Bp. Binks et al., Equilibrium and dynamic adsorption of C12E5 at the air-water surface investigated using ellipsometry and tensiometry, LANGMUIR, 16(23), 2000, pp. 8926-8931
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.