Adsorption of Sulfonated Trioxyethylenated Alcohols at Air/Solution Interface

Abstract

Surface properties and adsorption thermodynamics and kinetics at the air/aqueous solution interface of three homologous trioxyethylenated fatty alcohol sulfonates, C₁₂H₂₅(OC₂H₄)₃SO₃Na (C₁₂E₃S), C₁₄H₂₉(OC₂H₄)₃SO₃Na (C₁₄E₃S), and C₁₆H₃₃(OC₂H₄)₃SO₃Na (C₁₆E₃S), have been investigated. Effectiveness and efficiency of the adsorption increase with decreasing temperature. Addition of NaCl to the solutions results in a lower effectiveness and efficiency of the adsorption. Increases in the length of hydrophobic chain results in a higher adsorption efficiency, but has almost no effect on adsorption effectiveness. On the other hand, the adsorption kinetic parameters n and t* increase with increases in hydrophobicity of the surfactants and attractive interaction between the surfactant molecules. Apparent diffusion coefficients obtained from short‐time experimental data and from longer‐time experimental data are in good agreement for the systems investigated, indicating that the adsorption is a process controlled strictly by diffusion of surfactant molecules in the solution. Further more, with increasing length of hydrophobic chain by two –CH₂–groups and in the order: C₁₂ → C₁₄ → C₁₆, the change of free energy of adsorption (ΔG _ad ⁰) has been calculated to be −3.1 kJ/mol (C₁₂ → C₁₄) and −2.8 kJ/mol (C₁₄ → C₁₆) per –CH₂– group, respectively. The absolute changes are a little bit lesser than the usual value of 3.5 kJ/mol. This may be due to the relative difference of hydrophilicity/hydrophobicity of the polyoxyethylene unit. Finally, modeling of Π_cmc has been done by quantitative structure–property relationship (QSPR) methods, and the calculated results are in good agreement with the experimental data.

Keywords:

• Sulfonated trioxyethylenated alcohol
• Adsorption
• Surface properties
• Quantitative structure‐property relationship
• Dynamic surface tension
• Standard thermodynamic parameters

Acknowledgments

Support of this work by the Natural Science Fund Foundation of China (Grant No. 20243005, 69971014) and the Fund Foundation of Guizhou Provincial Governor (Qian, No. 2001‐6) is gratefully acknowledged.