The observation of minima in the surface tension-concentration curves for aqueous solutions of various surface-active materials has been the cause of considerable conjecture. Pownev and Addison (12), in particular, found welldefined minima in the surface tension of solutions of sodium salts of primary alcohol sulfates containing from twelve to eighteen carbon atoms. The minima are as much as 5 dynes per centimeter lower than the value reached at higher concentrations. These "anomalous" results have presented apparent disagreement with Gibbs' adsorption equation. The hypotheses proposed to reconcile such incongruities have been frequently discussed (1 to 12).Usually the explanations proposed fall into two groups: First, it has been claimed that there are experimental difficulties which result in errors in the methods used for measuring surface tension (3, 4, 7). On the other hand, the possible existence of abrupt changes in colligative properties of solutions of certain surface-active substances has been emphasized ( 12) and this could lead to minima in the surface tension-concentration curves.The data which we present here deal with solutions and procedures not covered by either of the above explanations.
PROCEDUREThe sulfated alcohols used in this work were of a particularly high degree of purity. The details of the methods used in their preparation are given in a report entitled "Some Properties Involving Surface Activity of Sodium Salts of Primar}* and Secondary Alcohol Sulfates" (5).
The preparation and properties are described for alternate members of a homologous and of an isomeric series of purified sodium salts of secondary alcohol sulfates, containing from 11 to 19 carbon atoms, and for a straight hydrocarbon chain with the sulfate group in various positions. The purified sodium salts of the primary alcohol sulfates with 10, 12, 14, and 16 carbon atoms are also examined.Measurements have been made of the surface tension and foaming, wetting, and detersive properties of solutions of these compounds in water and with added electrolytes. Solubilities of sodium salts of the secondary and primary alcohol sulfates studied are reported at 50 intervals from 20°to 4(j°C. The data are discussed from the point of view of correlating changes in the properties involving surface activity when the structure and molecular weight of the compound are changed.
Electromotive force measurements were made with sodium-respopsive glass electrodes and saturated KCl, calomel electrodes on mixtures of sodium alkyl sulfates. Two solution pairs were studied: sodium decyl and dodecyl sulfates and sodium dodecyl and tetradecyl sulfates. Critical micelle concentrations were selected from plots of p y a us. log c. The procedure of Mysels and Otter was used to establish the non-micellar concentrations in the micellar solutions of mixtures. The extent of the dissociation of sodium counterions by the micelles was estimated from the reduced data. From the activity coefficients strong interaction below the critical micelle concentration was inferred for all the mixtures except those containing more than 25 mole Y, c f sodium decyl sulfate.The present investigation examines the behavior of aqueous solutions of mixed colloidal electrolytes by means of electromotive force (e.m.f.) measurements based on a sodium-responsive glass electrode. Botr6, Crescenzi, and Mebe' carried out similar measurements for solutions of a few single colloidal electrolytes by using cationic organic membranes. In their discussion they treated the micelles as polyelectrolytes.It often is convenient to treat micellar solutions of colloidal electrolytes as a system of quasi-phases, one, an aqueous or non-micellar medium, within which a second, the micelles, is distributed. Lange2 recognized that in a micellar system, which contains two surface active Eiolutes, the critical micelle concentration (c.m.c.) varies with the relative amounts of the two solutes and that the locus of the c.m.c. was a kind of phase boundary of the aqueous medium. On the basis of a theoretical model, Mysels and Otter3 recently employed conductivity measurements to obtain the boundary of the mixed micelles and the tie lines to the aqueous medium; they discussed also the thermodynamics of the system.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.