  Lyotropic liquid crystalline quaternary mixtures of potassium laurate (KL),
potassium sulphate (K2SO4)/alcohol (n-OH)/water, with the alcohols having
different number of carbon atoms in the alkyl chain (n), from 1 octanol to
1-hexadecanol, were investigated by optical techniques (optical microscopy and
laser conoscopy). The biaxial nematic phase domain is present in a window of
values of n = nKL \pm 2, where nKL = 11 is the number of carbon atoms in the
alkyl chain of KL. The biaxial phase domain got smaller and the
uniaxial-to-biaxial phase transition temperatures shifted to the relatively
higher temperatures on going from 1-nonanol to 1-tridecanol. Moreover, these
new mixtures present high values of the birefringences comparing to other
lyotropic mixtures. This result is expected to be related to the micellar shape
anisotropy. Our results are interpreted assuming that alcohol molecules tend to
segregate in the micelles in a way that depends on the relative value of n with
respect to nKL . The larger the value of n, the more alcohol molecules tend to
be located in the curved parts of the micelle, favoring the uniaxial nematic
calamitic phase with respect to the biaxial and uniaxial discotic nematic
phases.
