Clarifying pair interaction potential between poloxamer 407 micelles solvated into phosphate-buffered saline in...

Poloxamer 407 (P407) is used as an invaluable pharmaceutical nanocarrier due to its stable hydrogel and specific micelle-micelle interaction. To reveal the core mechanisms of the nanocarrier function and the effects of saline on the interactions, the pair interaction potentials between P407 micelles in phosphate-buffered saline (PBS) are clarified in the sol-gel-sol transition states without pre-limiting any types of interaction forces. The clarified pair potentials show that the primary minimum of the micelle-micelle interaction deepens and shifts toward the far position during the sol-to-gel transition. This result experimentally demonstrates the core mechanism of the specific crystallization described by the Alder transition theory as the structural expansion between P407 micelles. The effects of PBS on the interactions are identified as a deeper primary minimum in the pair potential and a stronger attractive interaction between P407 micelles than those in water. In the gel crystallization, the spatial distance between P407 micelles in PBS expands less than in water. This behavior leads to the formation of a more fluctuating crystal at the mesoscopic length scale. It is suggested that the fluctuating crystal structure in PBS results in a lower temperature shift during the de-gelation transition by temperature rise.