A modified synthetic boundary expt. of anal. ultracentrifugation has been employed to examine, online, polyelectrolyte complex formation at flat interfaces yielding highly swollen membranes/networks. Systematic expts. with sodium alginate as a polyanion and chitosan and poly(L-lysine) as polycations identified the influence of concn., pH, molar mass, and polycation type on the membrane characteristics and the formation process. The membranes have been evaluated by five characteristics defined herein: total thickness, compactness, heterogeneity, symmetry, and growth. The results confirm the sensitivity of the method suited to elaborate general relationships for polyelectrolyte membrane design. [on SciFinder (R)]