Nuclear spin relaxation in a hexagonal lyotropic liquid crystal

Abstract

Proton relaxation study of paraffinic chain motions in a lyotropic liquid crystalThe 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 (N c) phase, we measure the orientation-dependent relaxation rates R lz and RIQ 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) X 10 -10 m 2 s -I at 25 ·C (activation energy 26 ± 2 kJ mol -I) and the counterion surface diffusion coefficient to (4.8 ± 0.9) X 10 -10 m 2 s -I at 25 ·C (a factor 2.8 smaller than in an infinitely dilute aqueous electrolyte solution). Furthermore, the flexibility of the cylindrical aggregates in the investigated E phase (aggregate volume fraction 0.27) is quantified in terms of an orientational order parameter ;:::0.9.

Accurate Anisotropic Water-Diffusion Measurements in Liquid Crystals

“…Our Stelar VTC87 temperature controller with its periment, it uses a constant gradient (to avoid errors due to gradient switching) and a longer (64-step in our case) phase flexible multiparameter regulation algorithm has been shown to provide good stability and quick response in other NMR cycle. The gradient strength in the CPTG-OE experiments was about 15 G/cm which led to a spread of the 2 H NMR probes (36). However, the bulk of the present gradient probe added a large thermal load to the regulation which made the frequencies of about 4 kHz.…”

NMR methods applied to anisotropic diffusion

“…On the other hand, a sample can often be orientated by magnetic field in a particular phase and thereafter by changing typically the temperature a new phase is reached which is not orientable but keeps the previously achieved uniform orientation. 55,57 Hence the relative orientation of the diffusion tensor and the gradient field can be simply changed even in an instrument equipped with a single gradient coil providing gradient in a single direction: the sample can be rotated instead.…”

Sequence-specific binding of counterions toB-DNA

“…A single Na ϩ site per 12 bp with an occupancy of 5% and a QCC of 1.3 MHz could produce a splitting as large as 1,500 Hz, depending on the orientation of the electric field gradient relative to the magnetic field (33,34). Reported 23 Na splittings from the cholesteric liquid-crystalline phase of ca.…”

“…Although these components may not represent distinct physical processes, the time scale is as expected for Na ϩ diffusion over the surface of the DNA duplex. This motion modulates the magnitude and orientation of the electric field gradient (motionally averaged by picosecond hydration shell dynamics) as the ion ''jumps'' across the grooves and diffuses around the helix axis (33,34).…”