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 pe ntaoxyethylene glycol ether (C12E5) at the air-water surface using both ell ipsometry and tensiometry. Two simple models for the monolayer structure we re used to derive the equilibrium adsorption isotherm from ellipsometric da ta and tested for their validity by comparison with neutron reflection and tension data. A monolayer model comprising a lower layer containing the eth oxy headgroups plus water and an upper layer containing the dodecyl tail gr oups was found to yield an isotherm consistent with all the data. The slow adsorption kinetics (minutes to hours) was monitored using both ellipsometr y and dynamic tension. Dynamic tension curves reconstructed from the ellips ometric data using the equilibrium adsorption isotherm were in agreement wi th measured dynamic tensions. The adsorption rate for a C12E5 monomer conce ntration corresponding to adsorption equal to 75% of the maximum adsorption was found to be diffusion controlled. Lower concentrations of C12E5 showed adsorption rates faster than diffusion-controlled adsorption, probably due to convection.