The effect of processing variables on the physical characteristics of non-ionic surfactant vesicles (niosomes) formed from a hexadecyl diglycerol ether

Niosomes are vesicles formed by self-assembly of non-ionic surfactants. In this investigation, the effects of processing variables, particularly tempe rature and sonication, on the physical characteristics and phase transition al 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 an d poly-24-oxyethylene cholesteryl ether (Solulan C24) in the molar ratios 9 1:0:9 and 49:49:2 were prepared by aqueous dispersion of films, followed by examination of 5(6)-carboxyfluorescein entrapment, particle size and morph ology. The thermal behaviour was examined using high sensitivity differenti al scanning calorimetry (HSDSC) and hot stage microscopy, while the effects of sonication were studied in terms of size and morphology, both immediate ly after preparation and on storing for 1 h at room temperature and 60 degr ees C, Polyhedral niosomes were formed from systems containing C(16)G(2) an d Solulan C24 alone, while cholesterol-containing systems formed spherical vesicles mixed with tubular structures; the polyhedral systems were found t o have a larger particle size and higher CF entrapment efficiency. HSDSC st udies 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 asc ribed to a membrane phase transition; no equivalent transition was observed for the cholesterol containing systems. Hot stage microscopy showed the po lyhedral vesicles to convert to spherical structures at similar to 48 degre es C, while on cooling the spherical vesicles split into smaller. structure s and reverted to the polyhedral shape at similar to 49 degrees C. Sonicati on resulted in the polyhedral vesicles forming spherical structures which u nderwent a particle size increase on storage at room temperature but not at 60 degrees C. The study suggests that the polyhedral vesicles undergo a re versible transition to spherical vesicles on heating or sonication and that this morphological change may be associated with a membrane phase transiti on. (C) 2000 Elsevier Science B.V. All rights reserved.