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

28 January 2022, Version 3
This content is a preprint and has not undergone peer review at the time of posting.


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.


Self assembly
polar solutes
graph theoretic analysis
molecular dynamics

Supplementary materials

Supporting information
Simulation snapshots, cluster analysis details under C1 and C2 conditions, cluster analysis results for studied systems, extended aggregation between water and acid molecules, short-range non-bonded interaction energy vs. simulation time plots, oxygen-oxygen RDF profiles for TODGA-water systems, diffusion coefficient and SASA results.


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