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PreprintCC BYChemRxiv

Dihydrogen Phosphate as a Hydrophilic Co-Anion in Multicomponent Cholinium Ionic Liquids for Directional Solvent Extraction

  • Guoyue Xu1,2
  • Andrew Wise3,2
  • Jiyoon Kim3,2
  • Kuang Gu3,2
  • Yong Zhang4,2
  • Brandon L. Ashfeld3,2
  • Tengfei Luo1,2
4 affiliations
  1. 1Department of Aerospace and Mechanical Engineering
  2. 2University of Notre Dame
  3. 3Department of Chemistry and Biochemistry
  4. 4Department of Chemical and Biomolecular Engineering
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Abstract

Directional solvent extraction (DSE) recovers water from brine with low-temperature heat, and its output depends on how much water the solvent takes up on heating and releases on cooling. In an ionic liquid (IL) solvent, adding a hydrophilic co-anion is one way to tune this behavior. Here we screened co-anions X in 5:1 mixtures of cholinium bis(trifluoromethylsulfonyl)imide ([Choline][NTf2]) with [Choline][X] and selected dihydrogen phosphate. We followed liquids with nominal [NTf2] : [H2PO4] ratios of 5:1, 10:1 and 20:1 over five extraction cycles and determined the freshwater yield (FWY) of each run from the mass of the phase collected after cooling. Every run with these liquids had an FWY above the ≈ 0.30 wt% ◦C−1 reported for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and the ≈ 0.31 wt% ◦C−1 reported for a hydroxide-containing cholinium formulation. FWY was 0.35–0.38 wt% ◦C−1 in the first cycle. When the recovered liquids were reused, it rose to 0.55–0.58 wt% ◦C−1 in the second or third cycle and then declined to 0.40–0.49 wt% ◦C−1 in the fourth and fifth cycles. In molecular dynamics simulations, dihydrogen phosphate interacted strongly with water, as reported for hydroxide, and was enriched around water relative to its share of the anions.

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Cite

BibTeXRIS
BibTeX
@misc{xu2026dihydrogen,
  title = {{Dihydrogen Phosphate as a Hydrophilic Co-Anion in Multicomponent Cholinium Ionic Liquids for Directional Solvent Extraction}},
  author = {Guoyue Xu and Andrew Wise and Jiyoon Kim and Kuang Gu and Yong Zhang and Brandon L. Ashfeld and Tengfei Luo},
  howpublished = {ChemRxiv},
  year = {2026},
  month = oct,
  doi = {10.26434/chemrxiv.15009828/v1},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15009828/v1},
  note = {CC BY},
}
RIS
TY  - UNPB
TI  - Dihydrogen Phosphate as a Hydrophilic Co-Anion in Multicomponent Cholinium Ionic Liquids for Directional Solvent Extraction
AU  - Guoyue Xu
AU  - Andrew Wise
AU  - Jiyoon Kim
AU  - Kuang Gu
AU  - Yong Zhang
AU  - Brandon L. Ashfeld
AU  - Tengfei Luo
PY  - 2026
DA  - 2026/10/04
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15009828/v1
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15009828/v1
AB  - Directional solvent extraction (DSE) recovers water from brine with low-temperature heat, and its output depends on how much water the solvent takes up on heating and releases on cooling. In an ionic liquid (IL) solvent, adding a hydrophilic co-anion is one way to tune this behavior. Here we screened co-anions X in 5:1 mixtures of cholinium bis(trifluoromethylsulfonyl)imide ([Choline][NTf2]) with [Choline][X] and selected dihydrogen phosphate. We followed liquids with nominal [NTf2] : [H2PO4] ratios of 5:1, 10:1 and 20:1 over five extraction cycles and determined the freshwater yield (FWY) of each run from the mass of the phase collected after cooling. Every run with these liquids had an FWY above the ≈ 0.30 wt% ◦C−1 reported for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and the ≈ 0.31 wt% ◦C−1 reported for a hydroxide-containing cholinium formulation. FWY was 0.35–0.38 wt% ◦C−1 in the first cycle. When the recovered liquids were reused, it rose to 0.55–0.58 wt% ◦C−1 in the second or third cycle and then declined to 0.40–0.49 wt% ◦C−1 in the fourth and fifth cycles. In molecular dynamics simulations, dihydrogen phosphate interacted strongly with water, as reported for hydroxide, and was enriched around water relative to its share of the anions.
ER  -