A Molecular-scale Perspective of Competitive Interactions Induced by Phase-modifiers in Organic Solvents

The self-assembly of amphiphiles leads to complex, multi-component soft-matter structures that underpin a wide-range of colloidal and engineering applications. Separations methods, for eg. liquid-liquid extraction, are heavily influenced by soft-matter self-aggregation. In addition to the amphiphile extractant, so-called "phase-modifier" amphiphiles are often added into the organic phase to influence the self-assembly process with the aim of preventing undesirable phase phenomena. It is believed that the behavior of a phase-modifier can be explained in the context of "cosurfactant" and/or "cosolvent" characteristics. However, there remain experimental challenges for identifying the true composition of aggregates, which results in dearth of molecular level knowledge on the phase-modifier's working principles. This work employs molecular dynamics with graph theoretical analyses to unravel the working mechanism of phase-modifiers. We focus upon N,N-dihexyloctanamide (DHOA) and tributyl phosphate (TBP) that are commonly used as phase-modifier with actinide selective amphiphile extractant N, N, N', N'-tetraalkyl diglycolamide (TODGA). We demonstrate that the hydrogen bonding ability of the modifiers introduces competition to the interactions among extractant and polar solvent molecules (e.g., water and nitric acid). As an important consequence, the modifiers limit the association of polar solvent molecules similar to a chaotropic agent, and therefore hinder the prerequisite steps of developing a microemulsion by restricting the formation of large polar cores. The hydrogen bonding ability of a phase-modifier allows it to sequester solvent molecules from the TODGA extractant, therefore decreasing the solvent-assisted self-aggregation of primary extractant and supporting the well-documented observation of increase in critical aggregate concentration from the prior experimental studies.