Molecular dynamics and network analysis reveal the contrasting roles of polar solutes within organic phase amphiphile aggregation

Abstract

Amphiphile self-assembly in non-polar media is often enhanced by polar co-solutes. This is observed within many biphasic separations processes, where amphiphiles mediate transport of water and acid into organic solution. A myriad of competitive intermolecular interactions have thus far prevented a fundamental understanding of the individual and dual roles of these polar solutes upon amphiphile self-assembly in non-polar media. Toward this end, the current work employs classical molecular dynamics and intermolecular network 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, preferential self-solvation leads to large (H2O)n clusters that cause TODGA clusters to sorb to the (H2O)n periphery and supports 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 and encouraging local aggregation. Prior experimental observations of enhanced TODGA self-assembly under these conditions are attributed primarily to the role of water rather than co-extracted HNO3, thus providing valuable new insight into the means by which extractant aggregation can be tuned within separations processes. The different roles of polar co-solutes, that derive from their individual hydrogen bonding capabilities and competitive interactions in the context of preferred solvation environments, is of fundamental importance amphiphile behavior in non-polar media.