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Langmuir 1998, 14, 5730-5739
Phase Behavior and Characterization of Micellar and Cubic Phases in the Nonionic Surfactant C〈17〉E〈84〉/Water System. A PFG NMR, SAXS, Cryo-TEM, and Fluorescence Study Bjo¨rn Ha˚kansson,*,† Per Hansson,† Oren Regev,‡ and Olle So¨derman† Division of Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, and Department of Chemical Engineering, Ben-Gurion University of the Negev, P.O. Box 653, 84105, Beer-Sheva, Israel Received January 20, 1998. In Final Form: June 22, 1998 An extensive study of the diffusion behavior of the C〈17〉E〈84〉/water system is presented. The surfactant, when mixed with water, forms a micellar phase below ≈13 wt % and a cubic phase between ≈20-60 wt % (at 25 °C). In addition to the pulsed field gradient (PFG) NMR technique (used to determine the selfdiffusion coefficients), the system has been investigated by small-angle X-ray scattering (SAXS), cryotransmission electron microscopy (cryo-TEM), and time-resolved fluorescence quenching (TRFQ). The self-diffusion coefficient (D) and the transverse relaxation time (T2) of the surfactant molecules decrease significantly when going from the micellar to the cubic phase. The results of the PFG NMR, SAXS, cryo-TEM, and TRFQ experiments show that the cubic phase is composed of discrete aggregates. On the basis of the rapid transverse 1H NMR relaxation in the cubic phase, it is argued that the micellar building blocks of the cubic phase are nonspherical. Furthermore, the SAXS data for four different concentrations of the surfactant (25, 35, 45, and 55 wt %) in the cubic phase can be indexed to the space group Im3m. If the data from the NMR and the SAXS measurements are combined, a lifetime of the surfactant monomers in the micelles of 8 and 7 ms was obtained at 25 and 35 wt % C〈17〉E〈84〉, respectively.
Introduction The structure of cubic liquid crystalline phases in surfactant systems has been the subject of numerous studies.1-4 With regard to the transport processes of the components making up the cubic phases, it has proven useful to distinguish between two classes of cubic phases; those that are bicontinuous and those that are built up of discrete micellar aggregates. The n-alkyl poly(ethylene glycol) ether surfactants are widely used as detergents and emulsifying agents. Today we have a rather detailed and profound understanding of the factors that govern phase behavior and microstructure in these systems.5,6 The system C12E12 was recently studied by Sakya et al.7 On the basis of SAXS measurements, they found three distinct micellar cubic phases in the C12E12/water system. The polar part of C12E12 is rather large. To further investigate the influence of the size of the polar headgroup, we have in this study focused our interest on the C〈17〉E〈84〉/ water system. This surfactant has a very long polar headgroup (in fact, it may be considered as a diblock copolymer). It is of commercial origin, and it has a distribution of molecular weights in both the nonpolar * Corresponding author. E-mail:
[email protected]. Telephone: +46 46 222 01 35. Fax: +46 46 222 44 13. † Lund University. ‡ Ben-Gurion University of the Negev. (1) Mariani, P.; Luzzati, V.; Delacroix, H. J. Mol. Biol. 1988, 204, 165. (2) Lindblom, G.; Rilfors, L. Biochim. Biophys. Acta 1989, 988, 221. (3) Fontell, K. Colloid Polymer Sci. 1990, 268, 264. (4) Fontell, K. Adv. Colloid Interface Sci. 1992, 41, 127. (5) Olsson, U.; Wennerstro¨m, H. Adv. Colloid Interface Sci. 1994, 49, 113. (6) Strey, R. Colloid Polym. Sci. 1994, 272, 1005. (7) Sakya, P.; Seddon, J. M.; Templer, R. H.; Mirkin, R. J.; Tiddy, G. J. T. Langmuir 1997, 13, 3706.
