2.5. Solvatochromic Solvent Parameters

The dyes 4-nitroanisole, N,N-diethyl-4-nitroaniline, 4-nitroaniline, and Reichardt’s dye (see Figure 2) were used to obtain the parameters π, β, and α. Ca 20 mg was added to 5 mL of water and stirred for at least one hour to obtain a concentrated solution for the dyes 4-nitroanisole and 4-nitroaniline. A concentrated solution of Reichardt’s dye was prepared in the same way but in ethanol instead of water due to poor solubility. Of these concentrated solutions, ca 0.2 μL was added to cuvettes containing 1.5 mL of the sample and stirred before being measured. N,N-diethyl-4-nitroaniline was added directly to the samples to be measured. The instrument used for measurements at room temperature was a SPECORD 205 UV-VIS spectrophotometer from Analytik Jena (Jena, Germany), and measurements with temperature control were conducted on a Cary 4000 spectrophotometer from Agilent. Plastic (Beijing, China) disposable cuvettes with a light path length of 12.5 mm were used throughout the measurements, and water was used as the background. The maximum absorption of the peaks of interest ranged from 0.1 to 1.1. The Kamlet-Taft parameters were calculated according to Equations (4)–(7) [21,22,23].

The structures of the solvatochromic dyes: (1) 4-nitroanisole; (2) N,N-diethyl-4-nitroaniline; (3) 4-nitroaniline; (4) Reichardt’s dye.

This study followed the methods described in the articles by Kamlet and Taft and used the dye pairs (2) and (3) to obtain β and the pair (1) and (4) to obtain α.