2D-3D transformations of amphiphilic monolayers influenced by intermolecular interactions: A Brewster angle microscopy study

The mechanism of the morphological 2D-3D transformations of insoluble supersaturated monolayers of long chain alkanoic acids, and their methyl ester and salt derivatives is studied directly at the air/water interface by means of Brewster angle microscopy. The phase changes occurring upon continuous overcompression of nonionic and charged monolayers are compared. The relationship between the shape of the monolayer isotherms in the region of the attained maximum surface pressure and the morphological features of the 2D-3D transformations is investigated. The effects of the hydrocarbon chain length variation (C-14, C-18, C-22), hydrophilic head groups ionization, strength of interfacial hydrogen bonding, temperature, and compression rate on the morphology of the 3D phase, separated from the overcompressed monolayers, are monitored. It is found that the interaction of the monolayers with the aqueous subphase (through head groups ionization or hydrogen bonding) has the strongest influence-on the morphology of the 2D-3D transformations for the shortest chain length (C-14) alkanoic acid homologue studied. The obtained results are discussed in relation to the nucleation-growth-collision theory of supersaturated monolayers (Vollhardt, D. Adv. Colloid Interface Sci. 1993, 47, 1.). It can be concluded that the morphological features of the overgrown 3D structures are determined by the balance between the monolayer compression rate and the nucleation and growth rates (sensitive to intermolecular interactions) of the 3D phase.