The structure of the micellar solutions of some amphiphilic compounds in pure water as determined by absolute small-angle X-ray scattering techniques

  • Reiss-Husson F
  • Luzzati V
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Abstract

Micellar solutions of several amphiphiles in water, without added electrolytes, have been studied by small-angle X-ray scattering methods, bawd upon intensity measurements on an absolute scale. The concentration range goes froin 5y0 to the appearance of a liquid crystalline phase (30 to 40% in most cases). Several arnphiphiles have been studied a t different temperatures: sodium lauryl sulfate (27 and 70'), sodium lauratc (70°), myris-tate (80°), palmitate (80'), stearate (go'), and oleate (KaO, 27'), and cetyltrimethylarn-monium chloride (CTACl, 27') and bromide (27, 50 arid 70'). The interpretation of the expcrirncrital curves is discussed. It is shown that over various concentration ranges the micelles may be assuiiied to be monodisperse, their size and shape being independent of concentration. The analysis of the curves leads to the determination of the structure of the micelles. The structure is dependcnt on various parameters: nature of the amphi-phile, temperature, arid conccntration. In most systems the micelles are spherical a t low coriccritratioris arid become rod-like a t high concentration. In two cases, the structure appears to be independent of the concentration: CTACl micelles are spherical from 5 to 38%; KaO micelles are rod-like from 4 to 20%. This polymorphism is discussed. Introduction For several years one of the main activities of our laboratory has been the study by X-ray diffraction of various structures of association colloids. Until now we have becn mainly interested in liquid crystals, especially those of concentrated (generally over 30%) aqueous systeins.2 We are now reporting on the struc-ture of iiiiccllar solutions which occur at lower concen-trations. We may note that, although the structure of micelles is quite well known, a t least for a few systems, at very low concentrations close to the critical riiicclle con-centration (c.rn.c.), there is little definitive inforniatiori about higher conccntrations, where the effects of cor-relations betwccn rnicellcs becoiiie predoiniiiant and the interpretation of hydrodynamic arid light-scattering experiments becomes uncertain. Structural analysis is bcst, done in this region of concentrations by small angle X-ray scattering, a technique which has been used by several authors. AIcl3aiii" and Harkiris"'l were pioneers in this field, even though their work is now only of historical interest sirice the interpretation of experimental findings lacked a theoretical b a s i ~ . ~ -~ Others'J have attempted a more rigorous interpreta-tion of the experimental curves by trying to recori-struct their form on the basis of models of spherical micelles showing niutual correlations. It proved, however, to be difficult to distinguish the effect of the shape of micelles from that of their correlations. We have undertaken this problem by using an iin-proved sinall-angle X-ray scattering technique, de-veloped in our laboratory, which is based upon intensity measurements on an absolute scale.

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Authors

  • F. Reiss-Husson

  • Vittorio Luzzati

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