According to theoretical quantum-chemical calculations and experimental studies from the group of phenolic compounds of secondary metabolites, it is flavonoids that exhibit higher anti-corrosion properties in water environments. At the same time, according to literature data, for the extraction of flavonoids from a number of polyphenolic compounds, it is effective to use solvents of increased polarity, rather than 70% aqueous solutions of polar solvents, namely 50-60%, so the criterion parameter for comparison with theoretical calculations of solubility indices was chosen quantitative the content of polyphenolic compounds and flavonoids/anthocyanins, which were quantitatively determined using different degrees of dilution of binary mixtures of i-PrOH, EtOH with water. On the basis of theoretical (determined by the COSMO-RS computer modeling method of the solubility index values log10(x_RS)of the main representatives of the group of secondary metabolite compounds) (table) and experimental studies (fig.), it was established that for more effective extraction of flavonoids and phenolic acids in the group of polyphenolic compounds, it is effective use of binary systems: 60 - 70% solutions of PrOH/H2O, EtOH/H2O. The value of relative solubility log10(x_RS) is shown in the table. Since the logarithm of the best solubility is set to 0, it can be seen that the best solvent for extracting all solutes was ethanol/isopropanol (100%), which is therefore taken as the reference and all other solvents are referenced to it. The results table contains cells of different colors according to the values log10(x_RS) of the relative solubility of the tested compounds in the solvent system: values log10(x_RS) highlighted in green indicate that these solvents have a higher solubility index (from 0 to −1) when compared to other ethanol/water ratios in the solvent system; yellow indicates a medium solubility index (-1 to -4) compared to that of the reference solvent, while red values indicate a low solubility index (>-4).
Table 1 – Calculated solubility indices (log 10(X_RS)) of polyphenolic compounds determined by the COSMO-RS computer simulation method.
According to calculations, phenolic acid is the most soluble of the group of studied polyphenolic compounds in the solvent system. Phenolic acids have a high solubility index from EtOH (100%) and i-PrOH (100%) to a mixture of EtOH/H2O (50:50) and i-PrOH/H2O (50:50) demonstrate moderate solubility in the rest of the solvent system, even in water (100%). As for the flavonoid class, flavones (naringenin) and flavonols (quercetin) have better values log 10(X_RS)compared to anthocyanins, showing a low solubility index only in EtOH/H2O (10:90) i-PrOH/H2O (20:90) and water (100 %); the most soluble of them is luteolin, which has high solubility values log 10(X_RS) from the reference solvent EtOH (100%) to EtOH/H2O (50:50), followed by quercetin, which is still well soluble in EtOH/H2O (60:40). The anthocyanins cyanidin-3-O-glucoside and peonidin-3-O-glucoside have the lowest solubility values, with high solubility only in EtOH (100%) and EtOH/H2O (90:10), but they still maintain an acceptable solubility index environment between EtOH/H2O (60:40) and EtOH/H2O (50:50). According to experimental data, the binary systems EtOH/H2O and i-PrOH/H2O (50:50), as well as the systems EtOH/H2O and i-PrOH/H2O (60:40) were the most effective for extracting the total content of flavonoids.
The obtained results are consistent with literature data, where for a wide list of different types of plant raw materials, the most effective is the use of aqueous solutions of ethanol [1-3]. Analysis of literature data shows that extracts containing aldehydes, ketones, terpene compounds/monoterpene phenols, esters, alcohols are usually effective when used as volatile/vapor phase corrosion inhibitors and are effective in neutral aqueous solutions, as they can provide a barrier mechanism of protective actions due to the formation of an adsorption film. Plant extracts with an increased content of phenolic acids and flavonoids usually exhibit inhibitory properties in acidic environments, however, their effectiveness in neutral water environments and water-salt solutions is not always high, since the main mechanism of action is based on the formation of a protective layer of complex compounds with iron cations, however, usually when their concentration in the solution is insufficient, the effect of stimulating the corrosion process occurs. In order to obtain a system of extractants with an optimal qualitative and quantitative composition, it is necessary to optimize the composition of the extractant mixture for the studied types of plant raw materials on the basis of the obtained experimental data. Thus, further efficiency of combining individual solvents i-PrOH, EtOH, H2O into a three-component system of extractants is expedient.
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