Comparison between phase behavior of anionic dimeric (gemini-type) and monomeric surfactants in water and water-oil

The phase behavior of dimeric (gemini-type) anionic snrfactant (sodium 1,2- bis(N-dodecanoyl beta-alanate)-N-ethane, GS) in water is similar to that of the corresponding monomeric surfactant (sodium N-dodecanoyl-N-methyl beta- alanate, MS). Aqueous micellar phase (Wm), hexagonal (H-1), bicontinuous cu bic (V-1), and lamellar (L alpha) phases are successively formed with incre asing the surfactant concentration. SAXS data for cross-sectional area per surfactant molecule in H-1 and L alpha phases reveal that GS molecules are more tightly packed in the aggregates than MS molecules, and this is due to the bridging of two hydrocarbon chains by the short spacer chain (-C2H4-) In the ternary water/surfactant/dodecane or m-xylene systems, the phase beh avior of both surfactants is also practically similar. Upon addition of oil , the H-1 phase is changed to the discontinuous micellar cubic (I-1) phase in both surfactant systems. The phase transition mechanism is discussed acc ording to the geometrical packing model. In the 3 wt % NaCl aqueous/MS or G S/cosurfactant-(hexanol or butanol)/dodecane systems, a single-phase microe mulsion is formed at an equal brine/dodecane ratio over a wide range of sur factant concentration in the MS systems whereas the L alpha phase is mainly formed and the microemulsion is produced only in a very narrow region in t he GS/hexanol system. By replacement of hexanol with butanol, wide microemu lsion regions appear, even in the GS system. Hence, GS molecules in the sur factant layer are considered to be packed more tightly than MS, even in the presence of oil.