On the nature of the intramicellar and intermicellar forces governing the liquid crystalline phase behavior of aqueous solutions of disklike micelles.

The liquid crystalline phase diagrams for aqueous solutions of the homologous series of surfactants CF3(CF2)nCO-2Cs+ (n=5–8) have been determined. They all exhibit the classical isotropic (I)-to-discotic nematic (ND) and ND-to-smectic lamellar (L) sequences of transitions with increasing concentration, as previously established for the n=6 system [N. Boden, S. A. Corne, and K. W. Jolley, J. Phys. Chem. 91, 4092 (1987)]. The effect of increasing n is to displace the transitions to higher temperatures. The behavior of all of the surfactant systems can be represented on a universal phase diagram. Both the I-to-ND and the ND-to-L transitions at corresponding concentrations are found to occur when the axial ratio of the disklike micelles attains a singular value in each case, irrespective of the value of n. The form of the experimental phase diagrams can be qualitatively understood in terms of a simple ‘‘zeroth’’ order theory which uses the results of Onsager’s theory [L. Onsager, Ann. NY Acad. Sci. 51, 627 (1949)] applied to disks to find the critical axial ratios in the coexisting isotropic and nematic phases and, separately, a dilute solution, self-assembly theory of disklike micelles [W. E. McMullen, A. Ben-Shaul, and W. M. Gelbart, J. Colloid Interface Sci. 98, 523 (1984)] to determine the temperatures at which these axial ratios are achieved at each concentration. The same treatment is also shown to account for the experimental phase behavior of mixed-chain-length systems. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]