The Contrasting Role of Water and Acid Within Organic Phase Amphiphile Aggregation

Abstract

The self-assembly of amphiphiles is often modified by the presence of co-solutes and significant study has examined this behavior in aqueous systems. Much less is known about the role of polar co-solutes upon amphiphile aggregation within non-polar media, however such conditions are relevant to a variety of industrial processes - not the least of which are separations systems like those found in liquid-liquid extraction (LLE). Therein, surface active amphiphiles extract water, acid, and other solutes of interest. Intriguing increases to amphiphile aggregates have been experimentally observed upon water and acid extraction, however a myriad of competitive intermolecular interactions have thus far prevented a fundamental understanding of the individual and dual role of these solutes upon amphiphile self-assembly. Toward this end, this work employs classical molecular dynamics and graph theory analyses to deconstruct the individual affects of water and nitric acid upon the self-assembly of N,N,N',N'-tetraoctyl-3-oxapentanediamide (TODGA), a prevalent amphiphile extractant used in metal ion separations. In the absence of acid, and at low water concentration, H2O is found to promote local dimer and trimer formation of TODGA, however as [H2O]org increases, the preferential solvation of water with itself causes the formation of large water clusters that serve to link large TODGA clusters on the periphery (causing extended aggregation). Addition of HNO3 to the humid solutions disrupts the water hydrogen bond network and inhibits the formation of large water clusters - thus preventing extended aggregation behavior. We rationalize the prior experimental observations as being attributed primarily to the role of water in the self-assembly of TODGA rather than co-extracted HNO3, thus providing valuable new insight into the means by which extractant aggregation can be tuned within LLE processes. In addition, this work differentiates the role of polar solutes upon amphiphile self-assembly via their individual hydrogen bonding capabilities and competitive interactions that disrupt preferred solvation environments.