Novel sorbitan monostearate organogels

Sorbitan monostearate, a hydrophobic nonionic surfactant, gels a number of organic solvents such as hexadecane, isopropyl myristate, and a range of ve getable oils. Gelation is achieved by dissolving/dispersing the organogelat or in hot solvent to produce-an organic solution/dispersion, which, on cool ing sets to the gel state. Cooling the solution/dispersion causes a decreas e in the solvent-gelator affinities, such that at the gelation temperature, the surfactant molecules self-assemble into toroidal inverse vesicles. Fur ther cooling results in the conversion of the toroids into rod-shaped tubul es. Once formed, the tubules associate with others, and a three-dimensional network is formed which immobilizes the solvent. An organogel is thus form ed. Sorbitan monostearate gels are opaque. thermoreversible semisolids, and they are stable at room temperature for weeks. The gels are affected by th e presence of additives such as the hydrophilic surfactant, polysorbate 20, which improves gel stability and alters the gel microstructure from a netw ork of individual tubules to star-shaped "clusters" of tubules in the liqui d continuous phase. Another solid monoester in the sorbitan ester family, s orbitan monopalmitate, also gels organic solvents to give opaque, thermorev ersible semisolids. Like sorbitan monostearate gels, the microstructure of the palmitate gels comprise an interconnected network of rodlike tubules. U nlike the stearate gels, however, the addition of small amounts of a polyso rbate monoester causes a large increase in tubular length instead of the "c lustering effect" seen in stearate gels. The sorbitan stearate and palmitat e organogels may have potential applications as delivery vehicles for drugs and antigens.