Thermodynamic Difference between Protocatechualdehyde and <i>p</i>‑Hydroxybenzaldehyde in Aqueous Sodium Chloride Solutions

The enthalpies of dilution of protocatechualdehyde and <i>p</i>-hydroxybenzaldehyde in the aqueous sodium chloride solutions were measured by using a mixing-flow microcalorimeter at 298.15 K. Densities of the ternary homogeneous systems at different temperatures (293.15, 298.15, 303.15, 308.15, and 313.15 K) were also measured with a quartz vibrating-tube densimeter. The homogeneous enthalpic interaction coefficients (<i>h</i>₂, <i>h</i>₃, and <i>h</i>₄) were calculated according to the excess enthalpy concept based on the calorimetric data. The apparent molar volumes (<i>V</i>_ϕ) and standard partial molar volumes (<i>V</i>_ϕ⁰) of the investigated system were computed from their density data. The variation trends in <i>h</i>₂ and <i>V</i>_ϕ⁰ with increasing salt molality were obtained and discussed in terms of the (solute + solute) and (solute + solvent) interactions. The experimental results showed that the molecular structures of protocatechualdehyde and <i>p</i>-hydroxybenzaldehyde, especially the number of hydroxyl groups, have evident influence on their thermodynamic properties. The thermodynamic data obtained in this work may be helpful for exploring the structure–function relationship of protocatechualdehyde and <i>p</i>-hydroxybenzaldehyde

Solvent and pH Dependences of Mixing Enthalpies of <i>N</i>‑Glycylglycine with Protocatechuic Acid

Protocatechuic acid (PA) is a natural phenolic compound which has been proven to have chemopreventive property against chemically induced carcinogenesis. The mixing enthalpies of PA with <i>N</i>-glycylglycine in sodium phosphate and potassium phosphate buffer solutions with different pH values have been investigated by mixing-flow isothermal microcalorimetry at <i>T</i> = 298.15 K. The heterotactic enthalpic interaction coefficients (<i>h</i>_<i>xy</i>) in the pH range of phosphate buffer solution from 3.0 to 8.0 have been calculated according to the McMillan–Mayer theory. Trends of the enthalpic pairwise interaction coefficients (<i>h</i>_<i>xy</i>) with increasing pH in both phosphate buffer solutions were obtained. The solvent and pH dependence of the <i>h</i>_<i>xy</i> were discussed in terms of molecular interactions between solvated solute molecules