Physicochemical properties of diethylammonium (DEA)-based protic ionic liquids and their aqueous solutions were obtained using experimental measurements of density (rho) at different temperatures from T = (293.15 to 313.15) K and the speed of sound (w) at 298.15 K. Four DEA-based protic ionic liquids (PILs) with carboxylates as anions were synthesized. The standard entropy (S-0) and lattice potential energy (U-POT) for pure PILs were estimated from molecular volumes at 298.15 K obtained using experimental density data. Apparent molar volumes (phi(v)), isobaric expansivity (alpha), isentropic compressibility (beta(S)), and isothermal compressibility (beta(T)) have been obtained for aqueous solutions of PILs using experimental density and speed-of-sound data. These data have been further used to understand the electrostriction and concentration dependence of internal pressure. The Passynski method has been used to estimate concentration-dependent hydration-numbers of PILs. The results obtained have been discussed in terms of concentration- and temperature-dependent hydrophobic hydration, hydrophobic ion association, water structural changes, and so on. It has been observed that hydrophobic hydration persists in the entire studied concentration and temperature ranges. A critical examination of the data and results obtained reveals that the studied bio-ionic liquids can act as potential candidates in the pharmaceutical industry and in protein chemistry.