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 dodecano l dissolved in an aqueous bulk phase. The surface pressure adsorption kinet ics Pi(t) and Brewster angle microscopy (BAM) studies are effectively combi ned for characterizing the two-dimensional phase transition. A characterist ic break point in the dynamic surface pressure curves indicates the phase t ransition which is largely affected by the dodecanol concentration in the a queous solution and on the temperature. Above a certain temperature and bel ow a corresponding bulk concentration, phase transition in the adsorbed mon olayer does not occur. After the phase transition point, formation and grow th of condensed phase domains are visualized by BAM. The surface pressure-a rea (Pi-A) isotherms of spread dodecanol monolayers show the characteristic main phase transition point indicating two-dimensional condensation of dod ecanol in the monolayer. At the same temperature, the surface pressure valu es of the main phase transition point in the spread dodecanol monolayer are approximately equal to those measured for the adsorbed monolayer. The expe rimental results can be well-described by the theory of the diffusion kinet ics of surfactant adsorption from solutions, which assumes the formation of small and large aggregates in the adsorbed monolayer. The parameters of th e theoretical model which describes the state of the spread dodecanol monol ayer indicate the formation of average dimers in the precritical expanded r egion of Pi-A isotherms, while in the transcritical region aggregated domai ns coexist in equilibrium with monomers and average dimers. The standard th ermodynamic characteristics calculated are used to compare the adsorption o f dodecanol from the aqueous solution and the dodecanol aggregation in the adsorbed monolayer.