Time-Dependent Phase Segregation of Dendrimer/n-Alkylthiol Mixed-Monolayers on Au(111): An Atomic Force Microscopy Study

The two-dimensional phase behavior of mixed-monolayers composed of poly(amidoamine) (PAMAM) dendrimers and n -alkylthiols adsorbed to Au(111) surfaces was studied using tapping mode atomic force microscopy (TM-AFM). Mixed-monolayers were prepared by sequential immersion of Au(111) substrates into ethanolic solutions of, first, dendrimers and then n -alkylthiols. Films were prepared by systematically varying the dendrimer generation (G4, G6, and G8) and the n -alkylthiol chain length (CH 3 (CH 2 ) x SH; x = 4, 11, 15). TM-AFM was used to monitor time-dependent morphological changes in the mixed-monolayers. Dendrimer monolayers immersed in hexane solutions of n -alkylthiols do not phase separate to an appreciable extent, which demonstrates the important role of dendrimer solvation in these processes. In contrast to the amine-terminated dendrimer monolayers, thiol-terminated dendrimers do not phase separate when exposed to ethanolic n -alkylthiol solutions, suggesting that one of the driving forces for this process is the difference in adsorption energies between the amine and thiol groups and the Au substrate.