Quaternary Ammonium Bromide Surfactant Oligomers in Aqueous

Growth and Branching of Charged Wormlike Micelles as Revealed by Dilution Laws ..... Influence of Spacer on Association Behavior and Thermodynamic ...
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Langmuir 2000, 16, 141-148

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Quaternary Ammonium Bromide Surfactant Oligomers in Aqueous Solution: Self-Association and Microstructure† Martin In and Veronique Bec Complex Fluids Laboratory, CNRS-Rhodia, Prospect Plains Road, Cranbury, New Jersey 08512-7500

Olivier Aguerre-Chariol Centre de Recherches d’Aubervilliers, Rhodia, 52, rue de la Haie coq, 93308 Aubervilliers, France

Raoul Zana* Institut C. Sadron (CNRS-ULP), 6 rue Boussingault, 67000 Strasbourg, France Received May 25, 1999. In Final Form: August 30, 1999 Surfactant oligomers are made up of x (g2) amphiphilic moieties connected at the level of, or close to, the headgroups by spacer group(s). This paper examines the effect of the degree of oligomerization x on the self-assembling of cationic surfactant oligomers at interfaces and in the bulk. The quaternary ammonium bromide surfactant oligomers investigated are made up of dodecyldimethyl- and dodecylmethylammonium bromide moieties connected by short polymethylene spacers -(CH2)s- (of carbon number s). The properties investigated are the surface occupied by the surfactant at the air/solution and silica/solution interfaces, the critical micellization concentration (cmc), the micelle ionization degree at the cmc, the micelle micropolarity and microviscosity, and the microstructure and rheology of the solution. As x is increased, the surfactant layers at interfaces become more dense, the cmc decreases, the micelle microviscosity increases, the micelle shape for oligomers with s ) 3 changes from spherical (x ) 1) to wormlike (x ) 2), branched wormlike (x ) 3), and ring like (x ) 4). Last the zero-shear viscosity of the oligomer aqueous solutions starts to increase very rapidly and by orders of magnitude at surfactant concentrations above a value C* that decreases as x is increased. On the contrary the micelle ionization degree at the cmc and micelle micropolarity are nearly independent of x. These results are discussed in terms of surfactant packing parameter. They emphasize once more the possibilities offered by surfactant oligomers in obtaining surfactant organized assemblies with new architectures and solutions with improved and adjustable properties.

Introduction Dimeric (gemini) surfactants are made up of two surfactant moieties connected at the level of, or very close to, the headgroups by a spacer group that may be hydrophilic, hydrophobic, flexible, or rigid.1 To the best of our knowledge, Bunton et al.2 were the first to report in 1971 the synthesis of quaternary ammonium bromide dimeric surfactants which they used to modify the rate of chemical reactions taking place at the surface of aqueous micelles. They characterized the synthesized surfactants only by their critical micelle concentration (cmc) values. The field of dimeric surfactants remained nearly untouched for about 15 years. After 1985 were published the first physicochemical investigations3-5 of cationic dimeric surfactants similar to those synthesized by Bunton et al.2 Nearly at the same time the group of Okahara in Japan reported6 the synthesis of anionic dimeric surfactants with * To whom correspondence should be addressed. † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry”. (1) Zana, R. In Structure-Performance Relationships in Surfactants; Esumi, K., Ueno, M., Eds.; Marcel Dekker, Inc.: New York 1997; Chapter 6, p 255. (2) Bunton, C. A.; Robinson, L.; Schaak, J.; Stam, M. F. J. Org. Chem. 1971, 36, 2346. (3) Devinsky, F.; Lacko, I. Tensides, Surfactants, Deterg. 1990, 27, 344 and references therein. (4) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (5) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451.

hydrophilic spacers and pointed out their surface properties.7 All these studies clearly showed two important features of dimeric surfactants with respect to the corresponding monomeric (conventional) surfactants, namely, much lower cmc values and a higher efficiency at reducing the surface tension of water. Since then the behavior of dimeric surfactants in aqueous solutions has been much investigated, particularly that of the alkanediyl-R,ω-bis(alkyldimethylammonium halide) surfactants. These are often referred to as m-s-m,2X-, m and s being the carbon numbers of the alkyl and alkanediyl groups, respectively, and X- the counterion neutralizing the ammonium headgroups. The effect of the spacer carbon number s on the micellization properties (cmc, size and shape of the micelles) and on the microstructure and rheology of the solution has been well demonstrated.4,8-10 In particular it has been shown the 12-2-12,2Br- surfactant forms very long wormlike micelles at a concentration as low as 1.5 wt % (about 25 mM), whereas the 12-3-12,2Br- surfactant requires a concentration 5 times larger for this purpose.8 Besides the 12(6) Zhu, Y.-P.; Mayusama, A.; Kirito, Y.; Okahara, M. J. Am. Oil Chem. Soc. 1991, 68, 539. (7) Zhu, Y.-P.; Mayusama, A.; Nakatsuji, Y.; Okahara, M.; Rosen, M. J. J. Colloid Interface Sci. 1993, 158, 40. (8) Zana, R.; Talmon, Y. Nature 1993, 362, 228. (9) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 10, 1448. (10) Kern, F.; Lequeux, F.; Zana, R.; Candau, S. J. Langmuir 1994, 10, 1714.

