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Cationic Gemini Surfactants with Oligo(oxyethylene) Spacer Groups and Their Use in the Polymerization of Styrene in Ternary Microemulsion Michael Dreja,† Wim Pyckhout-Hintzen,‡ Holger Mays,§ and Bernd Tieke*,† Institut fu¨ r Physikalische Chemie, Universita¨ t zu Ko¨ ln, Luxemburger Strasse 116, D-50939 Ko¨ ln, Germany, Institut fu¨ r Festko¨ rperforschung, Forschungszentrum Ju¨ lich (KFA), Postfach 1913, D-52425 Ju¨ lich, Germany, and Department of Physical Chemistry, Uppsala University, PO Box 532, S-75121 Uppsala, Sweden Received September 28, 1998. In Final Form: November 4, 1998 The aggregation and phase behavior, micelle and microemulsion structure, and the polymerization behavior of various binary and ternary systems containing water, (oligooxa)alkanediyl-R,ω-bis(dimethyldodecylammonium bromide) surfactants (12-EOx-12), and styrene are reported. The hydrophilic spacer chain length x in the cationic gemini surfactants was varied between 0 and 5. With increasing spacer chain length the surface area per molecule at the air-water interface increases, whereas the single-phase region of the ternary microemulsion systems is largest for x being 1 and decreases with increasing x in the investigated concentration range. Small-angle neutron scattering (SANS) and cryogenic temperature transmission electron microscopy (cryo-TEM) measurements indicate that the micelle and microdroplet dimensions decrease with increasing x. Polymerization was carried out at 25 and 60 °C using 60Co γ-rays. Spherical polystyrene particles in the nanometer size range were obtained, the particle size being dependent on the spacer length of the gemini surfactants and on the polymerization temperature. At 25 °C, particle size and molecular weight of polystyrene were maximum at medium spacer length, while at 60 °C, the particle size decreased with increasing x, in correspondence to the microstructural properties of the microemulsions observed by SANS. The experimental results suggest that the chemical structure of the surfactant molecules used in the microemulsion formulation has a strong influence on the particle properties after polymerization. This influence depends on the curvature of the interfacial film induced by the surfactant molecules.
Introduction Self-organized amphiphilic systems such as micelles, microemulsions, and thin interfacial films are useful as reaction media for the controlled formation of colloidal materials.1 For example, the mesoscopic domains of microemulsions can be used for polymerization reactions. Polymer colloids with high molecular weight (Mw > 106 g‚mol-1) and very small particle size (5-100 nm) can result if a hydrophobic monomer like styrene is polymerized in ternary oil-in-water (o/w) microemulsions.2-4 Many monomers have yet been investigated, and polymerization reactions have been carried out in the various microemulsion regimes.2,5 It turned out that o/w microemulsions are best suited for the polymerization of hydrophobic monomers, whereas water-in-oil (w/o) systems are preferred in the case of hydrophilic monomers.2,6 Also bicontinuous phases of hydrophobic monomers and hydrophilic comonomers have been polymerized, and it was * To whom correspondence should be addressed. E-mail for correspondence:
[email protected]. † Universita ¨ t zu Ko¨ln. ‡ Forschungszentrum Ju ¨ lich. § Uppsala University. (1) Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach Science Publishers: Philadelphia, PA, 1992. (2) Candau, F. In ref 1, p 215. (3) Antonietti, M.; Basten, R.; Lohmann, S. Macromol. Chem. Phys. 1995, 196, 441. (4) Full, A. P.; Kaler, E. W.; Aranello, J.; Puig, J. E. Macromolecules 1996, 29, 2764. (5) Sjo¨blom, J.; Lindberg, R.; Friberg, S. Adv. Colloid Interface Sci. 1996, 85, 125. (6) Barton, J. Prog. Polym. Sci. 1996, 21, 399. (7) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1995, 36, 2637.
