Interpretation of different molecular packing in the micelles of lyotropic nematic and cholesteric phases via mass density measurements

Lyotropic nematic and cholesteric mixtures were produced by dissolving amino acid based racemic surfactants, potassium N-dodecanoyl-dl-alaninate (dl-KDDA) and potassium N-dodecanoyl-dl-serinate (dl-KDDS), and their l-enantiomer surfactants (l-KDDA and l-KDDS) in a mixture of sodium sulfate (Na2SO4)/1-decanol (DeOH)/water, separately. Polarizing light microcopy (PLM) and mass density measurements were applied to each mixture. The textures of all phases were investigated by PLM. Mass density measurements showed that the cholesteric phases of both l-KDDA and l-KDDS exhibited larger mass density values than their dl-KDDA and dl-KDDS nematic phases, respectively. This result was supported by our previous small-angle X-ray scattering study. The difference between micelle volumes of cholesteric and nematic phases was explained by different molecular packing of dl- and l-forms of the same surfactant molecules in the achiral and chiral micelles, respectively. This indicated that mass density measurement may be a useful way to detect the different molecular packing in the micelles which causes the increase or decrease in the micelle size or volumes qualitatively as it is expected.