10.1021/la990645g CCC: $19.00 © 2000 American Chemical Society Published on Web 10/29/1999

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2-12,2Br- solutions show shear-induced structuration at concentrations between 1 and 2 wt %.11,12 This field has been recently reviewed.1,13 Trimeric quaternary ammonium halide surfactants were synthesized and investigated more recently.14,15 Thus the surfactant 12-3-12-3-12,3Br- has been shown to have a much lower cmc than the corresponding dimer 12-312,2Br- 14 and to form branched wormlike micelles.15 This paper further investigates the influence of the degree of oligomerization x on the micellization of surfactant oligomers and on the microstructure and rheology of their solutions by comparing the properties of the surfactant monomer, dodecyltrimethylammonium bromide (DTAB), dimer 12-3-12,2Br-, trimer 12-3-12-3-12,3Br-, and the newly synthesized tetramer 12-3-12-4-123-12,4Br- (even though the central spacer groups in this tetramer contains four methylene groups, this surfactant is considered in the following to correspond to the tetramer with s ) 3). Results are also presented for the trimeric surfactant with s ) 6, 12-6-12-6-12,3Br- synthesized as part of this work. They are compared to those for the corresponding dimer 12-6-12,2Br- and monomer 12-3,Br- (dodecylpropyldimethylammonium bromide) which have been much investigated.9,16,17 The results presented below concern the adsorption behavior at the air-solution and silica-solution interfaces, the values of the cmc, micelle ionization degree at the cmc, micropolarity and microviscosity, and the microstructure and rheology of the aqueous solutions of these surfactants. The micelle aggregation numbers of DTAB, 12-3-12,2Br-, 12-6-12,2Br-, and 12-3-12-3-12,3Br- have been reported.9,14 The present study involved only bromide surfactants. The bromide ion is omitted below when referring to the surfactants. Experimental Section Materials. The samples of 12-3-12 and 12-6-12 have been prepared as previously described.4 The surfactant trimers 123-12-3-12 and 12-6-12-6-12 and the tetramer have been prepared using a procedure different from that previously used for 12-312-3-12.14 This procedure uses commercially available polyamines which are first permethylated then fully quaternized. The polyamine with s ) 6 was permethylated using a mixture of formaldehyde and formic acid as reducing agent.18 For the polyamine with s ) 3, this reaction leads to a mixture of the permethylated polyamine and of compounds arising from its breakup. The permethylation was therefore performed using sodium borohydride NaBH4 as reducing agent and was carried out in concentrated aqueous sulfuric acid at low temperature.19 Permethylation of Bis(aminohexyl)amine (BAHA). A mixture of BAHA (Fluka), formalin (formaldehyde 37 wt % in water) and formic acid was heated at 100-110 °C for 30 h (HCHO/N ) 3.5; HCOOH/N ) 7). After the mixture was cooled, concentrated aqueous NaOH was added to increase the pH above 12. The water-insoluble permethylated amine was extracted with (11) Schmitt, V.; Scho¨ssler, F.; Lequeux, F. Europhys. Lett. 1990, 12, 697. (12) Oda, R.; Panizza, P.; Schmutz, M.; Lequeux, F. Langmuir 1997, 13, 6407. (13) Zana, R. In Novel Surfactants. Preparation, Applications and Biodegradability; Holmberg, C., Ed.; Marcel Dekker, Inc.: New York, 1998; Chapter 8, p 241. (14) Zana, R.; Le´vy H.; Papoutsi, D.; Beinert, G. Langmuir 1995, 11, 3694. (15) Danino, D.; Talmon, Y.; Le´vy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420. (16) Zana, R. J. Colloid Interface Sci. 1980, 76, 330. (17) Lianos, P.; Lang, J.; Zana, R. J. Colloid Interface Sci. 1983, 91, 276. (18) Clarke, H. T.; Gillepsie, H. B.; Weisshaus, S. Z. J. Am. Chem. Soc. 1933, 55, 4571. (19) Giumanini, A. G.; Chiavari, G.; Scarponi, F. Anal. Chem. 1976, 48, 484.