found that most of the characteristic structure can be retained after polymerization, especially if polymerizable surfactants are used.7-10 It was additionally shown that the molecular structure of the polymerizable surfactants is important for the resulting copolymer morphology.11,12 Nevertheless, progress in the field crucially depends on innovation from surfactant science, and a lack of suitable substances for further work is obvious. Therefore, we recently introduced the so-called dimeric or “gemini” surfactants of the alkanediyl-R,ω-bis(dimethylalkylammonium bromide) type (m-s-m surfactants) into microemulsion polymerization, which allows the fine-tuning of the phase behavior and leads to nanolatex particles with controlled properties.13 It was observed that the length of the hydrophobic alkylene spacer in the gemini surfactants decisively influences the microemulsion formation and that a long spacer leads to dramatic improvement of the solubilization properties compared with cationic single tail surfactants. This could be explained by changes of curvature in the interfacial film and the compatibility between parent microemulsion droplet structure and resulting spherical particle shape. Since there exists no further experience in microemulsions with gemini surfactants, we looked for suitable substances which show even more improved stabilization properties during polymerization. Especially, the effect (8) Gan, L. M.; Li, T. D.; Chew, C. H.; Teo, W. K.; Gan, L. H. Langmuir 1995, 11, 3316. (9) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C.; Pey, K. L. Polymer 1996, 37, 2801. (10) Gan, L. M.; Chew, C. H. Colloids Surf., A 1997, 123-124, 681. (11) Dreja, M.; Tieke, B. Macromol. Rapid Commun. 1996, 17, 825. (12) Dreja, M.; Pyckhout-Hintzen, W.; Tieke, B. Macromolecules 1998, 31, 270. (13) Dreja, M.; Tieke, B. Langmuir 1998, 14, 800.
10.1021/la981354v CCC: $18.00 © 1999 American Chemical Society Published on Web 12/24/1998
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of the chemical nature of the spacer unit was of substantial interest, since this unit is known to strongly influence aggregation behavior of gemini surfactants at the airwater interface.14-16 The effective headgroup area of dimeric surfactants with hydrophilic spacer groups gradually increases, whereas a maximum is found in the headgroup area for gemini surfactants with hydrophobic spacer chains. We therefore synthezised a consecutive series of cationic gemini surfactants with hydrophilic oligo(oxyethylene) spacer groups (m-EOx-m surfactants) and investigated their aggregation behavior in aqueous solution17 as well as their suitability in the preparation of polymerizable microemulsions:
Dreja et al. solution. 60Co γ-radiation was used for the polymerization. The dose rate was 52.8 krad/h or 0.528 kGy/h (1 rad ) 10-5 J/g ) 0.01 J/kg ) 0.01 Gy). To achieve a complete conversion, a γ-ray dose of 10 kGy was applied to each sample. Characterization Methods. Static surface tension measurements of aqueous surfactant solutions were performed using the Wilhelmy plate technique, Kru¨ss digital tensiometer K 10 T, 25 ( 0.1 °C. Samples were aged for 24 h at 25 °C before taking measurements. From the plot of γ versus the logarithm of the surfactant concentration, the surface excess concentration Γ can be derived, using the Gibbs equation
Γ)-
dγ 1 2.3nRT d log c
(
)
T
(1)
where n depends on the number of molecular species (n ) 2 for univalent ionic surfactants, n ) 3 for bolaamphiphiles). In the case of gemini surfactants, it is still unclear the use of which n is correct from a thermodynamically point of view.14 If Γ is known, the surfactant headgroup area asur can be calculated as
While m was 12, x was 0, 1, 2, 3, 4, and 5. Such (oligooxa)alkanediyl-R,ω-bis(dimethyldodecylammonium bromide) surfactants were also found to show interesting thermotropic phase behavior in the bulk phase and formation of lyotropic mesophases at high concentrations in water.18 In the following, the properties of the 12-EOx-12 surfactants will be described and discussed in comparison with those of the 12-s-12 surfactants with hydrophobic spacer chains described recently. To investigate the aggregation behavior and the microstructure of the micelles and microemulsions, surface tension and small-angle neutron scattering (SANS) experiments were carried out. Polymerization was initiated in the various systems using 60Co γ-irradiation at 25 and 60 °C with and without presence of a cross-linker component. The polymerized microemulsions were analyzed in terms of the resulting particle size and molecular weights. Experimental Part Materials. Styrene (Aldrich) was freshly distilled before use in order to remove inhibitor and oligomeric impurities. Dodecyltrimethylammonium bromide (DTAB, Fluka) and hydroquinone (Merck) were used as received. Water purified by a MilliQPLUS system was used for all experiments. Surfactant Synthesis. The gemini surfactants 12-EOx-12 with x ) 2, 3, 4, and 5 were prepared as previously described.18 In short, the corresponding oligo(oxyethylene)glycols HO-(CH2)2(EO)x-OH were brominated with PBr3 in “dry” dioxane. The resulting dibromides were quaternized with N,N-dimethyldodecylamine under reflux in “dry” ethanol. For product purification, the (oligooxa)alkane-R,ω-bis(dimethyldodecylammonium bromide) surfactants were recrystallized several times from acetone or ethyl acetate. 12-EOx-12 with x being 0 and 1 were also prepared according to other procedures.19,20 All products were received as white crystalline solids, and their purity was checked by 1H NMR (Bruker AC, 300 MHz) and elemental analysis. Polymerization. Polymerization in microemulsion was performed in closed glass vessels at 25 and at 60 °C, the latter in a thermostated (Lauda K4) Dewar vessel. The transparent samples were prepared by titrating the desired amount of styrene with or without 1,3-diisopropenylbenzene as cross-linker (molar ratio cross-linker/styrene 1:20) into an aqueous 10 wt % surfactant (14) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (15) Zhu, Y. P.; Matsuyama, A.; Kobata, Y.; Nakatsuji, Y.; Okahara, M.; Rosen, M. J. J. Colloid Interface Sci. 1993, 158, 40. (16) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 1149. (17) Mays, H.; Almgren, M.; Brown, W.; Dreja, M.; Tieke, B. To be published. (18) Dreja, M.; Gramberg, S.; Tieke, B. Chem. Commun. 1998, 1371. (19) Bunton, C. A.; Robinson, C., Schaak, J.; Stam, M. F. J. Org. Chem. 1971, 34, 780. (20) Devinsky, F.; Lacko, I.; Bitterova, F.; Tomeckova, L. J. Colloid Interface Sci. 1986, 114, 314.
