MONOLAYERS OF MIXED SURFACTANTS AT THE OIL-WATER INTERFACE, HYDROPHOBIC INTERACTIONS, AND STABILITY OF WATER-IN-OIL EMULSIONS

The present study involves investigation of monolayer properties of so rbitan monooleate and diethanolamine derivatives of polyisobutylene su ccinic anhydride of three different molecular weights, viz. 500, 700, and 1050, as single as well as binary mixtures at the oil-water interf ace. The average backbone carbon numbers of the polyisobutylene chains are 14, 22,and 34, respectively. The monolayer studies at the oil-wat er interface were carried out with a specially designed Langmuir troug h. The investigations were carried out at two different oil phases, vi z. heptane and paraffin oil. The packing efficiency of these surfactan ts (single or in combination) are found to be greatly influenced by th e oil chain length. Within the limits of experimental accuracy, the mi xed surfactant systems comprising sorbitan monooleate and polyisobutyl ene surfactants of three different hydrocarbon chain lengths follow th e rule of hydrocarbon chain length compatibility. The observed differe nces in packing features revealed the significance of oil penetration and cohesive interactions in the aliphatic layer of the surfactant fil m. The efficacy of these surfactants were further examined in water-in -oil emulsions and the results are consistent with the findings of mon olayer properties. At the condition of chain length compatibility, the mixed surfactant system produced emulsions with maximum stability.