Confinement effects on dipole-dipole interactions in nanofluids: the role of equilibrium fluctuations

When spin bearing molecules are enclosed in an elongated nanovoid, and each molecule diffuses across the whole volume in times much shorter than the characteristic times of NMR spin dynamics, a time independent average dipolar Hamiltonian emerges which couples the spins in each molecule equally with the spins in any other molecule. Baugh et al. [Science, 134, 1505 (2001)] have observed the remarkable orientation dependent linewidth in such a situation, and offered a convincing interpretation using the techniques of solid state NMR. In the present paper I show that, contrary to previous expectations, the same linewidth can be predicted with liquid NMR techniques, provided that one takes into account equilibrium fluctuations in the estimation of the dipolar field.