J. Phys. Chem. B 2006, 110, 20781-20788
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Evolution of a Lamellar Domain Structure for an Equilibrating Lyotropic Liquid Crystal Penny L. Hubbard, Kathryn M. McGrath, and Paul T. Callaghan* MacDiarmid Institute for AdVanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria UniVersity of Wellington, Wellington 6001, New Zealand ReceiVed: January 10, 2006; In Final Form: March 16, 2006
Aerosol OT/water exhibits a lamellar phase over a wide range of concentrations. We show, by magnetic resonance (NMR) and scanning electron microscopy (SEM), that the morphology of the lamellar phase varies significantly across that range and that the rate of equilibration depends strongly on concentration (25, 33, and 50 wt %) with, paradoxically, the faster equilibration at higher surfactant concentrations. We find that the 25 wt % sample exhibits a defect-rich local structure, characteristic of a superposed L3 character. Further into the lamellar region, at 33 wt %, this defect-rich structure persists heterogeneously, while, at 50 wt %, the lamellar phase domains are highly ordered. The NMR methods used here included 2H spectroscopy and the two-dimensional NMR method, diffusion-diffusion exchange spectroscopy (DEXSY). The latter was used to obtain quantitative information on the domain sizes and defects within the polydomain lamellar mesophase. Comparison of the NMR with the SEM results suggests that, at 25 wt % AOT, bilayer defects play an important role in influencing the 2H NMR and DEXSY NMR results.
Introduction Some lyotropic liquid crystals are known to exhibit a lamellar phase over a wide range of concentrations. Notable among these is the aqueous system Aerosol OT (bis(2-ethylhexyl) sodium sulfosuccinate). According to the binary phase diagram, the lamellar mesophase at room temperature extends from 17 wt % toward a transition to a viscous isotropic cubic phase between ∼78 and 82 wt % and a reversed hexagonal phase in the waterpoor part of the phase diagram.1-5 The lamellar structure may be extended down to 5 wt % with the addition of between 1.3 and 1.7 wt % of electrolyte.6,7 A further consequence of electrolyte addition is the existence of a mixed LR + L3 phase at salt concentrations above 2 wt % in the water-rich corner of the phase diagram.6,7,14-19 Therefore, not only does the phase diagram for AOT present opportunity for a wide range of bilayer spacing it also suggests a potential behavior rich in defect structure. It would be surprising if the naive picture of a lamellar phase comprising ideal bilayers, whose spacing simply varies with water content, were to hold true across such a wide swathe of concentration. Indeed the lamellar categorization for AOT is known to be problematic, and a number of prior studies indicate that various behaviors (lamellar repeat distance, Raman intensities, conductivity, and water diffusion coefficient) of this phase change anomalously over the concentration range.1,4,8-13 The characterization of defect and domain structures in lyotropic liquid crystals has long been a source of query and debate.13,20-23 Many microscopic techniques suffer from an inability to characterize samples at the macroscale; for example, transmission electron microscopy (TEM)24,25 and scanning electron microscopy (SEM) enable site-specific defects to be imaged and a qualitative picture of defect types to be obtained at length scales of 10 nm to 100 µm, but, even at low magnification, only a relatively small area of a sample can be investigated. Further, microscopy does not allow an assessment of domain connectivity or orientational symmetry. Scattering * E-mail:
[email protected].
