Extremely Slow Counterion Dynamics in Xanthan Liquid Crystal through ²³Na and ¹⁴N NMR

Abstract

To describe the structure and dynamics in a lyotropic xanthan liquid crystal, we have done 23 Na and 14 N NMR experiments. The spectra are consistent with a powder average over randomly oriented liquid-crystalline domains. From 14 N quadrupolar echo experiments it was concluded that the exchange among the several micron sized domains is slow with residence times in the millisecond range. Conventional spin-lock experiments ( 14 N and 23 Na) as well as spin-lock experiments filtered through the quadrupolar spin polarization order ( 23 Na only) revealed the existence of an extremely low frequency, submegahertz dispersion in the spectral densities pertaining to both sodium and ammonium counterions. The corresponding correlation times are on the order of tens of microseconds and agree with a structural correlation length on the order of 500 nm. The latter correlation times reflect the rearrangement of counterions among polymer segments over the same spatial extent as the polymer persistence length.

Direct detection of nitrogen-14 in solid-state NMR spectroscopy

“…The above publications spurred an increase in the use of 14 N NMR as a probe of LCs, lipid bilayers and related systems, with a number of reports studying 14 N linewidths and relaxation [180][181][182][183][184][185][186][187][188][189][190][191], and others investigating quadrupolar splittings [192][193][194][195][196][197][198][199][200][201][202][203][204][205][206][207][208][209][210].…”

Thermal relaxation and coherence dynamics of spin 3/2. I. Static and fluctuating quadrupolar interactions in the multipole basis

“…This often is the case in soft-condensed matter, where the correlation times can be rather long, but with relatively small values of the corresponding coupling constants. Equation [6] now becomes d*/dt ϭ Ϫi͓H* S , *͔ ϩ f ͑*͒ [7] with the relaxation superoperator…”

“…This inhomogeneous broadening makes an accurate measurement of the spectral density at zero frequency J 0 in ordered material difficult, if not impossible. Recently, it has been shown that in the case of a powder-type variation of the static quadrupolar coupling, slow dynamics is probed more efficiently by applying a lock through a radio-frequency field (6). Another method to effectively eliminate the inhomogeneous broadening is based on delayed acquisition (37)(38)(39).…”

“…This makes an accurate measurement of the natural line widths difficult, if not impossible. If the signals can not be separated to a sufficient extent, slow dynamics is probed more efficiently by applying a lock through an RF field as will be reviewed in part II (6,18).…”

“…Quadrupolar spin probes are becoming increasingly important in a wide range of applications ranging from the investigation of porous materials (1), (bio)polymers in the liquid state (2)(3)(4)(5)(6), through biological fluids (7), to the diagnosis of pathology in humans via magnetic resonance imaging (MRI) (8). Among those probes, the naturally occurring spin quantum number 3/2 nuclei 7 Li, 23 Na, 39 K, 87 Rb, 35 Cl, 81 Br, and 131 Xe constitute an important class.…”