(429f) Droplet Drying – One Molecule at a Time

Self-assembled structure of long-chain carboxylic acids in the shape of nano-spirals was observed by atomic force microscopy in the spin coated film on graphite. The nano-spirals formed when solvent mixture of methanol and m-cresol was used. In contrast, spin coating using only methanol as the solvent produced epitaxial films of stripe nanopatterns and using only m-cresol disordered structure. The nano-spirals have uniform line height of 0.7 nm and line width equaling twice the molecular chain length. The small size of the spirals, ~ 60 nm in diameter, is attributed to the fast solvent removal and thinning of the film. Due to the disparity in vapor pressure between the two solvents, droplets of m-cresol-rich liquid remaining on the substrate serve as templates for the self-assembly of carboxylic molecules at the solid/liquid/vapor interface. The spiral starts with a carboxylic acid dimer depositing at the droplet edge and the spiral grows one dimer at a time with each incoming dimer carbon skeleton plane facing the previous one and aligning along the triple phase line. The spiral ends with total depletion of either the solute or the solvent. Equal numbers of clockwise or counterclockwise inward rotating spirals were found. The self-assembly behavior of the long-chain carboxylic acids allows the visualization of the droplet vaporization process one molecule at a time.