In this work, four ammonium based ionic liquids were synthesized by neutralization process between N-methylcyclohexylamine (NMC) and acids such as pentanoic acid, hexanoic acid, heptanoic acid and octanoic acid. Synthesized ionic liquids were characterized by spectroscopic methods namely H-1 NMR, C-13 NMR and FT-IR. Further, thermophysical properties viz. density (rho) and speed of sound (u) values have been measured at atmospheric pressure (0.1 MPa) as a function of temperature over the range from (293.15 to 343.15) K. Moreover, refractive index was measured at T = 303.15 K and in turn molar refraction (R-m) and free volume (f(V)) were calculated using the Lorentz-Lorenz relation. The experimental density values fitted with linear equation and correlated with the expected density proposed by Gardas-Coutinho model. Knowledge of the experimental density and speed of sound are useful to calculate thermodynamic properties such as thermal expansion coefficient (alpha) isentropic compressibility (beta(S)) and free length theory (L-f). The strength of ionic interaction between the ions was estimated by calculating lattice potential energy (U-POT) and standard entropy (S-o) as proposed by Glasser. Thermal decomposition temperatures (T-d) were investigated using TG analysis. Finally, thermo physical properties of pure ILs were analyzed in terms of chemical structure of IL and its anionic part. (C) 2015 Elsevier Ltd. All rights reserved.