Phase transition and domain morphology in Langmuir monolayers of a calix[4]arene derivative containing no alkyl chain

The monolayer features of 5,11,17,23-tetra-tert-butyl-25,27-di(benzoylamidoethoxy-26,28-dihydroxyl calix[4]arene (DBAC) at the air/water interface are experimentally studied. Using combined studies of surface pressure-area (π-A) isotherms, surface pressure relaxation, and monolayer morphology by Brewster angle microscopy (BAM), it is demonstrated that this calix[4]arene type, having no long alkyl chain, nevertheless, can form stable Langmuir monolayers. The π-A isotherms show a pronounced plateau region between ∼90 and ∼60 A 2 /molecule after a linear increase of surface pressure. A first-order phase transition in the monolayer state takes place in the linear part of the pressure increase of the π-A isotherm for 5 °C ≤ T ≤ 25 °C clearly indicated at 5 °C by a small inflection region between 102 and 99 A 2 /molecule and the simultaneous formation of condensed phase domains. The differences of the densities of the coexisting fluid and condensed phase are small. Temperature increase reduces the tendency to form the quasicrystalline condensed monolayer phase. The relaxation studies reveal that the DBAC monolayers are not in equilibrium under the conditions of compression/decompression at which the π-A isotherms were recorded. The corresponding BAM studies provide the exact area value for the onset of the two-dimensional phase transition. Under these conditions well-shaped monolayered dendritic structures are formed. The pronounced plateau region of the compression/decompression isotherm between ∼90 and 60 A 2 /molecule represents the transition from a tightly packed monolayer to a bilayer/multilayer structure. Possible conformers of the DBAC molecule were calculated by structure optimization using force field Tripos 5.2. The dimensions calculated for the conformer with the low cross-sectional area are in reasonable agreement with the features of the monolayer obtained by the surface pressure and BAM studies.