Before we tell you more details about the pretty picture that has brought you here, you should know somethings about surfactants and the structures they can form. The following should serve as a poor man's introduction of the surfactants.

Surfactants are molecules which can be considered to be made up of two parts:

• A part that likes water e.g. an ionic group
• AND a part that hates water e.g. an organic chain.

When surfactant molecules are mixed in water, at very low concentrations nothing dramatic happens. They just dissolve. However as we add more and more surfactant molecules, above a critical concentration known as CMC, it becomes energetically favourable for the molecules to form aggregates such that the part that only the water liking part is in contact with water. The water hating parts bundle up together and a large variety of such molecular aggregates have been observed. The geometry and size of these aggregates depends on the shape of the molecules and the charges they carry.
Some of these structures are shown below. If you are interested further and wish to learn more, reference 1 is a good book.

Figure 1

surfactant aggregates

One would expect the two oppositely charged species to come together through electrostatic attraction and form disordered aggregates.

In our group we have been investigating the behavior of negatively charged polystyrene particles 1 micron in diameter in a mixture of a neutral (Triton X-100) and a cationic (didodecyl-dimethyl-diammonium bromide, which we shall call DDAB*) surfactant. The particles were introduced in the solutions at a very low concentration(~0.4%).
The particles' size was chosen so that they can be easily observed under an optical microscope and we chose the surfactants that were easily available at that time. Triton X-100 is shaped like the single tailed molecule like A in figure 1 above and forms micelles, while DDAB* is a double tailed molecule like B in the same figure. DDAB* forms bilayers and these bilayers can form vescicles. When the two surfactants are mixed, they form mixed bilayers and vesciles in addition to the micelles and DDAB* bilayers.

• When the nagatively charged polystyrene particles were introduced in a solution of Triton X-100, the particles remained uniformly dispersed and were seen to exhibit brownian motion. This was true for all concentrations (up to 2000 CMC) of Triton X-100 we tried (e.g. see figure 2).



Figure 2

• If we took a solution of DDAB* alone, the particles formed disordered aggregates. The size of the aggregates depended on the concentration of DDAB* used (e.g. see figure 3).



Figure 3

• However when we used a mixture of these surfactants, for a wide range of their relative ratios and over-all concentrations (above their CMC),
  initially the particles were nicely dispersed, but after a few weeks, several two dimensional crystallites (one of which is shown in figure 4) were observed in the solution amidst the randomly dispersed particles.



Figure 4

• I samples that are 55 to 70 CMC of DDAB* and 45 to 30 CMC Triton X-100 (CMC of Triton X-100 = 100 - CMC of DDAB*) one dimensional ordering in the form of rings was observed (see figure 6)



Figure 6

• Making use of the fluorescent surfactant we could see that particles on these curves are located at the boundaries of there vescicles (figure 7).



Figure 7

• In addition to the crystallites and rings vescicles covered with particles, forming hollow sphere like structures were also seen. One such vescicle parly adhearing to the glass cover slip but covered with particles otherwise can be seen in figure 8.



Thus one can reconstruct the process of the formation of the crystallites:
Certain negatively charged Polystyrene beads stck to the positively charged vescicles
Once the particles are stuck on the surface of the vescicle, their brownian motion perpendicular to the vescicle surface is highly restricted. The particles though are free to move (under brownian forces) along the vescicle surface and in time find an energitically favourible state (which is a crystallite).
After crystallite formation, it is difficult for all the particles in the crystallite to follow the fluctuations of the vescicle membrane and so the crystallites break off from the vescicles and float away.



• The presence of some salt (~25mM NaCl) makes it easier (in terms of time scale) to observe the intermediate stages of crystallite formation. The last figure (figures 9) show these stages.


The left frame shows some particles stuck on the vescicle.
In the middle one can see a crystallite on the surface of the vescicle.



Figure 9


At right is seen a crystallite fallen away from the vescicle.

Reference

• Intermolecular & Surface Forces by Jacob Israelachvili, second edition, Academic Press

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This work was completed by Laurence Ramos and Kapeeleshwar Krishana while they were post-docs with the Weitz Group. Check out what Laurence is doing now.