J. Phys. Chem. B 1998, 102, 9161-9167
9161
Characterization of the Nanostructures in Liquid Crystalline Mesophases Present in the Ternary System Brij-35/Dibutyl Ether/H2O by Small- and Wide-Angle X-ray Scattering R. Schwarzenbacher, M. Kriechbaum, H. Amenitsch, and P. Laggner* Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Steyrergasse 17, 8010 Graz, Austria ReceiVed: May 19, 1998; In Final Form: August 13, 1998
We investigated the liquid crystalline phase structures and their temperature-dependent dynamics in the ternary system composed of Brij-35 [poly(oxyethylene(∼23))lauryl ether, C12OE∼23], dibutyl ether (DBE) and 10 mM Na/K buffer. A sequence of schematic ternary phase diagrams at 10, 20, 30, 40, and 60 °C, derived from temperature-scanning small- and wide-angle X-ray scattering (SWAX), has been established. In the binary Brij-35/H2O system, the amphiphilic component Brij-35, being totally miscible with water, self-assembles as a function of its concentration into a normal micellar solution L1, a normal packed micellar cubic I1-phase region, and an extended region of various lamellar structures. The cubic I1 phases show diffraction patterns with Im3m (unit cell parameter aIm3m ) 85-94 Å) and Ia3d (aIa3d ) 162-167 Å) symmetry. Increasing temperature destabilizes the cubic phases between 15 and 55 °C, depending on the weight ratio of the used components. The lamellar phases show unit cell dimensions of a ) 179-181 Å. A phase transition from Lc (crystalline lamellar phase), Lβ (gel lamellar phase), to a isotropic micellar phase, here denoted as L2, occurs at temperatures from 15 to 35 °C, as a function of the water content. The DBE uptake of these structures is limited to 20-50%(w/w), which generates biphasic regions at higher DBE amounts, where a DBE excess phase containing approximately 2% (w/w) H2O and 0.02% (w/w) Brij-35 is formed. Increasing amounts of DBE leadswith exception of the Lc phasesto enhanced phase temperature stabilities in the waterlean region and induce normal hexagonal H1 (a ) 68-135 Å) and various cubic structures. We found the cubic space groups Im3m and Ia3d with unit cell dimensions aIm3m ) 83-112 Å and aIa3d ) 162-195 Å in the cubic I1 region and Pn3m (aPn3m ) 164 Å) and Ia3d (aIa3d ) 187-214 Å) as well as Fd3m (aFd3m ) 218-275 Å) in the cubic region with low water contents.
1. Introduction Owing to their particular aggregation and phase behavior and their various technological and biotechnological applications, nonionic surfactants such as Brij-35 [poly(oxyethylene(∼23))lauryl ether, C12OE∼23], have been the subject of extensive physical studies.1-21 Brij-35 is a mixture of poly(oxyethylene)lauryl ether molecules with an average of 23 oxyethylene units. Its lyotropic behavior is characterized by the rather high hydrophiliclipophilic balance (HLB) value of 16.9 and a phase inversion temperature (PIT) > 90 °C,26 indicating a very hydrophilic surfactant, with a reported critical micellar concentration (cmc) of about 0.05-0.2 g/L.2-5 Using light scattering methods for micellar solutions containing 50-100 g/L Brij-35, Phillies et al.16 reported a micellar hydrodynamic radius of ac ≈ 57 Å at 25 °C, with an outer coronal shell of 13 Å, and an aggregation number of approximately N ) 40. For 70 °C, these authors found values with ac ≈ 49 Å and N ) 63. These data indicate a change in micellar shape and size with increasing temperature caused by structural changes within the surfactant chains. The POE chains no longer fit into the tetrahedral structure of water, which causes a release of hydrate water molecules out of the micellar shell region and thus a micellar contraction. In the present study, we demonstrate that these findings are not * Corresponding author. Telephone: 43-316-812003. Fax: 43-316812367. E-mail:
[email protected].
restricted to micelles in solution but are also valid for the liquid crystalline phases. The principal objective of the current paper is the documentation of the liquid crystalline phases and their temperaturedependent dynamics, present in the ternary system composed of Brij-35, DBE, and H2O. Lyotropic liquid crystals consist of distinct, nanoscaled hydrophobic and hydrophilic domains separated by a surfactant interface, where structure and physical properties depend on composition, temperature, and pressure.22-30 SWAX is a particularly valuable tool to establish the existence of lyotropic phases and to obtain their structural parameters, both temperature- and pressure-controlled. The information on the liquid-crystalline phases given in this work is fundamental to understanding the influence of composition and structure of enzyme immobilization matrixes on biocatalytic processes and thereby serves as a rational basis for an optimized system design. This report is, therefore, mainly focused on the structures of the highly viscous liquid-crystalline phases in the ternary system Brij-35, DBE, and H2O. The phase boundaries of the two- and three-phase areas, especially in the Brij-35-lean regions, where emulsions occur, were not further examined. 2. Materials 2.1. Materials Used for the Investigation of the Phase Diagrams. The surfactant was poly(oxyethylene(∼23))lauryl ether (Brij-35) (Merck, CH3(CH2)11(OCH2)∼23OH, C12OE∼23), molar mass ∼1100 g, containing ∼3%(w/w) H2O. Dibutyl ether
10.1021/jp9822889 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/14/1998
9162 J. Phys. Chem. B, Vol. 102, No. 45, 1998
Schwarzenbacher et al.