techniques (SAXS and SALS) may provide greater insight at length scales on the order of 5-100 nm and allow macroscopic sampling, although the method is generally insensitive to structural lengths outside the range of the scattering wavevector. One particularly effective means of investigating lyotropic phase structure is the pulsed-gradient spin-echo (PGSE) NMR measurement of diffusion. Due to the short spin-spin relaxation time of the surfactant molecules (∼100 µs), the echo decay in the PGSE NMR experiment, dominated by the water and the various restrictions to water diffusion presented by the surfactant structures, can provide a useful signature of phase morphology. We have revisited the AOT/water system using a new 2D pulsed-gradient spin-echo NMR exchange technique (diffusion exchange spectroscopy or DEXSY). This method is sensitive to changes in diffusive behavior as water molecules diffuse between differently oriented domains in the polydomain sample. The method can, in principle, be used to reveal domain size, and is sensitive to lengths between 10 and 100 µm. We combine this approach with the measurement of the deuterium NMR spectrum in samples where the water is partially replaced with D2O. This technique is sensitive to structural anisotropy on the length scale traversed by a water molecule over the characteristic spectral time, the inverse of the deuterium quadrupolar splitting. For the AOT system, this length is on the order of 1 µm. Finally, we compare these NMR results with structures apparent in freeze-fracture scanning electron microscopy, where features on a length scale from 10 nm to 100 µm can be observed. Note that in this work we shall be concerned with the polydomain lamellar mesophase in which domains are locally anisotropic, but globally isotropic; here, the 2H NMR powder pattern reveals information about the bilayer spacing and degree of domain orientation.26-28 By contrast, the interpretation of diffusive behavior measured by pulsed-gradient spin-echo (PGSE) NMR is more subtle. In locally anisotropic environments, the standard Stejskal-Tanner expression29 must be modified to allow for the limited dimensionality of local diffusion, and the macroscopic distribution of the orientations
10.1021/jp0601872 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/04/2006
20782 J. Phys. Chem. B, Vol. 110, No. 42, 2006 of local domains.11,30 PGSE NMR experiments are generally performed with the magnetic field gradient direction along some chosen axis in the laboratory frame. For bilayer domains with directors along this axis, the water molecules will be strongly restricted in their diffusion, while for other orientations diffusion may appear much more rapid. This leads to an inherent “multiexponential” character to the signal decay, a character that can be represented as an apparent distribution of diffusion coefficients using standard inverse Laplace transformation methods (e.g., CONTIN31). By carrying out two pulsed-gradient spin-echo encodings in rapid succession, it is possible to investigate how this diffusion distribution changes with the time of encoding separation, thus visualizing whether water exchanges between slow and fast modes over the separation (mixing) time. This is the method employed here. Alternatively, one may apply successive encodings with differing gradient directions. By this means, it is possible to investigate the degree of orientation order in the locally anisotropic/globally isotropic polydomain phase. This latter method, known as diffusiondiffusion correlation spectroscopy (DDCOSY), is the subject of another article.32 Experimental Section Electron Microscopy Sample Preparation. Aerosol OT or bis(2-ethylhexyl) sodium sulfosuccinate (Sigma, purity 99%) was dissolved in distilled H2O. Weight ratios of 25, 33, and 50 wt % of surfactant were prepared. The samples were agitated on a vortex mixer for several minutes and sealed in sample tubes. The samples were then left to equilibrate at room temperature for ∼3 weeks and ∼3 months. Samples for the preparation of freeze-fracture replicas were frozen in Balzers “hats” in liquid nitrogen cooled Freon 22, fractured at -115 °C, and raised to -100 °C for 2 min each. Carbon and platinum were applied with electron beam guns in a Balzers 301. C and Pt were applied at 45° stationary, followed by C at 5° rotating. Replicas were cleaned and collected on uncoated copper mesh grids and slot grids. Scanning Electron Microscopy. The freeze-fracture replicas were secured on a specialized holder and loaded into the JEOL JSM-6500F field-emission scanning electron microscope. The images were obtained at an accelerating voltage of 10-15 kV and a probe current of 300 pA. The focal distance was 10 mm, and standard SEI digital images were obtained, with no tilt. Note that the results of the scanning electron microscopy (SEM) study at high magnification reveal the submicron topography of these bilayers while at low magnification a broader view of the grain boundaries of the lamellar domains is obtained. Comparison of the SEM technique with TEM showed that similar information is attained from both techniques; however, SEM proves to be more instructive when we require information at low magnification, allowing the entire replica to be viewed and not just the areas between the copper grid bars. Hence, SEM results for domain size may be readily compared with those of NMR. Nuclear Magnetic Resonance: Sample Preparation. Weight ratios of 25, 33, and 50 wt % AOT were dissolved in a mixture of 90:10 H2O with D2O to allow both 1H and 2H NMR experiments to be performed on the same samples. The samples were agitated on a vortex mixer for several minutes, sealed in 10 mm NMR tubes, and centrifuged to remove any trapped air. The samples were then left to equilibrate at room temperature for ∼3 weeks and ∼3 months before experiments were performed. Nuclear Magnetic Resonance: Experiments. The experiments were performed using an AVANCE 300 MHz Bruker
Hubbard et al.