and polar parts (hence we quote the average hydrocarbon and ethylene oxide chain sizes; see further Table 1). At 25 °C, C〈17〉E〈84〉 when dissolved in water forms a micellar phase at low concentrations (the cmc is 2.5 µM), which exists up to ≈13 wt %. Between ≈13-20 wt % there is a two-phase region. Above ≈20 wt % the surfactant forms a cubic liquid crystalline phase. This region extends up to ≈60 wt %. Above this concentration the surfactant is not miscible with water at the temperature used (25 °C). The main purpose of this study is to characterize the micellar and cubic phases of the C〈17〉E〈84〉/water system. To achieve this goal, we have used four different experimental techniques, namely pulsed field gradient (PFG) NMR, small-angle X-ray scattering (SAXS), cryo-transmission electron microscopy (cryo-TEM), and timeresolved fluorescence quenching (TRFQ). The first question we seek to answer is whether the cubic phase is bicontinuous or discrete. Some preliminary diffusion studies reported from a similar system3 have indicated that the cubic phase is bicontinuous. This would appear to be a surprising result, given the large headgroup of C〈17〉E〈84〉. Furthermore, we aim at determining which space group (or possibly, space groups) the cubic phase belongs to and how the surfactant molecules are arranged three dimensionally in the cubic phase. As mentioned above, one can also regard the molecule under study as a copolymer. In fact, since it aggregates, the aggregates can be considered to be “starlike” polymers. To compare the effect of the self-aggregation on the diffusion behavior, we have also performed self-diffusion measurements of a PEG/water system, where the PEG has roughly the same molecular weight as the surfactant, viz. E〈98〉. The outline of this paper is as follows. First, the Experimental Section is given. Subsequently, results and
S0743-7463(98)00081-X CCC: $15.00 © 1998 American Chemical Society Published on Web 09/04/1998
Properties of the Nonionic Surfactant C〈17〉E〈84〉/Water System
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Table 1. General Properties of the Surfactant and Polymer Used in This Study system C〈17〉E〈84〉 E〈98〉
c* b cmc [η]a Mw (g Mn (g polydispersity (µM) (wt %)-1 (wt %) mol-1) mol-1) index 2.5
0.17 0.13
5.9 7.7
4284 4546
3956 4329
1.083 1.050
a The estimated error in [η] is (5%. b Evaluated by means of eq 1.
discussion of the data, including necessary methodological details, are presented. Finally, some concluding remarks are given. Experimental Section Materials. The surfactant (trade name B08), henceforth referred to as C〈17〉E〈84〉, and the polymer (trade name PEG 4000 FL), henceforth referred to as E〈98〉, were supplied by Akzo Nobel AB, Stenungsund, Sweden, and were used without further purification (for some basic physical characteristic of the substances, see Table 1). The samples used in the viscosity, SAXS, cryo-TEM, and TRFQ measurements were mixed with Millipore-Q treated water. The heavy water (>99.8%) used for mixing the samples for the NMR measurements was purchased from Dr. Glaser AG, Basel, Switzerland. Viscosity Measurements. The viscosity was measured both for the surfactant and the polymer with an Ostwald viscometer. The sample solutions used were made by weighing and then mixing the components properly. The samples were then equilibrated before used. The thermostat bath, containing the viscometer, was held at 25.1 °C for all the measurements. PFG NMR Self-Diffusion Measurements. The self-diffusions of C〈17〉E〈84〉, E〈98〉, and water were measured with the FTPFG technique, monitoring the 1H spectra and following the procedures suggested previously.8,9 Most of the measurements were performed on a Varian Unity 400 spectrometer, equipped with a 360 MHz (8.45 T) Oxford wide bore magnet, using the spin-echo pulse sequence (see further discussion below). For the samples in the cubic phase the self-diffusion coefficients were small. In addition, the transverse relaxation rates were rapid. These samples were measured on a Bruker DMX 200 spectrometer, using the stimulated echo sequence.10 The latter spectrometer has considerably stronger gradients and in addition much less problems connected with eddy currents induced by the pulsed magnetic field gradients than the former. The gradient probe used on the Varian Unity 400 is home-built and has a gradient strength of approximately 0.18 T m-1 A-1, while a Bruker probe, which has a gradient strength of approximately 0.22 T m-1 A-1, was used on the Bruker DMX 200. All measurements were performed in 5 mm NMR tubes (with 250 µL sample solution in the tube). The samples for the NMR measurements in the micellar region were made by first weighing and then mixing the components properly in small vials. Subsequently, the solutions were transferred to NMR tubes. Samples in the cubic region were made by weighing the components directly into the NMR tubes. These were then centrifuged in order to equilibrate. All samples were equilibrated for some time (1-2 days) at 25 °C before used. The temperature was 25.0 (0.1 °C for all measurements. In the presentation of the data we will make use of the variable k, defined as k ≡ γ2g2δ(∆ - δ/3)) (for a definition of the symbols, see below). SAXS Measurements. The SAXS measurements were performed on a Kratky compact small angle system equipped with a position sensitive detector (OED 50M from M Braun, Graz, Austria) containing 1024 channels of width 53.0 µm. Cu KR radiation of wavelength 1.542 Å was provided by a Seifert ID300 X-ray generator, operating at 50 kV and 40 mA. A 10 µm thick nickel filter was used to remove the Kβ radiation, and a 1.5 mm tungsten filter was used to protect the detector from the primary beam. The sample-to-detector distance was 277 mm. The samples for the SAXS measurements were placed between (8) Stilbs, P. Prog. NMR Spectrosc. 1987, 19, 1. (9) So¨derman, O.; Stilbs, P. Prog. NMR Spectrosc. 1994, 26, 445. (10) Tanner, J. E. J. Chem. Phys. 1970, 52, 2523.
