Molecular Dynamics and Network Analysis Reveal the Contrasting Roles of Polar Solutes Within Organic Phase Amphiphile Aggregation

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 role 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\textsuperscript{$'$},N{$'$}-tetraoctyl-3-oxapentanediamide (TODGA), a prevalent amphiphile extractant used in metal ion separations. In the absence of acid, and at low water concentration, H$_2$O is found to promote local dimer and trimer formation of TODGA, however as [H$_2$O]$_{org}$ increases, preferential self-solvation leads to large (H$_2$O)$_n$ clusters that cause TODGA clusters to sorb to the (H$_2$O)$_n$ periphery and supports extended aggregation. Addition of HNO$_3$ 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 HNO$_3$, 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.