asur ) (NaΓ)-1
(2)
The single-phase regions of the microemulsion at 25 and 60 °C were determined visually by titrating styrene into watersurfactant mixtures in screw-capped glass tubes. Each sample was thoroughly homogenized using a Vortex mixer and thermostated in a water bath. The phase diagram at 60 °C was determined with styrene, to which a few parts per million of hydroquinone were added in order to prevent thermal polymerization. Phase boundaries were checked by preparing samples by weight at compositions slightly below and above the phase boundaries determined by titration. The accuracy of the boundaries was within (0.2 wt %, and the obtained values were independent of titration speed and stirring rate. Particle size and size distribution were determined at the temperature of 25 ( 0.1 °C with a Nicomp C 370 particle sizer (He-Ne laser, λ ) 632.8 nm) and a goniometer system with an ALV5000e correlator (ALV-GmbH Langen, Ar ion laser, λ ) 488 nm), both instruments yielding values in good agreement. The intensity fluctuations of scattered light were measured at the fixed angle of 90°. A Gaussian distribution function for the relaxation times (cumulant method) was fitted to the intensity correlation functions, and values were checked using a constrained regularization procedure (CONTIN, ALV). The resulting apparent diffusion coefficients were converted to the corresponding hydrodynamic intensity-weighted particle diameters using the Stokes-Einstein equation, assuming spherical particle shape and that the solvent has the viscosity of pure water. Molecular weights were measured using gel permeation chromatography (Waters 501) equipped with a UV detector. A set of two Styragel columns (HR1, HR5, Waters) and a Eurogel column (GPC 1000, Knauer) was used after calibration with polystyrene standards (Polyscience). The eluent was degassed tetrahydrofuran (THF); the flow rate was 1 mL/min. Small-Angle Neutron Scattering (SANS). SANS experiments were performed with the KWS1 instrument at the Forschungszentrum Ju¨lich GmbH (KFA) using the FRJ-2 DIDO research reactor (23 MW). The neutron wavelength λ was 7 Å with ∆λ/λ ) 0.21 ( 0.02, and the scattered intensities were measured over a scattering vector range q from 0.006 to 0.16 Å-1 using different sample-to-detector distances (2 m, 4 m, 8 m); |q| ) (4π/λ) sin(θ/2), where θ is the scattering angle. The detection was two-dimensional with a 64*64 channel array of 0.8 cm width each. The samples were put in quartz cells of 1 or 2 mm path length and tightly closed with Teflon stoppers. All measurements were performed at a constant temperature of 23 °C. Scattering from the samples was corrected for sensitivity and dark current of the detector, solvent, and empty cell scattering and normalized to sample thickness and transmission. The resulting intensities were radially averaged and placed on an absolute scale using a secondary Lupolen standard and software provided by the neutron facility. The absolute cross sections are shown as open points in Figure 4. The solid lines in these figures are calculated as described below.
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Langmuir, Vol. 15, No. 2, 1999 393
Table 1. Characteristic Properties of the 12-EOx-12 Surfactants in Aqueous Solutions Obtained from Surface Tension Measurementsa x in 12-EOx-12
cmc (10-4 mol‚L-1)
γcmc (mN‚m-1)
-[dγ/d log c]T
asur (n ) 2) (nm2)
asur (n ) 3) (nm2)
asur (SANS) (nm2)
1 2 3 4 5
8.0 9.2 8.6 9.0 12.3
38.2 41.0 40.8 42.1 42.0
28.9 23.7 23.1 20.6 17.8
0.655 0.800 0.818 0.921 1.065
0.983 1.200 1.230 1.382 1.597
1.308 (1.363) 1.417 (1.442) 1.648 (1.525) 1.826 (1.774)
a The surface area of the surfactant head groups resulting from SANS measurements of the 5 wt % micellar solutions is included for comparison (values in parentheses for 10 wt %).
