Characterization of Langmuir-monolayers of molecular clips by means of Brewster-angle-microscopy

Molecular clips are able to selectively bind electron deficient aromatic and aliphatic substrates, and these processes are usually investigated in dilute solutions of organic solvents. Caused by discrepancies between polar and hydrophobic groups, molecular clips are surface-active compounds and, in analogy to surfactants, they can form monomolecular films at the water surface. In this publication, we systematically investigated the self-association process and the phase-behaviour of three different molecular clips with the polar head groups -OCH₂COOH (a), -OCH₂COOEt (b), and -OCONHPh (c) by means of surface-pressure-area-isotherms and Brewster-angle-microscopy (BAM). We observed marked differences for all investigated surface-active compounds. The molecular surface areas of the three clips, determined from pressure-area-isotherms, could be traced back to the molecular diameters of the amphiphilic compounds. In several experiments we investigated the influence of diverse film compression and expansion steps. Hysteresis effects could be explained by different film morphologies. In a series of experiments we could show that the aromatic guest molecule 1,2,4,5-tetracyanobenzene (TCNB), which strongly binds to molecular clips, did not influence the phase diagrams and film structures.