An investigation into the critical surfactant concentration for solid solubility of hydrophobic drug in different polyethylene glycols

Abstract

Solid dispersions of 10% w/w griseofulvin in different polyethylene glycols (PEGs) with or without incorporation of alkali dodecyl sulphates (MDS) were prepared by the melting method. The investigations concerned the solid state (X-ray powder diffraction), the transition from solid to liquid state (Oscillating DSC) and the liquid state (low frequency dielectric spectroscopy). The critical concentrations of SDS for the formation of solid solutions in varying PEGs were evaluated. In PEG 3000 this formation occurs at 1.4% w/w SDS, whereas PEG 6000 and PEG 20 000 require solely 1.0% w/w SDS to transfer a dispersion into a solid solution. PEG 3000 was also investigated with the addition of MDS. The critical surfactant concentrations for the formation of solid solutions with the counterions Li +, Na + and K + were 1.0%, 1.4% and 2.1% w/w, respectively. The investigated systems had varying degrees of crystallinity. With the addition of SDS to PEGs with a range of molecular weights, the highest crystallinity was seen in the PEG 3000 sample. The different polymers contained different amounts of folded and extended chains which influences the amount of amorphous material within the polymer structure. When surfactants with different counterions were added to PEG 3000, the lithium sample showed the highest crystallinity. In the melt the Li + sample showed the lowest dielectric mobility. The results show that concentration and structure of surfactant together with the presence of folded and extended chains form the conditions for the formation of solid solutions.