The Determination of Critical Concentrations for the Formation of Soap Micelles by the Spectral Behavior of Pinacyanol Chloride

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Abstract
The absorption spectrum of pinacyanol chloride in aqueous solutions of anionic soaps changes sharply to that characteristic of its solutions in organic solvents over a short range of soap concentration. This effect is attributed to the formation of micelles, in whose hydrocarbon‐like layers or cores the dye is solubilized. The concentration of soap at which this spectral change occurs is taken as ``the critical concentration for the formation of micelles.'' In water the dye exhibits an absorption band at 5500A which disappears rapidly as the concentration of molecular soap increases. Thus, even 0.00006 molar potassium laurate gives a large effect. At about 0.0126 M the γ‐band at about 4800A reaches a high intensity, which falls, with further increase of concentration, very rapidly close to the critical concentration. At 2.3 to 2.4×10−2 for potassium laurate and 6×10−3 for the myristate the α‐band (ca. 6150A) and the β‐band (ca. 5700A) begin to increase rapidly in intensity and these are considered as the critical concentrations, since these are the prominent bands in the solution of the dye in an oil. Alkali oleate and dilinoleate soaps exhibit an additional band (ω‐band) at about 5200A. Some other critical concentrations determined by this method are 3.6–5.2×10−4 molar for sodium cetyl sulfate, 2.5–3.2×10−2 molar for potassium dehydroabietate, 7–12×10−4 molar for potassium oleate, and 2.5×10−4 molar for potassium dilinoleate. The values for the last two soaps are uncertain, since their behavior differs considerably from that displayed by the salts of saturated acids. All measurements were made at 25.8°C with the exception of the sodium cetyl sulfate which was investigated at 35.8°C. An absorption band at approximately 4800A appears in all the soap solutions measured below the critical concentration with the exception of potassium dilinoleate. This band is absent when salts or bases are added to the dye solution. A titrametric method has been developed for the determination of critical concentrations of soaps in the presence of added salts and hydrocarbons.

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