The influence of 1-butanol and electrolytic background on the properties of CTAB micelles as examined using a set of indicator dyes

The ionic equilibrium of six indicator dyes, bromophenol blue, thymol blue, sulfonefluorescein, n-decyl ester and methyl ether of fluorescein, and N,N′-di-n-octadecylrhodamine was studied in CTAB micellar solutions. It was revealed that the values of the apparent ionization constants, Kaapp, of the indicators are substantially influenced by additions of 1-butanol either at low or high bulk ionic strength created by NaBr additions. Whereas the pKaapp values at ionic strength of 0.05 M as a rule decrease to more or less extent as compared to the corresponding pKas in water, the penetration of the alcohol into the micellar pseudophase results in either increase or decrease in pKaapp. The dependence of pKaapp vs. log[Br−] at 0.80 M n-C4H9OH allows estimating the degree of counter-ion binding under such conditions: β = 0.40 ± 0.01. The distinct deviation of the last quantity from the corresponding value, β = 0.76 ± 0.02 registered in butanol-free CTAB micellar solution is caused by the decrease in the interfacial concentration of the cationic head groups within the Stern layer. The molar fraction of 1-butanol in micelles and the partition constant of this alcohol between the aqueous phase and micelles were estimated.