Mesophasen in gemischten Tensidsystemen / Mesophases in mixed surfactant systems

Abstract

Mixed surfactant solutions of the zwitterionic surfactant C 14 DMAO, the anionic surfactant SDS and the co-surfactant hexanol were studied in the whole molar mixing ratio. The single phase region of the dilute L α -phases was established. The lowest surfactant concentration for which the L α -phase is formed depends on the mole fraction of the anionic surfactant X a and shifts with increasing X a to higher surfactant concentrations. For a 100 mM surfactant solution, the L α -phase disappears for X a values above 0.3. At the overall surfactant concentration 100 mM the phase diagram of the quaternary surfactant system C 14 DMAO/SDS/hexanol/water is given. The phase boundaries of the L 1 -phase were determined by conductivity measurements. Conductivity decreases markedly in the two-phase L 1 /L α -region which contains vesicles. The strongly synergistic behaviour between C 14 DMAO and SDS is also observed in the presence of hexanol and is reflected in a minimum of the surface and interfacial tensions with increasing X a . The synergism is still marked at the phase boundary of the L 1 -phase in the presence of hexanol. The surface and interfacial tension values decrease continuously with the addition of hexanol and the interfacial tension reaches a value of 0.15 mN/m for X a ≤ 0.6 at the phase boundary. The phase boundary of the L 1 -phase is shifted to higher hexanol concentrations with increasing mole fractions of SDS. The addition of hexanol to slightly charged solutions leads to increasing rod length and zero shear viscosities (η°). The structural relaxation times and the η° values reach their highest values at the L 1 -phase boundary for X a = 0.15. Four different relaxation processes with different signs were observed with electric birefringence investigations as a function of the hexanol concentration and of the mixing ratio of C 14 DMAO and SDS. The first, third and fourth processes have a negative sign, the second process a positive sign. The 100 mM surfactant mixtures show the four electric birefringence signals between 0 < X a < 0.6 as a function of the hexanol concentration. In the L 1 /L α -two-phase region we observe for a constant surfactant concentration c° and constant X a micellar rods in the coexisting L 1 -phase and the anomaly of the electric birefringence. With increasing hexanol concentrations in the two phase solutions the length of the rod-like micelles decreases and for higher hexanol concentrations only globular micelles are found.