Octanoic acid

The following analytical standard grade commercial chemicals were used: heptanal, octanal, hexanoic acid from Aldrich Chemicals (Milwaukee, WI, USA), and nonanal, decanal, heptanoic acid and octanoic acid from Acros Organics (Geel, Belgium). Sodium sulphate, DCM, chloroform and diethyl ether were purchased from Fisher Scientific (Hampton, NH, USA) and sulfuric acid was purchased from Aldrich Chemicals (Milwaukee, WI, USA),
The stock solution incorporating standards was prepared by dissolving 1 μL of each standard in MilliQ water (Millipore, Bedford, MA, USA) in a 100 mL measuring flask. The working solutions were prepared by diluting the standard stock solution, in the range 3 to 80 μmol/L.

Octanoic acid (Acros Organics, Pittsburgh, PA, USA) and trihexylamine (Sigma Aldrich, St. Louis, MO, USA) were used as received for the preparation of the T6A OA organic mixture. Hydrochloric acid (37%, Merck, Kenilworth, NJ, USA), nitric acid (65%, Merck), bromic acid (47%, Merck), hydroioidic acid (55%, BDH Chemicals, Radnor, PA, USA), and perchloric acid (70%, Acros Organics) were diluted with Milli-Q water (deionized with Simplicity Millipore deionizer, Millipore, Burlington, MA, USA) to the pH values at which they were used. Sodium hydroxide (>99%, Merck) and potassium hydroxide solution (87.5%, J. T. Baker, Radnor, PA, USA) were similarly diluted.
trihexylamine and Octanoic acid were mixed in 1:1 mole ratio, based on weighings with an analytical balance. After initial mixing, samples were stirred 20+ hours, using a magnetic stirrer.
For the extraction experiments, 4.00 mL of the organic phase were mixed with an equal volume of the aqueous phase and shaken, overnight on a Beun De-Ronde wrist-shaker. Phases were then isolated using a Medilite centrifuge, and the resulting phases were separated by a syringe.
pH measurements of the aqueous phase were conducted with a Mettler-Toledo Seven2Go apparatus (Columbus, OH, USA) and a a Mettler-Toledo InLab Expert electrode. The pH meter was calibrated, prior to each round of measurements, using Certipur buffer standards (Merck).

Pb(II), Ni(II), Co(II), Zn(II), Mn(II), and Cd(II) solutions were prepared by dilution, starting from the stock solutions (1000 mg L⁻¹) Merck (Darmstadt, Germany) to achieve the intended concentrations. Standard metal solutions used for calibration of the AAS spectrophotometer were prepared by diluting their respective 1000 mg L⁻¹ stock solutions. For HDES preparation, L(-) menthol (CAS number: 2216-51-5; mass fraction purity: ≥99%), thymol (CAS number: 89-83-8; mass fraction purity: ≥99%), octanoic acid (CAS number: 124-07-2; mass fraction purity: ≥99%), dodecanoic acid (CAS number: 143-07-7; mass fraction purity: ≥99%), were obtained from Acros Organics (Antwerpen, Belgium) and decanoic acid (CAS number: 334-48-5; mass fraction purity: ≥99%) was procured from Alfa Aesar (Ward Hill, MA, USA). Disodium EDTA (CAS number: 6381-92-6; mass fraction purity: ≥99%) were obtained from Sigma-Aldrich (St. Louis, MO, USA).