A viscometric study of the micelles of sodium dodecyl sulfate in dilute solutions

Abstract

The viscosities of solutions of sodium dodecyl sulfate of concentrations up to 0.8 percent in distilled water and in 0.01- to 0.12-Af sodium chloride have been measured. By introducing the concept of a monomer saturation concentration it is possible to determine the intrinsic viscosity of the micelles at each concentration of sodium chloride. A method for the experimental determination of the monomer saturation concentration is presented. The dependence of the intrinsic viscosity on the salt concentration is discussed in terms of electroviscous and hydration effects. The data indicate the presence of spherical micelles in the solutions investigated. 1- Introduction Since the early experiments of Krafft [I}1 and McBain [2] on soap solutions, and the subsequent interpretation of their results in terms of the existence of micelles in such solutions, many researches [3, 4] have been undertaken to investigate the size and shape of these particles. It is now generally assumed that there are at least two types of micelles. In the concentration range between the onset of micelle formation (about 0.1 to 0.25 percent by weight) and about 1 to 2 percent, a small, highly charged and highly conducting micelle is thought to exist. This type of micelle has been discussed at some length by Hartley [5, 6]. He considered them as spheres, but there has been no general agreement on their shape. A large micelle, haying a structure discernible by X-ray diffraction, exists at higher concentrations (about 10 percent or greater). They have been discussed and investigated by McBain [2], Hess [7], Philippoff [8], and others [9]. Harkins [10] and his coworkers interpret their later X-ray diffraction data for concentrated solutions in terms of cylindrical micelles. Brady [11], however, utilizing a treatment considered by Corrin [12], interprets his data for concentrated sodium dodecyl sulfate solutions in terms of a radial distribution of spherical micelles. Recent light-scattering investigations by Debye [13] indicate that in the presence of high concentrations of electrolyte, the micelles of hexadecyl trimethyl ammonium bromide are rodlike. The purpose of this research was to investigate the viscometric behavior of dilute solutions of a pure, well-defined detergent, sodium dodecyl sulfate. Detergent concentrations up to 0.8 percent by weight were used. Distilled water and 0.01 to 0.12 M sodium chloride solutions were used as solvents. 2. Materials The' sodium dodecyl sulfate was synthesized, as described by Shedlovsky [14], from a vacuum-distilled sample of n-dodecyl alcohol. The chlorosulfonic acid used in the synthesis was distilled immediately before use. All other reagents used in the synthesis and subsequent purification conformed with Amerii Figures in brackets indicate literature references at the end of this paper. can Chemical Society specifications. Prior to making up solutions for the measurements, the detergent was extracted with diethyl ether for about 8 hr in a Soxhlet extractor. After a measurement, the detergent was recovered from solution in the following manner: The solution was evaporated to dryness and the residue taken up in ethyl alcohol. The alcohol solution was filtered and evaporated to dryness. This residue was then taken up in water and crystallized. Finally, the crystals were extracted with ether.