison to classical “monomeric surfactants”.19-21 Surprisingly, reports on mixtures of ionic gemini surfactants and organic counterions have been rare so far.21-28 In particular, low molar mass gelators of water and chlorinated solvents, which are based on specific (16) Cates, M. E. Macromolecules 1987, 20, 2289. (17) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (18) Yang, J. Curr. Opin. Colloid Interface Sci. 2002, 7, 276. (19) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205. (20) Zana, R., Xia, J., Eds. Gemini Surfactants: Synthesis, Interfacial and Solution-Phase BehaVior, and Applications; Dekker: New York, 2003. (21) Wattebled, L.; Note, C.; Laschewsky, A. Tenside, Surfactants, Deterg. 2007, 44, 25. (22) Oda, R.; Huc, I.; Schmutz, M.; Candau, S. J.; MacKintosh, F. C. Nature 1999, 399, 566. (23) Berthier, D.; Buffeteau, T.; Le´ger, J.-M.; Oda, R.; Huc, I. J. Am. Chem. Soc. 2002, 124, 13486. (24) Brizard, A.; Aime´, C.; Labrot, T.; Huc, I.; Berthier, D.; Artzner, F.; Desbat, B.; Oda, R. J. Am. Chem. Soc. 2007, 129, 3754. (25) Buwalda, R. T.; Engberts, J. B. F. N. Langmuir 2001, 17, 1054. (26) Jiang, N.; Li, P.; Wang, Y.; Wang, J.; Yan, H.; Thomas, R. K. J. Phys. Chem. B 2004, 108, 15385. (27) Thalody, B.; Warr, G. G. Aust. J. Chem. 2004, 57, 193.
10.1021/la701542k CCC: $37.00 © 2007 American Chemical Society Published on Web 08/21/2007
Effects of Organic Salts on Gemini Surfactants
Langmuir, Vol. 23, No. 20, 2007 10045
Figure 1. Studied gemini surfactants.
gemini surfactant-counterion systems such as ethanediyl-1,2bis(hexadecyldimethylammonium) and pure tartrate enantiomers, were described. Cryo-transmission electron microscopy (CryoTEM) studies of the gels revealed long and strongly entangled helical or twisted fibers.22-24 Also, the interaction of methyl orange with a series of gemini surfactants, namely, alkanediylR,ω-bis(dodecyldimethylammonium bromide) and a pyridiniumbased gemini surfactant, was studied.25 Transmission electron microscopy (TEM) showed vesicles in the aqueous phase in addition to crystals. More recently, cationic gemini surfactants with nucleotides as counterions were reported, and their behavior at the air-water surface as well as in bulk solutions was examined.28 Still, no general picture can be derived from these singular studies, for instance, concerning the effects of dimerization and of the spacer group on the properties of the aqueous mixtures. Therefore, we explored the effect of adding sodium salts with large organic anions, so-called hydrotropes, to a series of cationic gemini surfactants (Figure 1). The geminis are derived from the parent compounds n-dodecyltrimethylammonium chloride (DTAC) and benzyl-n-dodecyldimethylammonium chloride (BDDAC).29-31 In particular, they vary the spacer group, which is known to influence strongly the behavior of geminis. A variety of aromatic anions with carboxylate or sulfonate moieties were tested (Figure 2). The ratio of gemini surfactant to salt, that is, the ratio of ionic groups in both components, was fixed to 1, because strong effects, such as high viscosity solutions, are typically found for mixtures of ionic surfactants and organic salts when approaching charge equimolarity.12,18,32 Experimental Section Materials. Water for all experiments was purified by a Millipore Qplus water purification system (resistance 18 MΩ cm). nDodecyltrimethylammonium chloride (DTAC, g98%, Fluka) and n-hexadecyltrimethylammonium chloride (CTAC, 99%, Acros) were (28) Wang, Y.; Desbat, B.; Manet, S.; Aime´, C.; Labrot, T.; Oda, R. J. Colloid Interface Sci. 2005, 283, 555. (29) Laschewsky, A.; Lunkenheimer, K.; Rakotoaly, R. H.; Wattebled, L. Colloid Polym. Sci. 2005, 283, 469. (30) Laschewsky, A.; Wattebled, L.; Arotc¸ are´na, M.; Habib-Jiwan, J.-L.; Rakotoaly, R. H. Langmuir 2005, 21, 7170. (31) Wattebled, L.; Laschewsky, A.; Moussa, A.; Habib-Jiwan, J.-L. Langmuir 2006, 22, 2551. (32) Abdel-Rahem, R.; Gradzielski, M.; Hoffmann, H. J. Colloid Interface Sci. 2005, 288, 570.
