Thermodynamic Difference between Protocatechualdehyde and p-Hydroxybenzaldehyde in Aqueous Sodium Chloride Solutions

The enthalpies of dilution of protocatechualdehyde and p-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 (h2, h3, and h4) were calculated according to the excess enthalpy concept based on the calorimetric data. The apparent molar volumes (Vϕ) and standard partial molar volumes (Vϕ0) of the investigated system were computed from their density data. The variation trends in h2 and Vϕ0 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 p-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 p-hydroxybenzaldehyde.