We studied the dynamics of adsorption/desorption of soluble surfactant
(C(12)E(6)) into an insoluble n-dodecyl p-toluenesulfonate (DTS) mono
layer at an air/water interface through stress relaxation experiments.
The resulting surface tension changes due to a small imposed surface
area disturbance, as a function of time, are measured using a capillar
y wave probe. We derived two theoretical models for this insoluble/sol
uble mixed surfactant system. The first assumes that adsorption/desorp
tion of surfactant molecules in the presence of an insoluble monolayer
is controlled by diffusive matter interchange between bulk and surfac
e, while the second assumes that adsorption/desorption of the soluble
component is controlled by an energy barrier mechanism due to the inso
luble monolayer. It is found that measured stress relaxation experimen
tal data are in good agreement with the diffusion-controlled model, Bo
th the diffusion characteristic frequency, omega(0), and high-frequenc
y limit surface modulus, epsilon(0), were found to increase with incre
asing surface concentration of the insoluble monolayer. The increase i
n omega(0), or the surface relaxation rate, is shown to be related to
the static property (partial derivative C-2/partial derivative Gamma(2
))(Gamma 1), where Gamma(1), Gamma(2), and C-2 represent the surface c
oncentrations of DTS and C(12)E(6) and the bulk concentration of C(12)
E(6), respectively.