The surface adsorption and micelle formation of the mixed aqueous solutions of fluorocarbon and hydrocarbon surfactants: II. Sodium perfluorooctanoate-sodium decylsulfate system

Abstract

The adsorption and micellar behavior of the mixed solutions of sodium perfluo- rooctanoate (7CFNa) and sodium decylsulfate (C_(10)SNa) have been studied (the ionic strength of the solutions are kept at 0.1m by adding NaCl). The adsorption has been calculated from the surface (and interfacial) tension-concentration curves by applying Gibbs equation Γ_T=-(1/RT)dγ/d lnm_(7CF)=-(1/RT)dγ/d lnm_(C_(10)S~-) where Γ_T(=Γ_(7CF-)+Γ_(C_(10)S~-)) is the total amount of adsorption. It has been found that the cmc's of 7CFNa and C_(10)SNa are of nearly the same value (1.66×10~(-2)m and 1. 45 ×10~(-2)m, respectively), but γ_(cme) of 7CFNa solution is～23mNm~(-1) which is much lower than that of C_(10)SNa solution. This implies that 7CFNa would have a much higher surface activity than C_(10)SNa, and 7CFNa would be adsorbed preferentially. For instance, 7CFNa has a surface mole fraction of about 0.8 in the saturated adsorption layer of the 1: 1 molal mixed solution. The mole fractions of 7CFNa at the surface are always greater than those in the bulk solutions. The adsorption at the n-heptane-aqueons solution interface is quite different from that at the air-solution surface. Here C_(10)SNa is preferentially adsorbed owing to mutual phobicity between the HC-chain of n-heptane and the FC-chain of 7CFNa at the interface. All the cmc of 7CFNa(or C_(10)SNa) in the mixed solutions obtained from the γ-logm relations have nearly the same value and the cmc_T-x curves show a positive deviation from the ideal case. This further indicates that in the mixed solutions of fluorocarbon and hydrocarbon surfactants there is no completly miscible micelle but essentially the individual micelle of each surfactant due to the mutual phobicity between FC- and HC-chain in the micellization process.