DYNAMIC PENETRATION OF SURFACTANT INTO AN INSOLUBLE MONOLAYER

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.