Polymeric nanoparticles (PNPs) of Poly (ε-caprolactam) or nylon 6 are synthesized by interfacial deposition in aqueous medium (non-solvent) and two different mixed solvents (ethyl acetate + ethanol and ethyl acetate+chloroform) in the presence TWEEN-80 and SDS as stabilizers. The PNPs are characterized by FT-IR spectra, DSC and XRD measurements. The particle size is determined by TEM analysis and dynamic light scattering (DLS) method. The mixed solvents influence the size and physicochemical properties such as acoustical properties, degree of dispersion, melting temperature, and stability (zeta potential) of the suspensions. The measured ultrasonic velocity correlates with size and computed absorption co-efficient correlates with degree of dispersion of the PNPs. Experimental density and ultrasonic velocity values are compared with those calculated using three different theoretical models.