SPEAKER: Christine DeWolf (Professor, Department of Chemistry and Biochemistry, Concordia University, Canada)
TITLE: Pi-interactions in Langmuir Films: A Tale of Structured Monolayers and Multilayers (and Their Interconversion)
ABSTRACT: It has long been known that strong headgroup interactions will influence lipid and surfactant molecular self-assembly. In Langmuir films their influence is observed as changes to film structure, organization, morphology and rheology. In this seminar, our recent work probing the balance of competing pi-stacking and hydrogen bonding interactions will be presented with two systems, namely surfactants containing aminobenzoquinones and phenolic headgroups. In both cases, the presence of pi-stacking interactions yields strong directional interactions that dominate the phase behaviour and lead to highly rigid films with unusual film morphologies. Grazing incidence x-ray diffraction (GIXD) revealed that these were associated with the formation of crystalline headgroups, a feature not often observed for lipids or surfactants with small headgroups. The long-range ordering of the headgroups is not only evident from the diffraction patterns but also supported by computational studies. This pi-stacking driven headgroup organization then leads to the formation of molecular superlattices and crystalline 3-D structures. The latter, observed with the dialkylaminobenzoquinones, were observed to co-exist with both tilted and untilted monolayers and, despite the heterogeneity, was found to be a highly reversible and reproducible process. In the case of the phenols, the relative strength of the pi-interactions can be observed via the invariance of the unit cell parameters, despite compression, deprotonation and/or addition of an w-thiol. The latter, which creates a bola amphiphile, also induces controlled and regular multilayer formation. While the utility of introducing pi-stacking to control ordering is well known in liquid crystals, there are relatively fewer reports of the use for creating and controlling long-range ordering in monolayer films.
Meeting ID: 942 2008 0385
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