Effect of Nanoscale Patterning in Surfactant-Coated Nanoparticles On Their Interfacial Properties: A Molecular Dynamics Study

Experiments have shown that interfacial properties of substrates covered by mixed surfactant monolayers depend on the pattern formed by the adsorbed surfactants [1,2]. Interestingly, when a nanoparticle (NP) is coated with a mixture of hydrophilic and hydrophobic molecules in a striped pattern, the total work of adhesion of the NP varies non-monotonically with the hydrophilic stripe width [1]. The basis for this non-monotonic behavior is not clear. Here we present results from our systematic molecular dynamics study of interfacial properties of NPs covered with hydrophilic and hydrophobic ligands in aqueous solutions for a variety of surface patterns. Based on detailed statistical analysis of data from our simulations we infer that two competing effects viz. cavitation and enhanced hydrophilicity, lead to the non-monotonic behavior observed in experiments. With an increase in hydrophilic stripe-width accompanied by a decrease in the hydrophobic stripe-width, there is emergence of the cavitation effect [3] that leads to stress in the h-bond network of surrounding water and therefore decreases work of adhesion. However, an increase in hydrophilic stripe-width is also accompanied by an increase in h-bonding between water molecules and the hydrophilic ligands, and therefore leads to an increase in the work of adhesion. These two competing effects, both of which depend on stripe-width, combine in a complex way to result in the non-monotonic dependence of work of adhesion on stripe width. Finally, we provide a brief comparison between the experimental and simulational findings.

1. JJ Kuna, K Voïtchovsky, C Singh, PK Ghorai, H Jiang, S Mwenifumbo, MM Stevens, F Stellacci and SC Glotzer, “On the Role of Nanometer Scale Structure on Interfacial Energy”, submitted 2009.

2. A Centrone, E Penzo, M Sharma, JW Myerson, AM Jackson, N Marzari and F Stellacci, “The role of nanostructure in the wetting behavior of mixed-monolayer-protected metal nanoparticles”, PNAS, 9886, 105 (29), 2008.

3. D Chandler, “Interfaces and the driving force of hydrophobic assembly”, Nature, 640, 437, 2005.