Surfactant-Mediated Two-Dimensional Crystallization of Colloidal Crystals
Title: Surfactant-Mediated Two-Dimensional Crystallization of Colloidal Crystals
Authors: Laurence Ramos, T.C. Lubensky, Nily Dan, Philip Nelson, D.A. Weitz
Source: Science 286, 2325 (1999) http://www.sciencemag.org/content/286/5448/2325.full.pdf
In this article, the authors develop a new method of forming self-assembled ordered structures of particles in a colloidal suspension, using charged surfactants to stabilize the ordered structure. They find that using a specific surfactant causes the formation of robust two-dimensional crystals of latex particles suspended in solution. The authors briefly describe the physical properties of the crystals and suggest possible uses, including use as templates for growth of other nanostructured materials, such as photonic devices.
Self-assembly of particles requires careful balancing of attractive and repulsive forces and thermal motion of the particles. If the attractive forces between particles are too great compared to the available thermal energy, the particles will instantly stick to each other, forming a disordered aggregate state. Ordinarily, Coulombic interaction is considered too strong of an attractive force to form any kind of ordered state, and previous work with charged surfactants and particles seemed to support this idea. However, the authors realized that surfactants by themselves can self-assemble into several very interesting ordered structures, and that the interactions between these ordered structures and charged particles had not yet been investigated. The methods they chose to study this problem are outlined in the following sections.
The authors used a mixture of two surfactants: decyldimethylammonium bromide (DDAB), which is a double-chained, positively charged surfactant, and polyoxyethylene (9-10) p-tert-octyl phenol (or Triton X-100), which is a single-chained, charge neutral surfactant. They chose these two surfactants because they have similar head group areas (0.6 nm^2) and critical micelle concentrations. One key area in which the surfactants differ is that DDAB forms bilayer vesicles above its critical micelle concentration, while Triton X-100 forms ordinary micelles. In mixtures of DDAB and Triton X-100, the two surfactants form vesicles of mixed composition.
Results and discussion
The authors took these mixtures of DDAB and Triton X-100 and added negatively charged polystyrene latex spheres in small concentrations. They found that the resultant structure depended on the concentrations of each surfactant:
- If neither surfactant was above the critical micelle concentration, the latex spheres remained in suspension.
- In a solution consisting only of charge-neutral Triton X-100 micelles, the latex spheres remained in suspension.
- In a solution consisting of only positively-charged DDAB vesicles, the latex spheres formed a disordered aggregate state.
- In a solution consisting of vesicles of mixed composition, the latex spheres formed large, ordered, two-dimensional, single-layer crystals (Fig. 1).
This final behavior was completely unexpected, and so the authors performed several studies to examine the properties of these crystals. The authors found that the crystals were very robust and did not break apart even when subjected to flow or pulled on with laser tweezers. The authors repeated their experiments, using a fluorescent tag attached to the surfactant molecules, allowing them to obtain images showing that each negatively charged particle was completely surrounded by surfactant molecules of positive charge, due to Coulombic interactions. Figure 2 shows the images taken by a laser fluorescence scanning confocal microscope at different focus depths through the plane of the crystal.
The authors also came to the counterintuitive realization that these two dimensional crystals did not form at the air-water interface, but formed in the bulk solution. This phenomenon occurs because the crystals are forming on the surfaces of the vesicles, as shown in Figure 3. As the particles adsorb to the vesicle, they form two-dimensional rafts on the vesicle surface, which limits their growth to the two-dimensional plane of the surface. Strangely, not all the vesicles are fully covered when the system reaches equilibrium – the adsorption of charged particles on the vesicles is self-limited. This is thought to be because, as particles adsorb onto the positively charged surface of the surfactant vesicle, negatively charged counterions are released into the interior of the vesicle. These counterions cannot pass through the vesicle wall, and as they build up, they begin to affect the charge on the opposite surface of the vesicle from where the particles are, eventually making it unfavorable for more particles to interact with the vesicle surface.
This research has several interesting applications and new directions in which to go. There are several more complicated self-assembled surfactant structures which could be created by use of block copolymers or other surfactants. It could be interesting to see how charged particles interact with a very complex surface. As a processing technique, this colloidal crystallization could serve to make templates for other nanoporous or microporous devices.