The effect of processing variables on the physical characteristics of non-ionic surfactant vesicles (niosomes) formed...

Niosomes are vesicles formed by self-assembly of non-ionic surfactants. In this investigation, the effects of processing variables, particularly temperature and sonication, on the physical characteristics and phase transitional behaviour of two niosomal systems based on a hexadecyl diglycerol ether (C(16)G(2)) have been studied. Systems containing C(16)G(2), cholesterol and poly-24-oxyethylene cholesteryl ether (Solulan C24) in the molar ratios 91:0:9 and 49:49:2 were prepared by aqueous dispersion of films, followed by examination of 5(6)-carboxyfluorescein entrapment, particle size and morphology. The thermal behaviour was examined using high sensitivity differential scanning calorimetry (HSDSC) and hot stage microscopy, while the effects of sonication were studied in terms of size and morphology, both immediately after preparation and on storing for 1 h at room temperature and 60 degrees C. Polyhedral niosomes were formed from systems containing C(16)G(2) and Solulan C24 alone, while cholesterol-containing systems formed spherical vesicles mixed with tubular structures; the polyhedral systems were found to have a larger particle size and higher CF entrapment efficiency. HSDSC studies showed the polyhedral systems to exhibit an endotherm at 45.4 degrees C and a corresponding exotherm at 39.1 degrees C on cooling which were ascribed to a membrane phase transition; no equivalent transition was observed for the cholesterol containing systems. Hot stage microscopy showed the polyhedral vesicles to convert to spherical structures at approximately 48 degrees C, while on cooling the spherical vesicles split into smaller structures and reverted to the polyhedral shape at approximately 49 degrees C. Sonication resulted in the polyhedral vesicles forming spherical structures which underwent a particle size increase on storage at room temperature but not at 60 degrees C. The study suggests that the polyhedral vesicles undergo a reversible transition to spherical vesicles on heating or sonication and that this morphological change may be associated with a membrane phase transition.