(DBE) p.a. was obtained from Merck and used as received. Water was freshly bi-distilled for the preparation of 10 mM NaK phosphate buffer (pH 7.0) used in all the samples. 3. Methods 3.1. Sample Preparation for the Phase Diagrams. Fiftyfive samples of 30.0 ( 0.02 g each were prepared individually by weighing appropriate amounts of Brij-35, buffer, and dibutyl ether into Pyrex glass tubes (10 mL), which were sealed immediately with Teflon screw caps. The samples were heated to >80 °C (some 120 °C) under extensive vortexing until a visually homogeneous appearance was achieved and after slow cooling at 5000 rpm centrifuged (Sorval). Samples were kept to equilibrate at ambient temperatures (25 ( 2 °C) for at least 3 weeks before examination. 3.2. Simultaneous Small- and Wide-Angle X-ray Scattering (SWAX). The measurements were carried out in a SWAX camera (Hecus MBraun-Graz-X-ray Systems, Graz, Austria),31 which employs Kratky slit optics and two coupled linear position sensitive detectors (PSD, MBraun, Garching, Germany) for the small- and wide-angle ranges, respectively, covering the ranges of Bragg’s spacings of 10-1000 and 3.44.9 Å, respectively. The line focus (Cu KR radiation) of a Rigaku Rotaflex Ru200 rotating anode generator (Rigaku Corp., Japan) was used as the radiation source, operating at 0.9-2.5 kW (30-50 mA and 30-50 kV, depending on the scattering power of the system). The Cu Kβ radiation was eliminated with a 10 µm Ni filter. The detector position scaling was calibrated with the characteristic reflections of silver stearate (small-angle region, at d ) 48.78 Å) and p-bromobenzoic acid (wide-angle region, at d1 ) 4.67 Å, d2 ) 3.80 Å, and d3 ) 3.70 Å) as reference materials. Samples were measured in the temperature range from 5 to 65 °C with an equilibration time of at least 5 min and an exposure time of 5-10 min. Typical temperature protocols are described in the captions of Figures 2-4. A set of high-resolution temperature scans have been performed at the Austrian high-flux SAXS wiggler beamline32,33 of the electron storage ring ELETTRA (Trieste, Italy). By use of the 8-keV (corresponding to λ ) 1.54) setup of the double focusing wiggler beamline, one-dimensional diffraction patterns were recorded with a 100-mm-long linear position sensitive detector. Set to a sample-detector distance of 1200 mm, the beamline was aligned to work within a resolution limit between 350 and 13 Å, with point collimation. The d-scale was calibrated with a reference pattern of dry rat tail tendon collagen having a long period of 650 Å. The acquisition time for each SAXS pattern was 10 s/frame at a heating rate of 1 °C/min. In all experiments the samples were contained in a thin-walled 1-mm-diameter quarz-Mark capillary, held in a steel cuvette to provide good thermal contact to the PC-controlled Peltier heating system (Hecus MBraun-Graz-X-ray Systems, Graz, Austria). For highly viscous samples, a similar cuvette modified for pastes with a layer thickness of 0.75 mm and Mylar windows was used. 4. Results 4.1. Phase Diagrams. To illustrate the evolution of the phase equilibria in the full ternary system Brij-35, DBE, and H2O (buffer) with temperature, we have drawn a sequence of schematic isothermal phase diagrams within the phase prism in Figure 1. The underlying data derived from characteristic SAXS-patterns (Figure 2) are summarized in Table 1. The borderlines of the different phase regions are drawn with a
Figure 1. Schematic phase diagrams at five different temperatures, T1 < T2 < T3 < T4 < T5, illustrating the evolution of the phase equilibria of the ternary nonionic surfactant/cosurfactant/water system Brij-35, DBE, and H2O. For the present system the different temperatures correspond approximately to T1 ) 10 °C, T2 ) 20 °C, T3 ) 30 °C, T4 ) 40 °C, and T5 ) 60 °C. The concentrations are expressed in weight percent. The nomenclature of the various phases is mainly according to the system given in ref 23: L1, micellar solution; I1, cubic phase of close-packed spherical micelles; H1, normal hexagonal phase; V1, normal bicontinuous cubic phase; Lc, crystalline lamellar phase; Lβ, fluid lamellar phase; L2, isotropic surfactant liquid.
Nanostructures in the System Brij-35/Dibutyl Ether/H2O
J. Phys. Chem. B, Vol. 102, No. 45, 1998 9163
TABLE 1: Liquid Crystalline Phases in the Ternary System Brij-35/Dibutyl Ether/H2O. Results Obtained from Temperature-Dependent SAXS Experiments and Visual Inspectiona composition %(w/w) Brij-35 100 90 80 80 70 70 60 60 60 50 50 50 50 40 20 11
H2O 10 20 10 30 20 30 20 10 30 10
lamellar Lc,ss
10 10 10 20 30 20 40 50 30 40 70 86
20 20 10 3
hexagonal H1
a, Å
T, °C
a, Å
T, °C
∼179 ∼180 ∼179 ∼145 ∼181 ∼187 ∼180 ∼174 ∼145? ∼150 ∼140
