J. Phys. II France
Volume 4, Numéro 8, August 1994
Page(s) 1439 - 1448
DOI: 10.1051/jp2:1994209
J. Phys. II France 4 (1994) 1439-1448

Freeze-fracture observations in the L $_\alpha$ phase of a swollen surfactant in the vicinity of the L 3 and the L 1 phase transitions

P. Boltenhagen1, M. Kleman2 and O. D. Lavrentovich3

1  Laboratoire de Physique des Solides, Bât. 510, Université de Paris-Sud, 91405 Orsay Cedex, France
2  Laboratoire de Minéralogie-Cristallographie, Universités Pierre et Marie Curie (Paris VI) et Paris VII, 4 place Jussieu, case 115, 75252 Paris Cedex 5, France
3  Kent State University, Liquid Crystal Institute, Kent, Ohio, 44242, U.S.A.

(Received 2 July 1993, revised 13 April 1994, accepted 18 April 1994)

Freeze-fractured specimens of the L $_\alpha$ phase of a swollen quasi-ternary surfactant (cetylpyridinium chloride/hexanol/brine), have been observed by electron microscopy. The textures show the same features as those oberved by optical microscopy, viz. defects with negative Gaussian curvature (focal conic domains of the usual type) close to the phase boundary with the L 3 sponge phase, and defects with positive Gaussian curvature (focal conic domains of second species, mostly spherolites) close to the phase boundary with the L 1 micellar phase. We have studied in more detail this latter region : the density of spherolites increases as one approaches the L 1-L $_\alpha$ transition, up to the point where they seemingly fill the whole space ; their size distribution obeys an exponential law we explain by a simple thermodynamic model which predicts a transition when the elastic coefficient $\Lambda = 2 K + \overline{K}$ vanishes. Our experimental data yield indeed, with a reasonable precision, a very small value of $\Lambda d/k\rm _B$ T for a composition of the surfactant close to the phase boundary.

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