Characterization of micellar systems by the use of acoustic spectroscopy

Acoustic spectroscopy affords a new and unique way to characterize concentrated suspension and emulsion while avoiding the limitations imposed by dilution, an undesirable step, particularly with highly structured samples. This study sought to illustrate the potentialities of this technique by using it to characterize the self-assembling behaviour of Poloxamer 407 systems (3-25%, w/v), both alone or after the addition of various amounts of hydroxypropyl β-cyclodextrin (5-20%, w/v). Particle size and the microrheological extensional moduli (G′ and G″) of the systems were determined from acoustic parameters such as sound attenuation and speed. By monitoring the variation of the particle size and the rheological extensional moduli at increasing temperatures, it was possible to define and outline the Poloxamer 407 transitions and the effect of the HP β-CD on them. Poloxamer 407 micelle formation due to progressive dehydration occurred within a temperature interval of 15°C (including gelation) and was dependent on poloxamer concentration. Particularly, particle size of the aggregates changed within this interval. Mean diameters were 600 nm at the onset of micelle formation and decreased after the thermogel formation to more or less 75 nm. The presence of HP β-CD changed the basic self-assembling mechanism of Poloxamer 407 by increasing micelle formation and particularly thermogelation temperatures. The results confirm that acoustic spectroscopy offers a powerful method for characterizing heterogeneous systems, thus indicating its potential for applications in the pharmaceutical field.