Complex formation between the triblock copolymer E(71)G(7)E(71), where E denotes ethylene oxide, the central hydrophobic block, phenyl glycidyl ether, is denoted as G, and the subscripts denote the block length, the surfactant sodium dodecyl sulfate (SDS), and the penicillins cloxacillin and dicloxacillin, which can be considered as hydrotropes has been investigated by dynamic light scattering (DLS) and isothermal titration calorimetry (ITC). The different copolymer/amphiphile systems have been studied at a constant copolymer concentration of 10 g dm(-3) and at different amphiphile concentrations. For all systems, the relaxation time distributions show a well-defined single mode with a shift toward slightly faster times, which indicates that the electrostatic interactions between the complexes is minimum, and so it is possible to estimate a size in terms of the apparent hydrodynamic radii. For ITC the amphiphile solutions were titrated in two different forms: as monomers (below the cmc) and as aggregates. The data indicate that the interaction between the surfactant and the hydrotropes with the copolymer occurs even at the lowest SDS concentration measured. (c) 2005 Elsevier Inc. All rights reserved.