Cellulose acetate (CA) membranes have been widely used for water purification owing to several advantages, e.g., biocompatibility and low fouling rate. However, they suffer from a lower water flux compared to the other polymeric membranes. Therefore, in this study, CA membranes were blended with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (T-CNFs) containing quaternary alkyl ammonium (QA) carboxylates to improve their water flux. When increasing the alkyl chain length of the QAs, the positron lifetime and intensity of the CA membranes increased and decreased respectively, as revealed via positron annihilation lifetime spectroscopy. This indicated that the CA membranes had larger and fewer pores when using the T-CNFs containing QAs with longer alkyl chains. The pure water flux of these membranes also increased with the alkyl chain lengths of QAs although their rejection rate (Rj) decreased accordingly. However, they revealed a potentiality to be used as ultrafiltration membranes, allowing a 99% Rj for albumin. The tensile strength, strain to failure, and work of fracture of the CA membranes increased when blended with T-CNFs. Force measurements using the AFM colloidal probe technique showed that the adhesion between the membrane constituents depends on their surface chemistry. This indicated that the structural differences observed among the blended membranes may be due to the affinity between CA and T-CNF containing QAs with different alkyl chain lengths. This study demonstrates that the properties of CA membranes can be tailored by the addition of T-CNFs with different surface chemistries.