A Pluronic block copolymer in H2O and D2O: The isotope effect on phase transition

Pluronic block copolymers are thermosensitive amphiphilic materials capable of self-assembling with various phases in water environments, depending on temperature and concentration. The phase transitions of high-concentrated Pluronic solutions in water (H2O) and heavy water (D2O) were observed via experimental rheology and small-angle x-ray scattering, to discover the isotope effect on the system phase and conformational properties. Aqueous solutions of Pluronic F68 in the presence of diclofenac sodium were used as the test material, being efficient drug delivery systems. Such materials undergo a reversible thermal phase transition, moving from a micellar liquid phase to a body-centered cubic solid structure with increasing temperature. The substitution of water by heavy water as a solvent has a noticeable influence on the Pluronic phase behavior. The liquid-to-solid transition temperatures of the Pluronic-based hydrogels in D2O are about 10 °C lower than those in H2O. Nonetheless, the deuterium isotope does not alter the characteristic dimensions of the Pluronic F68 self-assemblies. These results evidence that the properties and phase behavior of amphiphilic materials as Pluronics can be hugely influenced by the solvent isotopic composition. The significance of the findings also hinges on the crucial role that experimental methodologies play in understanding the link between the system macroscopic flow properties and the corresponding microstructural evolution. Indeed, the common practices for experimentally characterizing Pluronic solutions—such as rheology and scattering techniques—often provide their bulk and microstructural description without discerning between the material preparation in water and deuterated water, implicitly assuming no thermodynamical and physical chemistry modifications by isotopic substitution.