Halloysite nanoclays (HNT) have been reported to have the potential to be utilized in many different applications such as drug delivery, environmental remediation, wastewater treatment and food packaging materials. In order to make HNTs more compatible in the medium they are incorporated into such as a polymeric matrix, purification and surface modification steps may be critically important to achieve the desired functionality. Non-homogeneous dispersion of pristine HNTs in many polymer matrices is a commonly observed phenomenon in polymer nanocomposites, negatively affecting the mechanical strength, gas permeation and thermal properties of final polymeric materials. Unfortunately, purification and sorting of HNTs for a uniform size distribution using conventional mineral separation techniques is difficult. In this study, HNTs were obtained with a uniform size distribution without any agglomerations by surface functionalization with polydopamine (PDA). The water solubility of PDA resulted in a stable colloidal suspension of PDA functionalized HNTs (DHNTs), which were first subjected to ultrasonic treatment, followed by centrifugation at different rpm values (2000, 6000 and 11000) to obtain HNTs that were purified according to their hydrodynamic volumes and sizes. Purified DHNTs were thermally treated at 350°C to remove polydopamine coating to obtain pure HNTs with a uniform size distribution. LDPE/HNT nanocomposites were prepared next, by melt-compounding using a twin-screw extruder. The dispersion of HNTs in the polymer matrix and the effect of homogenous-sized HNTs on mechanical and thermal properties as well as ethylene scavenging capacity of corresponding films were investigated in comparison with pristine HNTs.