Effects of the Polypropylene Oxide Number on the Surface Properties of a Type of Extended Surfactant

The surface properties of 5 extended surfactant C_12–14P _mE₂S solutions in pure water and 0.1 M NaCl were investigated through surface tension and electrical conductivity measurements. The surface properties measured include the critical micelle concentration (CMC), critical surface tension (γ_cmc), maximum surface excess concentration (Γ_max), minimum area occupied per surfactant molecule (A_min), and efficiency in surface tension reduction (pC₂₀). The CMC values of the 5 surfactants decreased with increasing polypropylene oxide number (PON) and were higher than those obtained in 0.1 M NaCl. The Γ_max values showed a downward trend whereas the A_min values exhibited an upward trend with increasing PON without NaCl. The Γ_max values were higher and the A_min values were lower than those obtained without 0.1 M NaCl. The CMC values increased at elevated temperatures. The CMC values of C_12–14P₃E₂S, C_12–14P₅E₂S, and C_12–14P₈E₂S were similar but were markedly lower than those of C_12–14E₂S at different temperatures. When PON was less than 12, the log CMC value decreased linearly with increasing PON in the absence of salt, and the relationship between pC₂₀ and PON was linear. But in the presence of 0.1 M NaCl, the log CMC value decreased exponentially with increasing PON.