used as received. The synthesis and purification of the dimeric surfactants (i-B-2, t-B-2, EO-2, o-X-2, m-X-2, and p-X-2) are described elsewhere.29 Acetylsalicylic acid (>99%), 2,5-dihydroxyterephthalic acid (98%), sodium benzoate (99%), and sodium 4-vinylbenzenesulfonate (99%) were purchased from Aldrich. 1-Hydroxy-2-naphthoic acid (99%), 3-hydroxy-2-naphthoic acid (>98%), 6-hydroxy-2-naphthoic acid (98%), 2,3-naphthalenedicarboxylic acid (95%), 2-naphthoic acid (>97%), sodium ptoluenesulfonate (“tosylate”, 99%), sodium 2-naphthalenesulfonate (96%), and disodium 2,7-naphthalenedisulfonate (95%) were from Acros. Diphenic acid (>95%), 2-hydroxy-1-naphthoic acid (97%), trans-cinnamic acid (99%), terephthalic acid (>99%), 4-vinylbenzoic acid (97%), and sodium salicylate (99.5%) were purchased from Fluka. All sodium salts were used as received. Free acids were suspended in water, neutralized by adding an equimolar volume of NaOH (0.1 M titrated solution from Merck), and freeze-dried prior to use. Mixtures of cationic surfactants and organic anions were prepared by dissolving the necessary amounts of each component separately in water and adding progressively one solution to the other one under stirring. The concentrations given in wt % refer to the total solids dissolved, that is, to the sum of surfactant and salt. Methods. 1H (300 MHz) NMR spectra were taken with a Bruker Avance 300 apparatus. Surface tensions were measured with a du Nou¨y ring tensiometer (Kru¨ss K12) at room temperature (about 296 K) taking into account the necessary modifications for the measurement of surfactant solutions.33 Critical micellization concentrations (cmc’s) were determined from the discontinuity in the surface tension versus log(concentration) curve. The conductivity of the surfactant solutions was measured at room temperature with a conductimeter MPC227 (Mettler Toledo, electrode NTC, 0-200.0 mS). The instrument was calibrated before measurements with both air and a standard solution (12.88 mS at 25 °C). Turbidimetry was performed with a temperature controlled turbidimeter model TP1 (E. Tepper, Germany) with heating and cooling rates of 1 °C‚min-1. Capillary viscometry was performed with a thermostated semiautomatic Ubbelohde viscometer (Schott, type 53110/I) at 30 °C. Rheological measurements were conducted with a thermostated dynamic stress rheometer DSR200 from Rheometrics (stress-controlled) with a cone plate (titanium, diameter 40 mm, cone angle 0.0401 radians with gap 0.053 mm) measuring system. Frequency sweep measurements were performed at a constant stress (chosen in the linear viscoelastic range) to measure the storage modulus (elastic) G′ and the loss modulus (viscous) G′′ as a function of the frequency typically over the range 0.05-2 Hz (above 2 Hz, the measurements are no longer (33) Lunkenheimer, K. Tenside, Surfactants, Deterg. 1982, 19, 272.
