Palm olein-in-water emulsions stabilised by span and tween surfactants as potential vehicles for drug delivery

Abstract

Palm olein emulsions were produced using the combinations of Span® and Tween® surfactants by mechanical homogenisation. Effects of the types of surfactants, concentrations, effective HLB and the types of viscosity modifiers and concentrations on the characteristics of the emulsions were investigated. With palm olein content of 20% (w/w), stable oil droplets were produced at HLB values ranging from 8.5 to 11.0. Optimal concentrations of surfactants ranged from 25 to 30% (w/w to oil) depending on the types of the Span®/Tween® mixtures. Among the viscosity modifiers used, Carbopol®940 was the most effective. Suitable concentrations of Carbopol®940 for the emulsions prepared with Span®20/Tween®20 ranged from 0.1 to 0.3% (w/w). Beyond this concentration, destabilisation of emulsion due to at least depletion of water molecules could have occurred as a result of competitive hydration between Carbopol®940 and the surfactants. The emulsions produced exhibited viscoelastic and pseudoplastic behaviour, with yield value ranging from 0.1 to 35.2 Pa. Depending on the concentration of Carbopol®940 and within the linear viscoelastic region, the emulsions were elastic in nature as shown by domination of storage modulus (G’) over the loss modulus (G”) and with tan δ < 1 in the frequency range of 0.01 to 10 Hz. These favourable rheological properties were induced by the formation of threedimensional network of Carbopol®940 molecules in the continuous aqueous phase, which also entrapped the oil droplets and thus increased the stability of the emulsion. To the optimised o/w emulsion formulation, active pharmaceutical ingredients and extracts of Cassia alata were incorporated. Betamethasone dipropionate and tolnaftate at concentrations of 0.5 mg/g and 1 mg/g respectively, did not affect the size of droplets and stability of the palm olein emulsions. Nonetheless, betamethasone dipropionate increased the viscosity and elasticity of the emulsion; while tolnaftate slightly reduced the viscosity, thixotropy and elasticity of the emulsion. Cassia alata extracts affected the emulsion stability. With up to 0.5 mg/g of ethanol extract and 0.25 mg/g of ethanol/water extract of Cassia alata, emulsions were stable. Further, Cassia alata extracts did not alter the pseudoplasticity of the emulsion despite decreasing viscosity with increasing concentration of the extracts. Desired rheological properties for the development of topical cream and lotion can be attained by changing the concentration of Carbopol®940 in palm olein-in-water emulsion, which is a potential vehicle for drug delivery.