SANS Analysis. SANS analysis was carried out in order to determine size, shape, and polydispersity of the micelles and microemulsions. For contrast enhancement, D2O was used in the neutron scattering experiments instead of H2O. Generally, the observed differential scattering cross section, dΣ(q)/dΩ ) I(q), of a dispersion of Np monodisperse particles is given by21,22
I(q) ) A np P(q) S(q) + B
(3)
where np ) Np/V is the number density of particles, P(q) is the particle form factor for each particle depending on the particle size and shape, S(q) is the interparticle structure factor arising from interparticle interference and depending on interparticle separation and interaction potentials, A is an instrumental constant, and B is a background constant that represents incoherent scattering. Form factors P(q) were computed assuming either spherical or prolate ellipsoidal particles; the inclusion of polydispersity did not effect the results. Intermicellar interactions were taken into account by computing S(q) using a HayterPenfold type approach including a repulsive Coulomb (Yukawa) potential and the local monodisperse approximation .23 Details of the fitting procedures are given in ref 12. Standard nonlinear least-squares fitting was used to determine the best parameter set for each sample. The incoherent background scattering due to the presence of hydrogen atoms was considered in the fitting procedure. Model functions were convoluted with a resolution function to correct for smearing due to finite apertures and wavelength distribution. The known volume fraction φ of the dispersed phase was used to specify the micelle dimensions via the semiaxes r and r of an ellipsoid of revolution, from which the surfactant and styrene aggregation numbers Nsur and Nsty can be calculated using molecular volumes.24 In all calculations, molecular dissolved surfactant was subtracted, and the hydration of the headgroup ions21 was taken into account. The concentration of the nonaggregated surfactant was assumed to be equal to the critical micelle concentration (cmc).
Results and Discussion Phase Studies of the Micellar and Microemulsion Systems. Figure 1a shows the plot of the surface tension γ versus the logarithm of molar surfactant concentration of the various gemini surfactants 12-EOx-12 at 25 °C. For sake of clarity, the curves have been shifted for each x by one power to lower concentration on the logarithmic scale. The curves show the expected behavior of increasing surface tension at concentrations below the cmc. Also, at concentrations below the cmc it can be seen that the increase of γ becomes less pronounced if the spacer length x is increased. In Table 1, the resulting characteristic values are compiled for the various 12-EOx-12 compounds. Surprisingly, the cmc and γcmc values do not show a monotonic increase as observed for a series of anionic gemini surfactants with hydrophilic spacer chains.15 We rather observe a stepwise increase for the cmc between (21) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022. (22) Chen, S.-H. Annu. Rev. Phys. Chem. 1986, 37, 351. (23) Pedersen, J. S. J. Appl. Crystallogr. 1994, 27, 595. (24) (a) Berr, S. S.; Coleman, M. J.; Jones, R. R. M.; Johnson, J. S., Jr. J. Phys. Chem. 1986, 90, 6492. (b) Berr, S. S. J. Phys. Chem. 1987, 91, 4760.
Figure 1. (a) Plot of the surface tension γ versus the logarithmic molar surfactant concentration for the various gemini surfactants 12-EOx-12 at 25 °C. For clarity, the curves have been shifted for each x to lower concentration by one power on the logarithmic scale. 12-EO1-12 is the reference surfactant where the concentration axis is correct. (b) Plot of the headgroup area asur obtained from surface tension measurements versus the spacer length x for n ) 3.
x ) 4 and 5 and for γcmc between x ) 1 and 2. The surface area per molecule at the air-water interface asur, which can be extracted from the linear slope in the plot of γ vs log surfactant concentration below the cmc (for details see experimental part), does monotonically increase with increasing x (Figure 1b), as reported for anionic geminis with hydrophobic spacer.15 According to Zana et al., two separate values of the surface area per molecule asur are
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Figure 2. Partial phase diagrams of the systems water/12-EOx-12 surfactant/styrene at 25 and 60 °C. The lines represent the compositions defining the boundary between one-phase and two-phase regions, 1 φ and 2 φ denote single-phase and two-phase regions. (a) x ) 1; (b) x ) 2; (c) x ) 3; (d) x ) 4; (e) x ) 5.
