Nonaqueous foam with outstanding stability in diglycerol monomyristate/olive oil system

We report the formation of nonaqueous foam with outstanding stability in diglycerol monomyristate (designated as C14G2) nonionic surfactant in olive oil at 25 °C. Effects of surfactant concentration and added water on foamability and foam stability have been investigated. The C14G2 forms the dispersion of surfactant solid in the dilute surfactant concentration regions at 25 °C and they are responsible for the outstanding stability of nonaqueous foams, as they tend to adsorb at the gas–liquid interface. It was found that foamability increases with surfactant concentration up to a certain point, and then level off. In the case of water added system, the foamability increases monotonically with the concentration of water added. Foam stability studies have shown that the foams are stable for a few minutes to several weeks depending on the surfactant concentration and amount of added water. Foam was stable for ∼20 min in 1 wt% C14G2/olive oil system, but it was stable more than a month in 10 wt% system. Foam stability of 5 wt% C14G2/olive oil system was decreased upon addition of 1 wt% water; foam was stable only for ∼1.5 h. It was found that the average size of the surfactant solid decreases with increasing surfactant concentration and thus foams with better stability could be observed at higher concentrations. The poor foam stability in the water added systems was caused due to structural transition of surfactant solid (α-solid) to lamellar liquid crystalline phase. The present results are obtained by optical microscopy, surface tension, and rheology data.