Adsorption of Fatty Acid Molecules on Amine-Functionalized Silica Nanoparticles: Surface Organization and Foam Stability.

Yingzhen Ma,Jin Gyun Lee,Anne-Laure Fameau,Yao Wu,William A Shelton,Lilin He,Gernot Rother,Bhuvnesh Bharti

doi:10.1021/acs.langmuir.0c00156

Abstract

The crucial roles of the ionization state and counterion presence on the phase behavior of fatty acid in aqueous solutions are well-established. However, the effects of counterions on the adsorption and morphological state of fatty acid on nanoparticle surfaces are largely unknown. This knowledge gap exists due to the high complexity of the interactions between nanoparticles, counterions, and fatty acid molecules in aqueous solution. In this study, we use adsorption isotherms, small angle neutron scattering, and all-atom molecular dynamic simulations to investigate the effect of addition of ethanolamine as a counterion on the adsorption and self-assembly of decanoic acid onto aminopropyl-modified silica nanoparticles. We show that the morphology of the fatty acid assemblies on silica nanoparticles changes from discrete surface patches to a continuous bilayer by increasing concentration of the counterion. This morphological behavior of fatty acid on the oppositely charged nanoparticle surface alters the interfacial activity of the fatty acid–nanoparticle complex and thus governs the stability of the foam formed by the mixture. Our study provides new insights into the structure–property relationship of fatty acid–nanoparticle complexes and outlines a framework to program the stability of foams formed by mixtures of nanoparticles and amphiphiles.

Highlights

The aggregative adsorption of amphiphilic molecules on colloids governs the optical properties, interfacial activity, and stability of the particles in the dispersion.[1−6] The selfassembled state of amphiphilic molecules on colloidal particles is dependent on the interaction between: (1) particle surface and unadsorbed amphiphilic molecules, (2) amphiphilic molecules adsorbed on the surface and unadsorbed molecules in bulk solution, and (3) among the molecules adsorbed on the surface.[7−9] These interactions determine the morphology of surface-adsorbed aggregates of amphiphilic molecules and govern the interfacial activity of the colloidal particles with adsorbed amphiphiles
This study presents the effect of the counterion-to-fatty acid molar ratio, R, on the self-assembled state of fatty acid molecules on aminopropyl-modified silica nanoparticles
We show that the maximum amount of fatty acid that can be adsorbed on the mSiO2 increases with increasing R

Summary

Introduction

The aggregative adsorption of amphiphilic molecules on colloids governs the optical properties, interfacial activity, and stability of the particles in the dispersion.[1−6] The selfassembled state of amphiphilic molecules on colloidal particles is dependent on the interaction between: (1) particle surface and unadsorbed amphiphilic molecules, (2) amphiphilic molecules adsorbed on the surface and unadsorbed molecules in bulk solution, and (3) among the molecules adsorbed on the surface.[7−9] These interactions determine the morphology of surface-adsorbed aggregates of amphiphilic molecules and govern the interfacial activity of the colloidal particles with adsorbed amphiphiles. The adsorption mechanism and morphology of the surface aggregates formed by synthetic amphiphiles, such as non-ionic surfactants, onto hydrophilic nanoparticles are well-known and characterized.[10−12] the equilibrium morphology of natural amphiphiles, such as fatty acids, adsorbed onto nanoparticles in the presence of organic counterions and their impact on the interfacial activity of nanoparticles are poorly understood. This lack in understanding is due to the complexity of the phase behavior of fatty acid molecules on the nanoparticles, which is highly dependent on the concentration of counterions in the dispersion medium.[13,14]. The self-assembled state and phase behavior of fatty acid in aqueous solution

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