EFFECTIVE FORCES BETWEEN DIAMAGNETIC AND PARAMAGNETIC-IONS IN D2O AT LOW AND MODERATE IONIC STRENGTHS - AN NMR RELAXATION STUDY

The dynamical behaviour of several pairs of dissociated, attractive an d repulsive, ions is investigated in aqueous solutions for ionic stren gths up to 1 mol-l(-1). The experimental information is provided by th e NMR longitudinal relaxation rates of the protons on the diamagnetic ions. The ionic solutions were chosen so that the main relaxation mech anism of these protons is due to the time fluctuations of their dipola r magnetic coupling with the electronic spins on the paramagnetic ions . This coupling strongly depends on the ion-ion potential of mean forc e (PMF) and on the ion self-diffusion coefficients. The interionic spa tial correlations and the associated PMF are derived from a new approx imation of the integral equations of the statistical mechanics of liqu ids. This formalism, which treats all the ions as discrete particles, rests on the infinite dilution PMF of the various ion pairs. It mixes a Born-Oppenheimer theory at infinite dilution with a sort of McMillan -Mayer approximation to take the ionic concentration into account. It goes beyond the Debye-Huckel screening theory, in which a continuous s creening charge distribution approximates the effects of the discrete surrounding ions. it is related to the concept of the local dielectric constants which replace the usual macroscopic dielectric constant and depend on the interionic distances. The self-diffusion coefficients o f the diamagnetic ions were measured by the NMR pulsed magnetic field gradient (PMFG) techniques applied to the resonant protons. In paramag netic solutions, where several protonated species coexist, special cau tion is required and this is discussed in detail. For all the investig ated solutions the theory well accounts for the observed variation of the NMR relaxation as a function of the ion charges, of the ionic stre ngth and of the NMR proton resonance frequency. The relaxation results predicted by the new approximation of the ion-ion PMF are compared wi th those derived from the simple Debye-Huckel screening formalism, whi ch can be reasonably used up to moderate ionic strengths.