Aqueous Solubility of Some Natural Phenolic Compounds

Abstract

In this work, the aqueous solubilities of two hydroxybenzoic acids (gallic and salicylic acid) and three phenylpropenoic acids (trans-cinnamic, ferulic, and caffeic acids) are addressed. Measurements were performed, as a function of temperature, between 288.15 and 323.15 K, using the shake-flask method for generating the saturated aqueous solutions, followed by compositional analysis by spectrophotometric and gravimetric methods. The pH values of the saturated aqueous solutions were measured by potentiometry. Additional thermodynamic properties, which are fundamental for a better understanding of the solubilization process, as well as necessary for the modeling studies, such as melting temperatures and fusion enthalpies were determined by differential scanning calorimetry (DSC). Apparent acid dissociation constants (Ka) were obtained by potentiometry titration. The measured data were modeled with the cubic-plus-association (CPA) equation of state (EoS). This EoS is applied, for the first time, for multifunctional associating molecules, and the results indicate that it can adequately be used to represent the measured and other literature data with satisfactory accuracy. 1. Introduction Fruits, vegetables, spices, and aromatic herbs are a natural source of phenolic compounds. These can be raw materials for the synthesis of different molecules with industrial interest, such as some drugs, cosmetics, antioxidants, antiseptics, and flavors; they also can be used in the preparation of resins, plasticizers, dyes, inks, and pharmaceutical products. 1 Many of these phenolic compounds have potential chemical and biological properties, such as antioxidant, chelating, free-radical scavenging, anti-inflammatory, antialergic, antimicrobial, antiviral, anticarcinogenic, chemoprevention (interfering with a disease process), and ultraviolet (UV) filtering properties. Most of them are phytochemical (they are not required for the normal functioning of the body, but they do have a beneficial effect on health, or an active role in the treatment of a disease). Because of their hydrogen-bonding ability and aromaticity, phenolic compounds can frequently act as free-radical scavengers, forming aryloxyl radicals (ArO ·). The stabilization of these radicals by other functional groups enhances the antioxidant