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Figure 2. Organic salts tested as additives to cationic dimeric surfactants. Abbreviations are in bold style. reliable). The obtained curves were fitted by the Maxwell model using the following equations:34 ω2τ2 G′(ω) ) G0 1 + ω2τ2
(1)
ωτ G′′(ω) ) G0 1 + ω2τ2
(2)
1 1 ) ω 2πν
(3)
The magnitude of the complex viscosity is given by |η*(ω)| )
xG′2(ω) + G′′2(ω) ) ω
η 0 ) G 0τ
(5)
Results and Discussion
where τ denotes the single stress relaxation time, G0 is the zeroshear modulus (or plateau modulus), and ω is the angular frequency (ω is equal to 2πν, where ν is the experimental oscillatory frequency in s-1). G0 is obtained from the high-frequency plateau in G′ (ω2τ2 . 1). The relaxation time is obtained as the reciprocal of the angular frequency at the crossover of G′ and G′′, since when G′(ω) ) G′′(ω) τ)
The zero-shear viscosity η0 is observed in the low-frequency limit as a plateau in η*. Any of these three parameters (η0, G0, and τ) can be determined from the other two according to
η0 1 + ω2τ2
(4)
Water Solubility of Stoichiometric Mixtures of Cationic Gemini Surfactants and Organic Salts. A series of salts with mono- and divalent aromatic anions was added in stoichiometric amounts to various gemini surfactants (Figures 1 and 2). Dodecyltrimethylammonium chloride (DTAC) and hexadecyl trimethylammonium chloride (CTAC) served as references for classical “monomeric” analogues. Visual analysis of the mixtures with 1 wt % solids shows a set of several scenarios, ranging from clear and low viscous solutions via clear but viscous solutions, hazy solutions of low viscosity, and turbid solutions that undergo slow precipitation to the rapid formation of voluminous precipitates. Though the results of the mixing experiments are complex, certain patterns seem to be recognizable. Apart from the addition (34) Ferry, J. D. Viscoelastic Properties of Polymers, 3rd ed.; Wiley: New York, 1980.
Effects of Organic Salts on Gemini Surfactants
Langmuir, Vol. 23, No. 20, 2007 10047
Table 1. Visual Appearance of Aqueous Solutions of Equimolar Mixtures of Cationic Surfactants and Anionic Aromatic Saltsa additive
DTAC CTAC i-B-2 t-B-2 EO-2 o-X-2 m-X-2 p-X-2
monocarboxylates benz vbenz cin sal acsal nc 2,1hnc 1,2hnc 3,2hnc 6,2hnc
+ + + + + +/v h h h +
+ +/v +/v +/v +/v +/v h h h +
+/v t + p +/v h p t p p
+/v t +/v +/v +/v h p p p p
+ +/v + +/v + t t t p +/v
+/v t t p +/v p t t p p
+ t + +/v +/v t t t p p
+ t t p p p t t p p
dicarboxylates ndc dp tere dht
+ + + +
+ + + +
p + + t
+/v + + +
+ + + +
t + + t
t + + t
t + p t
monosulfonates tos stys ns
+ + t
+/v +/v t
p p t
p t t
+/v t t
h p t
p t p
p p p
disulfonate nds
+
p
p
t
t
p
p
p
Concentration ) 1 wt % in total. (+) ) clear; (h) ) hazy/translucent, (t) ) turbid, precipitates on standing; (p) ) precipitate; (v) ) visual viscosifying effect. a
of some very hydrophobic singly charged anions derived from naphthalene, the mixtures of the organic salts with the “monomeric” reference DTAC are clear, low viscous solutions. The notable exception is the mixture of DTAC with 2-naphthoate (nc), which is clear but slightly viscous. When using the more hydrophobic homologue CTAC, the pattern is nearly the same, with one marked difference, namely that most of the clear solutions formed with sulfonated benzene derivatives or aromatic monocarboxylates are more or less viscous. With nc, even a transparent gel is formed. The situation is different for the gemini surfactants. A number of surfactant/salt mixtures, such as those with benzoate and benzene dicarboxylates, give clear aqueous solutions, though certain mixtures are viscous as will be discussed below. A few mixtures became hazy/translucent but without any visually detectable viscosity effect. Still, the addition of most of the salts to aqueous solutions of the gemini surfactants induced turbidity followed by precipitation, which could be instantaneous or could develop slowly within a day (Table 1). This concerns in particular the bulky organic sulfonates. This might be explained by the lower hydrophilicity of the sulfonate compared to the carboxylate moiety35 or by a stronger interaction of the cationic surfactant head groups with sulfonate anions compared to carboxylates according to their relative positions in the Hofmeister series.36 Out of the gemini surfactants studied, the largest variety of organic salts was tolerated by EO-2. This gemini disposes of a more flexible and more polar spacer group than the other ones. Still, whereas mixtures of EO-2 with benzoic acid and its derivatives stay clear, the more hydrophobic naphthoate anions studied, namely, nc, 2,1hnc, 1,2hnc, 3,2hnc, 6,2hnc, and ndc, generally induce precipitation when admixed (Table 1). Note that 3,2hnc is a structural analogue of sodium salicylate,37-39 and hence, it should be strongly adsorbed on the micellar surface with the carboxylate and hydroxyl groups protruding out of the micelle.32 As the naphthalene residue in 3,2hnc is expected to (35) Laughlin, R. G. J. Soc. Cosmet. Chem. Jpn. 1981, 32, 371. (36) Cacacem, M. G.; Landau, E. M.; Ramsden, J. J. Q. ReV. Biophys. 1997, 30, 241.