Figure 1. DEXSY PGSE NMR pulse sequence in which gradient pulse pairs are applied collinearly, along all possible axes. The encoding pairs are separated by an effective mixing time, τm′ ) τm + ∆.
spectrometer. A PC, running XWINNMR software, controls the 7 T superconducting 89 mm vertical bore magnet. The spectrometer is equipped with a complete Bruker micro-imaging accessory, powered by a Bruker GREAT-60 gradient power supply. A 10 mm 1H radio frequency (RF) coil (pulse length, 12-13 µs) was used in combination with the Bruker gradient coils. 2H NMR. 2H spectra were recorded on the samples at all concentrations. For these specta, 2048 points were collected in the spectral dimension, and at least 128 scans were acquired for each spectrum, with an acquisition time of 0.4 s per scan. The length of the RF pulse was 100 µs in a 20 mm coil. Deuterium quadrupole line profiles were recorded on a Bruker 300 MHz spectrometer, operating on a deuterium frequency of ∼46 MHz. DEXSY. The DEXSY experiment correlates the diffusion coefficients obtained by applying the gradient pulse pairs (pulse separation ∆) collinearly, each pair separated by a variable mixing time (τm). By changing τm, any changes in restrictions to free diffusion apparent in the two separate sets of gradient pulses (i.e., G1 vs G2) may be compared. Such changes might be expected to arise if water moves between differently oriented domains. Consequently, changes arising as the effective mixing time, τm′ ) τm + ∆, is incremented may be used to calculate domain size. The double pulsed-gradient spin-echo (PGSE) pulse sequence is shown in Figure 1. Three-dimensional (3D) data sets were acquired using 2048 data points in the spectral dimension and 32 data points in both diffusion dimensions. Two scans were co-added for each 3D data set. The experiment time for each correlation map was 2.25 h. The maximum gradient used was 8.5 G/mm, with a pulse duration (δ) of 6 ms (6.5 ms, allowing for the equivalent rectangular area including the gradient ramp time) and a diffusion encoding time (∆) of 20 ms. The temperature was stabilized at 298 ( 0.5 K using a heating element and an air cooling system, monitored by a thermocouple and regulated by a PID algorithm. The 3D data file containing the FID signals was Fourier transformed in the spectral dimension. The peak of interest (water, 4.7 ppm) was then integrated to produce a twodimensional matrix, whose rows and columns are labeled by the diffusion parameters, (γδG)2(∆ - δ/3). This matrix was then subjected to a 2D Laplace inversion to produce a correlation map of 25 × 25 logarithmically spaced diffusion intensities. In the DEXSY experiment, the gradients were always applied collinearly, with a variable effective mixing time (τm′). The transverse component of magnetization was stored along the longitudinal axis during this time to reduce T2 relaxation effects. A homogeneity-spoiling magnetic field gradient was applied between the 90° storage pulses to destroy any unwanted magnetization that remains in the xy-plane, as shown in Figure 1. The effects of eddy currents were reduced by the addition of a 10 ms delay before data acquisition. 2D Laplace Inversion. The 2D Laplace inversion method used is based on the method of Song et al. 33 and Venkatara-
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manan et al. 34 and employs a one-dimensionalization of the matrix combined with singular value decomposition to reduce dimensionality. The software has been extensively used in recent NMR studies.35-37 In our application of this approach, regularization was adjusted to minimize χ2 with maximum smoothing, in accordance with standard practice.31 The 32 × 32 data arrays were analyzed to produce 25 × 25 diffusion values logarithmically spaced across the chosen range. Birefringence. When viewing the samples between crosspolarizers the 25, 33, and 50 wt % samples can be seen to be birefringent. However, the 50 wt % sample exhibits a significantly more colorful pattern, typical of a more highly structured system. No obvious time evolution is apparent in the birefringence for any of these samples. Additionally, the 50 wt % sample was distinctly less turbid than the lower concentration samples. Results and Discussion Sets of data were obtained from samples at ∼3 weeks and ∼3 months after preparation. In the following sections, we show significant differences in the behavior of samples having similar equilibration time but different concentrations. 2H NMR. We begin by showing the comparative deuterium NMR spectra. The water deuteron is able to be used as a measure of orientational order because the nuclear quadrupole interaction is sensitive to the alignment of the local bond-related electric field gradients, with respect to the magnetic field. Then, the quadrupole interaction may be written as,38
HQ(t) )
3eVzzQ P (cos θ(t))[3Iz2 - I(I + 1)] 4p 2
(1)
