The goal of this work was to characterise the interfacial behaviour of amphiphilic beta-cyclodextrins modified at the primary face. It was considered that the oil/water system with spread modified cyclodextrins at the interface adequately mimics the interface encountered in the nanoprecipitation process, used for the preparation of nanocapsules. The effect of the presence of spread amphiphilic beta-cyclodextrins upon the interfacial behaviour of the Miglyol oil/water system was assessed from interfacial tension (gamma(i)) measurements at 20 degreesC. The results reveal the diminution of gamma(i) for all studied amphiphilic cyclodextrins. The highest reduction in gamma(i) was observed for the beta-CDs modified at the primary face for which the lowering in gamma(i) was comprised between 14 and 18 mN m(-1). For beta-CD-C-6, a beta-CD substituted at the secondary face, the decay of gamma(i) was much less pronounced (about 9 mN m(-1)). The results show also the influence of the hydrocarbon chain length upon the reduction in gamma(i): the cyclodextrin substituted with an aliphatic chain containing six carbons, '6-N-CAPRO-beta-CD', was more efficient than a beta-CD substituted with the aliphatic chain containing 14 carbons ('6-N-MYRISTO-beta-CD'). The presence of a NH amine function in the skeleton of the hydrocarbon chain conferred to the studied amphiphilic molecules an increased performance in lowering gamma(i) of the system. Surface pressure (pi)-area (A) compression isotherms of the modified CDs at the air/water interface provided additional light on the interfacial behaviour of these molecules. The data from the interfacial tension measurements corroborated those from the nanocapsule formation and provided evidence that substitution at the primary face with a relatively short aliphatic chain (six carbons) containing an amine function yielded homogenous and smaller in size nanocapsules. (C) 2002 Elsevier Science B.V. All rights reserved.