High permeance membranes selective towards small ions and solutes are required to produce high-quality product water. Conventional semi-aromatic polyamide thin-film composite (TFC) nanofiltration membranes prepared via interfacial polymerization of diamine, e.g., piperazine (PIP) and acid chloride, do not show the precise separation of small solutes (<200 Da). To achieve this, we introduced an aromatic diamine group, e.g., m-phenylenediamine (MPD), within the semi-aromatic polyamide structure to achieve selective separation between small solutes with enhanced permeation upon solvent activation. Herein, we report the fabrication of polyamide nanofilm composite membranes using a mixture of PIP and MPD as a diamine precursor and react with trimesoyl chloride (TMC) on top of the ultrafiltration (UF) supports. Nanofilm composite membranes prepared with 1.0 wt% PIP and 0.1 wt% MPD on crosslinked polyimide (XP84) UF support show a seven times higher selectivity of ∼ 59.0 between glycerol and glucose than the conventional nanofiltration membranes (selectivity of ∼ 8.2). Membranes also show a ∼ 55.8% increase in pure water permeance (PWP) with identical Na₂SO₄ rejection (99.27% – 99.53%) and an increased selectivity between NaCl to Na₂SO₄ (from 43.7 to 98.9) after solvent activation with DMF. Additionally, after solvent activation, mixed-diamine-based nanofilm composite membranes show a sharp and reduced molecular weight cut-off (MWCO ∼ 170 Da) compared to the nanofiltration membranes prepared from semi-aromatic polyamide. The permeation results conducted with small neutral solutes in the aqueous feed suggest the existence of pores of mean size ∼ 13.0 Å, with an enhancement of pore sizes ∼ 7 – 22.6% after DMF activation. The process of membrane fabrication is acceptable for large-scale manufacturing.