Facile surface modification of colloidal particles using bilayer surfactant assemblies: a new strategy for electrostatic complexation in Langmuir-Blodgett films

Abstract

Preliminary investigations have recently indicated that interdigitated bilayer assemblies of fatty acid molecules form spontaneously on colloidal silver particle surfaces while such bilayer structures are not observed on planar silver films. In this paper, this problem is probed further through contact angle and quartz crystal microgravimetry measurements of monolayer formation of lauric acid molecules on well-defined hydrophobic monolayers (formed from octadecanethiol chemisorbed on gold films) as a function of solution pH. The repulsive interaction between the ionized carboxylic acid groups in the lauric acid molecules prevents the formation of bilayer assemblies on planar surfaces. However, nanoscale surface curvature of colloidal particles permits interdigitation of the hydrocarbon chains in the bilayers, thereby maximizing the hydrophobic interaction as well as considerably reducing the electrostatic repulsive interactions of the headgroups, leading to stable bilayer assemblies. The strategy based on bilayer formation on colloidal particles is flexible and is used to derivatize colloidal silver particles with carboxylic acid and amine functional groups and thereafter electrostatically immobilize them at the air-hydrosol interface using the conjugate Langmuir monolayer. Good quality multilayer colloidal particle films can be deposited by the Langmuir-Blodgett technique, indicating that the bilayer assemblies on the colloidal particles are quite robust. This novel approach considerably extends the scope for the generation of nanoscale architectures using self-assembly of surface-modified colloidal particles.