Non-covalent Assembly of Nitroxide Spin Labels in Laponite Films: Formation of One-Dimensional Heisenberg Antiferromagnets

Lana Chavez
Northern Arizona University

Laponite is a synthetic layered silicate with a particle size of 25 nm and a basic chemical composition similar to the naturally occurring smectite clay Hectorite. Aqueous solutions of Laponite and nitroxide spin labels derived from 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), 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 show that the spin probes produce a signal whose linewidth varies as | 3cos²θ -1|4/3 where θ is the angle between the plane of the film and the applied magnetic field. These linewidth studies and supporting lineshape studies provide evidence for non-covalent assembly of nitroxide radicals in the Laponite film into structures that act as one-dimensional Heisenberg antiferromagnets. The films are metastable and after about 60 days the films no longer have the properties of one-dimensional Heisenberg antiferromagnets.