In et al. diethyl ether. This solution was dried on Na2SO4, the diethyl ether was evaporated and the permethylated BAHA was purified by distillation under vacuum (ca. 1 mm mercury) at 150 °C, with an overall yield of 80%. Permethylation of Bis(3-aminopropyl)amine (norspermidine) and N,N ′-Bis(3-aminopropyl)-1,4-tetramethylenediamine (Spermine). A solution of polyamine in 3 M aqueous sulfuric acid and formalin (HCHO/N ) 4) in an open flask cooled with an ice bath was treated with NaBH4. This reducing agent was added in small amounts in order that the temperature remained below 15 °C. This reaction was accompanied by smoke, foaming, and formation of hydrogen. Additions of water or sulfuric acid were sometimes required to lower the viscosity of the reacting mixture and to keep its pH low. Once the addition of NaBH4 was completed (NaBH4/N ≈ 2.5) volatile boron compounds which may distill with the amine were removed by three successive extractions with diethyl ether. Concentrated aqueous NaOH was added to increase the pH above 12 and the water-insoluble permethylated amine extracted with diethyl ether. The ether was eliminated by distillation. The permethylated norspermidine was purified by distillation under vacuum (ca. 1 mm mercury) at 110 °C with an overall yield of 75%. With the permethylated spermine the addition of diethyl ether at acidic pH brought about the precipitation of unidentified compounds that were filtered off. The permethylated spermine was not distilled in order to avoid a possible decomposition and additional losses and was obtained with a yield 65%. Quaternization of the Permethylated Polyamines. The quaternization reactions were carried out in acetonitrile at 80 °C for 2-4 days. The raw surfactant thus obtained precipitated out upon cooling, and addition of ethyl acetate completed this process. Purification and Characterization of the Surfactants. 12-3-12-3-12 and 12-6-12-6-12 were recristallized thrice from an ethyl acetate/ethanol mixture. 12-3-12-4-12-3-12 was recrystallized three times from an acetone/ethanol mixture. The results of elemental analysis and 13C NMR are summarized in Table 1. The silica used was a precipitated silica (Tixosil 103, lot 953793, from Rhone-Poulenc) with a specific area of 66 m2/g, measured using the BET method. The solutions were prepared using water from a Millipore MilliRho 3Plus apparatus (conductance above 18 MΩ). Methods. The cmc of the tetramer was measured using the electrical conductivity method and taken as the concentration corresponding to the break in the plot of the conductance of the solution vs surfactant concentration. The ionization degree at the cmc was taken as the ratio of the slopes of the plots above and below the cmc.16 The adsorption of the surfactant onto the silica surface was measured as follows. A series of 15 solutions of concentration ranging between 0.1 and 10 cmc were prepared for each surfactant. A weighted amount of silica was then added, and the suspensions were gently stirred during 20-30 h, a time sufficient for equilibration. The amount of added silica was such that the equilibrium concentration of the surfactant in the supernatant amounted to about 20-80% of the initial concentration. The suspensions were then centrifuged for 1 h at 2800 rpm. The surfactant concentration in the supernatant was determined by a colorimetric diphasic titration using disulfine blue as indicator and sodium dodecyl sulfate (SDS) as anionic surfactant.20 When added to the cationic surfactant/cationic dye mixture, SDS first complexes with the cationic surfactant and, when the latter is used up, forms a water-insoluble complex with the dye. The complex migrates to the organic phase (chloroform) which turns pink. This titration is commonly used for conventional (monomeric) surfactants. We checked with surfactant oligomer solutions of known concentration that the stoichiometry of the complexes is of one dodecyl sulfate ion per cationic oligomer headgroup. Nevertheless, for concentrations below 10-5 M, the detection of the titration point was difficult and the error became large. Thus, for the surfactant tetramer, the part of the adsorption isotherm at low surfactant concentration could not be obtained, but the maximum adsorbed amount was determined accurately. The (20) Cross, J. T. In Cationic Surfactants; Jungermann, E., Ed.; Marcel Dekker, Inc.: New York, 1970; Chapter 13, p 419.

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Table 1. Characterization of the Surfactant Oligomers by Elemental Analysis and surfactant molecular weight elemental analysisa N wt % C wt % H wt % Br wt % 13C NMR

a

13C

NMR

12-6-12-6-12 1033.22 g/mol

12-3-12-3-12 949.06 g/mol

12-3-12-4-12-3-12 1283.44 g/mol

4.09 (4.07) 61.16 (61.61) 11.15 (11.12) 21.58 (23.20)

4.39 (4.43) 59.35 (59.48) 10.98 (10.83) 24.29 (25.26)

4.32 (4.37) 59.39 (59.89) 10.98 (10.84) 23.83 (24.90)

14.1 CH3 m12 21.8 CH2 s2, s5 22.6 CH2 m2 and m11 22.9 CH2 m′2 and m′11 24.9 CH2 s3, s4 26.2 CH2 m3 26.4 CH2 m′3 29.3-29.6 CH2 m4-9, m′4-9 31.9 CH2 m10, m′10 48.8 CH3 h′ 51.0 CH3 h 61.8 CH2 m1 62.1 CH2 m′1 64.0 CH2 s1 64.5 CH2 s6

14.1 CH3 m12 18.5 CH2 s2 22.7 CH2 m2 and m11 23.0 CH2 m′2 and m′11 26.4 CH2 m3 26.6 CH2 m′3 29.4-29.7 CH2 m4-9, m′4-9 31.9 CH2 m10, m′10 50.1 CH3 h′ 51.2 CH3 h 58.1 CH2 s1 61.0 CH2 s3 64.5 CH2 m′1 66.4 CH2 m1

14.1 CH3 m12 18.5 CH2 s2 19.4 CH2 s′2,s′3 22.7 CH2 m2 and m11 23.0 CH2 m′2 and m′11 26.5 CH2 m3 26.6 CH2 m′3 29.4-29.6 CH2 m4-9, m′4-9 31.9 CH2 m10, m′10 49.4 CH3 h′ 51.2 CH3 h 58.0 CH2 s1 60.3 CH2 s’1, s’4 61.0 CH2 s3 64.1 CH2 m′1 66.9 CH2 m1

The calculated percentages in weight (wt %) are given in parentheses.

