Electronic Effects on the Self-Assembly of Monolayers from Surfactants Possessing Aromatic Headgroups

Surfactant monolayers at interfaces provide a platform to investigate molecular self-assembly in pseudo-two-dimensional systems. It has been previously reported by this group that Langmuir monolayers of octadecylgallate (ODG), a phenolic surfactant, display long-range headgroup–headgroup ordering observable using grazing-incidence X-ray diffraction. Such ordering is unusual in a surfactant with a small headgroup. To achieve rational design of such interfacial structures through tuning of the headgroup–headgroup distance and orientation, the cause of the ordering must be known. As these causes can be difficult to determine using experiment alone, herein, computational methods are employed. Models of gallate monolayers with 42 monomers per monolayer are calculated with and without (a) the ester linkage to the tail and (b) the hydroxyl groups meta to the tail, using GFN1-xTB, a semiempirical method to determine the effects and contributions of these moieties on the monolayer structure. The computed unit cell distances for the monohydroxy ester system mimic the ODG unit cell distances observed experimentally. Overall, there is substantially less variation in ring···ring stacking and horizontal offset (shear) distances in ester models than in alkyl models, and the packing on average is tighter. Frontier molecular orbitals are calculated for each type of monomer using ωB97X-D/6-31+G(d,p). Both the ester linkage and the meta- hydroxyl groups lower the HOMO–LUMO gap, and the ester moiety changes the character of the monomer LUMO. The ester also enables additional noncovalent interactions between monomers in the monolayer. It is hypothesized that these factors combine to explain the increased ordering observed.