SURFACE-CHEMISTRY AND MICROEMULSION FORMATION IN OIL-WATER SYSTEMS CONTAINING SODIUM TRI-NORMAL-ALKYLSULPHO-TRICARBALLYLATES

We have investigated the surface chemical behaviour and microemulsion formation in oil + water mixtures containing a homologous series of sodium tri-n-alkylsulphotricarballylates (tri-chain surfactants). The aqueous phase critical monomer concentration required for aggregate formation decreases with increasing surfactant chain length. The magnitude of the decrease is fairly typical in comparison with a range of ionic surfactants. The systems form microemulsions and the multiphase system progression Winsor I-III-II is induced by the addition of NaCl to the aqueous phase. Under these conditions the oil-water interfacial tension passes through a minimum. For the range of surfactants investigated, the aqueous phase [NaCl] required for minimum tension decreases approximately tenfold when the surfactant chain length is increased by one carbon atom on each of the three chains. The characteristic sizes of the middle phase microemulsions have been estimated from the phase compositions and the values scale approximately with the square root of the reciprocal of the minimum interfacial tension. The NaCl was found to partition unequally between the bulk aqueous phase and the dispersed water of the middle phase microemulsions. The magnitude of the partition coefficient was consistent with exclusion of NaCl from a zone of thickness approximately equal to twice the Debye length extending from the charged surfactant monolayer. These patterns of behaviour are qualitatively similar to those observed for a range of mono-and di-chain surfactants.