Association of sodium ions to aqueous alkylsulfate and alkanoate micelles in the presence of 1-alcohols

Electromotive force and conductivity measurements have been used to study the counter ion association to micelles of sodium dodecyl sulfate, sodium dodecanoate, and sodium octanoate. The conductivity and electromotive force measurements match very well as both is mainly a measure of the amount of free counter ions in the solutions. Addition of alcohols to these micellar solutions show some differences between octanoate micelles on one hand, and dodecyl sulfate and dodecanoate on the other. Propanol and pentanol addition lead to an increase in the sodium activity for dodecyl sulfate and dodecanoate solutions that are well above the c.m.c. while a decrease is observed for octanoate. Addition of higher alcohols show that the counter ion activity and the conductivity go through a maximum at low alcohol contents and then decreases. At high surfactant contents alcohol addition may induce a shape transition from small spherical micelles to larger rod- or disc-like micelles. It appears that the degree of counter ion association increases when these larger micelles form.