Mixed monolayer and mixed micelle formation in binary mixture of surfactants—Systems for trioxyethylenated dodecyl sulfonate and some polyoxyethylenated fatty alcohol ether nonionic surfactants

The interaction and synergism of some polyoxyethylenated fatty alcohol ether (POE) nonionic surfactants (C₁₂E₂, C₁₂E₃, C₁₀E₅, C₁₀E₇, where C_x indicates number of carbon atoms in the chain and E_y indicates number of oxyethylene glycol ethers) with trioxyethylenated dodecyl sulfonate (C₁₂E₃S) in mixed monolayer formation at the surface and in mixed micelle formation in aqueous solutions were studied at 25 and 40°C by calculating interaction parameters (β_α, β_M) from surface tension-concentration data by use of Rosen's equations based on the nonideal solution theory. All the systems investigated adapt reasonably well to the nonideal model, with negative values of β_σ and β_M (where M means micelle and σ refers to the air-liquid interface) indicating a favorable interaction between the mixed surfactants. Either at a monolayer or in a mixed micelle, the attractive interaction becomes stronger when the alkyl chain in the POE surfactant is longer, i.e., when the POE becomes more hydrophobic. The interaction increases in the order C₁₀E₇10E₅12E₃, C₁₂E₂. For the two C₁₀E _n (n= 5,7)/C₁₂E₃S systems, as temperature increases from 25 to 40°C, the interaction increases in a mixed micelle, but it decreases in a mixed monolayer. Synergism in mixed micelle formation exists for C₁₂E₃S/C₁₀E _n mixtures when X₁ ^M , the mole fraction of POE in a mixed micelle, is ≈0.4–0.8, whereas synergism does not occur in the systems of C₁₂E₃S/C₁₂E _m due to the large difference between CMC₁ and CMC₂, i.e., large |In(C ₁ ^M /C ₂ ^M )| value (where CMC=critical micelle concentration). The degree of synergism in mixed micelle formation is temperature independent and is 0.23, 0.18, and close to zero for C₁₀E₅/C₁₂E₃S, C₁₀E₇/C₁₂E₃S, and C₁₂E _m (m=2,3)/C₁₂E₃S systems, respectively. Synergism in surface tension reduction effectiveness occurs in C₁₂E₃S/C₁₂E₂ and C₁₂E₃S/C₁₂E₃ systems. The mole fractions of POE in the solution phase are 0.302 and 0.333 for the two mixtures at the point of maximum synergism.