An electron spin resonance study of non-ionic surfactant vesicles (niosomes).

Certain non-ionic surfactants form lamellar vesicles called niosomes. Being elastic and deformable, niosomes have been used as an efficient vehicle for transdermal drug delivery. However, dynamic properties of niosomes have not been studied extensively. In this study we used electron spin resonance (ESR) technique to measure the membrane fluidity of niosomes. In parallel with phospholipid liposomes, the ESR spectra of 5- and 16-doxyl stearate in niosomes of sorbitan monostearate (Span 60) and sorbitan monooleate (Span 80) showed that motion of the spin label was more restricted at the region near the headgroup than near the bilayer center. Cholesterol increased fluidity of Span 60 niosomes whereas it decreased fluidity of Span 80 niosomes. Dicetyl phosphate added at 10 mol% concentration as a stabilizer had a minimal effect on the membrane fluidity throughout the bilayer. We also used ESR technique to monitor the hydration-induced transformation of Span 60 proniosome gel to niosome and showed that the niosome prepared by hydration of proniosome gel was identical to the niosome obtained from a thin film hydration method. Finally the ESR spectra of Span niosomes were compared with those of polysorbate (Tween) niosomes and polyethoxy fatty ether (Brij) niosomes. Tween niosomes had a bulky headgroup and were much less rigid than Span niosomes. This effect of headgroup size on fluidity was also manifest in Brij niosomes where fluidity increased with the number of ethoxy units in the polyethoxy headgroup.