Vzz is the electric field gradient (efg) along the O-D bond principal axis, and Q is the nuclear quadrupole moment. P2 is the second Legendre polynomial where θ(t) is the angle between the bond axis and the magnetic field. The bilinear nature of the interaction with respect to the spin operators leads to a two line spectrum whose splitting is determined by HQ. Because the efg principal axis is associated with an interatomic bond direction within the molecule, θ(t) fluctuates due to molecular motion. Provided that the motion is fast compared with the interaction strength, 3eVzzQ/4p, the fluctuating Hamiltonian is motionally averaged to its mean. Consider the case in which the bond orientation (θ,φ) with respect to B0 can be decomposed into (θR,φR) with respect to an axis inclined at a polar angle R with respect to B0, where (θR,φR) fluctuates rapidly while R is static or varies slowly compared with the motional narrowing condition. Then, the spherical harmonic addition theorem may be used to factorize as follows.39
P2(cos θ(t)) ) P2(cos θR(t))fastP2(cos R)
(2)
In our case, R represents the angle of the local lamellar domain director with respect to the magnetic field while the scaling down of the interaction strength as the water molecule tumbles and diffuses between the bilayers is given by the order parameter P2(cosθR(t))fast. Typically, this value is around 0.01, giving water deuteron quadrupole splittings of around 1 kHz in lamellar phases. For powder samples (isotropic orientation of domains), the spectrum consists of a broad absorption curve with two marked peaks, the classic powder director pattern. Significant macroscopic alignment causes the powder pattern38 to shift toward a simple doublet spectrum. Figure 2 shows, at ∼3 weeks and ∼3 months equilibration,
Figure 2. 2H NMR spectra of 25, 33, and 50 wt % AOT in 10:90 D2O (with H2O) at 298 K and (a) 3 weeks and (b) 3 months after preparation. In each case, the higher concentration spectrum is broader. Fewer scans were used in part b, resulting in a lower signal-to-noise ratio.
the 2H NMR spectra for 25, 33, and 50 wt % AOT/water where 10% of the water is replaced with D2O, sufficient to give a good deuterium NMR spectrum but insufficient to perturb the phase structure.32 Note that the higher surfactant content samples exhibit a higher degree of alignment of D2O molecules. The 50 wt % spectra exhibit clear powder lamellae patterns, independent of equilibration time. By contrast, the 25 wt % sample at ∼3 weeks shows a broad singlet, while, at ∼3 months, the emergence of a quasipowder spectrum (perhaps a mixture of isotropic and powder pattern) is apparent. At ∼3 weeks equilibration, the 25 wt % spectrum is almost isotropic; this extreme deviation from powder lamellar spectral behavior is curious, given that this material is clearly within the region of the phase diagram labeled lamellar. We thus focus on this concentration in our subsequent measurements, noting that the high concentration sample shows no obvious difference, whatever method of examination is used, from ∼3 weeks to ∼3 months equilibration. Scanning Electron Microscopy. We now move our attention to the SEM images obtained from freeze-fracture replicas. The 25 wt % AOT in water samples, at magnifications of ×30 000 and ×550, at ∼3 weeks and ∼3 months after sample preparation, can be seen in Figure 3. Figure 3a at ×30 000 shows the lamellae at ∼3 weeks to be composed of sheets that are rich in defects, with features of the range of ∼100 nm. This type of image was consistently seen throughout the replicas prepared from the surfactant mixture at this concentration. At ∼3 months, there remains clear evidence for defects with features in the range of several hundred nanometers, although their amplitude appears noticeably attenuated in these EM images from the sample with this greater equilibration. At ×550 magnification, even at this coarser length scale, one can still observe evidence for underlying defect structures in the ∼3 week sample. By contrast, at ∼3 months equilibration, a smoother domain structure is apparent. These data for the 25 wt % samples reveal a defect-rich structure, which may contain lamellar layers but is dense with holes and bridges, more akin to a metastable L3 sponge phase than a typical LR phase.25,40-45 The quadrupolar broadened “isotropiclike” peak in the 2H spectrum for this sample is consistent with L3 character (Figure 2b), although it might equally be consistent with a lamellar phase in which there existed a distribution of interlamellar spacings. However, the electron microscopy strongly indicates an ∼100 nm defect structure, an observation which, we shall show, is consistent with diffusion measurements. At 3 months equilibration, the emergence of a more obvious lamellar powder spectrum is consistent with the emergence of greater lamellar characteristics, while local defect features are retained. We note that these samples do appear to be birefringent
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Figure 3. SEM images for 25 wt% AOT/water replicas at ×30 000 (a) ∼3 weeks after preparation and (b) ∼3 months after preparation and at ×550 (c) ∼3 weeks after preparation (note the shadow of the grid) and (d) ∼3 months after preparation.
