(465e) Diffusioosmotic Flow Reversals Due to Ion-Ion Electrostatic Correlations

Diffusioosmosis refers to the deterministic fluid motion induced by a solute concentration gradient. Existing theories of diffusioosmosis have neglected ion-ion electrostatic correlations which are important in multivalent or concentrated electrolytes. Here, we develop a mathematical model to compute the diffusioosmotic mobilities of binary symmetric electrolytes in a charged parallel-plate channel. We use the modified Poisson equation to model the ion-ion electrostatic correlations. We report two key findings. First, ion-ion electrostatic correlations can cause a unique reversal in the direction of diffusioosmosis. Such a reversal is not captured by existing theories, occurs at ≈ 0.4 M for a monovalent electrolyte, and occurs at a much lower concentration ≈ 0.003 M of a divalent electrolyte. Second, we predict a separate diffusioosmotic flow reversal, which is not due to electrostatic correlations but the competition between the underlying chemiosmosis and electroosmosis. However, electrostatic correlations can radically change how this flow reversal depends on the channel surface charge and ion diffusivity between a concentrated and a dilute electrolyte. The mathematical model developed here can be used to design diffusioosmosis, which is central to applications such as species mixing and separation, enhanced oil recovery, and reverse electrodialysis.