Interfacial activity of amino acid-based glycerol ether surfactants and their performance in stabilizing O/W cosmetic emulsions

Abstract

Biocompatible and biodegradable molecules of Phenyl- and Tyrosine-glycerol ether surfactants are good candidates for eco-friendly cosmetic emulsions. Both molecules have a hydrophobic alkyl chain with 12 carbon atoms, but different amino acids in the hydrophilic head: phenylalanine (PhGE12) and tyrosine (TyrGE12). This study reports on the activity of liquid/liquid interfaces with the glycerol ether surfactants being dissolved in the organic phase (olive oil) and the aqueous phase containing the non-ionic poly(oxyethelene) (20) sorbitan monooleate (trade name Tween 80). Measurements refer to a) dynamic surface tension by drop profile tensiometry and drop volume tensiometry and b) interfacial dilatational viscoelasticity by oscillating drop profile analysis. It is shown that PhGE12 attains much lower interfacial tension values and exhibits faster interfacial adsorption than TyrGE12 whereas interfacial dilatational viscoelasticity values are comparable between the two surfactants. Oil-in-water emulsions are prepared and their stability is assessed by monitoring the time evolution of creaming index. To interpret observations, measurements of the emulsions’ initial droplet size distributions and apparent bulk viscosity are employed. It is found that at similar surfactant concentrations, PhGE12 emulsions exhibit better stability than TyrGE12 emulsions. This might be attributed to the lower interfacial tension in PhGE12 emulsions that yield droplet size distributions richer in intermediate (∼50μm) and small size (∼0.5μm) droplets and to their higher apparent viscosities. On the other hand, increasing the concentration of either surfactant leads to a substantial increase of emulsions stability which is in line with the lower interfacial tension and higher apparent viscosity measurements but does not agree with the insensitive versus surfactant concentration droplet size distributions and interfacial dilatational viscoelasticity.