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Abstract: Elaboration and charaterization of a new dialysis membrane by sorption of polyelectrolytes nanolayers. This work aims at improving dialysis membrane selectivity and adaptability to the media to be treated by surface modifications using a polyelectrolyte. The poly(acrylonitrile-co-methallyl sulfonate) membrane, denoted AN69, has been modified by sorption of a cationic polyelectrolyte, the poly(allyldimethylammonium chloride), denoted PDADMA, on the membrane from its dilute solution. The modified membrane was characterized by measuring the selectivity towards lithium ions (Li+) in dialysis by measuring its transport number, the diffusional flow of few electrolytes, the hydraulic permeability and the streaming potential. The results showed that the membrane charge is reversed upon sorption of the PDADMA, indicating a drastic change in the membrane charge. The combination of Li+ transport numbers and streaming potential measurements show with a good evidence some modifications of the membrane on the surface and inside their pores; although the adsorption is reversible, it is so strong – due to multiple membrane–polyelectrolyte electrostatic interactions – that the layer is only removed by conditioning in NaCl aqueous solutions with ionic strength higher than 2 mol/L. The modified membrane appeared to be more selective toward ions Li+, Na+, K+, Ca2+, and Mg2+. Finally, this kind of surface modification can be easily engaged in order to better adapt the membrane surface to its end use. Furthermore, in the present article, we have described in detail, for the first time, a new streaming potential apparatus especially designed for this study. To cite this article: M. Pontié et al., C. R. Chimie 08 (2005) . [Copyright &y& Elsevier]