Counterion N.M.R. in heterogeneous aqueous systems

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Abstract
The molecular origin of the electric field gradient (efg) experienced by an atomic ion in a heterogeneous aqueous solution has been examined with the molecular dynamics simulation technique. A model system comparising 50 sodium ions and 1000 water molecules, spherically enclosed by a hydrophobic interface carrying 50 carboxylate groups, was simulated during 50 ps. Previously, this problem has been approached only within the continuum-solvent approximation. Our results show that the counterion efg is strongly affected by the local solvent structure and, hence cannot be adequately described in a continuum-solvent model. In particular, the so-called polarization factor, which accounts for solvent ‘screening’ of the efg in continuum models, was found to be highly dependent on the ionic configuration. For the counterion population analysed here, the polarization factor thus spans the entire range from no screening (corresponding to ϵ4 = 1 in an electrostatic continuum model) to complete screening (ϵr = ∞). The results of this study are relevant for the interpretation of counterion N.M.R. relaxation rates and quadrupolar line splittings from a variety of heterogeneous aqueous systems, ranging from solutions of polyelectrolytes (e.g. DNA), micelles or colloids to microemulsions and lyotropic liquid crystals.