Four modified hyperbranched polyethylene imines (PEIs) were synthesized by means of the alkylation of PEI. SAXS, viscosity, surface tension, and pyrene fluorescence emission were then used as techniques to examine the conformation and aggregation of the modified PEIs in aqueous solution, in the absence and presence of sodium dodecylsulfate (SDS). Analysis of the SAXS data showed that the radius of gyration decreases with an increase in the alkyl chain length of the polymer, while the viscosity data indicated a decrease in the intrinsic viscosity under the same conditions. The nonmodified PEI was not surface active, while the hydrophobically modified samples showed pronounced surface activity and the presence of hydrophobic domains. On addition of SDS, the onset of the formation of polymer-surfactant complexes was determined, indicating a decrease in the critical aggregate concentration with an increase in the alkyl chain length of the polymer backbone. (C) 2011 Elsevier Inc. All rights reserved.