Crosslinked Micellar Gel: A Promising New Material for Environmental Remediation and Controlled Drug Delivery
S. R. Kline, NCNR/CHRNS
Surfactants are commonly used in detergency applications where several hundred surfactant molecules aggregate to form a micelle consisting of a hydrophobic core that is shielded from its aqueous environment. Unwanted (often oily) solutes are spontaneously solubilized in the hydrophobic cores of the micelles, providing a homogeneous dispersion of oil in water where the oil is effectively concentrated inside the micelle. Unfortunately, the oil-containing micelles are typically too small (ca. 5 nm) to be removed from the water by conventional methods such as filtration or centrifugation. Recently, a crosslinked micellar gel has been synthesized at the NIST Center for Neutron Research (NCNR) that retains the features and functionality of surfactant micelles and benefits from the structural stability of a crosslinked polymer matrix. The material, a crosslinked micellar gel or "macro-micelle", is prepared by a simple one-step aqueous polymerization of the surfactant cetyltrimethylammonium 4-vinylbenzoate to which a small quantity of a crosslinker, divinyl benzene, has been added. As prepared, the gel is only 1 % surfactant and 99 % water by mass, but can easily be handled and cut to a desired shape.
Physical characterization of the micellar gel at the NCNR by small-angle neutron scattering has confirmed that the structure of the gel is that of cylindrical micelles between crosslinks, as shown in Figure 1. The micelles have a circular cross-section of 4 nm, and an average pore size of 40 nm. The ability of the micellar gel to solubilize cyclohexane is also shown in Figure 1, and the gel has a solubilization capacity similar to unpolymerized micelles. The most significant advantage of the micellar gel versus unpolymerized micelles is that once oily material has been solubilized and concentrated in the gel, the gel (and the contaminant) can simply be removed from the solution, since the gel is macroscopic in size. This material clearly has promise in use for wastewater cleanup (gasoline, for example). Other possible applications include a controlled release matrix for drug delivery, a porous matrix for gel electrophoresis, or a selective filter membrane. A patent is pending on the material and work is continuing toward defining a class of crosslinkable micellar materials.
Figure 1. SANS intensity from a crosslinked micellar gel, compared to the same gel after equilibrating with cyclohexane-saturated water. The "break" in the scattering curve indicative of the cylinder cross section occurs at a lower q-value for the cyclohexane-swollen gel, indicating that the cylinder diameter has increased to accommodate cyclohexane molecules. The additional scattering from the cyclohexane-swollen gel over the majority of the q-range is due to the increase in hydrocarbon volume fraction by the solubilized cyclohexane.
Last modified 10-June-2002 by website owner: NCNR (attn: Jeff Krzywon)