Hydrophobicity and thermodynamic derivatives for dilute aqueous solutions of amphiphiles

Abstract

Isobaric heat capacities per unit volume and densities were determined for 20 aqueous solutions of moderately large amphiphiles in the water-rich region. Measurements were made at atmospheric pressure in the temperature range (274.15-333.15) K, but for 6 of these systems, isobaric heat capacities per unit volume and densities were also obtained as a function of pressure within (0.13-60)MPa. From these data, magnitudes such as the isobaric molar heat capacity, excess heat capacity, apparent molar heat capacity or partial molar heat capacity were derived. Moreover, density measurements as a function of temperature and pressure provided data for the isothermal compressibility and isobaric thermal expansivity. Using this broad database, it is examined whether previously reported anomalous behaviors for tert-butanol and 2-butoxyethanol in water belong to a general scheme for aqueous solutions of amphiphiles. In the first place, a possible relation between the anomalous behavior of the studied solutions of amphiphiles and pure water anomalous thermodynamics, with the associated hypothetical second liquid-liquid critical point, is discussed. To this end, isobaric heat capacity, isothermal compressibility and isobaric thermal expansivity as a function of temperature and pressure are used. Further, the anomalous behavior, as observed in isobaric heat capacity as a function of composition and temperature, is interpreted in terms of hydrophobic effects. Molecular factors such as size, structure and degree of hydrophobicity are analyzed. Finally, the locus of extrema of various third derivatives of Gibbs energy in the x-T plane, also known as the Koga line, is studied with emphasis on two possible interpretations: (i) hydration of solute molecules and associated structural reorganization of pure water at higher temperatures and (ii) hydrophobic aggregation of solute molecules.