CCFE - Center for Complex Fluid Engineering - Carnegie Mellon University

Surfactant molecules self assemble to form aggregates, or micelles, in aqueous solution. Micelles in rod-like and worm-like conformations (see figure) have very high aspect ratios and unique solution properties, making them ideal for use as rheological modifiers and solubilizing agents. Other applications include use in medical applications such as drug delivery and membrane synthesis or use for templating in nano-material synthesis. Unfortunately, micelles exist in a precarious balance of hydrophobic and electrostatic forces. Small variations in system conditions affect this balance, making the aggregate structure extremely unstable and difficult to involve in multi-step processing. A method for engineering aggregates with both a stable, predictable structure and the unique properties of micelles will greatly increase their usefulness in engineering applications. One way to achieve this goal is to synthesize surfactants with vinyl groups on an associated counterion, thereby allowing the micelles to be polymerized, and rendering their structure insensitive to bulk conditions.
The system currently being studied in this research is cetyltrimethlyammonium 4-vinylbenzoate (CTVB), which consists of a cationic surfactant (CTA+) and polymerizable counterion (VB-). In solution, CTVB forms elongated, cylindrical, or worm-like, micelles. Upon addition of a free radical initiator, however, the counterions react resulting in shorter, rod-like polymerized aggregates. Small angle neutron scattering (SANS) measurements have shown that resulting aggregates maintain the same diameter as the initial micelles, which is fixed at 4 nm by the tail length of the chosen surfactant. Unlike the unpolymerized micelles, however, these aggregates are highly stable and insensitive to changes in temperature or concentration. NMR studies have confirmed the conversion of monomer to polymer and suggest that the polymerized counterions are immobilized within the aggregates.
Currently, we are quantifying the effects of varying polymerization conditions on product structure and dimensions. Changing the starting initiator concentration has a strong effect on the resulting aggregate length, which has been shown by static light scattering (SLS) to vary from 30 to 250 nm. The longer aggregates have shown signs of flexibility, while the shorter micelles appear rigid. By combining SLS and SANS data, all length scales will be characterized, allowing us to fully quantify the structure of several polymerized micelle systems. With this knowledge, we will be able to construct kinetic models for predicting and controlling the final structures generated through this complex, two-phase reaction.

The figure shows both SANS and SLS data for a 1 mg/mL sample of CTVB polymerized using 1% by weight CTVB and 7.5% initiator/CTVB. The data are fit with a simple rigid rod model for a cylinder of radius 2 nm and length 35 nm. The deviation at low-q is due to a combination of polydispersity and flexibility of the sample. These effects will be incorporated into future model fitting.