Equilibrium and dynamics of soluble surfactant penetration into a Langmuirmonolayer of a 2D aggregating homologue

The penetration dynamics of a dodecyldimethylphosphine oxid dissolved in th e aqueous substrate into a Langmuir monolayer of eicosanyldimethylphosphine oxide and the equilibrium characteristics of the mixed monolayer formed ar e experimentally and theoretically studied. For the theoretical description of the low-density gaseous monolayers, the generalized Szyszkowski-Langmui r equation of state and the adsorption isotherm equation were applied. Afte r the main phase transition, at which the gaseous phase starts to coexist w ith a condensed phase as a result of 2D aggregation, the monolayers are con sidered within the framework of a generalized Volmer equation and a quasich emical model for the a,aggregational equilibrium. The equations of state fo r the mixed monolayer are derived corresponding to different assumptions ab out the composition of 2D aggregates. The adsorption isotherm equations for soluble surfactant within the tightly packed monolayer are derived from th e simultaneous solution of the corresponding equations of state and Pethica 's equation. For the mixture of insoluble eicosanyldimethylphosphine oxide and soluble dodecyldimethylphosphine oxide, the experimental results were s hown to agree well with the theoretical predictions based on the characteri stics of these surfactants in the single-component solutions and monolayers . The model based on mixed aggregates is best suited to match the experimen tal values both for II-A and II-t isotherms.