**2. Materials and Methods**

## *2.1. Chemicals*

The chemicals used in this study—3-amino-1-propanol (AP), 2-(methylamino)ethanol (MAE), 2-(butylamino)ethanol (BEA), tetrabutylammonium bromide (TBAB), and tetrabutylammonium chloride (TBAC)—were purchased from Sigma-Aldrich. TBAC was purified by double crystallization from acetone by adding diethyl ether. All salts were dried under reduced pressure before use, TBAB at 323 K for 48 h while TBAC at 298.15 K for several days. The corresponding information and the chemical structures of the DESs components are presented in Table 1.

**Table 1.** Provenance, mass fraction purity, and chemical structures of the compounds studied.


<sup>a</sup> As stated by supplier. <sup>b</sup> After crystalization, determined by potentiometric titration.

*2.2. Preparation of DESs and Their Aqueous Solutions*

DESs were prepared by mass with the same molar ratio of 1:6 salt to amino alcohol. The weighing was done using an analytical balance (Mettler Toledo) with the precision

of 0.1 mg. The standard uncertainty in the mass fraction was estimated to be less than ±<sup>1</sup> × <sup>10</sup><sup>−</sup>4. The combinations of the quaternary ammonium salts and AP/MAE/BEA were mixed at 353.15 K for 1 h using a magnetic stirrer in a fume hood until a homogeneous and uniform liquid without any precipitate was formed. Water content of DESs was measured using a Mettler Toledo Coulometric Karl–Fischer titrator (899 Coulometer apparatus from Metrohm) and it was found to be less than 0.0016 mass fraction. Table 2 displays the abbreviation of chemicals and DESs along with their molar mass, molar ratio, mass fraction, and water content.

**Table 2.** Abbreviations of chemicals and DESs along with their molar mass, molar ratio, mass fraction, and water content.


<sup>a</sup> The standard uncertainty of DES mass fraction composition is 0.0001. <sup>b</sup> Water content of DESs in mass fraction determined by Karl Fisher titration with the standard uncertainty ±0.0001.

Deionized, double distilled, degassed water with a specific conductance of 1.15 × <sup>10</sup>−<sup>6</sup> <sup>S</sup>·cm−<sup>1</sup> was used for the preparation of aqueous mixtures of DESs. The water contents in DES was accounted for upon solution preparation.

#### *2.3. Physical Properties Measurements*

#### 2.3.1. Density and Speed of Sound

Measurements of density and speed of sound were performed by using a digital vibration-tube analyzer (Anton Paar DSA 5000, Graz, Austria) with proportional temperature control that kept the samples at working temperature with an accuracy of 0.01 K. Experimental frequency for the measurements of the ultrasonic speed was equal to 3 MHz. The apparatus was calibrated with double distilled deionized and degassed water and dry air at atmospheric pressure according to the apparatus catalog procedure. The experimental uncertainty of density and ultrasonic velocity measurements was better than 35 × <sup>10</sup>−<sup>3</sup> kg m−<sup>3</sup> and 2 × <sup>10</sup>−<sup>1</sup> m s<sup>−</sup>1, respectively.
