Neutron reflectivity (NR) and small angle neutron scattering (SANS) have been used to investigate the equilibrium surface adsorption behavior and the solution microstructure of mixtures of the anionic surfactant sodium 6-dodecyl benzene-4 sulfonate (SDBS) with the nonionic surfactants monododecyl octaethylene glycol (C12EO8) and monododecyl triiscosaethylene glycol (C12EO23). In the SDBS/C12EO8 and SDBS/C12EO23 solutions, small globular mixed micelles are formed. However, the addition of Ca2+ ions to SDBS/C12EO8 results in a transition to a vesicle phase or a mixed vesicle/micellar phase for SDBS rich compositions. In contrast, this transition hardly exists for the SDBS/C12EO23 mixture, and occurs only in a narrow composition region which is rich in SDBS. The adsorption of the SDBS/C12EO8 mixture at the air-solution interface is in the form of a mixed monolayer, with a composition variation that is not consistent with ideal mixing. In water and in the presence of NaCl, the nonideality can be broadly accounted for by regular solution theory (RST). At solution compositions rich in SDBS, the addition of Ca2+ ions results in the formation of multilayer structures at the interface. The composition range over which multilayer formation exists depends upon the Ca2+ concentration added. In comparison, the addition of a simple monovalent electrolyte, NaCl, at the same ionic strength does not have the same impact upon the adsorption, and the surface structure remains as a monolayer. Correspondingly, in solution, the mixed surfactant aggregates remain as relatively small globular micelles. In the presence of Ca2+ counterions, the variation in surface composition with solution composition is not well described by RST over the entire composition range. Furthermore, the mixing behavior is not strongly correlated with variations in the solution microstructure, as observed in other related systems.

We report the use of redox-active surfactants Fc(CH2)nN + (CH3)3‚Br -, where Fc ) ferrocene ) [η 5 -C5H5]-Fe[η 5 -C5H5] and n ) 8, 11, or 15, in a study of principles for active control of interfacial properties of aqueous solutions. By comparing the surface activity of Fc(CH2)11N + (CH3)3 with that of HO(CH2)11N + -(CH3)3 and CH3(CH2)11N + (CH3)3, we demonstrate that Fc(CH2)nN + (CH3)3 behave as unsymmetrical bolaform surfactants with one ionic "head" group (N + (CH3)3) and one nonionic "head" group (Fc), whereas ferrocenyl surfactants, when oxidized to Fc + (CH2)nN + (CH3)3, have properties of symmetrical bolaform surfactants such as N + (CH3)3(CH2)15N + (CH3)3. Oxidation of Fc(CH2)nN + (CH3)3 to Fc + (CH2)nN + (CH3)3 leads to changes in interfacial properties through three mechanisms, at least. First, near their critical micellar concentrations (cmc's), oxidation caused desorption of monolayers of Fc(CH2)8N + (CH3)3 and Fc(CH2)11N + (CH3)3 from the surfaces of their aqueous solutions, thereby recovering the surface tension of the aqueous electrolyte. We measured changes in surface tension as large as 23 mN/m. Second, at concentrations greater than the cmc, oxidation of Fc(CH2)11N + (CH3)3 caused little, if any, change in the excess surface concentration of surfactant. The accompanying increase in the density of charge within the monolayer did, however, cause a decrease in surface tension of 6 mN/m. Third, oxidation of Fc(CH2)15N + (CH3)3 caused its monolayers at the surface of water to change from condensed states to expanded ones at constant surface pressure. We infer the phase transition within the monolayer to be driven by changes in conformation that accompany the transfer of ferrocene, upon oxidation, from the outer region (side in contact with air) of the condensed monolayer into contact with the aqueous subphase.