Surfactant Crystals as Stimulable Foam Stabilizers: Tuning Stability with Counterions

Abstract

The demands on foam stability are variable and changing, which is why design of foams that are both ultrastable and stimulable is important. We study foams stabilized using surfactant particles made through precipitation of sodium dodecyl sulfate with alkali chlorides. We have previously shown that depending on the concentrations of surfactant and salt, the foams can be ultrastable or age like common surfactant foams. We now show that the adsorption of surfactant crystals changes with the type of salt added and how the crystals are made, as well as the surfactant concentration. We see differences in foam stability if the crystals are made prior to foaming or if they are formed concomitantly with foaming. The adsorption of the crystals is improved if the crystals are made during generation, possibly because of their smaller size. The foams destabilize when heated above the Krafft boundary. We show that through tuning the surfactant concentration and salt type or concentration, we can modulate the melting temperature, and hence the destruction temperature of foam between 22 and 50 °C. Precipitated surfactant particles are versatile alternatives to stabilize ultrastable and stimulable foams.

Aqueous and Oil Foams Stabilized by Surfactant Crystals: New Concepts and Perspectives

“…Since the crystals are reformed from molecules on cooling to below the solubility boundary, the foam stability can easily be alternated between low and high by appropriate tuning of the temperature through multiple temperature cycles (Figure a). In the same way, aqueous foams stabilized by surfactant crystals are thermoresponsive. , Below the Krafft point, crystalline particles are present and the foam is stable. However, above the Krafft temperature, the crystals disappear and micelles are present.…”

“…Ultrastable aqueous foams have also been obtained in the presence of SDS crystals in the presence of a high concentration of salts. − In the same way as for oil foams, the crystalline particles adsorb onto bubble surfaces, arresting coarsening and drastically reducing drainage by their accumulation in the liquid films between bubbles (Figure ). When the quantity of crystals is high at bubble surfaces, bubbles become nonspherical due to the jamming of particles, which prevents relaxation to a spherical shape.…”

“…This limited recent data clearly shows that the crystal size and shape have effects on the aqueous foam properties but in a nonmonotonous way, showing that more data and characterization are needed to understand the links between foam properties and those of the crystals. Moreover, differences in foam stability are observed if SDS crystals are made prior to foaming or if they are formed during foaming . The adsorption of the crystals is improved if crystallization occurs during foam generation.…”

“…The same conclusion was reached in a study of oil foams in fatty alcohol–sunflower oil mixtures (unpublished data). In aqueous foams containing SDS crystals, the type of salt used has an effect on the crystal size . KCl yields the smallest crystals and is the most efficient at producing SDS particles to stabilize foams compared with NaCl and RbCl.…”

“…At the time when the scientific community regained an interest in oleofoams, new aqueous liquid foam systems were discovered that were based on the partial stabilization of gas bubbles by crystalline surfactant particles . These aqueous foams are stabilized by sodium dodecyl sulfate (SDS) crystalline particles present at the air–water interface and in the liquid films between bubbles. − The crystals are obtained by adding a large amount of salt. It is important to note here that some papers described aqueous foam stabilization by a fluid lamellar L α phase or by liquid crystals. , The extreme stability of foams made with lamellar phases seems to be primarily due to the high viscosity of the foaming solution which slows down gravity drainage.…”