Electrostatic complexation of carboxylic acid derivatized silver colloidal particles with fatty amine Langmuir monolayers. Role of neutral spacer molecules in the monolayer

In a series of papers we have investigated the electrostatic complexation of carboxylic acid-derivatized metal colloidal particles with charged fatty amine Langmuir monolayers and the formation of Langmuir Blodgett (LB) films of the colloidal particles. The role of the subphase pH in controlling the cluster density at the air-water interface and in the builtup films was addressed. In this paper we advance our study to include the effects of an uncharged spacer molecule, octadecanol, on the colloidal particle complexation at the air-water interface in mixed Langmuir monolayers of octadecanol and octadecyl amine. The surface density of the charged amine molecules was controlled by varying the molar concentrations of amine and alcohol molecules in the Langmuir monolayer and the time dependence of the complexation followed using π-A isotherms, quartz crystal microgravimetry and UV-vis spectroscopy measurements. The equilibrium cluster density at the air-water interface and consequently in the LB films was enhanced as the fraction of amine molecules in the Langmuir monolayer increased. The complexation of the clusters at the air-water interface is slow, taking ∼15 h to reach equilibrium densities. Cluster density-dependent shifts in the surface plasmon resonance were observed in builtup LB films and have been explained in terms of an effective medium theory. Ellipsometry studies of the builtup multilayer films indicate that fairly uniform cluster density films can be deposited by the LB technique. The results presented in this paper are consistent with an electrostatic interaction-mediated process for the complexation of carboxylic acid-derivatized colloidal particles with fatty amine Langmuir monolayers.