RESUMEN
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: 12-2-12/DTAB, 12-2-12/C(12)E(6), 12-2-12/C(12)E(8), 12-3-12/C(12)E(8), Dim3/C(12)E(8), and Dim4/C(12)E(8) (12-2-12 and 12-3-12=dimethylene-1,2- and trimethylene-1,3-bis(dodecyldimethylammonium bromide), respectively; C(12)E(6) and C(12)E(8)=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/C(12)E(8) and SDS/C(12)E(8) (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/C(12)E(8) mixture but not for other dimeric surfactant/nonionic surfactant mixtures investigated. The aggregation numbers of the mixed reference systems DTAB/C(12)E(8) and SDS/C(12)E(8) vary monotonously with composition from the value of the aggregation number of the pure C(12)E(8) to that of the pure ionic component. In contrast, the aggregation number of the dimeric surfactant/C(12)E(8) 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. Copyright 2001 Academic Press.
RESUMEN
The mixed micellization between the anionic dimeric surfactant Dim1 (disodium 1,11-didecyl-3,6,9-trioxaundecane 1,11-disulfate) and the two nonionic surfactants C12E5 and C12E8 (penta- and octaoxyethylene monododecyl ethers, respectively) has been investigated. The cmc of the mixtures has been measured by fluorescence probing, using the fluorescent probe pyrene. Its variation with composition revealed synergism in micelle formation. The micelle aggregation (N) numbers have been measured using time-resolved fluorescence quenching at a total surfactant concentration of about 70 mM and at 25, 40, and 55°C. The differences in total aggregation number of the mixed micelles in the two systems reflect (i) the higher aggregation number of C12E5 micelles with respect to C12E8 and Dim1 micelles; (ii) the higher cloud temperature of C12E8 with respect to C12E5; and (iii) the opposite temperature dependences of N for Dim1 and nonionic surfactants micelles. The results show nonideal mixing behavior with a shallow minimum in the variation of the mixed micelle aggregation number with the mixture composition at a Dim1 mole fraction around 0.5. Copyright 1998 Academic Press. Copyright 1998Academic Press