Static and dynamic properties of calixarene monolayers at the air/water interface. 1. pH effects with p-dioctadecanoylcalix[4]arene

Abstract

Newly synthesized p-dioctadecanoylcalix[4]arene is shown to form stable monolayers at the air/water interface over a wide pH range (1.8-12). Surface pressure-area isotherms obtained with the Wilhelmy plate technique are used to infer that the molecular packing at the onset of film collapse is consistent with a conelike conformer having all of the phenol oxygens of the calixarenes adsorbed to the interface. Increasing pH leads to a stepwise dissociation to phenoxides and attendant expansion of the monolayer attributable to enhanced electrostatic repulsions among the negatively charged phenoxide polar heads. Analysis of the area per molecule as a function of pH reveals two distinct steps and an intermediate plateau. Considering the presence of two types of chemically inequivalent phenol groups and the potential for intramolecular hydrogen bonding, this behavior was interpreted in terms of acid-base equilibria in which the first step corresponds to an apparent surface pKa,1 ≈ 6.4 while the second step is consistent with an apparent surface pKa,2 ≈ 9.2 (assuming pKa,3, pKa,4 ≫ 12). As for the dynamics of the monolayers, the technique of surface light scattering (SLS) was employed to probe the surface viscoelasticity. Although pH had a dramatic effect on the molecular packing of p-dioctadecanoylcalix[4]arene monolayers, SLS results indicate that all films exhibited the limiting behavior of infinite lateral modulus dynamics when the surface pressure exceeds 1 mN·m-1, and such behavior is established to be independent of pH. From these findings, it is deduced that the packing of the aromatic rings and long octadecanoyl side chains, rather than electrostatic interactions of the phenoxide headgroups, is responsible for the lateral rigidity of such monolayers. © 1999 American Chemical Society.