The interfacial properties of poly-2-(dimethylamino) ethyl methacrylate-N-alkyl quaternized with octyl, decyl, dodecyl and tetradecyl bromide are studied from the chloroform to the water/chloroform interface. In order to estimate the entropy of the adsorption process, the interfacial tension was measured at several temperatures according to the Du Nouy method. The interfacial tension decrease is highly dependent on the hydrophilic-hydrophobic balance between the quaternary ammonium groups and the aliphatic side chains. The shorter the polyelectrolyte side chains the higher the interfacial activity. The excess interfacial concentrations, Gamma, are determined according to the Gibbs-Szyszkowski equation and the areas covered by monomer unit at the interface sigma are higher than those reported for condensed monolayers of single low molecular weight amphipathic molecules, such as alcohols and carboxylic acids. The free energies of adsorption, Delta G(ads)degrees, reveal that the adsorption process is comparable to that occurring in small molecules systems. The linear dependence of Delta G(ads)degrees with the number of carbon atoms on the side chain allows estimating the contribution per methylene side group between +0.32 and +0.39 kJ mol(-1). On the other hand, the contribution from the polar residue to Delta G(ads)degrees ranges between 40.4 and 45.7 kJ mol(-1). The dependence of the interfacial tension with temperature suggests that entropy is the driving force factor determining the adsorption process. (C) 2010 Elsevier B.V. All rights reserved.