View source for Surface roughness directed self-assembly of patchy particles into colloidal micelles

Daniela J. Kraft, Ran Ni, Frank Smallenburg

"Surface roughness directed self-assembly of patchy particles into colloidal micelles"

Entry by [[Fei Pu]], AP 225, Fall 2012


'''Keywords:''' [[self assembly]], [[colloids]], [[Monte Carlo]], [[Micelles]], [[Particles]]


==Summary==


This articles describes how [[self-assembly]] of colloidal particles into larger structures has potential for creating materials with unprecedented properties, such as full photonic [[band gaps]] in the visible spectrum. Colloidal particles with site-specific directional interactions, so called [[patchy particles]], are promising candidates for bottom-up assembly routes. 

An experiment with patchy colloidal particles was done based on independent surface roughness specific interactions. Smooth patches on rough colloids are shown to be exclusively attractive due to their different overlap volumes. The article discusses in detail the case of colloids with one patch that serves as a model for molecular surfactants with respect to their geometry and their interactions. These one-patch particles assemble into clusters that resemble surfactant micelles, called [[colloidal micelles]]. Similarities as well as differences between the colloidal model system and molecular surfactants are also discussed and quantified by employing computational and theoretical models.

==Materials and Methods==

Colloidal particles consisting of one smooth and one rough sphere were synthesized following a modified synthesis by Kim et al. (27). Roughness on the seed particles was obtained through adsorption of polystyrene particles nucleated during polymerization. The synthesized colloids were washed and redispersed in 0.3% w∕w aqueous polyvinyl alcohol (Mw ¼ 30–50 kg∕mol).

Monte Carlo simulations were used in the canonical ensemble (NVT) to calculate the probability distribution of the cluster. Also, The free energy of clusters of different sizes was calculated using grand-canonical Monte Carlo (GCMC) simulations on single clusters (41).

==Results & Discussion==

When two colloids overlap each other, the depletion entropy increases, and such phenomenon makes the colloids attract more closely. The effect of rough particles interacting with smooth particles and other rough particles was recorded and analyzed. At higher concentrations and thus stronger attractions, the roughness anisotropic colloidal particles spontaneously organized into clusters, in which the attractive parts constitute the core of the aggregate and the non-attractive rough sides are located at the outside. These structures look like micelles, as shown below in Figure 1. 

[[Image:Aggregate.jpg|thumb|center|400px|
Figure 1. The rough and smooth colloidal particles interact and spontaneously bond into clusters, which could be called [[colloidal micelles]]]]


Monte Carlo simulation was further done on the smooth and rough colloids, shown in Figure 2. As time went on, it seems like smooth particles attracted into clusters and formed the core of the micelles and that the rough particles stayed outside and surrounded the core. 

[[Image:Colloid Micelle.jpg|thumb|center|400px|
Figure 2. Smooth particles clustered on the inside of the micelles while the rough particles surrounded the outside.]]

Finally,cluster size distributions changed as interactions increased and geometry overlapped more. From Figure 3, it's clear that as density, p, increased, clusters were more prone and easily formed. When density was low at the beginning, the particles flowed freely and even repelled. Only when density reached a threshold that the surface energy and forces favored clustering. 

[[Image:Cluster size distributions.jpg|thumb|center|400px|
Figure 3. As density of particles increased, they overlapped more and formed clusters.]]

==Soft Matter Applications==

Due to the available variety of colloids and their straightforward assembly even between different patch sizes, it is expected that
these soft colloidal particles with smooth and rough surfaces could self-assemble in a controlled manner into superstructures with desired topology and properties. This has significant applications. For example, the virus macromolecules, protein subunits, and building cell blocks in our body are often complex and challenging to identify key elements for self-assembly processes. By mimicking such self-assembly processes on a colloidal scale, insights into the paramount elements that control the assembly can be obtained in situ and applied to build up superstructures with new and desirable properties. The findings in this article have fundamental and practical importance in the field of colloidal and macromolecular self assembly.