In this work, five 1,1,3,3-tetramethylguanidine (TMG)-based ionic liquids (ILs) with [TMG] cations as well as monocarboxylic acid anions {[CH3-(CH2)(n)-COOH], where n = 2, 3, 4, 5, and 6} were synthesized and characterized by H-1 NMR and C-13 NMR CO2 absorption capacity was studied for even number carboxylic acid anions [TMG] [But], [TMG] [Hex], and [TMG] [Oct]. The maximum absorption capacity of CO2 was observed for [TMG] [Oct], which clearly indicated that the increase in chain length increases the absorption capacity. The physicochemical properties such as density (rho), speed of sound (u), viscosity (eta) and refractive index (n(D)) were measured as a function of temperature over the range from 293.15 to 343.15 K at atmospheric pressure (0.1 MPa). The experimental density values were fitted with a second-order polynomial equation and correlated with the expected density proposed by the Gardas Coutinho model. The thermodynamic properties, such as thermal expansion coefficient (alpha), isentropic compressibility (beta(s)), and free length theory (L-f) were calculated. The strength of ionic interaction between the ions was estimated by calculating lattice potential energy (U-POT) and standard entropy (S degrees) from experimental data. The experimental viscosity values were fitted by applying the Vogel-Fulcher-Tammann equation and correlated with an Arrhenius-type equation. Thermal decomposition temperatures (T-d) were investigated using TG analysis. The intermolecular interactions of the ILs have been analyzed with the experimental refractive index data at various temperatures, and the effects of carboxylate anion chain length and temperature on physicochemical properties were also investigated.