Ion Binding in Liquid Crystals Studied by NMR V. Static Quadrupolar Effects for Alkali Nuclei

Abstract

Nuclear magnetic resonance spectra of ⁷Li⁺, ²³Na⁺, ³⁹K⁺, ⁸⁵Rb⁺, ⁸⁷Rb⁺ and ¹³³Cs⁺ counter-ions in lyotropic liquid crystalline phases were recorded. For all these nuclei it was observed that static quadrupolar interactions cause splittings of the NMR absorption into component signals, First-order ²³Na quadrupole splittings are strongly affected by sample temperature, phase structure, sample composition, macroscopic alignment and amphiphile end-group. The competitive binding of the alkali ions was studied by ⁷Li⁺, ²³Na⁺ and ¹³³Cs⁺ NMR. The splittings were assumed to depend on four factors, i.e. (a) the degree of counter-ion binding to the amphiphilic surfaces, (b) the quadrupole coupling constant, (c) the degree of orientation of the electric field gradients and (d) exchange of counter-ions between microcrystallites. It is found that the rate of exchange between microcrystallites depends strongly on the counter-ions present, that sodium ions exchange more rapidly than water molecules, and that the counter-ion exchange proceeds at a lower rate if D₂O is substituted for H₂O. The presence of the counter-ion exchange process, which was established by comparing the temperature dependences of splittings for powdered and aligned samples, makes it often impossible to interpret splittings obtained with powdered samples solely in terms of molecular interactions. Counter-ion quadrupole splitting studies with phospholipid-containing lamellar samples show that the method constitutes a novel possibility to study ion binding in biological membrane systems as well as models of these.