error on the value of the adsorbed amount of surfactant was dependent on the concentration and estimated to be of 5-10% for the plateau values. The surface tension γ of the surfactant solution was measured by the pendant drop volume method using a Lauda TVT1 tensiometer in the quasistatic mode. A drop with a certain volume is quickly formed at the tip of a capillary. With time, increasing surfactant adsorption at the drop surface decreases the interfacial tension. The drop falls right after the capillary force equals the drop weight. The next drop is then formed with a slightly smaller volume and its lifetime recorded, and the procedure is repeated until the drop lifetime diverges. For each drop volume, the measurement is repeated three times. The surface tension recorded (considered as the equilibrium one) corresponds to drop lifetime of about 2 min for the dimers and 20 min for the trimers. For the trimers, it cannot be excluded that the true equilibrium surface tension is actually slightly lower than the recorded one since the surface tension of the surfactant trimer solutions showed a very slow decrease with time. The surface area per surfactant, aM, was obtained from the slope of the linear part of the γ vs ln C plot below the cmc (C ) surfactant concentration), using the Gibbs equation, according to

aM ) nRT/NA(dγ/d ln C)T

(1)

where NA is Avogadro’s number. The constant n was taken as 2 for DTAB, 3 for dimeric surfactants, and 4 for trimeric surfactants.1 The micelle micropolarity was characterized by the value of the ratio I1/I3 of the intensity of the first and third vibronic peaks in the fluorescence emission spectra of micelle-solubilized pyrene.21,22 The micelle microviscosity was obtained using the fluorescent probe dipyrenylpropane (DPP).23-27 It was taken as the value of the product of the DPP excimer fluorescence lifetime, τE, by the ratio IM/IE of the intensities of fluorescence emissions of this probe at around 378 nm (IM, first vibronic peak of the monomer emission) and at around 480 nm (IE, excimer emission), (21) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (22) Zana, R. In Surfactant Solutions. New Methods of Investigation; Zana, R., Ed.; Marcel Dekker, Inc.: New York 1987, Chapter 5 and references therein. (23) Miyagishi, S.; Suzuki, H.; Asakawa, T. Langmuir 1996, 12, 2900. (24) Turley, W. D.; Offen, H. W. J. Phys. Chem. 1985, 89, 2933. (25) Zana, R.; In, M.; Le´vy, H.; Duportail, G. Langmuir 1997, 13, 5552. (26) Zachariasse, K. A. Chem. Phys. Lett. 1978, 57, 429. (27) Zachariasse, K. A.; Duveneck, G.; Busse, R. J. Am. Chem. Soc. 1984, 106, 1045.

at an excitation wavelength of 346 nm.23,25 Recall that DPP forms intramolecular excimers. This formation involves a motion of the two pyrenyl moieties which is reminiscent of the fluttering of the wings of a butterfly.26,27 The fluorescence intensities were measured using a Hitachi F 4010 spectrofluorometer. The lifetimes were measured using a single photon counting apparatus.25 The microstructure of the solution was investigated by means of transmission electron microscopy at cryogenic temperature (cryo-TEM).28 The crystalline surfactant powder was solubilized in water upon heating, up to 70 °C for the tetramer. After dissolution the solution was kept at room temperature for 15 h or 11 days before observation, to check its stability. A droplet of the solution was applied to a 3 mm diameter copper grid, coated by a holey carboned plastic film made hydrophilic by glow discharge in 0.1 Torr air, and blotted with a filter paper as to reduce the thickness of the liquid film down to about 30-200 nm, and forming menisca in the holes. The grid was then very rapidly plunged in liquid ethane at a temperature of -185 to -190 °C. Blotting and plunging were performed in a continuous stream of air saturated with water to prevent evaporation. This ultrafast cooling allowed complete vitrification of the specimen. The grids were transferred on a GATAN 626 cryo-specimen holder, using a cryo-transfer device. The observations were performed using a JEOL 1200 EX II transmission electron microscope equipped with a low-dose facility which reduced electron damage to a minimum. The specimens were equilibrated at about -160 °C in the microscope column and examined at 120 kV with a direct magnification not greater than 35 000. The micrographs shown below have been prepared by scanning the negatives at very high resolution (1800 dpi), adjusting the contrast, and increasing the magnification using the Photoshop software. They were printed on photographic paper at 300 dpi using a thermal transfer Kodak printer. The final aspect was generally better than a simple photographic reproduction, and the final magnification was around 100 000. The rheology of the solutions was investigated using a Rheometrics RFS II rheometer in the dynamic mode. A conicylinder geometry was used to test most of the samples, whereas a double wall Couette was used for the lowest concentrations. The solutions were loaded in the cell and allowed to relax for a sufficiently long time (up to several hours at concentration between 3 and 6 wt %, higher concentration samples were loaded at 50 °C and then cooled to 25 °C). The storage modulus, G ′, and the loss modulus, G ′′, were measured in the frequency range 10-3 to 102 rad/s. The linear regime was determined after the relaxation period by strain sweep experiments at frequencies ω (28) Talmon, Y. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 364.

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In et al.