provided in Table 1 for n ) 2 and n ) 3 in order to take into consideration the number of molecular species possibly involved, namely, one surfactant molecule and one (n ) 2) or two (n ) 3) counterions.14 If the absolute values of the surface area are compared with gemini surfactants with hydrophobic spacers of similar length, it is obvious that the hydrophilic spacer groups are most probably not fully extended at the air-water interface. We rather suppose that the spacer groups penetrate into the water phase, the more the longer the hydrophilic units
are. Therefore, it must be concluded that also the curvature of the surfactant aggregates is strongly dependent on the spacer length x. All 12-EOx-12 surfactants were found to form ternary microemulsions with water and styrene in the temperature range between 25 and 60 °C. In Figure 2, the partial phase diagrams of the various systems are shown at 25 and 60 °C. It can be seen that the single-phase microemulsion region always lies near to the water corner of the Gibbs’ phase triangle and that its extent is dependent on the
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Langmuir, Vol. 15, No. 2, 1999 395 Table 2. Parameters from SANS Spectra Analysisa
surfactant
% (w/w)
styrene
φ
r/Å
ξ/Å
γ
φ/10-18 cm-3
κK
z
Nsur
EO1 EO2 EO3 EO4 EO1 EO2 EO3 EO4 EO1 EO2 EO3 EO4
5 5 5 5 10 10 10 10 10 10 10 10
0 0 0 0 0 0 0 0 2 2 2 2
0.0469 0.0468 0.0468 0.0467 0.0945 0.0945 0.0945 0.0944 0.1092 0.1091 0.1091 0.1090
30.7 29.8 27.0 25.5 29.5 29.3 29.1 26.2 41.2 39.6 36.8 35.8
1.8 1.4 1.1 1.1 3.4 1.8 1.1 1.1 3.2 1.7 1.5 1.4
18 16 14 14 17 14 13 11 24 23 20 20
144 120 103 90 165 168 142 132 271 100 105 136
0.22 0.30 0.52 0.61 0.26 0.50 0.83 1.14 0.11 0.24 0.35 0.41
1.7 1.9 1.9 1.8 1.8 2.2 2.3 2.3 1.7 1.7 1.8 1.8
43 39 36 34 46 46 42 41 58 36 36 41
163 110 61 49 273 135 77 54 606 265 179 146
Nsty
z/Nsur
764 365 255 221
0.26 0.35 0.59 0.68 0.17 0.34 0.55 0.76 0.10 0.13 0.20 0.28
a Results from computer modeling assuming either polydisperse spheres or monodisperse ellipsoidal particles and a repulsive Yukawa potential. φ is the droplet volume fraction, r is the effective droplet radius, is the minor axis ratio of a prolate ellipsoid, ξ and γ are the Yukawa interaction parameter, φ is the number density of droplets, κK is the inverse Debye length times the droplet diameter, z is the charge of a single droplet, Nsur and Nsty are the aggregation numbers of the surfactant and styrene, respectively, and z/Nsur is the fraction of dissociated surfactant molecules in the droplet. r, , ξ, and γ are fit parameters.
spacer chain length x. The compound with x being 1 shows the largest single-phase region in the investigated concentration regime. Also, it can be seen that the edges of the single-phase regions are not much dependent on the temperature. This is most likely due to the high solubility of the 12-EOx-12 surfactants in water, in contrast to the 12-s-12 gemini surfactants with hydrophobic spacer groups, where a strong temperature dependence was observed except for s being 10. There, especially 12-2-12 (being identical with 12-EO0-12) shows only a very small single-phase region.13 If the spacer length x is increased to values larger than 1, the amount of the single-phase region gradually decreases, the ternary system with x being 5 exhibiting the smallest single-phase region. This means that a long hydrophilic spacer is not as efficient in the stabilization of o/w microemulsions as a long hydrophobic spacer group, for which a large single-phase region was observed.13 According to theoretical considerations of Safran et al., the slope of the boundary between the single- and two-phase region in ternary systems should be directly related to the curvature of the microemulsion droplets.25 With regard to the 12-EOx-12 surfactants, it must be concluded that the microemulsion droplets with 12-EO5-12 are most highly curved and thus exhibit the smallest droplet diameters. A short spacer chain obviously leads to a lower surfactant film curvature and hence to larger aggregates. Using small-angle neutron scattering (SANS), this can be experimentally verified as will be shown later. In Figure 3, the molar solubilization ratio of styrene per surfactant is plotted versus the surfactant concentration in aqueous solution for various surfactants at 60 °C. For comparison, the solubilization properties of the singletailed analogue, dodecyltrimethylammonium bromide (DTAB), are shown as well. It is obvious that the solubilization is maximum for the 12-EOx-12 surfactants with x being 1 and 2 and decreases with increasing spacer length x according to the phase behavior. However, the differences are less pronounced than in the case of the 12-s-12 surfactants. This is remarkable since the absolute increase in spacer length is about 3.8 Å for each x and only 2.5 Å for each s. Therefore, the differences in the solubilization properties between 12-EOx-12 and 12-s-12 are due to the change in the chemical nature of the spacer group. In the case of 12-EOx-12, the surfactant becomes more hydrophilic with increasing x. Structural Studies of Micellar and Microemulsion Systems. In the following, the results from SANS measurements of micellar solutions and monomeric microemulsion systems with 12-EOx-12 surfactants and
Figure 3. Plot of the molar solubilization ratio of styrene/ surfactant vs the surfactant concentration in wt % at 60 °C.
