Surface Rheology Investigation of the 2-D Phase Transition in n-Dodecanol Monolayers at the Water-Air Interface

n-Dodecanol is the last even linear chain alkanol soluble in water. Owing to this feature, previous investigations have shown in n-dodecanol monolayers at the water-air interface the existence of 2-D phase transitions, resulting in the formations of large molecular aggregates beyond a critical surface pressure. Here the rheological properties of this adsorption layer have been investigated in relation with the appearance of these phase transitions. To this aim, the surface dilational viscoelasticity has been measured as a function of the surface pressure by a specific oscillating drop procedure implemented on a pendant drop tensiometer. Different oscillation frequencies were investigated. The results show that the module and phase of the dilational viscoelasticity exhibit specific features that can be correlated to the different stages of the formation of a continuous liquidlike layer at an interface initially void from surfactant. In the regions of the gaslike 2-D phase the results agree with the prediction of the generalized Volmer model, which was already applied to interpret the equilibrium properties. In the region of coexistence between gaseous and liquid phases, just beyond the critical surface pressure, a model accounting for the presence of the aggregation process, previously developed by the authors, has suitably been applied to interpret the data. The comparison of the measured data with the model prediction allows the rate constants and other features of the aggregation process to be accessed.