confer more hydrophobicity to the system than sodium salicylate, it may penetrate deeper in the micelle. This effect is not limited to the gemini surfactant, but it is observed for the parent “monomeric” compound DTAC too. However, the hydrophobicity of the counterion is not the only factor determining the extent of the interactions (and micellar growth or precipitation) of cationic surfactant/salt systems. The orientation of substituents in the aromatic fragment is important too. Equimolar mixtures of the cationic dimer EO-2 with various isomers of hydroxynaphthoic acid (hnc) exhibit markedly different behaviors. Whereas mixtures with 2,1hnc, 1,2hnc, and 3,2hnc precipitate in water, the mixing of EO-2 and 6,2hnc gives a transparent, slightly viscous solution. Hence, the precise position of the hydroxyl group is essential. This is corroborated by the fact that the addition of parent sodium 2-naphthoate (nc) to solutions of EO-2 results in precipitation too. Presumably, the positioning of the carboxylate and hydroxyl group for 6,2hnc at opposite ends of the organic skeleton prevents the naphthalene core from penetrating the micellar surface like for nc or the other hnc isomers,40 because the hydroxyl group on the second aromatic cycle tries to stay in contact with water. The packing of the mixed system EO-2(6,2hnc)2 is thus less tight than that for the other isomers. Comparing the behavior of the gemini surfactants for a given organic anion, the spacer group is found to markedly influence the behavior (Table 1). For instance, EO-2 is the only gemini of the series which tolerates the admixture of 6,2hnc or sodium tosylate without precipitation. Concerning the model salt sodium salicylate, we find that mixtures with gemini surfactants i-B-2 and o-X-2 precipitate in aqueous solution, whereas mixtures with gemini surfactants EO-2, m-X-2, and t-B-2 remain clear (at least below 0.5 wt %). Accordingly, it seems that mixtures with gemini surfactants are increasingly prone to precipitation when the spacer group becomes shorter and, for a given length, more hydrophobic. The insolubility of the system p-X-2(sal)2, which seems at first sight to deviate from this correlation, may be governed by the high Krafft temperature of the particular molecular structure, namely 22-23 °C for p-X-229 and 44 °C for its bromide analogue.41 From the general survey, it is concluded that cationic gemini surfactants undergo strong interactions with aromatic anions, as could have been expected. The window of soluble surfactant/ organic salt mixtures is generally smaller for gemini surfactants than for their monomeric analogues, and the choice of an appropriate spacer group is crucial. Gemini EO-2 is less prone to precipitation in mixed systems with hydrophobic salts than the other tested, structurally related gemini surfactants. This may be due to the relative length, the higher flexibility, or the higher polarity of this spacer group. In any case, the use of monovalent anions with an aromatic nucleus larger than benzene seems problematic, and water-soluble mixtures of such combinations seem exceptional. Micellization and Surface Activity. Surface activity and micellization behavior of a given ionic surfactant structure depend sensitively on the chosen counterion.2,3 Though this effect is common wisdom, most studies of counterion effects have focused on inorganic ions, whereas systematic studies on organic (37) Mishra, B. K.; Samant, S. D.; Pradhan, P.; Mishra, S. B.; Manohar, C. Langmuir 1993, 9, 894. (38) Horbaschek, K.; Hoffmann, H.; Thunig, C. J. Colloid Interface Sci. 1998, 206, 439. (39) Cressely, R.; Hartmann, V. Eur. Phys. J. B 1998, 6, 57. (40) Abdel-Rahem, R. Tenside, Surfactants, Deterg. 2005, 42, 95. (41) Zana, R. J. Colloid Interface Sci. 2002, 246, 182.