styrene are described. In parts a and b of Figure 4, the scattering curves of micellar solutions of 5 and 10 wt % surfactant are plotted. In Figure 4c, the SANS curves of microemulsion systems with 10 wt % surfactant and 2 wt % styrene are shown. The curves show the characteristic interaction peaks for dispersions of charged particles, which are due to the corresponding peak in the interparticle structure factor S(q).21 Data analysis of the scattering curves (for details see experimental part and ref 12) yielded appropriate fitting curves, which are shown as solid lines. The detailed fitting parameters and resulting droplet properties are compiled in Table 2. Figure 4a shows that the maximum in the SANS curves of the 5 wt % micellar solutions shifts to larger q values when the spacer length x is increased. Usually, the peaks occur at q ∼ 2π/d, where d is the average distance between micelles. The observed behavior can be interpreted as a decrease of the micellar dimensions with increasing x. Also, the overall scattering intensity decreases slightly with x. From Table 2, it can be seen that the micelles with x being 3 or 4 can be very well modeled as spheres, whereas in the case of x being 2 and 1, a prolate ellipsoidal shape has to be assumed. With regard to the data for 10 wt % surfactant in aqueous solution, this trend is increased even more. The micelles grow in the minor axis direction but only slighly in the direction of the major axes. Obviously, a short spacer leads to an increase in micellar dimensions (25) Safran, S. A. In Micelles, Membranes, Microemulsions, and Monolayers; Gelbart, W. M., Ben- Shaul, A., Roux, D., Eds.; SpringerVerlag: New York, 1994.
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due to a smaller headgroup distance and a resulting lower curvature of the interfacial film. Thus, the micelles become more and more elongated. Similar observations were made by us and other groups on gemini surfactants with hydrophobic spacer groups, where it was found that a minimum in micellar size occurs for a medium spacer chain length and that the aggregates are largest for the shortest spacer chains.26-29 The data derived from the fit curves show that the micelles of 12-EO1-12 are highly anisotropic at 10 wt % with a minor axis ratio of being 3.4, which means rodlike aggregates. The calculated aggregation numbers Nsur also indicate the same behavior. Nsur decreases from 273 for x ) 1 to 53 for x ) 4. The effective surface charge z of a single micelle obtained from SANS increases strongly with x (see Table 2), which shows that the dissociation is very much dependent on the aggregate curvature and is high for spherical micelles. Similar values of z have been found by Zana et al. for 12-s-12 gemini surfactants of medium length (s ) 10-12) using conductivity measurements.30 From the geometrical model, the surface area per surfactant molecule asur can be calculated, which is included in Table 1 for comparison. It can be found that the asur values resulting from SANS measurements are larger than those at the air-water interface. This indicates a possible change in the spacer conformation in micellar aggregates, where a more subtle force balance governs the surfactant assembly. These results are well confirmed by cryo-TEM. These images show that the 12-EOx-12 aggregation behavior is strongly depending on the spacer length x of the surfactant, the concentration, and temperature. Polydisperse wormlike micelles are found with 12-EO1-12 at 25 °C, as depicted in Figure 5a. However, micellar solutions of the hydrophilic gemini surfactants with longer spacer exhibit more or less spherical aggregates. In Figure 5b, globular micelles of 12-EO4-12 at 25 °C are shown as an example. A detailed study using cryo-TEM and time-resolved fluorescence quenching to characterize the shape transitions will appear in a forthcoming publication.17 If styrene is added to the micelles, an o/w microemulsion is formed. From Figure 4c it can be seen that the overall scattering intensity increases, especially in the case where x is small. The peak positions are shifted to lower q values, which indicates a growth of the droplets upon styrene addition. From the data analysis, it can be concluded that the swelling of the micelles mainly occurs in the direction of the major axes of the ellipsoids. The increase of the average micelle radius r is about 10 Å, independent of the surfactant species. Only in the case where x is large, also the minor axis is increased when styrene is added. The aggregation numbers of the droplets are roughly doubled, and the effective droplet surface charge is reduced. It can be concluded that the structural properties of the micelles are of essential importance for the structure of the resulting microemulsion droplets. The solubilization of styrene then does not alter the droplet curvature very much and preserves the somewhat ellipsoidal aggregate structure, as earlier observed in a single-tail surfactant system with styrene12 and also in other systems.31
Figure 4. SANS spectra of (a) 12-EOx-12 micellar solutions at the concentration of 5 wt %, (b) 12-EOx-12 micellar solutions at the concentration of 10 wt %, and (c) microemulsions with 2 wt % styrene at the 12-EOx-12 concentration in water of 10 wt %. Open symbols represent the measured data, and solid lines are the intensities derived from the model (cf. legend of Table 2).
