(553d) Self-Assembly and Phase Transition of Amphiphiles at Solid-Liquid Interfaces

Thermodynamic state of adsorbed molecules governs colloidal stability and interfacial activity of nanoparticles. External stimuli, such as pH and temperature have been widely used to program the self-assembly of amphiphilic molecules in bulk solvent by altering their protonated/deprotonated state. While the molecular phase behavior of amphiphiles in aqueous solutions has been widely studied, the morphology and phase transition on highly curved solid-liquid nanoparticle interface is poorly understood. Here we use molecular dynamics simulations combined with small-angle neutron scattering experiments to investigate the effect of protonated/deprotonated state on the morphology and phase behavior of long-chain fatty acid molecules adsorbed on positively charged silica surface. We show that the addition of counter-ions controls the relative concentration of fatty acid molecules in protonated and deprotonated states. In addition, we present an operational state diagram identifying distinct morphologies of the adsorbed fatty acid molecules, and interrelate these states with nanoparticle interfacial activity. This study provides an understanding of the effect of curvature on the thermodynamic state of molecules including the role of electrostatic, hydrophobic and steric interactions on the self-assembly process at nano-interfaces.