Hydrophobic molecules infiltrating into the poly(ethylene glycol) domain of the core/shell interface of a polymeric micelle: evidence obtained with anomalous small-angle X-ray scattering.

Polymeric micelles have been extensively studied as nanoscale drug carriers. Knowing the inner structure of polymeric micelles that encapsulate hydrophobic drugs is important to design effective carriers. In our study, the hydrophobic compound tetrabromocathecol (TBC) was chosen as a drug-equivalent model molecule. The bromine atoms in TBC act as probes in anomalous small-angle X-ray scattering (ASAXS) allowing for its localization in the polymeric micelles whose shape and size were determined by normal small-angle X-ray scattering (SAXS). Light scattering measurements coupled with field flow fractionation were also carried out to determine the aggregation number of micelles. A core-corona spherical model was used to explain the shape of the micelles, while the distribution of bromine atoms was explained with a hard-sphere model. Interestingly, the radius of the spherical region populated with bromine atoms was larger than the one of the sphere corresponding to the hydrophobic core of the micelle. This result suggests that the TBC molecules infiltrate the PEG hydrophilic domain in the vicinity of the core/shell interface. The results of light scattering and SAXS indicate that the PEG chains at the shell region are densely packed, and thus the PEG domain close to the interface has enough hydrophobicity to tolerate the presence of hydrophobic compounds.