NMR Transversal relaxivity of aqueous suspensions of particles of Ln3+-based zeolite type materials

Abstract

A series of zeolite-type silicates containing stoichiometric amounts of Ln³⁺ ions in the framework (Ln-AV-9 materials), with composition (Na₄K₂)(Ln₂Si₁₆O₃₈)·10H₂O (Ln = Nd, Sm, Eu, Tb, Gd, Dy) has recently been synthesized and characterized. They form paramagnetic microparticles, which as aqueous suspensions have negligible water¹H longitudinal relaxivities (r₁) for all Ln³⁺ ions studied and quite large transverse relaxivities (r₂). In this work we further analysed the size distribution of the Ln-AV-9 particles and their r₂* and r₂ relaxivities. The r₂* relaxivity effects are explained by the static dephasing regime (SDR) theory. The r₂ relaxivities appear to be strongly dependent on the interval between two consecutive refocusing pulses (τ_CP) in the train of 180° pulses applied. For long τ_CP values, the r₂ of the systems saturates at a value, which is always an order of magnitude smaller than r₂*. These features are explained by a crude model, which takes into account the residual diffusion effect in the static dephasing regime. The large microparticles, although not efficient in T₁ relaxation, are quite effective in enhancing T₂ relaxation, particularly at high magnetic fields. The r₂* values and the saturation values for r₂ were found to increase linearly with B₀ and μ₀². The largest transversal relaxation rate enhancements were observed for Dy-AV-9 with a saturation value of r₂ of 60 s⁻¹mM⁻¹ and a r₂* value of 566 s⁻¹mM⁻¹ at 9.4 T and 298 K.