Amphiphile-Induced Anisotropic Colloidal Self-Assembly.

Marcel Rey,Taotao Yu,D Martin A Buzza,Nicolas Vogel,Karina Bley,Katharina Landfester

doi:10.1021/acs.langmuir.8b01382

Abstract

Spherical colloidal particles typically self-assemble into hexagonal lattices when adsorbed at liquid interfaces. More complex assembly structures, including particle chains and phases with square symmetry, were theoretically predicted almost two decades ago for spherical particles interacting via a soft repulsive shoulder. Here, we demonstrate that such complex assembly phases can be experimentally realized with spherical colloidal particles assembled at the air/water interface in the presence of molecular amphiphiles. We investigate the interfacial behavior of colloidal particles in the presence of different amphiphiles on a Langmuir trough. We transfer the structures formed at the interface onto a solid substrate while continuously compressing, which enables us to correlate the prevailing assembly phase as a function of the available interfacial area. We observe that block copolymers with similarities to the chemical nature of the colloidal particles, as well as the surface-active protein bovine serum albumin, direct the colloidal particles into complex assembly phases, including chains and square arrangements. The observed structures are reproduced by minimum energy calculations of hard core-soft shoulder particles with experimentally realistic interaction parameters. From the agreement between experiments and theory, we hypothesize that the presence of the amphiphilesmanipulates the interaction potential of the colloidal particles. The assembly of spherical colloidal particles into complex assembly phases on solid substrates opens new possibilities for surface patterning by enriching the library of possible structures available for colloidal lithography.

Highlights

Colloidal particles are useful building blocks to fundamentally study self-assembly phenomena1–4 as well as to engineer functional materials with a defined structure at the nanoscale.5–7 When adsorbed at a liquid interface, colloids are able to crystallize into ordered twodimensional lattices.1,8,9 Depending on the balance between attractive van der Waals and capillary forces and repulsive electrostatic and dipole forces, monodisperse spherical colloidal particles typically form close packed or non-close packed arrangements with hexagonal symmetry.3,11 These colloidal monolayers can be deposited onto a solid substrate, providing a strategy to create ordered nanoscale surface patterns in a simple and fast process over macroscopic dimensions
We investigated commercially available surfactants (Triton X-100, sodium dodecyl sulfate (SDS)), low molecular weight ionic block copolymers (BCP) poly(acrylic acid)-block-poly(methyl methacrylate) (poly(AA15-block-MMA15))55 and the surface active protein bovine serum albumin (BSA)
For large colloidal particles (d = 1.1 μm) we directly visualized the assembly in situ by mounting the Langmuir trough set-up on top of a conventional optical microscope equipped with a camera

Summary

Introduction

Colloidal particles are useful building blocks to fundamentally study self-assembly phenomena as well as to engineer functional materials with a defined structure at the nanoscale. When adsorbed at a liquid interface, colloids are able to crystallize into ordered twodimensional lattices. Depending on the balance between attractive van der Waals and capillary forces (arising from contact line undulations10) and repulsive electrostatic and dipole forces, monodisperse spherical colloidal particles typically form close packed or non-close packed arrangements with hexagonal symmetry. These colloidal monolayers can be deposited onto a solid substrate, providing a strategy to create ordered nanoscale surface patterns in a simple and fast process over macroscopic dimensions. Depending on the balance between attractive van der Waals and capillary forces (arising from contact line undulations10) and repulsive electrostatic and dipole forces, monodisperse spherical colloidal particles typically form close packed or non-close packed arrangements with hexagonal symmetry.. Depending on the balance between attractive van der Waals and capillary forces (arising from contact line undulations10) and repulsive electrostatic and dipole forces, monodisperse spherical colloidal particles typically form close packed or non-close packed arrangements with hexagonal symmetry.3,11 These colloidal monolayers can be deposited onto a solid substrate, providing a strategy to create ordered nanoscale surface patterns in a simple and fast process over macroscopic dimensions. The directionality of the capillary interaction forces can be manipulated with anisotropic particles or a defined curvature of the liquid interface to create square symmetries or particle chains. Liquid crystal interfaces can guide spherical colloids into 1D chains or 2D crystals. Topographically prestructured surfaces provide an alternative engineering to guide the assembly of colloids. Common to all of these approaches is that the anisotropy of the final assembly is externally imposed onto the colloidal particles via process conditions, force fields, or the properties of the substrate

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Discussion

Conclusion