Table 2. Values of the Surface Area per Dodecyl Group at the Air/Solution Interface (aM), of Γmax, and of the Surface per Dodecyl Chain (A) at the Silica/Solution Interface, for Surfactant Oligomers at 25 °C surfactant DTAB 12-2-12 12-3-12 12-6-12 12-3-12-3-12 12-6-12-6-12 12-3-12-4-12-3-12

aM (nm2/chain)

106Γmax (mol surfactant/m2)

106Γmax (mol chain/m2)a

Af (nm2/chain)a

0.59a (0.49)b (0.36)b 0.48a (0.52)c (0.72)c 0.49a 0.83a

3.5d 2.7,a 2.87,d 2.76e 2.3a 1.35,a 0.83e 1.75a 0.7a 1.3a

3.5 5.4 4.6 2.7 5.25 2.1 5.2

0.95 0.61 0.72 1.23 0.63 1.58 0.64

a This work. b From ref 29. c From ref 30. d From ref 31. e From ref 32. f Values calculated under the assumption that the adsorbed surfactant makes up a double layer at the silica surface.

) 1, 10, and 100 rad/s. The concentration was varied from 1 to 30 wt % for the dimer, from 0.5 to 25 wt % for the trimer, and from 0.25 to 2.5% for the tetramer. The zero shear viscosity, η0, was determined as the low-frequency value of the ratio G ′′/ω, when no frequency dependence could be detected. All measurements were performed at 25 °C, unless specified otherwise.

Results and Discussion Behavior of Surfactant Oligomers at Air/Solution and Silica/Solution Interfaces. The concentration dependence of the surface tension, γ, has been determined for 12-3-12 and the two surfactant trimers (not shown). The effect of time in measurements involving surfactant tetramer solutions was too important and did not permit us to obtain reliable values of the surface tension. No results are therefore presented for this surfactant. The values of the surface area, aM, per dodecyl chain at the air/solution interface, obtained from the linear part of the γ vs ln C plots below cmc using eq 1, are listed in Table 2. The present data for DTAB and 12-3-12 roughly agree with reported ones.29,30 The results for the s ) 3 surfactant series show that aM remains nearly constant as x is increased from 1 to 3. A similar result has been found for the s ) 2 surfactant series in going from x ) 2 to 3.29 The results for the s ) 6 series show a small increase of aM as x is increased from 2 to 3. Recall that the plot of aM versus s for the 12-s-12 dimers shows the presence of a maximum at s ) 10-12.30 Figure 1 shows typical adsorption isotherms (amount of adsorbed surfactant, Γ, as a function of the free surfactant concentration, Cf) determined for some of the investigated oligomers. The isotherm shape is similar to that reported in other studies, and the adsorption plateau is reached at a concentration slightly above the cmc.31,32 The mechanism of adsorption has been discussed.32,33 This work focuses on the values of the maximum amount of surfactant adsorbed, Γmax, and its dependence on x. Table 2 lists the values of Γmax (in moles of adsorbed surfactant per m2 and in moles of adsorbed chain per m2 in the third and fourth columns, respectively) for the surfactants investigated, obtained in this work, and some reported values.31,32 For the surfactant dimers and trimers Γmax is seen to decrease as s is increased, a behavior already reported for the same series of surfactant dimers.32 Also the value of Γmax, expressed in mole of adsorbed dodecyl chain per m2 (fourth column in Table 2), increases with x, up to x ) 3, but levels out for x ) 3 and 4. (29) Esumi, K.; Taguma, K.; Koide, Y. Langmuir 1996, 12, 4039. (30) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (31) Esumi, K.; Goino, M.; Koide, Y. J. Colloid Interface Sci. 1996, 183, 539. (32) Chorro, C.; Chorro, M.; Dolladille, O.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 1998, 199, 160. (33) Chorro, C.; Chorro, M.; Dolladille, O.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 1999, 210, 134.

Figure 1. Adsorption isotherms of surfactant oligomers: variations of the amount of adsorbed oligomer vs concentration of free oligomer Cf: (A) 12-3-12 (b) and (O)12-3-12-3-12; (B) 12-6-12 (b) and 12-6-12-6-12 (O) at 25 °C.

The values of Γmax per chain can be used to infer information on the structure of the adsorbed oligomer assemblies. Manne et al.34 have shown that 12-2-12 forms (34) Manne, S.; Scha¨ffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382.

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Langmuir, Vol. 16, No. 1, 2000 145

