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 pK a,1 ≈ 6.4 while the second step is consistent with an apparent surface pK a,2 ≈ 9.2 (assuming pK a,3, pK a,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.

Interfacial behavior of common food contact polymer additives

“…In this respect, the Irganox species studied in this work bear some resemblance to a family of calixarenes studied extensively by Esker et al [39,40]. Both the Irganox and calixarenes have phenol groups and attached hydrophobic, alkyl segments.…”

“…Furthermore, monolayers of both species exhibit a linear dependence of surface pressure on area and similar collapse pressures. However, the structure of the Irganox monomers-especially IN1010-and the calixarene monomers studied in references [39] and [40] are markedly different. Calixarenes in contact with water will orient themselves so that all the hydroxyl groups on the phenols are in contact with the subphase leading to strong intramolecular interactions.…”

Pyrogallol[4]arenes Show Highly Variable Amphiphilic Behavior at the Air–Water Interface Dependent Upon Side Chain Length and Branching

“…In 1988, Regen et al recognized that these macrocyclic amphiphiles could self-assemble at the air-water interface. [3] They reported stable monolayer formation on a Langmuir trough by both mercurated and alkylated calix [6]arenes derivatives. [3] Because these macrocycles have open interior space, they were attractive candidates as supramolecular filters and sensors.…”

“…Numerous attempts were made to stabilize both calix[-n]arene and calix[n]resocinarene monolayers. Throughout the remainder of the 1980s and the 1990s, the Langmuir trough was used to examine the stabilizing effects of varying the ionic composition, [4] ionic strength, [5] and pH [6] of the subphase on which the calix[n]arene derivatives were spread. Investigations into the effects of interior cavity size, [7] conformation, [8,9] and substitution [10][11][12][13][14][15][16] on monolayer formation and stability have been conducted by several groups during the past two decades.…”

“…[18] Molecular recognition has been demonstrated with various biologically relevant guests by using the Langmuir trough. These include studies by Hianik et al, [19] Hegner et al, [20] Pieles et al, [21] Konovalov et al, [22] and Faul et al [23] Calix [6]arenes have been especially well studied in solution, [12,24] at the air-water interface, [25] and in the solid state, [26] revealing self-assembly properties, gaseous permeation, and selective guest recognition. Applications have ranged from volatile molecule sensing [27] to the detection of neurotransmitters.…”