Figure 4. SEM images at ×30 000 ∼3 months after preparation for (a) 33 wt % AOT/water replicas and (b) 50 wt % AOT/water replicas.
when viewed through crossed polarizers, clearly indicating anisotropic character at the µm scale. Turning to the higher concentration samples, Figure 4 shows ×30 000 SEM images for 33 and 50 wt % at ∼3 months. The 50 wt % image was practically indistinguishable from SEM images obtained much earlier at ∼3 weeks, indicating a much more rapid equilibration for these higher concentration samples. It is clear that the 50 wt % concentration samples have fewer defects, consistent with “lamellarlike” morphology down to the 100 nm length scale. The 33 wt % sample shows features characteristic of those found for both 25 and 50 wt %. In Figure 5, we show a low magnification SEM image (×1000) for the 50 wt % sample. Again, domains are clearly visible where sharp lines define the grain boundaries whose domain sizes range from approximately 10 to 50 µm. At this point, we note that Coppola et al. used obstruction factors calculated from PGSE NMR13 to find 17 µm domain sizes after
perturbing a ∼50 wt % sample by shaking for 2 min, and they found 50 µm sizes after another month of equilibration, consistent with our SEM results. Analysis of the SEM images in Figure 3c and d suggests domain sizes ranging from ∼10 to 100 µm for the 25 wt % sample. DEXSY NMR. The scanning electron microscopy technique is informative, especially when seeking submicron structure on a local scale. However, it does not allow the exploration of the full sample behavior and is disadvantaged by the need to manipulate the sample extensively in obtaining the freezefracture replica. Conversely, NMR allows the original sample to be investigated and a global description of the sample to be ascertained. DDCOSY may be used to demonstrate the existence of a polydomain locally anisotropic sample or to indicate deviations of the director distribution from global isotropy. This method is the subject of ref 32 where it is shown that AOT/water does
Lamellar Domain Liquid Crystal Structure Evolution
Figure 5. SEM image at ×1000 of 50 wt % AOT/water replicas in which domain interfaces are clearly apparent.
Figure 6. Theoretical DEXSY maps for the case of an isotropic distribution of lamellar domains where Dpar ) 0.1Dperp, Dpar being the water diffusion rate normal to the bilayers (i.e., transbilayer). “Zero” and “full” refer to zero exchange and full exchange of water molecules between the domains, respectively.
indeed show deviations, at least for the higher concentration samples. Here, our focus is domain size rather than domain orientation. In particular, we seek to elucidate the paradoxical situation whereby SEM shows a large scale single domain structure for the 25 wt % sample (see Figure 3), while deuterium NMR spectroscopy (Figure 2) indicates isotropy, over the characteristic spectroscopic length scale (the distance diffused by water molecules over the time corresponding to the inverse of the spectral width of 1 kHz), i.e., ∼1 µm. For this purpose, DEXSY NMR is ideally suited. When the effective mixing time (τm′) is sufficiently short that no water molecules have time to diffuse from one domain to another, the 2D map is diagonal, revealing slow or fast diffusive motion, depending on the local domain orientation with respect to the applied magnetic field gradient. As the mixing time increased to approach the diffusive exchange time between domains, off-diagonal peaks appear in the spectrum. Since that exchange time can be converted to a length scale via a knowledge of the water diffusion coefficient, the domains can be “sized”. Callaghan and Fu´ro37 have shown that DEXSY may be used to estimate domain size in a nonionic surfactant lyotropic lamellar phase and also demonstrated the use of the method to measure domain size polydispersity via the effective mixing time (τm′) dependent positions of the offdiagonal peaks that appear in the correlation maps when τm′ exceeds the time for a solvent molecule to diffuse from one domain to another. Figure 6 shows the idealized (calculated) DEXSY maps for an isotropic distribution of lamellar domains in which diffusion normal to the domains (Dpar) is one-tenth of that within the 2D planes of water between the bilayers (Dperp),
