Modification of the properties of NaDS micellar solutions by adding electrolytes and non-electrolytes: investigations with decyl eosin as a pKa-probe

The following ways to modify the properties of the sodium dodecyl sulfate micellar pseudophase were used: (1) variation of NaCl concentrations in the bulk, (2) the complex formation of counter-ions with crown ethers, (3) the replacing of Na+ by Mg2+, accompanied by MgSO4 addition, (4) introduction of a non-ionic surfactant Tween 80 and of 1-pentanol. The apparent ionization constants, Kaa, of hydrophobic indicator decyl eosin (HR⇄R−+H+) were determined in micellar solutions under the mentioned conditions. All these additions were shown to reduce the pKaa, which was interpreted as being caused by the electric potential of the Stern layer growing more positive. The effects observed were compared with those registered earlier with the help of hydrophobic coumarin and cationic indicators; the reasons of diversities appearing in some cases are explained.