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Table 2. Surface Activity and Micellization Data of Cationic Surfactant Monomers and Dimers and of Their Mixtures with Various Organic Salts, Determined by Tensiometrya surfactant/ mixture
cmc (g/L)
cmc (mmol/L)
σcmc (mN/m)
DTACb CTACc
4.80 0.42
18.3 1.0
40.5 35
EO-2b t-B-2b o-X-2b
1.25 1.10 0.720
2.2 2.0 1.2
44.9 41.5 37.0
DTAC(sal)d CTAC(sal)
1.20 0.09
2.8 0.19
34.0
EO-2(sal)2 t-B-2(sal)2 EO-2(acsal)2 EO-2(6,2hnc)2 EO-2(tos)2
0.07 0.05 0.11 0.020 0.067
0.08 0.06 0.11 0.02 0.07
32.0 32.0 33.0 38.5 32.0
EO-2(tere) EO-2(dp) EO-2(dht) EO-2(ndc) o-X-2(ndc)
0.300 0.110 0.240 0.030 0.024
0.38 0.13 0.30 0.036 0.027
40.5 40.0 38.0 38.5 30.5
a The total molar mass for the mixtures corresponds to the sum of the molar masses of the chloride surfactant and of 1 or 2 equiv of the counterion (cf. corresponding abbreviations in Figure 2). b From ref 29. c From ref 59. d Measured by conductimetry.
Figure 3. Surface tension curves of dimeric surfactants EO-2 and o-X-2 mixed with the organic salt 2,3-naphthalene dicarboxylate (ndc): (b) EO-2(ndc) and (O) o-X-2(ndc). Vertical and horizontal lines are a guide for the eyes, respectively, positioning the cmc and σcmc values of pure dimeric surfactants: EO-2 (dashes) and o-X-2 (dots).29
counterions, for example, of cationic surfactants are limited.14,42-44 In particular, such studies involving gemini surfactants have been exceptional.26,45 We performed therefore surface tension measurements on various transparent mixtures of the investigated cationic surfactants (dimers and “monomers”) and salts in water to learn about the effect of the aromatic anions on surface activity and micellization (Table 2). By virtue of its high compatibility compared to the other dodecylammonium derived geminis, EO-2 was the surfactant of choice for these studies. The measurements show that the cmc and σcmc values of the surfactant dimers are strongly reduced by adding any of the organic salts compared to the data of the pure surfactant in water (Figure 3). This reflects synergistic effects for mixtures of cationic surfactants and aromatic anions. Large differences are found as a function of the (42) Underwood, A. L.; Anacker, E. W. J. Colloid Interface Sci. 1984, 100, 128. (43) Underwood, A. L.; Anacker, E. W. J. Colloid Interface Sci. 1987, 117, 296. (44) Sugihara, G.; Arakawa, Y.; Tanaka, K.; Lee, S.; Moroi, Y. J. Colloid Interface Sci. 1995, 170, 399. (45) Bakshi, M. S.; Kaur, N.; Mahajan, R. K.; Singh, J.; Singh, N. Colloid Polym. Sci. 2006, 284, 879.
