<p>Well-compatible with current industrial manufacturing and diverse applications, interfacial polymerization (IP) and IP-based polyamide nanofiltration membranes have evoked extensive research. Wherein, aside from tailoring pore sizes, regulating charged features of membranes remains challenging. Re-visiting principles of interfacial amide condensation, HCl byproducts are found exerting disparate effects in interfacial diffusion and ensuing amidation reaction. Exemplified by classic aqueous monomers–piperazine, H⁺ byproducts or equivalents, backflowing from IP zone and associating with amine groups of up-migrating piperazine or surrounding oligomers to alleviate the amidation, unexpectedly escort more amines to terminate polyamide chains for enhanced positive moieties. Resulting NF membranes synergize steric and charge effects for high Li⁺/Mg²⁺ selectivity of ~68 concurrent with water permeance of ~9 L m⁻² h⁻¹ bar⁻¹, superior in polyamide membranes. Hence, backflow H⁺ from the organic phase, typically cleared away in routine IP, is developed as the novel strategy to configure spatial charges of resulting membranes, further expanding IP’s versatility in fabricating functional membranes.</p>