Tuning Pluronic F127 phase transitions by adding physiological amounts of salts: A rheology, SAXS, and NMR investigation

Specific ion (Hofmeister) effects in colloid and biological systems represented a scientific challenge for more than 100 years. Recently, possible applications, based on their rationalization, are emerging. Here, Cl^-, SO₄^2-, SCN^- anions and Na⁺, K⁺, Mg²⁺ cations are added at physiological concentration (∼0.15 mol/kg) to Pluronic F127 20 wt% aqueous solutions to suitably tune phase transitions for a smart drug delivery platform. Rheological measurements, along with SAXS and NMR self-diffusion experiments, are used to carefully characterize the prepared F127/salt-based formulations. The critical micellar temperature (cmt), the hard-gel formation temperature (T_hg), liquid crystal structures, and self-diffusion coefficients are determined. The cmt and T_hg values of F127/salt formulations are lower than that of F127 20 wt% sample, following an anionic Hofmeister series: SO₄^2- < Cl^- < SCN^-. All added salts significantly increase storage modulus and complex viscosity with maximum values occurring at T around 40 °C. SAXS data confirm that added salts preserve cubic liquid crystal phases. NMR self-diffusion analysis highlights that the intermolecular interactions and mobility of F127 unimers/aggregates are ion specific at 16 °C but not at 40 °C. These findings suggest that F127/salt-based formulations may constitute a versatile thermosensitive platform for drug delivery able to assure sustained release in topical or surgery administrations, in the range of temperatures 30–45 °C.