counterion’s nature. For gemini EO-2, the cmc value decreases with changing counterions as follows: EO-2 . EO-2(tere) > EO-2(dht) > EO-2(dp) ≈ EO-2(acsal)2 > EO-2(sal)2 ≈ EO2(tos)2 > EO-2(ndc) > EO-2 (6,2hnc)2 (Table 2). In this series, the anions with the more hydrophobic aromatic skeleton give rise to considerably lower cmc’s. This is seen when comparing, for example, the monovalent anions 6,2-hydroxynaphthalene carboxylate and salicylate or the dianions naphthalene dicarboxylate and diphenate to terephthalate. However, for a given aromatic skeleton, the combined effect of, for example, the hydrophilicities of all substituents is not straightforward to rationalize. Concerning the effect of the aromatic anions on the surface tension at the cmc, the σcmc values decrease as follows: EO-2 > EO-2(tere) > EO-2(dp) > EO-2(ndc) ≈ EO-2 (6,2hnc)2 > EO-2(dht) > EO-2(acsal)2 > EO-2(sal)2 ≈ EO-2(tos)2. The series shows that the addition of dianions is generally less effective in reducing the σcmc value than the addition of two monovalent anions. The detailed effect of the substituents and the substitution pattern of a given aromatic nucleus on σcmc is complex. In any case, increasing the hydrophobicity of the anion is not a criterion to give particularly low σcmc values. This is exemplified by comparing EO-2(6,2hnc)2 to EO-2(sal)2. It is noteworthy that the addition of sodium salicylate and sodium tosylate provokes sensitively the same effects on the cmc and surface tension when mixed with EO-2. Otherwise, the ranking of the anions in the series for the σcmc values differs from the one for the cmc values. Hence, the cmc and σcmc can be modified independently by selecting the appropriate organic salt. Though this study focused on mixtures of organic salts with gemini EO-2, which produce optically clear solutions, the few other examples looked at demonstrate qualitatively that the structure of the gemini surfactant influences the properties of the mixtures. Namely, a marked effect of the spacer group on micellization and surface activity is observed. For instance, when disodium naphthalene dicarboxylate (ndc) is added to EO-2 (which has a long, flexible and polar spacer) or to o-X-2 (which has a short, rigid and comparatively hydrophobic spacer), two different surface tension isotherms are obtained (Figure 3). The cmc value is only slightly lower for the mixture o-X-2(ndc) than for EO-2(ndc) (Table 2), while the cmc values of the parent geminis o-X-2 and EO-2 differ by a factor of about 2.29 However, both gemini surfactants experience the same strong decrease of 6.5 mN/m of their σcmc value after adding ndc (Figure 3), thus maintaining the markedly higher surface activity of o-X-2 over EO-2.29 The opposite scenario is found when comparing the micellization and surface activity data for mixture EO-2(sal)2 with t-B-2(sal)2 (see Table 2). While the cmc values are already very close for the parent gemini surfactants, their σcmc values differ significantly. After addition of salicylate, however, both geminis have the same low cmc and σcmc values; that is, the surface activity of both geminis becomes nearly identical. These examples demonstrate that, on the one hand, the spacer group has an important role for the behavior of mixtures of gemini surfactants and organic salts. On the other hand, it is the precise pair of anion and spacer that decides on the effects obtained. Looking at the effect of the dimerization on mixtures of surfactants and organic salts, it is found that the reduction of the cmc is much more accentuated for the mixtures containing gemini surfactants. For instance, the cmc value of gemini EO-2 is reduced by a factor 18, whereas the cmc of monomer DTAC is only reduced by a factor of 4 when sodium salicylate is added in stoichiometric amounts (see Table 2). Looking at CTAC, which is the hexadecyl homologue of DTAC, its cmc is about 10 times
Effects of Organic Salts on Gemini Surfactants
Figure 4. Relative viscosity versus concentration: (9) EO-2(sal)2, (4) EO-2(6,2hnc)2, (0) EO-2(tos)2, and (b) EO-2. In the experimental conditions, the shear rate γ˘ is greater than 100 s-1. Dashed lines are a guide to the eyes.