bilayers on mica while 12-4-12 and, thus, 12-6-12 form cylindrical micelles. The values of Γmax per chain for 122-12, 12-3-12, 12-3-12-3-12, and 12-3-12-4-12-3-12 listed in Table 2 are close enough to suggest that those surfactants form bilayers onto silica. On the other hand, the values of Γmax for 12-6-12-6-12 being slightly smaller than for 12-6-12, the same kind of structure (cylindrical micelles) can be expected for these two surfactants in the adsorbed state. On the assumption that the surfactants investigated adsorb as bilayers on silica, the values of the surface A occupied by one surfactant dodecyl chain on the silica surface are readily obtained from the values of Γmax as A ) 1/2NAΓmax. The results listed in Table 2 show that A decreases significantly when x is increased from 1 to 3 and levels out for x between 3 and 4, at constant s ) 3. Note that the presence of an additional methylene group in the central spacer of 12-3-12-4-12-3-12 may have resulted in a larger value of A than for the tetramer 123-12-3-12-3-12 (compare A values for the surfactants with s ) 3 and s ) 6), which is the true tetramer in the s ) 3 series. The variations of aM and A with x are qualitatively similar. Nevertheless the values of A are significantly larger than those of aM (see Table 2) indicating denser surfactant layers at the air/solution than at the silica/ solution interface. Recall that the adsorption at the air/ water interface starts at much lower surfactant concentrations than at the silica/solution interface. In fact the air/solution interface is often saturated (as indicated by the start of the linear variation of dγ/d ln C) at C below cmc/10, whereas adsorption at the silica/solution interface becomes significant only at C slightly below cmc. This indicates a stronger surfactant affinity for the air/solution than for the silica/solution interface and may indicate that the surfactant layer is denser with the former. The decrease of the surface occupied by an oligomer alkyl chain at interfaces upon increasing x is likely to also occur at the micelle/water interface and, thus, to affect the self-association behavior of surfactant oligomers. This behavior is largely determined by the value of the surfactant packing parameter P.35 Recall that P ) vS/aSl where l is the length of the hydrophobic moiety, vS is the volume of the surfactant hydrophobic moiety, and aS is the surface area per surfactant available at the micelle/ solution interface.35 For surfactant oligomers of varying x, l is independent of x, vS ) xvD (vD ) volume of the surfactant alkyl chain, here dodecyl chains), and aS ) xaC (aC ) surface area available per alkyl chain at the micelle/ solution interface). Thus P reduces to vD/aCl. As aC decreases upon increasing x, P increases with x. In turn, this should result in an increase with x of the rate of micelle growth with surfactant concentration.35 Such a behavior was indeed reported for the surfactant series with s ) 39,15 but was not explained owing to the lack of results such as those in Table 2. Critical Micelle Concentration and Micelle Ionization Degree at the cmc. The results are listed in Table 3. A notable feature is the relative insensitivity of the ionization degree at the cmc, R, to the degree of oligomerization for both the s ) 3 and 6 series. The values of the free energy change upon micellization per mole of dodecyl chain, ∆G°M, have been calculated from the equation36

∆G°M ) RT(1/x + β) ln cmc - (RT ln x)/x

(2)

(35) Israelachvili, J. N.; Mitchell, J. D.; Ninham, B. W. J. Chem. Soc., Faraday Trans 1 1976, 72, 1525.

Table 3. cmc, Micelle Ionization Degree r at the Cmc, Micelle Micropolarity (I1/I3) and Microviscosity (τEIM/IE), and Free Energy Change upon Micellization Per Mole of Dodecyl Chain, of Surfactant Oligomers at 25 °C surfactant

cmc (mM)

15.1a 0.96b 0.16c 0.14 12-3-12-4-12-3-12 0.06 12-3 11a 12-6-12 1.03b 12-6-12-6-12 0.28

DTAB 12-3-12 12-3-12-3-12

a 0.25a 0.22b 0.24c 0.19 0.20 0.32a 0.33b 0.30

-∆G°M (kJ/mol)

I1/I3

τEIM/IE (ns)

18.3 20.8 21.5

1.41b 321d (72.6) 1.48b 918d (76.5) 1.44b 1726d (79.2)

22.6 18.9 18.8 19.0

1.45b 2370d (85.0) 1.46b 344d (74.7) 1.48b 711d (81.9) 1.51 1160 (89.3)

a From ref 16 b From ref 4. c From ref 14. values in parentheses are those of τE in ns.

d

From ref 25. The

Figure 2. Variation of the cmc of surfactant oligomers with x for the surfactant series with s ) 3 (0) and s ) 6 (9) at 25 °C.

where β ) (1 - R) is the degree of association of the counterions to the micelles. The cmc is expressed in moles of alkyl (dodecyl) chain per liter. Equation 2 is valid for dilute micellar solutions, at concentrations only very slightly above the cmc. Table 3 shows that the values of ∆G°M all are around -20 kJ/mol, irrespective of the values of x and/or s. The small differences seen between the different values may be real but may also arise from the error on β. Figure 2 shows the variations of the cmc with x, for the two surfactant series with s ) 3 and 6. Micelle Micropolarity and Microviscosity. Table 3 lists the values of the pyrene polarity ratio I1/I3 and of the product τEIM/IE that is proportional to the microviscosity.23-25 Figure 3 shows the variations of these two quantites with the degree of oligomerization for the two surfactant series with s ) 3 and 6. The variations of polarity are rather small and show no systematic trend with x. This may be due to the fact that the micellesolubilized pyrene is located in the micelle palisade layer22 whose chemical composition depends only little on the surfactant degree of oligomerization. This layer is essentially made up of quaternary ammonium headgroups, bromide counterions, water molecules, and R- and β-methylene groups of surfactant alkyl chains. The relative proportions of these various components probably vary (36) Zana, R. Langmuir 1996, 12, 1208.

