Mixed micellization of dimeric (gemini) surfactants and conventional surfactants - II. CMC and micelle aggregation numbers for various mixtures

The aqueous solutions of mixtures of various conventional surfactants and dimeric anionic and cationic surfactants have been investigated by electrical conductivity, spectrofluorometry, and time-resolved fluorescence quenching to determine the critical micelle concentrations and the micelle aggregation numbers in these mixtures. The following systems have been investigated: 122-12/DTAB, 12-2-12/Cl12E6, 12-2-12/C12E8, 12-3-12/C12E8, Dim3/ C12E8, and Dim4/C12E8 (12-2-12 and 12-3-12 = dimethylene-1,2- and trimethylene-1,3-bis(dodecyldimethylammonium bromide), respectively; C12E6 and C12E8 = hexa- and octaethyleneglycol monododecylethers, respectively: Dim3 and Dim4=anionic dimeric surfactants of the disodium sulfonate type, Scheme 1; DTAB = dodecyltrimethylammonium bromide). For the sake of comparison the conventional surfactant mixtures DTAB/C12E8 and SDS/C12E8 (SDS = sodium dodecylsulfate) have also been investigated (reference systems). Synergism in micelle formation (presence of a minimum in the cmc vs composition plot) has been observed for the Dim4/C12E8 mixture but not for other dimeric surfactant/nonionic surfactant mixtures investigated. The aggregation numbers of the mixed reference systems DTAB/C12E8 and SDS/C12E8 vary monotonously with composition from the value of the aggregation number of the pure C12E8 to that of the pure ionic component. In contrast, the aggregation number of the dimeric surfactant/C12E8 mixtures goes through a minimum at a low value of the dimeric surfactant mole fraction. This minimum does not appear to be correlated to the existence of synergism in micelle formation. The initial decrease of the aggregation number of the nonionic surfactant upon addition of ionic surfactant, up to a mole fraction of ionic surfactant of about 0.2 (in equivalent per total equivalent), depends little on the nature the surfactant, whether conventional or dimeric. The results also show that the microviscosity of the systems containing dimeric surfactants is larger than that of the reference systems. (C) 2001 Academic Press.