Equilibrium and surface rheology of two polyoxyethylene surfactants (CiEOj) differing in the number of oxyethylene groups

A study of equilibrium adsorption properties and surface dilational rheology of two polyoxyethylene surfactants of technical grade have been carried out. It has been shown, that equilibrium adsorption and surface rheology are greatly influenced by the oxyethylene chain. To explore the equilibrium properties, a pendant drop profile tensiometer was used. The equilibrium surface pressure isotherms can be explained in the framework of the reorientation model. The increase in the number of oxyethylene units leads to higher surface activity at low concentration when the surfactants adsorb in the state of maximum molar area. However, at higher concentration the adsorbed surfactant molecules reorient to a state of minimum molar area. In this state the surface activity is, on contrary, lower for C₁₀EO₁₄ due to higher hydrophilicity. Dilational rheology was studied by means of the oscillating drop method. The surfactant with higher EO units showed an enhanced elasticity. Moreover, whereas C₁₀EO₆ experimental data can be interpreted on the basis of the diffusional theory and a diffusion coefficient of 4 × 10⁻¹⁰ m² s⁻¹ was estimated, C₁₀EO₁₄ behavior is more complex maybe due to the multitude of possible adsorption conformations and is closer to that expected for polymer surfactants.