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Figure 3. Variation of the micropolarity I1/I3 (9, 0) and of the microviscosity τEIM/IE (b, O) of micelles of surfactant oligomers with the degree of oligomerization x for the surfactant series with s ) 3 (0) and s ) 6 (9) at 25 °C.

little with x since the micelle ionization degree R is nearly independent of x. The polarity appears to be somewhat larger for the s ) 3 than for the s ) 6 series. The slightly larger value of R for the s ) 6 series (see Table 3) may be responsible for this small difference. Contrary to the micropolarity, the microviscosity shows a strong dependence on both x and s (see Figure 3). This has been explained by the fact that the motion of the pyrenyl moieties required for the formation of the DPP excimer involves displacements of surfactant alkyl chains.25 This motion becomes increasingly difficult as x is increased because the motion of one alkyl chain then brings about the motion of the other chains of the oligomer, since these chains are connected and tethered at the micelle surface by the headgroups. This cooperative motion of the various chains of an oligomer is expected to become less cooperative, and thus the microviscosity is expected to show a smaller dependence on x, as the spacer carbon number s is increased. This is borne out by the results in Figure 3. Microstructure of the Aqueous Solutions of Surfactant Oligomers. DTAB micelles are known to remain spherical or spheroidal even at very high concentration and/or ionic strength.37 The dimer 12-3-12 has been shown to form spherical micelles at a concentration of 1.5 wt % and elongated micelles at 7 wt %.8 In a previous report which used a sample of the trimer 12-3-12-3-12 synthesized in a different manner from the one used in the present investigation, cryo-TEM showed the existence of branched wormlike micelles at a concentration of 2 wt %, in a specimen quenched from a temperature of 50 °C.15 Figure 4 (left) shows a micrograph obtained as part of this work with the newly synthesized sample of 12-3-12-3-12. Branched wormlike micelles can be seen, confirming the previous observation. The micrograph also shows that the specimen preparation has resulted in a shearing of the wormlike micelles, and the ensemble looks like domains of long, stretched, and oriented micelles, with domain boundaries consisting of entanglements and connections. The picture is thus illustrative of the fact that the junctions are able to slide to relax the stress induced by shear in the sample, as suggested by Appell and Porte.38 Figure 4 (right) shows a micrograph representative of a 1 wt % solution of 12-6-12-6-12. Only spherical or (37) Candau, S. J.; Hirsch, E.; Zana, R. J. Phys. (Paris) 1984, 45, 1263. (38) Appell, J.; Porte, G. J. Phys. Lett. (Paris) 1983, 44, L-689.

In et al.

spheroidal micelles are seen. This observation is consistent with the low values of the aggregation number of 12-612-6-12 micelles measured by time-resolved fluorescence quenching.39 This enormous difference in microstructure between 12-3-12-3-12 and 12-6-12-6-12 solutions illustrates once more the very strong influence of the spacer carbon number on the aggregation behavior of surfactant oligomers. Figure 5 shows a typical micrograph obtained with a 1 wt % solution of the tetramer. A large fraction of closedloop (ring) micelles can be seen, coexisting with wormlike micelles. The low surfactant concentration used together with the smaller dimension of a ring with respect to a wormlike micelle reduced micelle overlap and made possible this clear observation of coexisting rings and wormlike micelles. Systematic measurements of the contour length (perimeter) of the rings by image analysis are presently under way in order to check available theoretical treatments of the formation of closed-loop micelles under the condition of chemical equilibrium. At this stage it is simply recalled that whenever ring micelles were observed,15,40,41 they appeared as a minor component that represented a very small percentage in weight of the material that was visualized by cryo-TEM. This is true in particular for the surfactant trimer 12-3-12-3-12 for which most of the visualized structures were wormlike micelles with a few branches. Only one ring was observed.15 Thus the passage from the trimer 12-3-12-3-12 to the tetramer 12-3-12-4-12-3-12 brought about an enormous increase in the occurrence of rings. The tetramer is characterized by a smaller cmc than the trimer (see Table 3) and probably by a larger endcap energy,42 which determines the rate of micelle growth upon increasing surfactant concentration. These two parameters are important for the occurrence of rings,43 which can also be related to differences in surfactant packing parameter between dimers, trimers, and tetramers. Indeed, when x surfactant alkyl chains are connected at the level of the headgroups, as in the surfactant x-mers investigated, the packing of these chains into spherical micelles is more and more hindered for geometric reasons, as x is increased at constant spacer carbon number s (since the surface per alkyl chain at interfaces decreases upon increasing x). Thus the free energy penalty for packing oligomers in either spherical micelles or hemispherical encaps terminating rodlike micelles is expected to increase with x, at a given s. This is indeed what has been found experimentally in going from DTAB to 12-3-12 and 12-3-12-312.42 To avoid this penalty the oligomers form in solution a smaller number of longer micelles, thereby reducing the number of endcaps, as x is increased. Thus 12-2-12 and 12-3-12 form elongated micelles, 12-3-12-3-12 forms still longer elongated micelles with a few branches which eliminate one endcap each, and the tetramer forms elongated micelles and rings. The formation of a ring eliminates two endcaps, but some conformational entropy is lost in ring closure. Rheology of Aqueous Solutions of Surfactant Oligomers. The variations of the zero-shear viscosity, η0, with the surfactant concentration (expressed in wt % in order to allow an easier comparison between oligomers) are presented in Figure 6 for several oligomers. They reflect the increased tendency to micelle growth upon (39) Ka¨stner, U.; In, M.; Zana, R. J. Colloid Interface Sci., in press. (40) Lin, Z.; Scriven, L. E.; Davis, H. T. Langmuir 1992, 8, 2200. (41) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474. (42) In, M.; Warr, G.; Zana, R. Phys. Rev. Lett., in press. (43) In, M.; Aguerre-Chariol, O.; Zana, R. J. Phys. Chem. in press.

