Dynamic and equilibrium surface pressure of adsorbed dodecanol monolayers at the air/water interface

A first-order phase transition can occur in adsorbed monolayers of dodecanol dissolved in an aqueous bulk phase. The surface pressure adsorption kinetics Pi(t) and Brewster angle microscopy (BAM) studies are effectively combined for characterizing the two-dimensional phase transition. A characteristic break point in the dynamic surface pressure curves indicates the phase transition which is largely affected by the dodecanol concentration in the aqueous solution and on the temperature. Above a certain temperature and below a corresponding bulk concentration, phase transition in the adsorbed monolayer does not occur. After the phase transition point, formation and growth of condensed phase domains are visualized by BAM. The surface pressure-area (Pi-A) isotherms of spread dodecanol monolayers show the characteristic main phase transition point indicating two-dimensional condensation of dodecanol in the monolayer. At the same temperature, the surface pressure values of the main phase transition point in the spread dodecanol monolayer are approximately equal to those measured for the adsorbed monolayer. The experimental results can be well-described by the theory of the diffusion kinetics of surfactant adsorption from solutions, which assumes the formation of small and large aggregates in the adsorbed monolayer. The parameters of the theoretical model which describes the state of the spread dodecanol monolayer indicate the formation of average dimers in the precritical expanded region of Pi-A isotherms, while in the transcritical region aggregated domains coexist in equilibrium with monomers and average dimers. The standard thermodynamic characteristics calculated are used to compare the adsorption of dodecanol from the aqueous solution and the dodecanol aggregation in the adsorbed monolayer.