Enthalpies and heat capacities of solution of racemic N-methyl-substituted glycolurils in water at T = (278.15 to 313.15) K

• Enthalpies of solution of N-methylated chiral glycolurils in water were measured.
• Calorimetric experiments were performed at T = (278 to 318) K and p = 99.6 kPa.
• Standard molar heat capacities of solution of glycolurils in water were computed.
• A solute dissolution becomes more endothermic with decreasing its hydrophobicity.
• A solution heat capacity increases in proportion to number of methyl-substituents.

The molar enthalpies of solution of racemic (with enantiomer ratio of 1:1) 2-monomethyl-, 2,6-dimethyl-, and 2,4,6-trimethylglycolurils (2-MMGU, 2,6-DMGU and 2,4,6-TMGU, respectively) in water were measured calorimetrically in the temperature range between (278.15 and 313.15) K and at p = 99.6 kPa. Derived from experimental data, the standard (at infinite dilution) molar enthalpic characteristics of the dissolution process are positive by sign and increase with rising temperature as well as in a sequence of 2,4,6-TMGU < 2,6-DMGU < 2-MMGU. In the same way, the standard heat capacity of solution decreases distinctly, leading to the conclusion that the effect of hydrophobic hydration is more pronounced for the more N-methylated solute. It was suggested that, despite the presence of hydrophobic moieties (N-sited methyl groups) in the molecules considered, a hydrophilic constituent (via H-bonding) seems to be the predominant one in the total enthalpy effect of a solute hydration.

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