The aim. Study of the interaction of surfactants with poloxamer 338 (P338) and the effect of P338 on the properties of cream bases. Materials and methods. Solutions of the surfactants and P338 as well as cream bases were under study. The average hydrodynamic diameter (Dh) and zeta potential (ζ‑potential) were determined by the light scattering intensity and electrophoretic mobility of micelles. The electron paramagnetic resonance (EPR) spectra of spin probes in micelles, solvents and bases were obtained; the type of spectrum, isotropic constant (AN), rotational correlation times (τ) and anisotropy parameter (ε) were determined. Liquids and cream bases were studied by capillary and rotational viscometry; the flow behaviour and yield stress (t0), dynamic and apparent viscosity (η) as well as the hysteresis (thixotropic) area (AH) were determined. The microstructure of the bases was examined by optical microscopy. The strength of adhesion (Sm) was assessed by the pull-off test, and the absorption of water was studied by dialysis. Results. Under the impact of P338 the hydrodynamic diameters of micelles formed by cationic, anionic and nonionic surfactants decreased as well as the absolute values of their ζ‑potential became lower, but the microviscosity of the micelle nuclei increased. There was also a change in the structure of the aggregates of surfactant with fatty alcohols; EPR spectra, which were superpositions characteristic for the lateral phase separation, converted into triplets that indicated the uniform distribution of lipophilic probes in the surfactant phase. When the content of P338 increased to 17 %, the rheological parameters of the bases increased drastically, the flow behaviour and the microstructure changed. The bases had the consistency of cream within temperature range from 25 °C to 70 °C and completely restored their apparent viscosity, which had decreased under shear stress. P338 enhances the adhesive properties of the bases. Due to their microstructure, cream bases have a lower ability to absorb water compared to a solution and gel containing 17 % and 20 % P338, respectively. Conclusions. The structure of surfactant micelles and aggregates of surfactants with fatty alcohols changed under impact of P338 due to the interaction of surfactants with P338. As a result of this interaction, at a sufficiently high concentration of P338, the microstructure and flow behaviour of bases changed, their rheological parameters, which remain high at temperatures from 25 °C to 70 °C, increased significantly, and water absorption parameters decreased. The bases with P338 were more adhesive

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The aim of our study was to identify factors affecting the in vitro release of diclofenac sodium (DS) from hypromellose-based gels (HPMC). Materials and methods. Gels with HPMC and liquids without HPMC were studied by viscosity-rotating viscometer method and spin probe electron paramagnetic resonance spectroscopy. Rheograms were used to determine the flow behavior and the apparent viscosity, and the EPR spectra were used to determine the rotational correlation time (τ–1) of the dissolved spin probes. The in vitro release tests were performed using vertical diffusion cells according to a validated procedure. The assay of DS and isopropyl alcohol (IPA) in the receptor medium was performed by high performance liquid chromatography (HPLC) and gas chromatography (GC) according to validated procedures, and the water content was determined using semi-micro method. Results. The apparent viscosity of the gels increased with increasing HPMC content and depended on the HPMC grade. The high apparent viscosity of the gels did not affect the values of τ–1 of the dissolved spin probes. In viscous gels and Newtonian fluids, the composition of which corresponded to the dispersion medium of gels, the values of τ–1 were identical and were in the range of rapid rotation, which is a prerequisite for similar and rapid release of the dissolved substances from gels and liquids. It was shown that the HPMC-based gel and Newtonian liquid without HPMC in terms of in vitro release parameters DS and IPA were equivalent. During in vitro testing the release of dissolved DS increased with increasing its concentration in the gel and depended on the dispersed state of DS. When the content of IPA was changed from 45.0 % to 22.5 %, the water absorption by the gel and the release of IPA decreased, and the release of DS increased, which was due to the decrease in the solubility of DS in the gel. Conclusions. HPMC, which provided high apparent viscosity of the gels, did not affect the value of τ–1 of the dissolved spin probes and the in vitro release of DS from the gels. The gel and Newtonian liquid were equivalent in terms of in vitro release of DS and IPA. The release of DS altered proportionally with the concentration of DS and depended on its dispersed state. As the content of IPA decreased, the release of IPA decreased, but the release of DS increased because of the decrease in the solubility of the DS in the gel