Equilibrium and Dynamic Adsorption of C₁₂E₅ at the Air−Water Surface Investigated Using Ellipsometry and Tensiometry

We have investigated the equilibrium and dynamic adsorption of n -dodecyl pentaoxyethylene glycol ether (C 12 E 5 ) at the air−water surface using both ellipsometry and tensiometry. Two simple models for the monolayer structure were used to derive the equilibrium adsorption isotherm from ellipsometric data and tested for their validity by comparison with neutron reflection and tension data. A monolayer model comprising a lower layer containing the ethoxy headgroups plus water and an upper layer containing the dodecyl tail groups was found to yield an isotherm consistent with all the data. The slow adsorption kinetics (minutes to hours) was monitored using both ellipsometry and dynamic tension. Dynamic tension curves reconstructed from the ellipsometric data using the equilibrium adsorption isotherm were in agreement with measured dynamic tensions. The adsorption rate for a C 12 E 5 monomer concentration corresponding to adsorption equal to 75% of the maximum adsorption was found to be diffusion controlled. Lower concentrations of C 12 E 5 showed adsorption rates faster than diffusion-controlled adsorption, probably due to convection.