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    Strong Headgroup Interactions Drive Highly Directional Growth and Unusual Phase Co-Existence in Self-Assembled Phenolic Films

    Year: 2019

    Journal: ACS Appl. Mater. Interfaces, Volume 11, 1-Dec, page 45354–45363

    Authors: Lamarche, Renaud Miclette; DeWolfe, Christine

    Organizations: Natural Sciences and Engineering Research Council (NSERC) of CanadaNatural Sciences and Engineering Research Council of Canada; Canada Foundation for Innovation (CFI)Canada Foundation for Innovation; Division of Chemistry (CHE), National Science Foundation [NSF/CHE-1834750]; Division of Materials Research (DMR), National Science FoundationNational Science Foundation (NSF) [NSF/CHE-1834750]; U.S. DOEUnited States Department of Energy (DOE) [DE-AC02-06CH11357]

    Keywords: phenol; GIXD; Langmuir monolayer; crystalline headgroups; protonation state; herringbone packing; molecular lattice

    Self-assembled materials as surface coatings are used to confer functional properties to substrates, but such properties are highly dependent on molecular organization that can be controlled through tailoring the noncovalent interactions. For monomolecular films, it is well-known that strong, dipolar interactions can oppose line tension generating noncircular domain growth. While many surfactant films exhibit liquid crystalline arrangement of the alkyl chains, there are relatively few reports of crystalline headgroups. Here, we report the self-assembly of phenolic surfactants where the combination of hydrogen bonding and pi-stacking leads to a herringbone arrangement of the headgroups, generating a molecular super-lattice that can be observed using grazing incidence X-ray diffraction; such an arrangement has been previously proposed for related phenolic systems but never experimentally observed. We also investigated using pH to modulate the intermolecular interactions and the response of the system in terms of molecular organization. The first hydroxyl deprotonation does not appear to impact the structure but has significant impact on the domain size and morphology. Higher pH generates both strong directional domain growth and a loss of the molecular lattice structure, attributed to a second deprotonation. In contrast, a shorter chain surfactant, lauryl gallate, forms a liquid expanded phase that can contract upon deprotonation. In the condensed phase, the deprotonation kinetics are unusually slow for which an internal charge re-organization is proposed. The slow kinetics leads to the co-existence of three distinct phases for a single component system over relatively long timescales and provides evidence of a liquid-mediated polymorphic transformation process in two-dimensional, soft-matter films. This work has implications for understanding the long-range ordering in aromatic self-assembled structures and the mechanisms underlying Langmuir monolayer polymorphism.
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