Microstructure and Rheology of Lamellar Liquid Crystalline Phases

Abstract

We have investigated the microstructure and rheological properties of ternary surfactant mixtures in a salt solution. The surfactants were 6% sodium 4-dodecylbenzenesulfonate, 3% C13-15 ethoxylated alcohol with seven ethylene oxide (EO) units and 1% C13-15 ethoxylated alcohol with 2, 4, 7, 9, 11, 14, 20, or 25 EO units. The salt solution was 10% nitrilotriacetate‚H2O. Microstructural investigations (electron microscopy, light microscopy, confocal laser microscopy, conductivity measurements, and centrifugation) show that at rest the samples containing the surfactant with 2 EO to 9 EO units are dispersions of lamellar droplets (curved surfactant bilayers). The samples containing the surfactant with 11 EO to 25 EO units show a continuous lamellar structure (sheets of surfactant bilayers) with a small amount of lamellar droplets present. The change in several rheological parameters reflects this change in microstructure. The power law index from flow experiments at low shear rates changes from 0.1 for the lamellar dispersions to 0.4 for the continuous lamellar phases. Similar changes are observed in shear modulus and in the limiting strain for linear viscoelastic behavior. The continuous lamellar phase is converted to droplets by shearing at rates above 1 s -1 . The continuous lamellar structures will recover in about a week when the samples are allowed to relax. The nature of the droplets is highly dynamic. Confocal laser microscopy shows small fluctuations in droplet shape on a time scale of about 100 s. This time coincides with a characteristic time of around 100 s pertaining to a (shallow) peak in G′′.

Swollen and collapsed lyotropic lamellar rheology

“…Moreover, the moduli have nearly the same range of magnitude and are of one decade difference. Lam 1 has γ c range of 0.08-0.1%, such values have been previously reported for MLVs systems [28,29]. Second, the strain overshoot phenomena is absent in case of diluted Lam 2 (Fig.…”

Glycerol effects on the formation and rheology of hexagonal phase and related gel emulsion

“…The rheogram has shown in Fig. 3a (50 wt.% glycerol) is consistent with a typical lamellar structure [24][25][26][27]. Later we will come to this point.…”

The effect of octyl glucoside on rheological behavior of diluted and concentrated lamellar phases

“…A review of the rheology of the lamellar phase in surfactant systems has been recently published [3]. The change in several rheological parameters such as the power law index from flow experiments can reflect changes in the microstructure of lamellar phases [4,5]. The shear effect on the lamellar phase has been widely studied [6,7] and recently, reo-SANS or reo-SALS has been carried out [8] to determine the microstructure under shearing.…”