The effect of 1-alkanols on ionization of sodium dodecyl sulfate micelles

Abstract For an aqueous sodium dodecyl sulfate solution including small amounts of l-alkanols (C 4 −C 7 ), it is confirmed that a linear relation is valid between d ln ( x f ) dy f (CMC-depressing power of the alkanol) and K (partition coefficient of the alkanol between bulk water and micelle phases). From the linear relation, we estimate a constant value of θ defined as −d ln ( x f ) dy f = θ·K; θ (=0.69) is independent of the kind of alkanol and x f and y f are mole fractions of monomerically dissolving surfactant and alkanol, respectively: x f corresponds to the critical micelle concentration (CMC) of the surfactant. Some aspects of θ are discussed from the viewpoint that θ reflects an interaction of the surfactant and the alkanol in their mixed micelles. The effects of solubilized alkanol on the degree of ionization of ionic micelles (α) and on the electrical potential at the micellar surface (φ) are numerically estimated on the basis of θ and through thermodynamic relations. In the relations, a factor like the “osmotic coefficient” is introduced to correct the nonideality of monomer and micellar surfactant species, in place of the familiar activity coefficient. From this factor it is found that solubilized alkanol causes an increase in α (i.e., ∂α / ∂y m > 0) but has little influence on ψ (i.e., ∂ψ / ∂y m ≈ 0), where y m is the mole fraction of the alkanol in the micellar phase.