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

25 November 2021, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Hypothesis: Amphiphile self-assembly in non-polar media is often enhanced by polar co-solutes, as observed upon amphiphile mediated transport of water and acid into organic solution. Such co-extraction precludes understanding the individual roles of polar solutes upon self-assembly. Using this liquid-liquid extraction (LLE) system as a test-bed, we hypothesize that co-solute competition and hydrogen bond (HB) characteristics cause different size/shape distributions of assembled amphiphiles and alter self-assembly mechanisms in non-polar solvents. Experiments: Concentration dependent classical molecular dynamics simulation and intermolecular network analyses identified the correlating relationships between HB properties of H2O and HNO3 upon the aggregation of N,N,N,N-tetraoctyl-3-oxapentanediamide (TODGA), a prevalent LLE amphiphile extractant. Findings: Concentration dependent competition of hydrogen bonding fundamentally impacts amphiphile self-assembly in non-polar media. H2O bridges TODGA and enhances self-assembly, 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 form extended aggregation. HNO3 restricts the (H2O)n size by disrupting the HB network. At large [H2O]org, HNO3 modulates TODGA self-assembly from extended to local aggregation. We attribute prior experimental observations 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.

Keywords

Self assembly
amphiphiles
polar solutes
graph theoretic analysis
molecular dynamics
TODGA
ALSEP

Supplementary materials

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Description
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Supporting information
Description
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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