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Figure 4. Electron micrographs of solutions of trimers: left, 1.5 wt % 12-3-12-3-12 (bar ) 100 nm); right, 1 wt % 12-6-12-6-12 (bar ) 50 nm). Both solutions were quenched from room temperature. The 12-3-12-3-12 solution was kept for 11 days after its preparation and was briefly reheated at 50-60 °C before specimen preparation. In the 12-3-12-3-12 micrograph branching points are indicated by black arrows, and pieces of frost (dark spots) are indicated by white arrows. Enlargement 100 000.

Figure 5. Electron micrograph of a 1 wt % solution of the tetramer 12-3-12-4-12-3-12, quenched from room temperature, 15 h after preparation of the solution. The micrograph shows many rings. Pieces of frost are seen as dark spots. Bar ) 100 nm. Enlargement 120 000.

increasing concentration as x is increased or s decreased as discussed above. All plots show a low concentration range where the oligomer solutions are not sensibly more viscous than water, with η0 slowly increasing with C until the concentration reaches a value C* above which the viscosity increases rapidly by several orders of magnitude. Such a variation of the viscosity is expected from a recent theoretical treatment of the increase of the micelle length (aggregation number) with the concentration for ionic micelles.44 Recall that C* corresponds to the onset of the so-called semidilute regime where the wormlike micelles (44) MacKintosh, F. C.; Safran, S. A.; Pincus, P. Europhys. Lett. 1990, 12, 697.

start to overlap.44 C* coincides with the cross over concentration to rapid growth regime and varies as Ec-1/2.44 The values of C* listed in Table 4 are seen to increase with s at a given x and to decrease upon increasing x at a given s. At a higher surfactant concentration, CM (see values in Table 4), the viscosity goes through a maximum and then decreases. The results for the dimers 12-2-12 and 12-3-12 show the importance of the value of the spacer carbon number on the zero shear viscosity. Thus the viscosity increases by over 5 orders of magnitude with CM ≈ 7 wt % for 122-12, by only 4 orders of magnitude with CM ≈ 20 wt % for 12-3-12, but by 7 orders of magnitude for 12-3-12-3-12, with CM ) 5 wt %. Also the maximum in η0 is seen to be

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Extensive rheological studies of 12-3-12-3-12 solutions showed that the presence of branches enhanced the elasticity compared to strictly linear micelles. These results will be reported elsewhere.42 Concluding Remarks

Figure 6. Variation of the zero-shear viscosity of the oligomers with the surfactant weight percent: 12-2-12 (∆, from ref 10); 12-3-12 (b); 12-3-12-3-12 (O); 12-3-12-4-12-3-12 (9) at 25 °C. Table 4. Value of C* and CM for Selected Surfactant Oligomers surfactant

C* (wt %)a

CM (wt %)

12-2-12 12-3-12 12-3-12-3-12 12-3-12-4-12-3-12

1.6 ( 0.2 4.0 ( 0.5 1.3 ( 0.15 1.0 ( 0.15

7 20 5

a The value of C* has been taken as that where the viscosity starts increasing rapidly.

more narrow with 12-2-12 than with 12-3-12. This maximum has been attributed to a true decrease of the micelle length.10 It has also been proposed that the decrease of viscosity at higher C is due to the formation of branched wormlike micelles, on the basis of molecular dynamics simulations.45 (45) Karaborni, S.; Esselink, K.; Hilbers, P. A.; Smit, B.; Van Oss, N. M.; Zana, R. Science 1994, 266, 254.

The results presented above clearly show the benefits that surfactant oligomers can provide with respect to the corresponding (conventional) monomeric surfactants. Surfactant oligomers form denser layers both at the air/ solution and silica/solution interfaces. They micellize at critical micelle concentrations that decrease much upon increasing degree of oligomerization x. The micelle ionization degree at the cmc is nearly independent of x, because at the limit of very low concentration, very close to the cmc, all oligomers investigated form nearly spherical micelles.9,14 The micropolarity of surfactant oligomer micelles show no dependence on x, but the microviscosity increases nearly linearly with this parameter, owing to the cooperativity in the motion of the various chains of an oligomer. The micelle shape can be modified by acting on both x and s. Thus for short spacers illustrated by the s ) 3 series and dodecyl chains, one goes from spherical micelles at x ) 1 to linear wormlike micelles at x ) 2, branched wormlike micelles at x ) 3, and finally closedloop (ring) micelles at x ) 4. With a longer spacer such as s ) 6, there is practically no change of shape in going from 12-317 to 12-6-129 and to 12-6-12-6-1239 as the measured micelle aggregation numbers were all found to be close to that of the near-spherical DTAB micelles. Of course these changes of micelle shape have a strong impact on the rheological behavior of aqueous oligomer solutions, with the possibility of obtaining gel-like systems at low surfactant concentration, in the absence of salt. Acknowledgment. The authors gratefully ackowledge Professor F. Devinsky (University of Comenius, Bratislava, Slovakia) and Dr. G. Beinert (retired from the Institut C. Sadron, Strasbourg) for helpful advice for the synthesis of the surfactant trimers and tetramer. LA990645G