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

Abstract

Micellar solutions of several amphiphiles in water, without added electrolytes, have been studied by small-angle X-ray scattering methods, based upon intensity measurements on an absolute scale. The concentration range goes from 5% to the appearance of a liquid crystalline phase (30 to 40% in most cases). Several amphiphiles have been studied at different temperatures: sodium lauryl sulfate (27 and 70°), sodium laurate (70°), myristate (80°), palmitate (80°), stearate (90°), and oleate (NaO, 27°), and cetyltrimethylammonium chloride (CTACl, 27°) and bromide (27, 50 and 70°). The interpretation of the experimental curves is discussed. It is shown that over various concentration ranges the micelles may be assumed 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 dependent on various parameters: nature of the amphiphile, temperature, and concentration. In most systems the micelles are spherical at low concentrations and become rod-like at high concentration. In two cases, the structure appears to be independent of the concentration: CTACl micelles are spherical from 5 to 38%; NaO micelles are rod-like from 4 to 20%. This polymorphism is discussed.