Figure 1. I/I0 vs k (k ≡ γ2g2δ2(∆ - δ/3)) for the methylene protons in the EO units (b) and the methyl protons in the hydrocarbon chains (O), for a sample of 12 wt % C〈17〉E〈84〉. The larger scattering of the points for the methyl protons (O), is due to a lower S/N ratio for these protons. The broken line represents a nonlinear least-squares fit of eq 2 to the data, while the solid line represents a biexponential nonlinear least-squares fit (i.e., a sum of two exponentials, according to eq 2, with appropriate weighting factors) to the data.
Figure 2. Diffusion behavior vs concentration of the surfactant (b). Also included are the self-diffusion coefficients obtained from the biexponential ()) (the slow component) and the lognormal ([) fits, respectively, in the cubic phase (see text for details). The shaded area represents the two-phase region between the micellar and cubic phases. two plates of quartz positioned in a special sample holder. The temperature was held at 25.0 ( 0.1 °C for all measurements by the use of a Peltier element. The space between the sample and the detector was under vacuum during the measurements in order to minimize scattering from the air. In the structural analysis the peak positions of SAXS spectra were determined at the q values giving the maximum intensity (see Figure 7). No slit desmearing of the SAXS spectra were performed. Cryo-TEM Measurements. The specimen was prepared by blotting a 5 µL drop of the sample on a carbon-coated holey polymer film supported on a 300-mesh grid (Ted Pella Inc., Redding, CA) in a controlled environment vitrification system (CEVS),11 where the temperature was controlled by a bulb and the relative humidity was kept above 95% by a wet sponge to prevent sample evaporation. The specimen was blotted using filter paper. This resulted in the formation of a sample film suspended over the holey carbon film. Then the grid was vitrified very quickly in liquid ethane. The CEVS operation retained the original composition of the sample so that the original microstructures remain unaltered in the vitrified specimen. The (11) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87.
5732 Langmuir, Vol. 14, No. 20, 1998
Figure 3. I/I0 vs k (k ≡ γ2g2δ2(∆ - δ/3)) for the methylene protons in the EO units for a sample of 35 wt % C〈17〉E〈84〉 in the cubic phase. The broken line represents a biexponential nonlinear least-squares fit (i.e., a sum of two exponentials, according to eq 2, with appropriate weighting factors) to the data, while the solid line represents a nonlinear least-squares fit of eq 3 to the data.
Figure 4. I/I0 vs k (k ≡ γ2g2δ2(∆ - δ/3)) for the methylene protons in the EO units of the surfactant in samples of 25 (b) and 35 (O) wt % of C〈17〉E〈84〉, respectively. The solid and the broken lines represent nonlinear least-squares fits of eq 2 to the data. Please note the difference in the scale of the abscissa, as compared to Figure 3. vitrified specimen was then transferred by an Oxford cryo holder into a Philips CM120 Bio-Twin microscope operating at 120 kV in the conventional TEM mode with nominal underfocus of about 5 µm. The images were recorded under cryogenic conditions, using a Gatan multiscan CCD camera. The low dose mode of the electron beam was used to reduce radiation damage by the electron beam. Since the cubic phase in the studied system appears at rather low surfactant concentration, the samples could be spread on the holey carbon grid. It should be noted that in systems where the cubic phase exists at higher surfactant concentration (e.g. in C12E8) an “on the grid processing” is essential for successful sample preparation.12 The d-spacing analysis of the micrographs was carried out using software from Digital Micrograph. The distance of 10-20 planes was measured on the digitized micrograph (using the “line density” option in the software) and divided by the number of the planes. TRFQ Measurements. The fluorescence decay data were collected with the single-photon counting (SPC) technique. A detailed description of the technique and the experimental setup is given elsewhere.13 All measurements were performed at ambient temperature (≈20 °C). Following the excitation at 325 (12) Danino, D.; Talmon, Y.; Zana, R. J. Colloid Interface Sci. 1997, 186, 170.
Ha˚ kansson et al.
Figure 5. The diffusion behavior vs concentration for E〈98〉 (b) and C〈17〉E〈84〉 in the liquid phase (O). The solid line represents a nonlinear least-squares fit of eq 4, with the following obtained parameters: E〈98〉, D0 ) 1.033 ((0.004) × 10-10 m2 s-1, R ) 0.107 ( 0.003, and γ ) 0.92 ( 0.01; C〈17〉E〈84〉, D0 ) 2.60 ((0.02) × 10-11 m2 s-1, R ) 0.159 ( 0.007, and γ ) 1.21 ( 0.03. The arrows in the graph indicate the overlap concentration (c*) obtained from eq 1 (see Table 1).
Figure 6. D/D0 vs (mol D2O/mol EO) for water in the C〈17〉E〈84〉/ water (O) and the E〈98〉/water system (b). The solid and broken lines represent the results of nonlinear least-squares fits of eq 7 to the data. The number average of the molecular weights were used for calculating the numbers on the x-axis (see Table 1). nm the emission from the probe (pyrene) was selected using an interference filter (400 ( 5 nm). No deconvolution of the decays was necessary as the excitation pulse width was short (
