Phase transition beyond the monolayer collapse – The case of stearic acid spread at the air/water interface

Abstract The paper presents a detailed characterization of stearic acid film in various experimental conditions, recorded both in the pre- and post-collapse region, using – in addition to π–Α isotherms – visualization of film structure and thickness measurements (using Brewster angle microscope, BAM) supplemented with a precise analysis of compressibility modulus versus area (Cs−1- A) plots. π–Α isotherms recorded for unionized molecules exhibit a liquid-condensed (LC) state where the thickness (th) and the compressibility modulus (Cs−1) remain constant with monolayer compression along a plateau region, where a tilt angle of 73.5°, in respect to the water surface, was calculated for stearic acid molecule. Films subjected to subsequent compressions (beyond the collapse) and expansions show semi-reversible behavior as proved with BAM images. Upon the increase of carboxylic group ionization in alkaline pHs and increasing the speed of compression, the “spike” at the collapse changed its shape from sharp to rounded. Upon further compression, a long plateau transition has been observed. Within this post-collapse plateau, a three-fold increase in film thickness was observed, confirming the previously suggested model of trilayer formation. However, the visualization of film structure with BAM revealed that this process does not occur homogeneously, but through the nucleation of trilayer structures in the form of domains that undergo a continuous process of growth and coalescence within the post-collapse plateau region. The application of the adapted Clausius-Clapeyron equation indicates that the process of trilayer formation is endothermic and implies an increase of entropy, which has been additionally proven by PM-IRRAS.