Micellar-shape anisometry near isotropic-liquid-crystal phase transitions.

Abstract

Micellar phases of the sodium dodecyl (lauryl) sulfate (SLS)--water--decanol system have been studied by x-ray scattering in the isotropic (I) phase, with emphasis on the I\ensuremath{\rightarrow}hexagonal (${\mathit{H}}_{\mathrm{\ensuremath{\alpha}}}$) and I\ensuremath{\rightarrow}nematic-cylindrical (${\mathit{N}}_{\mathit{c}}$) lyotropic liquid-crystal phase transitions. Analysis of the scattering curves is made through modeling of the product P(q)S(q), where P(q) is the micellar form factor and S(q) is the intermicellar interference function, calculated from screened Coulombic repulsion in a mean spherical approximation. Results show that micelles grow more by decanol addition near the I\ensuremath{\rightarrow}${\mathit{N}}_{\mathit{c}}$ transition (anisometry \ensuremath{\nu}\ensuremath{\simeq}3) than by increased amphiphile concentration in the binary system near the I\ensuremath{\rightarrow}${\mathit{H}}_{\mathrm{\ensuremath{\alpha}}}$ phase transition (\ensuremath{\nu}\ensuremath{\simeq}2.4). These results compare well with recent theories for isotropic--liquid-crystal phase transitions.