The preparation, characterization, and application to nanofiltration of a membrane made of poly(acrylamidocaproic acid) partially neutralized with calcium using the thin-film composite membrane technique are reported in this study. The long hydrophobic side substituent prevents excessive swelling with water and binds the chain. The neutralized ionic group is designed to enhance the affinity of the membrane to water. By controlling the degree of neutralization, the performance of the membrane could be adjusted. The amount of water molecules participating in the hydration shell surrounding an ionic group was increased with increasing ionic concentration in the membrane. The exponential increase of flux with increasing degree of neutralization is due to the increase of the number and size of the hydration shell. The effect of the pressure and molecular weight of the solute on performance was also investigated. The surface interaction between the solute and the membrane seemed to mainly affect the characteristics of performance.