(26) Dreja, M.; Pyckhout-Hintzen, W.; Tieke, B. Unpublished results. (27) Hirata, H.; Hattori, N.; Ishida, M.; Okabayashi, H.; Frusaka, M.; Zana, R. J. Phys. Chem. 1995, 99, 17778. (28) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (29) (a) De, S.; Aswal, V. K.; Goyal, P. S.; Bhattacharya, S. J. Phys. Chem. 1996, 100, 11664. (b) Aswal, V. K.; De, S.; Goyal, P. S.; Bhattacharya, S.; Heenan, R. K. Phys. Rev. E 1998, 57, 776. (30) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (31) Imae, T. Colloids Surf. A 1996, 109, 291.
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Figure 5. Cryo-TEMs of (a, top) a 10 wt % 12-EO1-12 solution and (b, bottom) a 10 wt % 12-EO4-12 solution. The samples were frozen and vitrified from 25 °C by rapid transfer into liquid ethane. Only the upper parts of low sample thickness show single layers of micellar aggregates.
Polymerization Studies of the Microemulsions. Polymerization of styrene in ternary o/w microemulsion leads to polystyrene latex particles in the nanometer size range. In a recent study, it has been shown that the particle properties such as size, polydispersity, and molecular
weight are distinctly influenced by the microemulsion structure. Moreover, the mesostructure can be tailored by changing the spacer length s of the 12-s-12 surfactants with hydrophobic spacer groups.13 In the present study, styrene was polymerized using the novel 12-EOx-12
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a
b
c
Figure 6. Plots of the hydrodynamic radius Rh vs the gemini spacer chain length x after polymerization at (a) 25 °C in the presence of a cross-linker, (b) 25 °C without cross-linker, and (c) 60 °C in the presence of a cross-linker.
surfactants with hydrophilic spacer units. Polymerization was carried out in the presence of a cross-linker at 25 and 60 °C and without cross-linker at 25 °C. It is well-known that a cross-linker can reduce the particle size for entropic
Dreja et al.
reasons.3 According to a method reported by Antonietti and Hentze,32 different surfactant-to-styrene weight ratios (WR) were used for polymerization, based upon a 10 wt % aqueous surfactant solution. In Figure 6a, the hydrodynamic radius Rh of the polystyrene particles obtained after γ-ray polymerization in the various systems with cross-linker at 25 °C is plotted versus the spacer chain length x. Rh was determined by dynamic light scattering. In Figure 6b, the same is done for non-cross-linked particles. It is obvious that for each spacer length or surfactant system, the latex particle size increases with increasing styrene content, i.e., decreases with increasing weight ratio. For the non-cross-linked systems the particle size is horizontally shifted to somewhat larger hydrodynamic radii. The distribution width of the particle size was always around 0.2-0.3, and similar polydispersities were already observed for other surfactant systems. The area per surfactant molecule at the droplet interface, which can be extracted from the particle size using the Antonietti-Wu approach,3 was usually between 0.3 and 0.5 nm2. This is significantly smaller than the values found for the air-water interface (see Table 1) and is due to the fact that a highly compressed arrangement of the molecules is implicated in the model. If the particle sizes of the different 12-EOx-12 systems are compared, a maximum is found at the medium spacer length 2 for systems with cross-linker and 3 for non-crosslinked systems. Concerning the phase and solubilization behavior of the ternary systems, this observation is very similar to the 12-s-12 systems at 25 °C.13 We found two opposing effects which lead to such results. One of them is related to the solubilization properties, while solubilization refers to the amount of styrene per surfactant/ monomer aggregate. If the solubilization is high, the particle size may become large due to the higher stabilizing properties of the surfactant molecules at the oil-water and after polymerization the polymer-water interface. If at larger x the solubilization is low, the monomer droplets are smaller and the polymerization of a monomer droplet should rather lead to a small particle size. This however does not explain the small particle size at x e 2. In the case of the very short spacer with x being 0 or 1, the microstructure of the ellipsoidal monomer droplets and the resulting latex spheres is so different that it leads to a forced surfactant rearrangement, loss of free energy, lower stabilization, and smaller particle size after polymerization. The second effect results from interfacial rigidity and microdroplet mobility, which may also play an important role during polymerization, especially for the resulting molecular weights, which will be discussed below. A high droplet rigidity leads to a low entry rate for radical oligomers and thus to small particles of low molecular weight. If instead the interface is flexible, growing radicals can easily enter the droplets and, in correspondence to a heterocoagulation model for microemulsion polymerization, the molecular weight will be high.33 If the hydrodynamic radius Rh of the polystyrene particles resulting from polymerization at 60 °C is plotted versus the spacer length x, a different picture is obtained (Figure 6c). Here, a decrease of the particle size can be found with increasing x, the effect becoming smaller at larger x values. It can be concluded that the polymerization behavior at 60 °C corresponds with the microstructural properties of the system. The results suggest that at higher (32) Antonietti, M.; Hentze, H. P. Adv. Mater. 1996, 8, 841. (33) Kim, D. R.; Napper, D. H. Macromol. Rapid Commun. 1996, 17, 845.