J. Phys. Chem. B, Vol. 110, No. 42, 2006 20785 a fairly typical ratio. In Figure 6a, we see the pattern for zero exchange of molecules between domains, and in 6b, the pattern for complete exchange over the effective mixing time τm′ is shown. Figure 7 shows the 2D correlation maps obtained from the DEXSY NMR experiment on the 50 wt % AOT sample at a range of mixing times (τm′). The appearance of off-diagonal features at around 70 ms corresponds well with the lower limit of the domain sizes obtained from the SEM technique (see Figure 5). Note that the angle subtended by the two off-diagonal features in Figure 7c is less than the 90° of the fully exchanged theoretical map of Figure 6b, indicating incomplete exchange.36 The fastest diffusion coefficient is seen in Figure 7 on the order of Dperp ) 1 × 10-9 m2 s-1, corresponding to water freely diffusing normal to the lamellar director. The 70 ms mixing time corresponds therefore to a length of x(2Dperp × 0.07 s) ∼ 12 µm. Although it is not easy to get an average value from the SEM image, it is clear that this value corresponds well with the lower limit observed via the microscopy images. Conversely, the lower concentration AOT sample shown in Figure 8 (25 wt %) behaves differently. From 40-420 ms (longer times cannot easily be accessed because of relaxation effects), there are clearly off-diagonal features in the correlations. Even at the shortest mixing time, it is apparent, from the offdiagonal features, that the self-diffusion of the water is changing over τm′. Figure 9 shows the DEXSY results for the 25 wt % sample at the minimum available τm′ of 20 ms and, in this case, with the gradients applied along all three laboratory axes. In each case, off-diagonal behavior is evident, although less pronounced than for τm′ ) 40 ms. By contrast, the τm′ ) 20 ms DEXSY equivalents for 50 wt % (see Figure 10) are all diagonal. This 20 ms exchange time, apparent at 25 wt %, corresponds to an effective domain length scale of 5 µm. This length is somewhat shorter than the corresponding size measured by SEM. It would however be consistent with our interpretation of the 2H NMR spectroscopy data for 25 wt %, data which also suggest a much shorter length scale over which the order parameter of the local domain structure is sampled compared with that observed in the SEM images for the 25 wt % sample (Figure 3c and d). We suggest that these DEXSY and 2H NMR spectroscopic discrepancies are associated with the dense ∼100 nm defect structure. The NMR tells us that orientational order (2H NMR) is substantially attenuated for diffusion distances on the order of 1 µm (the characteristic distance diffused by water molecules over the time corresponding to the inverse of the spectral width), while the facility to leak from domain to domain via defects (DEXSY) is facilitated for distances on the order of 5 µm. Note that the similarity of the (xx,yy,zz) DEXSY maps in both Figure 9a-c and Figure 10a-c suggests that both the 25 and 50 wt % samples are isotropic on a macroscopic scale. However, small deviations from this apparent global isotropy can be observed using DDCOSY.32 The results obtained regarding anomalies in the 25 wt % samples may be compared with anomalies found in the study by Coppola et al.13 on a range of AOT/water concentrations. These authors found that the lower concentration samples (∼25 wt %) were much more susceptible to perturbation by agitation than those at high concentration (∼60 AOT %). It may be that the relative fluidity of the low concentration AOT/water mixtures is consistent with a mesophase which is more sensitive to perturbation and therefore fluctuations in the self-organized structure of the surfactant molecules due to undulations in the bilayers, dense defects, and metastability.
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Figure 7. DEXSY correlation maps of 50 wt % AOT in 90% H2O (10% D2O) at 298 K, encoded along the x-direction with an exchange time of (a) 40, (b) 70, and (c) 420 ms, ∼3 weeks after preparation. A similar result is seen after ∼3 months equilibration.
Figure 8. DEXSY correlation maps of 25 wt % AOT in 90% H2O (10% D2O) at 298 K, encoded along the x-direction with an exchange time of (a) 40, (b) 70, and (c) 420 ms, ∼3 weeks after preparation. A similar result is seen after ∼3 months equilibration.
