Evidence for Synergism in Nonionic Surfactant Mixtures: Enhancement of Solubilization in Water-in-Oil Microemulsions

It is well-known that certain mixtures of surfactants can provide better performance than pure surfactants for a wide variety of applications,1,2 and thus it is expected that enhanced solubilization ofwater inwater-in-oil (w/o) microemulsions will also be achieved with certain surfactant mixtures. The formation of w/o microemulsions involves dissolving an aqueous phase into an oil phase, creating a transparent and thermodynamically stable suspension of dropletswith diameters in the range of 10100nm. It is desirable to accomplish thiswithaminimum amount of surfactant,3 and in order to achieve this goal, mixtures of surfactants canbeused. We intend toaddress different surfactant mixtures that exhibit synergism in the solubilizationofwater inw/omicroemulsions, showing the existence of at least two different mechanisms of synergism. Synergism in surfactants may be defined as any situation where mixtures of surfactants have superior properties when compared to the properties of any of the single components alone. Although certain mixtures of dissimilar surfactants canbe expected to showsynergism, enhancement in properties may also occur withmixtures of similar structures. Shinoda et al.4,5 havedemonstrated thatmacroemulsionsmadewithnonionic surfactants that have been purified to a single poly(ethylene oxide) chain length are generally less able to solubilize the dispersed phase into the continuous phase, when compared to macroemulsions made with surfactant having the same average length, but an ethylene oxide size distribution. Ordinarily, onewouldnot expect strong synergistic effects in mixtures of nonionic surfactants, as synergism in anionic/nonionic surfactantmixtures has beenattributed to Coulombic, ion-dipole, or hydrogen-bonding interactions among the polar groups. Nonionics, which have minimum intermolecular interactions, should have, by comparison, the lowest synergism of all mixtures.6 In light of this previous work, we are able to demonstrate strong synergism in nonionic surfactant mixtures. When formulating water-in-oil microemulsions, one mustaccount for several factors, including thepartitioning of thesurfactant (or its components)betweentheoil,water, and interfacial domains (Figure 1). For maximum solubilization it is desirable to have most of the surfactant at the interface between the oil and water, rather than dissolved in the oil or water phases. Increasing the interfacial area should also increase solubilization. Synergism in microemulsion solubilization has been previously reported for a nonionic-anionic system designed for pharmaceutical applications, using mixtures of AOT (sodium bis(2-ethylhexyl)sulfosuccinate) and Arlacel 20 (sorbitan laurate) in different proportions.7,8 The present paper reports similar improvements using nonionicnonionic surfactant mixtures for solubilization of water in water-in-oil microemulsions and provides evidence for two different synergism mechanisms.