Noncovalent assembly of nitroxide spin labels in Laponite films: Formation of one-dimensional Heisenberg antiferromagnets

Laponite is a synthetic layered silicate with a particle size of 25 nm and a basic chemical composition similar to that of the naturally occurring smectite clay hectorite. Aqueous solutions of Laponite and nitroxide spin labels derived from 2,2,6,6-tetramethyl-1-piperidinyloxyl form gels which when air-dried form shiny orange films. Phase contrast scanning force microscopy shows that the freshly prepared films have surfaces with islands of spin label embedded in the Laponite matrix. X-ray diffraction shows a spacing between clay layers of about 1.54 nm and an interstratified phase with spacings of 0.9 nm. Angle-dependent electron spin resonance of the films shows that the spin probes produce a signal whose line width varies as |3 cos² θ - 1|^4/3 where θ is the angle between the perpendicular to the plane of the film and the applied magnetic field. These line width studies and supporting line shape studies provide evidence for noncovalent assembly of nitroxide radicals in the Laponite film into structures which act as one-dimensional Heisenberg antiferromagnets. The films are metastable, and the antiferromagnet properties degrade considerably over about 60 days.