GIANT RODLIKE REVERSED MICELLES FORMED BY SODIUM BIS(2-ETHYLHEXYL) PHOSPHATE IN N-HEPTANE

The solution behaviors of sodium bis(2-ethylhexyl) phosphate (NaDEHP) in n-heptane were investigated by light-scattering and viscosity measu rements. NaDEHP forms giant rodlike reversed micelles, with a radius o f gyration as large as 53 nm, which violently contrasts with the liter ature view that the average micellar aggregation numbers in nonaqueous or apolar media are much smaller (seldom exceeding 20) than those in aqueous media. Significantly, a small amount of water plays the role o f an antimicellar growth agent; i.e., the reversed micellar size decre ases remarkably when ''dry'' solutions are exposed to humid air from w hich water vapor is absorbed or when bulk water is directly added-a be havior which is distinctly opposite to that for sodium bis(2-ethylhexy l) sulfosuccinate/apolar medium systems. Thus, the literature views th at large micelles can only be found in aqueous media and that the surf actant headgroups in reversed micelles are linked together by hydrogen bonds are misleading. It is suggested that the primary contribution t o the driving force for the growth of rodlike NaDEHP reversed micelles is long-range electrostatic interactions among the headgroups of the surfactant molecules and their counterions, and a possible mechanism f or the effect of water is also discussed.