(171h) Elucidating Tunable Diffusioosmotic Flow Reversals Due to Ion-Ion Electrostatic Correlations

Diffusioosmosis is 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 a general mixture of 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, we show that, in a single monovalent or multivalent electrolyte, ion-ion electrostatic correlations can cause a unique reversal in the direction of diffusioosmosis. Such a reversal is not captured by existing theories and occurs between O(10-3) and O(0.1) molar concentration, highlighting its practical relevance. Second, we show that the diffusioosmosis reversal persists in a general mixture of electrolytes. The ionic strength of the mixture at which the diffusioosmosis reversal occurs is different depending on the concentration of the individual electrolyte, underscoring the tunability of the reversal. The mathematical model developed here can be used to design diffusioosmosis, which is central to applications such as species separation, enhanced oil recovery, and blue energy harvesting.