Anisotropic adsorption of molecular assemblies on crystalline surfaces

Orientational order of surfactant micelles and proteins on crystalline templates has been observed but, given that the template unit cell is significantly smaller than the characteristic size of the adsorbate, this order cannot be attributed to lattice epitaxy. We interpret the template-directed orientation of rodlike molecular assemblies as arising from anisotropic van der Waals interactions between the assembly and crystalline surfaces where the anisotropic van der Waals interaction is calculated using the Lifshitz methodology. Provided the assembly is sufficiently large, substrate anisotropy provides a torque that overcomes rotational Brownian motion near the surface. The probability of a particular orientation is computed by solving a Smoluchowski equation that describes the balance between van der Waals and Brownian torques. Torque aligns both micelles and protein fibrils; the interaction energy is minimized when the assembly lies perpendicular to a symmetry axis of a crystalline substrate. Theoretical predictions agree with experiments for both hemi-cylindrical micelles and protein fibrils adsorbed on graphite.