Solubility and solid phase studies of isomeric phenolic acids in pure solvents

Abstract

The solubilities of gallic, protocatechuic, gentisic or α-resorcylic acids were measured in nine pure solvents (water, methanol, ethanol, 1-propanol, 2-propanol, 2-butanone, ethyl acetate, acetonitrile and dimethylformamide) at 298.15 K and 313.15 K, using the analytical isothermal shake-flask method. Additionally, solid phase studies of the selected phenolic acids were carried out using differential scanning calorimetry (DSC) and X-ray diffraction (XRD), giving access to important data on melting properties as well as on the structure of the solid phase before and after the dissolution. The NRTL-SAC model coupled to the reference solvent approach (RSA) were applied to correlate the solubility data in a set of seven solvents and, after used to predict the solubility in 1-propanol and dimethylformamide. Average relative deviations (ARD) between 28 and 40% for the correlation and between 16 and 59% for the predictions were obtained. These values are within the order of magnitude usually found for such type of semipredictive models, using a limited set of data.

Solvent and temperature effects on the solubility of syringic, vanillic or veratric acids: Experimental, modeling and solid phase studies

“…In the case of systems containing alcohols, the solubility decreases with an increase of the alky chain of the alcohol. Surprisingly, much higher solubilities were found for systems with DMF, behavior that was also observed for dihydroxybenzoic acids [4]. In all cases, the solubility increases with temperature.…”

“…An effective alternative is the use of predictive and semi-predictive thermodynamic models to estimate SLE diagrams. In this context, the NRTL-SAC model proposed by Chen and Song [30] has shown reliable results for predictions of solubility of phenolic compounds in water and organics solvents [4,29,[31][32][33]. Alternately, due to the high uncertainty of the melting enthalpies of the studied acids, the reference solvent approach (RSA) was combined with the NRTL-SAC model [32,34] under the same conditions as mentioned before.…”

“…The solubility experiments were carried out by the isothermal shake-flask method, which is described thoroughly elsewhere [4,48]. Saturated solutions containing a small amount of solid in excess were prepared in a flask with around 50 ml of solvent.…”

“…Due to its semi-predictive nature and successful application in previous studies [29][30][31][32][48][49][50], the NRTL-SAC model was chosen to describe the solubility of the solutes studied in this work in pure solvents. A detailed description of the model, with its fundaments and equations, is presented elsewhere [30,49].…”

“…Following our previous work on the solubility of hydroxybenzoic acids (gallic, α-resorcylic, gentisic and protocatechuic acids) [4], in this work our focus is on benzoic acids containing the methoxy group, namely syringic, vanillic and veratric acids. Interesting structural differences can be seen in Fig.…”