The hydrophobic and hydrophilic interactions in the system 2,4,6-trimethylpyridine–water in the vicinity of and above the lower critical solution temperature

The speed of sound and density for the system 2,4,6-trimethylpyridine+water were measured within the whole concentration range and for temperatures from 277 to 303 K. The molar volume, isentropic compressibility and the respective excess and partial quantities were calculated. The partial molar volume of 2,4,6-trimethylpyridine below the lower critical solution temperature (LCST) depends on the mixture concentration in a way typical of mixed solutes in water. The compressibility data confirm the mixed hydrophobic–hydrophilic nature of 2,4,6-trimethylpyridine. A qualitative model was proposed to account for the experimental results. In the low amine concentration range, the hydrophobic hydration is pronounced, while at the higher concentrations, hydrophilic interactions of hydrogen bond type are predominating. In dilute aqueous solutions, the water molecules probably form a clathrate-like structure around the 2,4,6-trimethylpyridine molecule that is most probably hydrogen-bonded to the host water network. An increase of the amine concentration causes a decay of the quasi-clathrate because of the shortage of water, while the N⋯HO bonds remain in principle intact. At temperatures above the LCST, only the water-rich phase exhibits properties characteristic of hydrophobic phenomena.