Surface and foaming properties of polyoxyethylene glycerol ester surfactants

Polyoxyethylene glycerol esters derived from cocoa oil are non-ionic surfactants obtained from a renewable source which fulfil the environmental and toxicological requirements to be used as ecofriendly foaming and/or emulsifying agents. This paper reports a study on the equilibrium adsorption, surface rheology and foaming properties of two commercial polyoxytheylene glycerol ester surfactants which differ in the number of ethylene oxide (EO) groups. Dynamic and equilibrium surface pressure values were obtained with a drop profile tensiometer. The oscillating drop technique was used to study the dilatational rheology of adsorbed surfactant layers. The foaming properties of aqueous solutions of these surfactants were characterized in a commercial foam scan column. Dynamic surface pressure measurements showed two adsorption processes in which a slower condensation of the adsorbed film followed a fast diffussion step. Surface rheology and equilibrium surface pressure data fitted an extended Frumkim model which takes into account the compressibility of the adsorbed layer. The polyoxyethylene glycerol ester with the highest number of EO groups turned out to be more surface active, leading to lower cmc and higher adsorption constant. Both surfactants exhibited similar overall foam capacity at a given concentration. However, the time evolution of the liquid volume in the foam and the size of the air bubbles pointed to the formation of a more homogeneous and stable foam with smaller bubbles for the most hydrophilic polyoxyethylene glycerol ester studied. This is considered to be related to its surface dilatational rheology properties.