Chain-length dependence of lipophilic force: comparison with the two-body van der Waals' force

Hydrophobic dodecanethiol capped Au nanoparticles (AuNPs) form two-dimensional patterns in monolayers of amphiphilic fatty acids ([Formula: see text]) at the air water interface. An interplay between the various lipophilic interactions, in turn, decides the NP cluster size, where stronger NP-monolayer and monolayer-monolayer attraction in fatty acid monolayers with longer tail length oppose nanoparticle aggregation resulting in a decrease in cluster size in both in-plane and out-of-plane directions. The decrease in the in-plane cluster size is steepest for 14  <  n  <  18, n being the total number of C atoms in the fatty acid, and levels off for higher fatty acids and cannot be explained on the basis of the two body van der Waals pair potential atleast at initial phases of pattern formation. The potential can be used only at later times, closer to stability.