Abstract
Linear viscoelastic tests as well as transient and steady flow experiments were carried out on lamellar liquid crystalline samples of poly (oxyethylene) alcohol/water/heptane systems. The effect of surfactant and heptane concentrations on the rheological properties of the lamellar mesophase was investigated. The mechanical spectrum inside the linear viscoelastic regime shows, in all cases, a well-developed plateau region in the whole frequency range studied. The values of the dynamic functions were higher for intermediate surfactant or heptane concentrations indicative of a major development of the elastic network in the midrange of existence of the lamellar phase. Transient and steady flow experiments point out a shear-induced evolution of the lamellar microstructure. Different power law regions with different values of the flow index were detected in the viscosity versus shear rate plots. These shear-induced structural modifications were confirmed by using polarizing microscopy in an optical shearing cell. Structural modifications appear to be highly influenced by shear rate. In general, applying relatively high constant shear rates, the alignment of the bilayers followed by the appearance of the “oily streaks” structure was observed. Appearance of shear-induced vesicles occurs at high heptane content, as indicates the texture of close-packed monodisperse spherulites detected by polarizing microscopy.
Highlights
Aqueous and non-aqueous solutions of nonionic surfactants (Tiddy, 1980; Robles-Vásquez, 1993; Dimitrova, 1995; Cordobés, 1997) may form different lyotropic liquid-crystalline phases as a result of interactions among association structures or micellar aggregates
The phase diagrams of polyoxyethylene nonionic surfactants exhibit a succession of different lyotropic liquid-crystalline phases (Mitchell, 1983; Franco, 1995; Moros, 2001)
The values of γc depend of the linear viscoelastic function selected for its determination
Summary
Aqueous and non-aqueous solutions of nonionic surfactants (Tiddy, 1980; Robles-Vásquez, 1993; Dimitrova, 1995; Cordobés, 1997) may form different lyotropic liquid-crystalline phases as a result of interactions among association structures or micellar aggregates. These phases present static and dynamic properties related to the crystalline (long-range positional and orientational order, elasticity,...) and liquid states (viscosity, concentration fluctuations, etc.). The lamellar lyotropic phase, Lα, appears at rather high concentrations when the interface tends to be planar, surfactant with more hydrophobic chains favors their appearance at low concentrations.
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