Figure 9. DEXSY correlation maps of 25 wt % AOT in 90% H2O (10% D2O) at 298 K with an exchange time of 20 ms for gradient encoding in (a) xx, (b) yy, and (c) zz, ∼3 weeks after preparation. Similar results were found for samples equilibrated for ∼3 months.
Figure 10. DEXSY correlation maps of 50 wt % AOT in 90% H2O (10% D2O) at 298 K with an exchange time of 20 ms for gradient encoding in (a) xx, (b) yy, and (c) zz, ∼3 weeks after preparation. A similar result is seen after ∼3 months equilibration.
The appearance of an L3-like morphology in the freezefracture replicas of the 25 wt % lamellar phase and the neither characteristically isotropic nor split powder pattern in the 2H spectra allow a greater understanding of the inherent nature of the AOT system. A number of reports have placed emphasis on extending the basic phase behavior of AOT via the addition of a salt, such as NaCl or NaNO3.6,7,14-19 These reports have shown that for concentrations of AOT below approximately 30 wt % the formation of an L3 phase may be induced for salt concentrations of 2 wt %. Initially, the phase coexists with the lamellar phase observed when no salt is present but, on increasing salt concentration, exists as a pure phase. In regards to deviation from simple LR character, we note also an earlier observation of disrupted or highly porous character in lamellar phases of both ionic and nonionic surfactant systems, in optical,
X-ray, and neutron scattering studies.46,47 The results reported here, when taken in context with these earlier reports, indicate a definite structural relationship between the L3 phase and the lamellar phase formed in this region of the phase diagram. We do however note that the L3 phase of AOT is asociated with negative curvature, favored when the surfactant headgroups are screened by addition of brine.48 We are of course dealing here with AOT/water, so that only the Na+ counterions are available for screening. The spontaneous curvature necessary for the formation of regions of “L3 character” is not expected at 25 wt %, but then, neither is the metstable character of these low concentration samples. Our results would seem to indicate that, even in the absence of salt, the Coulomb repulsion between the headgroups, coupled with the conformational freedom of the hydrocarbon chains, is sufficiently accommodating to enable a
Lamellar Domain Liquid Crystal Structure Evolution lamellar phase rich in defects having a more three-dimensionally connected morphology. Further, due to the slow equilibration time for the 25 wt % AOT sample, it would appear that at least one route to equilibration of the lamellar phase is via a metastable phase with a higher concentration of membrane connections, i.e., a morphology that is closer to an L3 phase than the lamellar phase, where annealing of these defects is kinetically hindered. It is important to emphasize here that our direct NMR observations are based on the behavior of water molecules. Any interconnected structure which we deduce (i.e., the L3 character) must necessarily be a feature of the water phase, but not necessarily of the surfactant bilayer component. In a true L3 phase, both water and surfactant exhbibit an interpenetraing interconnected morphology. We do note however that it would be possible for the water phase alone to exhibit interconnected morphology, if there existed significant water pools as defects in the bilayers. Such a picture is indeed suggested in Figure 8 of ref 46. While the focus of the present work has been to contrast the 25 and 50 wt % samples, we do note that, in many respects, the behavior at 33 wt % is intermediate. Certainly, the 33 wt % samples appear fully equilibrated after 3 months, with a clear lamellar behavior evident in both 2H NMR spectroscopy and DEXSY measurements (not shown here). However, when viewed form the perspective of the high resolution SEM images, some aspects of the L3 character, so apparent at 25 wt %, are obvious, though, we assert, insufficient to cause significant deviation from a dominant lamellar morphology. Conclusions We have shown here a high degree of consistency between measurements by the three methods of SEM, 2H NMR spectroscopy, and DEXSY NMR, though each accesses morphological features of the phases at differing length scales. First, the 2H NMR spectroscopy shows that the 25 wt % surfactant phase is anomalous, exhibiting local L3 characteristics, whereas the 33 and 50 wt % samples exhibit classic powder patterns, as expected of an ideal polydomain lamellar phase. The high resolution SEM images, in which a defect-rich structure is found in the 25 wt % sample, are entirely consistent with this finding. Also consistent is the evolution of both the SEM-visualized defect pattern and the 2H NMR spectrum as the 25 wt % sample is aged to ∼3 months equilibration. Second, the domain size seen in low resolution SEM data for 50 wt % AOT agrees well with that found in the mixing time dependence of the DEXSY experiment. Similarly, the apparently smaller effective domain size (
