Energetics of poloxamer micellization at normal and high pressures

Abstract

Micellization of a poloxamer P105 in diluted aqueous solutions was first investigated by means of high-sensitivity differential scanning calorimetry at different pressures up to ∼160 MPa. The temperature dependence of partial heat capacity of the poloxamer clearly indicates that the unimers have the random coil conformation while the micelles are built of a “dry” PO core and hydrated EO shell. Independently of the polymer concentration, the micellization temperature increases upon increasing pressure while the enthalpy decreases. Based on these dependences, the micellization increments of the partial volume Δtv as well as the coefficients of thermal volume expansion and compressibility, Δtα and Δtχ, of the poloxamer were calculated. They prove that the dehydration of PO hydrophobic blocks makes the main contribution to the micellization energetics. We demonstrate that poloxamer micelles are formed via a nucleation stage of unimer dimerization followed by consecutive energetically favored attachment of unimers to the dimer and further to the growing micelle.