Per-Ola Quist

Micelle size and order in lyotropic nematic phases from nuclear spin relaxation

Nuclear magnetic resonance (NMR) relaxation of quadrupolar nuclei is introduced as a new method for determining micelle size and nematic order in lyotropic nematic mesophases from the dependence of the spin relaxation rates on molecular diffusion over the curved micelle surface. The approach is illustrated by an experimental study of two uniaxial nematic phases (the calamitic NC and discotic ND phases of the sodium dodecyl sulphate/decanol/water system) using two nuclei: 2H in the α‐deuterated surfactant and 23Na in the counterions. The two nuclei yield similar results: an apparently temperature independent axial ratio of 3–4 in both phases and a nematic order parameter which decreases from ca. 0.9 (0.75) at the lowest temperature to ca. 0.6 (0.5) at the highest temperature in the NC (ND) phase. As compared to the predictions of the Maier–Saupe theory, the nematic order parameter in the NC phase is considerably larger and decreases more strongly as the nematic–isotropic transition is approached.

Nuclear spin relaxation in a hexagonal lyotropic liquid crystal

The hexagonal (E) phase in the sodium dodecyl sulphate (SDS)/decanol/water system is investigated by 2H and 23Na nuclear magnetic resonance (NMR) of the selectively deuterated SDS and the sodium counterion. Using macroscopically oriented E phase samples, prepared from the magnetically aligned nematic (NC) phase, we measure the orientation‐dependent relaxation rates R1Z and R1Q as well as the line shape of both nuclei. The orientation dependence of the lab‐frame spectral densities, determined from the relaxation rates, allow us to separate contributions from different types of molecular motion. In particular, we find a dominant contribution from molecular diffusion around the cylindrical aggregate. From this contribution we determine the lateral diffusion coefficient of SDS to (1.4±0.2)×10−10 m2 s−1 at 25 °C (activation energy 26±2 kJ mol−1 ) and the counterion surface diffusion coefficient to (4.8±0.9)×10−10 m2 s−1 at 25 °C (a factor 2.8 smaller than in an infinitely dilute aqueous electrolyte solution). ...

First order transitions to a lyotropic biaxial nematic

Abstract The phase diagram of the sodium dodecylsulphate/decanol/water system is studied by2H NMR spectroscopy in the range between the calamitic nematic (N+ C) and discotic nematic (N− D) phases. In this narrow range a nematic biaxial phase (NBX) is observed. The phase transitions between the nematic phases are all of first order. The shape of the surfactant aggregates in the nematic phases varies with composition and temperature.

Microstructure and dynamics in lyotropic liquid crystals. Principles and applications of nuclear spin relaxation

Abstract Nuclear spin relaxation of quadrupolar nuclei provides access to a wide range of properties of lyotropic liquid crystals, ranging from the molecular ordering and dynamics at the interface to the macroscopic viscoelastic behaviour. We emphasize here the unique capability of the spin relaxation method to provide detailed geometric and dynamic information relating to the microstructure of lyotropic liquid crystals, i.e. the metric, curvature, and fluctuations of the dividing interface that separates polar and non-polar regions. This information is conveyed to the spin system via the translational diffusion of surfactants or counterions over the interface. The general principles of the spin relaxation method, as applied to lyotropic liquid crystals, are described, with emphasis on the model-independent information content of the relaxation observables and on the relation to microstructure. Specific results for lamellar, hexagonal, cubic, and nematic phases are also described.

N.M.R. lineshapes from quadrupolar nuclei in biaxial lyotropic structures: Elliptic rod with nonuniform molecular distribution and orientational order

The appearance of the N.M.R. spectrum of a quadrupolar nucleus in a lyotropic liquid crystal reflects the molecular organization in the surfactant aggregates. In particular, the spectral lineshape is a sensitive probe of deviations from circular cross-section in rod-like aggregates. This circumstance has been exploited by Chidichimo, Doane et al. to characterize the so-called ribbon phase, using a model based on the assumption that the aggregates are built from lamellar and hemi-cylindrical sections. In this work, we present an alternative model based on an elliptic-rod geometry. Using this model, we relate the N.M.R. observables to the eccentricity of the cross-section and to the inhomogeneity in the local orientational order and the molecular distribution. The two models differ mainly in two respects: (i) the elliptic geometry does not have the unphysical curvature discontinuity of the ribbon geometry, and (ii) unlike the ribbon model, the elliptic model can always distinguish between effects of aggrega...

Water dynamics and aggregate structure in reversed micelles at sub-zero temperatures. A deuteron spin relaxation study

The microemulsion phase of the system AOT–D2O–2,2,4-trimethylpentane (TMP) has been studied by water 2H longitudinal and transverse spin relaxation measurements at three frequencies and at temperatures down to –29 °C. Additional information was obtained from n.m.r. signal intensities and quadrupolar line splittings, differential scanning calorimetry and small-angle X-ray scattering. The results establish the existence of reversed micelles with unfrozen aqueous cores containing between 2 and 4 water molecules per AOT. As the temperature is lowered, these micelles grow into very long rods. The rate of water reorientation in the core, obtained directly from the frequency-dependence of the longitudinal relaxation rate, is two orders of magnitude lower than that of bulk water at the same temperature.

Molecular segregation and aggregate shape in a lyotropic rectangular phase

Abstract The microstructure of the rectangular phase in the system sodium decylsulphate/decanol/water is investigated by means of deuterium NMR. By analysing the lineshape from selectively deuteriated decylsulphate and decanol, we separate the effects of (i) the shape anisotropy of the aggregate cross-section and (ii) the inhomogeneous distribution of the two surfactants within the aggregate. The aspect ratio of the cross-section is determined to 1.39±0.01, substantially smaller than previous estimates. We find no evidence for anisotropic growth of the aggregate cross-section in the hexagonal phase, as previously suggested. Rather, the aggregate shape appears to change abruptly at the hexagonal-rectangular phase transition with little change (with temperature) thereafter. The distribution of decylsulphate and decanol within the aggregates of the rectangular phase is highly non-uniform; the decanol concentration is 3 times higher in the central lamellar region than in the curved edges, while the decylsulph...

Anisotropic 23Na spin relaxation in liquid crystals. Determination of all nine spectral densities for a hexagonal lyotropic phase

Abstract The orientation dependence of three independent 23Na relaxation rates is determined from measurements on the counterions in a magnetically aligned hexagonal lyotropic liquid crystal. The data allow us to determine the nine director-frame spectral densities that comprise the full information content of the spin-relaxation behavior of a quadrupolar nucleus in a uniaxial phase. The model-independent director-frame spectral densities provide important information about the time scale and rotational symmetry of the anisotropic motions in the phase. More detailed information is obtained by invoking a specific dynamic model that separates dynamic and structural factors. The six high-frequency spectral densities are quantitatively accounted for in terms of fast local motions and counterion surface diffusion around the cylindrical surfactant aggregates, while the three zero-frequency spectral densities contain large contributions from diffusion-modulated director fluctuations.