Hydrosoluble cyclodextrin/poloxamer polypseudorotaxanes at the air/water interface, in bulk solution, and in the gel...

The interactions between poloxamer 407 (Pluronic F127, PF127) and two hydrophilic derivatives of beta-cyclodextrin, i.e., hydroxypropyl-beta-cyclodextrin (HPbetaCD; molar substitution (MS) 0.65) and methylated-beta-cyclodextrin (MbetaCD; MS 0.57), were characterized by means of surface tension measurements, pi-A isotherms, isoperibol microcalorimetry, 1H NMR, and rheometry. An effective complexation of poloxamer with the two CDs was evidenced as a change in the surface pressure of the pi-A isotherm of PF127 on a subphase of CD solution, with a positive excess being observed at the expanded region and a negative excess at the collapsed region. Such changes indicated that when the CD lies with the main axis perpendicular to the interface at low pressure no complexation occurs, but as the pressure increases and the CDs eventually change their arrangement to be with the main axis parallel to the interface, the amphiphilic copolymer can form polypseudorotaxanes. Addition of CDs to PF127 micellar solutions led to the exothermic rupture of micelles, a shift in the cmc toward higher values, changes in the chemical shifts of H3, H4, and H5 of MbetaCD and of the methyl groups of PF127, and an increase in the gel temperature. The interaction was stronger between poloxamer and MbetaCD, compared to HPbetaCD, with the stoichiometry of the polypseudorotaxanes being preferably ca. 1:20 in both cases. SEM images revealed formation of nanorods of stacked polypseudorotaxanes. Complexation with a high affinity constant between unimers and CDs in bulk solution was also evidenced by competitive displacement of methyl orange. Feasible structural models of the PF127:CD polypseudorotaxanes at both the air-water interface and in the bulk solution are proposed.