Orientation of rod molecules in selective slits: a density functional theory

Abstract

A density functional theory (DFT) is used to investigate molecular orientation of rod fluids inselective slits. The DFT approach combines a modified fundamental measure theory (MFMT)for excluded-volume effect, the first-order thermodynamics perturbation theory for chainconnectivity and the mean-field approximation for van der Waals (vdW) attraction. To studythe molecular orientation, the intramolecular bonding orientation function is introducedinto the DFT. First, we investigate the orientation of the surfactant-like rod molecule ofAB6 (i.e. ABBBBBB)in a nanoslit of H = 20σ, where the walls selectively adsorb segment ‘A’. It is observed that, with the increase ofthe surface energy of the wall to head segment (i.e. ‘A’ segment) of the rod molecule, therod molecules adsorbed on the wall present the perpendicular orientation gradually, andassemble into a smectic-A-like monolayer finally. In addition, we also explore themolecular orientation of the rods with both end segments preferring to the wall,i.e. AB8A andAB7A, in ananoslit of H = 10σ.Interestingly, the AB8A rod monolayer is compatible with either a smectic-A-like or a smectic-C-like organization, butAB7A rod molecules exhibit the smectic-A-like organization. The orientation factor of theAB7A rod molecule reaches1, suggesting that AB7A rod molecules self-assemble into an ordered structure with perfectly perpendicularorientation to the wall.