Swollen and collapsed lyotropic lamellar rheology.
We have investigated linear rheological properties and the structure-flow relationship of the swollen (Lam(1)) and collapsed (Lam(2)) lamellar phases, formed on didodecyldimethylammonium bromide (DDAB)/lecithin/water ternary system at 25 degrees C. Both lamellar phases behaved like Bingham fluids and showed remarkable yield stresses. At rest the Lam(1) phase, which is characterized by densely packed vesicles whose sizes increase as the water content decreases in accordance to evolution of (2)H NMR spectral profiles of D(2)O, resulted in a strong elastic gel-like response. On the other hand, the Lam(2) phase, formed at high surfactant concentrations, showed a weak-gel viscoelasticity and (2)H NMR spectral patterns which are typical of planar bilayered structures. The increase of the quadrupole splitting as the water content decreases was assumed as a strong evidence of size increasing of the lamellar domains. We have demonstrated that by using dynamic rheology and the derived relaxation time spectra, along with (2)H NMR spectra of D(2)O, it is possible to differentiate between equilibrium lamellar structures occurring in a broad interval of total surfactant concentration. In addition, a shear-thickening regime, observed at intermediate shear-rate values, highlighted the onset of out-equilibrium lamellar structures which were present both on Lam(1) and Lam(2) phases.
Youssry
Mohamed
M
Department of Chemistry, University of Calabria, Arcavacata di Rende (CS), I-87036, Italy. youssry@unical.it
Coppola
Luigi
L
Nicotera
Isabella
I
Morán
Carmen
C