lower than that of DTAC (Table 2) due to its much longer alkyl chain. When sodium salicylate is added, the critical aggregation concentration of CTAC is decreased by a factor of 4-5, comparable to the effect for DTAC (Table 2). Accordingly, when comparing the situation to the case of gemini EO-2, one finds for the pure compounds that the cmc of CTAC is lower by a factor of ∼3 (on a weight basis) than that of EO-2. However, in the mixture with sodium salicylate, the cmc of EO-2(sal)2 becomes lower than that of CTAC(sal) on a molar and even on a weight base. This demonstrates the very strong synergism when mixing dimeric surfactants with organic salts: dimerization obviously enhances the associative behavior. Viscosifying Effects. Sodium salicylate is the most common anion used for the formation of wormlike micelles with cationic surfactants.4,12,14,15 When surveying the mixing of the cationic geminis in aqueous solution with various aromatic anions, it was qualitatively noticed that several gemini/salt pairs exhibit thickening (Table 1). This includes anions other than salicylate. Viscosifying effects were observed in the case of dimer EO-2 (in decreasing order) for mixtures with sodium vinylbenzoate, salicylate, 6-hydroxynaphthalene-2-carboxylate, and tosylate (Figure 4). A mixture EO-2/sodium vinylbenzoate (0.5 wt %) gives a slightly stronger viscosifying effect (η0 ) 0.60 Pa·s) than the corresponding mixture with sodium salicylate (η0 ) 0.44 Pa·s). This order correlates neither with the hydrophobicity of the anions (e.g., 6,2hnc is less efficient than sal, though it is more hydrophobic) nor with the cmc value and the surface activity, which are virtually the same for EO-2(sal)2 and EO-2(tos)2. Clearly, the detailed positioning of the molecular blocks is important. This is exemplified by the comparison of the mixtures EO-2(vbenz)2 and EO-2(cin)2, which contain isomeric anions and which behave as strongly thickening and not thickening, respectively (see Table 1). Other aromatic anions which can thicken aqueous solutions of dimeric surfactants are sodium benzoate, sodium cinnamate, and sodium vinylbenzoate (Figure 5). Again, the correct pairing of the gemini structure and anion is crucial. For example, sodium cinnamate induces thickening for t-B-2 but not for EO-2, whereas its isomer vinylbenzoate (cf. Figure 2) behaves oppositely; that is, it thickens only solutions of EO-2 but not those of t-B-2 for which it induces precipitation. For a given organic salt, the effects of dimerization and of the spacer group, that is, of the surfactant structural parameters, were surveyed qualitatively by capillary viscometry (see the Supporting Information). Whereas mixtures of sodium salicylate with DTAC (1:1) do not show a significant change in the solution viscosity up to 1 wt %, stoichiometric mixtures with the dimers EO-2, m-X-2, or t-B-2 give highly viscous solutions at
Langmuir, Vol. 23, No. 20, 2007 10049
Figure 5. Shear viscosity η versus shear rate γ˘ for (O) o-X-2(benz)2 2 wt %, (2) t-B-2(benz)2 2 wt %, (9) t-B-2(cin)2 1 wt %, and (3) EO-2(vbenz)2 0.5 wt % at 25 °C.
Figure 6. Shear viscosity η versus shear rate γ˘ at 25 °C for the mixtures (O) EO-2(sal)2 0.5 wt %, (b) EO-2(sal)2 1.0 wt %, (4) CTAC(sal) 0.5 wt %, and (2) CTAC(sal) 1.0 wt %.
concentrations as low as 0.3 wt %. The thickening effect reflects a strong growth of the micelles, which is generally only observed for conventional cationic surfactants with longer, at least tetradecyl, alkyl chains. Thus, the analogous mixture with a long chain homologue of DTAC, namely, CTAC, was compared (Figure 6). The zero-shear viscosity values of CTAC(sal) are higher than those for the system EO-2(sal)2 as model dodecyl chain gemini. This may be explained by the longer hydrophobic chain of CTAC, which confers a higher cross-sectional diameter to the formed wormlike micelles, and hence a higher volume fraction in solution. Still, shear-thinning is observed at lower shear rates for the hexadecyl classical surfactant than for the dodecyl chain gemini surfactant. Thus, from a certain flow rate on (ca. 10 s-1 at 1 wt %), the shear viscosity for EO-2(sal)2 becomes higher than that for CTAC(sal). This suggests that the supramolecular structures containing the dimer complexes disentangle less easily at high flow rates. This might be a consequence of the chemical link between two surfactant fragments. Hence, long chain surfactants are not needed to achieve high solution viscosity when using gemini surfactants. Medium chain geminis suffice to produce solutions with almost comparable viscoelastic properties and additional interesting features such as, for example, shear thinning only at higher shear rates. Other organic salts were examined to corroborate the observed enhancement effect of the dimerization of surfactant fragments. For instance, sodium benzoate produces viscous aqueous solutions in combination with o-X-2 at fairly low concentrations (