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The same is true for the 12-EOx-12 microemulsions. In Figure 7a, the weight average Mw of the molecular weight obtained after polymerization without cross-linker at 25 °C is plotted versus the weight ratio. The expected linear increase of Mw with monomer load, i.e., decreasing WR value, is found. It is interesting that for the different surfactant systems the slope of the curves is nearly constant. If in the systems with different x value the weight ratio is kept constant, the effect of the microstructure of the monomer systems on the polymerization can be studied. In Figure 7b, a plot of Mw versus the gemini spacer length x is shown. It is found that the maximum in the molecular weight occurs at a medium spacer length of 3, which very well corresponds with the maximum of the polystyrene particle size in Figure 6b. It also confirms earlier findings that a spherical microdroplet structure is a prerequisite for high molecular weights. A medium spacer chain length constitutes a compromise between the ability of the surfactant molecules to stabilize large amounts of monomer in almost spherical droplets and the ability to support the polymerization process by a high exchange mobility. If 12-s-12 surfactants are used, the same is found for a medium spacer length s being 6 to 8.13 If a hydrophilic spacer group is present, and if the oxygen atoms in the spacer units are neglected, a spacer length x of 2 to 3 exactly corresponds to six to eight CH2 groups. Therefore, the polymerization behavior of ternary microemulsions with cationic gemini surfactants can be regarded as rather independent of the chemical nature of the spacer groups, as long as the flexibility in the interface is sufficient. Conclusions
Figure 7. Plot of the weight average Mw of the molecular weight vs (a) the surfactant to styrene weight ratio (WR) and (b) the gemini spacer chain length x after polymerization at 25 °C without cross-linker.
temperature the original droplet structure is retained more easily due to the compensation of the free energy loss; i.e., the droplets are large at low x values and small for high x, as shown by the SANS measurements. Likewise, for the lowest weight ratio the particle size increases with x, which is due to the decrease of the single-phase region. Here, a transition from microemulsion to emulsion polymerization takes place, and the particle size cannot be controlled anymore. The molecular weights of the non-cross-linked latex particles were determined by size exclusion chromatography. It has been found earlier that the molecular weights are low at low styrene contents in the parent microemulsion system and increase with increasing styrene load.
In our study, novel cationic gemini surfactants of the oligooxaalkanediyl-R,ω-bis(dimethyldodecylammonium bromide) type (12-EOx-12) with flexible hydrophilic spacer groups are reported. These surfactants show an interesting aggregation behavior in aqueous solution and are useful for the preparation of polymerizable microemulsions at both 25 and 60 °C. The size of the single-phase region, the micellar and microemulsion droplet size, and the resulting particle size after polymerization induced by 60Co γ-rays depend on the spacer length x. The curvature of the aggregates increases with x. Comparing the resulting latex particle properties with those obtained by polymerization in microemulsions with 12-s-12 surfactants, one can conclude that a flexible spacer group of medium length is necessary for an efficient stabilization of microemulsion polymerization, which results in small particles sizes and high molecular weigths. Acknowledgment. The authors thank Professor R. Strey for kindly allowing us to use his tensiometer. M. Pyrasch is thanked for help in surfactant synthesis and polymer characterization. Financial support by the Deutsche Forschungsgemeinschaft (Project Ti219-5.1) is gratefully acknowledged. LA981354V
